Post by: jakesilove on January 28, 2016, 08:00:25 pm
HSC CHEMISTRY Q&A THREAD
To go straight to posts for the new syllabus, click here.
What is this thread for?
If you have general questions about the HSC Chemistry course or how to improve in certain areas, this is the place to ask! 👌
Who can/will answer questions?
Everyone is welcome to contribute; even if you're unsure of yourself, providing different perspectives is incredibly valuable.
Please don't be dissuaded by the fact that you haven't finished Year 12, or didn't score as highly as others, or your advice contradicts something else you've seen on this thread, or whatever; none of this disqualifies you from helping others. And if you're worried you do have some sort of misconception, put it out there and someone else can clarify and modify your understanding!
There'll be a whole bunch of other high-scoring students with their own wealths of wisdom to share with you. So you may even get multiple answers from different people offering their insights - very cool.
To ask a question or make a post, you will first need an ATAR Notes account. You probably already have one, but if you don't, it takes about four seconds to sign up - and completely free!
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Original post.
Before you can ask a question, you'll have to make an ATAR Notes account here. Once you've done that, a little 'reply' button will come up when you're viewing threads, and you'll be able to post whatever you want! :)
Hey everyone!
A lot of you will have met me at the HSC Head Start lectures, where I lectured in 2U and 3U Maths, Physics and Chemistry.
My role on these forums is to help you. The HSC syllabus is tricky, nuanced and pretty damn huge. To help you out, I thought it would be a great idea to have a forum where you can just post questions, and myself or other forum members can post answers!
This is a community, so we want you to feel like you can post any type of Chemistry, no matter how "basic" you might think it is. Remember, IF YOU'RE HAVING TROUBLE WITH A TOPIC, THERE ARE THOUSANDS OF OTHERS HAVING THE SAME ISSUE. The best way to learn Maths is by looking through practice questions, and their associated answers. I honestly think a forum like this, and a place where I could always go to have difficult questions answered would have helped me in my HSC year.
Remember that Chemistry is a bloody difficult course. There will be lots of answers to the same questions, and I'll try give you the best or easiest to remember ones.
I got an ATAR of 99.80, and a mark of 93 in the Chemistry course. There are similar forums for a bunch of other subjects, so make sure to take a look at them as well!
Hey everyone!
A lot of you will have met me at the HSC Head Start lectures, where I lectured in 2U and 3U Maths, Physics and Chemistry.
My role on these forums is to help you. The HSC syllabus is tricky, nuanced and pretty damn huge. To help you out, I thought it would be a great idea to have a forum where you can just post questions, and myself or other forum members can post answers!
This is a community, so we want you to feel like you can post any type of Chemistry, no matter how "basic" you might think it is. Remember, IF YOU'RE HAVING TROUBLE WITH A TOPIC, THERE ARE THOUSANDS OF OTHERS HAVING THE SAME ISSUE. The best way to learn Maths is by looking through practice questions, and their associated answers. I honestly think a forum like this, and a place where I could always go to have difficult questions answered would have helped me in my HSC year.
Remember that Chemistry is a bloody difficult course. There will be lots of answers to the same questions, and I'll try give you the best or easiest to remember ones.
I got an ATAR of 99.80, and a mark of 93 in the Chemistry course. There are similar forums for a bunch of other subjects, so make sure to take a look at them as well!
Post by: Happy Physics Land on January 28, 2016, 09:47:35 pm
Post by: jakesilove on January 28, 2016, 10:19:34 pm
In 2007 HSC exam for chemistry, there was a question regarding "explain one benefit of car batteries lasting several years" and the notes from marking centre recommended that "Better responses clearly linked the benefit to a property of lead or sulfuric acid. Weaker responses did not give a specific benefit, but rather general statements that could have many meanings, such as ‘convenient and reliable’. The better responses explained such words in the context of their answers." I have some trouble trying to relate the property of lead or sulfuric to the question. Please Help Jake!!!
Hey Happy Physics Land (love the name by the way).
Great question, because it's the perfect example of the fact that sometimes teachers mark question in a way that is arbitrary and, to be frank, just wrong. The question, word for word, is "Explain one benefit of car batteries lasting several years." (from the 2007 paper, question 20b). This is framed in relation to an image of a Lead-Acid battery, however really the question doesn't ask specifically about a car battery with components that are Sulfuric acid or Lead. So really, an answer such as durability, convenience etc. should answer the question.
For a two mark question, make sure you talk about the positive attribute AND the reason its beneficial (ie. convenience of not having to replace battery regularly, resulting in greater costs to consumer, and safety, so that the battery doesn't die mid-drive).
The way the answer that you quote is phrased makes it seem like the question was "Explain how the components of the above battery make them suitable for car batteries". I really think the markers got that wrong, and in more recent exam papers you won't find such obvious misrepresentation of what the question was asking for.
The answer to the question "Explain how the components of the above battery make them suitable for car batteries" has many answers, including
1. The battery does not discharge much voltage whilst not in use, unlike many other forms of re-chargeable batteries
2. Results in a high voltage-discharge when required
3. Not easily subject to internal damage through corrosion etc.
However, like I've said I don't think these answers should have been required given the question. That being said, including as much information as possible is always a good idea when answering Chemistry questions, and so throwing one of these in could be a good idea if you want to play it safe.
I hope this helps: remember that you're only supposed to answer the question and, in this case, I think the marker got it wrong.
Jake :)
Post by: Happy Physics Land on January 28, 2016, 11:23:30 pm
Best Regards
Happy Physics Land
Post by: thushan on January 29, 2016, 12:39:23 pm
Will lurk here and help answer questions too! But I will be coming from a Victorian slant.
Post by: ghey04 on January 31, 2016, 10:35:52 am
Post by: Happy Physics Land on January 31, 2016, 11:19:34 am
Hey! I was wondering for the q "Analyse a recently developed biopolymer in terms of progress and method of production. Provide an evaluation of its potential of use" (5 marks) in Production of Materials (basically the dotpoint I guess), how would you write up the answer? I'm getting thrown off which part I should focus and write more on.
Hello ghey04:
I am a year 12 student currently undertaking chemistry, just like yourself, and I am happy to help you out :) ! Ok so let's break down this question first since it consists of so many parts. Essentially the question asks us to do 5 things:
1. Identify this biopolymer (and perhaps even the year it was first developed)
2. How this biopolymer has developed in the society (i.e. is it widely used? Use statistics? state a use with slight references to its properties as an example. Is it becoming increasingly economical to produce?)
3. Identify and describe the method of production (Identify the bacteria/biomass required to produce this biopolymer (VERY IMPORTANT), describe the every single step of the procedure that is taken to produce this biopolymer)
4. Describe its current usage (list about 2 uses with slight reference to its properties)
5. Evaluate (i.e. make a judgement) the potential of the biopolymer in the future, and state your reason to back up your argument (e.g. this biopolymer has a significant potential for the society because as current technologies improve, this biopolymer can be produced more efficiently and economically. In conjunction with its ability to bio-degrade and the fact that it is produced from renewable resources, the production process of this biopolymer will be very sustainable and hence this biopolymer has a large potential to replace petrochemical polymers in both the contemporary and future society)
In regards to which part you should focus on, I think that both parts should be the same amount of writing. However, because you will be describing the method of production, and because the first question consists of essentially 3 parts, then I would put more concentration upon the first question. The ratio of writing between the two questions I would recommend is about 4 : 3. But please dont ignore the importance of the second question because it assesses how we can theoretically apply what we have learnt to predict the continual progress of the biopolymer into the future.
I hope this helps ghey04 and I wish you the best of luck throughout your HSC year!
Best Regards
Happy Physics Land
Post by: jakesilove on January 31, 2016, 11:23:45 am
Hey! I was wondering for the q "Analyse a recently developed biopolymer in terms of progress and method of production. Provide an evaluation of its potential of use" (5 marks) in Production of Materials (basically the dotpoint I guess), how would you write up the answer? I'm getting thrown off which part I should focus and write more on.
Hey ghey04!
Fantastic question, because really what a lot of Chemistry questions come down to is knowing what you are actually supposed to include in your answer.
I would always start off by underlining the important words. In this case, you need to analyse... progress and methods of production, and evaluate potential use. So that's exactly how I would structure your answer.
Start off by discussing the development of the biopolymer. Talk about its history/discovery, and any progress that has been made. You can summarise this in maybe two or three sentences. Then discuss how you produce the polymer, again in two or three sentences. Finally, discuss potential uses. I would definitely use subheadings for a 5 mark questions: teachers absolutely love it!
For instance, the structure would be:
History/Progress
Two/three sentences summarising important points. Try to include some specific facts!
Method of production
Two/three sentences explaining, fairly specifically, how we CURRENTLY develop the polymer. Try to include specific names of compounds etc.
Potential use
Two/three current uses, including WHY the biopolymer is useful in that scenario. For instance, polystyrene is a good material for a cup because it is an insulator, and will not heat up when boiling liquid is poured in.
I hope that helps! If you want to write up a response and post it here based on the above I am happy to comment on it :)
Jake
Post by: ghey04 on January 31, 2016, 11:28:18 pm
Post by: wesadora on February 07, 2016, 12:40:34 am
Br2(l) + 2Cl-(aq) --> 2Br-(aq) + Cl2(g)
Will this reaction occur spontaneously? Justify your response using relevant half-equations (3 marks)
Firstly, what defines a 'spontaneous reaction' in regards to galvanic cells, and how would you go about this question :/
Thanks!
Post by: jakesilove on February 07, 2016, 09:44:08 am
Consider the chemical equation below:
Br2(l) + 2Cl-(aq) --> 2Br-(aq) + Cl2(g)
Will this reaction occur spontaneously? Justify your response using relevant half-equations (3 marks)
Firstly, what defines a 'spontaneous reaction' in regards to galvanic cells, and how would you go about this question :/
Thanks!
Hey Wesadora!
Below is my solution to this (very difficult) question. Usually, you could get similar questions regarding metals (Sodium and Potassium Nitrate etc.) but I've never seen one like this. Hope it helps!
(http://i.imgur.com/wZuKnNZ.png?1)
Jake
Post by: Johny1234567 on February 07, 2016, 10:41:15 am
I need help with the following question:
35mL of 0.86 M sulfuric acid was placed into a 25mL solution of 1.00 M sodium hydroxide.
b) Calculate the concentration hydrogen or hydrogen ions after this reaction. You may assume the reaction is complete. (3 marks)
c) Calculate the pH of the final solution. (1 mark)
d) If 5 drops of methyl orange was placed into the NaOH solution before the reaction took place, explain the colour changes that occur over the course of the experiment. (3 marks)
Any help would be greatly appreciated!
Post by: jakesilove on February 07, 2016, 11:43:20 am
Hello!
I need help with the following question:
35mL of 0.86 M sulfuric acid was placed into a 25mL solution of 1.00 M sodium hydroxide.
b) Calculate the concentration hydrogen or hydrogen ions after this reaction. You may assume the reaction is complete. (3 marks)
c) Calculate the pH of the final solution. (1 mark)
d) If 5 drops of methyl orange was placed into the NaOH solution before the reaction took place, explain the colour changes that occur over the course of the experiment. (3 marks)
Any help would be greatly appreciated!
Hey Johny!
These sort of "Concentration" questions are very typical, however this is one of the most difficult ones I've seen (mainly because of the difficulty of the chemical equation). However, if you are able to follow this working out and do lots of practice questions, you'll find this to be easy marks in the HSC! Hope that this helps!
If you don't quite understand a step, maybe submit another question that is similar, but easier. Once you have the general working out style, you'll be totally fine :)
(http://i.imgur.com/XSdZ1HL.png?1)
(http://i.imgur.com/fXnbvvS.png?1)
Thanks for the great question!
Jake
Post by: Happy Physics Land on February 07, 2016, 12:30:06 pm
Hey Johny!
These sort of "Concentration" questions are very typical, however this is one of the most difficult ones I've seen (mainly because of the difficulty of the chemical equation). However, if you are able to follow this working out and do lots of practice questions, you'll find this to be easy marks in the HSC! Hope that this helps!
If you don't quite understand a step, maybe submit another question that is similar, but easier. Once you have the general working out style, you'll be totally fine :)
(http://i.imgur.com/XSdZ1HL.png?1)
(http://i.imgur.com/fXnbvvS.png?1)
Thanks for the great question!
Jake
Farrrrrr Jake you are on Fire..... Loving those explanations!
Post by: Johny1234567 on February 07, 2016, 01:14:00 pm
Hey Johny!
These sort of "Concentration" questions are very typical, however this is one of the most difficult ones I've seen (mainly because of the difficulty of the chemical equation). However, if you are able to follow this working out and do lots of practice questions, you'll find this to be easy marks in the HSC! Hope that this helps!
If you don't quite understand a step, maybe submit another question that is similar, but easier. Once you have the general working out style, you'll be totally fine :)
(http://i.imgur.com/XSdZ1HL.png?1)
(http://i.imgur.com/fXnbvvS.png?1)
Thanks for the great question!
Jake
Hello Jake,
Your explanation was clear and concise. Thank you !
Post by: Sharms on February 07, 2016, 03:52:24 pm
I am dedicated towards learning its just I never really go on with chemistry and all the concepts
Post by: jakesilove on February 07, 2016, 04:02:56 pm
Just completed the first term of HSC and got 45% in my chem exam, how hard would it be to to change into senior science having missed the whole first term and quickly approaching the half yearlies?
I am dedicated towards learning its just I never really go on with chemistry and all the concepts
Hey Sharms!
I'm sorry to say that I actually don't know anything about Senior Science, in terms of difficulty or anything else. Really the only thing I can recommend is talking to your respective teacher about changing subjects.
Sorry that I can't be of more help! Maybe someone else on the forum has more information :)
Jake
Post by: wesadora on February 07, 2016, 04:19:07 pm
Post by: kathywong on February 09, 2016, 12:52:19 am
My teacher said that "half life and the type of radiation they emit are not chemical properties so im kinda lost. Please help thanks
Post by: jakesilove on February 09, 2016, 09:33:34 am
Hi, what are some chemical properties of radioisotopes? The syllabus dot point says "explain their use in terms of their properties".
My teacher said that "half life and the type of radiation they emit are not chemical properties so im kinda lost. Please help thanks
Hey Kathy!
In this case I would respectfully disagree with your teacher. Half life and type of radiation they emit is certainly a chemical property, two of which you should ALWAYS include in any band 6 response. To repeat; they are chemical properties!.
However, there are definitely more chemical properties you can discuss if you want to achieve top marks. Things like state (for Medicine, useful if the compound is a liquid etc.) and ability to latch onto certain compounds in the body (such as Glucose) so that you can track its movement and thereby detect various diseases and illnesses.
The main this to get out of this is that your teacher is wrong. If they want you to include even more, then definitely do, but in the HSC half life and type of radiation is definitely a chemical property.
I hope this helps! Absolutely great question, because often your teachers can be slightly misguided in their approach to the syllabus.
Jake :)
Post by: g98 on February 09, 2016, 06:54:34 pm
I have a quick question about studying in general:
What is your opinion on music while you study?
And your lectures in Sydney were great by the way!
Thank you!
Post by: jakesilove on February 09, 2016, 06:58:48 pm
Hey Jake!
I have a quick question about studying in general:
What is your opinion on music while you study?
And your lectures in Sydney were great by the way!
Thank you!
Hey g98!
Great question! Actually, the second time I've been asked that today oddly enough.
Throughout my HSC year, I literally listened to Music non-stop. Like seriously, if there wasn't music coming out of my room, I was probably asleep. I didn't feel distracted by it, and it helped time go more,,, manageable?
Once, I tried listening to Classical music and to be completely honest, it actually helped a lot! Still, I found it heaps boring so decided to revert back to Alt-J.
My overall recommendation is just to do whatever you feel comfortable with. If music distracts you, keep it off. If it helps you pass the time, blast it out the windows. There's no "set rule" or particular method that will help you study better.
I know this isn't a very specific answer, but I hope it helps! Great question by the way.
Jake
Post by: Happy Physics Land on February 09, 2016, 07:10:53 pm
Hey Jake!
I have a quick question about studying in general:
What is your opinion on music while you study?
And your lectures in Sydney were great by the way!
Thank you!
Actually if you just type up "study concentration music" on youtube, there's a whole list of those and pretty much all of them have a brain stimulation effect that would help you to concentrate. My recommendation is to turn it to a volume where you can still hear someone calling you but loud enough to cover all the unnecessary noises in the surrounding environment.
Post by: zoe.mckeon on February 10, 2016, 07:07:11 pm
I'm just wondering what you found to be an effective study method for Chemistry? I found during Year 11 that I did study but I could not remember much when in an exam, and what I did know was hard to apply to a question....
Thanks for your help.
Post by: jakesilove on February 10, 2016, 09:24:01 pm
Hey Jake,
I'm just wondering what you found to be an effective study method for Chemistry? I found during Year 11 that I did study but I could not remember much when in an exam, and what I did know was hard to apply to a question....
Thanks for your help.
Hey Zoe!
Fantastic question, and one which I will soon be devoting an entire resource too. Studying is the hardest part of any subject, and making sure that you tailor your study to each subject is absolutely vital.
I totally agree with you though: the hardest part of studying for Chemistry in particular isn't learning the content; rather, it is applying that content to a standard exam question!
My best advice is to DO PAST PAPERS and COMPLETE PAST QUESTIONS. When you finish a topic, don't just take notes, learn the content etc. Find past questions relating to the topic, answer them, mark them yourself (using marking guidelines), re answer the question so that it's perfect, and then include that answer in your notes.
I would say that, until you've done the above, you haven't actually finished a topic. Don't wait until you finish an entire block of the syllabus (eg. The Acidic Environment etc.); as soon as you finish a dot point or two, try answering questions.
Once you done this enough, you'll know exactly what each question is asking. It's all about interpreting what the markers want, and the only way to do that is to do enough past papers to know exactly what's going on.
To summarise
- Write notes
- Learn the content
- Do past paper questions relating to the topic
- Mark question
- Perfect answer
- Include answer in your notes
I really hope that this helps! Great question, and great to see the activity on this forum! Looking forward to more great questions, and answers from the community!
Jake :)
Post by: thushan on February 11, 2016, 09:00:16 am
Post by: kathywong on February 11, 2016, 08:13:28 pm
-are transuranic elements and radioisotopes produced in exactly same way ie.using nuclear reactor and particle accelerator?
-Is it not necessary for nuclear fission to occur in nuclear reactor and nuclear fusion to occur in particle accelerator ?
-Will knowing the location discovered and how it is produced for recent discoveries of elements be enough?
I have an assessment coming up so would you mind checking if my notes are correct and if the main important details are mentioned? thank you
Describe how transuranic elements are produced
Neutron bombardment:
- Natural elements are immersed in a nuclear reactor and bombarded with neutrons (or other particles) to produce higher elements as nuclear fission occurs
Particle accelerator
-transuranic element atomic number>96 are made by bombarding heavy nucleus with high speed small positive nucleus (e.g. He, C or Boron) in accelerators, synchrotrons, and cyclotrons.
Describe how commercial radioisotopes are produced
Nuclear reactors
-Bombard target nuclei with slow neutrons produced by fission of uranium-235. The nucleus absorbs neutrons hence producing a neutron-rich radioisotope.
Particle accelerators
-A machine uses electric and magnetic fields to accelerate charged particles such as protons, or nuclei (eg.Helium) or ion of other atoms (eg. Hydrogen-2 ions, Helium-3 ions) to collide with target nucleus. Nuclear fusion occurs and neutron-deficient radioisotope is produced.
eg. (nuclear fusion) Fluorine-18 is prepared in a cyclotron by bombarding nitrogen-14 with helium nuclei.
Identify instruments and processes that can be used to detect radiation
-Photographic film- Darkening of photographic film indicated the presence of radioactivity. The darkening is due to formation of silver halide crystals as radiation is absorbed by silver salts in the film.
-Cloud Chamber - device that consists a container of supersaturated vapour of water or alcohol. As radiation travels through the device, it ionises surrounding air molecules which causes vapour molecules to condense onto these ions, to produce visible cloud tracks/ trails.
-Scintillation counter:
- When certain substances like ZnS absorbed energy from alpha, beta and gamma rays, they emit flashes of light which can be collected and amplified in a photo multiplier. Thus, an electrical pulse can be generated and recorded by a counter
-Used to measure the amount of radiation exposed to a person who has taken a body scan
Geiger-Muller counter
- Best for detection of alpha and beta radiation but cannot tell difference between alpha and beta particles
- Consists of a sealed glass/metal tube with inside filled with argon gas and with a thin mica window at one end to allow particles to enter
- Particles enter ionise gas molecules (e.g. argon), emitting electrons which accelerate towards the positive electrode, and ionising more gas as they proceed in their path and create an electrical pulse
- GM counter records the electrical pulse and convert it to audible clicks
Medical and radioisotopes uses and chemical properties
Cobalt-60: (industrial/medical)
Chemical properties:
Chemically inert
Gamma emitter
Long half-life of 5.3 years
Relating uses to chemical properties:
- it is an emitter of gamma rays which can penetrate the materials such as metals, paper, rolled steel and detected on the other side using radioactive film, therefore can measure thickness of these materials or detect faults in metal casting and cracks in aircraft wings.
-The gamma radiation it emits used to sterilise food eg. Strawberries to minimise spoilage and extend shelf life, and medical supplies such as disposable syringes and bandages. Food on a conveyor belt is gamma irradiated by passing it through a chamber containing a safely shielded Co-60 source.
Benefits
-it is chemically inert therefore has relatively low emission of radiation also limits the potential damage to anyone working with the radioisotope.
-potent gamma emitter - Gamma rays can easily penetrate hermetically sealed packaging and the contents, killing harmful microbes such as bacteria, viruses and fungi. They have sufficient energy to destroy bacteria but not enough to make food radioactive.
-has a reasonably long half-life of 5.3 years since it is chemically inert hence do not require frequent replacement/do not need to be produced on site and can be readily used to treat cancer patients
-Provides effective treatment of many cancerous tumours in sensitive organs because it releases beta and gamma radiation which can easily penetrate and kills abnormal cells
-Cheaper methods of diagnosis and treatment of medical illnesses and cancer
-Provides non-invasive, convenient diagnostic techniques to trace medical illnesses and problems
-medical sterilisation which is more effective than the use of antiseptics and heat treatment
Concerns
-There are concerns that gamma radiation can destroy vitamin content in food and may lead to the formation of harmful compounds in the food. In addition, workers must be protected from irradiation.
-Continued exposure to radiation can lead to diseases such as tissue damage, tumours, cancer and genetic damage
-Without further research, the long-term effects in irradiation of food, are unknown
Technetium-99m (medical):
Chem properties:
-short half-life of 6 hours
-emits low energy gamma
-can be attached to a range of biological carriers
-relatively unstable and reactive
Relating chemical prop to uses:
-used as a biological tracer for diagnostic imaging. It can be attached to biological molecule which will concentrate in the targeted organ such as brain, kidney, bones liver and spleen and injected into the body. Then, it will emit gamma radiation which can easily penetrate and detected in various part of body using a gamma camera (scintigraphy) and pinpoint blood clots, constrictions, heart defects and size and location of cancer growth, etc.
eg. Tc-99m phosphonates is used to diagnose skeletal bone problems
Benefits:
-short half-life of 6 hours which is short enough to minimise the exposure of radiation to patient and long enough to examine metabolic processes occurring in the body
-emits low energy gamma radiation which minimises damage to healthy cells/tissues, but can still be detected in the body by a gamma ray sensitive camera.
-Can be attached to a range of biological carriers which will concentrate in a targeted organ
-relatively reactive, so it can be reacted to form a compound with chemical properties that leads to concentration in the organ of interest such as the heart, liver, lungs or thyroid.
-Can be made on site from molybdenum-99 in a transportable generator – Mo-99 is a product of nuclear fission in a nuclear reactor
-Easy to detect using small quantities
-Provides a range of non-invasive, convenient diagnostic techniques to trace medical illnesses and problems and patient experiences little or no discomfort during the test
-Cheaper methods of diagnosis and treatment of medical illnesses and cancer
-Scintigraphy can be used to produce real time images of body sections which enables observation from all angles instead of Xrays which are static images
Problems:
-It is produced in a nuclear reactor from Molybdenum which is not as dangerous as Plutonium-241 but usual safeguards are necessary (lead shielding) as long term exposure to any radiation can lead to diseases such as tissue damage, tumours, cancer and genetic damage (which leads to deformities in offspring)
Evaluation:
-Tc-99m may cause damage to tissues, or cancer when it is exposed in long term. However, it can be minimised with proper safeguards and in addition its short half-life and low frequency gamma radiation. More importantly, it can give value information concerning a wide variety of metabolic disorders or cancers without the need for invasive surgery. Therefore, there are more benefits associated with the use of Tc-99m and this makes it the most widely used medical radioisotope.
Americium-241 (industrial)
Chemical properties:
-Emits alpha and gamma radiation
-long half-life of 432.7 years
Relating uses to chem prop: (medical and industrial):
-Used in smoke detectors because it is emits alpha and gamma radiation.
When no smoke is present, the alpha particles ionise nitrogen and oxygen in the air in the detector.
When smoke is present, the smoke absorbs the alpha particles emitted, so the rate of ionisation drops and this sets off the alarm.
-Gamma radiation released from Am-241 can be used for indirect analysis of materials radiography and for quality control in manufacturing fixed gauges eg. It has been used to measure glass thickness to help create flat glass
-Gamma rays also provide diagnosis of thyroid function
-It is recently suggested for use as a denaturing agent in plutonium reactor fuel rods to render the fuel unusable for conversion to nuclear weapons
Benefits:
-Long half-life of 432.7 years so do not need frequent replacement
-Alpha particles do not themselves pose a health hazard – as they are absorbed in a few cm of air or by the detector itself
Problems:
-emits alpha and gamma radiation which can be a serious health hazard if ingested or inhaled
-Dangerous if swallowed as it would concentrate in the skeleton and continue to emit radiation which could cause some cellular damage
-Production is dangerous as it is made through the decay of plutonium-241 in a nuclear reactor which is a highly radioactive element and emits high energy gamma radiation as it decays. Hence, special precautions must be taken when handling plutonium, otherwise it can cause cancer and death
Evaluation:
-It is necessary as it can save lives by warning inhabitants of fires
Sodium-24 (industrial)
Chemical properties:
-Soluble in water
-Emit beta and gamma radiation
-Half life of 15 hours
Relating uses to chem prop:
-it is soluble in water and and used as a leak detector in damaged underground water, gas and oil pipes as it emits beta and gamma radiation. Minimal radiation is detected if it is from inside a pipe, but if the tracer leaks into the soil large radiation are detected
Benefits:
-emits beta radiation - it does not pose a major health hazard to living things because the radiation will be absorbed by the pipe itself or by its surrounding/ pipe leaks can be located without having to dig up entire pipeline.
-half-life of 15 hours – lasts long enough to find leaks and short enough so that it does not cause any serious permanent pollution to water bodies
-Non-toxic to humans or animals- although it may form organic compounds and be absorbed into the blood stream, this will not strongly affect the health of humans or animals due factors such as its life span and low intensity emission of gamma radiation
Problems:
-Not a naturally occurring isotope hence relies upon nuclear reactors for production which may cause disastrous consequences if large amount of harmful radiation is accidently released
-Radiation cause undesirable reactions in living tissue and so can cause tissue damage, cancers and/or genetic damage leading to deformities in offsprings.
Evaluation:
-It is certainly valuable in detecting leakages in industry despite some minor disadvantages which can be minimised/eliminated with strict safety precautions
Strontium-90 (industry)
Chemical prop:
-Half-life of 28 years
-emits B radiation
Uses:
-used in thickness gauges to monitor the thickness of sheet materials. Radiation from the Sr-90 is passed through the material to a detector. The intensity of the radiation detected is an indicator of the thickness of the material.
Benefits:
-Long half-life of 28 years so it can be used for extended periods of time so no need frequent replacement
Problem:
-It is similar to Calcium therefore has tendency to replace calcium and this can lead of leukaemia and bone cancer
Iodine-131 (medical)
Chemical prop:
-Half-life of 8 days
-Can be attached to biological molecule
-Emit Beta radiation
Relating uses to chem prop:
-used in medicine to diagnose and treat thyroid cancers.
-When it is tagged to NaI and injected into bloodstream, it will accumulate in the thyroid gland where it undergoes beta and Gamma decay to kill cancer cells or identify thyroid diseases by detection
Benefits:
-Relatively short half-life of 8 days which is long enough for treatment to be done but not too long such that the patient is exposed to excess radiation.
Describe recent discoveries of elements
Copernicium-227
-half-life of 0.24 milliseconds.
-It was discovered in 9th of February in 1996 at the GSI in Darmstadt Germany
-element was created by firing accelerated zinc-70 nuclei at a target made of lead-208 nuclei in a heavy ion accelerator
Post by: jakesilove on February 11, 2016, 08:50:47 pm
My questions:
-are transuranic elements and radioisotopes produced in exactly same way ie.using nuclear reactor and particle accelerator?
-Is it not necessary for nuclear fission to occur in nuclear reactor and nuclear fusion to occur in particle accelerator ?
-Will knowing the location discovered and how it is produced for recent discoveries of elements be enough?
I have an assessment coming up so would you mind checking if my notes are correct and if the main important details are mentioned? thank you
Describe how transuranic elements are produced
Neutron bombardment:
- Natural elements are immersed in a nuclear reactor and bombarded with neutrons (or other particles) to produce higher elements as nuclear fission occurs
Particle accelerator
-transuranic element atomic number>96 are made by bombarding heavy nucleus with high speed small positive nucleus (e.g. He, C or Boron) in accelerators, synchrotrons, and cyclotrons.
Describe how commercial radioisotopes are produced
Nuclear reactors
-Bombard target nuclei with slow neutrons produced by fission of uranium-235. The nucleus absorbs neutrons hence producing a neutron-rich radioisotope.
Particle accelerators
-A machine uses electric and magnetic fields to accelerate charged particles such as protons, or nuclei (eg.Helium) or ion of other atoms (eg. Hydrogen-2 ions, Helium-3 ions) to collide with target nucleus. Nuclear fusion occurs and neutron-deficient radioisotope is produced.
eg. (nuclear fusion) Fluorine-18 is prepared in a cyclotron by bombarding nitrogen-14 with helium nuclei.
Identify instruments and processes that can be used to detect radiation
-Photographic film- Darkening of photographic film indicated the presence of radioactivity. The darkening is due to formation of silver halide crystals as radiation is absorbed by silver salts in the film.
-Cloud Chamber - device that consists a container of supersaturated vapour of water or alcohol. As radiation travels through the device, it ionises surrounding air molecules which causes vapour molecules to condense onto these ions, to produce visible cloud tracks/ trails.
-Scintillation counter:
- When certain substances like ZnS absorbed energy from alpha, beta and gamma rays, they emit flashes of light which can be collected and amplified in a photo multiplier. Thus, an electrical pulse can be generated and recenient diagnostic techniques to trace medical
Relating uses to chem prop:
-used in medicine to diagnose and treat thyroid cancers.
-When it is tagged to NaI and injected into bloodstream, it will accumulate in the thyroid gland where it undergoes beta and Gamma decay to kill cancer cells or identify thyroid diseases by detection
Benefits:
-Relatively short half-life of 8 days which is long enough for treatment to be done but not too long such that the patient is exposed to excess radiation.
Describe recent discoveries of elements
Copernicium-227
-half-life of 0.24 milliseconds.
-It was discovered in 9th of February in 1996 at the GSI in Darmstadt Germany
-element was created by firing accelerated zinc-70 nuclei at a target made of lead-208 nuclei in a heavy ion accelerator
Hey Kathy!
They look like great notes. I will tell you for sure though that you have way too much content. There is no way you need to memorise that many radioisotopes, and you can definitely ease up on the specific dates and examples. Still, the content itself is great! Just figure out how many isotopes you actually need (ie. Two for med/Two for industrial) and limit your notes to that so you don't need to do unnecessary study.
The answer to all of your questions at the start of your post is that you really don't need to know the answer to that. Nowhere in the curriculum does it require you to have an in depth understanding of the production of transuranic elements (having a basic understanding of particle accelerators etc. is absolutely enough). Remember you are not supposed to be learning all of Chemistry, just the Chemistry in the curriculum dotpoints.
So my overall tip is that it looks like you're doing a little too much study/note taking. It's great to see that you're enthusiastic, but there's not need to go overboard.
Jake :)
Post by: kathywong on February 12, 2016, 09:33:26 pm
why does increasing in pressure have no effect ? is it because they're not gas molecules
Why does adding Fe(NO3)3(s)/Na(OH)(s) to solution result more intense violet colour because it thought adding solid/liquid won't affect concentration unless they're aqueous or gaseous
Post by: Happy Physics Land on February 12, 2016, 10:15:10 pm
equilibrium --->>> Iron (III) ions (yellow colour) reacts with salicylic acid C7H6O3(s) producing Fe(c7h5o3)(aq)(violet colour) + H+ (hydrogen ion)
why does increasing in pressure have no effect ? is it because they're not gas molecules
Why does adding Fe(NO3)3(s)/Na(OH)(s) to solution result more intense violet colour because it thought adding solid/liquid won't affect concentration unless they're aqueous or gaseous
Hey Kathy:
What a great question, honestly it just covers so many aspects of Le Chatelier's principle and I would indeed see this as a hard question. I really appreciate that you are actually asking this interesting question!
First I will have to congratulate you on correctly stating that the increase in pressure has no effect on the position of the equilibrium because there are no gas molecules involved in this system. Pressure would only affect gas particles because doesnt matter whether we increase or decrease the volume, the amount of the solid and liquids would always stay constant (after all, if you imagine having a rock in a coke can, the rock is not gonna get larger when you pull open the liquid to decrease the pressure). So you have correctly stated that the increase in pressure has no impact due to a lack of gas molecules involved.
Ok the second question is actually a lot more interesting. when you add Fe(NO3)3 into the solution, it is EXTREMELY CRUCIAL to recall something we've learnt back in year 11 chemistry. YES, DISSOCIATION, THATS WHAT IM TALKING ABOUT. Because Fe(NO3)3 is an ionic substance, it is going to undergo dissociation when added to a solution (because of water), forming Iron (III) ions and NO3- ions. This increase in concentration of iron ions then disturbs the system. According to Le Chatelier's Principle, the equilibrium will shift to right, favouring the product side to minimise this disturbance. Hence more Fe(C7H5O3)(aq) are produced and consequently the solution becomes more intensely violet since the product has a violet colour.
Such an awesome question, thanks for posting this. If you still have any concerns or confusions please dont hesitate to ask! :)
Best Regards
Happy Physics Land
Post by: Happy Physics Land on February 13, 2016, 12:32:10 am
is it a good idea to use other people s notes to study (but type own notes for questions like discuss, evaluate) because it takes too much time for me to make my own notes and it might not be as good as others
one ques about nuclear chem: will fusion always occur in reactor and fission occur in particle accelerator?
Hey Amanda:
Yep, nice question. Actually this question has been posted for quite a few times now on this forum, and Jake and I kinda have our own individual ways of responding to your concerns. Jake recommends to effectively use other people's notes from a variety of sources and choose and pick the best information for the particular syllabus dot point. For me, I usually type up my own notes. I would update my notes for every single subject (especially content based subjects such as chemistry and physics) every week. In this way I only have to spend 3 hours a week to basically make notes and revise through everything. As your notes accumulate you will be much better prepared for your exams. But of course this is only my approach and when Jake comes around he can give you more insights.
Regarding the nuclear chemistry question, I may like to inform you that this is definitely not always the case. In fact in nuclear reactors, fission would often be involved. I do understand where your perspective came from though, since our final desired radioisotope would only be produced through fusion with neutrons, which you have correctly stated. But if you consider how neutron bombardment is achieved, you can see the slight inaccuracy in your statement (sorry if lm being a bit too critical!)
Neutron bombardment which takes place in nuclear reactors actually begins with the FISSION of Uranium. When we add a neutron to Uranium-235, an extremely unstable state Uranium-236 is formed, which will only exist for 10^-12 seconds before splitting into fission fragments such as Barium-146 and Krypton-92. During this process, excess neutrons are also produced which then becomes the source for neutron bombardment. So as you can see, fission DO indeed take place inside nuclear reactors and fission of Uranium-236 is essential for fusion between neutron and other nuclei to produce the desired radioisotope.
In particle accelerators or cyclotrons you would have positive particles such as an alpha-particle or neutrons which are a result of spallation (fragmentation of other elements) being accelerated into a target nuclei to form a new radioisotope. Similar to what happens in nuclear reactors, we dont just get those neutrons or alpha particles from nowhere. We need another element to undergo fission to produce there particles that would allow us to form new isotopes. So your second statement about particle accelerators would be quite right, particle accelerators involve predominantly the nuclear process of fission.
So anyways I hope my explanation is clear to you and I really liked the question you asked because it is quite confusing since they are quite complicated to understand (those fancy gadgets these days). If you have any further concerns, please dont hesitate to ask!
Best Regards
Happy Physics Land
Post by: jakesilove on February 13, 2016, 03:08:56 pm
Hey Amanda:
Yep, nice question. Actually this question has been posted for quite a few times now on this forum, and Jake and I kinda have our own individual ways of responding to your concerns. Jake recommends to effectively use other people's notes from a variety of sources and choose and pick the best information for the particular syllabus dot point. For me, I usually type up my own notes. I would update my notes for every single subject (especially content based subjects such as chemistry and physics) every week. In this way I only have to spend 3 hours a week to basically make notes and revise through everything. As your notes accumulate you will be much better prepared for your exams. But of course this is only my approach and when Jake comes around he can give you more insights.
Regarding the nuclear chemistry question, I may like to inform you that this is definitely not always the case. In fact in nuclear reactors, fission would often be involved. I do understand where your perspective came from though, since our final desired radioisotope would only be produced through fusion with neutrons, which you have correctly stated. But if you consider how neutron bombardment is achieved, you can see the slight inaccuracy in your statement (sorry if lm being a bit too critical!)
Neutron bombardment which takes place in nuclear reactors actually begins with the FISSION of Uranium. When we add a neutron to Uranium-235, an extremely unstable state Uranium-236 is formed, which will only exist for 10^-12 seconds before splitting into fission fragments such as Barium-146 and Krypton-92. During this process, excess neutrons are also produced which then becomes the source for neutron bombardment. So as you can see, fission DO indeed take place inside nuclear reactors and fission of Uranium-236 is essential for fusion between neutron and other nuclei to produce the desired radioisotope.
In particle accelerators or cyclotrons you would have positive particles such as an alpha-particle or neutrons which are a result of spallation (fragmentation of other elements) being accelerated into a target nuclei to form a new radioisotope. Similar to what happens in nuclear reactors, we dont just get those neutrons or alpha particles from nowhere. We need another element to undergo fission to produce there particles that would allow us to form new isotopes. So your second statement about particle accelerators would be quite right, particle accelerators involve predominantly the nuclear process of fission.
So anyways I hope my explanation is clear to you and I really liked the question you asked because it is quite confusing since they are quite complicated to understand (those fancy gadgets these days). If you have any further concerns, please dont hesitate to ask!
Best Regards
Happy Physics Land
Love you HPL.
Just remember everyone that you don't really need to know all this stuff in as much depth as HPL has described above; but that shouldn't stop you from extending yourself! If there isn't an explicit dotpoint on the topic, you can't really be assessed on it. Look at past papers to figure out how much information you need to remember!
Jake
Post by: curious.egg on February 13, 2016, 09:35:49 pm
How would you use the equilibrium constant to describe the position of the equilibrium? Please help!!
Question:
" 2NO(g) ⇌ N2(g) + O2(g) 0.800 moles of NO was placed into a 2.00L vessel at 2000 degrees Celsius and the equilibrium concentration of N2 was found to be 0.198 mol/L. Calculate the equilibrium constant for this reaction and use this value to describe the position of the equilibrium. "
(industrial chemistry)
Post by: amandali on February 13, 2016, 09:48:46 pm
Post by: jakesilove on February 13, 2016, 09:51:56 pm
Helloo!
How would you use the equilibrium constant to describe the position of the equilibrium? Please help!!
Question:
" 2NO(g) ⇌ N2(g) + O2(g) 0.800 moles of NO was placed into a 2.00L vessel at 2000 degrees Celsius and the equilibrium concentration of N2 was found to be 0.198 mol/L. Calculate the equilibrium constant for this reaction and use this value to describe the position of the equilibrium. "
(industrial chemistry)
Hey Mr. Egg!
I'll premise my answer with the fact that I didn't actually do Industrial chemistry, and despite having done Equilibrium constants at University would have no idea how to actually calculate that equilibrium constant.
However, your question isn't to calculate the constant but how we would then use this value to describe the position of the equilibrium. My university level Chemistry can definitely help with that part!
An equilibrium constant is used to describe whether there will be more of the products, more of the reactants, or an equal amount of both in a reaction.
The rule is as follows.
If
K > 1
then the equilibrium favors the products.
If
K < 1
then the equilibrium favors the reactants.
And that's it! So if you got an equilibrium constant of 1.1 in your calculations, you would just say that the position of the equilibrium lies slightly on the right (the side of the products). If you got an equilibrium constant of 0.1, you would say that the position of the equilibrium lies largely on the left (the side of the reactants).
Hope my explanation made sense!
If anyone that has actually DONE Industrial Chemistry would like to contribute, that would be supremely helpful!
Jake
Post by: jakesilove on February 13, 2016, 10:00:48 pm
"given a geiger counter, outline a procedure by which you could determine the proportions of alpha and beta emissions from a sample of ac-277" 3 marks im not sure what to write about is it just describing the radiation entering the tube, ionising air molecules and generating audible clicks
Hey Amandali!
This is a really tricky question, and to be honest one that I think falls largely outside the syllabus. Still, you could use your working knowledge of Geiger counters to come to some sort of answer!
I definitely agree that, first, you would explain how Geiger counters actually work (radiation entering the tube, ionising air molecules and generating audible clicks etc.). However, since it says "outline a procedure" and it is worth three marks, I would recommend actually pretending it was some sort of experiment. Knowing that ac-277 is radioactive, I would just spout something vague like "1. Place ac-277 in a location such that its radiation does not negatively effect human life. 2. Taking adequate precautions, direct a Geiger counter towards the ac-277. 3. As the (radiation entering the tube, ionising air molecules and generating audible clicks etc.), a certain value can be read and recorded from the scale placed on the Geiger counter."
Basically, just make sure that you identify that a specific amount of radiation can be detected (as per the scale on the counter) and that this is some sort of procedure, to make sure you get full marks!
It is really important that you read the question thoroughly and address each individual word. Hope this helps!
Jake
Post by: FD121 on February 13, 2016, 10:14:59 pm
Post by: jakesilove on February 13, 2016, 11:13:55 pm
Any advice for Gravimatric Analysis SAC? It is a prac write up.
Hey FD121!
I think you're looking for the VCE Chemistry thread: This is the HSC Chemistry thread! As I don't know what is in your curriculum, I can't really answer your question.
Maybe repost your question here: VCE Chemistry Question Thread
Jake
Post by: amandali on February 14, 2016, 04:27:17 pm
Isn't this to much for a 4 marker ques ?
Commercial isotopes are used in medicine, industry and research. It can be produced in nuclear reactors where the target nuclei is bombarded with slow-moving neutrons which are then absorbed by the nucleus, thus forming a neutron-rich isotope.
eg.the medical isotope Cobalt-60 is produced in nuclear reactor (equation)
Neutron-deficient isotopes can be made in particle accelerators where the target nuclei collides with accelerated positively charged particles, causing fusion to occur.
eg.fluorine-18 is produced in particle accelerator
(equation)
Transuranic elements which have atomic masses greater than 92, can also be produced in nuclear reactor and particle accelerator, however, the target nuclei is usually already large
eg. Neptunium-93 is made in reactor
(equation)
eg.Curium-242 is produced in particle accelerator (equation)
Post by: jakesilove on February 14, 2016, 04:57:51 pm
The question is "Describe how commercial radioisotopes are produced, and how transuranic elements are produced" - 4 marks
Isn't this to much for a 4 marker ques ?
Commercial isotopes are used in medicine, industry and research. It can be produced in nuclear reactors where the target nuclei is bombarded with slow-moving neutrons which are then absorbed by the nucleus, thus forming a neutron-rich isotope.
eg.the medical isotope Cobalt-60 is produced in nuclear reactor (equation)
Neutron-deficient isotopes can be made in particle accelerators where the target nuclei collides with accelerated positively charged particles, causing fusion to occur.
eg.fluorine-18 is produced in particle accelerator
(equation)
Transuranic elements which have atomic masses greater than 92, can also be produced in nuclear reactor and particle accelerator, however, the target nuclei is usually already large
eg. Neptunium-93 is made in reactor
(equation)
eg.Curium-242 is produced in particle accelerator (equation)
Hey Amandali!
I definitely don't think that's too much for a four marker. I think it's always best to be SURE you get every single mark. By including loads of scientific jargon (which you did), and including great examples (which you did), you will absolutely get full marks. I think that is probably slightly longer than absolutely necessary, but in this case there really isn't much harm spending an extra 30 seconds to be sure you don't lose a mark.
Jake
Post by: amandali on February 16, 2016, 07:57:53 am
"a small amount of pure sodium metal was dropped into 1.5 L of water. the collected gas occupied a volume of 18.6L at 25 degrees and 100 kpa. calculate concentration of hydroxide ions(aq) after reaction" ans:1mol/L
Post by: RuiAce on February 16, 2016, 08:48:39 am
hi how do u do this question
"a small amount of pure sodium metal was dropped into 1.5 L of water. the collected gas occupied a volume of 18.6L at 25 degrees and 100 kpa. calculate concentration of hydroxide ions(aq) after reaction" ans:1mol/L
The reaction between sodium and water is given as thus:
2 Na + 2 H2O -> 2 NaOH + H2
The gas collected is simply hydrogen gas. As we are at 25deg 100kPa, we have the molar volume Vm = 24.79
Hence, to determine the moles of H2:
n = V/Vm = 18.6/24.79 = 0.7503025413... mol
For each mole of H2 yielded, 2 moles of NaOH are yielded. Hence, moles of NaOH:
n = 2 * n(H2) = 1.500605083... mol
Note that one mole of {OH-} is present in one mole of NaOH, so the above answer gives us the moles of hydroxide ions present.
Hence, to determine the concentration, use C = n/V where V is the volume of water here
C = 1.500605083.../1.5 = 1.000403388 mol L^-1
= 1.0 mol L^-1 (correct to 2 sig. fig.)
Post by: jakesilove on February 16, 2016, 10:23:14 am
The reaction between sodium and water is given as thus:
2 Na + 2 H2O -> 2 NaOH + H2
The gas collected is simply hydrogen gas. As we are at 25deg 100kPa, we have the molar volume Vm = 24.79
Hence, to determine the moles of H2:
n = V/Vm = 18.6/24.79 = 0.7503025413... mol
For each mole of H2 yielded, 2 moles of NaOH are yielded. Hence, moles of NaOH:
n = 2 * n(H2) = 1.500605083... mol
Note that one mole of {OH-} is present in one mole of NaOH, so the above answer gives us the moles of hydroxide ions present.
Hence, to determine the concentration, use C = n/V where V is the volume of water here
C = 1.500605083.../1.5 = 1.000403388 mol L^-1
= 1.0 mol L^-1 (correct to 2 sig. fig.)
RuiAce, looks like you'll be a big name on this community! Love your "Maths Challenge" forum, and this is a great answer, perfectly set out, that will really benefit everyone!
The only thing I would add, which I'm sure you would have done in an actual answer but didn't because of the formatting, is to make sure to always include states when writing out a chemical reaction. It's just an easy way to lose marks unnecessarily, so keep an eye out for that!
Again, thanks for posting the response! Keep at it!
Jake
Post by: RuiAce on February 16, 2016, 12:04:36 pm
RuiAce, looks like you'll be a big name on this community! Love your "Maths Challenge" forum, and this is a great answer, perfectly set out, that will really benefit everyone!
The only thing I would add, which I'm sure you would have done in an actual answer but didn't because of the formatting, is to make sure to always include states when writing out a chemical reaction. It's just an easy way to lose marks unnecessarily, so keep an eye out for that!
Again, thanks for posting the response! Keep at it!
Jake
Thank you! I'll stay around here more and do my best :)
As for the comment, yes I agree most definitely. I think if I were writing it out I would've just subconsciously written down the states.
Post by: Maz on February 16, 2016, 09:53:45 pm
i did an experiment in class the other day- and i have a validation on it in the next couple of days...i was wandering if someone could please help me out? please
The experiment was on equilibrium and Le CHatelier's principle
first we got K2CrO7 and added HCl solution to it...then to the same solution we added the base NaOH. originally the K2CrO7 was yellow, with added HCl it went to a darker colour (around orange), then when we added NaOH to the solution it went back to yellow..
i did the first part of the experiment ok...but i need help with this bit
the question is: By referring to the collision theory, account for the observed colour change that occurred when NaOH solution and the HCl solution were added to the K2CrO7 solution
I'd appreciate any help :)
Post by: wesadora on February 16, 2016, 10:34:05 pm
"Using the tabulated reduction potentials, calculate a cell emf for the disproportionation of H2O2 and into O2 and H2O.
thanks!
-Wes
Post by: RuiAce on February 16, 2016, 10:43:31 pm
Hi humans :)
i did an experiment in class the other day- and i have a validation on it in the next couple of days...i was wandering if someone could please help me out? please
The experiment was on equilibrium and Le CHatelier's principle
first we got K2CrO7 and added HCl solution to it...then to the same solution we added the base NaOH. originally the K2CrO7 was yellow, with added HCl it went to a darker colour (around orange), then when we added NaOH to the solution it went back to yellow..
i did the first part of the experiment ok...but i need help with this bit
the question is: By referring to the collision theory, account for the observed colour change that occurred when NaOH solution and the HCl solution were added to the K2CrO7 solution
I'd appreciate any help :)
Before I proceed with your question, do you mean K2CrO4 or K2Cr2O7? Because CrO7 does not exist whereas CrO42- and Cr2O72- are the chromate and dichromate ions respectively.
Either way, the only disadvantage here is that I cannot formulate an equation. I will just call it potassium compound for now.
It would appear, as though the HCl reacted with the potassium compound in an equilibrium reaction. The addition of HCl shifts this equilibrium to the right according to LCP, favouring the forward reaction which produces this new orange colour. But when we introduced NaOH, because NaOH will react with the HCl in a non-equilibrium reaction (we assert that the reaction goes to completion: NaOH(aq) + HCl(aq) -> NaCl(aq) + H2O(l)), we remove the amount of HCl present in the mixture with the potassium compound. As the HCl got reduced, the equilibrium shifted to the left according to LCP, favouring the reverse reaction and reintroducing new potassium compound (yellow colour remade).
Post by: RuiAce on February 16, 2016, 10:47:43 pm
Got a really short question: (yet surprisingly complicated, I find :-\ )
"Using the tabulated reduction potentials, calculate a cell emf for the disproportionation of H2O2 and into O2 and H2O.
thanks!
-Wes
Your question seems rather peculiarly worded and I can't make sense out of it. So these are the only tips I can provide at this point:
a) Hydrogen peroxide is a weak acid
b) This reaction in the standard reduction potential might be useful:
1/2 O2(g) + 2 H+ + 2 e- ⇌ H2O(l)
c) I found this equation on Wikipedia just now:
2 H2O2(l) -> 2 H2O(l) + 2 H2(g)
Note that H+ is an acid
Post by: Maz on February 19, 2016, 12:04:53 am
im a little confused on how to do concentration vs time graphs...could you please help me out?
if a reaction is 2NO2
N2O4 (g) + 57kJ
how do you sketch the concentration vs time graphs for the effect of changing volume on the system and
the effect of changing temperature
how would these be different for rate vs time graphs for the same things
thankyou so much :)
Post by: RuiAce on February 19, 2016, 09:01:48 am
hey
im a little confused on how to do concentration vs time graphs...could you please help me out?
if a reaction is 2NO2
N2O4 (g) + 57kJ
how do you sketch the concentration vs time graphs for the effect of changing volume on the system and
the effect of changing temperature
how would these be different for rate vs time graphs for the same things
thankyou so much :)
So at this point, because you don't have an exact amount of info on the concentrations of NO2 OR N2O4, we are unable to draw very precise concentration-time graphs. We can only make our graphs relative.
We will assume that the mixture is already at equilibrium. Consider the reaction: N2O4 (g) ⇌ 2 NO2 (g). Note that technically I have flipped your equation around.
According to secondary sources, the equilibrium naturally lies WELL to the right at lower temperatures. This means, initially the concentration of NO2 is far superior to that of N2O4.
Because heat is released when dinitrogen tetroxide is produced, we realise that this reaction above is endothermic. Higher temperatures favour the production of NO2, and lower temperatures favour the production of N2O2. Diagram 1 will therefore show what happens when heat is added to the system.
According to Le Chatelier's Principle, the equilibrium will shift to the right to minimise the changes, thereby increasing the concentration of NO2.
Firstly, all of the reactants given are gaseous. Hence, changes in pressure (and by consequence, volume) will affect ALL the substances in the equation. Therefore, we immediately proceed to the equation.
Note that on the left of the reaction, we only have 1 mol of gas. Whereas, on the right, we have 2. This means that if we increase the pressure, the system wants to eliminate the amount of gas present and thus the equilibrium will shift to the left, producing more N2O4. On the contrary, if we decrease the pressure, there is more room for gas to exist so the equilibrium will shift to the right and produce more NO2.
By convention, when we talk about changing the pressure we mean adding more NO2 AND N2O4 in, or taking some of BOTH out. But another way to change the pressure is to change the volume of the vessel. Note that if we DECREASE the volume of the vessel, we give LESS room for the gas to occupy. Hence, DECREASING the volume of the vessel is essentially the same as INCREASING the pressure. On the contrary, this means when we INCREASE the volume of the vessel, we DECREASE the pressure.
Thus, by decreasing the volume of the vessel we promote yield of N2O4. And vice versa for increase.
Now, let's consider the formula C=n/V. If you look at this formula, you will realise that if we decrease the volume of the vessel, we effectively INCREASE the concentration of EVERYTHING momentarily! This happens because obviously the amount mof moles present already can't be changed.
Analogy: Say at equilibrium we had 1 mol of NO2 in a 1L container. The concentration of NO2 is 1mol L^-1.
But then we decrease the volume of the vessel to 500mL (1/2 of a litre). The concentration, at the time we decreased it, effectively becomes 2mol L^-1 now, because we still have 1 mol of nitrogen dioxide present!
So if we decrease the volume of the vessel, momentarily we will have a SPIKE in the concentration graph. THEN, the system will try to adjust its equilibrium according to LCP.
This can be seen in diagram 2.
Now, the rate of reaction is dependent on two factors. We will now assume that a system has NOT YET achieved equilibrium, and is trying to.
1. Add a catalyst.
As you know, the catalyst speeds up the rate of reaction by providing an alternate pathway requiring a lower activation energy. Hence, if we add a catalyst, we are going to force the equilibrium to be achieved more quickly. Note that both the forward and reverse reaction use the same catalyst.
2. Add an inert impurity.
Say we pumped some argon gas into the reaction chamber. You could say that this is increasing the pressure as well. BUT, there is a difference.
Argon will not react, so it will not cause havoc (oxygen being introduced can cause explosions in all sorts of reactions e.g. Haber process for production of ammonia). But argon has a counter effect. When we increase the pressure by pumping in actual equilibrium mixture (NO2 and N2O4) we are promoting reactions on one side. But pumping in argon favours NEITHER side. Hence, if a system is not at equilibrium, the presence of argon is only going to SLOW DOWN the rate of reaction.
HOWEVER... In both situations, where the equilibrium is actually AT is unaffected. This only changes the rate, not the final result.
See diagram 3.
- Diagrams are coming. I don't have pen and paper with me beside me but what I do have is an ortho appointment so I'm running short of time right now
Post by: RuiAce on February 19, 2016, 03:29:40 pm
Post by: Maz on February 19, 2016, 07:18:37 pm
(http://uploads.tapatalk-cdn.com/20160219/69fc6f91b608211889200eae93edea43.jpg)
So at this point, because you don't have an exact amount of info on the concentrations of NO2 OR N2O4, we are unable to draw very precise concentration-time graphs. We can only make our graphs relative.
We will assume that the mixture is already at equilibrium. Consider the reaction: N2O4 (g) ⇌ 2 NO2 (g). Note that technically I have flipped your equation around.
According to secondary sources, the equilibrium naturally lies WELL to the right at lower temperatures. This means, initially the concentration of NO2 is far superior to that of N2O4.
Because heat is released when dinitrogen tetroxide is produced, we realise that this reaction above is endothermic. Higher temperatures favour the production of NO2, and lower temperatures favour the production of N2O2. Diagram 1 will therefore show what happens when heat is added to the system.
According to Le Chatelier's Principle, the equilibrium will shift to the right to minimise the changes, thereby increasing the concentration of NO2.
Firstly, all of the reactants given are gaseous. Hence, changes in pressure (and by consequence, volume) will affect ALL the substances in the equation. Therefore, we immediately proceed to the equation.
Note that on the left of the reaction, we only have 1 mol of gas. Whereas, on the right, we have 2. This means that if we increase the pressure, the system wants to eliminate the amount of gas present and thus the equilibrium will shift to the left, producing more N2O4. On the contrary, if we decrease the pressure, there is more room for gas to exist so the equilibrium will shift to the right and produce more NO2.
By convention, when we talk about changing the pressure we mean adding more NO2 AND N2O4 in, or taking some of BOTH out. But another way to change the pressure is to change the volume of the vessel. Note that if we DECREASE the volume of the vessel, we give LESS room for the gas to occupy. Hence, DECREASING the volume of the vessel is essentially the same as INCREASING the pressure. On the contrary, this means when we INCREASE the volume of the vessel, we DECREASE the pressure.
Thus, by decreasing the volume of the vessel we promote yield of N2O4. And vice versa for increase.
Now, let's consider the formula C=n/V. If you look at this formula, you will realise that if we decrease the volume of the vessel, we effectively INCREASE the concentration of EVERYTHING momentarily! This happens because obviously the amount mof moles present already can't be changed.
Analogy: Say at equilibrium we had 1 mol of NO2 in a 1L container. The concentration of NO2 is 1mol L^-1.
But then we decrease the volume of the vessel to 500mL (1/2 of a litre). The concentration, at the time we decreased it, effectively becomes 2mol L^-1 now, because we still have 1 mol of nitrogen dioxide present!
So if we decrease the volume of the vessel, momentarily we will have a SPIKE in the concentration graph. THEN, the system will try to adjust its equilibrium according to LCP.
This can be seen in diagram 2.
Now, the rate of reaction is dependent on two factors. We will now assume that a system has NOT YET achieved equilibrium, and is trying to.
1. Add a catalyst.
As you know, the catalyst speeds up the rate of reaction by providing an alternate pathway requiring a lower activation energy. Hence, if we add a catalyst, we are going to force the equilibrium to be achieved more quickly. Note that both the forward and reverse reaction use the same catalyst.
2. Add an inert impurity.
Say we pumped some argon gas into the reaction chamber. You could say that this is increasing the pressure as well. BUT, there is a difference.
Argon will not react, so it will not cause havoc (oxygen being introduced can cause explosions in all sorts of reactions e.g. Haber process for production of ammonia). But argon has a counter effect. When we increase the pressure by pumping in actual equilibrium mixture (NO2 and N2O4) we are promoting reactions on one side. But pumping in argon favours NEITHER side. Hence, if a system is not at equilibrium, the presence of argon is only going to SLOW DOWN the rate of reaction.
HOWEVER... In both situations, where the equilibrium is actually AT is unaffected. This only changes the rate, not the final result.
See diagram 3.
- Diagrams are coming. I don't have pen and paper with me beside me but what I do have is an ortho appointment so I'm running short of time right now
Thankyou so so so much :)
Post by: katherine123 on February 20, 2016, 08:54:34 pm
CH3COOH --> CH3COO- + H+
HCl --> H+ + Cl-
Post by: RuiAce on February 20, 2016, 09:03:21 pm
why does acetic acid produce the least concentration of H+ ? doesnt it produce the same amount as HCl since the formula is
CH3COOH --> CH3COO- + H+
HCl --> H+ + Cl-
No. Because acetic acid is weak and hydrochloric acid is strong.
HCl → H+ + Cl-
CH3COOH ⇌ H+ + CH3COO-
Hydrochloric acid fully ionises in solution. (The reality is that it's about 90-99%, but we assume it goes to 100%).
However, with acetic acid, only about 1.3% or so ionises.
Post by: jakesilove on February 21, 2016, 12:52:10 pm
why does acetic acid produce the least concentration of H+ ? doesnt it produce the same amount as HCl since the formula is
CH3COOH --> CH3COO- + H+
HCl --> H+ + Cl-
Just to build on RuiAce's answer, basically the way you assess the answer to this multiple choice question is with an understanding of "Weak" and "Strong" Acids.
Strong acids ionise completely in solution. Like RuiAce says, that means that approximately 90-99% of the HCl converts into the products (ie. Hydrogen ions and Chloride ions).
Weak acids do not ionise completely in solution. Like RuiAce says, that means that approximately 1.3% of the Acetic acid converts into the products.
In the HSC, you sort of just need to know which acids are weak, and which are strong. From memory, the only strong acids you work with are HCl and Sulfuric acid. The rest, you can assume, are weak! Therefore, as HCl is the only Strong acid in the list, it will produce the highest concentration of Hydrogen ions, and therefore have the lowest pH.
Jake
Post by: RuiAce on February 21, 2016, 01:34:00 pm
Just to build on RuiAce's answer, basically the way you assess the answer to this multiple choice question is with an understanding of "Weak" and "Strong" Acids.
Strong acids ionise completely in solution. Like RuiAce says, that means that approximately 90-99% of the HCl converts into the products (ie. Hydrogen ions and Chloride ions).
Weak acids do not ionise completely in solution. Like RuiAce says, that means that approximately 1.3% of the Acetic acid converts into the products.
In the HSC, you sort of just need to know which acids are weak, and which are strong. From memory, the only strong acids you work with are HCl and Sulfuric acid. The rest, you can assume, are weak! Therefore, as HCl is the only Strong acid in the list, it will produce the highest concentration of Hydrogen ions, and therefore have the lowest pH.
Jake
A tiny bit to add on again: Nitric acid is strong.
But yep, the list does not go beyond HCl, H2SO4 and HNO3 with regards to the HSC.
(However, you can assume that HBr and HI are also strong. Beause Cl, Br and I are all halogens (group A VII elements) There is a trap though - HF is weak!)
To fully answer the question though: To justify why citric acid is stronger than acetic obviously depends on the fact that citric acid (2-hydroxypropane-1,2,3-tricarboxylic acid) is triprotic and has three protons to donate, whereas acetic acid (ethanoic acid) only has one. But I think we all already knew that as the answer was disregarded immediately.
Post by: amandali on February 22, 2016, 11:47:39 pm
eg.Student mixed 20ml of 0.02mol/L of H2SO4 with 500ml of 0.001mol/L of Ca(OH)2 find the ph
so you will round the answer to 1 sig fig because of "0.02ml/0.001ml"
Will i lose marks if i don't round them enough eg. 4.1231x10^-2 when its meant to be 4.12x10^-2
Post by: RuiAce on February 23, 2016, 08:36:48 am
do you round of answer according to the given data with lowest sigfigCorrect. In the question you provided, because 1 sig. fig. is the lowest amount of figures given, you must round it to that.
eg.Student mixed 20ml of 0.02mol/L of H2SO4 with 500ml of 0.001mol/L of Ca(OH)2 find the ph
so you will round the answer to 1 sig fig because of "0.02ml/0.001ml"
Will i lose marks if i don't round them enough eg. 4.1231x10^-2 when its meant to be 4.12x10^-2
In terms of losing marks? Some examiners care, and others don't. Hence, not rounding correctly is a risk that you can't take.
Post by: katherine123 on February 24, 2016, 04:29:05 pm
The solubility of calcium hydroxide is 0.12 g per 100ml of water at 25 degrees. Calculate maximum pH of a solution of Calcium hydroxide assuming that the addition of the solid changes the volume only negligibly. ans: 11.5
Post by: jakesilove on February 24, 2016, 06:40:22 pm
How do you solve this? thanks ^^
The solubility of calcium hydroxide is 0.12 g per 100ml of water at 25 degrees. Calculate maximum pH of a solution of Calcium hydroxide assuming that the addition of the solid changes the volume only negligibly. ans: 11.5
Hey Katherine!
I actually get a different answer (12.5) and have looked through my working a few times and am not sure where I've gone wrong. Perhaps your answers are wrong? Otherwise, hopefully someone in the community can help me out!
(http://i.imgur.com/gRiFDzh.png?1)
Jake
Post by: IkeaandOfficeworks on February 24, 2016, 08:40:37 pm
Post by: jakesilove on February 24, 2016, 08:46:54 pm
I just wanted to ask about how to answer an ASSESS question. Like the structure (i.e how you start the response, what to include etc.) in answering these questions. Thank you!
Hey!
To answer an assess question, you essentially need to include an extra step with is a "judgement" or "reasoning" step.
You won't be required to conclude with some sort of actual assessment. For instance, at the end of an "assess the use of Ethanol as an alternate fuel source" you don't need to say "Ethanol is definitely good as an alternative fuel source". Rather, at the end you should say something like "Ethanol has many benefits as an alternate fuel source, however further research is required to increase the efficiency" etc.
The difference between "Explain" and "Assess" is the "reasoning" step. For instance, with ethanol, if you need to assess advantages and disadvantages it is not enough to just state that Ethanol has a lower heat of combustion. Rather, you need to say "As Ethanol has a lower heat of combustion, it is less productive than regular fuel when used. This is a negative result, as it means an individual needs to purchase more fuel to travel the same distance, as compared to Octane". Notice I've explained WHY it is a disadvantage. That is the main extra component of an "assess" question!
Hope this helps!
Jake
Post by: IkeaandOfficeworks on February 24, 2016, 10:27:53 pm
Post by: Maz on February 25, 2016, 07:47:01 pm
i have a test on equilibrium tomorrow and i was wandering if someone could please help me?
The general idea is that if a system is in equilibrium the foreword and reverse reactions are occurring at the same rate...
if a reactant is added the system will move to reduce and counter-react this added reactant by producing more products.
but what happens if the newly added reactant is so much that the other reactant doesn't even have enough particles available to
react with it? does the excess just exist in a non-reacted state in the system? and how do you determine that, just by being given a
equilibrium reaction?
Would really appreciate and help with this...
thankyou so so much in advance :)
Post by: jakesilove on February 25, 2016, 08:08:24 pm
Hey
i have a test on equilibrium tomorrow and i was wandering if someone could please help me?
The general idea is that if a system is in equilibrium the foreword and reverse reactions are occurring at the same rate...
if a reactant is added the system will move to reduce and counter-react this added reactant by producing more products.
but what happens if the newly added reactant is so much that the other reactant doesn't even have enough particles available to
react with it? does the excess just exist in a non-reacted state in the system? and how do you determine that, just by being given a
equilibrium reaction?
Would really appreciate and help with this...
thankyou so so much in advance :)
You're absolutely right! Any "excess" particles just sit there in an unreacted state. You can only figure this out if you are given molar values. For instance, in the reaction between Hydrochloric acid and Sodium Hydroxide, the ratio of the two reactants is 1:1. Therefore, if there are more moles of one than the other, the "excess" will just sit there gathering dust (not literally, obviously!).
In a generic "Extra reactant x is added, what will happen", you can assume that this "excess" principle is totally irrelevant. It is only when you are expected to do molar calculations that you should even think about principles like excess reagents etc.
Hope this helps!
Jake
Post by: Maz on February 25, 2016, 09:41:21 pm
You're absolutely right! Any "excess" particles just sit there in an unreacted state. You can only figure this out if you are given molar values. For instance, in the reaction between Hydrochloric acid and Sodium Hydroxide, the ratio of the two reactants is 1:1. Therefore, if there are more moles of one than the other, the "excess" will just sit there gathering dust (not literally, obviously!).it helps a lot
In a generic "Extra reactant x is added, what will happen", you can assume that this "excess" principle is totally irrelevant. It is only when you are expected to do molar calculations that you should even think about principles like excess reagents etc.
Hope this helps!
Jake
thankyou! :)
Post by: amandali on February 26, 2016, 03:53:46 pm
Post by: jakesilove on February 26, 2016, 03:59:26 pm
why does redox reaction occur when the cathode and anode are not separated?
Hey Amandali!
Redox reactions like the one in the picture you've attached actually don't need to be separated! As long as there are two dissimilar metals and a way for ions to travel back and forth (in this case, the solution connecting the two is that method) then a reaction will occur. The separation of the typical Galvanic cell is not actually required. Electrons still travel from one substance to the other, and ions travel in the opposite direction in the solution.
Jake
Post by: RuiAce on February 26, 2016, 07:25:43 pm
Hey Katherine!
I actually get a different answer (12.5) and have looked through my working a few times and am not sure where I've gone wrong. Perhaps your answers are wrong? Otherwise, hopefully someone in the community can help me out!
(http://i.imgur.com/gRiFDzh.png?1)
Jake
Hey Jake, with this question I found out your mistake.
When I typed 0.12/Ans into the calculator, where Ans=Molar Mass of Ca(OH)2, I found the answer to be 1.619...x10-3
Post by: RuiAce on February 26, 2016, 07:27:05 pm
Thanks Jake! One more question, how different is an Evaluate question from an Assess question? Thanks again!To answer this question, it doesn't.
Post by: RuiAce on February 26, 2016, 07:33:30 pm
why does redox reaction occur when the cathode and anode are not separated?
Hey Amandali!To reinforce what Jake said here, note that for the seperated galvanic cell, the salt bridge is there only for a migration of ions. Note that in the single unit, the migration of ions is equally possible!
Redox reactions like the one in the picture you've attached actually don't need to be separated! As long as there are two dissimilar metals and a way for ions to travel back and forth (in this case, the solution connecting the two is that method) then a reaction will occur. The separation of the typical Galvanic cell is not actually required. Electrons still travel from one substance to the other, and ions travel in the opposite direction in the solution.
Jake
Post by: amandali on February 26, 2016, 10:41:50 pm
Assuming no volume change, what is the pH of final solution
ans: 11.4
what i did was:
-show that HNO3 is limiting reagent with moles (5*10^-3)
-n(H20) = (5*10^-3) since 1 mol of HNO3 produce 1 mole of H20
-find n(H+)= n(H20)
-[H+]= n/v
- pH=-log(H+)
Post by: Happy Physics Land on February 26, 2016, 11:08:42 pm
0.300g of solid NaOH was added to 1.00L of 5.00*10^-3 mol/L HNO3
Assuming no volume change, what is the pH of final solution
ans: 11.4
what i did was:
-show that HNO3 is limiting reagent with moles (5*10^-3)
-n(H20) = (5*10^-3) since 1 mol of HNO3 produce 1 mole of H20
-find n(H+)= n(H20)
-[H+]= n/v
- pH=-log(H+)
Hey Amanda:
Good question. Ok so for me firstly what I would have done is to establish a balanced neutralisation equation.
NaOH (s) + HNO3 (aq) --> NaNO3 (aq) + H2O (l)
So clearly with this equation no further balancing is required. Now we need to work out the moles of HNO3 and moles of NaOH to determine the limiting reagent.
n of NaOH = 0.3/(22.9 + 16 + 1.008) = 0.0075173 moles
n of HNO3 = CV = 5 x 10^-3 x 1 = 0.005 moles
Ok so quite clearly since the molar ratio in the balanced neutralisation equation is 1:1:1:1, HNO3 is the limiting reagent, as you have correctly identified, well done!
Now theres an extra step here we need to be careful of, because the H+ ions and OH- ions that are dissociated from HNO3 and NaOH would neutralise one another and become H2O, we need to calculate the amount of moles of NaOH that is in excess of:
Excess n(NaOH) = 0.0075173 - 0.005 = 0.0025173 moles
Assuming theres no volume change, the concentration of NaOH would be C = 0.0025173/1 = 0.0025173 mol/L
So now we know that there are 0.0025173 moles per litre of OH- ions in the solution. In order to figure out the concentration of H+ ions and the pH values, we need to use the water constant (Kw = 1.0 x 10^-14).
Kw = [H+] [OH-]
[H+] = 1.0x10^-14/0.0025173 = 3.97251 x 10^-12
Now we can calculate the pH:
pH = -log(3.97251 x 10^-12)
Hence pH = 11.4
Hope you understood my reasoning there amanda, a very good question indeed! If you have any further queries please dont hesitate to ask! :)
Best Regards
Happy Physics Land
Post by: katherine123 on February 27, 2016, 08:43:01 am
I dont know what went wrong with my working
I found:
- final [H+]=10^-1
since HCl --> H+ + Cl-
- [HCl] = 10^-1
- initial v(HCl) = 25*10^-3 L
- initial [HCl] = 25*10^-3*0.5 = 12.5*10^-3
using CiVi=CfVf
Vf=0.003L
Post by: RuiAce on February 27, 2016, 08:54:57 am
A student is given 25ml of 0.50mol/L of HCl(aq). He was asked to prepare a solution with pH=1.0 . What will be the final volume of the solution? Ans: 125ml
I dont know what went wrong with my working
I found:
- final [H+]=10^-1
since HCl --> H+ + Cl-
- [HCl] = 10^-1
- initial v(HCl) = 25*10^-3 L
- initial [HCl] = 25*10^-3*0.5 = 12.5*10^-3
using CiVi=CfVf
Vf=0.003L
[H+]=10-pH=10-1mol L-1
Strong acid - Assume the acid fully ionises (Degree of ionisation = 100%)
Note: Degree of ionisation is why this happens. NOT because HCl → H+ + Cl- because what if you had acetic acid: CH3COOH ⇄ H+ + CH3COO-
Therefore [HCl] = [H+] = 10-1mol L-1
In the dilutions formula, we now have Vf=10-1mol L-1 ----> This you already have
Vi=2.5*10-2 L ----> This you already have
Ci=0.50 mol L-1 ---->This is what you did that confused me. I am unsure as to why you chose to do V * C when the concentration was given already. The formula n=CV does allow us to find the moles of acid present, but the concentration was already given to us.
0.50 * 2.5*10-2 = 0.1 * Vf
Vf = 0.125 L
Post by: RuiAce on February 27, 2016, 09:00:17 am
Hey Amanda:
Good question. Ok so for me firstly what I would have done is to establish a balanced neutralisation equation.
NaOH (s) + HNO3 (aq) --> NaNO3 (aq) + H2O (l)
So clearly with this equation no further balancing is required. Now we need to work out the moles of HNO3 and moles of NaOH to determine the limiting reagent.
n of NaOH = 0.3/(22.9 + 16 + 1.008) = 0.0075173 moles
n of HNO3 = CV = 5 x 10^-3 x 1 = 0.005 moles
Ok so quite clearly since the molar ratio in the balanced neutralisation equation is 1:1:1:1, HNO3 is the limiting reagent, as you have correctly identified, well done!
Now theres an extra step here we need to be careful of, because the H+ ions and OH- ions that are dissociated from HNO3 and NaOH would neutralise one another and become H2O, we need to calculate the amount of moles of NaOH that is in excess of:
Excess n(NaOH) = 0.0075173 - 0.005 = 0.0025173 moles
Assuming theres no volume change, the concentration of NaOH would be C = 0.0025173/1 = 0.0025173 mol/L
So now we know that there are 0.0025173 moles per litre of OH- ions in the solution. In order to figure out the concentration of H+ ions and the pH values, we need to use the water constant (Kw = 1.0 x 10^-14).
Kw = [H+] [OH-]
[H+] = 1.0x10^-14/0.0025173 = 3.97251 x 10^-12
Now we can calculate the pH:
pH = -log(3.97251 x 10^-12)
Hence pH = 11.4
Hope you understood my reasoning there amanda, a very good question indeed! If you have any further queries please dont hesitate to ask! :)
Best Regards
Happy Physics Land
I will suggest an alternate route at the final steps. However, take note that this requires the same amount of working so it's preference as to what you use.
---> Substitute directly into pOH = -log10[OH+]
---> Use pH + pOH = 14
Note that this is essentially the process of using Kw just in a different way. (And you use subtraction instead of division)
Post by: katherine123 on February 27, 2016, 10:11:37 am
[H+]=10-pH=10-1mol L-1
Strong acid - Assume the acid fully ionises (Degree of ionisation = 100%)
Note: Degree of ionisation is why this happens. NOT because HCl → H+ + Cl- because what if you had acetic acid: CH3COOH ⇄ H+ + CH3COO-
Therefore [HCl] = [H+] = 10-1mol L-1
In the dilutions formula, we now have Vf=10-1mol L-1 ----> This you already have
Vi=2.5*10-2 L ----> This you already have
Ci=0.50 mol L-1 ---->This is what you did that confused me. I am unsure as to why you chose to do V * C when the concentration was given already. The formula n=CV does allow us to find the moles of acid present, but the concentration was already given to us.
0.50 * 2.5*10-2 = 0.1 * Vf
Vf = 0.125 L
so is it okay if i write "assuming full ionisation and since HCl is monoprotic" (without HCl-->H+ + Cl-)
and will i lose marks if i omit the equilibrium sign for weak acids in exam?
Post by: RuiAce on February 27, 2016, 02:01:49 pm
so is it okay if i write "assuming full ionisation and since HCl is monoprotic" (without HCl-->H+ + Cl-)
and will i lose marks if i omit the equilibrium sign for weak acids in exam?
Assuming full ionisation due to HCl being strong but yes also include monoprotic if you want full accuracy.
And yes.
Post by: amandali on February 28, 2016, 07:10:19 am
- For chemistry of batteries: is it okay if i make a table containing oxidation, reduction equation and voltage without describing in words
- "Evaluate it in comparison.." - so does that mean i talk about both problems and benefits and then judge which one is better
- For assess/evaluate ques in general - is it okay to state advantages only or do i have to include both advantages and disadvantages
This is my plan:
• Dry cell
Ad:
- first small and easily-portable cell
- cheap
Dis
- does not give constant voltage and has a short-life therefore used for infrequent used electrical devices eg. Torches, radios
Impacts on society
- increase development of electrical devices
• Button cell
Ad:
- Smaller than dry cell and has long lifespan thus used in smaller, frequently used devices eg. Hearing aids and cameras
- provides the same initial voltage (1.6V) as dry cell but maintains constant voltage over longer period of time since [OH-] remains constant so more useful for above mentioned devices
Dis
- made of silver so more expensive than dry cell
Impacts on society
- small size, long-lifespan, stable voltage allows it to be used in small, sensitive devices
• Evaluate – neither is problematic environmentally
- KOH in button cell is caustic
- Zn in dry cell is toxic to plants
BUT they are contained in small quantities hence do not pose serious concern
Large scale (industrial) will be problematic
Post by: katherine123 on February 28, 2016, 08:34:24 am
Which one requires the greatest volume of base to change the colour of indicator
a)citric acid
b)acetic acid
c)sulfuric acid
d)HCl
i dont understand why it's A :/
Post by: RuiAce on February 28, 2016, 08:56:51 am
Equal volumes of four 0.1mol/L acids were titrated with the same sodium hydroxide solution.
Which one requires the greatest volume of base to change the colour of indicator
a)citric acid
b)acetic acid
c)sulfuric acid
d)HCl
i dont understand why it's A :/
Because the reaction is with sodium hydroxide (strong base), the reaction ALWAYS goes to completion as a strong base will fully react with both a strong acid, AND a weak acid.
I.e. (Assume aqueous states for all but water)
HCl + NaOH → NaCl + H2O
H2SO4 + 2 NaOH → Na2SO4 + 2 H2O
CH3COOH + NaOH → CH3COONa + H2O
C3H4OH(COOH)3 + 3 NaOH → C3H4OH(COO)3Na3 + 3 H2O
Because all reactions go to completion, we are fully dependent on the amount of protons they can donate. It can be seen that citric acid is triprotic, as opposed to sulfuric being diprotic and the rest being monoprotic. This is why the answer is citric: You have more protons to neutralise. (Note that the volumes and concentrations are constant, so the moles of acid are constant. It's the moles of H+ that differs.)
Post by: RuiAce on February 28, 2016, 09:02:19 am
Name ONE type of cell, other than dry-cell or lead-acid cell, you have studied. Evaluate it in comparison with either the dry-cell or lead-acid cell, in terms of chemistry and the impact on society. Include relevant chemical equations in your answer.
- For chemistry of batteries: is it okay if i make a table containing oxidation, reduction equation and voltage without describing in words
- "Evaluate it in comparison.." - so does that mean i talk about both problems and benefits and then judge which one is better
- For assess/evaluate ques in general - is it okay to state advantages only or do i have to include both advantages and disadvantages
This is my plan:
• Dry cell
Ad:
- first small and easily-portable cell
- cheap
Dis
- does not give constant voltage and has a short-life therefore used for infrequent used electrical devices eg. Torches, radios
Impacts on society
- increase development of electrical devices
• Button cell
Ad:
- Smaller than dry cell and has long lifespan thus used in smaller, frequently used devices eg. Hearing aids and cameras
- provides the same initial voltage (1.6V) as dry cell but maintains constant voltage over longer period of time since [OH-] remains constant so more useful for above mentioned devices
Dis
- made of silver so more expensive than dry cell
Impacts on society
- small size, long-lifespan, stable voltage allows it to be used in small, sensitive devices
• Evaluate – neither is problematic environmentally
- KOH in button cell is caustic
- Zn in dry cell is toxic to plants
BUT they are contained in small quantities hence do not pose serious concern
Large scale (industrial) will be problematic
Yes. Many candidates draw up a table in the exam room to answer these types of questions. Although this is not my preference as I liked writing lengthy responses, many band 6s have been obtained using tables and "dot points" everywhere.
You must always discuss advantages and disadvantages to be able to make an informed assessment of what you are talking about. You cannot only speak about one side.
And then, you should always make some comparison. Because when you are evaluating, you need to communicate exactly why as to one cell is better/worse than the other.
The plan is mostly good, however you could have more impacts on society for the dry cell - the question specifies this so this should be more important. One may argue that for the dry cell, the impact on society is more prominent amongst devices that do require low voltages for operation and it's mainly aided in just their manufacture, however the impact is limited when it comes to larger scale powering.
More importantly, observe that your question asked for the chemistry of the cells, to which you seemingly have forgotten. The chemistry of the cell refers to the exact equations of the cell that occur at the anode and cathode (and if there is an equation #3 that occurs elsewhere, that too).
Post by: jakesilove on February 28, 2016, 09:22:46 am
Name ONE type of cell, other than dry-cell or lead-acid cell, you have studied. Evaluate it in comparison with either the dry-cell or lead-acid cell, in terms of chemistry and the impact on society. Include relevant chemical equations in your answer.
- For chemistry of batteries: is it okay if i make a table containing oxidation, reduction equation and voltage without describing in words
- "Evaluate it in comparison.." - so does that mean i talk about both problems and benefits and then judge which one is better
- For assess/evaluate ques in general - is it okay to state advantages only or do i have to include both advantages and disadvantages
This is my plan:
• Dry cell
Ad:
- first small and easily-portable cell
- cheap
Dis
- does not give constant voltage and has a short-life therefore used for infrequent used electrical devices eg. Torches, radios
Impacts on society
- increase development of electrical devices
• Button cell
Ad:
- Smaller than dry cell and has long lifespan thus used in smaller, frequently used devices eg. Hearing aids and cameras
- provides the same initial voltage (1.6V) as dry cell but maintains constant voltage over longer period of time since [OH-] remains constant so more useful for above mentioned devices
Dis
- made of silver so more expensive than dry cell
Impacts on society
- small size, long-lifespan, stable voltage allows it to be used in small, sensitive devices
• Evaluate – neither is problematic environmentally
- KOH in button cell is caustic
- Zn in dry cell is toxic to plants
BUT they are contained in small quantities hence do not pose serious concern
Large scale (industrial) will be problematic
Hey!
I definitely agree with RuiAce when they says that, for a top level response, you should probably have a few more advantages/disadvantages up your sleeve. I think that answering these questions in a table is the best way to ensure you get marks, as it is abundantly clear where marks should be allocated based on your subheading. The only thing I would add is a society heading, with advantages and disadvantages within that, and then an environment heading, with the same headings under that.
The reason for this is just the way the dotpoint is worded. You'll likely need to talk about just one, or both, so splitting them up makes it far clearer that you've actually addressed the question!
That being said, your general approach and structure is very good. Make sure to include the Chemistry if asked for the chemistry!
Good luck! Memorisation sections like this were the bane of my existence in Year 12...
Jake
Post by: Chemystery on February 28, 2016, 05:19:32 pm
I've left my studying to last minute and have a chem test tomorrow on half the prelim course (Chemical Earth and Metals) and was wondering what would be the best way to smash through some last-minute revision? After seeing your other post i know I won't leave things last minute again in the future, thank gosh this is only preliminary haha!
Post by: Happy Physics Land on February 28, 2016, 06:59:37 pm
Hey Jake!
I've left my studying to last minute and have a chem test tomorrow on half the prelim course (Chemical Earth and Metals) and was wondering what would be the best way to smash through some last-minute revision? After seeing your other post i know I won't leave things last minute again in the future, thank gosh this is only preliminary haha!
Hey Chemystery:
Yeah its certainly not a very good idea to be doing last minute revisions. But like you said luckily its preliminaries and this perhaps is a lesson that can be learnt for year 12. In year 12 its definitely not a good idea to study the night before. Even studying the week before would still be a little short. Personally I recommend to start revising bit by bit a month beforehand and more work during the 2 weeks before the exam. This way you wont feel too pressurised the days before your exam.
But I would give you some ideas of what you should probably focus on studying right now:
Chemical Earth:
- Trends in periodic tables
- Bohr's atomic model
- Valence Electrons and energy shells
- Lewis Dot Diagram for ionic and covalent bonds as well as for individual atoms
- Ionic Bonds and Covalent Bonds
- Atomic radius and Effective charge
- Physical and electrical properties of ionic compounds
- Physical, electrical and heat properties of metals
- Properties of Covalent network (e.g. diamond) substance
- Properties of Covalent molecular substances
- The multiple spheres of Earth (e.g. atmosphere, lithosphere, hydrosphere etc.) and the dominate molecules and elements that exist within each sphere
- What is a physical change? List some physical properties of a substance
- What is a chemical Change? List some chemical properties of a substance
- How do you know when a chemical change occurs?
- Distinguish between compound, solution, mixture and solute
- Boiling points and melting points
- All the separation techniques and their industrial/home uses
Metals:
- The historical development of metals
- The smelting of iron from Haematite / copper from Chalcopyrite
- Metals discovered throughout the centuries
- What are alloys?
- Provide some examples of alloys, and state their industrial/home uses
- Balancing chemical equations
- Reaction of metal + water
- Displacement reaction
- Reaction of metal + oxygen
- Reaction of metal with dilute acid
- Activity series of metals
- Full ionic, net ionic equations and half ionic equations (oxidation equation and reduction equation)
- Redox reactions
- Be familiar with how to calculate moles using molar mass and mass and how to calculate mass using molar mass and moles
- Avogadro and Gay Lussac's law memorised
- Calculate Volume using moles and molar volume, calculate moles using volume and molar volume
- Empirical formula and molecular formula, and how to find the molecular formula when the question provides you with molar mass and the empirical formula
- Gravimetric analysis
- Electronegativity
- Ionisation energy (1st, second and third)
- Electrical conductivity of substances
Thats pretty much all you will have to study for, and it will be an immense amount of content if you only have one day. Just try to skim through textbook content for every dotpoint above and you would at least have some knowledge of what should be going on. Anyways, good luck for tomorrow!
Best Regards
Happy Physics Land
Post by: jakesilove on February 28, 2016, 07:16:36 pm
Hey Jake!
I've left my studying to last minute and have a chem test tomorrow on half the prelim course (Chemical Earth and Metals) and was wondering what would be the best way to smash through some last-minute revision? After seeing your other post i know I won't leave things last minute again in the future, thank gosh this is only preliminary haha!
Hey!
So firstly, obviously I'm going to suggest you don't leave it to the night before again.
However, moving on, I recommend using your set of notes (or finding someone else's set of notes), going through and handwriting out (under subheadings, in extremely summarised form, with colours) all of the information that you don't know. Then, just read over that booklet that you've written out. The act of writing out information will make it far more likely that you remember the content tomorrow!
Really, best of luck. You definitely need it, but like you say it is only preliminary!
Jake
Post by: RuiAce on February 28, 2016, 07:17:42 pm
Hey Chemystery:
Yeah its certainly not a very good idea to be doing last minute revisions. But like you said luckily its preliminaries and this perhaps is a lesson that can be learnt for year 12. In year 12 its definitely not a good idea to study the night before. Even studying the week before would still be a little short. Personally I recommend to start revising bit by bit a month beforehand and more work during the 2 weeks before the exam. This way you wont feel too pressurised the days before your exam.
But I would give you some ideas of what you should probably focus on studying right now:
Chemical Earth:
- Trends in periodic tables
- Bohr's atomic model
- Valence Electrons and energy shells
- Lewis Dot Diagram for ionic and covalent bonds as well as for individual atoms
- Ionic Bonds and Covalent Bonds
- Atomic radius and Effective charge
- Physical and electrical properties of ionic compounds
- Physical, electrical and heat properties of metals
- Properties of Covalent network (e.g. diamond) substance
- Properties of Covalent molecular substances
- The multiple spheres of Earth (e.g. atmosphere, lithosphere, hydrosphere etc.) and the dominate molecules and elements that exist within each sphere
- What is a physical change? List some physical properties of a substance
- What is a chemical Change? List some chemical properties of a substance
- How do you know when a chemical change occurs?
- Distinguish between compound, solution, mixture and solute
- Boiling points and melting points
- All the separation techniques and their industrial/home uses
Metals:
- The historical development of metals
- The smelting of iron from Haematite / copper from Chalcopyrite
- Metals discovered throughout the centuries
- What are alloys?
- Provide some examples of alloys, and state their industrial/home uses
- Balancing chemical equations
- Reaction of metal + water
- Displacement reaction
- Reaction of metal + oxygen
- Reaction of metal with dilute acid
- Activity series of metals
- Full ionic, net ionic equations and half ionic equations (oxidation equation and reduction equation)
- Redox reactions
- Be familiar with how to calculate moles using molar mass and mass and how to calculate mass using molar mass and moles
- Avogadro and Gay Lussac's law memorised
- Calculate Volume using moles and molar volume, calculate moles using volume and molar volume
- Empirical formula and molecular formula, and how to find the molecular formula when the question provides you with molar mass and the empirical formula
- Gravimetric analysis
- Electronegativity
- Ionisation energy (1st, second and third)
- Electrical conductivity of substances
Thats pretty much all you will have to study for, and it will be an immense amount of content if you only have one day. Just try to skim through textbook content for every dotpoint above and you would at least have some knowledge of what should be going on. Anyways, good luck for tomorrow!
Best Regards
Happy Physics Land
Wow this was a nice summary of the prelim chem course... now how do you still even remember all of this...
I have to be given a question to know what to do...
Post by: Happy Physics Land on February 28, 2016, 07:30:10 pm
Wow this was a nice summary of the prelim chem course... now how do you still even remember all of this...
I have to be given a question to know what to do...
Thats why you make your own notes in prelims mate
Post by: RuiAce on February 28, 2016, 08:10:07 pm
Thats why you make your own notes in prelims mate
BACK IN MY DAYS...
Ok,
Back when I was in prelim... I either winged it... or read other people's notes (biology)... and went into the exam... and did well in everything except English.
Post by: ProfLayton2000 on February 29, 2016, 04:56:05 pm
Regarding 5-7 mark 'assess' or 'evaluate' questions, is there a set structure you would recommend?
Post by: amandali on February 29, 2016, 05:45:12 pm
Dry cell is a small, easily-portable and cheap cell. It has a short-life and provides a non-constant voltage hence it is used for low-energy drained devices like torches and radios. Whereas, silver oxide cell provides constant voltage over longer period of time than dry cell since [OH-] remains constant hence it is used to operate sensitive devices like hearing aids and watches rather than dry cell. Silver oxide cell is also smaller and has a long-lifespan than dry cell which makes it more useful for above mentioned devices. However, it is made of silver so it is more expensive than dry cell.
The electrolyte KOH in silver oxide cell is caustic and the outer zinc casing of dry cell is toxic to plants but there are present in small quantities hence neither cell is problematic to the environment.
Including a table:
Dry cell: oxidation, reduction equations
SIlver oxide cell : oxidation, reduction equations
-how much am i suppose to write for 7 marks ques
-I feel like it s a bit brief but i dont know what else to add
Post by: jakesilove on February 29, 2016, 05:53:53 pm
Hi there (and you have no idea how glad I am this website exists)
Regarding 5-7 mark 'assess' or 'evaluate' questions, is there a set structure you would recommend?
I'm glad that you find these forums helpful!
I think it really depends on the question itself. For an ethanol question, a table with advantages and disadvantages always works best. You will always tailor it for what sort of question it is (are there two clear sides to the argument, and so can you use a table? Are there multiple important components (eg. for your Battery) and so can you use subheadings?).
The important part is that you actually explain WHY an aspect is good/bad etc. It is not enough to say Ethanol has a lower molar heat of combustion: Explain how this has a negative impact on consumers.
Also, make sure you address EACH COMPONENT of the question. It is really easy to skip a word in a question that ends up being 3 marks worth of content. Underline the important part of the question, and address everything.
I don't think that there is a set structure. For an assess/evaluate, you will almost always have advantages/disadvantages, so I think a table/subheadings is always appropriate. However I think the best way to make sure you are prepared for extended response question is to do one standard question from each topic, re write it until it is perfect (marked by us or your teachers) and then put it in your notes.
Good luck!
Jake
Post by: jakesilove on February 29, 2016, 05:57:44 pm
Name one type of cell, other than dry cell or lead acid cell. Evaluate it in comparison with either the dry cell or lead-acid cell, in terms of chemistry and the impact on society. Include relevant chemical equations in your answer. 7 marks
Dry cell is a small, easily-portable and cheap cell. It has a short-life and provides a non-constant voltage hence it is used for low-energy drained devices like torches and radios. Whereas, silver oxide cell provides constant voltage over longer period of time than dry cell since [OH-] remains constant hence it is used to operate sensitive devices like hearing aids and watches rather than dry cell. Silver oxide cell is also smaller and has a long-lifespan than dry cell which makes it more useful for above mentioned devices. However, it is made of silver so it is more expensive than dry cell.
The electrolyte KOH in silver oxide cell is caustic and the outer zinc casing of dry cell is toxic to plants but there are present in small quantities hence neither cell is problematic to the environment.
Including a table:
Dry cell: oxidation, reduction equations
SIlver oxide cell : oxidation, reduction equations
-how much am i suppose to write for 7 marks ques
-I feel like it s a bit brief but i dont know what else to add
Hey Amandali!
I think that this is an almost perfect response. Whilst it may seem brief, remember that when you handwrite all of that it will be nearly two thirds of a page! The only think that I would absolutely make clearer is the general impact on society. Have one sentence saying "Dry cell had a huge/small impact on society because of (insert property here) allowing for (insert affect here). Similarly/conversely, the Silver cell".... etc. Just make it utterly clear that you're answering the specific question.
Really, though, I think that this is a very good answer overall. Decide whether subheadings/table could work, write it out a few times and maybe send through your final answer, and we can assess its final mark!
Jake
Post by: ProfLayton2000 on February 29, 2016, 06:31:34 pm
I'm glad that you find these forums helpful!
I think it really depends on the question itself. For an ethanol question, a table with advantages and disadvantages always works best. You will always tailor it for what sort of question it is (are there two clear sides to the argument, and so can you use a table? Are there multiple important components (eg. for your Battery) and so can you use subheadings?).
The important part is that you actually explain WHY an aspect is good/bad etc. It is not enough to say Ethanol has a lower molar heat of combustion: Explain how this has a negative impact on consumers.
Also, make sure you address EACH COMPONENT of the question. It is really easy to skip a word in a question that ends up being 3 marks worth of content. Underline the important part of the question, and address everything.
I don't think that there is a set structure. For an assess/evaluate, you will almost always have advantages/disadvantages, so I think a table/subheadings is always appropriate. However I think the best way to make sure you are prepared for extended response question is to do one standard question from each topic, re write it until it is perfect (marked by us or your teachers) and then put it in your notes.
Good luck!
Jake
Unbelievably helpful. Thank you.
Post by: jakesilove on February 29, 2016, 10:47:02 pm
Unbelievably helpful. Thank you.
I'm really glad! Keep on posting great questions like this; as the community grows, we'll be able to answer more and more questions, and release more and more resources!
Jake
Post by: katherine123 on March 02, 2016, 07:16:09 am
Explain why the two solutions have different pH values 3 marks
HCl has lower pH since it fully ionises while citric partially ionises but HCl is monoprotic and citric is triprotic though :/
Post by: RuiAce on March 02, 2016, 08:10:58 am
0.1 mol/L solution of HCl has pH of 1.0, whereas, a 0.1mol/L solution of citric acid has pH of 1.6
Explain why the two solutions have different pH values 3 marks
HCl has lower pH since it fully ionises while citric partially ionises but HCl is monoprotic and citric is triprotic though :/
Citric acid's degree of ionisation is only about 8.6% whereas hydrochloric acid's is virtually 100%. This difference is huge. You'd need to concentrate citric acid by a factor of about 12 to make it's pH similar to HCl.
Post by: jakesilove on March 02, 2016, 08:20:54 am
0.1 mol/L solution of HCl has pH of 1.0, whereas, a 0.1mol/L solution of citric acid has pH of 1.6
Explain why the two solutions have different pH values 3 marks
HCl has lower pH since it fully ionises while citric partially ionises but HCl is monoprotic and citric is triprotic though :/
Hey all!
Recalling that this is a three mark question, I would definitely recommend including more information. Here is what I would write, including where I think the marks lie.
HCl is a strong acid, which means it ionises completely in solution (in that the equilibrium between HCl and Water lies completely to the right) [1]. As the concentration of the solution is 0.1 mol/L, and the ratio between HCl and Hydrogen ions is 1:1, the pH will be
Conversely, Citric acid is a weak acid, which means it does not ionise completely in solution (in that the equilibrium between Citric acid and Water lies largely to the left) [2.5]. Whilst the concentration of the solution is the same, as it does not ionise completely the ratio between moles of acid and moles of Hydrogen ions will not be 1:1 (it will be a:b, a>>b). Therefore, the pH of this solution will be higher than the pH of an equally concentrated solution of HCl [3].
I think this question would also be easier if you write out the ionisation of each acid, but honestly I can't figure out how to make that look nice on the forums. Just make sure to fully explain the concepts in order to get the marks!
Hope this helps.
Jake
Post by: William3558 on March 02, 2016, 02:18:32 pm
A student titrated an aliquot of standard sodium carbonate solution with HCl acid in a burette.
Would the concentration determined for HCl be higher than, lower than or unchanged from the actual value if the student had previously washed with water, but not dried:
a) The pipette used to deliver the aliquot of sodium carbonate solution
The answer was higher. But, wouldn't the water reduce the concentration and amount of mol in the sodium carbonate? Therefore, the amount of mol coming out of the HCl in the burette be less than expected, thus the concentration is less than expected?
b)
The answer was unchanged. As water is added afterwards into the flask anyway.
c) The burette
The answer was lower. As this would reduce the concentration of the HCl from its actual value
I think I might be overthinking it...
Post by: katherine123 on March 02, 2016, 02:51:34 pm
Post by: jakesilove on March 02, 2016, 03:33:20 pm
Hi, I need some help with explaining an answer.
A student titrated an aliquot of standard sodium carbonate solution with HCl acid in a burette.
Would the concentration determined for HCl be higher than, lower than or unchanged from the actual value if the student had previously washed with water, but not dried:
a) The pipette used to deliver the aliquot of sodium carbonate solution
The answer was higher. But, wouldn't the water reduce the concentration and amount of mol in the sodium carbonate? Therefore, the amount of mol coming out of the HCl in the burette be less than expected, thus the concentration is less than expected?
b)
The answer was unchanged. As water is added afterwards into the flask anyway.
c) The burette
The answer was lower. As this would reduce the concentration of the HCl from its actual value
I think I might be overthinking it...
Hey William!
So, let's start with the pipette. Like you say, if there is excess water in the pipette, the sodium carbonate will become more dilute. If it is more dilute, then you will require LESS HCl to neutralise the solution (and thus end the titration). If the number of moles of acid used in LESS than it should be, that means it is going to be calculated as MORE concentrated. Think of it this way: If you figure out (through calculation) that 1 mol is required to neutralise the base, but all of a sudden you release LESS than a mole of acid (because the base is more diluted than expected), you will still assume that, for instance, there is one mole of acid in the 15mL of liquid you release. When you do your
calculation, your value for concentration will be HIGHER than it should be.
If there is extra liquid in the Burette, then the Acid becomes more dilute. Therefore, it requires MORE acid to neutralise the base. This means, again, that you will increase V but keep n the same, resulting in a lower concentration (as per the formula)
Hope this helps! If I haven't explained it enough, let me know :)
Jake
Post by: jakesilove on March 02, 2016, 03:37:55 pm
i dont get why it is B
Hey Katherine!
This is a really tough question, but hopefully my explanation makes sense.
The list is ordered in DECREASING strength. HCl is the strongest acid, and HCN is the weakest acid. You can tell because, whilst they have the same concentration, they ionise to different extents, with HCl ionising completely (as it is strong) and HCN hardly ionising at all (as it is very weak).
An acid and a base will react when, first, it breaks up into its components (ie. HCl ionises to become a Hydrogen ion and a Chlorine ion and NaOH breaks up into a sodium ion and a Hydroxide ions) and then these ions react with each other. However, HCl will ionise completely, straight away, without the need for a huge amount of solvent. This is because it is strong. Conversely, weak bases will not ionise completely without a lot of solution. This is why the weakest acid will take the most amount of base to react completely! It just takes a long time to ionise, as opposed to stronger acids.
Hope that this makes sense!
Jake
Post by: William3558 on March 02, 2016, 04:17:21 pm
Hey William!
So, let's start with the pipette. Like you say, if there is excess water in the pipette, the sodium carbonate will become more dilute. If it is more dilute, then you will require LESS HCl to neutralise the solution (and thus end the titration). If the number of moles of acid used in LESS than it should be, that means it is going to be calculated as MORE concentrated. Think of it this way: If you figure out (through calculation) that 1 mol is required to neutralise the base, but all of a sudden you release LESS than a mole of acid (because the base is more diluted than expected), you will still assume that, for instance, there is one mole of acid in the 15mL of liquid you release. When you do your
calculation, your value for concentration will be HIGHER than it should be.
If there is extra liquid in the Burette, then the Acid becomes more dilute. Therefore, it requires MORE acid to neutralise the base. This means, again, that you will increase V but keep n the same, resulting in a lower concentration (as per the formula)
Hope this helps! If I haven't explained it enough, let me know :)
Jake
Just what I needed to hear! Thanks! Very appreciated. ;D
Post by: grace_joseph on March 03, 2016, 05:51:11 pm
Also, are there any rules to determining whether a substance is a strong/weak acid/base when just given it's formula?
Thanks so much! :)
Post by: jakesilove on March 03, 2016, 06:27:30 pm
Hi! I have an upcoming practical exam on titrations with a written component on the Acidic Environment Syllabus 1 (indicators) and Syllabus 4. I was just wondering if you had any tips for studying for these sort of exams, as well as any helpful information on those syllabus's? Sorry, this is really vague and open-ended, I'm just a bit uncertain as to how I prepare!
Also, are there any rules to determining whether a substance is a strong/weak acid/base when just given it's formula?
Thanks so much! :)
Hey Grace!
I'll answer your last question first, as honestly it's just easier; no, there is no way to know whether something is a strong/weak substance based entirely on its formula (as far as I know, and definitely as far as your Curriculum goes). The only way you could be required to 'know' is if it is a substance you regularly use in the Syllabus (ie. HCl and H2SO4 are strong, Citric is weak etc.) or, experimentally, you could tell based on the pH of a substance.
What I mean by that is you could create a dilute/concentrated solution, test its pH, and determine its strength. If you put 0.1 Mol of monoprotic acid into 1L of water, you would expect the concentration of
As for the general "How do I study for a practical exam!" question, I think that's definitely harder to answer. Have a working understanding of the dot points, to which I can only suggest writing your own notes, talking to friends about the dotpoints, and answering past HSC questions on those specific dotpoints. Then, make sure you are very, very comfortable with the terms Reliability, Validity and Accuracy in terms of first-hand experiments. If you're not sure, Reliability refers to repetition, Validity refers to experimental design testing the required variable (ie. controlling variables etc.) and Accuracy refers to the precision of instruments. Then, make sure you know which the independent and dependent variables are. Look through potential practical tasks that can be drawn from the dotpoints in those two sections, in case you can guess the question.
Other than that, I don't really have specific tips! If you have a good idea of the chemistry in those sections, understand experimental design, and bring a Ruler for your graph, you'll be totally fine!
An extra note: make sure to write out procedures in past tense, in case you have to!
Let me know if there is anything (more specific, preferably aha) else I can help you with!
Jake :)
Post by: grace_joseph on March 03, 2016, 06:42:46 pm
Massive help!
Grace :)
Post by: William3558 on March 03, 2016, 09:15:13 pm
Post by: jakesilove on March 03, 2016, 09:32:45 pm
Quick Question, In a back titration with standard sodium hydroxide added in excess to the solution with ammonium... When you boil the solution to evaporate all of the ammonia. Why do you need to keep adding water and keep a constant volume? Is it so the solution doesn't go dry?
Hey William!
I think you are looking for the VCE Question Thread (which is here). We hardly do any of that in the HSC!
Jake
Post by: amandali on March 06, 2016, 12:56:48 am
H3X <--> X- + 3H+
and can you check my response thanks:
Compare the environmental sustainability of producing ethanol from petroleum and sugar cane. Support you answer with relevant chemical equations
Petroleum is not environmentally sustainable as it is a non-renewable resource hence will eventually deplete in the future. Moreover, the production of ethanol from petroleum is unsustainable as it involves the process of fractional distillation C6H12O6(aq)--> C6H12O6(l) and catalytic cracking C4H8(l)-->C2H2(g) + C2H6 (g)which requires enormous amount of energy that are commonly produced by the combustion of fossil fuels which releases CO2 that contributes to global warming.
In contrast, sugar cane may be sustainable because it can be regrown once harvested thus a renewable resource. However, its production is not completely carbon neutral as petroleum may be combusted to produce energy to operate crop harvesters and trucks which releases more CO2 than it is absorbed to make ethanol.
Post by: Happy Physics Land on March 06, 2016, 01:22:50 am
Am i allowed to wrote this condensed ionisation equation for citric acid in hsc exam:
H3X <--> X- + 3H+
and can you check my response thanks:
Compare the environmental sustainability of producing ethanol from petroleum and sugar cane. Support you answer with relevant chemical equations
Petroleum is not environmentally sustainable as it is a non-renewable resource hence will eventually deplete in the future. Moreover, the production of ethanol from petroleum is unsustainable as it involves the process of fractional distillation C6H12O6(aq)--> C6H12O6(l) and catalytic cracking C4H8(l)-->C2H2(g) + C2H6 (g)which requires enormous amount of energy that are commonly produced by the combustion of fossil fuels which releases CO2 that contributes to global warming.
In contrast, sugar cane may be sustainable because it can be regrown once harvested thus a renewable resource. However, its production is not completely carbon neutral as petroleum may be combusted to produce energy to operate crop harvesters and trucks which releases more CO2 than it is absorbed to make ethanol.
Hey Amanda:
Great questions! In regards to your first question, l understand that you would be doing that in order to reduce time consumption. But it is crucial that we dont risk anything in our hsc exams, because you never know who will be marking your essay. Hence it is always good to write the full formula out despite it will cost you several seconds more.
Still referring to your equation, there are some errors that has been made using the condensed formula:
1) After ionisation/donating all the protons, the charge of X wouldnt be X-, but X3-, because citric acid has lost the three protons in its carboxylic bond.
2) The process of ionisation of citric acid is gradual, i.e. it cant be represented in 1 step, because in the chemistry that we learn, the three protons cannot be donated all at once. I.e. it would follow a three-step sequence of: C6H8O7(aq) <---> C6H7O7-(aq) + H(aq), C6H7O7-(aq) <---> C6H6O7(2-)(aq) + H+(aq), C6H6O7(2-)(aq) <---> C6H5O7(3-)(aq) + H+(aq). So yeah this is why I dont recommend the condensed method (unless your teacher reinforce it) because it can yield careless mistakes and it doesnt show the full chemical process.
Referring to your response now, I quite like it because you definitely compared sugar and petroleum to some depth and you've also provided the correct equation of catalytic cracking. If this is a 4 mark question I would definitely give it a 3 and if lm a strict marker l would give it 2. What l recommend you can improve on is to maintain a balance between the amount of pros and cons you give to petroleum and those you give to sugar cane. What you have done there is good, but if we write more on petroleum than sugar cane it would sound a bit biased in terms of your understanding. What you can add to enhance your statement of sugar cane being a environmentally sustainable source is to say "during the cultivation of sugar canes, photosynthesis takes place (provide an equation of photosynthesis) which consumes the carbon dioxide from atmosphere, reducing the amount of greenhouse gas and alleviating global warming". This would make your reasoning stronger for sugar cane. With the equation for fractional distillation, you can include it but that is not the point. A better equation to include is the combustion of ethanol which clearly demonstrates that carbon dioxide is released and it is released a large quantity (2CO2, meaning that for every mole of ethanol combusted, two moles are released).
But Amanda you have been posting a lot of questions on AN which really shows how dedicated you are to your studying. If I sounded a little harsh please dont be discouraged because right now you are doing a super awesome job. If you have any further questions please dont hesitate to ask!
Best Regards
Happy Physics Land
Post by: jakesilove on March 06, 2016, 09:07:43 am
Am i allowed to wrote this condensed ionisation equation for citric acid in hsc exam:
H3X <--> X- + 3H+
and can you check my response thanks:
Compare the environmental sustainability of producing ethanol from petroleum and sugar cane. Support you answer with relevant chemical equations
Petroleum is not environmentally sustainable as it is a non-renewable resource hence will eventually deplete in the future. Moreover, the production of ethanol from petroleum is unsustainable as it involves the process of fractional distillation C6H12O6(aq)--> C6H12O6(l) and catalytic cracking C4H8(l)-->C2H2(g) + C2H6 (g)which requires enormous amount of energy that are commonly produced by the combustion of fossil fuels which releases CO2 that contributes to global warming.
In contrast, sugar cane may be sustainable because it can be regrown once harvested thus a renewable resource. However, its production is not completely carbon neutral as petroleum may be combusted to produce energy to operate crop harvesters and trucks which releases more CO2 than it is absorbed to make ethanol.
I completely agree with HPL's assessment of your answer, and I just have one thing to add. Definitely make sure that you write roughly the same amount for each "half" of the question, and part of that is chemical equations. Make sure to include some sort of chemical equation for the second half. Also, I think it is important to identify that it is biomass (ie. Cellulose) that is important in the production of ethanol from Sugar Cane; it is then easy to claim that this is renewable, as biomass makes up 50% of all plant matter!
Good answer overall though; however I would definitely expand on your second half.
Jake
Post by: lazydreamer on March 06, 2016, 12:42:34 pm
I'm glad that you find these forums helpful!
I think it really depends on the question itself. For an ethanol question, a table with advantages and disadvantages always works best. You will always tailor it for what sort of question it is (are there two clear sides to the argument, and so can you use a table? Are there multiple important components (eg. for your Battery) and so can you use subheadings?).
The important part is that you actually explain WHY an aspect is good/bad etc. It is not enough to say Ethanol has a lower molar heat of combustion: Explain how this has a negative impact on consumers.
Also, make sure you address EACH COMPONENT of the question. It is really easy to skip a word in a question that ends up being 3 marks worth of content. Underline the important part of the question, and address everything.
I don't think that there is a set structure. For an assess/evaluate, you will almost always have advantages/disadvantages, so I think a table/subheadings is always appropriate. However I think the best way to make sure you are prepared for extended response question is to do one standard question from each topic, re write it until it is perfect (marked by us or your teachers) and then put it in your notes.
Good luck!
Jake
hello, sorry it was an oldish post but a question about using tables...
Could i get away with incorporating tables for questions like assess, compare/contrast, discuss etc? it's so much easier to organise information that way and looks better than a huge chunk of words, would markers appreciate that or...nah? haha
aaand because i don't have time to write a detailed paragraph
thanks
Post by: Happy Physics Land on March 06, 2016, 01:00:52 pm
hello, sorry it was an oldish post but a question about using tables...
Could i get away with incorporating tables for questions like assess, compare/contrast, discuss etc? it's so much easier to organise information that way and looks better than a huge chunk of words, would markers appreciate that or...nah? haha
aaand because i don't have time to write a detailed paragraph
thanks
Hey LazyDreamer:
Definitely yes for compare and contrast. If you use a table for compare and contrast and include the suitable information and suitable titles, then its almost a guaranteed full mark. Cautions needs to be taken with questions that involve evaluate, discuss or assess. These questions require a logical flowing response, i.e. you need to use linking words. Even if you are using a table for these questions, you would still need linking words and words that show cause and effect. If you are using a table for evaluate a assess, you must also make two clear judgments in your response in order to secure full marks. But yes, generally saying, a table is a great way to reduce time consumption whilst still effective presenting your response to the question.
Best Regards
Happy Physics Land
Post by: lazydreamer on March 06, 2016, 01:29:55 pm
Hey LazyDreamer:
Definitely yes for compare and contrast. If you use a table for compare and contrast and include the suitable information and suitable titles, then its almost a guaranteed full mark. Cautions needs to be taken with questions that involve evaluate, discuss or assess. These questions require a logical flowing response, i.e. you need to use linking words. Even if you are using a table for these questions, you would still need linking words and words that show cause and effect. If you are using a table for evaluate a assess, you must also make two clear judgments in your response in order to secure full marks. But yes, generally saying, a table is a great way to reduce time consumption whilst still effective presenting your response to the question.
Best Regards
Happy Physics Land
thanks for answering, i'll keep that in mind for pre-trials and see how i go :)
and i agree that for evaluate/discuss/assess qns i would need to include a couple of sentences at the end that makes a judgement. As for linking cause and effect, couldn't that be included in the table, eg |Cause|Effect|Evaluation|
btw
Quote
If you are using a table for evaluate a assess, you must also make two clear judgments in your response in order to secure full marks.what do you mean by 2 clear judgments?
Post by: Happy Physics Land on March 06, 2016, 01:38:47 pm
thanks for answering, i'll keep that in mind for pre-trials and see how i go :)
and i agree that for evaluate/discuss/assess qns i would need to include a couple of sentences at the end that makes a judgement. As for linking cause and effect, couldn't that be included in the table, eg |Cause|Effect|Evaluation|
btw what do you mean by 2 clear judgments?
Ok so this might sound a little dumb but for example if you are asked to assess the impact of polyethylene on the society, in your first dotpoint/establishing sentence, you must state that polyethylene has had a significant/mediocre/insignificant impact upon the society (making a judge). At the end of your answer, l would literally write "Final Judgement: Polyethylene has had a tremendous impact upon the society". You can just include one judgment if you want, but for me, having a judgment at the start and having another one in the end helps to make your statements sound stronger.
Post by: lazydreamer on March 06, 2016, 02:14:21 pm
Ok so this might sound a little dumb but for example if you are asked to assess the impact of polyethylene on the society, in your first dotpoint/establishing sentence, you must state that polyethylene has had a significant/mediocre/insignificant impact upon the society (making a judge). At the end of your answer, l would literally write "Final Judgement: Polyethylene has had a tremendous impact upon the society". You can just include one judgment if you want, but for me, having a judgment at the start and having another one in the end helps to make your statements sound stronger.
not at all ;D reads like an essay paragraph to me: thesis statement that answers the qn, then supporting points, then a link back to the question. Makes sense i guess, thanks!
Post by: jakesilove on March 06, 2016, 03:11:42 pm
not at all ;D reads like an essay paragraph to me: thesis statement that answers the qn, then supporting points, then a link back to the question. Makes sense i guess, thanks!
Just wanted to back up HPL here: Totally agree with what he's said! If you have any further questions, about this or anything else, please don't hesitate to post :)
Jake
Post by: lazydreamer on March 06, 2016, 03:32:26 pm
Just wanted to back up HPL here: Totally agree with what he's said! If you have any further questions, about this or anything else, please don't hesitate to post :)
Jake
this site is great, much easier to get help than on BoS (which is blocked on the school laptops lol) so when i found a forum which was actually unblocked with a helpful community, it felt like i died and went to hsc heaven. Thanks, i think i'll be using this consistently throughout the year :)
Post by: Happy Physics Land on March 06, 2016, 07:18:14 pm
this site is great, much easier to get help than on BoS (which is blocked on the school laptops lol) so when i found a forum which was actually unblocked with a helpful community, it felt like i died and went to hsc heaven. Thanks, i think i'll be using this consistently throughout the year :)
Hahaha its good to know that we are helping you lazydreamer!!! Your involvement will make our community an even greater place than it was! :D
Happy Posting!
Post by: RuiAce on March 07, 2016, 10:27:28 am
Ok so this might sound a little dumb but for example if you are asked to assess the impact of polyethylene on the society, in your first dotpoint/establishing sentence, you must state that polyethylene has had a significant/mediocre/insignificant impact upon the society (making a judge). At the end of your answer, l would literally write "Final Judgement: Polyethylene has had a tremendous impact upon the society". You can just include one judgment if you want, but for me, having a judgment at the start and having another one in the end helps to make your statements sound stronger.Will add my 2 cents to this.
To be honest, I reckon when giving the evaluation a good evaluation could be something such as
Polyethylene has benefited society appreciably through it's wide ranges of applications (give reasoning) however further improvements could still be made in shifting away from these resources and using biopolymer alternatives.
(Usually evaluate questions at this time of the year seem to be ethanol though so I would say overall, it is the renewability of the resource that ultimately makes ethanol a viable alternate fuel source.)
(Do forgive me though - I'm sleep-deprived right now so I don't feel like reading too much of what I missed.)
Post by: Happy Physics Land on March 07, 2016, 05:06:10 pm
Hi Jake!
We did an experiment in class where we titrated sodium carbonate and hydrochloric acid, creating a neutral salt (I think... it's a strong acid and strong base right?). Our teacher told us that methyl orange was the best indicator, but this doesn't make sense to me. Shouldn't we use bromothymol blue or phenolphthalein as the indicator as the equivalence point would be around pH 7?
I thought maybe it was to do with the presence of carbon dioxide as that's an acidic oxide, but not sure if that's right or not. Unless I've gotten something completely wrong!
Thanks!
Hey Grace:
That was a great experiment you did, and this actually is a pretty interesting question. One thing I can assure you is that the existence of CO2(g) as an outcome of this reaction definitely wouldnt affect which indicator to use, because CO2(aq) is the one that would react with water to form H2CO3(aq) which is acidic. Of course you may wanna argue that theres an equilibrium equation of CO2(g) --> CO2(aq) and some CO2(g) may have converted into CO2(aq) due to slight changes in temperature or pressure. Even if this equilibrium reaction does take place, its usually almost negligible and it definite wouldnt influence your choice of indicators.
In regards to which indicator to use, l think that your teacher hasnt been very accurate with her words. When we test the pH of things, we normally use two indicators at the same time to ascertain the pH range of this substance that we are testing. (If you need further elaboration on that point just ask again). So we dont usually only use one indicator, say for example we do use methyl orange and the NaCl we are testing in this case is a neutral substance, then the methyl orange would turn yellow. But hey, methyl orange's pH is defined over 3.1-4.4, so the yellow colour can possibly mean its a neutral substance, but at the same time it can also mean that NaCl is weakly acidic or even basic! Thats why we needs a second indicator for the purpose of determining the salt's pH range. So personally, if you have an option over which indicators use, I would use methyl red which ranges from 4.4 -6.3 pH and phenolphthalein which ranges from 8.3 -11.0 pH. So to test this salt is neutral, you would see that methyl red shows a yellow colour, and phenolphthalein showing no colour (i.e. colourless). This means that the salt has a pH that ranges from 6.3 to 8.3, and hence you can justify that the salt is neutral or just a little bit acidic or basic.
Thats my response to your question, if Jake comes around soon he can probably give you a better response than I can. But if you have any questions, dont hesitate to ask! :)
Best Regards
Happy Physics Land
Post by: neeshy900 on March 07, 2016, 06:57:33 pm
Post by: Happy Physics Land on March 07, 2016, 08:19:18 pm
Any tips on titration prac exams?? Stressing out :( :(
Hello Neeshy:
Yes I can definitely understand how those titration prac exams can be extremely concerning, but dont worry, we are all here to help you! I will provide you with some specific tips below and hope they help.
Preparation of a Primary Standard Solution:
- A standard solution is a solution of known composition and concentration used for quantitative analytical procedures (in this case, titration)
- Standard solution made by dissolving a primary standard volume of water
- Solids substances are desirable for preparing standard solution because they are easier to weigh
- It is also desirable for the substances to be easily obtained in its pure form, which reduces the cost and makes it easier to perform calculations with minimal errors involving the existence of impurities
- Substances with large molar mass is desirable to allow for reduced weighing errors
- And finally, the substance must also have a high water solubility - obviously, because the substance is required to be dissolved in water for standard solution preparation
*Procedure for preparing primary standard solutions may be obtained from your textbook
Dilution Calculations
- Since we are adding only solvent, moles of solute remains unchanged
- Before we dilute (solution 1, concentrated) the equation that applies is n1 = C1V1
- After we dilute, V and C have both changed, hence the equation that applies is n2=C2V2
- But since the moles of solute is the same before and after, hence the equation that applies is C1V1 = C2V2
General Overview of Neutralisation Reaction
- HEAT is released in neutralisation reactions between strong acids and strong bases (e.g. NaOH + HCl, the most common example)
- ACID + BASE ----> SALT + WATER
- Neutralisation reaction between strong acid and strong base is essentially the reaction of a proton and OH ion to form water
Titration
- An analytical process which involves finding the volume of a solution of accurately known conc. that is required to react completely with a known volume of a solution of unknown concentration. Hence it is a volumetric analysis.
- Summary of Procedure:
1. A known volume of a solution of unknown conc. placed in a conical flask
2. A solution of known conc. added until all of the analyte has reacted, volume is recorded
3. Using the mole ratio of the reaction the unknown conc. is calculated
Calculation of Concentration of an Unknown Solution:
1. Write a balanced equation
2. Calculate number of moles of reactant A (n=CV)
3. Using the moles of reactant A and the stoichiometric molar ratio in the equation, calculate the number of moles of Reactant B used
4. Use C=n/V to calculate the conc. of Reactant B
These are basically all I have to say, a bit content heavy, if Jake happens to come around he can give you more advices. Anyways good luck my friend, if you have any questions, dont hesitate to ask!
Best Regards
Happy Physics Land
Post by: RuiAce on March 07, 2016, 08:25:28 pm
Hi Jake!
We did an experiment in class where we titrated sodium carbonate and hydrochloric acid, creating a neutral salt (I think... it's a strong acid and strong base right?). Our teacher told us that methyl orange was the best indicator, but this doesn't make sense to me. Shouldn't we use bromothymol blue or phenolphthalein as the indicator as the equivalence point would be around pH 7?
I thought maybe it was to do with the presence of carbon dioxide as that's an acidic oxide, but not sure if that's right or not. Unless I've gotten something completely wrong!
Thanks!
Sodium carbonate is not a strong base...
Methyl orange was used because you're titrating a WEAK base with a strong acid!
_________________________
HPL mentioned to use something in it's pure form to prepare a standard solution. An example of this would be to use ANHYDROUS sodium carbonate, not hydrated sodium carbonate.
Post by: RuiAce on March 07, 2016, 08:30:33 pm
Hey Grace:
That was a great experiment you did, and this actually is a pretty interesting question. One thing I can assure you is that the existence of CO2(g) as an outcome of this reaction definitely wouldnt affect which indicator to use, because CO2(aq) is the one that would react with water to form H2CO3(aq) which is acidic. Of course you may wanna argue that theres an equilibrium equation of CO2(g) --> CO2(aq) and some CO2(g) may have converted into CO2(aq) due to slight changes in temperature or pressure. Even if this equilibrium reaction does take place, its usually almost negligible and it definite wouldnt influence your choice of indicators.
In regards to which indicator to use, l think that your teacher hasnt been very accurate with her words. When we test the pH of things, we normally use two indicators at the same time to ascertain the pH range of this substance that we are testing. (If you need further elaboration on that point just ask again). So we dont usually only use one indicator, say for example we do use methyl orange and the NaCl we are testing in this case is a neutral substance, then the methyl orange would turn yellow. But hey, methyl orange's pH is defined over 3.1-4.4, so the yellow colour can possibly mean its a neutral substance, but at the same time it can also mean that NaCl is weakly acidic or even basic! Thats why we needs a second indicator for the purpose of determining the salt's pH range. So personally, if you have an option over which indicators use, I would use methyl red which ranges from 4.4 -6.3 pH and phenolphthalein which ranges from 8.3 -11.0 pH. So to test this salt is neutral, you would see that methyl red shows a yellow colour, and phenolphthalein showing no colour (i.e. colourless). This means that the salt has a pH that ranges from 6.3 to 8.3, and hence you can justify that the salt is neutral or just a little bit acidic or basic.
Thats my response to your question, if Jake comes around soon he can probably give you a better response than I can. But if you have any questions, dont hesitate to ask! :)
Best Regards
Happy Physics Land
Unnecessary. Bromothymol blue will be GREEN at around pH7, which is a very clearly distinguishable colour from blue and yellow. As with litmus with purple and blue/red.
The rule of thumb is:
Strong B Weak A -> Phenolphthalein
Strong B Strong A -> Litmus/Bromothymol blue
Weak B Strong A -> Methyl orange
Post by: Happy Physics Land on March 07, 2016, 09:03:30 pm
Unnecessary. Bromothymol blue will be GREEN at around pH7, which is a very clearly distinguishable colour from blue and yellow. As with litmus with purple and blue/red.
The rule of thumb is:
Strong B Weak A -> Phenolphthalein
Strong B Strong A -> Litmus/Bromothymol blue
Weak B Strong A -> Methyl orange
Umm, you reckon l should just delete my post?.....
Post by: lazydreamer on March 07, 2016, 09:06:24 pm
Any tips on titration prac exams?? Stressing out :( :(
Great run-down from HPL! You know it's a titration, so make sure you know exactly how to carry it out, step-by-step. Play it out in your mind if you want. That will give you some confidence and reassurance going into it. But don't get complacent, 'cause anything can happen!
A few tips, not sure if it'll help but...:
- There are different ways to rinse your burette, pipette and standard flask, make sure you know them!
- If you can, label your glassware, good lab. practice and prevents confusion, which you don't need in an exam yea?
- Don't forget to close the stopcock when rinsing the burette, lol
- Rinse down the edges of the flask as you titrate, esp. nearing the end-point, in case some analyte/titrant gets stuck on the sides, which will result in error
ummm that's all i can think of right now, best of luck in your exam, i'm sure you'll do great :D
Post by: RuiAce on March 07, 2016, 09:06:58 pm
Umm, you reckon l should just delete my post?.....
No real point. Once it's posted it's there. It's still a learning experience and who knows if someone else will make a similar mistake.
Post by: amandali on March 09, 2016, 05:24:16 pm
HSO3- + H20 <--> SO3(2-) + H3O+
HSO3- + H2O <--> H2SO3 + OH-
to show that hydrogen sulfite ion is amphiprotic
Post by: RuiAce on March 09, 2016, 06:03:19 pm
why is this unacceptable
HSO3- + H20 <--> SO3(2-) + H3O+
HSO3- + H2O <--> H2SO3 + OH-
to show that hydrogen sulfite ion is amphiprotic
Technically it should be. Because you are still showing how it can act as a base or an acid according to B-L theory.
Easiest thing to do, however, is to demonstrate it reacting with a different acid and a different base just to clear up ambiguity.
HSO3- + HCl(aq) -> H2SO3(aq) + Cl-
HSO3- + OH- -> SO32- + H2O(l)
If you got marked incorrect by a teacher, you will have to dispute it with them
Post by: Happy Physics Land on March 09, 2016, 11:06:36 pm
The question is: with the aid of appropriate equations, explain why the dihydrogen phosphate ion H2PO4- is amphiprotic, yet an aqueous solution of KH2PO4 has a greater pH than 7?
Thank you very much in advance :D !!!
Post by: jakesilove on March 10, 2016, 07:12:35 am
Hey guys, need a hand here if anyone dont mind!
The question is: with the aid of appropriate equations, explain why the dihydrogen phosphate ion H2PO4- is amphiprotic, yet an aqueous solution of KH2PO4 has a greater pH than 7?
Thank you very much in advance :D !!!
Hey HPL!
In a nutshell, here is the way you need to answer the question in order to get full marks. There is some guess work here, but I only make fair assumptions based (lol) on the question.
First, you need to write out equations showing Dihydrogen Phosphate acting as both an acid (proton donor) and base (proton acceptor). To do this, write an equation with the Dihydrogen Phosphate and any acid, and then Dihydrogen Phosphate and any base. You need to explain what an amphiprotic substance is (ability to donate AND ability to accept protons), and then relate it to your equations.
Then, all you really need to say is that Dihydrogen Phosphate is a STRONGER base than water. As such, the water will act as an acid (as it is also amphiprotic), donating protons to the Dihydrogen Phosphate (which acts as a base). This produces hydroxide ions (due to the water losing a proton) and therefore the pH is greater than 7.
Hope this helps!
Jake
Post by: Happy Physics Land on March 10, 2016, 06:40:23 pm
Hey HPL!
In a nutshell, here is the way you need to answer the question in order to get full marks. There is some guess work here, but I only make fair assumptions based (lol) on the question.
First, you need to write out equations showing Dihydrogen Phosphate acting as both an acid (proton donor) and base (proton acceptor). To do this, write an equation with the Dihydrogen Phosphate and any acid, and then Dihydrogen Phosphate and any base. You need to explain what an amphiprotic substance is (ability to donate AND ability to accept protons), and then relate it to your equations.
Then, all you really need to say is that Dihydrogen Phosphate is a STRONGER base than water. As such, the water will act as an acid (as it is also amphiprotic), donating protons to the Dihydrogen Phosphate (which acts as a base). This produces hydroxide ions (due to the water losing a proton) and therefore the pH is greater than 7.
Hope this helps!
Jake
Ahhhh ok thank you Jake! We actually had a pretty intense discussion on the most suitable equations to include and my teacher agreed with your reasoning as welll mmmm. Thank you so much Jake, greatly appreciated! :D
P.S. nice pun!
Post by: amandali on March 13, 2016, 03:47:20 pm
Since Z increases as temperature decreases so :
X (g) + Y (g) + heat <--> Z(g)
Post by: Happy Physics Land on March 13, 2016, 07:03:21 pm
isnt this reaction endothermic
Since Z increases as temperature decreases so :
X (g) + Y (g) + heat <--> Z(g)
Hey Amanda!
Quite a tricky question actually, took me a while to figure out.
So we know the equilibrium equation is here is a certain moles of X + a certain moles of Y = a certain moles of Z. For convenience's sake, lets just assume first that the equation is X + Y <--> Z.
So as we can see, as temperature increases in all three trends, the yield of Z decreases. Le Chatelier's principle tells us that as temperature of system increases, the system is disturbed and hence the equilibrium shifts to the endothermic reaction in order to consume the heat in the system to minimise this disturbance. Since yield of Z decreases as temperature increases, the yield of X and Y are favoured, and according to Le Chatelier we know that the endothermic process is favoured. Therefore we can conclude that the reverse process of Z---> X + Y is endothermic. Hence the forward process of X + Y ---> Z becomes exothermic.
We know now that the answer definitely cant be A or C because they both display an endothermic reaction. So this leaves us with B and D. Great! Now lets have a look at the pressure. As pressure increases, the yield of Z actually increases (as you can see, the graph representing 200 atm pressure is way above the two other graphs). According to La Chatelier's principle, as pressure increases, the reaction would favour the side with the least moles of gas in order to alleviate the pressure by having less gas particles occupying a reduced volume. Since in the graph the production of Z is favoured and according to Le Chatelier's principle the side with least moles of gas should be favoured, then it can be concluded that Z must have less moles of gas than the reactants X and Y.D is the only answer that suits both criteria of being exothermic and Z being the side with least moles of gas.
Hence the correct answer is D.
If you have any further questions dont hesitate to ask! :)
Best Regards
Happy Physics Land
Post by: amandali on March 13, 2016, 08:03:00 pm
Post by: Neutron on March 13, 2016, 08:07:33 pm
can anyone explain how to determine whether an acid/base can act as a buffer pair? thanks
Hey Amandali! I believe a buffer pair is commonly made up of a weak acid and its conjugate base (so for example, CH3COOH and CH3COO- are buffer pairs! Hope that makes sense :D
Neutron
Post by: Neutron on March 13, 2016, 08:09:32 pm
Sorry I have quite a long question, I was wondering whether someone could explain how a dry cell battery works or a lead-acid battery cell, I've read some textbooks about it but I'm still a bit confused! And also, what's a good way of knowing what citric acid and sodium hydroxide forms? Like what the products are.. Thank you! :D
Neutron
Post by: Happy Physics Land on March 13, 2016, 08:33:01 pm
can anyone explain how to determine whether an acid/base can act as a buffer pair? thanks
Hey Amanda:
Good question! Actually a few weeks before this "buffer" concept was a little confusing for me too. But yes, like what neutron said, a buffer pair would be a weak acid and the conjugate base of that weak acid. You may ask, why not a strong acid? Because strong acids dont produce reversible reactions! Buffers are required to be weak acids or bases because we need the equilibrium reaction for excess acids/bases to be converted into neutral substances in order to minimise the added acid/base's impact upon the normal pH level of the system.
In our blood, H2CO3 and HCO3- are prevalent examples of buffers. Other buffer pairs can include CH3COOH/CH3COO- or H2PO42-/HPO4-.
Best Regards
Happy Physics Land
Post by: Happy Physics Land on March 13, 2016, 09:00:25 pm
Hey guys!
Sorry I have quite a long question, I was wondering whether someone could explain how a dry cell battery works or a lead-acid battery cell, I've read some textbooks about it but I'm still a bit confused! And also, what's a good way of knowing what citric acid and sodium hydroxide forms? Like what the products are.. Thank you! :D
Neutron
Hey Neutron!
First of all, thank you for answering amanda's question! It's really great to see everyone helping each other out! I certainly give you credit for that!
Ok, so for me, my chem teacher didnt really explain that dotpoint, so I really had to chew my jacaranda textbook and collect online informations. Im suspecting that you are perhaps thrown into the same situation too? (correct me if lm wrong haha) But dont worry, Im here to help!
If you understood galvanic cells, you should be able to reason with the chemical functionality of a dry cell battery. All it is, is just the electrode in the anode half-cell being oxidised and the electrode in the cathode half-cell being reduced, and hence producing a voltage. It works exactly like your galvanic cells, except with more complicated substances. In a dry-cell battery, the positive cathode at the centre consists of an inert graphite rod surrounded by MnO2(s) powder. The negative anode consists of the zinc casing of the cell. And there is a thick layer of moist electrolyte paste NH4Cl(aq) between the graphite rod and MnO2(s). What basically happens is essentially the reaction between manganese dioxide powder and Ammonium chloride and zinc casing and ammonium chloride . (2MnO2(s) + 2NH4+(aq) + 2e- --> Mn2O3(s) + H2O(l) + 2NH3(aq)) In this equation, the original oxidation state of Mn is +4 but in the product, the oxidation state of Mn becomes +3. This means that Mn has been reduced. In the zinc casing and ammonium chloride reaction (Zn(s) + 2NH4Cl-(aq) --> ZnCl2(s) + 2NH3(aq) + H2(g)), the original oxidation state of Zinc was 0, and after the reaction, its oxidation state becomes 2+. This means that Zinc has been oxidised. This redox reaction then produces a voltage, just like what happens in a galvanic cell.
In regards to citric acid + sodium hydroxide, you can very easily deduce what happens either by using the rule "acid + base --> salt + water" or just simply memorise what I got below! :)
C6H8O7(aq) + NaOH(s) --> NaC6H5O7(aq) + 3H2O(l)
Hope my answer has helped with your understanding of these areas of chemistry, and they are indeed hard concepts to master. Once again, thanks for being helpful in the community and if you have any further questions, please dont hesitate to ask! :)
Best Regards
Happy Physics Land
Post by: katherine123 on March 13, 2016, 10:05:30 pm
-If all products are weak eg. H2PO4- + H3O+ --> H3PO4 + H20 then the reaction goes to completion
-some weak some strong eg. H2PO4- +H20 <--> OH- (strong) + H3PO4 (weak)
then reaction undergoes equilibrium
-weak acid eg. HF will always produce a strong conjugate base ( F- )
Post by: Happy Physics Land on March 13, 2016, 10:12:55 pm
Am i right in saying that:
-If all products are weak eg. H2PO4- + H3O+ --> H3PO4 + H20 then the reaction goes to completion
-some weak some strong eg. H2PO4- +H20 <--> OH- (strong) + H3PO4 (weak)
then reaction undergoes equilibrium
-weak acid eg. HF will always produce a strong conjugate base ( F- )
Hey Katherine:
In regards to your first two points, I find it a bit difficult to memorise it that way. All you need to know is whenever there is a strong acid or a strong base involved in the reactants, then the reaction is always going to go to completion because strong bases/acids completely ionise in water. Whenever both reactants are weak acids/bases, the reaction is always going to become reversible (i.e. equilibrium reaction) because they dont fully ionise in water.
I do agree with your third point though! Very well stated! Weak acids will for strong conjugate bases because they only partially ionise in water.
Hope this give you a clearer insight into acid-base reactions! :)
Best Regards
Happy Physics Land
Post by: qwerty222 on March 13, 2016, 11:04:34 pm
if i wanted to do some self-study at home wat textbook would u recommend?
Post by: Happy Physics Land on March 14, 2016, 12:01:34 am
hello,
if i wanted to do some self-study at home wat textbook would u recommend?
Hey qwerty222!
Jacaranda, definitely jacaranda.
It has the most detailed information on each dotpoint, a whole load of content, if you make notes out of the textbook and be familiar with them, you will be very familiarised with all your modules. The two disadvantages are its cost (60-70 dollars l remember) and the overwhelming amount of content which may bore you. If you want to obtain a fast grasp onto the chemistry knowledge I will recommend Excel chemistry textbook because it has all the information you would need for each dotpoint and present it in very succint manner. But of course it is not as extensive as jacaranda. I havent really used any other textbooks as just yet, but l do recommend these two textbooks, especially jacaranda.
Best Regards
Happy Physics Land
Post by: katherine123 on March 14, 2016, 10:45:29 pm
Post by: Happy Physics Land on March 14, 2016, 11:25:43 pm
why is HNO3 and NO2- not a conjugate acid/base pair and conjugate acid-base buffer pair
Hey Katherine!
A conjugate acid/base pair will have exactly the same chemical composition except it will either lose or gain a proton (i.e. a H+ atom). So in this case, when HNO3(aq) loses a H+ atom, it becomes NO3-, not NO2-. So HNO3 and NO3- in this case will be conjugate pairs.
Post by: katherine123 on March 15, 2016, 06:50:46 am
Post by: Happy Physics Land on March 15, 2016, 08:52:00 am
Is this info wrong in saying that CH3COO- since weak acids always produce strong conjugate bases
I would say that the information is wrong. The chemistry that we have been taught in year 12 tells us that a weak acid would yield a strong conjugate base. Hence CH3COO- is likely to be a strong conjugate base.
Sorry, update. The information is actually correct. We got taught that a weak acid would yield a strong conjugate base. But just like what rui said, only an extremely weak acid would yield a strong conjugate base. However, the conjugate base of the weak acetic acid is still stronger in comparison to water (i.e. it will react with water) despite compared to other bases it is still a weak base. Sorry for my misunderstanding of the knowledge!
Post by: WLalex on March 15, 2016, 09:06:35 am
Thanks
Post by: RuiAce on March 15, 2016, 10:10:57 am
Is this info wrong in saying that CH3COO- since weak acids always produce strong conjugate bases
The information is very much correct.
A weak acid (CH3COOH) yields a weak base conjugate CH3COO-
A strong acid (HCl), specifically, however has an extremely weak conjugate base (Cl-).
I would say that the information is wrong. The chemistry that we have been taught in year 12 tells us that a weak acid would yield a strong conjugate base. Hence CH3COO- is likely to be a strong conjugate base.
It actually isn't, basically.
No pun intended.
Post by: Happy Physics Land on March 15, 2016, 10:13:22 am
The information is very much correct.
A weak acid (CH3COOH) yields a weak base conjugate CH3COO-
A strong acid (HCl), specifically, however has an extremely weak conjugate base (Cl-).
It actually isn't, basically.
No pun intended.
But isnt CH3COOH a weak acid?
Post by: RuiAce on March 15, 2016, 10:14:35 am
Hi, i was wondering if you have any resources/information of the lead-acid battery. The chemistry of it was quite confusing to me and the way the teacher taught it to us was in a hurry and she didn't really explain much. We also did the silver button cell
Thanks
Note that with such cells, you don't have to listen to your teacher. You can choose yourself to do the dry cell over lead-acid, and pick another cell over the button cell. (However, admittedly the button cell, whilst not the one I did, is probably the easiest to do.)
EasyChem offers some decent notes on the cell. http://www.easychem.com.au/production-of-materials/electrochemical-methods/comparison-of-battery-cells. I didn't study this one (and my HSC was last year) so I can't offer something comprehensive - I will leave that to another user.
Take strong note that the 'chemistry' of a cell specifically refers to the reactions at the anode and cathode (and any electrolyte if applicable) - thus it's mostly the EQUATIONS.
Post by: RuiAce on March 15, 2016, 10:14:58 am
But isnt CH3COOH a weak acid?
Acetic acid is very much a weak acid, yes.
And the acetate ion is ALSO a weak base.
You need to be aware of the fact that the conjugate of only an EXTREMELY weak acid/base is a strong base/acid.
This is because the degree of ionisation of a strong acid/base is virtually 100% (at least 90% but we assume 100% in our course), thus the presence of it's conjugate is NOT going to reform into the original thing. I.e. this reaction can't happen: Cl- + H3O+ → HCl(aq) + H2O(l). Only the reverse reaction can happen.
As opposed to the acetate ion, where we do have the equilibrium happening and both substances can form due to a low degree of ionisation.
CH3COO- + H3O+ ⇌ CH3COOH(aq) + H2O(l)
Post by: katherine123 on March 15, 2016, 06:02:10 pm
Acetic acid is very much a weak acid, yes.
And the acetate ion is ALSO a weak base.
You need to be aware of the fact that the conjugate of only an EXTREMELY weak acid/base is a strong base/acid.
This is because the degree of ionisation of a strong acid/base is virtually 100% (at least 90% but we assume 100% in our course), thus the presence of it's conjugate is NOT going to reform into the original thing. I.e. this reaction can't happen: Cl- + H3O+ → HCl(aq) + H2O(l). Only the reverse reaction can happen.
As opposed to the acetate ion, where we do have the equilibrium happening and both substances can form due to a low degree of ionisation.
CH3COO- + H3O+ ⇌ CH3COOH(aq) + H2O(l)
so basically
Very strong/weak acid/base produces very weak/strong conjugate base/acid
Moderately strong/weak acid/base produce moderately weak/strong base/acid ?
Post by: Happy Physics Land on March 15, 2016, 06:41:21 pm
so basically
Very strong/weak acid/base produces very weak/strong conjugate base/acid
Moderately strong/weak acid/base produce moderately weak/strong base/acid ?
Hey Katherine!
Very strong acid/base would produce very weak conjugate base/acid. Very weak acid/base would produce very strong base/acid. With weak acids and weak bases such as CH3COOH and NH3, their conjugate bases and acids will be strong, but it is only strong when compared to water. When compared to other strong bases or acids they might still be weak. To say "Moderately strong/weak acid/base produce moderately weak/strong base/acid" is justifiable but I think its better to say that these bases and acids are weak/strong compared to water. So yes I would largely agree with your statement here. Sorry for my initial incorrect response on this by the way!
Best Regards
Happy Physics Land
Post by: RuiAce on March 15, 2016, 07:24:42 pm
so basically
Very strong/weak acid/base produces very weak/strong conjugate base/acid
Moderately strong/weak acid/base produce moderately weak/strong base/acid ?
Not exactly the best way to memorise it. Because there's no such thing as very strong or moderately strong.
There's only strong, weak and extremely weak.
Conjugate of a strong acid/base is an extremely weak base/acid.
Conjugate of a weak acid/base is a weak base/acid.
Post by: katherine123 on March 15, 2016, 10:20:06 pm
Discuss factors that must be considered when using neutralization reactions to safely minimise damage in chemical spills 4 marks
Acid can be neutralised by adding solid amphiprotic substance eg. NaHCO3. Since it is a weak acid/base it will not constitute a hazard when it is in excess.
as an acid neutralise base: HCO3- (aq)+ OH- (aq)< -> CO3(2-)(aq) + H2O (l)
as a base neutralise acid: HCO3(-)(aq) + H30+(aq) <-> H2CO3 (aq) + H2O(l)
As a powder, it provides a great surface area for quick absorption of liquid acid. Moreover, it produces effervescence in acid therefore complete neutralisation of concentrated acid can be known when fizzing stops. HCO3- + H3O + --> CO2 + 2H2O
Its disadvantages has minimal adverse impact as CO2 produced has negligible contribution to atmospheric CO2 and calcification of water pipes that is causes can be easily managed
Post by: RuiAce on March 16, 2016, 10:00:11 am
Is this okay?
Discuss factors that must be considered when using neutralization reactions to safely minimise damage in chemical spills 4 marks
Acid can be neutralised by adding solid amphiprotic substance eg. NaHCO3. Since it is a weak acid/base it will not constitute a hazard when it is in excess.
as an acid neutralise base: HCO3- (aq)+ OH- (aq)< -> CO3(2-)(aq) + H2O (l)
as a base neutralise acid: HCO3(-)(aq) + H30+(aq) <-> H2CO3 (aq) + H2O(l)
As a powder, it provides a great surface area for quick absorption of liquid acid. Moreover, it produces effervescence in acid therefore complete neutralisation of concentrated acid can be known when fizzing stops. HCO3- + H3O + --> CO2 + 2H2O
Its disadvantages has minimal adverse impact as CO2 produced has negligible contribution to atmospheric CO2 and calcification of water pipes that is causes can be easily managed
On the note of NaHCO3 being a weak acid/base, you should contrast it to the disadvantage of using a strong acid/base. One of them is yes, causing a hazard of it's own when used in excess, but the other is that the process of neutralisation is highly exothermic and it's only worse when a strong acid/base is used. This occurs due to the reaction H+ + OH- -> H2O(l) having ∆H = approx -57 kJ mol-1, which is reduced to about -42 kJ mol-1 when a weak acid/base is present.
Another suggestion when neutralising the acid, however this being more or so on larger scale spills, is to soak the acid up into sand first. Have the sand absorb the acid, and then still using NaHCO3, neutralise it at a much safer location.
Post by: katherine123 on March 16, 2016, 03:14:02 pm
explain why a solution of CH3COONa is basic, while a sodium sulfate solution of the same concentration has a pH of 7. Write ionic equations to describe any reactions
Post by: RuiAce on March 16, 2016, 06:04:53 pm
how do you answer this ques
explain why a solution of CH3COONa is basic, while a sodium sulfate solution of the same concentration has a pH of 7. Write ionic equations to describe any reactions
Preface. We know that sulfuric acid is a strong acid. It's first proton will always ionise no matter what.
H2SO4 + H2O(l) → HSO4- + H3O+
But we obviously know that sulfuric acid is diprotic. It has two protons that it's willing to donate. However, the second hydrogen atom does not always get fully ionised - it goes into equilibrium
HSO4- + H2O(l) ⇌ SO42- + H3O+
Except, under normal circumstances this equilibrium lies well to the right. The amount of sulfate ions present will always be appreciably greater than those of hydrogen sulfate. In fact, it almost ionises to completion as well.
The consequence is that whilst H2SO4 is obviously strongly acidic, HSO4- is reasonably acidic and SO42- is only a tiny bit basic.
In Na2SO4, obviously the sodium ion Na+ is neutral. But because the sulfate ion is only somewhat basic, it doesn't adjust the pH of the solution much at all. It is essentially neutral, just not exactly (which the question mistakenly implied).
By contrast, for sodium acetate, the acetate ion is definitely basic. (Obviously, it's a weak base. But it's not as weak as the sulfate ion.)
CH3COO- + H3O+ ⇌ H2Ol + CH3COOH(aq)
A simple analysis of that equation clearly shows why a solution of sodium acetate will be basic.
Post by: king_sanj on March 16, 2016, 06:49:29 pm
Post by: Happy Physics Land on March 16, 2016, 07:09:24 pm
Hey Jake! For HSC chem, in the Production of Materials module, would we need to know anything about Hydrobromous acid in terms of the Bromine water test used to distinguish between alkenes and alkanes?
Hey King_sanj:
Jake is temporarily away for a period of time, so I can answer the question here if you wouldnt mind. In Production of Materials alone, we wouldnt need to know anything about Hydrobromous acids (HBr), because when we are doing the Bromine water test we are using only Br2(aq). Because the reaction between bromine water and alkenes is an addition process, it doesnt form hydrobromous acid and hence it is not necessary to know about HBr for the module. However, what you do need to know is the final product between the alkene and the Br2(aq), because oftentimes you would be asked to provide with an equation of the reaction between the alkene and bromine water. For module 2 Acidic Environment you WOULD need to know about HBr and how it is a weak acid compared to other ones such as H2SO4 and HCl.
Best Regards
Happy Physics Land
Post by: RuiAce on March 16, 2016, 07:43:44 pm
Hey King_sanj:
Jake is temporarily away for a period of time, so I can answer the question here if you wouldnt mind. In Production of Materials alone, we wouldnt need to know anything about Hydrobromous acids (HBr), because when we are doing the Bromine water test we are using only Br2(aq). Because the reaction between bromine water and alkenes is an addition process, it doesnt form hydrobromous acid and hence it is not necessary to know about HBr for the module. However, what you do need to know is the final product between the alkene and the Br2(aq), because oftentimes you would be asked to provide with an equation of the reaction between the alkene and bromine water. For module 2 Acidic Environment you WOULD need to know about HBr and how it is a weak acid compared to other ones such as H2SO4 and HCl.
Best Regards
Happy Physics Land
I know what he's referring to.
In truth, whilst we prepare it using bromine itself, Br2, what we actually get and use for the practical is what is called bromine water - HOBr.
Observe the following equation:
Br2(l) + H2O(l) -> HOBr(aq) + HBr(aq)
Now, hydrobromic acid will not affect the experiment in any way whatsoever. It may be worth noting that it's a strong acid, but it just sits there.
We are only interested in bromine water - HOBr, which actually has the same molecular formula as hyprobromous acid (HBrO) - not hydrobromous acid. Hydrobromous acid isn't actually a thing. Technically, the experiment used to distinguish between the alkane and alkene series actually uses bromine water. This is where the colloquial name "bromine water experiment" is derived from.
For the sake of having an alkene, I will use hex-1-ene, with formula C6H12(l).
The HSC (generally) accepts your usage of bromine itself in the equation you use:
Alkene: C6H12 + Br2 -> C6H12Br2
But the ACTUAL equation of what goes on, is believe it or not, that with the bromine water
Alkene: C6H12 + HOBr -> C6H12BrOH
Realistically speaking, the second equation would be more accurate.
However, in terms of what bromine water HOBr actually is? That is beyond the scope of the syllabus. It is merely a tool for us to use in our Production of Materials topic.
Aside:
Naturally, we would use hexane as our contrast for the experiment:
Alkane: C6H14 + Br2 -(UV)-> C6H12Br2 + H2
Alkane: C6H14 + HOBr -(UV)-> C6H12BrOH + H2
Post by: qwerty222 on March 18, 2016, 04:08:44 pm
Hey qwerty222!
Jacaranda, definitely jacaranda.
It has the most detailed information on each dotpoint, a whole load of content, if you make notes out of the textbook and be familiar with them, you will be very familiarised with all your modules. The two disadvantages are its cost (60-70 dollars l remember) and the overwhelming amount of content which may bore you. If you want to obtain a fast grasp onto the chemistry knowledge I will recommend Excel chemistry textbook because it has all the information you would need for each dotpoint and present it in very succint manner. But of course it is not as extensive as jacaranda. I havent really used any other textbooks as just yet, but l do recommend these two textbooks, especially jacaranda.
Best Regards
Happy Physics Land
Thank you! :D
Post by: katherine123 on March 18, 2016, 05:46:38 pm
Post by: Happy Physics Land on March 18, 2016, 06:13:46 pm
I have asked my teacher about moderately weak/strong acids/base producing moderately conjugates and she told me that I would not come across those except the strong and weak acids/base. Is this true? She also told me to assume CH3COOH is a weak acid and produces strong conjugate in the exam.
Hey Katherine:
If you have a look at past hsc problems, they are usually pretty straight forward. In 2009 multiple choice question 7, there was a question on the conjugate base of HSO4-. In 2014 question 10 there was a question on identifying the conjugate pair. So generally there wont be a lot of questions on solely conjugate pairs, but they usually come with buffers in our systems. For me I would just prepare for everything and yes you should write that CH3COOH produces strong conjugate in the exam but say that its strength is relative to water
Post by: DrShellgon on March 18, 2016, 08:10:29 pm
Post by: sire123 on March 18, 2016, 10:00:41 pm
23a) is what Im after, cheers.
Post by: RuiAce on March 18, 2016, 10:05:03 pm
I have asked my teacher about moderately weak/strong acids/base producing moderately conjugates and she told me that I would not come across those except the strong and weak acids/base. Is this true? She also told me to assume CH3COOH is a weak acid and produces strong conjugate in the exam.If your teacher said to assume that then even if it's incorrect go with what she said. Because that is what will get you marks in the exam
Post by: RuiAce on March 18, 2016, 10:11:58 pm
Hi, how does shaking a can of soda release CO2 in relation to the equilibrium concept?
Consider the reaction CO2(g) + H2O(l) ⇌ H2CO3(aq) ∆H = -'ve
As we know (from being taught), the dissolution of carbon dioxide is indeed exothermic. That is, the more carbon dioxide dissolves, the more heat gets released.
When we are shaking a can of soda, we are providing the entire system with kinetic energy. As you would expect, since this is a closed environment some of this kinetic energy is transformed into heat energy. What is heat energy? Heat.
As we introduce heat into the system, according to Le Chatelier's principle the system will readjust it's equilibrium to counteract the disturbance in the system. In this case, as heat is released, when more heat is present the reverse reaction will be favoured. Effectively, this produces a greater concentration of CO2
Post by: RuiAce on March 18, 2016, 10:25:26 pm
Hey could anyone explain q23 of 2013 chem hsc paper?
23a) is what Im after, cheers.
Please copy and paste the actual question accompanying the reference to the HSC next time. :)
Our data sheet positions the reduction of silver at a much lower point than the reduction of lead. From this, we can infer that lead is indeed a more reactive metal than silver. Thus, we can anticipate that a metal displacement reaction will happen, where lead takes the place of silver. The equation can be determined easily (keeping in mind that the typical valence for lead is 2+) as follows:
Pb(s) + 2 AgNO3(aq) -> Pb(NO3)2(aq) + 2 Ag(s)
Initial moles of lead:
n(Pb) = m/M = 20.72/207.2 = 0.1 mol
Initial moles of silver nitrate:
n(AgNO3) = C*V = 0.1 * 1 = 0.1 mol
Observe the mole ratio:
1 mol of Pb reacts with 2 mol of AgNO3.
Obviously we don't have 2 mol of AgNO3 here, we only have 0.1. Let's fix up our mole ratio a bit then.
0.05 mol of Pb reacts with 0.1 mol of AgNO3.
This looks better.
Now, firstly observe that after 0.05 mol of Pb reacts, we still have a remaining 0.05 mol. Hence, n(Pb) = 0.0500 mol. That's another part of the question done.
(Take note that because the minimum amount of significant figures the question gives is 4, we must answer with 4 s.f.)
Then, observe immediately after that all of our moles of AgNO3 must've reacted (this is obviously not an equilibrium reaction, so everything reacts). Hence, n(AgNO3) = 0 mol
Obviously, 1 mol of Ag+ is present in 1 mol of AgNO3 as silver nitrate is only composed of one silver ion!
Thus, n(Ag+) = 0 mol
Now, the yield for 1 mol of Pb and 2 mol of AgNO3 is
1 mol of Pb(NO3)2, and 2 mol of Ag
This just means:
The yield for 0.05 mol of Pb and 0.1 mol of AgNO3 is
0.05 mol of Pb(NO3)2, and 0.1 mol of Ag
This directly tells us that the moles of Ag we have is:
n(Ag) = 0.100 mol
But observe that 1 mol of Pb(NO3)2 yields:
1 mol of Pb2+, and
2 mol of NO3-
Hence, n(Pb2+) = 0.0500 mol
and, n(NO3-) = 0.100 mol
Post by: sire123 on March 18, 2016, 10:39:40 pm
And would this be called a neutral species equation?
Post by: liiz on March 19, 2016, 01:08:01 am
I was just wondering whether I could please get some advice on how to structure and approach this question: "Assess current developments in the use of biopolymers (5 marks)". I'm not sure whether I'm meant to just refer to one biopolymer and its use or kind of just a general idea about it all and provide examples? Thanks!!
Post by: RuiAce on March 19, 2016, 06:31:27 am
Orrite ill upload the question next time as well! Thanks Rui, but are NO3- ions the spectator ions?
And would this be called a neutral species equation?
Certainly. To verify that, take note that the initial moles and final moles of the nitrate ions are both 0.100 mol.
And no. A neutral species equation is one which does not involve any changes in the charge. Note that this would actually be a redox reaction, where lead is being oxidised and silver ions are being reduced.
Post by: RuiAce on March 19, 2016, 06:33:16 am
Hello :)
I was just wondering whether I could please get some advice on how to structure and approach this question: "Assess current developments in the use of biopolymers (5 marks)". I'm not sure whether I'm meant to just refer to one biopolymer and its use or kind of just a general idea about it all and provide examples? Thanks!!
There are factors about biopolymers (e.g. renewability, possible feasible properties) that hold true for all biopolymers. However, in the scope of the chemistry course you will only be asked to make specific reference to one. It may also help to contrast to traditional petroleum-derived polymers such as the obvious polyethylene.
The two most common ones I've found in the HSC are polyhydroxybutyrate and polylactic acid
Post by: sire123 on March 19, 2016, 01:03:15 pm
A chemist analysed aspirin tablets for quality control. The initial step of the analysis was the standardisation of a NaOH solution. Three 25.00 mL samples of a 0.1034 mol L–1 solution of standardised HCl were titrated with the NaOH solution.The average volume required for neutralisation was 25.75 mL.
(a) Calculate the molarity of the NaOH solution.
...
...
...
...
Three flasks were prepared each containing a mixture of 25 mL of water and 10 mL
of ethanol. An aspirin tablet was dissolved in each flask. The aspirin in each solution
was titrated with the standardised NaOH solution according to the following equation:
C9H8O4(aq) + NaOH(aq) → C9H7O4Na(aq) + H O(l) 2
The following titration results were obtained.
Tablet Volume (mL)
1 16.60
2 16.50
3 16.55
(b) (i) Calculate the average mass (mg) of aspirin per tablet.
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...
...
...
...
...
...
(ii) Why was it necessary to include the ethanol in the mixture?
...
...
Post by: missrissole on March 19, 2016, 06:20:51 pm
Q: Why would the neutralisation of HCl and Na2CO3 require the addition of methyl orange indicator? I thought the best suited indicator would be bromothymol blue because this titration involved a strong base and a strong acid - hence a neutral salt? (or is Na2CO3 a weak base)?
Got me reeaaallll caught up. Thanks in advance! 8)
Post by: RuiAce on March 19, 2016, 06:43:24 pm
And also, I have another q:
A chemist analysed aspirin tablets for quality control. The initial step of the analysis was the standardisation of a NaOH solution. Three 25.00 mL samples of a 0.1034 mol L–1 solution of standardised HCl were titrated with the NaOH solution.The average volume required for neutralisation was 25.75 mL.
(a) Calculate the molarity of the NaOH solution.
...
...
...
...
Three flasks were prepared each containing a mixture of 25 mL of water and 10 mL
of ethanol. An aspirin tablet was dissolved in each flask. The aspirin in each solution
was titrated with the standardised NaOH solution according to the following equation:
C9H8O4(aq) + NaOH(aq) → C9H7O4Na(aq) + H O(l) 2
The following titration results were obtained.
Tablet Volume (mL)
1 16.60
2 16.50
3 16.55
(b) (i) Calculate the average mass (mg) of aspirin per tablet.
...
...
...
...
...
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(ii) Why was it necessary to include the ethanol in the mixture?
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a)
NaOH(aq) + HCl(aq) -> H2O(l) + NaCl(aq)
The reaction immediately shows that the mole ratio between sodium hydroxide and hydrochloric acid is one to one.
n(NaOH) = n(HCl)
Using n=CV
[NaOH] * V(NaOH) = [HCl] * V(HCl)
[NaOH] * 25.75mL = 25.00mL * 0.1034 mol L-1
[NaOH] = 0.100388... mol L-1
Note that for this part of the question you would round to 4 significant figures and give your answer as 1.004*10-1 mol L-1 but for the remainder of the question you use the UNROUNDED answer in your calculator
b) (i) This can obviously be done by inspection, but as the HSC requires proper calculations the average is obviously
V(NaOH) = (16.60+16.50+16.55)mL/3 = 0.01655L
Important aside: It's not that easy to copy and paste a table. In the actual HSC question, we assume that the tablet is the same, but rather human error caused the differences in the calculated volumes for NaOH that it was titrated against. So the average volume is the volume of NaOH used for titration.
Observe that the mole ratio is once again one-to-one. This means
n(C9H8O4)
= n(NaOH)
= [NaOH] * V(NaOH)
= answer from part a) * 0.01655
= 1.6614278...*10-3 mol
Using the fact that m = n*MM
m = 1.6614278...*10-3 * 180.154
= 0.2993g (correct to 4 s. f. here)
b) (ii) The question was generous here to make this a one mark question. And so they should, because they didn't give us a diagram of what aspirin looks like on the molecular level.
So the only logical answer to using ethanol in the mixture is to promote the solubility of aspirin (that's the answer). Just by looking at C9H8O4 we can make a few assumptions:
a) There's quite a fair few carbons. Expect some kind of a chain, or benzene ring (tbh, yes benzene ring in the actual molecule) which ethanol will easily dissolve.
b) The 4 oxygens could all form -OH groups for all we know but obviously we don't know. Even then, both water AND ethanol interact through hydrogen bonding and dipole-dipole interactions when hydroxyl groups are present, so you can still safely assume that ethanol increases solubility.
But basically, it's cause there's obviously going to be carbon chains, which essentially makes ethanol a more effective solvent.
Heyyyaaa there!! I have a Question that has been bugging me.. would love to get your advice / help with it! :))
Q: Why would the neutralisation of HCl and Na2CO3 require the addition of methyl orange indicator? I thought the best suited indicator would be bromothymol blue because this titration involved a strong base and a strong acid - hence a neutral salt? (or is Na2CO3 a weak base)?
Got me reeaaallll caught up. Thanks in advance! 8)
Na2CO3 is of course, a weak base. The only strong bases in the course are essentially NaOH, KOH and all the other group 1 hydroxides.
Post by: Happy Physics Land on March 19, 2016, 07:16:15 pm
Hello :)
I was just wondering whether I could please get some advice on how to structure and approach this question: "Assess current developments in the use of biopolymers (5 marks)". I'm not sure whether I'm meant to just refer to one biopolymer and its use or kind of just a general idea about it all and provide examples? Thanks!!
Hey liiz:
The way I would first approach this question is to highlight key words, its really useful because it tells you what to focus on. Assess tells us that we are making a judgment, current developments tells us we need to perhaps provide some statistics or relate the emergence of biopolymers to what is happening in the world right now. Use of biopolymers should tell us that we should provide two examples of uses of biopolymers (relate it to the polymer's properties). A lot of people would start to talk about how a specific biopolymer is made, and explain the complicated chemistry behind the production process and its properties. These descriptions however wouldnt reward you any marks because our focus is "current development" and "uses of biopolymer".
Personally, this is how I would structure this response, but of course there are many approaches that you can adopt.
1. Evaluate the current development (i.e. something like "use of biopolymers in commodities have undergone significant growth over the recent years (insert a statistic if possible))
2. Provide examples of uses of biopolymers (2 examples, include specific examples if possible, e.g. because of its rigid property, PLA has been extensively used in Danone yoghurt cups)
3. Describe the reasons in the current world which have encouraged the development of biopolymers (e.g. the depletion of the non-renewable petroleum have resulted in the rising in price of fossil fuels, making the production of biopolymers increasingly economical)
4. List 3 advantages of biopolymers in general
5. Assess the limitations of biopolymers (e.g. despite these positive aspects about biopolymers, companies are currently experiencing some difficulties with manufacturing them in large amounts and the process of production biopolymers is still not as efficient as using petrochemicals.)
6. Restate your assessment of the development of biopolymers (Judgment: There has a been a rapid and significant current development of biopolymers. I know this sounds a bit clumsy, but by writing this, the teacher knows you are achieving the "assess" part of the question)
Best Regards
Happy Physics Land
Post by: Happy Physics Land on March 19, 2016, 07:25:30 pm
Heyyyaaa there!! I have a Question that has been bugging me.. would love to get your advice / help with it! :))
Q: Why would the neutralisation of HCl and Na2CO3 require the addition of methyl orange indicator? I thought the best suited indicator would be bromothymol blue because this titration involved a strong base and a strong acid - hence a neutral salt? (or is Na2CO3 a weak base)?
Got me reeaaallll caught up. Thanks in advance! 8)
Hey missrissole!
Completely agree with what Rui said, just want to drop in my two pennies here. Na2CO3 definitely is a weak base, and like what Rui said, majority of the strong bases come from hydroxides of group 1 alkali metals and group 2 alkaline earth metals. Since HCl is a strong acid, the pH of reaction mixture at equivalence point would be below 7 (i.e. acidic). Hence methyl orange would be the best indicator here because it is suitable for use in the lower acidic range. Bromothymol blue is useful indicator for strong acid/strong base titration because the equivalence point of the reaction has a pH of 7. Bromothymol blue displays a green colour for pH range 6.0 - 7.6, hence it would be useful for evaluating neutral equivalence points.
Hope my answer helps! If you have further questions, please dont be hesitant to ask! :D
Best Regards
Happy Physics Land
Post by: DrShellgon on March 21, 2016, 09:31:04 pm
How about pure solids? Why do pure solids not affect equilibrium? E.g. what if we put in a block of pure sodium, or lithium, won't it react with a particular side of the equilibrium and cause some sort of shift?
Also thanks for your previous answer :)
Post by: RuiAce on March 21, 2016, 09:35:15 pm
Hi, I was wondering, why do pure liquids not affect equilibrium? Isn't there a possibility that the pure liquid, say it's like a pure acid, could react with the reactants or the products in an equilibrium? How about pure water? Won't this potentially dilute either the reactants or the products, causing a change in concentration and causing the equilibrium to shift?
How about pure solids? Why do pure solids not affect equilibrium? E.g. what if we put in a block of pure sodium, or lithium, won't it react with a particular side of the equilibrium and cause some sort of shift?
Also thanks for your previous answer :)
Solids don't carry much kinetic energy for them to start reacting with other things by themselves. All that you do by adding a solid is somewhat alter the surface area for the solid to react; solid molecules aren't going to bump into other liquid/gaseous/aqueous substances just like that.
Now, what exactly do you mean by pure liquids? Do you mean something such as 18M sulfuric acid (H2SO4(l))?
__________________
In terms of dilutions.
Aqueous solution substances are called that because, they are, dissolved water. All water does is reduce the concentration of the substance (out of the definition of dilution).
Post by: liiz on March 22, 2016, 06:26:30 pm
Post by: Johny1234567 on March 22, 2016, 07:01:36 pm
hello :) wondering if someone could please help explain this question to me "Explain why ethanol will dissolve in water and in pentane?" - I know it's a simple question but I'm still struggling to understand the whole polar and non-polar concept! Thanks so muchFirstly, hydrocarbons are non-polar substances. Water are polar substances. Remember the theory behind 'like dissolves like', i.e polar substances dissolve other polar substances and vice versa.
Ethanol, if you draw out the structure, has a non-polar carbon end, as well as a polar hydoxyl functional group. Because ethanol has both non-polar and polar-ends, it will dissolve in water (polar) and pentane (non-polar).
Post by: Johny1234567 on March 22, 2016, 07:05:14 pm
P.S Not entirely necessary but would prefer if someone completed the entire question so I can confirm my answers.
Thanks!
Post by: Happy Physics Land on March 22, 2016, 07:26:49 pm
hello :) wondering if someone could please help explain this question to me "Explain why ethanol will dissolve in water and in pentane?" - I know it's a simple question but I'm still struggling to understand the whole polar and non-polar concept! Thanks so much
Hey Liiz!
Sure thing! Ethanol will dissolve in water because of its hydrophilic (means water-liking) -OH group which allows it to interact with water molecules through hydrogen bonding. Ethanol will dissolve in pentane because of its non-polar hydrocarbon group which is able to interact with other non-polar substances such as pentane through dispersion forces.
This would usually be a 2 mark question and if you just write what I wrote above in the exam they are easy 2 marks to get! If you want me to expand on what I said, please dont hesitate to tell me! :)
Best Regards
Happy Physics Land
Post by: curious.egg on March 22, 2016, 08:33:03 pm
For the prac where we had to make a natural indicator, my school used purple cabbage and in the procedure, we added salt and ethanol when we were crushing it. Why was salt and ethanol added? Please help!
Post by: RuiAce on March 22, 2016, 08:48:41 pm
Attached is a question in chemical monitoring and management. I've done up to e) but am stuck on f). Could someone please help me c:I have never seen the term "reaction efficiency" coined before in the HSC. Such a question will not be asked in this course.
P.S Not entirely necessary but would prefer if someone completed the entire question so I can confirm my answers.
Thanks!
a) Hydrogen and nitrogen introduced into the system. According to the reversible equation of the production of ammonia, as we initially has a concentration of ammonia of 0 mol L-1 it's unsurprising that ammonia begins to get produced. The system then reaches equilibrium.
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
b) and c) A downward spike in the concentration of all 3 substances suggests that the volume of the system has been increased, which is equivalent to a decrease in pressure. As there are 4 moles of gas on the left and 0 on the right, LCP predicts that the equilibrium shifts to the left, which explains the gradual increase in the concentrations of nitrogen and hydrogen.
d) and e) An upward spike in only the concentration of hydrogen means that excess hydrogen was introduced into the system. Trivially, LCP predicts that the equilibrium will shift to the right to minimise this disturbance.
Post by: RuiAce on March 22, 2016, 08:50:07 pm
Hellooo!!
For the prac where we had to make a natural indicator, my school used purple cabbage and in the procedure, we added salt and ethanol when we were crushing it. Why was salt and ethanol added? Please help!
This is a highly specific question to which you will have to ask your teacher to determine what exactly were the impacts of the substances.
I performed this experiment using red cabbage and I did not require the addition of any of these reagents.
Post by: DrShellgon on March 22, 2016, 10:39:16 pm
Solids don't carry much kinetic energy for them to start reacting with other things by themselves. All that you do by adding a solid is somewhat alter the surface area for the solid to react; solid molecules aren't going to bump into other liquid/gaseous/aqueous substances just like that.
Now, what exactly do you mean by pure liquids? Do you mean something such as 18M sulfuric acid (H2SO4(l))?
__________________
In terms of dilutions.
Aqueous solution substances are called that because, they are, dissolved water. All water does is reduce the concentration of the substance (out of the definition of dilution).
Yep, by pure liquids, I meant adding reagants like the H2SO4 you mentioned, I'm just wondering, won't it react with a particular side of the equilibrium and thus cause a shift?
Also with adding just normal water to the equilibrium, am I correct to assume that since it's just water, the dilution it causes to both sides of the equilibrium is equal and thus it will overall have no effect on the equilibrium, similar to how a catalyst, when added, simply speeds up the rate of reaction and has no overall effect on the equilibrium?
Post by: Sine on March 23, 2016, 12:07:01 am
is incorrect identification of end point okay (and If you have any others)
Post by: RuiAce on March 23, 2016, 07:58:56 am
Yep, by pure liquids, I meant adding reagants like the H2SO4 you mentioned, I'm just wondering, won't it react with a particular side of the equilibrium and thus cause a shift?
Also with adding just normal water to the equilibrium, am I correct to assume that since it's just water, the dilution it causes to both sides of the equilibrium is equal and thus it will overall have no effect on the equilibrium, similar to how a catalyst, when added, simply speeds up the rate of reaction and has no overall effect on the equilibrium?
Ok I had to do a bit of research to determine a formal answer.
The whole basis of Le Chatelier's Principle and all factors of dynamic equilibrium do not strictly relate to the moles of the substance, rather, the concentrations of the substances.
Effectively, gas concentrations can be measured as they will essentially be the moles of the substance over the volume of the vessel.
An aqueous substance's concentration can also be measured as it is the moles of the substance over the volume of the entire fluid they're dissolved in.
Solids and liquids, on the other hand, cannot be defined in terms of a concentration. You can add more moles of solid or liquid to a system, but this doesn't change their concentrations. The reason for this is, what is there to change the concentration with respect to?
- For solids, just adding more of them in really doesn't do much. They'll just sit there.
- For liquids, if you add more liquid what can you take the concentration of liquid with respect to? You increase the moles of the liquid, but you don't have something to take a volume with respect of, to find a concentration.
If your school elects to take up the industrial chemistry option topic, you will also be introduced to the equilibrium constant of a system, and find that in this constant (determines the ratio of reactants to products) no solid or liquid is included either.
Post by: RuiAce on March 23, 2016, 08:04:06 am
experimental procedure errors for a back titration?Back titration specifically is not something important in the HSC course.
is incorrect identification of end point okay (and If you have any others)
Due to the nature of what it is compared to regular titration, the experimental procedure errors are similar to that of normal titration. This would include the incorrect identification of the endpoint as well as anything else (problematic method - equipment washed wrongly).
Post by: Maz on March 23, 2016, 09:41:18 pm
when you suffer from an acid stomach, the fluid in your stomach can contain hydrochloric acid at
a concentration of 2.5x10-4 mol L-1
to reduce the amount of acid in your stomach you are advised to take an antacid tablet that contains 5mg of aluminium hydroxide and 5mg of magnesium hydroxide. if at this time you have 1.5L of fluid in your stomach calculate the concentration of hydrogen ions and hydroxide ions in you stomach after you have taken the tablet.
if you took a second antacid tablet what would the hydrogen and hydroxide ion concentration be now?
i'd really appreciate the help...
thankyou :)
Post by: katherine123 on March 24, 2016, 06:45:09 pm
Le Chatalier’s principle applies in step 1 and 2 since reactions are in equilibrium, but it is not applied in step 3 since reaction goes to completion.
At step 1, low pressure and low temperature will cause reaction to shift towards the formation of NO(g). When heat is removed, reaction, being exothermic, will shift to the right to make up some of the reduced heat. When pressure is reduced, the reaction shifts right as 9 moles of gas react to form 10 moles of gas which results in increase in pressure.
The answer include “these conditions reduce the rate of reaction” which I don’t really understand
At step 2, the production of NO2(g) is favoured at low temperature and high pressure. This is because the removal of heat will cause the exothermic reaction to shift right to counteract some of the lost heat, and increase in pressure cause reaction to shift right since 3 moles of gas reacts to form 2 moles of gas which reduce some of the increased pressure.
At step 3, high pressure and high temperature will favour the formation of HNO3. The increase in pressure also increases the collision between gas molecules and water, and increase in temperature will increase the average kinetic energy of particles which leads to more successful collisions, hence reaction will shift right.
I don’t get this bit included in the answer “as this step is not an equilibrium, the highest yield and rate of formation of nitric acid will occur when reactants undergo more collisions per second. “
-Doesn’t change in pressure, temperature, concentration or volume have no effect on reactions that go to completion?
-Pressure only applies for gas molecules right?
Post by: Happy Physics Land on March 24, 2016, 07:54:52 pm
Hi can u check my 6 marks response? question is attached below
Le Chatalier’s principle applies in step 1 and 2 since reactions are in equilibrium, but it is not applied in step 3 since reaction goes to completion.
At step 1, low pressure and low temperature will cause reaction to shift towards the formation of NO(g). When heat is removed, reaction, being exothermic, will shift to the right to make up some of the reduced heat. When pressure is reduced, the reaction shifts right as 9 moles of gas react to form 10 moles of gas which results in increase in pressure.
The answer include “these conditions reduce the rate of reaction” which I don’t really understand
At step 2, the production of NO2(g) is favoured at low temperature and high pressure. This is because the removal of heat will cause the exothermic reaction to shift right to counteract some of the lost heat, and increase in pressure cause reaction to shift right since 3 moles of gas reacts to form 2 moles of gas which reduce some of the increased pressure.
At step 3, high pressure and high temperature will favour the formation of HNO3. The increase in pressure also increases the collision between gas molecules and water, and increase in temperature will increase the average kinetic energy of particles which leads to more successful collisions, hence reaction will shift right.
I don’t get this bit included in the answer “as this step is not an equilibrium, the highest yield and rate of formation of nitric acid will occur when reactants undergo more collisions per second. “
-Doesn’t change in pressure, temperature, concentration or volume have no effect on reactions that go to completion?
-Pressure only applies for gas molecules right?
Hey Katherine!
A very good answer, pretty much encompasses everything. You have provided the correct answers and correct reasoning. If I am to recommend anything, I would suggest putting in "According to Le Chatelier's principle" every time you refer to an equilibrium shift and at the end of your reasoning just put "... to minimise the disturbance". If I am to be picky, I would be careful with your final statement in step 3 where you stated "hence the reaction will shift right". Step three goes to completion, hence the reaction would always be going to the right, it would be considered incorrect to state "shift right" because this would indicate that you are considering this equation as an equilibrium reaction.
1. To answer your first concern, when you decrease your temperature (i.e. if you put the equilibrium mixture in a cold condition), the particles would be deprived of their kinetic energy and hence there will be less effective collisions. As we know from year 11 chemistry, rate of reaction is dependent upon the amount of effective collisions that take place, hence decrease in temperature would reduce rate of reaction. Less pressure (i.e. increase in volume) would provide a larger amount of space for gas molecules to flow around, hence decrease the chance of effective collision taking place (imagine yourself in a crowded party that takes place in a small space, there is a high chance you would walk into someone. Now imagine yourself in the same party with same amount of people in a much larger space, it is less likely that you would walk into someone). Hence decreased pressure would also cause reduced rate of reaction.
2. You have essentially said it in your answer without realising :D ! You have correctly state that rise in temperature and rise in pressure would both promote effective collisions and with more effective collisions occurring between reactants, more products will be formed and the rate of reaction will increase! Temperature rise and pressure certainly changes the position of equilibrium, but they apply universally to all reactions! You may like to have a look at your year 11 content on the factors that influence the rates of reactions. Think about when we havent learnt equilibrium reactions back in year 11, and how we applied those factors such as concentration, pressure and temperature to complete reactions.
3. In our HSC chemistry, we can assume that pressure only applies for gases, because it has the greatest influence on gases than any other states of matter (However in reality, pressure do have an impact upon liquids. This is what we call hydraulic pressure and is a dominant concept in mechanical and automotive industries. But in this case you dont have to worry about this too much.) Because we increased pressure, gas particles would be more concentrated in a smaller amount of space and hence it's not the water molecules that are colliding into gas particles, but it is the gas particles that are colliding into water molecules to generate the products. If you wish, you may also mention that there is some impact of pressure upon water molecules, but for the sake of HSC, you dont have to include it.
Hope my explanation was clear! If you have further questions please dont hesitate to ask! :D
Best Regards
Happy Physics Land
Post by: amandali on March 24, 2016, 08:05:42 pm
Post by: Happy Physics Land on March 24, 2016, 08:57:42 pm
Hi i dont really understand how to determine suitable indicator to use for solutions.
Hey Amanda!
If we have a look at the indicator. It is able to distinguish between strong acids and weak acids (0.1M HCl reflects a pink colour, 0.001M HCl reflects a pale yellow colour), however it is unable to differentiate between neutral substances and bases (colour observed for distilled water is the same as the colour observed for bases). This means, that any reactions that would have an equivalence point of 7 or more than 7 would be unsuitable because there wont be any colour change to indicate whether neutrality has been achieved or basicity has been achieved. Hence we can first eliminated B) which is a strong acid + strong base reaction. This is going be neutral and hence the indicator wont be suitable. We can eliminate C) because this is a weak acid + weak base reaction, which will also yield a pH around 7 (be mindful however, it is difficult to conduct titration with weak acid + weak base due to a lack of distinct colour change, so in fact no indicator would be very suitable). We can also eliminate D) because this is a weak acid + strong base reaction which would yield an equivalence point above pH 7. As mentioned before, because there is no colour change to distinguish between neutral equivalence point and basic equivalence point, therefore D would also be unsuitable.
So this leaves us with A as our answer. However, before we put that down as our answer, we must also check whether we can determine the equivalence point of A using our indicator. Clearly, there is a colour change from strong acid to weak acid. Hence we are able to determine the equivalence point around pH 3 - 6 (i.e. weak acid pH range). Since A) is a strong acid + weak base reaction, its equivalence point would be around pH 3 - 6 (weak acid pH range) and hence A is our final answer.
Great question! Hope you understood my explanation! If you are still a little confused about some of my explanations please dont hesitate to ask! :)
Best Regards
Happy Physics Land
Post by: amandali on March 24, 2016, 09:46:35 pm
Post by: katherine123 on March 25, 2016, 11:23:18 am
Is it too long and how do you allocate marks?
Question: 7 marks With reference to the underlying chemistry and with relevant equations, assess the impacts on society of two uses of ethanol
Ethanol is a renewable resource produced by fermentation of sugary crops such as sugar can and corn
C6H12O6(aq) --> 2C2H6O(aq) + 2CO2(g)
Carbon dioxide and water is released when ethanol is combusted hence it is a useful car fuel.
C2H6O(l) + O2(g)--> 2CO2(g) +3H2O(l)
Theoretically, the carbon dioxide released from combustion of ethanol balances with the CO2 absorbed during photosynthesis. Hence, ethanol is considered carbon neutral as it does not lead to increase in CO2 in the atmosphere, thus prevents global warming and has a significant positive impact on society.
However, fossil fuels are used during the farming process, distillation and distribution of ethanol. Hence, it does lead to a net increase in CO2 into the atmosphere. Moreover, production of ethanol requires the clearing of vast areas of arable land which can impact on food production, causes environmental problems like soil erosion and salinity and destructs animal habitats.
Weighing up the pros and cons, using ethanol as fuel has a negative impact on society as it does lead to increase in CO2 when the energy output of its production is taken into consideration which can elevate global warming and in addition it requires clearing of arable land which also leads to severe environmental problems.
Ethanol is also used as solvent for polar substances like water, and non-polar substances like oil and grease. It has non-polar region (-C2H3) which interacts with the non-polar substances via dispersion forces and also has a polar-region (OH-) which forms hydrogen bonds or dipole-dipole interact with polar substances.
(including a diagram of ethanol bonding with polar/non-polar sub)
Therefore, ethanol is used as a solvent in cosmetic, perfumes and paints which create a positive impact on society by making lives easier with these products being available for use.
However, ethanol is derived from petroleum involving the process of cracking and dehydration. The mining of petroleum is environmentally damaging as it can cause problems like contamination of soil, erosion and oil leaks. Moreover, ethanol is volatile and flammable thus explosion may occur when it comes into contact with heat or flames if it is not stored properly which pose a severe risk to society.
Weighing up the pros and cons, ethanol is not good to be used as solvents as it poses a great risk to society due to its flammability which outweighs its positive impact of making lives easier.
Post by: katherine123 on March 25, 2016, 11:25:12 am
Hey Katherine!
A very good answer, pretty much encompasses everything. You have provided the correct answers and correct reasoning. If I am to recommend anything, I would suggest putting in "According to Le Chatelier's principle" every time you refer to an equilibrium shift and at the end of your reasoning just put "... to minimise the disturbance". If I am to be picky, I would be careful with your final statement in step 3 where you stated "hence the reaction will shift right". Step three goes to completion, hence the reaction would always be going to the right, it would be considered incorrect to state "shift right" because this would indicate that you are considering this equation as an equilibrium reaction.
1. To answer your first concern, when you decrease your temperature (i.e. if you put the equilibrium mixture in a cold condition), the particles would be deprived of their kinetic energy and hence there will be less effective collisions. As we know from year 11 chemistry, rate of reaction is dependent upon the amount of effective collisions that take place, hence decrease in temperature would reduce rate of reaction. Less pressure (i.e. increase in volume) would provide a larger amount of space for gas molecules to flow around, hence decrease the chance of effective collision taking place (imagine yourself in a crowded party that takes place in a small space, there is a high chance you would walk into someone. Now imagine yourself in the same party with same amount of people in a much larger space, it is less likely that you would walk into someone). Hence decreased pressure would also cause reduced rate of reaction.
2. You have essentially said it in your answer without realising :D ! You have correctly state that rise in temperature and rise in pressure would both promote effective collisions and with more effective collisions occurring between reactants, more products will be formed and the rate of reaction will increase! Temperature rise and pressure certainly changes the position of equilibrium, but they apply universally to all reactions! You may like to have a look at your year 11 content on the factors that influence the rates of reactions. Think about when we havent learnt equilibrium reactions back in year 11, and how we applied those factors such as concentration, pressure and temperature to complete reactions.
3. In our HSC chemistry, we can assume that pressure only applies for gases, because it has the greatest influence on gases than any other states of matter (However in reality, pressure do have an impact upon liquids. This is what we call hydraulic pressure and is a dominant concept in mechanical and automotive industries. But in this case you dont have to worry about this too much.) Because we increased pressure, gas particles would be more concentrated in a smaller amount of space and hence it's not the water molecules that are colliding into gas particles, but it is the gas particles that are colliding into water molecules to generate the products. If you wish, you may also mention that there is some impact of pressure upon water molecules, but for the sake of HSC, you dont have to include it.
Hope my explanation was clear! If you have further questions please dont hesitate to ask! :D
Best Regards
Happy Physics Land
Does yield apply for reactions in equilibrium If not, why?
Post by: Happy Physics Land on March 25, 2016, 07:02:15 pm
Whats the difference between PHB and PHB-V? Do i need to know how to draw the structure of these polymers ?
Hey Amanda!
Sorry for the late reply! PHB and PHBV are both common biopolymers that possess contrasting physical properties! PHB is otherwise known as 3-hydroxybutyrate, which is stiff and brittle. Because of these properties, PHB have limited usage in commodities and hence we dont usually discuss about PHB. In contrast, PHBV, or biopol, is stronger and more flexible. Because of these desirable physical properties, PHBV have widespread use in medical implants (prosthetic devices) , specialty packaging and tissue repairing. The structure of these polymers are beyond the scope of chemistry syllabus and looking at past questions they have never asked for the structure of these, so no you wouldnt need to, but I guess you can look up the structure if you are interested and just in case they do ask you in an exam. What you definitely do need to know is the specific name of the bacteria that is involved in the production of PHB and PHBV, in this case the bacteria is called Cupriavidus Necator.
Good luck studying! :)
Best Regards
Happy Physics Land
Post by: Happy Physics Land on March 25, 2016, 07:08:47 pm
Does yield apply for reactions in equilibrium If not, why?
Hey Katherine!
Yes yield definitely do apply for reactions in equilibrium (as stated by le chatelier's principle, shifting of equilibrium to a certain side will favour the yield of that side), and it also applies to complete reactions as well! With the changes in temperature, changes in concentration and change in temperature, both equilibrium reactions and complete reactions will have different yields of products. You can refer to my previous answer for explanations on this! :)
Best Regards
Happy Physics Land
Post by: Happy Physics Land on March 25, 2016, 07:37:51 pm
Hi can u check my response
Is it too long and how do you allocate marks?
Question: 7 marks With reference to the underlying chemistry and with relevant equations, assess the impacts on society of two uses of ethanol
Ethanol is a renewable resource produced by fermentation of sugary crops such as sugar can and corn
C6H12O6(aq) --> 2C2H6O(aq) + 2CO2(g)
Carbon dioxide and water is released when ethanol is combusted hence it is a useful car fuel.
C2H6O(l) + O2(g)--> 2CO2(g) +3H2O(l)
Theoretically, the carbon dioxide released from combustion of ethanol balances with the CO2 absorbed during photosynthesis. Hence, ethanol is considered carbon neutral as it does not lead to increase in CO2 in the atmosphere, thus prevents global warming and has a significant positive impact on society.
However, fossil fuels are used during the farming process, distillation and distribution of ethanol. Hence, it does lead to a net increase in CO2 into the atmosphere. Moreover, production of ethanol requires the clearing of vast areas of arable land which can impact on food production, causes environmental problems like soil erosion and salinity and destructs animal habitats.
Weighing up the pros and cons, using ethanol as fuel has a negative impact on society as it does lead to increase in CO2 when the energy output of its production is taken into consideration which can elevate global warming and in addition it requires clearing of arable land which also leads to severe environmental problems.
Ethanol is also used as solvent for polar substances like water, and non-polar substances like oil and grease. It has non-polar region (-C2H3) which interacts with the non-polar substances via dispersion forces and also has a polar-region (OH-) which forms hydrogen bonds or dipole-dipole interact with polar substances.
(including a diagram of ethanol bonding with polar/non-polar sub)
Therefore, ethanol is used as a solvent in cosmetic, perfumes and paints which create a positive impact on society by making lives easier with these products being available for use.
However, ethanol is derived from petroleum involving the process of cracking and dehydration. The mining of petroleum is environmentally damaging as it can cause problems like contamination of soil, erosion and oil leaks. Moreover, ethanol is volatile and flammable thus explosion may occur when it comes into contact with heat or flames if it is not stored properly which pose a severe risk to society.
Weighing up the pros and cons, ethanol is not good to be used as solvents as it poses a great risk to society due to its flammability which outweighs its positive impact of making lives easier.
Hey Katherine!
Ok so let's start off breaking down the question and analysing the mark distribution:
Straight away, 2 marks for providing two equations for the two uses. (However, if your content isnt satisfactory, teacher may only award one mark for the equations because the equations are meant to be there to support your explanations. If your explanation isnt too desirable, it deprives the meaning of having an equation included).
1 mark for assessing (i.e. making a judgment)
2 marks for identifying the use of ethanol and explaining its impact on society (you may not be awarded any mark for just simply identifying the use, nor would you be awarded any mark for explaining impacts upon environment, but something you can relate to is that CO2 continuously contribute to greenhouse effect, and this will making the Earth environment inhospitable for human society.)
2 marks for identifying a second use of ethanol and explaining its impact on society.
What I do like about your response, is your frequent use of cause and effect language and your ability to make several judgments. These are evidences of you clearly understanding how to answer an explain/assess question! Well done! You have also included several social impacts and have also explained why these impacts are positive/negative. Another positive aspect of your response is the fact that you have provided the chemistry principles behind these uses, such as your successful identification of the -OH hydroxyl group and the hydrocarbon group. This answers the part "using underlying chemistry".
A fatal error (please dont be scared off by this harsh tone :) its still an articulate response!) in your writing is that you are overly focusing on environmental impacts. This is a question asking about social impacts, as soon as you mention the word environment, you are off-topic. If this question is on "society and environmental impacts", I would give 7/7 for your response, beyond all doubts. Even if you want to include environmental impacts, you will still need to ultimately relate it back to effect upon society, just like what you have done with the global warming concept. When you talked about arable land, I thought you would follow up with the idea "the use of land and agricultural products for the production of ethanol would create competition with human consumption" (this is a good idea for social impact if you wanna put it in). But instead you made a great point on environmental impact, but again it's not relevant to the question.
A small technicality error is your chemical formula for ethanol. You wrote C2H6O, but the more correct way of expressing it is C2H5OH. Sure, these two formulae have exactly the same number of hydrogen atoms, but we must not ignore the significance of the -OH hydroxyl group in ethanol, because that is what allows ethanol to dissolve in water! By writing C2H6O, you are ignoring the significance of -OH and it leaves a chance for the teacher to deduct you a mark there!
Overall, I would personally give a 5/7. Your teacher may give a 4-5/7. So essentially your response is a brand 5 range response, which is definitely good at this stage. If you apply my feedbacks, this would definitely be a band 6 response! :) Well done overall!
Best Regards
Happy Physics Land
Post by: atarz on March 25, 2016, 11:27:56 pm
Post by: RuiAce on March 25, 2016, 11:51:40 pm
I'm very confused... I have done two practice half-yearly tests from James Ruse. One said to use a stopper in the production of an ester while another said not to. Also I have seen esterification using thermometers. What should be used in esterification and why?
The whole point of the condenser is to not only allow for high temperatures, but ALSO to avoid a buildup of pressure within the system. Otherwise, the system will explode. You will have glass everywhere.
The stopper, is a beautiful way to PROMOTE this buildup of pressure. Therefore it is very very very VERY bad for the process of reflux.
Now, perhaps a bit more importantly, if you look carefully at your diagram that is not a stopper. That is just the top of the condenser. I think you just got the diagram confused.
Note: The thermometer is optional. I've never refluxed with one but it's probably advantageous if you want your system to be heated to a specific temperature for reflux. It may promote safety, but I don't believe it's necessary.
Post by: atarz on March 26, 2016, 10:09:06 am
The whole point of the condenser is to not only allow for high temperatures, but ALSO to avoid a buildup of pressure within the system. Otherwise, the system will explode. You will have glass everywhere.
The stopper, is a beautiful way to PROMOTE this buildup of pressure. Therefore it is very very very VERY bad for the process of reflux.
Now, perhaps a bit more importantly, if you look carefully at your diagram that is not a stopper. That is just the top of the condenser. I think you just got the diagram confused.
Note: The thermometer is optional. I've never refluxed with one but it's probably advantageous if you want your system to be heated to a specific temperature for reflux. It may promote safety, but I don't believe it's necessary.
Post by: amandali on March 26, 2016, 04:38:27 pm
b) acetic acid = weak, monoprotic
c)sulfuric acid=diprotic , strong
d) HCl = monoprotic strong
ive managed to rule out HCl and acetic acid but im not sure why its citric
isnt citric really weak so its going to partially ionise producing much more less H+ than H2SO4
Post by: Happy Physics Land on March 26, 2016, 05:38:41 pm
a) citric acid = weak, triprotic
b) acetic acid = weak, monoprotic
c)sulfuric acid=diprotic , strong
d) HCl = monoprotic strong
ive managed to rule out HCl and acetic acid but im not sure why its citric
isnt citric really weak so its going to partially ionise producing much more less H+ than H2SO4
Hey Amanda!
Well citric acid isnt REALLY weak, like l mean its still weak but its definitely not the weakest acid you will ever see. But then thats not the point in this question! Here, the amount of base required will depend on the stoichiometry of the reaction (i.e. the number of ionisable protons in the acid). Since citric acid has the most protons per mole (because its triprotic), it would require the most amount of base to change the colour of indicator.
But yes I do understand where your reasoning came from. Keep in mind however this question is about a neutralisation reaction, not so much about whether an acid is weak or strong. If we compare the balanced chemical equation for citric acid + NaOH (C6H8O7(aq) + NaOH(aq) --> NaC6H5O7(aq) + 3H2O(l)) and the balanced chemical equation for sulfuric acid + NaOH (H2SO4(aq) + 2NaOH(aq) --> Na2SO4(aq) + 2H2O(l)), we can see that 3 moles of NaOH is required to neutralise one mole of citric acid, and 2 moles of NaOH are required to neutralise one mole of citric acid. So I think this explains why the answer is citric acid not sulfuric acid more clearly for you.
If you still find it a bit hard to understand, please let me know! :)
Best Regards
Happy Physics Land
Post by: RuiAce on March 27, 2016, 01:44:56 am
a) citric acid = weak, triprotic
b) acetic acid = weak, monoprotic
c)sulfuric acid=diprotic , strong
d) HCl = monoprotic strong
ive managed to rule out HCl and acetic acid but im not sure why its citric
isnt citric really weak so its going to partially ionise producing much more less H+ than H2SO4
Precisely what HPL was trying to say.
Because we're titrating with a strong base, the reaction goes to completion REGARDLESS of the strength of the acid.
So since the acids are both equal in volume AND equal in concentration, the triprotic one will take more NaOH to effectively be neutralised.
Post by: amandali on March 27, 2016, 09:25:17 am
- Is qualitative an indirect evidence and quantitative a direct evidence
- Am i allowed to omit the spectator ions for carbonate, iron reaction (below)
- is it enough to know the effects of acid rain as the only qualitative evidence
-For "discuss" questions i dont need a concluding statement right ?
There has been an increase in the concentration of the oxides of nitrogen and sulfur in the atmosphere. Assess both the evidence to support this statement and the need to monitor these oxides 8 marks
Oxides of sulfur has increased in concentration from burning sulfide ores to extract metals CuS(s) + O2(g) -->Cu(s) + SO2(g) which releases SO2(g)
Whereas, acidic oxides of Nitrogen has increased mainly due to internal combustion engines and coal burning power stations.
N2(g) + O2(g) -->2NO(g)
2NO(g) + O2(g) -->2NO2(g)
These oxides the react with the water in the atmosphere:
SO2 (g) + H2O(l) -->H2SO3(aq)
2NO2(g) + H2O (l)-->HNO2(aq) + HNO3 (aq)
The evidence for the increase in these oxides comes from both direct of measurements of gas concentrations in the atmosphere and indirectly through effect of the increase in acid rain due to these oxides. The quantitative measurements are accurate and reliable due to the development of technologies in recent time. For eg. levels of SO2 and NO2 gases measured by analysing the gases trapped in bubbles of Antarctic ice which is a sample of the atmosphere from the past 200 years.
The indirect evidence comes from the effects of acid rain formed by these oxides. The acids H2SO3(aq) and HNO2(aq)/HNO3(aq) react and dissolves carbonates, which destructs historically important buildings and statues made of limestone and marble thus this becomes a record for the increase in levels of these oxides. 2H+(aq)+CaCO3(s)-->CO2(g) + H2O(l)+ Ca2+(aq) . Steel bridges and other steel structures also show evidence of damage due to acid rain since it reacts iron and dissolves it. 2H+(aq)+ Fe(s)-->H2(g)+Fe2+(aq). However, these evidences are less reliable as they could be resulted due to other oxides or factors.
It is vital to record the monitor and record increasing levels of pollution so that we will know when to act and reduce the levels of NOx and SO2 as they are respiratory irritants therefore can cause asthma and other respiratory diseases and thus have a devastating impact on population’s health. They also lead to formation of acid rain which will cause cuticle on leaves to strip off leading to degradation of forest.
Overall, the direct and indirect evidence both shows the increase in concentration of these oxides, however, they are not completely reliable as the concentration of oxides changes over time and they are not representative of all parts of the world.
Post by: Happy Physics Land on March 27, 2016, 01:05:28 pm
Hi can u check my response?
- Is qualitative an indirect evidence and quantitative a direct evidence
- Am i allowed to omit the spectator ions for carbonate, iron reaction (below)
- is it enough to know the effects of acid rain as the only qualitative evidence
-For "discuss" questions i dont need a concluding statement right ?
There has been an increase in the concentration of the oxides of nitrogen and sulfur in the atmosphere. Assess both the evidence to support this statement and the need to monitor these oxides 8 marks
Oxides of sulfur has increased in concentration from burning sulfide ores to extract metals CuS(s) + O2(g) -->Cu(s) + SO2(g) which releases SO2(g)
Whereas, acidic oxides of Nitrogen has increased mainly due to internal combustion engines and coal burning power stations.
N2(g) + O2(g) -->2NO(g)
2NO(g) + O2(g) -->2NO2(g)
These oxides the react with the water in the atmosphere:
SO2 (g) + H2O(l) -->H2SO3(aq)
2NO2(g) + H2O (l)-->HNO2(aq) + HNO3 (aq)
The evidence for the increase in these oxides comes from both direct of measurements of gas concentrations in the atmosphere and indirectly through effect of the increase in acid rain due to these oxides. The quantitative measurements are accurate and reliable due to the development of technologies in recent time. For eg. levels of SO2 and NO2 gases measured by analysing the gases trapped in bubbles of Antarctic ice which is a sample of the atmosphere from the past 200 years.
The indirect evidence comes from the effects of acid rain formed by these oxides. The acids H2SO3(aq) and HNO2(aq)/HNO3(aq) react and dissolves carbonates, which destructs historically important buildings and statues made of limestone and marble thus this becomes a record for the increase in levels of these oxides. 2H+(aq)+CaCO3(s)-->CO2(g) + H2O(l)+ Ca2+(aq) . Steel bridges and other steel structures also show evidence of damage due to acid rain since it reacts iron and dissolves it. 2H+(aq)+ Fe(s)-->H2(g)+Fe2+(aq). However, these evidences are less reliable as they could be resulted due to other oxides or factors.
It is vital to record the monitor and record increasing levels of pollution so that we will know when to act and reduce the levels of NOx and SO2 as they are respiratory irritants therefore can cause asthma and other respiratory diseases and thus have a devastating impact on population’s health. They also lead to formation of acid rain which will cause cuticle on leaves to strip off leading to degradation of forest.
Overall, the direct and indirect evidence both shows the increase in concentration of these oxides, however, they are not completely reliable as the concentration of oxides changes over time and they are not representative of all parts of the world.
Hey Amanda!
I will just start off answering your questions first:
1. Both qualitative and quantitative are direct evidences (not necessarily first-hand evidences though!). Qualitative requires you to describe this evidence. So for example in this case if you mentioned photochemical smog, you would need to account for this evidence and explain how this evidence affects monitoring oxide levels in the future. Quantitative evidence requires numbers, either from data that the question perhaps gives you in an exam or from data you've already known beforehand. Just making up some statistics here, for example you can say about 100000 tonnes of nitrogen oxides are contributed by internal combustion engines or 5000 tonnes of nitrogen oxides are measured to be released everyday or something like that.
2. With extended response questions like this, unless they ask you for a net ionic equation, you only need to include a balanced chemical equation. Of course, it is up to you whether you would want to delete spectator ions or not! For me, I usually wouldnt unless the spectator ions really causes confusion.
3. No definitely not! Know at least 3 evidences, because questions like "assess the evidence for nitrogen oxides/sulfur oxides" are extremely common in exams. You would need around 3-4 evidences and account for those evidences and what they suggest about what we should do in the future to produce less of those oxides. Some evidences are NOAA flask and gas chromatograph measurements, increase in coal burning power plants and photochemical smog. There! You already have 3 more evidences besides acid rain!
4. Personally I do, but it doesnt make such a big impact upon your final mark for the response if you dont include a concluding statement. You've gotta present both sides (i.e. positive and negative) of the question and then have one short last sentence that expresses your opinion (e.g. the benefits of _______ outweighs the disadvantages of ________ and hence _________ definitely benefits the society more).
Ok so I've had a look at your response now, and I will just point out some of the good stuff and some of those not-so-good stuff that you can improve on. But look, overall I reckon this would be a response somewhere between band 5 - band 6, and your teacher might choose to give you a high band 5. You have fluently illustrated what the question asks you to do and demonstrated a good understanding of those evidences and their impacts. You have in your establishing sentences accounted for the occurrence of your acidic rain and stated briefly the reason for concentrations of these oxides in atmosphere (to be brief with these things is good because they dont directly answer the question but you are providing examiners with backgrounds). I must also applaud you on your assessment of the evidences which is the question! Many candidates would forget or include very little judgments in their responses. What I often tell people to do is in the end just explicitly state the word judgment and state what your judgment is (i.e. "Judgment: __________________________"). It sounds a bit clumsy but it tells the teacher you are answering the assess part of the question!
Ok now lets move onto the defects of your response. The biggest defect in your response is that your evidences are limited. You have mentioned two evidences: gas measurement and a whole tonne of info on acidic rain. This imbalance between the amount of explanation you put on gas measurement and the amount of explanation you put on acidic rain makes the examiner feel like "okay so this girl really knows her acidic rain stuff but she's just stating this gas measurement for the sake of stating it". I would recommend to either take out some information about acid rains and put more information into gas measurement. Or you can just simply add more information in gas measurement. To make your response stronger, you should really include another evidence such as photochemical smog.
Another defect is about your monitoring part of the question. You have used cause and effect language to explain the need for monitoring these acidic oxides but like what I said before, you are focusing a little too much on the evidence part and you are not balancing out your monitoring part. You can add one more point about the necessity to monitor these oxides (for example Nitrogen oxide's greenhouse effect is 100 times stronger than carbon dioxides) and you can also relate photochemical smog to your statement of acidic oxides being respiratory irritants.
Ok so out of 8 I would give a 6-7/8, and your teacher would most likely give you a mark like that as well (definitely not anything below 6). Overall its a very good piece of writing, well done! :D
Best Regards
Happy Physics Land
Post by: katherine123 on March 27, 2016, 01:32:15 pm
Is initiator and catalyst the same (organic peroxide for the production of LDPE)?
Analyse why ethylene is such as important starting material for the chemical industry, in your answer, include relevant chemical equations, and a description of new materials and fuels that can be prepared from ethylene. 7 marks
Ethene contains a reactive double bond which enables it to react readily with many substances forming polyethylene which is a starting material for plastics, a useful product which is widely used to make lives easier
[CH2CH2]n --> -[CH2-CH2-]-
The 2 types of polyethylene include LDPE and HDPE. HDPE is produced at a low gas pressure and temperature of 60 degrees with a Ziegler Natta catalyst. The catalyst attaches to the monomers which enable monomers to link and form a long chain. Once the activated chains collide, the chain growth stops and polyethylene is formed with an unbranched, crystalline structure which makes it rigid and brittle. Thus, it is used to make garbage bins, toys and garbage bags. LDPE is also produced in a similar way but under high gas pressure and temperature of 300 degrees with organic peroxide initiator. The chains formed are branched thus cannot pack closely together like HDPE which makes it soft and flexible. Hence, it is used to make glab wraps and plastic squeeze bottles.
It can also be converted to ethanol by heating it with steam at 300 degrees and high pressure using phosphoric catalyst as the catalyst. CH2CH2(g) + H20(l) --> CH3CH2OH(aq). Ethanol is a useful solvent for perfume, cosmetics, and paints. Although the ethanol that is currently used as a fuel blend originates from fermentation of sugary crops, ethanol may be sourced from ethene in the future if a more economic method for its production is developed.
Post by: amandali on March 27, 2016, 04:41:17 pm
Hey Amanda!
I will just start off answering your questions first:
1. Both qualitative and quantitative are direct evidences (not necessarily first-hand evidences though!). Qualitative requires you to describe this evidence. So for example in this case if you mentioned photochemical smog, you would need to account for this evidence and explain how this evidence affects monitoring oxide levels in the future. Quantitative evidence requires numbers, either from data that the question perhaps gives you in an exam or from data you've already known beforehand. Just making up some statistics here, for example you can say about 100000 tonnes of nitrogen oxides are contributed by internal combustion engines or 5000 tonnes of nitrogen oxides are measured to be released everyday or something like that.
2. With extended response questions like this, unless they ask you for a net ionic equation, you only need to include a balanced chemical equation. Of course, it is up to you whether you would want to delete spectator ions or not! For me, I usually wouldnt unless the spectator ions really causes confusion.
3. No definitely not! Know at least 3 evidences, because questions like "assess the evidence for nitrogen oxides/sulfur oxides" are extremely common in exams. You would need around 3-4 evidences and account for those evidences and what they suggest about what we should do in the future to produce less of those oxides. Some evidences are NOAA flask and gas chromatograph measurements, increase in coal burning power plants and photochemical smog. There! You already have 3 more evidences besides acid rain!
4. Personally I do, but it doesnt make such a big impact upon your final mark for the response if you dont include a concluding statement. You've gotta present both sides (i.e. positive and negative) of the question and then have one short last sentence that expresses your opinion (e.g. the benefits of _______ outweighs the disadvantages of ________ and hence _________ definitely benefits the society more).
Ok so I've had a look at your response now, and I will just point out some of the good stuff and some of those not-so-good stuff that you can improve on. But look, overall I reckon this would be a response somewhere between band 5 - band 6, and your teacher might choose to give you a high band 5. You have fluently illustrated what the question asks you to do and demonstrated a good understanding of those evidences and their impacts. You have in your establishing sentences accounted for the occurrence of your acidic rain and stated briefly the reason for concentrations of these oxides in atmosphere (to be brief with these things is good because they dont directly answer the question but you are providing examiners with backgrounds). I must also applaud you on your assessment of the evidences which is the question! Many candidates would forget or include very little judgments in their responses. What I often tell people to do is in the end just explicitly state the word judgment and state what your judgment is (i.e. "Judgment: __________________________"). It sounds a bit clumsy but it tells the teacher you are answering the assess part of the question!
Ok now lets move onto the defects of your response. The biggest defect in your response is that your evidences are limited. You have mentioned two evidences: gas measurement and a whole tonne of info on acidic rain. This imbalance between the amount of explanation you put on gas measurement and the amount of explanation you put on acidic rain makes the examiner feel like "okay so this girl really knows her acidic rain stuff but she's just stating this gas measurement for the sake of stating it". I would recommend to either take out some information about acid rains and put more information into gas measurement. Or you can just simply add more information in gas measurement. To make your response stronger, you should really include another evidence such as photochemical smog.
Another defect is about your monitoring part of the question. You have used cause and effect language to explain the need for monitoring these acidic oxides but like what I said before, you are focusing a little too much on the evidence part and you are not balancing out your monitoring part. You can add one more point about the necessity to monitor these oxides (for example Nitrogen oxide's greenhouse effect is 100 times stronger than carbon dioxides) and you can also relate photochemical smog to your statement of acidic oxides being respiratory irritants.
Ok so out of 8 I would give a 6-7/8, and your teacher would most likely give you a mark like that as well (definitely not anything below 6). Overall its a very good piece of writing, well done! :D
Best Regards
Happy Physics Land
Is it sufficient if i add this to the gas measurement
Moreover, direct measurements are made by statutory bodies such as NSW EPA and scientists researching the atmosphere. NOx were found to be part of pollution generated over large cities due to use of internal combustion engines since industrial revolution
NOx produces photochemical smog after reacting in the presence of sunlight which is a toxic pollutant that causes respiratory diseases
Post by: jakesilove on March 27, 2016, 05:14:21 pm
Please check my response thanks
Is initiator and catalyst the same (organic peroxide for the production of LDPE)?
Analyse why ethylene is such as important starting material for the chemical industry, in your answer, include relevant chemical equations, and a description of new materials and fuels that can be prepared from ethylene. 7 marks
Ethene contains a reactive double bond which enables it to react readily with many substances forming polyethylene which is a starting material for plastics, a useful product which is widely used to make lives easier
[CH2CH2]n --> -[CH2-CH2-]-
The 2 types of polyethylene include LDPE and HDPE. HDPE is produced at a low gas pressure and temperature of 60 degrees with a Ziegler Natta catalyst. The catalyst attaches to the monomers which enable monomers to link and form a long chain. Once the activated chains collide, the chain growth stops and polyethylene is formed with an unbranched, crystalline structure which makes it rigid and brittle. Thus, it is used to make garbage bins, toys and garbage bags. LDPE is also produced in a similar way but under high gas pressure and temperature of 300 degrees with organic peroxide initiator. The chains formed are branched thus cannot pack closely together like HDPE which makes it soft and flexible. Hence, it is used to make glab wraps and plastic squeeze bottles.
It can also be converted to ethanol by heating it with steam at 300 degrees and high pressure using phosphoric catalyst as the catalyst. CH2CH2(g) + H20(l) --> CH3CH2OH(aq). Ethanol is a useful solvent for perfume, cosmetics, and paints. Although the ethanol that is currently used as a fuel blend originates from fermentation of sugary crops, ethanol may be sourced from ethene in the future if a more economic method for its production is developed.
Hey!
Yes, initiator and catalyst can be thought of as the same thing.
I like your answer; I think it goes into sufficient depth for most areas that you discuss. However, I would add something small about the PRODUCTION of ethene: Part of it's significance in the chemical industry is the ease by which it can be produced, particularly through catalytic cracking of fuel fractions. This is a really important reason as to why we love Ethene: not only is it great (as you've described) because of its reactivity, but also because of its potential abundance! Other than that, and perhaps rewording some of your sentences (read it outloud, and see if anything seems a bit rambly) I think you have covered lots of vital areas in answering the question.
Jake
Post by: Happy Physics Land on March 27, 2016, 06:36:15 pm
Is it sufficient if i add this to the gas measurement
Moreover, direct measurements are made by statutory bodies such as NSW EPA and scientists researching the atmosphere. NOx were found to be part of pollution generated over large cities due to use of internal combustion engines since industrial revolution
NOx produces photochemical smog after reacting in the presence of sunlight which is a toxic pollutant that causes respiratory diseases
Hey Amanda!
Yes if you add these your response would sound way stronger! Because now you are accounting for your evidences more and proposing a bit more new ideas! Well done! To get a full 8/8 I reckon you can just add in a few examples of how to monitor those oxides (e.g. look up if theres a legislation regarding the release of NOx or SOx pollutants, and then you can say "because of these evidences which demonstrate the negative consequences of these oxides, hence these emissions must be monitored. For example, the government body has participated in this monitoring process through this legislation _________, " This just makes the teacher realise that you have a great insight into the question and displays understandings towards those monitoring actions.
Im gonna use an analogy here (hehe :) no hates ). For me, I reckon originally your response was like a finely-made cake with icings sprinkled everywhere. Now after you apply the feedback and added in those stuff your response is like a finely-made cake that is beautifully decorated with a layer of evenly distributed icing. So now what you have to do is to just sprinkle some rainbow coloured sugar thingy on top!
Best Regards
Happy Physics Land
Post by: grace_joseph on March 27, 2016, 06:39:21 pm
Would you please be able to explain why this is the answer to the 2011 HSC paper q25 (attachment 1 and 2)? I don't really know why you need two equations, or even where the equations came from. I understand buffers but have never seen them proved using water before
Also, would you mind explaining question 15 of this paper to me when you could? (attachment 3) The answer is A :)
Thanks very much Jake :)
Post by: jakesilove on March 27, 2016, 07:07:10 pm
Hi Jake,
Would you please be able to explain why this is the answer to the 2011 HSC paper q25 (attachment 1 and 2)? I don't really know why you need two equations, or even where the equations came from. I understand buffers but have never seen them proved using water before
Also, would you mind explaining question 15 of this paper to me when you could? (attachment 3) The answer is A :)
Thanks very much Jake :)
Hey Grace!
Great question, as it really does seem like the equations were pulled out of nowhere! However, when you consider what a buffer is actually MEANT to do, the equations start to make sense!
A buffer is something that resists small changes in pH, which is vitally important for things like living organisms and cells. All the equations display are the acidic and basic nature of the buffer: in the first equation, you can clearly is the substance is a proton donor, whilst in the second you can clearly see that the substance is a proton acceptor. As such, the system can be both acidic and basic, and therefore resist small changes in pH! I hope that this makes sense: if you're ever unsure as to how to write out buffer equations, just add water to each part of the buffer and show the acidic and basic properties of the buffer!
As for Question 15, I don't think I can really answer it without the rest of the question. However, I assume that there are equal moles of strong and weak acid? In that case, remember that the important part of neutralisation reactions is the number of MOLES of the acid, as the number of moles of the base will need to equal that (in whatever ratio the chemical equation states) in order for neutralization to occur. For instance, a monoprotic WEAK acid of 0.1mol/L will require the same amount of NaOH to neutralise it as a monoprotic STRONG acid of 0.1mol/L, as the concentration is the important part in this reaction.
I hope this helps! Feel free to ask more questions so I can help clarify your understanding :)
Jake
Post by: grace_joseph on March 27, 2016, 08:02:29 pm
And sorry I forgot to add the first part of question 15! It's attached now but I still don't understand why a weak acid would require the same amount of base as a strong one, as when we do titrations I thought it was established that one drop of NaOH may overpower acetic acid but not have any effect on HCl?
Sorry! I really appreciate any help you can give, even if I still don't get it :D
Grace
Post by: amandali on March 27, 2016, 09:07:58 pm
how do these 2 definitions describe neutralization differently
Water and ethanol are both used as solvents. Explain the diff and sim. in their solvent behaviour in terms of their molecular structures. Include a diagram 4 marks
I wrote my answer in a table (similarities+differences)
Is there anything else i can add to differences or is that sufficient
Similarities
• Both water and ethanol are polar molecules
• Both consists of polar hydroxyl group (-OH) which can attract and dissolve polar substances eg. HCl through dipole-dipole attraction, hydrogen bonding or dispersion forces.
differences
• Ethanol has a non-polar region ie ethyl group (-CH2CH3) whereas water does not thus only ethanol can dissolve (some) non-polar substances eg. Hexane via dispersion forces. Water does not have non-polar regions therefore is a poor solvent for non-polar substances
Including a diagram of ethanol attract both polar and non-polar sub. and water attracting polar sub.
Post by: jakesilove on March 27, 2016, 10:36:47 pm
Hi Jake! Thank you very much for the buffer answer, yes I get it now :)
And sorry I forgot to add the first part of question 15! It's attached now but I still don't understand why a weak acid would require the same amount of base as a strong one, as when we do titrations I thought it was established that one drop of NaOH may overpower acetic acid but not have any effect on HCl?
Sorry! I really appreciate any help you can give, even if I still don't get it :D
Grace
Hey Grace!
When it comes to titrations, it is the Mol/L that is important. Think about it this way: let's pretend I titrate 25mL of monoprotic acid (0.1mol/L) with XmL of monoprotic base (0.1mol/L).
The equation is something like Acid + Base --> Salt + Water, and the ratio of Acid:Base will be 1:1.
Using our formulas, we know that 25mL of 0.1 mol/L acid will contain 0.0025 moles (0.1*0.025). Because it is a 1:1 reaction, we require 0.0025 moles of base, which will require a quantity of 25mL (as expected).
However, nowhere here did we discuss what specific acids or bases we used! The only important thing is the concentration: you can have a really concentrated weak acid, and a very dilute strong acid! It would be EASIER to have a strong, concentrated acid, but definitely not necessary. When it comes to Titrations, only the numbers matter.
Jake
Post by: RuiAce on March 27, 2016, 10:46:05 pm
Hey!
Yes, initiator and catalyst can be thought of as the same thing.
I like your answer; I think it goes into sufficient depth for most areas that you discuss. However, I would add something small about the PRODUCTION of ethene: Part of it's significance in the chemical industry is the ease by which it can be produced, particularly through catalytic cracking of fuel fractions. This is a really important reason as to why we love Ethene: not only is it great (as you've described) because of its reactivity, but also because of its potential abundance! Other than that, and perhaps rewording some of your sentences (read it outloud, and see if anything seems a bit rambly) I think you have covered lots of vital areas in answering the question.
Jake
Just popping in some chat Jake...
I didn't even realise that the peroxide initiator in LDPE and catalyst in HDPE worked the same way until AFTER the HSC!!
Post by: amandali on March 28, 2016, 01:40:52 am
Evaluate the success of the current usage of ethanol as an alternative car fuel obtained from sugar cane 4 marks
Currently, ethanol is used directly as a fuel in Brazil as there is enough arable land to grow sugary crops from which ethanol is derived. The advantage of ethanol is that it is renewable and abundant as it is produced from the fermentation of crops such as sugar cane, corn, wheats, C6H12O6 (aq) --> 2CH3CH2OH(aq) + 2CO2 (g) , as opposed to the hydrocarbon fuels like octane which is produced from non-renewable fossil fuels. Hence, ethanol has an unlimited use while hydrocarbon fuels are limited and are predicted to deplete within the next few decades. Moreover, ethanol combusts cleanly due to presence of oxygen in its molecule which enables a more complete combustion. This mean that less harmful respiratory irritants like C(s) and CO(g) that causes respiratory diseases are produced.
CH3CH2OH(l) + 3O2 (g) --> 2CO2 (g) + 3H2O(l) .
Meanwhile, it is not widely used countries like Australia as it lacks arable land to grow sugary crops and it is not economical to produce ethanol. The disadvantage of ethanol is that its production requires cultivating and clearing vast areas of arable land hence this will affect food production and give rise to environmental effects such as land erosion and destruction of animal habitats. An immense amount of energy is also required to manufacture fertilisers, plant and harvest the crop and distil ethanol thus it is not economically viable to do so.
Weighing the pros and cons, it is unlikely that ethanol will not be used as an alternative fuel unless an economic viable method for converting cellulose to glucose is discovered.
Post by: jakesilove on March 28, 2016, 11:18:40 am
Please check my response thanks
Evaluate the success of the current usage of ethanol as an alternative car fuel obtained from sugar cane 4 marks
Currently, ethanol is used directly as a fuel in Brazil as there is enough arable land to grow sugary crops from which ethanol is derived. The advantage of ethanol is that it is renewable and abundant as it is produced from the fermentation of crops such as sugar cane, corn, wheats, C6H12O6 (aq) --> 2CH3CH2OH(aq) + 2CO2 (g) , as opposed to the hydrocarbon fuels like octane which is produced from non-renewable fossil fuels. Hence, ethanol has an unlimited use while hydrocarbon fuels are limited and are predicted to deplete within the next few decades. Moreover, ethanol combusts cleanly due to presence of oxygen in its molecule which enables a more complete combustion. This mean that less harmful respiratory irritants like C(s) and CO(g) that causes respiratory diseases are produced.
CH3CH2OH(l) + 3O2 (g) --> 2CO2 (g) + 3H2O(l) .
Meanwhile, it is not widely used countries like Australia as it lacks arable land to grow sugary crops and it is not economical to produce ethanol. The disadvantage of ethanol is that its production requires cultivating and clearing vast areas of arable land hence this will affect food production and give rise to environmental effects such as land erosion and destruction of animal habitats. An immense amount of energy is also required to manufacture fertilisers, plant and harvest the crop and distil ethanol thus it is not economically viable to do so.
Weighing the pros and cons, it is unlikely that ethanol will not be used as an alternative fuel unless an economic viable method for converting cellulose to glucose is discovered.
Hey Amanda,
I would add in some more aspects of ethanol as an ALTERNATIVE CAR FUEL, specifically things like the erosion of engines, specific heat capacity etc. that effects its current usage in Australia (limiting it to E10 etc.). Other than that, a really good response.
Jake
Post by: katherine123 on March 28, 2016, 12:50:39 pm
at 10min can it be increase in temperature as well (endothermic) although its due to increase in CO concentration as indicated by the sharp increase
Post by: jakesilove on March 28, 2016, 01:15:04 pm
how do you know that there is change in pressure? i know that change in concentration is indicated by sharp drop or increase
at 10min can it be increase in temperature as well (endothermic) although its due to increase in CO concentration as indicated by the sharp increase
Hey!
You know that there is a change in pressure when there is no sharp drop, and the equilibrium clearly shifts to one side. For instance, at four minutes, there is clearly no sharp drop and yet there is an obvious shift to the products! This could either be because of temperature, concentration or pressure.
Because there is no sharp drop/increase, it isn't a change in concentration. As such, you would talk about the fact that it could be a change in temperature, a change in pressure, or both. Because there are more moles of gas on the side of the products, the only reason products would increase in concentration is if you DECREASE the pressure of the system (by Le Chatelier's). As it is endothermic, an increase in temperature will cause a shift to the right, and therefore this could explain the results at 4 minutes as well. There is no way to tell which of these options occurred, so you need to talk about both!
At 10 minutes, the reason for the change is (as you've identified) because of a removal of CO. This will cause a shift to the right, as per Le Chatelier's, giving the results on the graph. There is no clear change in temperature, just a classic change in concentration.
Post by: Maz on March 28, 2016, 06:53:18 pm
can i please get some help with this question? i got an answer of 120g when the answer is actually 10.5g - big difference haha :P
the question is
Calculate the mass of quicklime (calcium oxide), which will react completely with 250mL of 1.50mol L-1 hydrochloric acid.
thank you so much :)
Post by: grace_joseph on March 28, 2016, 06:55:34 pm
Thank you so much!
Grace :)
Post by: lazydreamer on March 28, 2016, 07:19:38 pm
Hi Jake! Would you please be able to explain to me why the answer for 7 is A and why the answer for 11 is C?
Thank you so much!
Grace :)
In Q7, you need to look at how many hydrogen protons are donated by the acid to NaOH. So basically you need to know the protic nature of the acids. The answer is citric acid because it is triprotic, therefore 3mols of acid is needed for 1mol NaOH compared to the other mono/di-protic acids. Degree of ionisation is irrelevant to answering the question, i know, i've seen that Q before
For 11, you'd need to deduce that the compound formed is an alkene (only alkenes discolor bromine water immediately). Then you'd need to be able to link this with the dehydration of an alkanol, by heating with an acid, which produces an alkene. Therefore the answer is C, the alkanol
Post by: jakesilove on March 28, 2016, 07:26:00 pm
Hi Jake! Would you please be able to explain to me why the answer for 7 is A and why the answer for 11 is C?
Thank you so much!
Grace :)
Totally agree with Lazydreamer's answer! Let me know if you need any further clarification :)
Jake
Post by: jakesilove on March 28, 2016, 07:34:09 pm
hey,
can i please get some help with this question? i got an answer of 120g when the answer is actually 10.5g - big difference haha :P
the question is
Calculate the mass of quicklime (calcium oxide), which will react completely with 250mL of 1.50mol L-1 hydrochloric acid.
thank you so much :)
Hey!
My answer is below, I hope that it helps! Let me know if you need any further clarification. Always just follow the steps that I've outlined: Chemical formula, application of equations, solution!
(http://i.imgur.com/oBzntsh.png?1)
It should read "the difference between my answer and the 10.5g".
Jake
Post by: lazydreamer on March 28, 2016, 07:38:58 pm
So in the Examples, why does the numbering start where it starts? Like why in 3 does it start closer to the methyl but in 4 it starts at the ethyl instead?
Thanks :D
Post by: RuiAce on March 29, 2016, 10:58:51 am
Now i got a question
So in the Examples, why does the numbering start where it starts? Like why in 3 does it start closer to the methyl but in 4 it starts at the ethyl instead?
Thanks :D
Firstly, the ethyl is written before the methyl because we want to maintain alphabetical order. E lies before M in the English the alphabet. But it seems like you already knew that.
We would like to choose, so that we always minimise the sum of the locant numbers in the molecule.
For Q3, if we count from left to right we will have
3-ethyl-5-methyl-hex-3-ene
3 + 5 = 8
If we count right to left we will have
4-ethyl-2-methyl-hex-3-ene
4 + 2 = 6
So we favour the latter.
For Q4, firstly keep in mind that there is an ethyl group on the right, because the double bond is what gives pentadiene it's name.
Now, using the same process
If we count from left to right we will have
4-ethyl-2,3-dimethyl-1,4-pentadiene
If we count from right to left we will have
2-ethyl-3,4-dimethyl-1,4-pentadiene
And here we have a problem. From maths, we know that 4+2+3=2+3+4. Oh dear!
Which is exactly the point I call you out for being a selective school student (or attempt to) and having to do harder questions.
The reason why the second one is favourable is that in the event that your sum of locants are the same (very unlikely to occur in the HSC), we again treat alphabetical order as priority. E lies before M, so the ethyl will get the lower number here.
Post by: RuiAce on March 29, 2016, 11:06:42 am
how do you know that there is change in pressure? i know that change in concentration is indicated by sharp drop or increase
at 10min can it be increase in temperature as well (endothermic) although its due to increase in CO concentration as indicated by the sharp increase
For questions like this there exists a rule of thumb
- No spike, and then things change -> Temperature change. If the nature of the reaction as exothermic or endothermic is given, you can work out if heated or cooled
- Spike in every substance -> Pressure change. Downward spike always means increase in pressure (or alternatively decrease in volume of the vessel)
- Spike in only one substance -> Concentration change. One of the things has been tampered with. Could be you introduced more or suddenly extracted a ton depending on spike up or down.
Post by: RuiAce on March 29, 2016, 11:10:55 am
In Q7, you need to look at how many hydrogen protons are donated by the acid to NaOH. So basically you need to know the protic nature of the acids. The answer is citric acid because it is triprotic, therefore 3mols of acid is needed for 1mol NaOH compared to the other mono/di-protic acids. Degree of ionisation is irrelevant to answering the question, i know, i've seen that Q beforeTo build on:
For 11, you'd need to deduce that the compound formed is an alkene (only alkenes discolor bromine water immediately). Then you'd need to be able to link this with the dehydration of an alkanol, by heating with an acid, which produces an alkene. Therefore the answer is C, the alkanol
It is irrelevant, but you should be aware that the reason why it is irrelevant is due to the titration with a strong base. The strong base being present, regardless of the strength of the acid, forces the reaction to go to completion no matter what.
Post by: indiago on March 29, 2016, 06:04:43 pm
I have come across similar questions to this one but I can't seem to figure out how to do them. Please help !!!
pH 2.00 of 0.01M HCl.
Calculate the pH after 20 mL of 0.01 mol L-1 hydrochloric acid is diluted with 180 mL of water.
Thanks, India
Post by: lazydreamer on March 29, 2016, 11:02:01 pm
Firstly, the ethyl is written before the methyl because we want to maintain alphabetical order. E lies before M in the English the alphabet. But it seems like you already knew that.
We would like to choose, so that we always minimise the sum of the locant numbers in the molecule.
For Q3, if we count from left to right we will have
3-ethyl-5-methyl-hex-3-ene
3 + 5 = 8
If we count right to left we will have
4-ethyl-2-methyl-hex-3-ene
4 + 2 = 6
So we favour the latter.
For Q4, firstly keep in mind that there is an ethyl group on the right, because the double bond is what gives pentadiene it's name.
Now, using the same process
If we count from left to right we will have
4-ethyl-2,3-dimethyl-1,4-pentadiene
If we count from right to left we will have
2-ethyl-3,4-dimethyl-1,4-pentadiene
And here we have a problem. From maths, we know that 4+2+3=2+3+4. Oh dear!
Which is exactly the point I call you out for being a selective school student (or attempt to) and having to do harder questions.
The reason why the second one is favourable is that in the event that your sum of locants are the same (very unlikely to occur in the HSC), we again treat alphabetical order as priority. E lies before M, so the ethyl will get the lower number here.
HAHAHA quite the opposite, i was struggling with naming organic compounds so i went on that website to practice and came across something that made me more confused...but your explanation made it clearer so thanks! :D
Post by: RuiAce on March 30, 2016, 12:17:07 am
Hello !!!
I have come across similar questions to this one but I can't seem to figure out how to do them. Please help !!!
pH 2.00 of 0.01M HCl.
Calculate the pH after 20 mL of 0.01 mol L-1 hydrochloric acid is diluted with 180 mL of water.
Thanks, India
Logical consideration:
The dilution increases the volume from 20mL to 200mL. Note that 200 = 20 * 10
Because it's been diluted to a ten-fold value critically speaking, the pH goes up by 1. pH = 3.
This is because the pH scale is logarithmic (pH = -log10[H3O+])
Formal method:
According to the dilutions formula C1V1=C2V2
0.01 mol L-1 * 20 mL = [HCl]final * 200 mL
0.001 mol L-1 = [HCl]final
As HCl is both monoprotic and a strong acid
[H3O+]=[HCl]=0.001 mol L-1
So pH = -log10[H3O+] = -log10(0.001) = 3
Post by: katherine123 on March 30, 2016, 04:55:07 pm
- 3 volumetric flasks - each with volume capacity of 100ml, 205ml and 500ml
-2.2g anhydrous NaHCO3 molar mass of 84.01
You are asked to prepare a 0.250mol/L solution of NaHCO3
Only one of volumetric flask is useful for the task
Choose one appropriate flask and justify your choice with calculations
what i did was i calculated the moles of NaHCO3 and find the volume required to have all the moles in the flask which turned out to be 105ml so i thought 250 ml is the most suitable since it can contain all the volume of solution
but the answer is 100ml
2nd ques: NaOH absorbs moisture from air. If it was weighed and prepared for use as basic solution in X, what effect will this have?
a) calculated acid concentration will be correct
b) calculated acid concentration will be too low
c) calculated acid concentration will be too high
d) Ph at end point will be much lower than it should be
Post by: jakesilove on March 30, 2016, 06:09:02 pm
During an investigation you are provided with:
- 3 volumetric flasks - each with volume capacity of 100ml, 205ml and 500ml
-2.2g anhydrous NaHCO3 molar mass of 84.01
You are asked to prepare a 0.250mol/L solution of NaHCO3
Only one of volumetric flask is useful for the task
Choose one appropriate flask and justify your choice with calculations
what i did was i calculated the moles of NaHCO3 and find the volume required to have all the moles in the flask which turned out to be 105ml so i thought 250 ml is the most suitable since it can contain all the volume of solution
but the answer is 100ml
2nd ques: NaOH absorbs moisture from air. If it was weighed and prepared for use as basic solution in X, what effect will this have?
a) calculated acid concentration will be correct
b) calculated acid concentration will be too low
c) calculated acid concentration will be too high
d) Ph at end point will be much lower than it should be
Hey Katherine!
I agree that, for the first part, you would require 105mL to get the required concentration. However, remember that a volumetric flask has to be FILLED UP COMPLETELY in order to be accurate and effective. Therefore, your options are to use slightly less water (100mL), and therefore get a concentration slightly higher than required, or use much more water (250mL) and get a concentration much lower then required. Obviously, using the 100mL will get you closer to the desired value, and so we choose the first volumetric flask!
Okay, let's talk about the second question. If the NaOH has been measured to weigh MORE than it actually is (due to water), then in your calculations you will use a basic solution with a HIGHER concentration than it actually is. If you use LESS base than you think you are using to neutralise the SAME amount of acid, then the acid is going to be weaker than you expect (for instance, you might think it neutralises 0.5 moles of base, but it is actually only neutralising 0.45 moles of base. As C=n/V, the concentration will lower if n lowers). Therefore, the answer is c) as the actual concentration is lower than the calculated concentration.
Jake
Post by: Maz on March 31, 2016, 07:12:04 pm
can you please help me with this one- it's a titration...
water from a domestic bore that sprayed onto a nearby fence was observed to leave orange brown deposits on it's surface. this indicated that the water contained iron compounds. the water was analysed for iron by titration in the following way: Aisr was bubbled through a 250 mL sample of water for several hours to convert all the dissolved iron into Fe3+ then boiled to precipitate all the calcium ions as calciu carbonate. the solid was removed by filtration then 10mL of standard 0.103mol L-1 NaOH was added to the bore water which precipitated all of the iron in the sample and left some excess NaOH in the solution.
After the mixture was filtered it required 27.34mL of standard 0.0277 mol L-1 HCl acid for neutralisation. if the density of the bore water was 1.01g mL-1 calculate the concentration of iron in the bore water measured in parts per million...
so far i have just converted the HCl to moles giving: 0.000757318 moles...
i really don't know what to do after that...please can you help me?
Post by: jakesilove on March 31, 2016, 07:37:07 pm
hi
can you please help me with this one- it's a titration...
water from a domestic bore that sprayed onto a nearby fence was observed to leave orange brown deposits on it's surface. this indicated that the water contained iron compounds. the water was analysed for iron by titration in the following way: Aisr was bubbled through a 250 mL sample of water for several hours to convert all the dissolved iron into Fe3+ then boiled to precipitate all the calcium ions as calciu carbonate. the solid was removed by filtration then 10mL of standard 0.103mol L-1 NaOH was added to the bore water which precipitated all of the iron in the sample and left some excess NaOH in the solution.
After the mixture was filtered it required 27.34mL of standard 0.0277 mol L-1 HCl acid for neutralisation. if the density of the bore water was 1.01g mL-1 calculate the concentration of iron in the bore water measured in parts per million...
so far i have just converted the HCl to moles giving: 0.000757318 moles...
i really don't know what to do after that...please can you help me?
Hey!
Unfortunately, I don't have time right now to do a full solution (although can write one up later, if no-one else has), however I can quickly explain the general method.
Knowing the moles of HCl, we can write out the reaction between HCL and NaOH to figure out the number of moles of NaOH that was in excess in solution (knowing that it is a 1:1 ratio).
Now that we have the EXCESS NaOH, we can figure out the amount of NaOH that reacted with Iron (figure out the total moles of NaOH added, and then subtract the excess).
We can write the reaction between Iron and NaOH. Knowing the number of moles of NaOH (from above) that reacts, we can figure out the number of moles of Iron!
Sorry that I can't go into much more detail right now.
Jake
Post by: g98 on March 31, 2016, 08:09:34 pm
Post by: Rd2487 on April 01, 2016, 07:06:10 pm
I just had a question about classifying salts ]
When you explain the salts nature how would you structure your response like lets say NH4CL?
So would you start saying that Cl is a weak conjugate anion of the strong acid HCl and hence it would not further hydrolyze.
Whereas the ammonium is a strong conjugate cation of the weak base ammonia hence it would further hydrolyze.
Now i get confused on this step after providing the reaction of NH4 with water. Would you say that because H30+ ions are produced in solution, it would make the salt acidic ??
Can you please provide a way to structure this response
Thanks
Post by: RuiAce on April 01, 2016, 07:55:16 pm
Hi JakeAs I have previously explained somewhere in this thread (not too sure where exactly).
I just had a question about classifying salts ]
When you explain the salts nature how would you structure your response like lets say NH4CL?
So would you start saying that Cl is a weak conjugate anion of the strong acid HCl and hence it would not further hydrolyze.
Whereas the ammonium is a strong conjugate cation of the weak base ammonia hence it would further hydrolyze.
Now i get confused on this step after providing the reaction of NH4 with water. Would you say that because H30+ ions are produced in solution, it would make the salt acidic ??
Can you please provide a way to structure this response
Thanks
The conjugate of something strong (HCl, NaOH, HNO3) is something extremely weak (respectively, Cl-, Na+, NO3-).
Whereas the conjugate of something weak (CH3COOH, NH3) is something that's also weak, but definitely not extremely weak (respectively CH3COOH-, NH4+)
However, when it comes to classifying salts as acidic/basic/neutral, one may actually safely assume substances such as Cl- and Na+ are neutral when writing exam responses. The reason for this is that the impact of these ions in the overall result is so negligible, it's almost non-existent.
Such a question would be about 3 marks at most (some may be lenient to give 2). Here is what I would say (not the BEST answer - I'm not currently doing the HSC)
The Cl- ion is a neutral anion. It's presence will not affect the acidity of the salt.
However, NH4+ is an acidic cation as it is able to react with water to form it's conjugate base:
NH4+ + H2O(l) ⇌ H3O+ + NH3(aq)
The acidic nature of ammonium chloride therefore occurs as a result of only the acidic ammonium ion impacting, as that of the chloride ion is negligible.
Post by: browny2409 on April 02, 2016, 01:49:05 pm
When detailing the development of Acid-Base theories (Lavoisier, Davy, Arrhenius and Brřsted-Lowry) there was one HSC question which asked how each developed from the other
I am aware of what each theory, but could you help in clarifying exactly how each theory improved the former
Thanks
Nathan
Post by: jakesilove on April 02, 2016, 02:18:49 pm
Hi,
When detailing the development of Acid-Base theories (Lavoisier, Davy, Arrhenius and Brřsted-Lowry) there was one HSC question which asked how each developed from the other
I am aware of what each theory, but could you help in clarifying exactly how each theory improved the former
Thanks
Nathan
Hey Nathan!
Essentially, all you need to know is that each subsequent theory was developed as a result of an unexplained finding! For instance, Lavoisier believed that acids contained Oxygen molecules (as you know). However, Davy noticed that HCl acted as an acid, and therefore obviously Lavoisier's theory was inadequate! As such, Davy developed his own theory.
Each development has similar examples that you can use, however I wouldn't bother. The only question you could really get either a multiple choice, asking you to determine which theory is outlined in the question, or a 'describe Arrhenius's theory of Acids and Bases' etc.
How many marks was that question you found? I would say you just need to outline the general aspects of each theory, and saying that they built on each other as new scientific information came to light. You're not expected to know examples of each: merely that they become increasingly complex,and increasingly accurate as they tend to encapsulate more and more actual acids.
Jake
Post by: RuiAce on April 02, 2016, 03:13:56 pm
Hi,
When detailing the development of Acid-Base theories (Lavoisier, Davy, Arrhenius and Brřsted-Lowry) there was one HSC question which asked how each developed from the other
I am aware of what each theory, but could you help in clarifying exactly how each theory improved the former
Thanks
Nathan
During the HSC I wrote a full 8 marker on acid-base theories. I was able to find it somehow.
Please refer to below:
Past theories of acids and bases tend to purely rely on what was believed to be the chemical composition of all acids and their properties. Lavoisier's theory stated that acids were substances which contained hydrogen, as he found that when compounds involving oxygen dissolved in water, they produced acidic properties, such as CO2, NO2 and P4O10. This also meant that some acids were recognised to be acids, e.g. H2SO4. However, this is now known to be significantly flawed, as some substances containing oxygen are in fact basic (e.g. Na2CO3, Na2O) and why some other acids did not contain oxygen (e.g. HCl). Davy then proposed that acids were substances that contained hydrogen through the discovery of HCl, through means of dissolving gaseous hydrogen chloride into water and producing a highly acidic solution. Davy also found that metals could displace the hydrogen in acids: Metal + Acid -> Salt + Hydrogen gas. Yet, this was also flawed because other substances such as CH4 are not acidic, yet also contain hydrogen.
Arrhenius then developed a theory that was, at the time, much more revolutionary and a more accurate definition of acids, and bases. Arrhenius showed that acids were substances that when dissolved in water, ionised to form H+ ions, and similarly for bases albeit OH- ions. Arrhenius was also able to initiate the definitions of relative strengths of acids through his theory from an understanding of degrees of ionisation, and also demonstrated how acids and bases can undergo a neutralisation process: Acid + Base -> Salt + Water. This is reasonably accurate, and is sometimes taught as a simplified model of how acids and bases work, however it did not cater for other bases such a Na2CO3 and NH3.
The most accurate definition of acids and bases was proposed by Johannes Bronsted and Thomas Lowry. The Bronsted-Lowry theory defines acids and bases in terms of their chemical behaviour, that is, acids were substances that could donate a proton/protons, whereas bases could accept them. This theory focused much more on the properties of acids and bases rather than their chemical composition and an understanding of what conjugate acids and bases were. By consideration of the ability to donate/accept H+, it was then finally understood why substances were acidic, basic or neutral. (It also led into the identification of amphiprotic species with reference conjugate acids and bases.)
Post by: RhyannahHamer on April 03, 2016, 05:46:09 pm
I'm doing notes under the syllabus right now, and I don't think I've got any sufficient notes on the following dot point:
'describe the difference between a strong and a weak acid in terms of an equilibrium between the intact molecule and its ions'
If you could help me out, it'd be much appreciated.
Thanks :)
Post by: jakesilove on April 03, 2016, 10:07:59 pm
Hey there,
I'm doing notes under the syllabus right now, and I don't think I've got any sufficient notes on the following dot point:
'describe the difference between a strong and a weak acid in terms of an equilibrium between the intact molecule and its ions'
If you could help me out, it'd be much appreciated.
Thanks :)
Hey!!
Basically, for that dot point I would have a few things up your sleeve.
Definitions of strong and weak acids
Always start by defining things. A strong acid is, by definition, an acid that ionises completely in solution. A weak acid is just any acid that does NOT ionise completely in solution (and usually only ionises to a tiny extent, eg. 1-10%).
Formulas
But what do the above statements even mean? The easiest way to describe strong and weak acids in terms of equilibrium is to use equations. So, let's do that!
Let's start by looking at a strong acid. I always like to use HCl, because its the one used most often in the HSC.
We write the reaction of HCl and Water to show ionisation.
(http://i.imgur.com/CM4B2wb.png?1)
Now, we know that HCl is a strong acid, and so ionises completely. In terms of equilibrium, this means that the reaction lies entirely to the right!
Now, let's look at weak acids or weak bases. You can use whatever weak acid or base you want.
First, we write out the equation.
(http://i.imgur.com/mc7JMgy.png?1)
Now, we know that Ammonia is a weak base. Therefore, in terms of equilibrium, we can say that the reaction lies largely to the left!
That's really all you need to know in terms of acids/bases and equilibrium. Hope it helps!
Jake
Post by: Maz on April 05, 2016, 01:10:31 am
i've tried to do this titration question a couple of times- i just don't know how to do it :(
can you please help me?
the question:
9.32g of 'coudy ammonia' was dissolved in water and made up to 250mL in a volumetric flask. 20mL portions of this solution were tritrated with 0.98 mol L^-1 HCl. An average of 25.8 mL HCl was required for the neutralisation using methyl orange indicator. Calculate the percentage by mass of ammonia in the 'cloudy ammonia'.
i'd really appreciate it if you could help me out...
thankyou :)
Post by: browny2409 on April 06, 2016, 09:38:43 am
Just another question, when summarising the chemistry of the fermentation process (Production of Materials), do we need to mention acid hydrolysis and enzyme digestion, because i have heard of these terms but don't have them in my notes - all i refer to is the invertase enzyme and the zymase enzyme brought about by the yeast
Thanks
Nathan
Post by: jakesilove on April 06, 2016, 09:52:51 am
hey
i've tried to do this titration question a couple of times- i just don't know how to do it :(
can you please help me?
the question:
9.32g of 'coudy ammonia' was dissolved in water and made up to 250mL in a volumetric flask. 20mL portions of this solution were tritrated with 0.98 mol L^-1 HCl. An average of 25.8 mL HCl was required for the neutralisation using methyl orange indicator. Calculate the percentage by mass of ammonia in the 'cloudy ammonia'.
i'd really appreciate it if you could help me out...
thankyou :)
Hey!
I'm so sorry for the late response, I totally missed your question! This is a tough one, that requires multiple applications of the titration formulas. My working is below, I hope it helps!
(http://i.imgur.com/amSlGC0.png?1)
(http://i.imgur.com/RDtuK5Q.png?1)
Jake
Post by: jakesilove on April 06, 2016, 09:59:14 am
Thanks for your pervious answer,
Just another question, when summarising the chemistry of the fermentation process (Production of Materials), do we need to mention acid hydrolysis and enzyme digestion, because i have heard of these terms but don't have them in my notes - all i refer to is the invertase enzyme and the zymase enzyme brought about by the yeast
Thanks
Nathan
Hey Nathan,
Whenever I tutor students, and whenever I answered questions on this topic in my HSC, I always recommend one method for exactly this problem: Mention the terms, but that's it.
An important step in the process is going from Cellulose to Glucose. At the moment, this process is expensive and time consuming, and requires either acid hydrolysis and enzyme digestion. However, you definitely don't need to know more information about either of those processes. I would definitely mention that they are expensive and time consuming, and name-drop the terms, but that's it.
Not only would you get full marks by mentioning it, but as it is really an 'addition' to the course the marker will look really favorably on you for going above and beyond, despite the fact that maybe you don't understand the terms you are using (as I definitely don't either!).
Post by: Neutron on April 06, 2016, 10:21:39 am
Got my Chemistry half yearly today and I've got a stupid question D: This is the question:
Which of the following aqueous solutions has a pH greater than 7?
a) Sodium ethanoate
b) Sodium chloride
c) Ammonium nitrate
d) Ammonium chloride
What's a good way to tell if you're unsure of the compound? Thanks!
Neutron
Post by: jakesilove on April 06, 2016, 10:32:41 am
Hey guys!
Got my Chemistry half yearly today and I've got a stupid question D: This is the question:
Which of the following aqueous solutions has a pH greater than 7?
a) Sodium ethanoate
b) Sodium chloride
c) Ammonium nitrate
d) Ammonium chloride
What's a good way to tell if you're unsure of the compound? Thanks!
Neutron
Hey!
EDIT: I was mistaken when answering this question: see below posts for the correct answer!
I don't think that's a stupid question at all! I think the most difficult part of the HSC Chemistry curriculum are sections where you just kind of need to memorise content, or rules, to get marks in a question.
Let's approach the multiple choice you've posed. First, I think it is fair to eliminate B straight away, as we know that NaCl is a neutral salt. It kind of has to be: we wouldn't want to be putting acidic/basic substances or our chips.
Next, I would try to recall that anything with an -oate on the end is know as an 'alkanoic acid' (from the Esterifaction part of your curriculum). Knowing that, I think it would be fair to eliminate A as well, since it will be acidic.
As for the last two, I honestly have no idea how to decide which is basic. I hope someone else in the forums can help me out, but I can't think of a way to distinguish between them in terms of any HSC level chemistry. I'm really sorry about that: at least at this stage you have a 50/50 chance of being right!
Again, so sorry about my terrible response.
Jake
Post by: Neutron on April 06, 2016, 11:53:45 am
Neutron
Post by: IntelxD on April 06, 2016, 12:03:45 pm
Hey guys!
Got my Chemistry half yearly today and I've got a stupid question D: This is the question:
Which of the following aqueous solutions has a pH greater than 7?
a) Sodium ethanoate
b) Sodium chloride
c) Ammonium nitrate
d) Ammonium chloride
What's a good way to tell if you're unsure of the compound? Thanks!
Neutron
Sodium ethanoate will dissociate into sodium ions and ethanoate ions in an aqueous solution. Ethanoate (also known as acetate) is the weak conjugate base of ethanoic acid. Thus, ethanoate will react with the H+ ions, lowering [H+] and slightly increasing the pH of the solution above 7. Ammonium in option c and d is a weak acid and so will have the opposite effect. You should also note that ions such as chloride, sodium and nitrate do in fact have acidic or basic properties. They however are so weak that their effect on the pH of the solution is negligible and usually produces opposing effects (eg, NaCl) leaving the pH unchanged.
Post by: helloworld on April 06, 2016, 12:25:24 pm
Post by: HopefulLawStudent on April 06, 2016, 12:50:42 pm
Post by: helloworld on April 06, 2016, 01:14:22 pm
Post by: RuiAce on April 06, 2016, 05:43:55 pm
Hey guys!
Got my Chemistry half yearly today and I've got a stupid question D: This is the question:
Which of the following aqueous solutions has a pH greater than 7?
a) Sodium ethanoate
b) Sodium chloride
c) Ammonium nitrate
d) Ammonium chloride
What's a good way to tell if you're unsure of the compound? Thanks!
Neutron
We're looking for the basic salt. Because ammonium nitrate and ammonium chloride are acidic, they're automatically out. Why did I eliminate them in a matter of 1 second?
NO3- and Cl- are the conjugates of strong acids. That means they're extremely weak; virtually neutral. So they don't impact.
And it just so happens that substances such as Na+ and K+ are neutral cations. Whereas NH4+ is actually weakly acidic.
(Note, if I had ammonium acetate that would be different. That'd be around neutral.)
And then, like mentioned, sodium chloride is neutral. This is trivial, but it's not the first thing I notice due to the difference between sodium ions and ammonium ions.
Now, I think Jake made a mistake. Whilst ethanoic acid (aka acetic acid) is acidic, the ethanoate (acetate) ion is consequently basic. It is the conjugate to acetic acid, and is thus what causes the basic properties. Just like Intel mentioned.
Post by: jakesilove on April 06, 2016, 10:06:20 pm
Now, I think Jake made a mistake.
You're absolutely right, I'm sorry about that! Your responses all look fantastic, thanks for helping out!
Jake
Post by: Johny1234567 on April 10, 2016, 08:29:00 am
a) The reactants are placed into the reaction vessel and are allowed to react for a longer period of time.
b) The reaction first reacts to equilibrium, and then the reaction vessel is heated.
c) The formation of ammonia with and without a catalyst, assuming all other conditions are the same. You do not need to show how hydrogen/ nitrogen vary on this graph.
Post by: RuiAce on April 10, 2016, 09:01:42 am
Draw separate concentration-time graphs for the following situations relating to the Haber Reaction.a) As the Haber process does effectively take a HUGE amount of time to reach equilibrium, show this by making the process seemingly unable to obtain equilibrium in graph 1, but then achieving it in graph 2.
a) The reactants are placed into the reaction vessel and are allowed to react for a longer period of time.
b) The reaction first reacts to equilibrium, and then the reaction vessel is heated.
c) The formation of ammonia with and without a catalyst, assuming all other conditions are the same. You do not need to show how hydrogen/ nitrogen vary on this graph.
b) Recall that the equation 3 H2(g) + N2(g) ⇌ 2 NH3(aq) with ∆H=-92 kJ mol-1
The forward reaction is exothermic. Hence, if we introduce heat into the system, according to Le Chatelier's principle the system will attempt to readjust it's equilibrium to counter the change by shifting it to the left.
From a graph that's already attained equilibrium, suddenly the concentrations of the reactants (hydrogen and nitrogen) must go up a bit, and that of ammonia will go down.
c) Catalyst speeds up the rate of reaction. Hence, you achieve the same equilibrium, but you achieve it faster. This just means that on the graph, say the equilibrium is at 3. You increase up to 3 more quickly, but you end up with the same horizontal line anyway.
I will let someone else post relevant diagrams
P.S. I haven't been helping much these days cause I want to focus on mid sems
Post by: Johny1234567 on April 10, 2016, 11:05:20 am
Calculate the yield of the Haber reaction that used 30 kg of Nitrogen gas in excess hydrogen gas to form 1.2 kg of ammonia.
Post by: jakesilove on April 10, 2016, 11:22:15 am
ty, one more question:
Calculate the yield of the Haber reaction that used 30 kg of Nitrogen gas in excess hydrogen gas to form 1.2 kg of ammonia.
Hey!
Below is my solution. Hope it helps!
(http://i.imgur.com/ykfGQYA.png?1)
Jake
Post by: chloe9756 on April 14, 2016, 04:20:07 pm
The solubilities at 25C of three white barium salts, in grams per 100mL of water are:
Barium Sulfate 0.00025
Barium Nitrate 10.1
Barium Iodide 220
15g of a white powder was prepared by mixing 5.0g of each of these three barium salts. Your task to to obtain a sample of pure barium sulphate and a sample of virtually pure barium nitrate. Write the method for the separation process. Explain at each stage what happens to the mixture.
Firstly, when the question says to obtain PURE barium sulphate and VIRTUALLY PURE barium nitrate, how should that change the process you choose to use. Also, the answers specifically says that you must add more than 55mL of water, why is this so? And it also says after separating barium sulphate through filtration, barium nitrate will stay dissolved but barium iodide will crystallise out. Why is this so, doesn't barium iodide have a higher solubility. Thank you :)
Post by: katherine123 on April 20, 2016, 03:16:56 am
0.132g of pure carboxylic acid (R-COOH) was dissolved in 25ml of water and titrated with 0.120mol/L NaOH solution
A volume of 14.80ml was required to reach the endpoint of the titration.
The carboxylic acid could be:
A)HCOOH
B)CH3COOH
C)C2H5COOH
D)C3H7COOH
ans:C
question 2: Oxalic acid C2O4H2
A 25.0ml solution of oxalic acid reacts completely with 15.0ml of 2.50mol/L of NaOH
what is the concentration of the oxalic acid solution?
ans: 0.750M
Post by: mmadeleine on April 23, 2016, 06:29:55 pm
"The electrolyte in car batteries is sulfuric acid. A student decided to determine the concentration of this acid in a well-charged car battery by taking exactly 2 mL by pipette and titrating it with 1.16 mol/L sodium hydroxide solution. 17.1 mL was needed to reach the equivalence point. Calculate the molarity of the sulfuric acid in the battery."
Thanks!
Post by: jakesilove on April 23, 2016, 07:22:08 pm
Hi, just a bit stuck on this titration question - I don't really understand how to do it so any help would be appreciated. :)
"The electrolyte in car batteries is sulfuric acid. A student decided to determine the concentration of this acid in a well-charged car battery by taking exactly 2 mL by pipette and titrating it with 1.16 mol/L sodium hydroxide solution. 17.1 mL was needed to reach the equivalence point. Calculate the molarity of the sulfuric acid in the battery."
Thanks!
Hey!
Below is the general approach for all titration questions. I would recommend becoming quite comfortable with the method. Hope this helps!
(http://i.imgur.com/LBOLUSM.png?1)
Jake
Post by: Johny1234567 on April 24, 2016, 02:03:46 am
Na2CO3 + H3O^+ -> CO2(g) + 2Na^+ + H2O
So, since sodium carbonate reacts with hydronium, the disturbance is that it decreases the hydronium concentration. This means it needs to shift right to increase hydronium concentration, correct? While this would increase CO2(aq) thus increasing solubility, there would be an increase in CO2(g) as well due to the reaction as stated above. Would the increase in CO2(g) also be treated as a disturbance, thus leading to an increase in solubilty. Or, does it simply decrease carbon dioxide solubity since it's a gas. Need some help please !
Post by: RuiAce on April 24, 2016, 08:51:05 am
What is the effect on solubility of adding concentrated sodium carbonate to the carbon dioxide equilibrium.
Na2CO3 + H3O^+ -> CO2(g) + 2Na^+ + H2O
So, since sodium carbonate reacts with hydronium, the disturbance is that it decreases the hydronium concentration. This means it needs to shift right to increase hydronium concentration, correct? While this would increase CO2(aq) thus increasing solubility, there would be an increase in CO2(g) as well due to the reaction as stated above. Would the increase in CO2(g) also be treated as a disturbance, thus leading to an increase in solubilty. Or, does it simply decrease carbon dioxide solubity since it's a gas. Need some help please !
The carbon dioxide equilibrium is just this:
H2CO3(aq) ⇌ H2O(l) + CO2(g)
Therefore I believe either you started the question wrongly, or the question was given misleadingly.
Introduction of sodium carbonate (which is a basic substance) will automatically cause it to react with the carbonic acid. According to LCP, because the concentrations of carbonic acid falls down, the equilibrium will shift to the left to minimise this disturbance. This implies that more carbon dioxide will be converted to carbonic acid, and thus solubility should increase.
Post by: RuiAce on April 24, 2016, 08:54:00 am
hey, this is a really simply question but i can't manage to get my head around a few things:
The solubilities at 25C of three white barium salts, in grams per 100mL of water are:
Barium Sulfate 0.00025
Barium Nitrate 10.1
Barium Iodide 220
15g of a white powder was prepared by mixing 5.0g of each of these three barium salts. Your task to to obtain a sample of pure barium sulphate and a sample of virtually pure barium nitrate. Write the method for the separation process. Explain at each stage what happens to the mixture.
Firstly, when the question says to obtain PURE barium sulphate and VIRTUALLY PURE barium nitrate, how should that change the process you choose to use. Also, the answers specifically says that you must add more than 55mL of water, why is this so? And it also says after separating barium sulphate through filtration, barium nitrate will stay dissolved but barium iodide will crystallise out. Why is this so, doesn't barium iodide have a higher solubility. Thank you :)
Making an assumption that this is indeed preliminary chemistry.
If the solubility of barium sulfate is as poor as THAT compared to the other substances, then filtration is automatically the best way to go. This is because you will need TONS of water for all of that barium sulfate to dissolve, whereas the other substances only require few. Thus, filtration should be used to seperate barium sulfate without any further thought.
Filtration only separates the barium sulfate. The barium nitrate and barium iodide remain dissolved in solution. I have absolutely no clue where they gathered that barium iodide will crystallise out.
Post by: RuiAce on April 24, 2016, 09:08:06 am
Help with this ques thanks
0.132g of pure carboxylic acid (R-COOH) was dissolved in 25ml of water and titrated with 0.120mol/L NaOH solution
A volume of 14.80ml was required to reach the endpoint of the titration.
The carboxylic acid could be:
A)HCOOH
B)CH3COOH
C)C2H5COOH
D)C3H7COOH
ans:C
question 2: Oxalic acid C2O4H2
A 25.0ml solution of oxalic acid reacts completely with 15.0ml of 2.50mol/L of NaOH
what is the concentration of the oxalic acid solution?
ans: 0.750M
For Q2, oxalic acid is indeed, diprotic.
2 NaOH(aq) + C2O4H2(aq) → Na2C2O4(aq) + 2 H2O(l)
Hence the mole ratio is given
n(NaOH) = 2n(C2O4H2)
Thus, using n = CV
[NaOH] * V(NaOH) = 2* [C2O4H2] * V(C2O4H2)
2.50 mol L-1 * 15.0 mL = 2 * [C2O4H2] * 25.0 mL
[C2O4H2] = 0.75 mol L-1 as the answers requested
______________________________________
Q1 is one of those typical rigorous questions that the HSC puts in, where you essentially have to do up to 4 calculations to find the answer.
We wish to determine if methanoic acid (formic acid), ethanoic acid (acetic acid), propanoic acid or butanoic acid was used in the titration. We firstly note that ALL of the carboxylic acids are monoprotic.
Acid + NaOH(aq) → Sodium salt + H2O(l)
In every case, the mole ratio is 1 to 1.
Thus, we have
n(Acid) = n(NaOH)
We will apply n=CV to the sodium hydroxide, but n=m/M to the acid:
m/M = CV
0.132g / M = 0.0148 L * 0.120 mol L-1
M = 74.32432... g mol-1
Now that we know the molar mass, we can simply use the periodic table to find out which acid is the correct acid.
It turns out that propanoic acid is correct:
M = 2*12.01 + 5*1.008 + 12.01 + 16.00 + 16.00 + 1.008 = 74.078
The tiny amount of inaccuracy should've been anticipated since the start.
Post by: Johny1234567 on April 24, 2016, 12:50:49 pm
The carbon dioxide equilibrium is just this:I thought the CO2 equilibrium consisted of 4 equations in the following sequence:
H2CO3(aq) ⇌ H2O(l) + CO2(g)
Therefore I believe either you started the question wrongly, or the question was given misleadingly.
Introduction of sodium carbonate (which is a basic substance) will automatically cause it to react with the carbonic acid. According to LCP, because the concentrations of carbonic acid falls down, the equilibrium will shift to the left to minimise this disturbance. This implies that more carbon dioxide will be converted to carbonic acid, and thus solubility should increase.
1. CO2(g) ⇌ CO2(aq)
2. CO2(aq) + H2O(l) ⇌ H2CO3(aq)
3. H2CO3(aq) ⇌ H+(aq) + HCO3-(aq)
4. HCO3-(aq) ⇌ H+(aq) + CO3^2-(aq)
Post by: RuiAce on April 24, 2016, 02:55:07 pm
We are talking about the carbon dioxide equilibrium, whereby the second equation (with carbon dioxide as a gas) is predominant. The third and fourth equations technically do occur as a result of Brřnsted-Lowry acid base theory.
However, their presence is negligible to this question, as the carbonic acid equilibrium is only specific to the interaction of carbon dioxide with water. This produces a substance that is mostly acidic, not basic.
Worthwhile mention: the hydrogen carbonate ion buffer makes greater use of the generation of the carbonate ion, and the carbonic acid equilibrium
Post by: Johny1234567 on April 24, 2016, 06:03:36 pm
Post by: katherine123 on April 26, 2016, 10:56:20 pm
Post by: RuiAce on April 27, 2016, 06:56:00 am
esters are polar but why are they non-water soluble?
Whilst the components of esters are quite appreciably polar, that is, the alkanol and alkanoic acid seperate, the ester itself is only a tiny bit polar. The polarity of the ester occurs as as the carboxylic acid group (-COOH) does contain one extra polar oxygen.
However, this is the only site of polarity. That OH part was dismantled when the ester was formed (note that water is a product of esterification). As this is the only site, the solubility of the substance will not be that great as soon as the chains start becoming longer in length. Much more of the molecule consists of carbon chains which are non-polar and do not readily dissolve into water.
(Some of the very short chained esters still somewhat dissolve into water.)
Post by: Happy Physics Land on April 27, 2016, 02:04:24 pm
Whilst the components of esters are quite appreciably polar, that is, the alkanol and alkanoic acid seperate, the ester itself is only a tiny bit polar. The polarity of the ester occurs as as the carboxylic acid group (-COOH) does contain one extra polar oxygen.
However, this is the only site of polarity. That OH part was dismantled when the ester was formed (note that water is a product of esterification). As this is the only site, the solubility of the substance will not be that great as soon as the chains start becoming longer in length. Much more of the molecule consists of carbon chains which are non-polar and do not readily dissolve into water.
(Some of the very short chained esters still somewhat dissolve into water.)
Essentially there is still partial solubility but as the chain length increases this solubility involving -COOH carboxylic group can be considered negligible
Post by: RuiAce on April 27, 2016, 08:17:27 pm
Essentially there is still partial solubility but as the chain length increases this solubility involving -COOH carboxylic group can be considered negligibleAn ester doesn't have a -COOH functional group that's an alkanoic acid
Post by: JellyBeanz on April 27, 2016, 08:26:51 pm
An ester doesn't have a -COOH functional group that's an alkanoic acid
I think he's referring to the COO section when an ester is formed through the condensation reaction between an alkanoic acid and alkanol.
Post by: RuiAce on April 27, 2016, 09:07:53 pm
I think he's referring to the COO section when an ester is formed through the condensation reaction between an alkanoic acid and alkanol.
The "COO" group (obviously it isn't really one) loses most of it's impact mainly cause it's right in the middle of the molecule. Partial solubility is definitely present but if it's more towards the middle it is slightly limited from impacting as much as if it were on the end.
If it were on the end, an entire tail (or head) would be dissolved. Kinda the principles behind how soaps work (industrial chemistry option for the HSC). But being in the middle it will hardly dissolve a bit as the molecule would have to bend too much for the solubility to be appreciable.
Post by: vaish_vj on April 30, 2016, 12:30:18 pm
My second 3/4 chem sac is regarding aspirin and oil of wintergreen (methyl salicylate).
Is they key structural difference between the two the fact that aspirin lacks a distinct hydroxyl functional group or the positioning of the methyl functional group on aspirin?
Post by: jakesilove on April 30, 2016, 04:07:21 pm
Hey!
My second 3/4 chem sac is regarding aspirin and oil of wintergreen (methyl salicylate).
Is they key structural difference between the two the fact that aspirin lacks a distinct hydroxyl functional group or the positioning of the methyl functional group on aspirin?
Hey! I think you're looking for the VCE Chemistry question board! Sorry, won't be able to help you out here :)
Jake
Post by: HighTide on May 02, 2016, 07:19:25 am
Hey!Apologies for invading the HSC thread... the structural difference would be more towards the functional groups present. Methyl salicylate has a hydroxyl group whilst aspirin has a carboxyl group. But also, if you draw it out, you'll see another difference in the components to form the ester.
My second 3/4 chem sac is regarding aspirin and oil of wintergreen (methyl salicylate).
Is they key structural difference between the two the fact that aspirin lacks a distinct hydroxyl functional group or the positioning of the methyl functional group on aspirin?
Post by: Maz on May 02, 2016, 07:45:32 pm
i have a titration assessed practical coming up soon
the assessment is the we have to find the percentage of household ammonia in a cleaning product by tritrating it against HCl... with the solution diluted to 100mL and an aliquot of 10mL
I've done the calculation part to one...but we are also given density...and i don't quite know where the density fits into it?
the HCl has a concentration of 0.1082
Average volume of HCl titrate used
= 4.85 cm3
No. of moles of HCl reacted = 0.1082 x (4.85 x10-3)
= 5.248 x 10-4 mol
From equation, mole ratio of HCl ≡ NH3 is 1:1,
no. of moles of NH3 reacted = 5.248 x 10-4 mol
No. of moles of NH3 in volumetric flask (diluted) = (100 ÷ 10) × 5.248 x 10-4 mol
= 5.248 x 10-3 mol
No. of mole of NH3 in 10mL (undiluted) = 5.248 x 10-3 mol
Concentration of NH3 = (5.248 x 10-3) ÷ (10 x 10-3) = 5.248 x 10-1 moldm-3
Molar mass of NH3 = 14.01 + 3 (1.0079)
= 17.0337 g mol-1
Therefore, weight-volume percentage of NH3 in window cleaner
= ((17.0337 x 5.248 x 10-1)/ 103) × 100%
= 0.894%
Percentage of ammonia in the window cleaner is: 0.894%
found the percentage of ammonia...but didnt use the density given...could you please tell me where i went wrong?
i'd really appreciate the help
Thankyou :)
Post by: RuiAce on May 02, 2016, 09:10:49 pm
Hey
i have a titration assessed practical coming up soon
the assessment is the we have to find the percentage of household ammonia in a cleaning product by tritrating it against HCl... with the solution diluted to 100mL and an aliquot of 10mL
I've done the calculation part to one...but we are also given density...and i don't quite know where the density fits into it?
the HCl has a concentration of 0.1082
Average volume of HCl titrate used
= 4.85 cm3
No. of moles of HCl reacted = 0.1082 x (4.85 x10-3)
= 5.248 x 10-4 mol
From equation, mole ratio of HCl ≡ NH3 is 1:1,
no. of moles of NH3 reacted = 5.248 x 10-4 mol
No. of moles of NH3 in volumetric flask (diluted) = (100 ÷ 10) × 5.248 x 10-4 mol
= 5.248 x 10-3 mol
No. of mole of NH3 in 10mL (undiluted) = 5.248 x 10-3 mol
Concentration of NH3 = (5.248 x 10-3) ÷ (10 x 10-3) = 5.248 x 10-1 moldm-3
Molar mass of NH3 = 14.01 + 3 (1.0079)
= 17.0337 g mol-1
Therefore, weight-volume percentage of NH3 in window cleaner
= ((17.0337 x 5.248 x 10-1)/ 103) × 100%
= 0.894%
Percentage of ammonia in the window cleaner is: 0.894%
found the percentage of ammonia...but didnt use the density given...could you please tell me where i went wrong?
i'd really appreciate the help
Thankyou :)
Density = Mass / Volume
So if they give you the density and you have the volume, you can use it to find the mass
Take note that n = m/M, not n = V/M. Also take note that only WATER has a density of 1. For water, m = V
Post by: Maz on May 02, 2016, 09:21:02 pm
Density = Mass / Volume
So if they give you the density and you have the volume, you can use it to find the mass
Take note that n = m/M, not n = V/M. Also take note that only WATER has a density of 1. For water, m = V
so using that method could you bypass all the parts where in my solution i have worked it out through moles...
would u just do density x av titre (volume)= mass?
Post by: amandali on May 03, 2016, 12:13:08 am
2. You are given a titration graph of weak acid + strong base with curve ending at the basic region
is the reason for that due to weak acid producing strong conjugate base which reacts with H2O to produce OH- that increases pH
Post by: fluffchuck on May 03, 2016, 07:00:00 pm
Do both ionic and covalent compounds contain intermolecular and intramolecular bonds? Thanks!!
Post by: Elizawei on May 03, 2016, 10:49:35 pm
Hey guys,
Do both ionic and covalent compounds contain intermolecular and intramolecular bonds? Thanks!!
Yup, they both do ;)
EDIT: SORRY didn't realise this was the HSC chem thread, oops :P
Post by: katherine123 on May 03, 2016, 11:46:30 pm
N2(g) + 3H2(g) <--> 2NH3(g)
[NH3(g)] decreases as NH3(g) --> NH3(l) so does it shift to right because the concentration of NH3(g) has decreased or because the reaction is under high pressure so it will shift to reduce the number of gas molecules to reduce pressure
Post by: amandali on May 03, 2016, 11:58:36 pm
a) since pressure is inversely proportional to volume, the decrease in volume will increase the pressure. According to Le chatalier's principle, the equilibrium will shift right so that there will be fewer total gaseous molecules in the system since for every 2 molecules that react only 1 is formed which reduces pressure. This shift will cause an increase in concentration of Z(g) and also heat released as forward reaction is exothermic.
b) catalyst does not affect the position of equilibrium because it lowers the activation energy which increases the rate of both forward and reverse reactions equally. Therefore, equilibrium state is reached sooner but the yield of product is not increased.
Post by: Rishi97 on May 04, 2016, 10:37:54 am
Can you check my response? thanks
a) since pressure is inversely proportional to volume, the decrease in volume will increase the pressure. According to Le chatalier's principle, the equilibrium will shift right so that there will be fewer total gaseous molecules in the system since for every 2 molecules that react only 1 is formed which reduces pressure. This shift will cause an increase in concentration of Z(g) and also heat released as forward reaction is exothermic.
b) catalyst does not affect the position of equilibrium because it lowers the activation energy which increases the rate of both forward and reverse reactions equally. Therefore, equilibrium state is reached sooner but the yield of product is not increased.
Good responses :)
Just a small fix up is needed for response 2. The questions asks about the effect of a catalyst on the equilbrium mixture. However, you started off talking about the position of equilbrium straight away. This is correct but maybe just structure it a bit differently. So start of by explaining the role of a catalyst and then apply it to the situation/question given. For eg. a catalyst is a substance that increases the rate of reaction by lowering the activation energy. The catalyst increases the rate of both the forward and reverse reactions. Thus, the equilbrium state is reached sooner but the amount of yield is not affected.
Hope this makes sense :) great effort just need a bit of fixing up...but if your teacher is not a harsh marker, you may have still gotten the marks :)
Post by: meretz.etai on May 05, 2016, 11:26:50 am
Im having some problems understanding the idea equilibrium, can help explain?
Post by: Happy Physics Land on May 08, 2016, 01:24:20 pm
Hey Jake,
Im having some problems understanding the idea equilibrium, can help explain?
Hey meretz!
Clearly I'm not Jake, but I would be more than happy to help you out here! In year 11, all the equations that we have learnt about are all complete reactions, which means that the reaction only goes one way because the reaction would proceed until all the reactants are used up. A good example is combustion reaction (2C8H18(l) + 25O2(g) ----> 18H2O(l) + 16CO2(g). So if you think about it, when you burn a fuel, do you ever see the products H2O and CO2 reacting to form the original fuel again? Definitely not. In contrast an equilibrium reaction is basically any reactions that do not go to complete and therefore the reaction can be reversed. For instance, the Haber Process N2(g) + H2(g) --> / <--- NH3(g) is a reversible process. In this case, when nitrogen reacts with hydrogen to form ammonia, some ammonia decomposes back into nitrogen and hydrogen as well. The amount of reactants/products forms are governed by Le Chatelier's principle which talk about the impact of temperature, pressure and concentration upon the equilibrium reaction. Be mindful that this does not affect complete reactions because it is a ONE-WAY reaction and therefore the products cannot decompose to form reactants again.
When we talk about chemical equilibrium, we are referring to a state of a close chemical system in which the concentrations of both reactants and products do not change with time and the rate of the forward reaction is equal to the rate of the reverse reaction. For instance, consider a sealed bottle of soda water. There is dynamic equilibrium happening because carbon dioxide is dissolving in the liquid and carbon dioxide being form as gas in the air above the liquid simultaneously. The equilibrium is maintained unless concentration changes, pressure changes or temperature changes which will cause a disturbance.
Post by: hannahboardman98 on May 11, 2016, 07:39:14 pm
A particular brand of vinegar claimed to 4.3% acetic acid. 10mL of the vinegar was diluted to make a 100mL solution. The claim was tested using a titration where 16mL of 0.1M sodium hydroxide was needed to neutralise 25mL of the 10% vinegar solution. Calculate the molarity (moles/L) and the percentage (w/v, ie - g/100mL = % ) of acetic acid in the original vinegar. Comment on the 4.3% claim.
Post by: jakesilove on May 12, 2016, 02:17:22 pm
Hi, I am really struggling to answer the question below. Please help!?
A particular brand of vinegar claimed to 4.3% acetic acid. 10mL of the vinegar was diluted to make a 100mL solution. The claim was tested using a titration where 16mL of 0.1M sodium hydroxide was needed to neutralise 25mL of the 10% vinegar solution. Calculate the molarity (moles/L) and the percentage (w/v, ie - g/100mL = % ) of acetic acid in the original vinegar. Comment on the 4.3% claim.
Hey! I've attached my answers below. These sorts of questions can be tough, as they require a whole bunch of steps and a lot of maths. Once you've done a few of them, however, you should be able to work through others!
(http://i.imgur.com/Umatd5x.png?1)
Jake
Post by: Loki98 on May 12, 2016, 05:20:52 pm
I have a titration practical coming up to determine the ethanoic acid content in a vinegar sample.
In the marking criteria it asks for "specific equations for the reactions that occur during the lab," but i only know of the neutralisation reaction. What could the other possible equations be?
Also for the conclusion, it asks to "identify area(s) of the lab most likely to cause experimental error and propose solutions to problems encountered in the investigation." I have asked my teacher about errors such as Naoh being hygroscopic and altering the results, the end point not being the same as the equivalence point and how colour changes of the indicator are not instant, I have also asked about the indicator changing the pH of the solution and also how each individual can have a unique colour perception when determining the end point. But, my teacher has rejected all of those suggestions and also said no human errors and equipment errors can be written. What other possible errors and improvements are there? Am i misreading something from the criteria?
Also, what should i write about the validity and accuracy of the experiment?
-Thanks
Post by: jakesilove on May 12, 2016, 06:12:04 pm
Hey,
I have a titration practical coming up to determine the ethanoic acid content in a vinegar sample.
In the marking criteria it asks for "specific equations for the reactions that occur during the lab," but i only know of the neutralisation reaction. What could the other possible equations be?
Also for the conclusion, it asks to "identify area(s) of the lab most likely to cause experimental error and propose solutions to problems encountered in the investigation." I have asked my teacher about errors such as Naoh being hygroscopic and altering the results, the end point not being the same as the equivalence point and how colour changes of the indicator are not instant, I have also asked about the indicator changing the pH of the solution and also how each individual can have a unique colour perception when determining the end point. But, my teacher has rejected all of those suggestions and also said no human errors and equipment errors can be written. What other possible errors and improvements are there? Am i misreading something from the criteria?
Also, what should i write about the validity and accuracy of the experiment?
-Thanks
Hey!
I'm sure that the marking criteria will really only require you to know neutralisation reactions for a Titration prac like this. I wouldn't worry about other areas of Chemistry for that :)
I'll have to think a bit more about your error question, because those sound like all the important errors to me. The dotpoint must mean something else; I feel like the 'area(s) of the lab' part makes it a bit weird and ambiguous, and I don't really know what the suggest. Surely, somehow, it is talking in a more general sense about the actual physical features of the lab. Maybe thinks like the floor being at a slope, causing your reading on the Burrette to be skewed? Air pressure? Humidity? Light causing increased reaction time? Temperature? I really don't know what the suggest, because it feels like you nailed all the important things and we're just pulling at strings.
For validity, you want to talk about whether you actually measured what you were trying to measure. In real terms, that means you want to have controlled all of the variables in your experiment. Things like correctly washing your equipment, keeping the room at a constant temperature, the hydroscopicness of NaOH etc (all of which change the result of your experiment) fall into this category.
For accuracy, you want to talk about your measuring devices. How accurate was your scale, did you account of paralax error, how accurate are your eyes. Essentially, it asks to what extent your measured values are close to the real value.
I'm sorry I couldn't be more helpful for the substance of your question! Maybe ask your teacher to explain the dotpoint; you not understanding it isn't your fault, its theirs.
Jake
Post by: Happy Physics Land on May 12, 2016, 06:43:56 pm
Hey,
I have a titration practical coming up to determine the ethanoic acid content in a vinegar sample.
In the marking criteria it asks for "specific equations for the reactions that occur during the lab," but i only know of the neutralisation reaction. What could the other possible equations be?
Also for the conclusion, it asks to "identify area(s) of the lab most likely to cause experimental error and propose solutions to problems encountered in the investigation." I have asked my teacher about errors such as Naoh being hygroscopic and altering the results, the end point not being the same as the equivalence point and how colour changes of the indicator are not instant, I have also asked about the indicator changing the pH of the solution and also how each individual can have a unique colour perception when determining the end point. But, my teacher has rejected all of those suggestions and also said no human errors and equipment errors can be written. What other possible errors and improvements are there? Am i misreading something from the criteria?
Also, what should i write about the validity and accuracy of the experiment?
-Thanks
Hey Loki!
Reading your description, I'm assuming that you are conducting a titration involving acetic acid and NaOH? Ok, then let's get started!
When the teacher asks you for equations, you need to know 3 equations: (for convenience sake lm not gonna write in states, but you should write them in an exam)
1. The Balanced chemical equation (CH3COOH + NaOH ----> NaCH3COO + H2O)
2. The net ionic equation (CH3COO- + Na+ ----> NaCH3COO)
3. Hydrolysis equations of salt (CH3COO- + H2O ---> CH3COOH + OH-)
Equation 1 and 2 are pretty straight forwards. Equation 3 is also significant because it shows why the endpoint would be slightly basic. Common exam questions would ask you for whether the endpoint is basic, acid or neutral and would ask you to identify a suitable indicator. If your teacher wants to differentiate between band 5 students from band 6 students, they would ask you why the endpoint would be basic. In this case, referring to equation 3 from above, since hydroxide ion is given off, the endpoint would hence be basic.
With your second question identifying the possible errors in the experiment, I would personally have written a human error or equipment error (it's kinda weird how your teacher is completely rejecting these errors). But look if your teacher says none of these can be included, then we have to find some other flaws in the experiment. One thing you can perhaps talk about is impurities from the atmosphere entering the conical flask or the burette and this can cause slight differences. Another thing maybe carrying out titration at the wrong temperature, which would affect experimental outcome because some indicators are quote sensitive to surrounding temperature. I'm a little confused by the meaning of "area(s) of the lab", because Im not too sure whether your teacher is talking about the experiment or the physical places in the lab that would cause error. Can you please enquire the teacher and come back to us on that? That would be helpful.
With the validity and accuracy part its quite easy, I will list some things for you to consider and you can write it out in a formal manner:
Validity:
- the extent to which the experiment tests the hypothesis
- Is there only one independent variable involved? If yes, then the experiment is valid
- Does your methods answer the aim? If yes, then the experiment is valid
- Is there a very significant difference between your result and your hypothesis? If yes, then the experiment is invalid. (This may sound like a point for accuracy, but accuracy is only concerned with relatively small deviations from your hypothesis. A significant deviation would mean that you are doing something wrong)
Accuracy:
- Did you thoroughly clean your equipments? Did you do it for multiple times? If yes, accurate
- Are your instruments correctly calibrated? If yes, accurate
- Is your weighing instrument presenting a precise quantity? (i.e. the value it displays does not fluctuate) If yes, accurate
- Are you biased in your reading of the scale? If yes, inaccurate
- Is the scale on your pipette and burette correct? If yes, accurate
- Are you reading from the bottom of meniscus (i.e. parallax error)? If yes, accurate
Then talk about how to minimise the impact of these inaccuracies if they exist.
Hope my answer helped! Remember to come back to us after you have enquired the teacher about that second point! :D
Best Regards
Happy Physics Land
- Are all other variables controlled? If yes, then the experiment
Post by: Happy Physics Land on May 12, 2016, 06:45:29 pm
Hey!
I'm sure that the marking criteria will really only require you to know neutralisation reactions for a Titration prac like this. I wouldn't worry about other areas of Chemistry for that :)
I'll have to think a bit more about your error question, because those sound like all the important errors to me. The dotpoint must mean something else; I feel like the 'area(s) of the lab' part makes it a bit weird and ambiguous, and I don't really know what the suggest. Surely, somehow, it is talking in a more general sense about the actual physical features of the lab. Maybe thinks like the floor being at a slope, causing your reading on the Burrette to be skewed? Air pressure? Humidity? Light causing increased reaction time? Temperature? I really don't know what the suggest, because it feels like you nailed all the important things and we're just pulling at strings.
For validity, you want to talk about whether you actually measured what you were trying to measure. In real terms, that means you want to have controlled all of the variables in your experiment. Things like correctly washing your equipment, keeping the room at a constant temperature, the hydroscopicness of NaOH etc (all of which change the result of your experiment) fall into this category.
For accuracy, you want to talk about your measuring devices. How accurate was your scale, did you account of paralax error, how accurate are your eyes. Essentially, it asks to what extent your measured values are close to the real value.
I'm sorry I couldn't be more helpful for the substance of your question! Maybe ask your teacher to explain the dotpoint; you not understanding it isn't your fault, its theirs.
Jake
Man the one time that I actually get to answer a chem question and you beat me to it
Post by: RuiAce on May 12, 2016, 08:28:01 pm
Man the one time that I actually get to answer a chem question and you beat me to it
I'm just too overloaded with work right now or I'd address every chemistry question. For now, I'm focusing on my specialty (even though it's not my top interest).
Post by: cajama on May 12, 2016, 09:03:33 pm
I'm not sure if this has been asked before but I do three sciences; Chemistry, Biology and Physics and I often get my scientists mixed up between the subjects (especially with Chemistry and Biology). What information of the scientist is sufficient for me to know?
Post by: jakesilove on May 12, 2016, 09:31:46 pm
Hi!
I'm not sure if this has been asked before but I do three sciences; Chemistry, Biology and Physics and I often get my scientists mixed up between the subjects (especially with Chemistry and Biology). What information of the scientist is sufficient for me to know?
Hey!
I can't really help you with Biology, but in terms of learning scientist information in general, you don't need to know very much detail. If it's an important dot-point, tested often (eg. Physicist in the Space section of the curriculum), then you should know at least some history, their contribution to the scientific field, and maybe some extra facts the throw in and sound smart. This applies similarly to the Haber process for Chemistry. When it comes to less important dotpoints, almost never tested (eg. that one weird one about a chemist at the start of Chemical Monitoring and Management), you don't need to worry too much. Know what sort of science they're involved with, maybe some chemical processes they use.
Overall, you don't need to know very much. Mainly, know context and contribution. There aren't many dotpoints requiring you to know about the contribution of specific scientists, so hopefully as you learn each one a bit better you won't get them as mixed up!
This all being said, there are some dotpoints that are person-related and require quite a bit of knowledge (as sometimes you can get a 5-7 marker on them). For instance, Westinghouse vs Edison. Basically just look through some past papers and decide how significant that person is; often, you won't need to know very much.
Jake
Post by: cajama on May 12, 2016, 10:15:25 pm
Hey!
I can't really help you with Biology, but in terms of learning scientist information in general, you don't need to know very much detail. If it's an important dot-point, tested often (eg. Physicist in the Space section of the curriculum), then you should know at least some history, their contribution to the scientific field, and maybe some extra facts the throw in and sound smart. This applies similarly to the Haber process for Chemistry. When it comes to less important dotpoints, almost never tested (eg. that one weird one about a chemist at the start of Chemical Monitoring and Management), you don't need to worry too much. Know what sort of science they're involved with, maybe some chemical processes they use.
Overall, you don't need to know very much. Mainly, know context and contribution. There aren't many dotpoints requiring you to know about the contribution of specific scientists, so hopefully as you learn each one a bit better you won't get them as mixed up!
This all being said, there are some dotpoints that are person-related and require quite a bit of knowledge (as sometimes you can get a 5-7 marker on them). For instance, Westinghouse vs Edison. Basically just look through some past papers and decide how significant that person is; often, you won't need to know very much.
Jake
Ahh thanks! ;D
Post by: Happy Physics Land on May 12, 2016, 10:40:33 pm
Hi!
I'm not sure if this has been asked before but I do three sciences; Chemistry, Biology and Physics and I often get my scientists mixed up between the subjects (especially with Chemistry and Biology). What information of the scientist is sufficient for me to know?
Hey Cajama:
Adding onto Jake's point, in chemistry there is actually a syllabus dotpoint which demands for students to know about the works of different chemists and the importance of their collaboration. To satisfy this dotpoint you would need to be familiarised with the various fields of chemistry such as analytical chemists, industrial chemists, environmental chemists, and radiography chemists. It is also recommended that you look up the techniques each one would use in their individual specialties. For example, analytical chemists use the technique of AAS (Atomic Absorption Spectroscopy) to qualitatively detect the presence of lead in water. It is also recommended that you know the name of a chemist that works for a particular field (This actually came up in a HSC exam a few years ago) and you would be required to talk about the details of the works that relate to the chemist.
Post by: smiley2101 on May 15, 2016, 12:16:37 pm
Post by: RuiAce on May 15, 2016, 03:41:05 pm
why is the answer B? thank you!!!!! :)
Using n=m/M, for every 1 gram of substance x we have y moles of it:
x is CO, y = 1/28.01 mol
x is CH4, y = 1/16.042 mol
x is C2H2, y = 1/26.036 mol
x is C2H6, y = 1/30.068 mol
So since the heat q released of reaction is just enthalpy change * moles:
CO: 1/28.01 * 233 = 8.318 kJ
CH4: 1/16.042 * 890 = 55.479 kJ
C2H2: 1/26.036 * 1300 = 49.93 kJ
C2H2: 1/30.068 * 1560 = 51.88 kJ
So the answer is B
Post by: ProfLayton2000 on May 16, 2016, 06:00:23 pm
So in the acidic env. topic we were told that phosphoric acid is weak, nitric acid is strong etc. Is there a definite list of strong/weak acids/bases we need to know for the HSC course?
Post by: jakesilove on May 16, 2016, 06:56:04 pm
(Hydrogen Iodide) there,
So in the acidic env. topic we were told that phosphoric acid is weak, nitric acid is strong etc. Is there a definite list of strong/weak acids/bases we need to know for the HSC course?
Hey!
Yep there certainly is! The acids that you need to know are strong acids are Nitric acid, Sulfuric acid and Hydrochloric acid. You can assume that any other acid you are given is weak (there are very few strong acids!).
Similarly, when it comes to strong bases you only really need to know that Sodium Hydroxide is weak.
Hope that helps!
Post by: RuiAce on May 16, 2016, 09:01:07 pm
Hey!Whoops.
Yep there certainly is! The acids that you need to know are strong acids are Nitric acid, Sulfuric acid and Hydrochloric acid. You can assume that any other acid you are given is weak (there are very few strong acids!).
Similarly, when it comes to strong bases you only really need to know that Sodium Hydroxide is weak.
Hope that helps!
@ProfLayton2000
Anyway, as Jake has pointed out, the main ones that you need to know are:
Strong acids:
HNO3
HCl
H2SO4
However, it should be quite obvious as to why HBr and HI are also strong. (What may be surprising instead, is that HF is in fact, weak. This occurs due to the fact fluorine is the most electronegative element.)
Strong bases are simple. Group I hydroxides and that is it.
Post by: Neutron on May 16, 2016, 09:14:54 pm
I was learning about coordinate covalent bonds today and I was wondering how you can tell which pairs form a product that does contain a coordinate covalent bond? Sorry if this seems simple but here's an example of a question:
Which of the following pairs form a product containing a coordinate covalent bond?
a) Ca and 2H+
b) H2O and H+
c) Ag+ and Cl-
d) NH4+ and OH-
Thank you! you're all amazing :D
Neutron
Post by: RuiAce on May 16, 2016, 09:35:27 pm
Hey guys!
I was learning about coordinate covalent bonds today and I was wondering how you can tell which pairs form a product that does contain a coordinate covalent bond? Sorry if this seems simple but here's an example of a question:
Which of the following pairs form a product containing a coordinate covalent bond?
a) Ca and 2H+
b) H2O and H+
c) Ag+ and Cl-
d) NH4+ and OH-
Thank you! you're all amazing :D
Neutron
The hydronium H3O+ is known to contain a coordinate covalent bond.
Otherwise, the process of elimination works as follows:
c) is definitely wrong as AgCl is an example of an ionic compound
a) is an impossibility.
d) is wrong because NH4+ and OH- will react in a neutralisation reaction to produce H2O and NH3
Therefore the answer must be b)
Post by: katherine123 on May 17, 2016, 09:38:47 pm
the ans: include adding HNO3 and i dont get the purpose of that isnt acid added just for like metal carbonates
don't we normally identify what ions are present in the solution rather than what type of compound it is
2. do we differentiate Fe(OH)2 and Fe(OH)3 because Fe(OH)2 goes from white to brown and then Fe(OH)3 stays consistently brown
3.the solubility rules differ across different sources
i got mine from the matrix website but i dont know if its a good idea
Post by: RuiAce on May 17, 2016, 10:06:21 pm
1. what is the method to identify that it's CaSO4(s)
the ans: include adding HNO3 and i dont get the purpose of that isnt acid added just for like metal carbonates
don't we normally identify what ions are present in the solution rather than what type of compound it is
2. why does Fe(OH)2 (s) turn from white to brown
Not enough info
If these are your cation/anion tests, you need to give the whole entire question.
(For Q1 however, if that IS the entire question, then CaSO4 can be VERIFIED by an acid test, but not IDENTIFIED.)
Post by: RuiAce on May 19, 2016, 10:36:38 pm
Q2: The hydroxide test to determine which iron ion (lol) is present is a bit ambiguous.
In the laboratory, sometimes what happens is that Fe2+ shows up in a colour it's not meant to show up. Fe3+ is technically meant to stay brown as you stated, but I think when I did the test for Fe2+ I actually ended up with green.
Q3: ALWAYS keep in mind that multiple cation have MORE THAN ONE acceptable test pathway to determine its nature. Some sources refer to "secondary tests" because they provide further evidence to what they call a "primary test"
Post by: Johny1234567 on May 20, 2016, 11:09:44 pm
I know how to do it with concentrated sulfuric acid, but the aqueous symbol for dilute throws me off since I'm not quite sure about the oxidation half. Help appreciated :)
Post by: Sheikie on May 21, 2016, 04:31:21 pm
Post by: RuiAce on May 21, 2016, 06:19:25 pm
What is the half-equations illustrating the reaction between iodide ions and dilute sulfuric acid?This is a bit confusing. Which galvanic cell are you interested in? Or what's the full question.
I know how to do it with concentrated sulfuric acid, but the aqueous symbol for dilute throws me off since I'm not quite sure about the oxidation half. Help appreciated :)
Alternatively, give the half equations to concentrated sulfuric acid for a reference point for me
hey, I was wondering what is the best way to study for chemistry pracs?
Studying for pracs is a matter of:
- Knowing the aim
- Knowing the method
- Only knowing results if its relevant (however this is often more than not)
- Knowing limitations to the experiment
- Identifying controls/variables, as well as relating validity/reliability/discussion
Post by: jakesilove on May 21, 2016, 06:47:15 pm
hey, I was wondering what is the best way to study for chemistry pracs?
Hey!
I think that, generally, there is little to study when it comes to chemistry pracs. Most of the time, you aren't being assessed on specific, actual knowledge of the course; rather, on your analytical skills, your ability to form a discussion and sometimes your ability to do calculations.
Other than being comfortable with a titration, in case you are asked to do one, I think that the main this to worry about is being able to write balanced chemical formulas, use equations from throughout the course, and understanding what accuracy, reliability and validity are. This latter point is especially important, in case you are asked (as often occurs) to write out a discussion for your prac.
You need to be able to write a risk assessment (risk, precaution, response) and be able to graph any results you find.
I think that's about it; I didn't elaborate on each of the above notes because I figured that you probably understand most of them, but if there is any part of this answer that I can expand on (accuracy, reliability and validity for example) please let me know! Happy to help out however I can.
Jake
Post by: RuiAce on May 21, 2016, 07:06:01 pm
Hey!
I think that, generally, there is little to study when it comes to chemistry pracs. Most of the time, you aren't being assessed on specific, actual knowledge of the course; rather, on your analytical skills, your ability to form a discussion and sometimes your ability to do calculations.
Other than being comfortable with a titration, in case you are asked to do one, I think that the main this to worry about is being able to write balanced chemical formulas, use equations from throughout the course, and understanding what accuracy, reliability and validity are. This latter point is especially important, in case you are asked (as often occurs) to write out a discussion for your prac.
You need to be able to write a risk assessment (risk, precaution, response) and be able to graph any results you find.
I think that's about it; I didn't elaborate on each of the above notes because I figured that you probably understand most of them, but if there is any part of this answer that I can expand on (accuracy, reliability and validity for example) please let me know! Happy to help out however I can.
Jake
Kinda need both in my opinion. I distinctively recall panicking when a past paper question said "outline a experiment to produce an ester in a laboratory"
Post by: jakesilove on May 21, 2016, 07:14:39 pm
Kinda need both in my opinion. I distinctively recall panicking when a past paper question said "outline a experiment to produce an ester in a laboratory"
I totally agree in terms of needing to know the actual pracs etc. when it comes to examination, but I assume the question was about an in-class practical task? A prac exam?
Jake
Post by: RuiAce on May 21, 2016, 08:56:52 pm
I totally agree in terms of needing to know the actual pracs etc. when it comes to examination, but I assume the question was about an in-class practical task? A prac exam?
Jake
Oh touche. It does give off that vibe.
I generally just assume the final exam though because what you have to memorise for that overlaps into what you have to know for a practical exam anyway
Post by: therealqwerty on May 23, 2016, 08:13:22 pm
Post by: RuiAce on May 23, 2016, 08:52:51 pm
Can you explain how to do this question, thanks :)Should be Q and R I reckon.
Q is the most reactive because not only does it burn in oxygen and react with acid, but it even reacts with water. (It might be aluminium)
R is the least reactive because it just reacts slowly with oxygen
S is in the middle
So the difference in Eo values for Q and R should be the greatest
Post by: jakesilove on May 23, 2016, 09:19:41 pm
Should be Q and R I reckon.
Q is the most reactive because not only does it burn in oxygen and react with acid, but it even reacts with water. (It might be aluminium)
R is the least reactive because it just reacts slowly with oxygen
S is in the middle
So the difference in Eo values for Q and R should be the greatest
Yep I agree! You always want to look for a reaction between a substance highest on the standard reduction potential table, and a substance lowest on the standard reduction potential table. As this table is also a marker of reactivity, you want a substance that is very reactive, and a substance that is not very reactive, to make the best possible galvanic cell.
Jake
Post by: therealqwerty on May 24, 2016, 07:48:01 am
Should be Q and R I reckon.
Q is the most reactive because not only does it burn in oxygen and react with acid, but it even reacts with water. (It might be aluminium)
R is the least reactive because it just reacts slowly with oxygen
S is in the middle
So the difference in Eo values for Q and R should be the greatest
Thanks :)
Post by: angela99 on May 24, 2016, 06:53:07 pm
any tips or layouts?
Post by: RuiAce on May 24, 2016, 07:28:09 pm
my teacher constantly tells me that my prac sections in the exam are my weakest.. im not really sure how to study the pracs we do in class or how to write notes on them
any tips or layouts?
Studying for pracs is a matter of:
- Knowing the aim
- Knowing the method
- Only knowing results if its relevant (however this is often more than not)
- Knowing limitations to the experiment
- Identifying controls/variables, as well as relating validity/reliability/discussion
Hey!...
I think that, generally, there is little to study when it comes to chemistry pracs. Most of the time, you aren't being assessed on specific, actual knowledge of the course; rather, on your analytical skills, your ability to form a discussion and sometimes your ability to do calculations.
Other than being comfortable with a titration, in case you are asked to do one, I think that the main this to worry about is being able to write balanced chemical formulas, use equations from throughout the course, and understanding what accuracy, reliability and validity are. This latter point is especially important, in case you are asked (as often occurs) to write out a discussion for your prac.
You need to be able to write a risk assessment (risk, precaution, response) and be able to graph any results you find.
I think that's about it; I didn't elaborate on each of the above notes because I figured that you probably understand most of them, but if there is any part of this answer that I can expand on (accuracy, reliability and validity for example) please let me know! Happy to help out however I can.
Jake
Post by: ProfLayton2000 on June 02, 2016, 09:01:27 pm
Post by: jakesilove on June 02, 2016, 09:32:11 pm
How do you assess the validity of an experiment?
Hey there!
For validity, you're essentially asking yourself did the experiment test what you were trying to test. In other words, did the set up actually answer the aim of the experiment?
This often takes into account things like whether or not you have controlled your variables, whether there may be any impurities in your substances, whether you put the right things in the right place and whether you used the correct kind of glassware. An example would be the experiment where you test the specific heat capacity of specific substances (ethanol etc.). Most of the heat DOESN'T get transferred into the water; most of it is lost to the environment! So, you're not REALLY testing what you're trying to test, greatly limiting the validity of the experiment.
If you have any specific questions, regarding specific pracs, let me know! Hope this helped :)
Jake
Post by: browny2409 on June 04, 2016, 03:15:32 pm
Thanks
Nathan
Post by: RuiAce on June 04, 2016, 04:00:28 pm
Hi, there is a dotpoint about the reliability of the sulphate fertiliser prac - what exactly else would you talk about for reliability other then repeatability?
Thanks
Nathan
I would talk about grammar.
Just kidding :P But not really - in a first hand investigation, repetition is the main requirement for reliability to be achieved. The more you repeat the experiment, the more reliable it is.
As for a dot point, yes. Refer to Chemical Monitoring and Management section 3. The dot points are #3 and #4 of the third column.
(Since your results involve a numeric analysis, if your figures are all over the place then that's clearly not as reliable as figures such as 70g, 72g and 69.5g. So if validity or accuracy crumbles, in this case reliability may also be damaged. This is probably the reason why reliability has a seperate dot point for this practical.)
Post by: relativity1 on June 06, 2016, 02:35:15 pm
-how to improve accuracy/reliability of the molar heat of combustion prac
-why weak acids have lower molar heats of neutralization than strong acids (something to do with ionisation i think)
Post by: Adriaclya on June 06, 2016, 03:12:30 pm
Thanks!
Post by: RuiAce on June 06, 2016, 05:31:59 pm
couple of questions
-how to improve accuracy/reliability of the molar heat of combustion prac
-why weak acids have lower molar heats of neutralization than strong acids (something to do with ionisation i think)
Reliability is trivial. Keep repeating the experiment.
Accuracy can be a bit harder though. For this prac, accuracy ties down to validity a lot. Most of the time your molar heat of combustion prac will be quite noticeably inaccurate because heat loss to the surroundings is so hard to perfectly control. It's almost impossible to create the perfect environment in the laboratory to perform a fully accurate experiment here.
Hi guys, i am really confused about moles, molar masses and such. Can someone please enlighten me?That doesn't really give me much direction as to where you should be headed.
Thanks!
Avogadro's number: 6.022 * 1023 = NA
1 mol of a substance = Avogadro's number times the number of molecules of that substance.
I.e. 1 mol of O2 is the same as saying you have 6.022 * 1023 oxygen molecules present.
The fundamental equation of chemistry is n=m/M where m is the mass of a substance present, and M is the molar mass of said substance. The molar mass can be extracted straight off the periodic table.
Post by: Johny1234567 on June 12, 2016, 01:52:39 am
Critically analyse the environmental concerns posed by the Solvay Process and the way in which these are addressed. (7 marks)
Post by: ProfLayton2000 on June 15, 2016, 05:05:23 pm
"Chemists are involved in monitoring and managing the environment. They deal with the reactants, products and conditions of reactions that occur in the environment around us. To do this they identify and measure chemicals around us. Discuss equipment and methods used by chemists to monitor our environment" (7 marks)
What do you suggest I include in my answer (and could you recommend a structure )?
Post by: jakesilove on June 15, 2016, 08:06:37 pm
Hi there,
"Chemists are involved in monitoring and managing the environment. They deal with the reactants, products and conditions of reactions that occur in the environment around us. To do this they identify and measure chemicals around us. Discuss equipment and methods used by chemists to monitor our environment" (7 marks)
What do you suggest I include in my answer (and could you recommend a structure )?
Hey!
This is an awfully vague, slightly absurd question to be asked. Still, for a 7 marker, it's a beast of a thing so it's important that we are able to tame it! I would probably recommend talking about at least two big hitter issues, discussing what they are, how scientists monitor it and why scientists monitor it.
The most obvious choices are the production of CFCs (and depletion of Ozone) and polluting waterways. In terms of structure, I would start with a quick introduction (Scientists monitor the environment and use chemical equations etc. etc. to protect etc. etc.). Then, throw a subheading "Ozone" in. Talk about what Ozone is, why it's important, what CFCs are, why they were significant, how we detected them, change over time etc. MAKE SURE TO TALK ABOUT EQUIPMENT used to monitor this process: This is specifically in the question, and forgetting to do so could lose you marks. Then, I'd do the same thing with waterways. Be fairly brief; you want to get across the fact that chemicals are scary, and can have huge impacts on our environment, and that monitoring them is the only way to be sure we don't all die. At the same time, mention specific information if you can (dates, equipment, processes). It's possible that talking about two separate things is a bit much; perhaps one would be sufficient. Either way, as long as you get the point across you'll get the marks! If you feel like writing up a response, I'd be happy to take a look at it :)
Let me know if I can elaborate on any of this or answer any other questions!
Jake
Post by: katherine123 on June 17, 2016, 02:37:13 am
Post by: Jakeybaby on June 17, 2016, 08:15:30 am
why is the answer B?It's irrelevant to the fact that one is weak and one is strong. It's due to both acids having the same concentration, therefore will both be titrated by the base at the same rate. The equivalence point will be the same as the same about of base will be needed to reach equivalence.
Post by: jakesilove on June 17, 2016, 09:06:37 am
It's irrelevant to the fact that one is weak and one is strong. It's due to both acids having the same concentration, therefore will both be titrated by the base at the same rate. The equivalence point will be the same as the same about of base will be needed to reach equivalence.
Absolutely on point! Think about it this way: When you're doing the maths (ie. C=n/v, moles=mass/molar mass etc.), the 'strong/weak' nature of the acid doesn't come into play at all. All you care about is the concentration, and the molar ratio, which is exactly the same between the two substances! Thanks for the response Jakey, quality stuff.
Jake
Post by: nimasha.w on June 17, 2016, 11:25:14 pm
Post by: Jakeybaby on June 17, 2016, 11:51:04 pm
hi! i'm stuck on trying to find a ppt test to distinguish between Ca2+ and Ba2+
If you could use a flame test, this would determine the difference, as Ca2+ would produce a 'brick-red' coloured flame whereas Ba2+ would produce a 'yellow-bright green' coloured flame.
Sorry, realised what ppt. means.
You could add NaOH to determine between the two if a precipitate test is the way to go.
Ca2+ will produce a white precipitate, whereas Ba2+ will not produce a precipitate due Ba(OH)2 being soluble.
Post by: RuiAce on June 17, 2016, 11:53:17 pm
hi! i'm stuck on trying to find a ppt test to distinguish between Ca2+ and Ba2+
Add F-
The one that precipitates is the Ca2+
(This is assuming you have to use precipitation. Otherwise a flame test is really easy.)
Post by: m_chan98 on June 19, 2016, 11:43:03 am
Post by: jakesilove on June 19, 2016, 12:00:48 pm
Hi! So I was wondering whether someone could provide just a bit of a summary of what solubility rules I need to know? I've consulted a few different sources but each of them are different. Also, I understand that the compounds are soluble, but soluble in what?? Thanks!!! :)
That's a seriously good question! I memorised the following solubility rules, which are really easy to remember once you start using the pneumonic device outlined below.
(http://i.imgur.com/wF8DyMR.png?1)
Those are the main solubility rules. I think that there are some others (eg. something to do with Silver Nitrate precipitating a white powder specifically) which discuss the COLOUR of the precipitate, but the actual precipitation is all outlined above. I wouldn't be learning more rules than what I've posted above.
As for your second questions; I assume it's just in water. I've tried doing some research, but everything comes up blank. However, you definitely don't need to think about that!
Let me know if I can be more specific/helpful regarding any of this,
Jake
Post by: amandali on June 19, 2016, 12:56:14 pm
CH3COOH(aq) + H2O(l) <---> H3O+(aq) +CH3COO-(aq)
The addition of OH- from NaOH will react with H3O+ which consequently reduces the concentration of H3O+ . This will cause the equilibrium to shift right to increase the concentration of H3O+. Hence, a buffer system is established which minimises pH increase.
The answer given has the equation the other way around so is this one correct as well?
Post by: jakesilove on June 19, 2016, 01:00:29 pm
can you check my answer ?
CH3COOH(aq) + H2O(l) <---> H3O+(aq) +CH3COO-(aq)
The addition of OH- from NaOH will react with H3O+ which consequently reduces the concentration of H3O+ . This will cause the equilibrium to shift right to increase the concentration of H3O+. Hence, a buffer system is established which minimises pH increase.
The answer given has the equation the other way around so is this one correct as well?
Hey! Your equation is definitely fine; as it's an equilibrium, you can write it whichever way you want. The only thing I would add is a brief explanation of acidity/basidity. Whist it may be obvious to you that an increase in H30+ is an increase in H+, which increased acidity (decreasing pH), perhaps throw in a single sentence saying that. This is just to make sure you give a really comprehensive response!
Great answer though,
Jake
Post by: RuiAce on June 19, 2016, 02:41:37 pm
can you check my answer ?
CH3COOH(aq) + H2O(l) <---> H3O+(aq) +CH3COO-(aq)
The addition of OH- from NaOH will react with H3O+ which consequently reduces the concentration of H3O+ . This will cause the equilibrium to shift right to increase the concentration of H3O+. Hence, a buffer system is established which minimises pH increase.
The answer given has the equation the other way around so is this one correct as well?
Also be sure to explicitly say "Le Chatelier's principle" whenever you're referring to equilibrium behaviour.
Post by: jakesilove on June 19, 2016, 02:46:34 pm
Also be sure to explicitly say "Le Chatelier's principle" whenever you're referring to equilibrium behaviour.
Ah yes of course: every single equilibrium answer should have something like "By Le Chatelier's principle (which says that a system undergoing a change with shift in equilibrium to minimise that change)...." etc.
Post by: smiley2101 on June 22, 2016, 04:34:39 pm
Hey guys!! Why is the answer A and not B?Thanks so much !
Post by: RuiAce on June 22, 2016, 05:13:42 pm
This means that the moles of hydrogen gas are used up at TWICE the rate the moles of CO are.
Hence the dotted curve represents the concentration of H2 and the blocked curve represents that of CO, because the concentration of H2 should be lower if it goes down more quickly.
Because the dotted curve is the one that spikes up, we know that H2 was what was added. Not CO.
Post by: anotherworld2b on June 24, 2016, 08:48:31 pm
I was wondering could I ask for help in regards to writing a scientific report (investigation) on molar mass?
I wasn't sure if I could ask this here :/
Design an investigation to determine the composition and molar mass of an unknown white substance
I dissolved the unknown bicarbonate (given info its a bicarbonate) with hydrochloric acid then boiled the substance
using a Bunsen burner to isolate the element. I later used a flame test to identify the substance as sodium bicarbonate
I wasn't sure what to put in the introduction of my report and the depth and what I would need to write to explain the concepts used
I am also confused about how to determine the composition and molar mass of the sodium bicarbonate
I tried to calculate the mass and mole of substances but I'm not sure if I am approaching it properly.
HELP is greatly appreciated TT ^ TT
1. Mass of tube + NaHCO3
51.6g
2. Mass of tube
47.6g
3. Mass of NaHCO3
4g
4. Moles of NaHCO3
0.0459 gmol -1
5. Mass of tube + NaCl
50.7g
6. Mass of NaCl
3.1g
7. Moles of NaCl
0.0530 gmol -1
Mass of heating basin + water
71.5g
Mass of water
20.8g
Mass of carbon dioxide
23.9g
Ratio: Moles NaCl /Moles NaHCO3
0.0530 : 0.0459
Mass of NaCo3 = 51.6 - 47.6 = 4g (used in experiment)
Mass of NaHCO3 = 22.98977 + 1.00794 + 12.0107 + (15.9994x3)
= 22.98977 + 1.00794 + 12.0107 + 47.9982
= 84.00661
Mole of NaHCO3 = 4/ 84.00661 = 0.0459
Post by: jakesilove on June 25, 2016, 02:08:10 pm
Hello :)
I was wondering could I ask for help in regards to writing a scientific report (investigation) on molar mass?
I wasn't sure if I could ask this here :/
Design an investigation to determine the composition and molar mass of an unknown white substance
I dissolved the unknown bicarbonate (given info its a bicarbonate) with hydrochloric acid then boiled the substance
using a Bunsen burner to isolate the element. I later used a flame test to identify the substance as sodium bicarbonate
I wasn't sure what to put in the introduction of my report and the depth and what I would need to write to explain the concepts used
I am also confused about how to determine the composition and molar mass of the sodium bicarbonate
I tried to calculate the mass and mole of substances but I'm not sure if I am approaching it properly.
HELP is greatly appreciated TT ^ TT
1. Mass of tube + NaHCO3
51.6g
2. Mass of tube
47.6g
3. Mass of NaHCO3
4g
4. Moles of NaHCO3
0.0459 gmol -1
5. Mass of tube + NaCl
50.7g
6. Mass of NaCl
3.1g
7. Moles of NaCl
0.0530 gmol -1
Mass of heating basin + water
71.5g
Mass of water
20.8g
Mass of carbon dioxide
23.9g
Ratio: Moles NaCl /Moles NaHCO3
0.0530 : 0.0459
Mass of NaCo3 = 51.6 - 47.6 = 4g (used in experiment)
Mass of NaHCO3 = 22.98977 + 1.00794 + 12.0107 + (15.9994x3)
= 22.98977 + 1.00794 + 12.0107 + 47.9982
= 84.00661
Mole of NaHCO3 = 4/ 84.00661 = 0.0459
Hey!
I have to admit, I'm not 100% sure what this investigation is actually asking you to do. Looks like you're having particular trouble with the introduction, depth, and explanation, so I'll just generally talk about that here. Hope it helps!
You don't really need an introduction for a scientific report. You want to have an aim, a hypothesis, equipment, a method, results (potentially calculations) and a discussion. Don't worry too much about having an introduction; just make sure to include the above listed features.
In terms of depth, just make sure your method includes every step that you used, including recording data, washing glassware, repetition etc. You don't really need to explain WHY you do anything until the discussion.
In the discussion, definitely talk about your method. Particularly, outline the solubility rules/flame tests that you knew to be true, and why you used them in this specific example. You basically want to go through accuracy, reliability and validity in every discussion. Talk about the flaws of the experiment, where sources of error lie, that kind of thing. This really comes down to your interpretation of the question, and I don't think I have enough information to properly give you advice for that.
Just make sure to go through the key components of a standard scientific paper. Make sure to include tables, graphs etc. with all the correct units, and make sure that your method is in past tense. Those sorts of things just make everything seem super professional. Your calculations seem right, but that's only a tiny part of an experimental report. Just as an example, check out the report someone else posted here!
Let me know if you have other specific questions; sorry that I have been extremely vague and general with my response, I'm just not sure what you have or haven't done yet.
Jake
Post by: Maz on June 25, 2016, 09:19:45 pm
could you please help me?
I have a fuel cell test on Monday and I was just wondering if you had any idea what kind of questions they generally ask. It is on the hydrogen-oxygen and methanol fuel cell specifically :)
Also,
Would you happen to know if there are any variations in electrolytes used (if any) and chemical reasons for these variations?
I'd really appreciate any ideas
Thankyou :)
Post by: Happy Physics Land on June 25, 2016, 11:23:18 pm
Hey,
could you please help me?
I have a fuel cell test on Monday and I was just wondering if you had any idea what kind of questions they generally ask. It is on the hydrogen-oxygen and methanol fuel cell specifically :)
Also,
Would you happen to know if there are any variations in electrolytes used (if any) and chemical reasons for these variations?
I'd really appreciate any ideas
Thankyou :)
Hey mq123! Thanks for asking (it's been a long time since I lasted posted in the forums)
I didnt really do fuel cell for my secondary investigation on batteries, I will try my best to help you here but if you dont seem to benefit from it Im extremely sorry and I hope someone more professional can be here to provide better advices.
In direct methanol fuel cell:
- Fundamental mechanism involves the exchange of protons
- Main fuel being methanol, which offers the advantage of being portable, energy-dense and chemically stable under normal temperature and pressure
- Methanol is reacted to hydrogen through steam reforming
- Common operation temperature 50 - 120 degrees celsius
- Know about how methanol cross-over reduces battery efficiency
- CO2 produces at the anode, negatively impacting the environment
- Anode equation: CH3OH + H2O --> CO2+6H[/sup]+[/sup] + 6e-
- Cathode equation: 6H+ + 6e- --> 3H2
- Overall cell equation: CH3OH + H2O --> CO2 + 3H2
- Know about the structure and purpose of: Oxygen flow field, anode/cathode backing, anode/cathode current collector
- Requires a continuous source of fuel and oxygen to sustain the chemical reaction, hence usually used in an ambient condition
In Hydrogen-Oxygen Cell:
- Anode equation: 2H2 --> 4H+ + 4e-
- Cathode equation: O2 + 4e- + 4H+ --> 2H2O
- Overall cell equation: 2H2 + O2 --> 2H2O
- Energy is released in the form of heat and electricity
- Unused H2 is circulated/reused for maximum efficiency
- Proton exchange reaction
- Know the purpose of proton exchange membrane
- Know the catalyst used at both the cathode and anode
Common exam questions will often involve:
- asking about the structure/composition of the battery
- asking about the chemistry behind the functioning of battery (I.e. need to state anode and cathode equations, overall equation and the working process that leads to redox reaction)
- asking about the impact of the battery on society and environment
- asking about the applications/practicality of the battery, advantages and disadvantages
- asking about cost and compare this cost to the more common dry cell batteries
Hope this helps! :)
Best regards
Happy Physics Land
Post by: anotherworld2b on June 28, 2016, 01:58:26 pm
Could I get a reply as soon as possible? I have to hand in my assignment tomorrow :'(
We used 4g of NaHCO3 and got 3.1g of NaCl
NaHCO3(s) + HCl(aq) → NaCl (s) + H2O(l) + CO2(g)
Info I have: ( not entirely sure its correct)
1. Mass of tube + NaHCO3
51.6g
2. Mass of tube
47.6g
3. Mass of NaHCO3
4g
4. Moles of NaHCO3
0.0459 gmol -1
5. Mass of tube + NaCl
50.7g
6. Mass of NaCl
3.1g
7. Moles of NaCl
0.0530 gmol -1
Mass of heating basin + water
71.5g
Mass of water
20.8g
Mass of carbon dioxide
23.9g
Ratio: Moles NaCl /Moles NaHCO3
0.0530 : 0.0459
Post by: amandali on June 28, 2016, 08:38:09 pm
In this option you studied one natural product that was not a fossil fuel. Describe the issues associated with shrinking world supplies of this natural product, and evaluate progress being made to solve the problems identified. 7 marks
A natural product is one that is used with little or no modification. An example include raw rubber which is a naturally occurring addition polymer formed from the polymerisation of 2-methyl-1,3-butadiene (isoprene) and obtained from sap of rubber trees. Many issues arise as its supply starts to shrink thus a replacement is required. In the early 20th century, the demand for rubber outstripped supply as rubber trees can only produce a certain amount of rubber each year. Rubber was needed for tyres for military vehicles thus its limited supply adversely affected countries’ performance at war. In ww2, Japan had control of rubber producing areas which threatened the supply of natural rubber in other countries. Moreover, automobile industry was growing and car manufacturers needed rubber. Due to the rising in demand for rubber, a first replacement for rubber is produced which is Styrene Butadiene rubber (SBR) formed from monomers of 1,3-butadiene and styrene. It is vulcanised with short Sulfur chains forming cross-links between polymer chains. It is more favourable than raw rubber as it has the improved properties ie. more durable, more resistant to chemical attack, and stronger. The progress made solved the problems as it increased the supply of rubber such that its demand is being met and cost is able to be maintained low.
Post by: jakesilove on June 28, 2016, 11:14:13 pm
Hi I was wondering could I get help in calculating the molar mass and composition of sodium bicarbonate?
Could I get a reply as soon as possible? I have to hand in my assignment tomorrow :'(
We used 4g of NaHCO3 and got 3.1g of NaCl
NaHCO3(s) + HCl(aq) → NaCl (s) + H2O(l) + CO2(g)
Info I have: ( not entirely sure its correct)
1. Mass of tube + NaHCO3
51.6g
2. Mass of tube
47.6g
3. Mass of NaHCO3
4g
4. Moles of NaHCO3
0.0459 gmol -1
5. Mass of tube + NaCl
50.7g
6. Mass of NaCl
3.1g
7. Moles of NaCl
0.0530 gmol -1
Mass of heating basin + water
71.5g
Mass of water
20.8g
Mass of carbon dioxide
23.9g
Ratio: Moles NaCl /Moles NaHCO3
0.0530 : 0.0459
Hey,
Unfortunately, I can't do calculations for you, and I won't be able to get to this properly tonight. I definitely can't do your assignments for you. All I can recommend is having a proper think about what you're actually trying to find, and utilising the equations at your disposal to the best of your ability.
Good luck,
Jake
Post by: jakesilove on June 29, 2016, 10:22:36 am
can you check my response thanks
In this option you studied one natural product that was not a fossil fuel. Describe the issues associated with shrinking world supplies of this natural product, and evaluate progress being made to solve the problems identified. 7 marks
A natural product is one that is used with little or no modification. An example include raw rubber which is a naturally occurring addition polymer formed from the polymerisation of 2-methyl-1,3-butadiene (isoprene) and obtained from sap of rubber trees. Many issues arise as its supply starts to shrink thus a replacement is required. In the early 20th century, the demand for rubber outstripped supply as rubber trees can only produce a certain amount of rubber each year. Rubber was needed for tyres for military vehicles thus its limited supply adversely affected countries’ performance at war. In ww2, Japan had control of rubber producing areas which threatened the supply of natural rubber in other countries. Moreover, automobile industry was growing and car manufacturers needed rubber. Due to the rising in demand for rubber, a first replacement for rubber is produced which is Styrene Butadiene rubber (SBR) formed from monomers of 1,3-butadiene and styrene. It is vulcanised with short Sulfur chains forming cross-links between polymer chains. It is more favourable than raw rubber as it has the improved properties ie. more durable, more resistant to chemical attack, and stronger. The progress made solved the problems as it increased the supply of rubber such that its demand is being met and cost is able to be maintained low.
Hey! I didn't do this area of Chemistry, but I think your answer is spot on! The only thing I would mention is that the last part of the question asks you to 'evaluate'. When you talk about SBR, you've mentioned its benefits, but are there any detrimental aspects? Impact on the environment, cost effectiveness, timeline etc? It is good to mention something like this, even if you decide that, overall, the process is beneficial.
Jake
Post by: zoeh on June 29, 2016, 05:30:06 pm
I just have a few questions about electrolysis because it wasn't really explained well in class…
When the electrodes used are inert how does this impact on the reaction? Is the anode and cathode still the 2 electrodes? And where does the oxidation occur? For example if graphite electrodes were used in copper sulfate electrolyte would copper reduce at the cathode and what would be oxidised?
thanks so much
Post by: jakesilove on June 29, 2016, 09:04:01 pm
Hi,
I just have a few questions about electrolysis because it wasn't really explained well in class…
When the electrodes used are inert how does this impact on the reaction? Is the anode and cathode still the 2 electrodes? And where does the oxidation occur? For example if graphite electrodes were used in copper sulfate electrolyte would copper reduce at the cathode and what would be oxidised?
thanks so much
Hey! Basically, if an electrode is inert, it is non-reactive (bit of a tautology there, sorry). It won't oxidise or reduce at any point in the reaction. Generally, Graphite or Platinum electrodes are used to signify that the electrode is to be inert. However, something else needs to be happening for this cell to move forward.
An inert electrode acts only as a location for electrons to move from one place to another, and thus for some kind of oxidation/reduction to occur. For instance, if you had a half-cell with a platinum electrode in water, the platinum doesn't actually do anything; it is the water that is oxidised or reduced! This goes for any solution in which an inert electrode is placed (the standard one used in the HSC, I believe, is bubbling Hydrogen gas through a liquid. The gas attaches to the Platinum electrode, is oxidised or reduced by the inflow/outflow of electrons, and changes states accordingly).
I hope that this clears things up for you! If you see an inert electrode, you need to look for something else to move the reaction forward. Then, compare this other substance to the other half cell, using your table of reduction potentials, in order to determine which side oxidises and which side reduces (as with any standard galvanic cell).
Let me know if I can clear any of this/anything else up!
Jake
Post by: zoeh on June 29, 2016, 10:58:24 pm
Hey! Basically, if an electrode is inert, it is non-reactive (bit of a tautology there, sorry). It won't oxidise or reduce at any point in the reaction. Generally, Graphite or Platinum electrodes are used to signify that the electrode is to be inert. However, something else needs to be happening for this cell to move forward.Thanks so much that clears it up! :)
An inert electrode acts only as a location for electrons to move from one place to another, and thus for some kind of oxidation/reduction to occur. For instance, if you had a half-cell with a platinum electrode in water, the platinum doesn't actually do anything; it is the water that is oxidised or reduced! This goes for any solution in which an inert electrode is placed (the standard one used in the HSC, I believe, is bubbling Hydrogen gas through a liquid. The gas attaches to the Platinum electrode, is oxidised or reduced by the inflow/outflow of electrons, and changes states accordingly).
I hope that this clears things up for you! If you see an inert electrode, you need to look for something else to move the reaction forward. Then, compare this other substance to the other half cell, using your table of reduction potentials, in order to determine which side oxidises and which side reduces (as with any standard galvanic cell).
Let me know if I can clear any of this/anything else up!
Jake
Post by: conic curve on June 29, 2016, 10:58:35 pm
Thanks
Post by: anotherworld2b on June 30, 2016, 09:51:32 am
I was unsure how to calculate molar mass, moles and ect and what formula I needed to use.
I am still quite confused about molar mass and moles calculations in general.
Could I have help in understanding when to use what formula and where to use it?
Post by: jakesilove on June 30, 2016, 08:31:16 pm
What are the most important things in prelim chem (other than chemical equations and moles) which are important in HSC chem (and will reappear in HSC chem)?
Thanks
There is very little actual crossover between the courses. In fact, I'd hesitate to say that there is absolutely none, whatsoever. What is important is a good understanding of chemical reactions (ie. Neutralization reactions, that kind of thing), a working knowledge of chemical terms (solutions, mixtures, moles, gas etc.) and not much else. Don't worry too much about the content you are learning now, and its relation to the HSC course. Just learn it for your exams, and then move on to the HSC curriculum later this year!
Jake
Post by: jakesilove on June 30, 2016, 08:44:15 pm
I'm sorry if my question seemed like I was taking advantage of this thread.
I was unsure how to calculate molar mass, moles and ect and what formula I needed to use.
I am still quite confused about molar mass and moles calculations in general.
Could I have help in understanding when to use what formula and where to use it?
That's not a problem :)
Okay, so let's do a brief refresher of moles, molar mass and mass, and how to calculate each from the other.
A mole is a collection of atoms (in fact, it is an exact number of atoms, Avagadro's number. This isn't important in the HSC, but knowing this makes understanding the concepts a lot easier). 2 moles is two times Avagadro's number of atoms, 1/2 a mole is 50% Avagadro's number etc. As you will imagine, each element's atom will have a different mass. So, if a single atom of Nitrogen was heavier than a single atom of Helium, you would expect a mole of Nitrogen to weigh more than a single mole of Helium (despite them having the same number of atoms!). The molar mass of a substance is the mass of one mole of the element.
So, let's put this all together into some calculations. The formula for relating all of this is
So, if we had 3g of Carbon, which has a molar mass of approximately 12g, then we have 1/4 moles of Carbon!
Easy.
Now, there's some more complicated maths that I think your assignment required you to do. Say you had a reaction between two substances. You can balance the equation easily, by adding numbers out the front of each reactant and product to make sure the same number of elements/atoms are on each side. A 2 signifies 2 moles, a 1 signifies 1 mole etc. Now, say you had a reaction that looks something like this:
We know that for any two moles of A, we need 1 mole of B to react, which will produce 1 mole of C. Therefore, if we know we have 2g of A, we can use the above formula to figure out how many moles that is (using the molar mass). We then know we need HALF as many moles of B, and can figure out the relevant mass of B (using it's molar mass). This is stuff that's really important to the HSC syllabus; I'm not sure what level you're at but hopefully this sounds familiar.
That's the basics of the whole thing. I could go into much more depth, but that really is a bulk of the skills in this course. If what I've written above doesn't make sense, or you're having trouble with anything, please feel free to post specific questions because at the moment it's hard to gauge where you're having problems. If none of this is making any sense, I would talk to your teacher about having a few lessons with them. It's a big topic area, and it's important you understand what's going on.
Hope that this helped, at least a little.
Jake
Post by: MysteryMarker on July 02, 2016, 09:03:54 pm
Which Reaction involved the formation of a coordinate covalent bond?
a) H+ + OH- → H2O
b) Mg + 2H+ →Mg2+ + H2
c) C2H4 + H2 → C2H6
d) CH4 + Br2 → CH3Br + HBr
I originally thought the answer was b as the magnesium atom technically 'provides' both electrons to form the bonding pair for the hydrogen atom.
Cheers broskies.
Post by: RuiAce on July 03, 2016, 07:31:06 pm
Hey guys, just got a past trial paper question that i don't really understand. I don;t know what the answer is but i've been told it is A. Can anyone explain to me how this is so, and if possible, maybe through the aid of a lewis electron dot diagram? Thanks Guys.B is wrong because if you react an acid with a metal you get a salt plus the hydrogen gas. E.g.
Which Reaction involved the formation of a coordinate covalent bond?
a) H+ + OH- → H2O
b) Mg + 2H+ →Mg2+ + H2
c) C2H4 + H2 → C2H6
d) CH4 + Br2 → CH3Br + HBr
I originally thought the answer was b as the magnesium atom technically 'provides' both electrons to form the bonding pair for the hydrogen atom.
Cheers broskies.
Mg(s) +2 HCl(aq) → MgCl2(aq) + H2(g)
C is obviously wrong because alkene → alkane cannot possibly become a coordinate bond
D is wrong mainly because it's just the formation of a CFC (or rather, BCFC here).
The answer is A. It should have been established clearly that the species H3O+, that is, the hydronium ion features a coordinate bond between one of the hydrogens and the oxygen. This is because the acid H+ isn't actually present. What's actually present is indeed, the hydronium ion. The more accurate way equation A is written is this:
H3O+ + OH- ⇌ 2 H2O(l)
Post by: trixqwe on July 03, 2016, 10:59:48 pm
Compare an 'amphiprotic' substance and an 'amphoteric' substance. Use examples and equations to support your answer.
Thanks in advance!!
Post by: RuiAce on July 04, 2016, 09:30:42 am
Hi there! I'm a little stuck on how to answer this question and I would appreciate any help :)Technically amphiprotic substances are all amphoteric, but not vice versa. Just like how all squares are rectangles.
Compare an 'amphiprotic' substance and an 'amphoteric' substance. Use examples and equations to support your answer.
Thanks in advance!!
Amphoteric just means it exhibits both acidic and basic properties. You can take Al2O3 and compare two reactions. When aluminium oxide reacts with an acid, it's exhibiting basic properties. When it reacts with a base, it's exhibiting acidic properties. (Don't remember the equations right now, but either Al2O3 or ZnO is included in the Jacaranda textbook.)
Amphiprotic specifically relates to B-L theory. That is, an acid is a proton donor, whereas a base is a proton acceptor. Amphiprotic substances are both. An example is your HCO3- ion (or if that's too inadequate, just consider NaHCO3).
It's easy to show that if you react sodium hydrogen carbonate with hydrochloric acid, you get water, carbon dioxide and sodium carbonate. Whereas if you react it with sodium hydroxide, you get sodium carbonate and water.
Post by: katherine123 on July 06, 2016, 02:27:30 am
i)
1.add 150ml of water into measuring cylinder A and 100ml of water into measuring cylinder B
2.transfer 10ml of water using 10ml pipette from measuring cylinder A to B (forward process)
3.Transfer 5ml of water using 5ml pipette from measuring cylinder B to A (backward process)
4.Repeat step 2 and 3 (one transfer cycle) until the volume of water remain constant in both measuring cylinder (equilibrium)
ii)Information collected is highly valid given that it is a qualitative model of an equilibrium reaction. In this procedure, it can be seen that the process occurs 2 directions (water transferred from measuring cylinder A to B or B to A) which is similar to an equilibrium. It is also observed that the volume of water in both measuring cylinder remain constant after transferring, like an equilibrium with no macroscopic changes. Moreover, no water is removed from the system which is similar to equilibrium which is a closed system
Post by: MysteryMarker on July 06, 2016, 11:53:35 am
The method for determining organic nitrogen within waterways is to convert it into ammonia. The nitrogen concentration is then found through acid-base titration. Is this in enough detail for exams or should i know about how they convert the nitrogen into ammonia? (heat in sulphuric acid and stuff.)
Post by: katherine123 on July 06, 2016, 04:45:36 pm
what info does the polarity of electrode give?
Post by: conic curve on July 06, 2016, 04:51:01 pm
Thanks
Post by: RuiAce on July 06, 2016, 11:04:58 pm
I did part i but not sure how to part ii) what are the negative points?Providing the question could help here, thanks. I am not sure where "negative points" comes from just by looking at the answer.
i)
1.add 150ml of water into measuring cylinder A and 100ml of water into measuring cylinder B
2.transfer 10ml of water using 10ml pipette from measuring cylinder A to B (forward process)
3.Transfer 5ml of water using 5ml pipette from measuring cylinder B to A (backward process)
4.Repeat step 2 and 3 (one transfer cycle) until the volume of water remain constant in both measuring cylinder (equilibrium)
ii)Information collected is highly valid given that it is a qualitative model of an equilibrium reaction. In this procedure, it can be seen that the process occurs 2 directions (water transferred from measuring cylinder A to B or B to A) which is similar to an equilibrium. It is also observed that the volume of water in both measuring cylinder remain constant after transferring, like an equilibrium with no macroscopic changes. Moreover, no water is removed from the system which is similar to equilibrium which is a closed system
Hey guys, just got a question on eutrophication and the methods used to determine the concentration of phosphates and nitrates/nitrogen.Back when I did the course, all I memorised was "Kjeldahl method" and nothing about the specifics at all. It is all that I felt was necessary.
The method for determining organic nitrogen within waterways is to convert it into ammonia. The nitrogen concentration is then found through acid-base titration. Is this in enough detail for exams or should i know about how they convert the nitrogen into ammonia? (heat in sulphuric acid and stuff.)
You may find more useful information from another person.
im quite confused about the electrolysis of molten, concentrated and aqueous of NaClThe polarity of the electrode determines which half-equation is occurring at the electrode. (Always remember that the polarity of the cathode/anode flip for an electrolytic cell; instead of a positive cathode and negative anode, we consider a negative cathode and positive anode.)
what info does the polarity of electrode give?
Recall the mnemonic RED-CAT AN-OX.
This means that the reduction occurs at the cathode, whereas the oxidation occurs at the anode.
We analyse three cases:
Case 1: Molten NaCl.
The only species present are Na+ and Cl-. This means that sodium ions will be reduced to sodium metal, and chloride ions will be oxidised to produce chloride gas.
+'ve anode: 2 Cl- -> Cl2(g) + 2e-
-'ve cathode: Na+ + e- -> Na(s)
Case 2: Aqueous solution of NaCl.
The two species present are as above, and H2O as well now. We use our data sheet to determine what is more likely to occur.
-'ve cathode: -2.71V is required to reduce sodium ions back to sodium. But -0.83V is required to reduce water into hydrogen gas and base (hydroxide ions). Clearly, it will take considerably less voltage for the latter equation to occur. Hence, provide water is present, that will ALWAYS be the reaction.
2 H2O(l) + 2 e- -> 2 OH- + H2(g)
+'ve cathode: -1.36V is required to oxidise chloride ions to chloride gas. -1.23V is required to oxidise water into acid (hydrogen ions) and oxygen gas. There is a voltage difference, however it's quite small, so we cannot say for sure which reaction will occur. We seperate up our cases:
Subcase 1: Concentrated aqueous solution (generally >2M, although some sources say >1M)
There is a good abundance of chloride ions present, hence they will be oxidised.
2 Cl- -> Cl2(g) + 2e-
Subcase 2: Dilute aqueous solution (generally <0.1M)
Not many chloride ions are present at all. The oxidation of water will be more dominant here.
2 H2O(l) -> 4 H+ + O2(g) + 4 e-
(Note: The electrolysis of dilute NaCl solution is basically the electrolysis of water.)
Sometimes with examples in chemical equations, I get confused with the states. Like sometimes h2o it is in liquid state and sometimes it is in gaseous state. How do you know when it's in liquid state and when it's in a gaseous state (using h2o as an example)?
Thanks
Whilst a bit of this will require your knowledge of chemistry (e.g. aluminium reacts specifically with STEAM), more often than not it should be implied as to what state water is in. You may choose to provide examples to voice your confusion, which I can walk through.
Post by: vm1997 on July 07, 2016, 12:56:32 pm
"Describe the tests for any factors that need to be specifically monitored in these areas/with these activities to ensure that we have safe drinking water" I can only name a couple of tests that need to be done for this.
Briefly, what tests can be done for this and what's the chemistry behind them?
"Identify the organisations that monitor the wwaterways in NSW and describe the types of chemistry that is used in the monitoring process" I know a few suc as SAC, but WHAT TESTS DO THEY DO??
Post by: RuiAce on July 07, 2016, 06:35:42 pm
Few questions on Water quality managementThe HSC expects you to know each of the following tests and how we can use it to determine overall water quality.
"Describe the tests for any factors that need to be specifically monitored in these areas/with these activities to ensure that we have safe drinking water" I can only name a couple of tests that need to be done for this.
Briefly, what tests can be done for this and what's the chemistry behind them?
"Identify the organisations that monitor the wwaterways in NSW and describe the types of chemistry that is used in the monitoring process" I know a few suc as SAC, but WHAT TESTS DO THEY DO??
- Concentration of common ions: Easy, just use your cation/anion tests and gravimetric analysis.
- Total dissolved solids: This is just an arbitrary way of saying how much solid is there in the water, measured in ppm. We could use gravimetric analysis but it's not that good since the amount of solid present generally isn't much. So we use the fact that the things dissolved are ions and will conduct electricity - thus we use a conductivity meter
- Hardness: This measures the concentration of Mg2+ and Ca2+. We use EDTA titration here.
- Turbidity: The cloudiness of water (on a macroscopic level) is sort of how clear it is. We take measurements of turbidity using a "turbidity tube". We need to know how turbid the water is to assess the ease of aquatic plants in photosynthesizing as an example.
- Acidity: This is obvious. And we can use a pH probe where possible.
- Dissolved oxygen: This is how much oxygen there is in the water. We use Winkler titration to find out this value. Harmful anaerobic bacteria grow when DO is low.
- Biochemical oxygen demand: By definition, "Biochemical Oxygen Demand is the DO necessary completely breakdown organic matter in the water through aerobic bacteria." To measure BOD, we take a sample of water and record the initial DO. We then leave the sample in the dark for 5 days before taking another value. The difference is the BOD.
I 100% rote-learnt the catchment dot point though. You can find information on that (and also more information on the tests) here. https://www.scribd.com/doc/49281194/The-Student-s-Guide-to-HSC-Chemistry
Post by: ssarahj on July 08, 2016, 06:35:11 pm
You can find information on that (and also more information on the tests) here. https://www.scribd.com/doc/49281194/The-Student-s-Guide-to-HSC-Chemistry
cheers for the link to that guide, it looks super helpful!!! :)
Post by: katherine123 on July 11, 2016, 11:26:57 pm
Providing the question could help here, thanks. I am not sure where "negative points" comes from just by looking at the answer.Back when I did the course, all I memorised was "Kjeldahl method" and nothing about the specifics at all. It is all that I felt was necessary.
forgot to add the pic
Post by: amandali on July 11, 2016, 11:55:23 pm
A natural product is one that is used with little or no modification. An example is raw rubber which is a naturally occurring addition polymer formed from the polymerisation of 2-methyl-1,3-butadiene (isoprene) and obtained from sap of rubber trees. Many issues arose when its supply started to shrink thus a replacement was required. In the early 20th century, the demand for rubber outstripped supply as rubber trees can only produce a certain amount of rubber each year. Rubber was needed for tyres for military vehicles thus its limited supply adversely affected countries’ performance at war. In WW2, Japan had control of rubber producing areas in South East Asia which threatened the supply of natural rubber in other countries. Another issue that caused the rising demand of rubber was due to the growing of automobile industry thus car manufacturers needed more and more rubber. In order to solve the limited supply of rubber, a first replacement for rubber was produced which is Styrene Butadiene rubber (SBR) formed from monomers of 1,3-butadiene and styrene. It is vulcanised with short Sulfur chains forming cross-links between polymer chains. It is more favourable than raw rubber as it has the improved properties ie. more durable, more resistant to chemical attack, and stronger. This progress made to increase supply of rubber has been effective as the demand for synthetic rubber as a total % of rubber is around 80%. Hence, allowing demand of rubber to be met while maintaining a low cost for production. However, synthetic rubber made from petrochemicals are non-biodegradable, therefore production from biodegradable polymers is taken into consideration.
since it says "evaluate progress being made" does the last sentence suffice the negative point of using synthetic rubber or is it okay to just focus on the positives
Post by: RuiAce on July 12, 2016, 08:57:24 am
forgot to add the picIf by negative points you just mean how it might not be invalid, well one thing could be that it doesn't accurately display the rate at which equilibrium is achieved. Two weaker arguments may be that the model also only demonstrates how an equilibrium is achieved, but we don't know if the forward reaction is forcibly going twice as fast (or something) as the reverse reaction. Nor do we know if the final ratio of products to reactants is right.
In this option you studied one natural product that was not a fossil fuel. Describe the issues associated with shrinking world supplies of this natural product, and evaluate progress being made to solve the problems identified. 7 marksYeah. From memory when I wrote about rubber, with the negatives of SBR I just wrote about how it's still being made out of petrochemicals, and why that's bad (which is something you should know from the production of materials topic).
A natural product is one that is used with little or no modification. An example is raw rubber which is a naturally occurring addition polymer formed from the polymerisation of 2-methyl-1,3-butadiene (isoprene) and obtained from sap of rubber trees. Many issues arose when its supply started to shrink thus a replacement was required. In the early 20th century, the demand for rubber outstripped supply as rubber trees can only produce a certain amount of rubber each year. Rubber was needed for tyres for military vehicles thus its limited supply adversely affected countries’ performance at war. In WW2, Japan had control of rubber producing areas in South East Asia which threatened the supply of natural rubber in other countries. Another issue that caused the rising demand of rubber was due to the growing of automobile industry thus car manufacturers needed more and more rubber. In order to solve the limited supply of rubber, a first replacement for rubber was produced which is Styrene Butadiene rubber (SBR) formed from monomers of 1,3-butadiene and styrene. It is vulcanised with short Sulfur chains forming cross-links between polymer chains. It is more favourable than raw rubber as it has the improved properties ie. more durable, more resistant to chemical attack, and stronger. This progress made to increase supply of rubber has been effective as the demand for synthetic rubber as a total % of rubber is around 80%. Hence, allowing demand of rubber to be met while maintaining a low cost for production. However, synthetic rubber made from petrochemicals are non-biodegradable, therefore production from biodegradable polymers is taken into consideration.
since it says "evaluate progress being made" does the last sentence suffice the negative point of using synthetic rubber or is it okay to just focus on the positives
These are not my notes, but basically they were all I memorised for section 1 of industrial chemistry.
(One thing that you will find in these notes at the very end is how progress is being made to develop an alternative to styrene-butadiene rubber, but made out of biomass. This is an example of even further progress to counter the drawbacks of SBR, and may or may not be worth mentioning in your 7-mark response. At this point though, because I'm too out of shape I won't critique your response for now, sorry.)
Post by: conic curve on July 12, 2016, 10:17:41 am
CHeers
Post by: anotherworld2b on July 12, 2016, 07:59:02 pm
Post by: RuiAce on July 12, 2016, 08:28:50 pm
I tried to balance question e but im not sure why i got it wrongLine 2 is correct.
C3H8(g) + 5 O2(g) -> 3 CO2(g) + 4 H2O(g)
You can use WolframAlpha to show you the answer to balancing out the equations. (Just make sure to use an equal sign.)
Could someone here please explain to me the important concepts in the metals module (i.e. Moles, spectator ions, half equations, limiting and excess, ionic equations, etc) I struggle to understand the maths behind this and require a lot of help (it'd be great if you could provide examples as well)Most of this stuff is quite fundamental to chemistry. Make sure you have read through your textbook carefully enough.
CHeers
In SI base units, the measurement of the mole is used to measure the quantity of particles that exist. This is opposed to the mass, which measures how much matter there IS in a particle.
1 mol is defined as the number of carbon atoms in exactly 12 grams of carbon solid. The correct value is approximately 6.022 * 1023 which you can find on your data sheet. This number is famously known as Avogadro's number.
NA = 6.022 * 1023 mol-1
This leads to the fundamental relationship:
where n = moles of a substance (measured in mol)
N = the actual number of atoms there are
NA = Avogadro's number
A number's quantity of particles is directly linked to the mass of the substance present. The relationship is given as
where:
n - moles of a substance present (measured in mol)
m - the mass of a substance given (measured in g for the sake of chemistry)
M - molar mass (measured in g mol-1)
By definition, the molar mass is just the mass divided by the quantity present. This is essentially the above formula rearranged (M=n/m). For historical reasons, it has units g mol-1 instead of kg mol-1.
However, every individual element has a different atomic number and atomic mass. The molar mass will differ depending on what species we have.
The number the molar mass (for monatomic elements) is just its atomic mass. (There is a conversion between atomic mass and molar mass but this we will ignore). Hence, the molar mass for, say, carbon, is 12.01 g mol-1. The molar mass of O is 16.00 g mol-1.
The molar mass of compounds is just that of the individual elements added together. E.g. the molar mass of PbSO4 equals
M(Pb) + M(S) + 4 * M(O)
= 207.2 + 32.07 + 4*16
= 303.27 g mol-1
As for the rest? Asking for all that in the one go is THOROUGHLY exhaustive and you will need to provide questions to set a basis. In the future, please also ask one or two questions (especially if they're too related to the same thing) at a time. Just like with the trigonometry questions - all of those at once was quite overloading.
Post by: Sahar8642 on July 12, 2016, 08:46:29 pm
Need Help for this q.
Its worth 5 marks
A sample of lemon juice is to be analysed in the laboratory. A student took 25.00 mL of the juice
and diluted it to 250.00 mL. Exactly 25.00 mL of the diluted lemon juice is titrated with standard
0.1045 mol L-1
sodium hydroxide solution using phenolphthalein as the indicator. An average titre
of 24.05 mL of sodium hydroxide was required
Assuming that the lemon juice contained only citric acid (molar mass = 192.1 g/mol), calculate
the concentration in mol L-1 of citric acid in the undiluted lemon juice.
Thank You!
Post by: RuiAce on July 12, 2016, 08:58:49 pm
Hey,We always start with an equation. Even if you forget the formula for citric acid (2-hydroxypropane-1,2,3-tricarboxylic-acid) you should be well aware of how it is triprotic.
Need Help for this q.
Its worth 5 marks
A sample of lemon juice is to be analysed in the laboratory. A student took 25.00 mL of the juice
and diluted it to 250.00 mL. Exactly 25.00 mL of the diluted lemon juice is titrated with standard
0.1045 mol L-1
sodium hydroxide solution using phenolphthalein as the indicator. An average titre
of 24.05 mL of sodium hydroxide was required
Assuming that the lemon juice contained only citric acid (molar mass = 192.1 g/mol), calculate
the concentration in mol L-1 of citric acid in the undiluted lemon juice.
Thank You!
The reaction goes to completion as we have a strong base.
C3H4OH(COOH)3(aq) + 3 NaOH(aq) -> C3H4OH(COO)3Na(aq) + 3 H2O(l)
We may determine the (average) moles of NaOH solution.
n(NaOH)
= CV
= 0.1045 mol L-1 * 0.02500 L
= 2.6125 * 10-3 mol
1 mol of NaOH reacts with 1/3 mol of citric acid.
n(C3H4OH(COOH)3) = n(NaOH)/3 = 8.7083333333... * 10-4 mol
This gives us the moles of citric acid used in the reaction. We know that 25 mL of citric acid was used in the reaction. Hence the concentration of the citric acid is
C2
= n/V
= (8.7083333333... * 10-4 mol) / (0.025 L)
= 0.03483333... mol L-1
Notice that this is the concentration of citric acid AFTER dilution. To obtain the concentration BEFORE dilution, we use the concentrations formula C1V1 = C2V2
C1 * 25.00 mL = 0.03483333... mol L-1 * 250.00 mL
C1 = 0.34833333... mol L-1
= 3.483 * 10-1 mol L-1 (4 sig. fig.)
(There may be a slight calculation error somewhere but the method should be good.)
Post by: anotherworld2b on July 12, 2016, 11:28:45 pm
Post by: kristine.faelnar on July 12, 2016, 11:32:00 pm
So I was making my notes for my option topic (Forensics) and I came across this syllabus point and I have no bloody idea what I'm doing. I don't really understand it and it hasn't been gone through much in class.
"Perform a first-hand investigation and gather first-hand information to identify the range of solvents that may be used for chromatography and suggest mixtures that may be separated and identified by the use of these solvents."
Please help!!
Post by: conic curve on July 12, 2016, 11:47:12 pm
Hi guys!!
So I was making my notes for my option topic (Forensics) and I came across this syllabus point and I have no bloody idea what I'm doing. I don't really understand it and it hasn't been gone through much in class.
"Perform a first-hand investigation and gather first-hand information to identify the range of solvents that may be used for chromatography and suggest mixtures that may be separated and identified by the use of these solvents."
Please help!!
I believe that is a practical
Post by: kristine.faelnar on July 12, 2016, 11:53:10 pm
I tried finding notes online as well and I couldn't find anything
Post by: RuiAce on July 13, 2016, 07:49:52 am
How would you do these questions?These were...a bit awkward to read... sideways and upside-down photo
Q4: Not really important at all. One thing to note about the ammonium ion is just the coordinate covalent bond from the nitrogen to the fourth hydrogen ion. Yes, the coordinate bond is to a hydrogen ion (aka. a proton), not a hydrogen atom. This hydrogen ion basically gives that extra +1 charge.
Whereas for hydroxide, the hydroxide ion is formed when water undergoes self ionisation. What happens is that one of the hydrogens in H2O essentially "comes off", but it doesn't take its electron with it. The hydrogen and oxygen were initially covalently bonded, but the hydrogen comes off and leaves its electron with oxygen. This causes the -1 charge.
I have no idea how this relates to the fertiliser practical. Maybe the rest of the textbook could help here.
Post by: conic curve on July 13, 2016, 08:49:59 am
I have never done it before, so I have no idea what to do for that :(
I tried finding notes online as well and I couldn't find anything
Just wait until your teacher does it in class and then you compose the practical
Post by: RuiAce on July 13, 2016, 10:22:34 am
I have never done it before, so I have no idea what to do for that :(If you haven't done a practical yet purely because you're not that far into the syllabus yet, you cannot be asked it in the exam. If you do, take it up to the teacher or faculty coordinator.
I tried finding notes online as well and I couldn't find anything
(Unfortunately I have no clue about forensic chemistry.)
Post by: RuiAce on July 13, 2016, 10:42:13 am
Did you do Industrial chemistry?Yes
Posts like these aren't necessary. Please delete it.
(You're free to private message me about this stuff all you like though :) )
Post by: conic curve on July 13, 2016, 11:10:52 am
Yes
Posts like these aren't necessary. Please delete it.
(You're free to private message me about this stuff all you like though :) )
Oh I was just asking ahaha :D
Anyways if anyone here is generous enough could you please explain to me spetator ions, ionic equations, half equations, OILRIG, etc all with examples (if it's too much do 1-2 at a time). I seriously suck at those and I want to continue chemistry in year 12 (I heard it comes up in year 12 BTW and that's the main reason why I'm worried)
Thanks so much :)
Post by: HighTide on July 13, 2016, 11:24:47 am
Oh I was just asking ahaha :DAlright, although this is a question, you should probably have a go at these first. A simple google search, or using a textbook would give you the answers.
Anyways if anyone here is generous enough could you please explain to me spetator ions, ionic equations, half equations, OILRIG, etc all with examples (if it's too much do 1-2 at a time). I seriously suck at those and I want to continue chemistry in year 12 (I heard it comes up in year 12 BTW and that's the main reason why I'm worried)
Thanks so much :)
Spectator ions don't participate in the reaction. So if have like a compound like sodium sulfate and barium nitrate, the precipitate would be barium sulphate, the sodium ions and nitrate ions are spectator ions as they are not involved in the main reaction, but they are in the solution.
Ionic equations are kind of self explanatory. If you have a specific question about it, I can elaborate.
Half equations: One for oxidation and another for reduction since both occur spontaneously.
OILRIG is just oxidation is losing electrons. Reduction is gaining electrons.
Alright examples you don't really need at this point. But since you're keen on learning this (it does indeed come up in year 12), you should check out videos from Khan Academy, or other youtube sites because they introduce you to the basics. These basics will suffice for year 11 and year 12. If you have any specific question after that, I'd be happy to answer it.
Post by: conic curve on July 13, 2016, 02:12:49 pm
Alright, although this is a question, you should probably have a go at these first. A simple google search, or using a textbook would give you the answers.
Spectator ions don't participate in the reaction. So if have like a compound like sodium sulfate and barium nitrate, the precipitate would be barium sulphate, the sodium ions and nitrate ions are spectator ions as they are not involved in the main reaction, but they are in the solution.
Ionic equations are kind of self explanatory. If you have a specific question about it, I can elaborate.
Half equations: One for oxidation and another for reduction since both occur spontaneously.
OILRIG is just oxidation is losing electrons. Reduction is gaining electrons.
Alright examples you don't really need at this point. But since you're keen on learning this (it does indeed come up in year 12), you should check out videos from Khan Academy, or other youtube sites because they introduce you to the basics. These basics will suffice for year 11 and year 12. If you have any specific question after that, I'd be happy to answer it.
How come spectator ions don't take place in the reaction?
Yeah I know OILRIG, I just tend to get confused with examples. Usually does it require the periodic table?
Post by: WLalex on July 13, 2016, 03:19:10 pm
Yeah I know OILRIG, I just tend to get confused with examples. Usually does it require the periodic table?
No it does not, or not to my knowledge, require the periodic table. Basically when looking at an equation you are trying to determine the characteristics of the reactants ad products in determining which has lost (oxidation) and gained (reduction) electrons....HINT: look at the change in states/charges
As a rule of thumb, most of the time that is...the metal e.g. Mg will be oxidised
Hope this is somewhat helpful
Alex
Post by: RuiAce on July 13, 2016, 03:23:19 pm
Spectator ions are present because they're a part of the species. However, after the reaction is complete, nothing has happened to them. For example:
CuSO4(aq) + Zn(s) -> Cu(s) + ZnSO4(aq)
In the reaction, copper (II) ions turned to copper solid
Zinc solid turned to zinc ions
The sulfate ion did nothing. The species present are copper(II) sulfate and zinc sulfate, but the sulfate itself never got altered. It remained as SO42-.
We call this a spectator ion due to the fact that it is there just to ensure the reaction happens. It doesn't participate in the reaction.
It isn't a condition either because it doesn't HAVE to be there for the reaction to happen.
____________
As to why they don't take part in the reaction? Note that the definition of spectator ion is something THAT does not take part. It's just there because it's there in the mixture.
Post by: wesadora on July 13, 2016, 08:57:22 pm
Spoiler
"A student used this equipment [experimental setup for alkanol's heat of combustion prac: i.e. retort stand, clamp, spirit burner, water, thermometer, conical flask] to heat 250g of water. The mass of the spirit burner, which contained ethanol, decreased from 296.52g to 295.95g.
Given the heat of combustion of ethanol is 29.7kJ/g, calculate the maximum possible change in the temperature of the water."
--> this question I'm confused about the kJ/g...seeing ∆H = Q/n for kJ/mol, i just thought of doing ∆H=Q/m as it was in kJ/g (kilojule per gram), and solving for Q, as: 29.7 = Q/0.57 (0.57 is mass of ethanol burned)...yeah i got lost :(
Given the heat of combustion of ethanol is 29.7kJ/g, calculate the maximum possible change in the temperature of the water."
--> this question I'm confused about the kJ/g...seeing ∆H = Q/n for kJ/mol, i just thought of doing ∆H=Q/m as it was in kJ/g (kilojule per gram), and solving for Q, as: 29.7 = Q/0.57 (0.57 is mass of ethanol burned)...yeah i got lost :(
Spoiler
"Ozone concentrations are measured in Dobson units (DU). DU are the standard way to express ozone concentration in the stratosphere. A concentration of one DU means there would be 2.7x1020 ozone molecules in a layer of air that was one square metre in area and 0.01mm thick.
A baseline value of 220 DU is chosen as the starting point for an ozone hole in the stratosphere since total ozone values of less than 200 DU were not found in historic obsevations over Antarctica over 1979.
Which of the following concentrations, in moles per cubic metre (molm'3), is most nearly equivalent to 220 DU?"
A) 0.05 molm-3
B) 10 molm-3
C) 5000 molm-3
D) 10 000 molm-3
--> this one i am just lost in.
A baseline value of 220 DU is chosen as the starting point for an ozone hole in the stratosphere since total ozone values of less than 200 DU were not found in historic obsevations over Antarctica over 1979.
Which of the following concentrations, in moles per cubic metre (molm'3), is most nearly equivalent to 220 DU?"
A) 0.05 molm-3
B) 10 molm-3
C) 5000 molm-3
D) 10 000 molm-3
--> this one i am just lost in.
Post by: RuiAce on July 13, 2016, 09:12:37 pm
I'm having trouble with a following two questions in one of my school's past trial exams:If they give you the enthalpy change per gram, then ∆H=-Q/m (don't forget the negative) is correct.Spoiler"A student used this equipment [experimental setup for alkanol's heat of combustion prac: i.e. retort stand, clamp, spirit burner, water, thermometer, conical flask] to heat 250g of water. The mass of the spirit burner, which contained ethanol, decreased from 296.52g to 295.95g.
Given the heat of combustion of ethanol is 29.7kJ/g, calculate the maximum possible change in the temperature of the water."
--> this question I'm confused about the kJ/g...seeing ∆H = Q/n for kJ/mol, i just thought of doing ∆H=Q/m as it was in kJ/g (kilojule per gram), and solving for Q, as: 29.7 = Q/0.57 (0.57 is mass of ethanol burned)...yeah i got lost :(Spoiler"Ozone concentrations are measured in Dobson units (DU). DU are the standard way to express ozone concentration in the stratosphere. A concentration of one DU means there would be 2.7x1020 ozone molecules in a layer of air that was one square metre in area and 0.01mm thick.
A baseline value of 220 DU is chosen as the starting point for an ozone hole in the stratosphere since total ozone values of less than 200 DU were not found in historic obsevations over Antarctica over 1979.
Which of the following concentrations, in moles per cubic metre (molm'3), is most nearly equivalent to 220 DU?"
A) 0.05 molm-3
B) 10 molm-3
C) 5000 molm-3
D) 10 000 molm-3
--> this one i am just lost in.
Q = mc∆T = 16.929 kJ
Continue.
_________________________________
This question tests how well you know your units.
Tell me if I am wrong.
Post by: wesadora on July 13, 2016, 09:17:38 pm
Thanks Rui, also 1 more quick question:
I just came across a question using crystalline oxalic acid (aka oxalic acid dihydrate) as a primary standard. This comes in the form (COOH)2.2H2O
I'm confused because in class we used it and my teacher said to include the 2H2O in the molar mass, while i SWEAR my chem tutor said not to. And this makes a BIG difference in the calculation (obviously) ._.
Post by: RuiAce on July 13, 2016, 09:18:53 pm
That's correct, answer was D. I'll have a look at the working and redo the first question and see what I get.I'd say to include the molar mass of the dihydrate. I will ask around because I haven't done this type of question in ages.
Thanks Rui, also 1 more quick question:
I just came across a question using crystalline oxalic acid (aka oxalic acid dihydrate) as a primary standard. This comes in the form (COOH)2.2H2O
I'm confused because in class we used it and my teacher said to include the 2H2O in the molar mass, while i SWEAR my chem tutor said not to. And this makes a BIG difference in the calculation (obviously) ._.
This post will be deleted.
________
A response I got
Well I guess if crystalline oxalic acid (just (COOH)2 without the hydrates) was used, adding it to water to get those extra dihydrates shouldn't impact the experimental results, so I think he shouldn't include it in his calculations. Because, according to me, adding the oxalic acid to the water just causes the acid to essentially dissolve in the water and pick up that dihydrate (similar to changing from a solid to aqueous subscript) and so it should be treated as just a change of state.
But I'm not completely sure.
Post by: ProfLayton2000 on July 13, 2016, 10:51:39 pm
So on the HSC Chem 2011 paper q29b: "Why does the neutralisation of any strong acid in an aqueous solution by any strong base always result in a heat of reaction approximately -57kJ mol^-1
Could you please explain why it is what it is?
Post by: RuiAce on July 13, 2016, 10:58:45 pm
Hi there,Because:
So on the HSC Chem 2011 paper q29b: "Why does the neutralisation of any strong acid in an aqueous solution by any strong base always result in a heat of reaction approximately -57kJ mol^-1
Could you please explain why it is what it is?
a) The reaction goes to completion between two completely ionised substances
b) The reaction H3O+ + OH- -> 2 H2O(l) has ∆H=-57 kJ mol-1 (approximately).
That's all you need. See if you can realise how that just builds up to an answer.
Post by: anotherworld2b on July 14, 2016, 12:44:28 am
Post by: jakesilove on July 14, 2016, 10:16:29 am
I was wondering for chemcial equations how do you know the state of each substance/compound whether it is aqueoud, liquid, gas or solid?
Hey!
To some extent, it's just intuition after you've done a hell of a lot of practice. The only liquid that you really deal with is water: Assume water is liquid, unless it is clearly steam (in which case it is a gas). Anything else, in like a 'liquid' form, but containing metal ions etc. will always be aqueous. That means any acidic solution, any basic solution, any ions in solution, will be aqueous. Gas, again, is usually obvious from the question or from having done heaps of past papers. Things like hydrogen gas, oxygen, nitrogen (all diatomic) will be in a gaseous form. I can't give much more advice than that, except to say DO HEAPS OF PAST PAPERS and this will all because clear. Think about what is actually happening in the reaction, and you'll be sweet :)
Jake
Post by: RuiAce on July 14, 2016, 12:09:06 pm
I was wondering for chemcial equations how do you know the state of each substance/compound whether it is aqueoud, liquid, gas or solid?On top of what Jake said, some are trivial whilst others are logic.
E.g. The water you produce in neutralisation between an acid and base has to be a liquid. You're mixing two liquids together.
As opposed to the combustion of ethanol where you're achieving such high temperatures and water must have been boiled already.
An example of what I call a special case I know is that aluminium refuses to react with liquid water, but rather steam.
Post by: katherine123 on July 15, 2016, 08:22:01 pm
when talking about conditions for certain process involving equilibrium do i have to talk about what happens if (temperature, pressure) is too low or too high or do i just need to talk about the compromise temperature (the temperature is not too high or low thus favours the forward reaction)?
Post by: conic curve on July 15, 2016, 08:37:44 pm
Also this would increase (what I said above) the atomic radius as the outermost electrons are placed further from the nucleus. Why is this the case?
Post by: zsteve on July 15, 2016, 08:43:28 pm
When going down a group in the periodic table, a new electron shell is added to the atom. Why is this the case, is this "by definition"
Also this would increase (what I said above) the atomic radius as the outermost electrons are placed further from the nucleus. Why is this the case?
The periodic table was constructed in such a way that patterns in the elements are made obvious and systematic (to a limited degree of course, trends break down as you go down a group).
Every time a new electron shell is added, the number of valence electrons can be thought of as being 'reset', hence elements in the same group have the same number of valence electrons and have similar (although by no means identical) properties.
As more electrons are added, they need to occupy higher energy levels which are thus more distant from the nucleus. This is a consequence of orbital theory which restricts the number of electrons in a particular energy state. The lowest energy state in an element (1s) can hold 2e-, once this is filled, it goes to the next orbital (2s), and so on, with the electrons becoming progressively more distant from the nucleus.
Across a period, as you gain more electrons (and consequently more protons), the atomic radius actually decreases because the pull on each valence electron becomes greater (due to more protons)
Post by: conic curve on July 15, 2016, 08:46:09 pm
Post by: zsteve on July 15, 2016, 08:50:08 pm
Another question but why did Mendeleev leave gaps in his periodic table?
That was purely due to the fact that the 'next' element actually skipped a property, and Mendeleev believed that there was a missing element which hadn't been discovered which belonged there. Later on, that element was discovered.
Post by: Happy Physics Land on July 16, 2016, 06:44:57 pm
Another question but why did Mendeleev leave gaps in his periodic table?
Because not all the elements can be found in the order as we see them now - some are more difficult to synthesise or extract than other due to the technological deficiency of the time.
Post by: Happy Physics Land on July 16, 2016, 07:00:06 pm
can you check my response attached below thanks
when talking about conditions for certain process involving equilibrium do i have to talk about what happens if (temperature, pressure) is too low or too high or do i just need to talk about the compromise temperature (the temperature is not too high or low thus favours the forward reaction)?
Hey Katherine,
Your response is certainly very detailed and encompasses the essential things you need to know. When you get asked about equilibrium questions involving ammonia, you usually talk about why there is a compromise. I see you have already included those into your response so thats very good.
A couple of things that would be beneficial for you to add:
- You want to be more specific with the type of catalyst you are using. Iron oxide is ok, but l think its better to say Fe3O4 (magnetite). This shows the teacher you really know your stuff.
- With temperature, you dont want very high temperature also because it can damage the catalyst. Since catalysts are quite expensive to replace and relatively vulnerable, you want to preserve them in good conditions by using lower temperatures.
- You must also add the liquidification of ammonia as a part of your conditions. By this l mean constantly turning ammonia gas into liquid to remove gaseous ammonia from the equilibrium and hence the equilibrium will shift right, favouring the production of more ammonia. You touched on this in your last condition but its very important to mention LIQUIDIFICATION of ammonia.
Other than those points, I think this is a very good table to study for ammonia section of chemical monitoring and management. Kudos to you, you have done a very good job!
Best Regards
Happy Physics Land
Post by: RuiAce on July 17, 2016, 08:24:40 am
Hey Katherine,Yeah, I agree with the use of "magnetite" over iron oxide. Btw liquefaction is the scientific word for condensation.
Your response is certainly very detailed and encompasses the essential things you need to know. When you get asked about equilibrium questions involving ammonia, you usually talk about why there is a compromise. I see you have already included those into your response so thats very good.
A couple of things that would be beneficial for you to add:
- You want to be more specific with the type of catalyst you are using. Iron oxide is ok, but l think its better to say Fe3O4 (magnetite). This shows the teacher you really know your stuff.
- With temperature, you dont want very high temperature also because it can damage the catalyst. Since catalysts are quite expensive to replace and relatively vulnerable, you want to preserve them in good conditions by using lower temperatures.
- You must also add the liquidification of ammonia as a part of your conditions. By this l mean constantly turning ammonia gas into liquid to remove gaseous ammonia from the equilibrium and hence the equilibrium will shift right, favouring the production of more ammonia. You touched on this in your last condition but its very important to mention LIQUIDIFICATION of ammonia.
Other than those points, I think this is a very good table to study for ammonia section of chemical monitoring and management. Kudos to you, you have done a very good job!
Best Regards
Happy Physics Land
However, I do think 400deg C (and 200 atm pressure) is fine in itself. At this temperature, the catalyst is safe. The more important thing is that the catalyst remains pure as was mentioned, because if the catalyst is impure, then the exposed surface decreases, thus limiting the impact of the catalyst. Pretty sure that's all we mean by an impure catalyst.
____________
@Katherine: Regarding impurities, it is good that you mentioned things such as O2 which should ALWAYS be removed to avoid violent explosions in the reaction vessel, even inert gases such as argon should be monitored because they still slow down the rate of reaction. They just happen to be a "counter-catalyst" - they slow both the forward AND reverse rate. (Which is why they're not as significant in my opinion but still important)
Post by: katherine123 on July 17, 2016, 08:52:57 pm
2013 There is a compromise between maximising yield and minimising the environmental impact of industrial processes. Justify this statement with reference to the production of sulfuric acid. 7 marks
The production of sulfuric acid is achieved through the mining and contact process which involves the maximisation of yield with environmental considerations throughout the process.
Mining: Sulfur is extracted through Frasch process in which super-heated steam and high-pressure air is forced into an underground sulfur deposit, melting the Sulfur and forcing it to the surface where it solidifies. Although this process produces high purity sulfur, it causes land subsidence and requires the removal of trees for land, thus damaging natural ecosystems. Mining also requires energy which is obtained by burning of fossil fuels which releases CO2 thus contributing to global warming and enhanced greenhouse effect.
Contact process step 1: Sulfur obtained by Frasch process is heated in dry air in a combustion furnace, forming SO2
S(s) +O2(g) --> SO2(g) + heat. Heat produced via this exothermic reaction is used to generate steam to produce electricity thus minimising environmental impacts as it reduce the reliance on fossil fuels thus minimising the amount of CO2 released.
Contact process step 2: SO2 is passed through a catalytic converter and several beds of V2O5 catalyst where it is converted into SO3. 2SO2(g) + O2(g) -->2SO3(g) +heat. The conditions of the reaction are manipulated to maximise yield of SO3 with environmental considerations. The forward reaction is exothermic thus the increase in temperature will force the equilibrium to favour backward reaction in order to reduce some of the increased heat according to Le Chatelier’s principle (LCP) which would reduce the yield of SO3. If the temperature is too high, the catalyst will be damaged which makes it less efficient and if temperature is too low, the rate of reaction will decrease which leads to the decrease in yield of SO3. Thus, a compromise temperature of approx. 400 degrees is maintained to ensure temperature is low enough to encourage forward reaction and high enough for particles to have sufficient kinetic energy to undergo reaction. Heat released via the exothermic reaction is used to generate electricity and unreacted gases are recycled back into the converter to reduce the necessity to mine for sulfur thus minimising environmental impacts of enhanced greenhouse effect and land subsidence. High pressure will favour the forward reaction (LCP) in order to reduce some of the increase in pressure thus increasing yield of SO3. However, low pressure of 1-2atm is used instead as it will be dangerous and costly if it is too high. V2O5 catalyst lowers the activation energy which increases the rate of reaction and enables lower temperature to be used, thus lowering the energy requirement and cost, and moreover reducing the production of energy from burning of fossil fuels which releases CO2 which minimises environmental impact. The concentration of oxygen is maintained high to force equilibrium to the right (LCP) in order to reduce some of the increased concentration of Oxygen thus increasing yield of SO3.
Contact process step 3: SO3(g) is dissolved in concentrated H2SO4 to produce oleum: SO3(g) + H2SO4(l) -->H2S2O7(l). Water is then added to H2S2O7 to form concentrated H2SO4: H2S2O7(l) + H2O(l) -->2H2SO4(l).
Do I need to include catalyst even though it doesn’t increase the yield
and do i need to be more specific about the compromise temperature:
- Initial gas stream is at 1000 degrees after combustion of sulfur.
- Gas stream is cooled to about 550 degrees and passed over bed of V2O5 catalyst which produces 70% conversion of SO2 to SO3
- Gas stream is further cooled to 400 degrees and passed over a second catalytic bed, producing 97% conversion
- After conversion of SO3 to H2SO4 (via oleum), remaining gas stream is passed over final bed of V2O5 to produce 99.7% conversion
Assess the impact of atomic absorption spectroscopy (AAS) on the scientific understanding of the effects of trace elements 4 marks
Before AAS was developed there were no other techniques sensitive enough to measure low concentrations of trace elements. Therefore, scientists were not able to detect and determine concentration of trace elements. However, after the development of AAS, which measures intensity of certain wavelengths of light from the electromagnetic spectrum (different element absorb specific wavelengths of light), the concentration of trace element can be determined very accurately as low as 0.01 ppm. The lamp used in AAS emits specific wavelengths that corresponds to the element being measured thus absorption of element is not affected by presence of other elements. This allows scientist to measure concentration and rectify deficiency of specific trace element (eg. animal health could not be maintained due to Cobalt deficiencies in pasture in coastal SW Australia) by providing fertilisers or dietary supplements to animals to enhance plant growth and animal and human health. Thus, AAS has enabled scientists to detect and measure concentration of trace elements which led to the increase in scientific understanding of the effects of trace elements.
Post by: RuiAce on July 17, 2016, 09:35:39 pm
Please check my response thanksI reckon for questions like this the focus is mainly on the Contact process for the production of sulfuric acid rather than the Frasch process for extraction of sulfur. This is because step 2 is where the equilibrium reaction happens. However, as you mentioned, the Frasch process does indeed come with environmental considerations such as land subsidence. Another point I would include here is that if the superheated steam leaks you are potentially causing thermal pollution which may lead to global warming. Also, the sulfur collected must be stored ASAP to ensure that it doesn't oxidise by itself and leak SO2 into the atmosphere.
2013 There is a compromise between maximising yield and minimising the environmental impact of industrial processes. Justify this statement with reference to the production of sulfuric acid. 7 marks
The production of sulfuric acid is achieved through the mining and contact process which involves the maximisation of yield with environmental considerations throughout the process.
Mining: Sulfur is extracted through Frasch process in which super-heated steam and high-pressure air is forced into an underground sulfur deposit, melting the Sulfur and forcing it to the surface where it solidifies. Although this process produces high purity sulfur, it causes land subsidence and requires the removal of trees for land, thus damaging natural ecosystems. Mining also requires energy which is obtained by burning of fossil fuels which releases CO2 thus contributing to global warming and enhanced greenhouse effect.
Contact process step 1: Sulfur obtained by Frasch process is heated in dry air in a combustion furnace, forming SO2
S(s) +O2(g) --> SO2(g) + heat. Heat produced via this exothermic reaction is used to generate steam to produce electricity thus minimising environmental impacts as it reduce the reliance on fossil fuels thus minimising the amount of CO2 released.
Contact process step 2: SO2 is passed through a catalytic converter and several beds of V2O5 catalyst where it is converted into SO3. 2SO2(g) + O2(g) -->2SO3(g) +heat. The conditions of the reaction are manipulated to maximise yield of SO3 with environmental considerations. The forward reaction is exothermic thus the increase in temperature will force the equilibrium to favour backward reaction in order to reduce some of the increased heat according to Le Chatelier’s principle (LCP) which would reduce the yield of SO3. If the temperature is too high, the catalyst will be damaged which makes it less efficient and if temperature is too low, the rate of reaction will decrease which leads to the decrease in yield of SO3. Thus, a compromise temperature of approx. 400 degrees is maintained to ensure temperature is low enough to encourage forward reaction and high enough for particles to have sufficient kinetic energy to undergo reaction. Heat released via the exothermic reaction is used to generate electricity and unreacted gases are recycled back into the converter to reduce the necessity to mine for sulfur thus minimising environmental impacts of enhanced greenhouse effect and land subsidence. High pressure will favour the forward reaction (LCP) in order to reduce some of the increase in pressure thus increasing yield of SO3. However, low pressure of 1-2atm is used instead as it will be dangerous and costly if it is too high. V2O5 catalyst lowers the activation energy which increases the rate of reaction and enables lower temperature to be used, thus lowering the energy requirement and cost, and moreover reducing the production of energy from burning of fossil fuels which releases CO2 which minimises environmental impact. The concentration of oxygen is maintained high to force equilibrium to the right (LCP) in order to reduce some of the increased concentration of Oxygen thus increasing yield of SO3.
Contact process step 3: SO3(g) is dissolved in concentrated H2SO4 to produce oleum: SO3(g) + H2SO4(l) -->H2S2O7(l). Water is then added to H2S2O7 to form concentrated H2SO4: H2S2O7(l) + H2O(l) -->2H2SO4(l).
Do I need to include catalyst even though it doesn’t increase the yield
and do i need to be more specific about the compromise temperature:
- Initial gas stream is at 1000 degrees after combustion of sulfur.
- Gas stream is cooled to about 550 degrees and passed over bed of V2O5 catalyst which produces 70% conversion of SO2 to SO3
- Gas stream is further cooled to 400 degrees and passed over a second catalytic bed, producing 97% conversion
- After conversion of SO3 to H2SO4 (via oleum), remaining gas stream is passed over final bed of V2O5 to produce 99.7% conversion
Whilst the 1000deg C for step 1 isn't really much of a hassle, including it is better than not as it shows you have better understanding of the actual Contact process. This reinforces your point that sufficient heat energy will indeed be produced to be converted to electricity.
With the catalyst beds for step 2, generally I only include the first and fourth with their relative percentages. But yes I would DEFINITELY include that one because the idea is 30% or 3% emission is unacceptable by environmental guidelines, but 0.3% is okay. The cooling...well yes I would include it. And you've already integrated it in there somehow as well.
Post by: anotherworld2b on July 18, 2016, 02:21:14 am
Post by: RuiAce on July 18, 2016, 09:55:54 am
I was wondering if i could get help with these questions XDCan we please not have so many questions waved around at once?
For the first one, you know that complete combustion involves the production of carbon dioxide and water.
C6H12(l) + O2(g) -> CO2(g) + H2O(g)
Use logic to determine what order to balance in:
C: Reactants - 6. Products - 1. Therefore 6 in front of CO2
H: Reactants - 12. Products - 2. Therefore 6 in front of H2O
O: Products - 6*12 + 6*6 = 18. Reactants - 2. Therefore 9 in front of O2
C6H12(l) + 9 O2(g) -> 6 CO2(g) + 6 H2O(g)
The second one is more open-ended, as there exists more than one equation for incomplete combustion. The products must include at least one of C and CO, however they can include any of C, CO or CO2 at once.
__________
Answer to Q5 is incomplete because CO is there
__________
Reactivity is a chemical property as it is essentially a measure of how likely it will form a new substance. Note that physical properties do not involve formation of a new substance.
For the net ionic equations, try writing complete equations first. (Remember: Metal + Acid -> Salt + Hydrogen)
Post by: katherine123 on July 18, 2016, 12:37:01 pm
6 marks
Chemists can assist in reversing or minimising the environmental problems caused by technology and the human demand for products and services. With reference to this statement, assess the need for chemists to collaborate when monitoring the environmental impact of a named electrochemical cell.
Chemist need to collaborate when monitoring the impact of materials so that human demand for products and services does not adversely affect the environment. An example of technology include the lead-acid battery is an electrochemical cell which produces high currents thus used as car batteries. However, the materials such as lead contained within the battery may cause serious environmental concerns when disposed. High concentration of lead in organism can lead to disruption of normal metabolic processes by affecting enzymes thus causing brain damage and damage to other organs and reproductive system. Therefore, levels of lead need to be monitored in the environment with chemists of different specialities collaborating. An environmental chemist will determine the levels of lead in the environment and provide information relevant to the setting of industry standards and environmental regulations relating to lead concentration in atmospheric emissions. Analytical chemist will accurately detect and measure the amount of lead present at times during the manufacturing and recycling process. This is done through the use of the technique AAS (atomic absorption spectroscopy) which can determine lead concentration in very low concentration. While the damage caused by the leaking of lead into the environment cannot be reversed, industrial and material chemists can minimise the environmental problems by providing alternative production methods and products. For example, the development of lithium batteries can reduce the environmental damage.
Therefore, there is a great need for chemists to collaborate to minimise the damage caused by this technology as some chemists monitor the environment while others use the results of this to develop environmental friendly alternatives.
Post by: EEEEEEP on July 18, 2016, 12:56:16 pm
please check thanks
6 marks
Chemists can assist in reversing or minimising the environmental problems caused by technology and the human demand for products and services. With reference to this statement, assess the need for chemists to collaborate when monitoring the environmental impact of a named electrochemical cell.
Chemist need to collaborate when monitoring the impact of materials so that human demand for products and services does not adversely affect the environment. An example of technology include the lead-acid battery is an electrochemical cell which produces high currents thus used as car batteries. However, the materials such as lead contained within the battery may cause serious environmental concerns when disposed. High concentration of lead in organism can lead to disruption of normal metabolic processes by affecting enzymes thus causing brain damage and damage to other organs and reproductive system. Therefore, levels of lead need to be monitored in the environment with chemists of different specialities collaborating. An environmental chemist will determine the levels of lead in the environment and provide information relevant to the setting of industry standards and environmental regulations relating to lead concentration in atmospheric emissions. Analytical chemist will accurately detect and measure the amount of lead present at times during the manufacturing and recycling process. This is done through the use of the technique AAS (atomic absorption spectroscopy) which can determine lead concentration in very low concentration. While the damage caused by the leaking of lead into the environment cannot be reversed, industrial and material chemists can minimise the environmental problems by providing alternative production methods and products. For example, the development of lithium batteries can reduce the environmental damage.
Therefore, there is a great need for chemists to collaborate to minimise the damage caused by this technology as some chemists monitor the environment while others use the results of this to develop environmental friendly alternatives.
>> Good • Demonstrates a thorough knowledge and understanding of the identifiedroles of chemists and the environmental impact of a correctly named electrochemical cell
>> GOOD • Assesses the need for collaboration between chemists
>> Good • Demonstrates coherence and logical progression of ideas with correct scientific terminology used
Post by: onepunchboy on July 19, 2016, 03:49:57 am
Post by: jakesilove on July 19, 2016, 09:20:13 am
I have a 25% assessed report to write up, 1000 +/- 200 words about biopolymers, my chosen PFA. I have attached an image of the issue I have been presented with.
The actual task is requires a critical discussion of the science relevant to the PFA, all in a concise report with an introduction, background information, the actual synopsis and a relevant conclusion.
I just want to know what you would recommend in regards to the format and structure of my report, as well as what you think is vital to include in order to maximise my marks.
Pls help!! (http://uploads.tapatalk-cdn.com/20160718/32d91f6d526fd523cd82774b1b9b640a.jpg)
Hey!
For a report like this, it is important to include a number of things. I would recommend subheading by topic, and using tables and graphs where possible (noting that graphs won't increase your word count!).
I think history is important. When did you biopolymer start being produced? How efficient was it? When was it discovered? For what purpose was it produced. Then, it is easy to cite advances in technology that has brought it to the stage it is at today. You should clearly indicate how these developments have been useful to both the production and use of the polymer; if the production has become more environmentally friendly/renewable, that is important as it 1) protects the environment and 2) allows for a sustainable, replenishing supply of the polymer!
Make sure to include chemistry. What is actually happening, what catalyst is being used, what does the reaction look like etc. Try to include chemical terminology as much as possible, as this brings the assignment away from a 'society and the environment' answer and towards a more chemical sound, scientific one.
The aspects that you've written in your question are of course crucial. Use that structure, but within "background information" I would include history or discover, development and uses (just as examples). You need to think about what is specifically important for your biopolymer, after doing fairly extensive research.
It's always hard to tell where marks lie for an assignment like this. All I can recommend is doing lots of research, including what you think is important, and if you go way over word count trying to take out stuff that doesn't directly lend itself to the question. Include lots of statistics to impress the marker (what is the reaction yield? What year did developments take place? Who discovered the process?).
Good luck! Let me know if you have any more specific questions, noting that we can't do your assignment for you of course!
Jake
Post by: jakesilove on July 19, 2016, 09:24:19 am
Can someone give me a basic run down on how aas works. Ive read the textbook but still very confusing to me
Hey! You really only need to know what AAS is in a very basic sense; textbooks often go into great depth, and the fact is you just don't need to know that. So, here's what I learnt for my HSC.
AAS is a technique to identify absolutely minuscule (ppm) amounts of heavy metals. First, a calibration curve must be calculated, in which a sample is tested that DOES NOT have any of the heavy metal present. The liquid will be 'aspirated' or 'vaporized', by being fed into a flame. Then, light is shone on one side of the flame, and detected on the other. From this, a curve is established (based on the readings on the detector). Then, the liquid which contains some heavy metal is tested. The same process is undergone (aspiration, taking readings from the detector). However, as there are now heavy metal particles floating around in the flame, some on the light will be reflected/refracted. This will lead to a different curve being present, which can be compared to the original curve. We can therefore establish the concentration of the heavy metal!
Obviously it's much more complicated that this. You also need to think about the effects of AAS (allowing us to detect small quantities = good, as we can protect society and the environment) as well as potential issues (only detect heavy metals, destroys sample, expensive to run etc.). Let me know if you need any further help!
Hope my explanation made sense.
Jake
Post by: RuiAce on July 19, 2016, 09:28:10 am
Can someone give me a basic run down on how aas works. Ive read the textbook but still very confusing to meAAS:
0. Remember the diagram off by heart. This is one of those scenarios where remembering the diagram actually helps, as mentioned in Jake's lecture.
1. The hollow cathode lamp shines into the flame
1. a) Every individual element has it's own absorption spectrum. The hollow cathode lamp must be shining light corresponding to the absorption spectrum of each element. Hence, every element has its own cathode lamp.
2. A standardised solution (this just means that its concentration is KNOWN) is fed into the flame.
2. a) This is used to calibrate the AAS machine.
3. Light passing through the flame will get absorbed by the metal ions and reemitted. I'm not sure if you need to know about the physics here for the HSC chemistry course, but I can explain it later if you wish so.
4. A collimator focuses the light
5. The light reaches a prism and is dispersed.
6. The photomultiplier picks up the light. Calibration is done.
7. Repeat steps 2 to 6 with the solution of unknown concentration.
Absorption is a measure of the amount of light absorbed relative to the reference beam. It is proportional to the concentration of the metal sample.
(Lel. Jake beat me here.)
Post by: onepunchboy on July 19, 2016, 11:19:22 am
Post by: anotherworld2b on July 19, 2016, 12:02:46 pm
I was wondering if the substance was sodium bicarbonate and you react it with Hydrochloric.
Would it be correct to assume that this reaction will follow the basic reaction
Acid + Metal Carbonate -> salt + carbon dioxide + water
I heard that bicarbonates wont react with HCL? Is that true?
In the method we then boiled the solution that is produced to get the sodium salt. Using the salt we conducted a flame test to validate the metal.
I was wondering what are the chemistry concepts behind this method?
Post by: RuiAce on July 19, 2016, 05:55:27 pm
For a lab report we were assigned to do we had to identify an unknown bicarbonate.No it definitely reacts.
I was wondering if the substance was sodium bicarbonate and you react it with Hydrochloric.
Would it be correct to assume that this reaction will follow the basic reaction
Acid + Metal Carbonate -> salt + carbon dioxide + water
I heard that bicarbonates wont react with HCL? Is that true?
In the method we then boiled the solution that is produced to get the sodium salt. Using the salt we conducted a flame test to validate the metal.
I was wondering what are the chemistry concepts behind this method?
https://www.youtube.com/watch?v=dJ9EyW58ntA
You can use HCl to test for the presence of HCO3- OR CO32-. However you can't distinguish between which one is there as easily.
Note that test for HCO3- is not explicitly stated in the syllabus.
Post by: tanishap on July 19, 2016, 07:23:35 pm
I came to the repeat Chem lecture last week at UTS and you showed us this totally boss document that was literally 3 pages of all the reactions we had to memorise, but I forgot where to find it. Can you just let me know where I can find that chemistry bible please? :))
Thanks heaps! :)
Post by: jakesilove on July 19, 2016, 07:26:04 pm
Hey Jake!
I came to the repeat Chem lecture last week at UTS and you showed us this totally boss document that was literally 3 pages of all the reactions we had to memorise, but I forgot where to find it. Can you just let me know where I can find that chemistry bible please? :))
Thanks heaps! :)
You can find it right HERE! Go mental :)
Jake
Post by: marynguyen18 on July 19, 2016, 10:58:06 pm
Post by: onepunchboy on July 19, 2016, 11:19:14 pm
Hey can someone explain the answer to this question, i put d but its wrong hehe
Thanks
Post by: RuiAce on July 19, 2016, 11:35:11 pm
(http://uploads.tapatalk-cdn.com/20160719/9b79abe4b72f78e7148701c8789b7ee0.jpg)Have you tried calculating the resultant pH for A and B?
Hey can someone explain the answer to this question, i put d but its wrong hehe
Thanks
The concentration of H+ is going to go down heaps if you react it with a base.
(Can't see the value on the top right corner to do the calculations)
The dilution just raises the pH by 1
Post by: onepunchboy on July 19, 2016, 11:37:21 pm
Post by: RuiAce on July 19, 2016, 11:45:18 pm
D) The dilution factor by 10 will change the pH into 3
Take B)
HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)
nHCl initial = 0.1L * 0.01 mol L-1 = 10-3 mol
nNaOH initial = 0.01L * 0.1mol L-1 = 10-3 mol
Everything's going to be reacted. The final solution will be neutral with a pH of 7
(You may want to test out the CaCO3 reaction as well just incase the change in pH is greater. But I doubt it will be.)
Post by: amandali on July 20, 2016, 04:33:24 pm
Post by: RuiAce on July 20, 2016, 05:07:30 pm
are these the correct and main steps for Solvay process?Yep I believe those are correct. Try to have all of those equations memorised.
Post by: conic curve on July 21, 2016, 10:58:34 am
I'm finding that module really hard and I don't want to drop chemistry in the future. I feel that I don't have that level of understanding in chemistry
Also
1. why does alcohol dissolve in both polar and non polar?
2. Why does water dissolve covalent compounds?
Thanks ;D
Post by: RuiAce on July 21, 2016, 11:05:07 am
How do you do well in the water module?The first question is more of a production of materials question that draws on knowledge for water.
I'm finding that module really hard and I don't want to drop chemistry in the future. I feel that I don't have that level of understanding in chemistry
Also
1. why does alcohol dissolve in both polar and non polar?
2. Why does water dissolve covalent compounds?
Thanks ;D
As a rule of thumb, likes dissolve likes
The hydroxyl group (-OH) can be involved with extensive hydrogen bonding between other O, N and F molecules. Because the -OH is also polar, it also engages in dipole-dipole interactions with other polar molecules.
But the long carbon chain is non-polar. The linear non-polar chain is what will dissolve with other non-polar substances such as alkanes, through dispersion forces.
(Dispersion forces are as a result of one side of the molecule temporarily becoming more polar than the other, thereby forming a dipole. Dipole-dipole interactions happen when there is a permanent dipole going on. Hydrogen bonding is only separate from dipole-dipole interactions because it's much stronger; about 1/10 the strength of an actual covalent bond)
Q2 is wrong. Water dissolves certain covalent compounds. It does not dissolve stuff like methane or propane.
The covalent compounds it dissolves are all polar because water IS a polar molecule
Post by: conic curve on July 21, 2016, 03:59:45 pm
The first question is more of a production of materials question that draws on knowledge for water.
As a rule of thumb, likes dissolve likes
The hydroxyl group (-OH) can be involved with extensive hydrogen bonding between other O, N and F molecules. Because the -OH is also polar, it also engages in dipole-dipole interactions with other polar molecules.
But the long carbon chain is non-polar. The linear non-polar chain is what will dissolve with other non-polar substances such as alkanes, through dispersion forces.
(Dispersion forces are as a result of one side of the molecule temporarily becoming more polar than the other, thereby forming a dipole. Dipole-dipole interactions happen when there is a permanent dipole going on. Hydrogen bonding is only separate from dipole-dipole interactions because it's much stronger; about 1/10 the strength of an actual covalent bond)
Q2 is wrong. Water dissolves certain covalent compounds. It does not dissolve stuff like methane or propane.
The covalent compounds it dissolves are all polar because water IS a polar molecule
TBH I find that the water module is a hard module and things are very difficult for me to grasp, especially things like VSPER theory, electron pair geometry, spatial geometry, etc
Is this "like dissolves like" something by definition/convention or is there a reason as to why "like dissolves like"
It says here that water is a polar molecule because of it's shape: http://www.triangularwave.com/a1b1.htm and that one side is positively charged and one side is negatively charged. Why is this? (again is this "by definition")
Post by: RuiAce on July 21, 2016, 04:40:36 pm
TBH I find that the water module is a hard module and things are very difficult for me to grasp, especially things like VSPER theory, electron pair geometry, spatial geometry, etcYes that's correct.
Is this "like dissolves like" something by definition/convention or is there a reason as to why "like dissolves like"
It says here that water is a polar molecule because of it's shape: http://www.triangularwave.com/a1b1.htm and that one side is positively charged and one side is negatively charged. Why is this? (again is this "by definition")
If instead, water were a linear molecule like CO2, there would not be a region for high electron density. Recall that electrons like to exist in pairs. However, like charges are obviously going to repel each other - no pair of electrons wants to hug another pair. So that extra 2 electrons in oxygen (that are not covalently bonded) will make advantage of the molecule being bent, and cluster at the top of the oxygen in H2O instead.
It is more easily shown if I drew a diagram tbh though.
Water does take a bit of effort to make sense out of yeah.
Post by: conic curve on July 21, 2016, 04:46:55 pm
Yes that's correct.
If instead, water were a linear molecule like CO2, there would not be a region for high electron density. Recall that electrons like to exist in pairs. However, like charges are obviously going to repel each other - no pair of electrons wants to hug another pair. So that extra 2 electrons in oxygen (that are not covalently bonded) will make advantage of the molecule being bent, and cluster at the top of the oxygen in H2O instead.
It is more easily shown if I drew a diagram tbh though.
Water does take a bit of effort to make sense out of yeah.
Try explaining with this picture: https://www.google.com.au/search?hl=en&biw=1366&bih=667&site=webhp&tbm=isch&q=water+molecule+structure&sa=X&ved=0ahUKEwivhsaR-oPOAhXCHZQKHddwB_0QhyYIIw&dpr=1#imgrc=J0EWfEKYgTnznM%3A
(if this helps obviously)
Edit: I'll redo them
http://www.chem1.com/acad/sci/wat-images/H2O.png
http://study.com/cimages/multimages/16/factsaboutwater_molecularstructureimage2.png
http://www.biology.arizona.edu/biochemistry/tutorials/chemistry/graphics/water.gif
http://witcombe.sbc.edu/water/images/chemistrydipolescicx.jpg
Post by: RuiAce on July 21, 2016, 04:53:39 pm
Post by: Jakeybaby on July 21, 2016, 07:03:11 pm
(http://i1164.photobucket.com/albums/q566/Rui_Tong/Capture_zps4wqhzana.png)Oh wow, I love it
Post by: anotherworld2b on July 22, 2016, 01:24:13 am
I got the answers for a and b but i didnt know how to do c and d.
Help is greatly appreciated
Post by: RuiAce on July 22, 2016, 01:32:22 am
How would you do q5c and d?Consider the original balanced equations to know what is going on first.
I got the answers for a and b but i didnt know how to do c and d.
Help is greatly appreciated
c) This is a metal and acid reaction. We know that:
Metal + Acid -> Salt + Hydrogen gas
So treating our species appropriately:
Zn(s) + H2SO4(aq) -> ZnSO4(aq) + H2(g)
Have any charges changed?
Zn: It has went from zinc solid to the zinc ion
SO4: The sulfate ion is as is
H: The hydrogen ion has become hydrogen gas.
So we have Zn(s) + 2 H+ -> Zn2+ + H2(g)
c) This is a precipitation reactants. But it's made easier because they give us the reactants and the product.
Pb(NO3)2 + 2 KNO3(aq) + PbI2(s)
Have any changed occurred?
Pb: It stayed as Pb2+ but it precipitated into the product PbI2(s) which cannot be split!
NO3: The nitrate ion is as is
K: The potassium ion is as is
I: It stayed as I- but it precipitated into the product PbI2(s) which cannot be split!
So we have Pb2+ + 2 I- -> PbI2(s)
_____________
An alternate way to approach these questions is to write out the complete ionic equation based off the balanced equation and then cancel out any spectator ions.
For c) this would be
Zn(s) + 2 H+ +
Post by: conic curve on July 22, 2016, 01:16:14 pm
Q. A soluble molecular substance such as sugar (e.g. sucrose C11H22O11) is mixed with water
a. Identify the forces that mist break for it to dissolve
I was not sure of this and was thinking about intermolecular or intramolecular but I seriously don't know.
b. Describe the attraction that then develops between the free water and sugar molecules
I'm not sure of this. Is it Hydrogen bonding?
Thanks guys ;)
Post by: RuiAce on July 22, 2016, 01:25:52 pm
I need help with the water topic since I find it quite difficultDraw out the molecular structure of sucrose. It is full of polar OH bonds everywhere. The fact that they are polar means that sucrose molecules will engage in dipole-dipole interactions (and maybe a small amount of hydrogen bonding) with each other. (Do you see why?)
Q. A soluble molecular substance such as sugar (e.g. sucrose C11H22O11) is mixed with water
a. Identify the forces that mist break for it to dissolve
I was not sure of this and was thinking about intermolecular or intramolecular but I seriously don't know.
b. Describe the attraction that then develops between the free water and sugar molecules
I'm not sure of this. Is it Hydrogen bonding?
Thanks guys ;)
If water is to dissolve sucrose, the intermolecular forces between sucrose molecules must be broken as water has to start interacting intermolecularly with sucrose instead. (No intramolecular bonds should be broken or we no longer have sucrose).
Because the polarity of water is much stronger than that of sucrose, it will overcome the intermolecular forces between sucrose molecules and form extensive hydrogen bonding with sucrose molecules instead, thereby dissolving the substance.
Process of thought:
a) Water has to break something. What is the only logical thing to break? In fact, what even is there to break is a better question
b) It is hydrogen bonding. Just draw out the shape of sucrose and water and you should be able to realise how the polar -OH groups on sucrose can interact with water.
Post by: amandali on July 22, 2016, 01:48:53 pm
dont know how to do q17 b
Post by: RuiAce on July 22, 2016, 02:00:42 pm
(http://uploads.tapatalk-cdn.com/20160721/7c99943ac16cb6eb2771db26da2e8a0d.jpg)Please give the answers to a next time if you could do it. Would make life easier :) (especially since I might've gotten it wrong).
dont know how to do q17 b
Because the equilibrium constant is a decently large number, at equilibrium there should be more of the products than the reactants.
Firstly, our calculations implicitly verify this as in the event the concentration of oxygen is greater, there would indeed be more of SO2 being converted into SO3 (90% instead of 50%).
How has Le Chatelier's principle been obeyed here? I reckon it's actually quite simple. We are saying that if there's more O2 (the reactant), then the forward reaction should be favoured. The equilibrium shifts towards the right, converting SO2 to SO3 in the process. If the concentration of O2 is greater, indeed we should anticipate that the forward reaction will occur to a greater extent.
Post by: marynguyen18 on July 22, 2016, 09:25:58 pm
Post by: conic curve on July 22, 2016, 09:27:07 pm
Hey i was just wondering how would you structure a 4-7 mark question on Cellulose?
It really depends on what the question is and what the marking criteria is
Post by: marynguyen18 on July 22, 2016, 09:34:08 pm
Post by: RuiAce on July 22, 2016, 09:41:57 pm
What if the question on Cellulose is related to it as being an alternative to petrochemicals and its potential as a raw material for chemicals used in industry?Think about why it can be an alternative to petrochemicals. This goes back down to the fact that natural production of ethylene comes from fossil fuels = bad for reasons you should already know.
Cellulose is a renewable resource. If you want to produce ethanol, all you have to do is ferment it and done. And you can easily get more cellulose from dead plant matter.
(But you should always analyse the negatives of this process, e.g. lots of arable land needed and the energy required to trigger fermentation is massive.)
Essentially, the part about being an alternative to petrochemicals is a process of outweighing the advantages and disadvantages against burning fossil fuels. The raw materials part can be related to things such as
- Ethanol: Very powerful solvent used in a variety of scenarios
- Polyethylene: A common polymer that we use today for plastics
Also, make sure to give a judgement if it ever asks you to evaluate/assess.
Post by: marynguyen18 on July 22, 2016, 09:46:18 pm
Post by: RuiAce on July 22, 2016, 09:49:01 pm
E.g. if the question is about the potentials of cellulose in being a replacement, you would recollect the info you have and say hence it is most likely that it is a good/bad alternative. Or something like overall, it is (say, adequate) in that it is able to...
Your judgment doesn't have to be "correct" - it just has to make sense and link to the question.
Post by: conic curve on July 22, 2016, 09:50:30 pm
Thank you so much! Also when you make a judgement does it mean that you have to say what you think would be a good alternative?
Judgement really means to give your own opinion
Anyways can anyone give me tips on how to answer hydrocarbon questions because I really suck at that
Thanks
Post by: RuiAce on July 22, 2016, 09:59:18 pm
Judgement really means to give your own opinionBefore commencing, your prefixes of meth-, eth-, prop-, but-, pent- etc. should be known off by heart first.
Anyways can anyone give me tips on how to answer hydrocarbon questions because I really suck at that
Thanks
The alkane series only has single bonds. The key characteristic of the alkenes series is that somewhere in there, there is a double bond.
A complete structural formula includes the bonds. 1-pentene is basically saying that there is a double bond, coming off carbon #1.
The correct answer is
C=C-C-C-C
with H's coming off the carbons. (Note: if you have a double bond, do not draw in too many H's)
A condensed formula just writes out what's there. For 1-pentene it would be
CH2CHCH2CH2CH3
For heptane this would be
CH3CH2CH2CH2CH2CH2CH3 (count the carbons! The prefix matters!)
For 1-heptene this would be
CH3CH2CHCHCH2CH2CH3
This one might not be obvious. It helps to draw out the complete structural formula whenever you're stuck.
Molecular formulae are easy. Alkanes have formula CnH2n+2, whereas alkenes have formula C2H2n
I'm leaving stuff unanswered because I want you to do some work yourself.
Post by: conic curve on July 22, 2016, 10:03:37 pm
Before commencing, your prefixes of meth-, eth-, prop-, but-, pent- etc. should be known off by heart first.
The alkane series only has single bonds. The key characteristic of the alkenes series is that somewhere in there, there is a double bond.
A complete structural formula includes the bonds. 1-pentene is basically saying that there is a double bond, coming off carbon #1.
The correct answer is
C=C-C-C-C
with H's coming off the carbons. (Note: if you have a double bond, do not draw in too many H's)
A condensed formula just writes out what's there. For 1-pentene it would be
CH2CHCH2CH2CH3
For heptane this would be
CH3CH2CH2CH2CH2CH2CH3 (count the carbons! The prefix matters!)
For 1-heptene this would be
CH3CH2CHCHCH2CH2CH3
This one might not be obvious. It helps to draw out the complete structural formula whenever you're stuck.
Molecular formulae are easy. Alkanes have formula CnH2n+2, whereas alkenes have formula C2H2n
I'm leaving stuff unanswered because I want you to do some work yourself.
Yes I know that but when it comes to answering them, I get stuck
Could you at least give me hints on them?
What does it mean by like 3-methy-2-difluro-1-ene (or something similar to that)?
Post by: RuiAce on July 22, 2016, 10:13:30 pm
Yes I know that but when it comes to answering them, I get stuckShould've been more specific about methyl groups and that stuff, yeah.
Could you at least give me hints on them?
What does it mean by like 3-methy-2-difluro-1-ene (or something similar to that)?
Firstly, let's clear up a nomenclature: 1-propene and prop-1-ene mean the same thing. The convention is to use the latter (very retarded change they implemented a few years ago, but we use it).
Let's fix your question into something more manageable.
3-methy-1,2-difluro-hept-1-ene
Getting a CFC (or rather HCFC) with a methyl group present is pretty darn rare but we can still do it.
We start by identifying the prefix: hept. This means there are 7 carbons
Note that it's hept-1-ene. Hence, there is a double bond coming off carbon 1
C=C-C-C-C-C-C
We are told that a methyl group comes off carbon number 3. We can add that in.
C
C=C-C-C-C-C-C
We are told that a fluorine comes off carbon 1, AND carbon 2. We can draw them in wherever appropriate
Cl C
C=C-C-C-C-C-C
Cl
Then just chuck all the appropriate hydrogens in there last.
Post by: conic curve on July 22, 2016, 10:20:02 pm
Should've been more specific about methyl groups and that stuff, yeah.
Firstly, let's clear up a nomenclature: 1-propene and prop-1-ene mean the same thing. The convention is to use the latter (very retarded change they implemented a few years ago, but we use it).
Let's fix your question into something more manageable.
3-methy-1,2-difluro-hept-1-ene
Getting a CFC (or rather HCFC) with a methyl group present is pretty darn rare but we can still do it.
We start by identifying the prefix: hept. This means there are 7 carbons
Note that it's hept-1-ene. Hence, there is a double bond coming off carbon 1
C=C-C-C-C-C-C
We are told that a methyl group comes off carbon number 3. We can add that in.
C
C=C-C-C-C-C-C
We are told that a fluorine comes off carbon 1, AND carbon 2. We can draw them in wherever appropriate
Cl C
C=C-C-C-C-C-C
Cl
Then just chuck all the appropriate hydrogens in there last.
Thanks so much :D
Post by: anotherworld2b on July 23, 2016, 07:07:15 pm
Post by: RuiAce on July 23, 2016, 07:35:14 pm
I've tried to answer q6 b and c but i got them wrong and i dont know whyYour answer to b) is unsimplified. The correct answer should be that without the 2's
H+ + OH- -> H2O(l)
You are safe to cancel out the numbers because this is a net IONIC form we're talking about.
_________________________________
Your working out for c) is wrong in the products. Here are proposed amendments.
1. Seperate CH3COOH(aq) into H+ and CH3COO- for better clarity in the reactants first
2. You wrote acetic acid when it's not there. It's only the acetate ion. Thus, replace the mistaken acetic acid with CH3COO-.
You should get MgO(s) + 2 H+ -> Mg2+ + H2Ol
Post by: anotherworld2b on July 23, 2016, 11:09:18 pm
I was also wondering how would you do q 8? I dont understand the question itself.
Your answer to b) is unsimplified. The correct answer should be that without the 2's
H+ + OH- -> H2O(l)
You are safe to cancel out the numbers because this is a net IONIC form we're talking about.
_________________________________
Your working out for c) is wrong in the products. Here are proposed amendments.
1. Seperate CH3COOH(aq) into H+ and CH3COO- for better clarity in the reactants first
2. You wrote acetic acid when it's not there. It's only the acetate ion. Thus, replace the mistaken acetic acid with CH3COO-.
You should get MgO(s) + 2 H+ -> Mg2+ + H2Ol
Post by: RuiAce on July 23, 2016, 11:19:04 pm
I got the answer for q6 b but the answer for q6 is different.CH3COOH(aq) is acetic acid. I disagree with the answer because the fact it is acetic acid means that it is definitely dissociated.
I was also wondering how would you do q 8? I dont understand the question itself.
Unless the question is being pedantic. Their answer isn't "completely" unjustified because some unsplit CH3COOH is in there as well. So I can see where they're coming from. But the question should've made that clearer - I don't like these ambiguous questions.
___________________
Q8 is combining equations. You have to do some maths here.
a) Equation 2 has just one CO in the reactants. But equation 1 has two COs in the products
So we have to multiply equation 2 by two to merge the equations:
2 CO(g) + 2 FeO(s) -> 2 Fe(l) + 2 CO2(g)
Substitute equation 1 in there to get:
C(s) + CO2(g) + 2 FeO(s) -> 2 Fe(l) + 2 CO2(g)
Cancel out the duplicated carbon dioxide:
C(s) + 2 FeO(s) -> 2 Fe(l) + CO2(g)
It's sort of like simultaneous equations: Perform (1) + 2*(2)
I'll let you have a try at the other two first. If you're still stuck, however (especially with 8c - that one is trickier), just post again
Post by: Aliceyyy98 on July 24, 2016, 12:44:05 am
I have a quick question about how coordinate covalent bonds work? Could someone please explain to me, perhaps using examples that would be great :) thanks in advance!
Cheers
Post by: RuiAce on July 24, 2016, 01:09:36 am
Hello!The basic idea is that whereas in your normal covalent bonds, two atoms share an electron with each other, in a coordinate covalent bond one atom shares BOTH of the electrons with the other.
I have a quick question about how coordinate covalent bonds work? Could someone please explain to me, perhaps using examples that would be great :) thanks in advance!
Cheers
E.g. The first one you must know of is ozone. In O2, there's a double bond between the two oxygen molecules (there are two covalent bonds, so both atoms share two electrons with each other). However in O3, that third oxygen gets TWO electrons from one of the other oxygen atoms.
It's more easily demonstrated with a Lewis dot structure diagram.
Post by: Aliceyyy98 on July 24, 2016, 01:17:19 am
The basic idea is that whereas in your normal covalent bonds, two atoms share an electron with each other, in a coordinate covalent bond one atom shares BOTH of the electrons with the other.
E.g. The first one you must know of is ozone. In O2, there's a double bond between the two oxygen molecules (there are two covalent bonds, so both atoms share two electrons with each other). However in O3, that third oxygen gets TWO electrons from one of the other oxygen atoms.
It's more easily demonstrated with a Lewis dot structure diagram.
Post by: RuiAce on July 24, 2016, 01:22:35 am
Hi thank you for you answer, just to add on, how would you know if a molecule has a covalent coordinate bond or not without a diagram or anything :(A pro-tip is to try drawing them all with covalent bonds first. If you can't successfully FORCE them to have a full outer shell (8 electrons) using only normal covalent bonds, there's probably a coordinate covalent bond somewhere.
However, the following three are ones that you just need to know that there is a coordinate covalent bond:
- Ozone O3: From one oxygen to the third oxygen
- Hydronium ion H3O+: From the oxygen to the third hydrogen
- Ammonium ion NH4+: From the nitrogen to the fourth hydrogen
Post by: angiezhang9 on July 24, 2016, 10:00:56 am
I don't understand this multiple choice question from the 2014 Catholic Trials.
An experiment was performed to determine the volume of carbon dioxide gas released during the fermentation of a glucose solution. The reaction took place over several days in a flask containing yeast that was immersed in a water bath at 28 degrees and stoppered with cotton wool. Which of the following factors would most significantly compromise the validity of this experiment?
A) The evaporation of water
B) The temperature of the water bath
C) Measurement error associated with determining the mass lost
D) Lack of oxygen due to the presence of the cotton wool plug
(The answer is A)
Thanks heaps! :)
Post by: RuiAce on July 24, 2016, 10:13:55 am
Hey :)One again, CSSA is giving very dodgy questions.
I don't understand this multiple choice question from the 2014 Catholic Trials.
An experiment was performed to determine the volume of carbon dioxide gas released during the fermentation of a glucose solution. The reaction took place over several days in a flask containing yeast that was immersed in a water bath at 28 degrees and stoppered with cotton wool. Which of the following factors would most significantly compromise the validity of this experiment?
A) The evaporation of water
B) The temperature of the water bath
C) Measurement error associated with determining the mass lost
D) Lack of oxygen due to the presence of the cotton wool plug
(The answer is A)
Thanks heaps! :)
Consider the answers:
D) is obviously wrong - The whole process of fermentation is dependent on anaerobic conditions. By removing oxygen, we are promoting the validity of the experiment.
C) is asking for the wrong thing - A measurement error affects the accuracy of the experiment, not the validity.
B) can be a bit more tricky. Obviously the best conditions for fermentation is at 37°C, but the water bath is at 28°C. This does not promote the fermentation of glucose and is also a candidate for why the experiment is invalid.
However, we can't just use this our basis. To consider the validity, look at the aim.
What we want to determine, is the volume of carbon dioxide released IN fermentation. The fermentation itself? It just has to proceed! Note that at some temperature like 28°C the reaction will still proceed, just that it will be hindered by a slow process.
Therefore B is out. But why is the answer A?
Post by: Happy Physics Land on July 24, 2016, 12:02:14 pm
Hey :)
I don't understand this multiple choice question from the 2014 Catholic Trials.
An experiment was performed to determine the volume of carbon dioxide gas released during the fermentation of a glucose solution. The reaction took place over several days in a flask containing yeast that was immersed in a water bath at 28 degrees and stoppered with cotton wool. Which of the following factors would most significantly compromise the validity of this experiment?
A) The evaporation of water
B) The temperature of the water bath
C) Measurement error associated with determining the mass lost
D) Lack of oxygen due to the presence of the cotton wool plug
(The answer is A)
Thanks heaps! :)
Hey Angie!
Rui has asked me to supplement his answer so please have a read what I wrote below :)
A is the correct option simply because all we are trying to measure is the volume of CO2 produced. If water evaporates (especially since we are carrying out the experiment under 28 degrees here, so a large chance that some water would have evaporated) then we obtain a measurement that includes a combined mass loss of CO2 and H2O. Validity means "are we measuring what we want to measure" and this is clearly not the case if water has also evaporated. Keep in mind, WE ONLY WANT TO MEASURE THE MASS OF CARBON DIOXIDE.
Explaining this one step further. When you conducted this experiment what you would have done is to measure the mass of fermentation mixture before fermentation has taken place, and the mass of fermentation mixture after fermentation has taken place. The loss in mass after fermentation you would assume to be CO2 (You would probably have carried out the experiment with a limewater flask connected to the fermentation mixture as well). But keep in mind that even at room temperature, water evaporation still occurs, meaning that whilst CO2 is being evaporated, water is too! So when you measure the lost weight, thats a combined weight of evaporated CO2 and WATER. So evidently we are not measuring JUST the mass of CO2 and therefore the experiment becomes invalid.
So, how do we improve validity? We can do one of the two things:
1. Keep a carton of water under the same temperature as your fermentation mixture. Make sure the volume of the water is the same as the volume of your fermentation mixture. Measure the loss in the amount of water at the same time when you measure the loss in mass of the fermentation mixture, subtract this from the measured weight loss of the fermentation mixture. Why are we doing this? Because we want to see how much water has evaporated under the same conditions and in an equal amount of time as fermentation takes place. This way we can measure only the mass of CO2 produced.
2. This is an easier method. Simply connect your fermentation mixture to a conical flask filled with limewater. When CO2 is released it reacts with Ca(OH)2 (lime) to form a milky liquid CaCO3. By measuring the increase in the weight of the limewater and divide this weight by the molar mass of CO2 we can work out how many moles of CO2 has been produced and therefore calculate the volume of CO2 produced. Make sure when you do this experiment that everything is enclosed to prevent CO2 from escaping into the air. Why do we not worry about water evaporation in this case? Because it doesnt affect anything! It does not react or enter the limewater and therefore all we are measuring here is the amount of CO2 produced!
A bit long (quite long tbh) for a multiple choice answer, but I thought I would just make everything clearer for you. :)
Best Regards
Happy Physics Land
Post by: angiezhang9 on July 24, 2016, 02:08:32 pm
Hey Angie!
Rui has asked me to supplement his answer so please have a read what I wrote below :)
A is the correct option simply because all we are trying to measure is the volume of CO2 produced. If water evaporates (especially since we are carrying out the experiment under 28 degrees here, so a large chance that some water would have evaporated) then we obtain a measurement that includes a combined mass loss of CO2 and H2O. Validity means "are we measuring what we want to measure" and this is clearly not the case if water has also evaporated. Keep in mind, WE ONLY WANT TO MEASURE THE MASS OF CARBON DIOXIDE.
Explaining this one step further. When you conducted this experiment what you would have done is to measure the mass of fermentation mixture before fermentation has taken place, and the mass of fermentation mixture after fermentation has taken place. The loss in mass after fermentation you would assume to be CO2 (You would probably have carried out the experiment with a limewater flask connected to the fermentation mixture as well). But keep in mind that even at room temperature, water evaporation still occurs, meaning that whilst CO2 is being evaporated, water is too! So when you measure the lost weight, thats a combined weight of evaporated CO2 and WATER. So evidently we are not measuring JUST the mass of CO2 and therefore the experiment becomes invalid.
So, how do we improve validity? We can do one of the two things:
1. Keep a carton of water under the same temperature as your fermentation mixture. Make sure the volume of the water is the same as the volume of your fermentation mixture. Measure the loss in the amount of water at the same time when you measure the loss in mass of the fermentation mixture, subtract this from the measured weight loss of the fermentation mixture. Why are we doing this? Because we want to see how much water has evaporated under the same conditions and in an equal amount of time as fermentation takes place. This way we can measure only the mass of CO2 produced.
2. This is an easier method. Simply connect your fermentation mixture to a conical flask filled with limewater. When CO2 is released it reacts with Ca(OH)2 (lime) to form a milky liquid CaCO3. By measuring the increase in the weight of the limewater and divide this weight by the molar mass of CO2 we can work out how many moles of CO2 has been produced and therefore calculate the volume of CO2 produced. Make sure when you do this experiment that everything is enclosed to prevent CO2 from escaping into the air. Why do we not worry about water evaporation in this case? Because it doesnt affect anything! It does not react or enter the limewater and therefore all we are measuring here is the amount of CO2 produced!
A bit long (quite long tbh) for a multiple choice answer, but I thought I would just make everything clearer for you. :)
Best Regards
Happy Physics Land
Thanks so much for clearing this up. It makes sense now :) RuiAce and Happy Physics Land
Post by: Happy Physics Land on July 24, 2016, 03:02:33 pm
Thanks so much for clearing this up. It makes sense now :) RuiAce and Happy Physics Land
No worries angie, what we do here the best is to clear things up! :)
Post by: anotherworld2b on July 25, 2016, 01:10:39 am
CH3COOH(aq) is acetic acid. I disagree with the answer because the fact it is acetic acid means that it is definitely dissociated.
Unless the question is being pedantic. Their answer isn't "completely" unjustified because some unsplit CH3COOH is in there as well. So I can see where they're coming from. But the question should've made that clearer - I don't like these ambiguous questions.
___________________
Q8 is combining equations. You have to do some maths here.
a) Equation 2 has just one CO in the reactants. But equation 1 has two COs in the products
So we have to multiply equation 2 by two to merge the equations:
2 CO(g) + 2 FeO(s) -> 2 Fe(l) + 2 CO2(g)
Substitute equation 1 in there to get:
C(s) + CO2(g) + 2 FeO(s) -> 2 Fe(l) + 2 CO2(g)
Cancel out the duplicated carbon dioxide:
C(s) + 2 FeO(s) -> 2 Fe(l) + CO2(g)
It's sort of like simultaneous equations: Perform (1) + 2*(2)
I'll let you have a try at the other two first. If you're still stuck, however (especially with 8c - that one is trickier), just post again
For the carbon dioxide in q8 a how do you know to cancel it out? Also does it matter which equation you subsitute?
I am not sure how to do part c :-[
Post by: RuiAce on July 25, 2016, 09:17:50 am
For the carbon dioxide in q8 a how do you know to cancel it out? Also does it matter which equation you subsitute?I didn't. It appeared on both the products AND the reactants once I did simultaneous equations, so that's when I knew to cancel them out.
I am not sure how to do part c :-[
c) seems to have two correct answers. I'm not sure which direction to head either, actually. Can we please have the answers?
(It's because I'm not sure whether we want Cl to cancel out, or ClO to cancel out.)
Post by: leila_ameli on July 25, 2016, 01:35:03 pm
I need a some help with this question:
Bond energy is the energy needed to break the chemical bond between two atoms. The bond energies for each of the two allotropes of oxygen are shown in the table.
Oxygen Ozone
Bond Energy (kJ mol-1) 498 364
Account for the difference in bond energy for these two allotropes of oxygen.
Post by: Aliceyyy98 on July 25, 2016, 01:48:44 pm
Could someone please the concept of buffers? with an example and preferably equations to illustrate? thanks!
Post by: Maz on July 25, 2016, 04:01:01 pm
Hi guys,Hey human
Could someone please the concept of buffers? with an example and preferably equations to illustrate? thanks!
Hopefully, I can help a bit...
Buffers are solutions with the ability to resist pH change when small quantities of acid/base are added to them. Buffers are generally a solution which contains a weak acid-base conjugate pair (a weak acid and its conjugate base or vice-versa). These acids and bases and co-exist without neutralising each other, as neutralisation generally occurs in an acid-base reaction, yet they can still react to neutralise any strong acid or strong base added to the buffer.
An example of a buffer solution could include a 2mol ethanoic acid solution, and 2 mol of sodium ethanoate; then making the solution u to 1L. The buffer contains a high concentration of both weak acid CH3COOH and its conjugate weak base, CH3COO-.
The equilibrium reaction is shown below:
CH3COOH (aq) + H20 (l) <----> CH3COO- +H3O+(aq)
Buffer behaviour can be predicted via Le Chateliers Principle (LCP). Eg, if you add a base to the buffer will neutralise some of the H3O+ present, causing its concentration to fall. This falling concentration causes the equilibrium reaction to shift right, as explained via LCP, (forward reaction favoured) o replace some of the lost hydronium concentration, and preventing too much of a significant fall. This is the property of a buffer being able to resist some changes in pH. If you flood the buffer with acid/base this will not work, and a pH change will result (each buffer has a buffer capacity)
Post by: jakesilove on July 25, 2016, 04:25:06 pm
heyy
I need a some help with this question:
Bond energy is the energy needed to break the chemical bond between two atoms. The bond energies for each of the two allotropes of oxygen are shown in the table.
Oxygen Ozone
Bond Energy (kJ mol-1) 498 364
Account for the difference in bond energy for these two allotropes of oxygen.
Hey!
Essentially, all I would be talking about to answer this question is the reactivity of each alloptrope. As Ozone has a lower bond energy, it will be easier to break/react. Oxygen will be more difficult to break/react. Clearly, Ozone is more reactive than Oxygen. We can explain this by saying that coordinate covalent bonds are easier to break than normal covalent bonds. This explains why Ozone only really forms in higher levels of the atmosphere, where even more reactive Oxygen free radicals are present to react. I think this is as much as an answer would require in the HSC!
Jake
Post by: MysteryMarker on July 25, 2016, 09:03:03 pm
During the electrolysis of dilute NaCl, why is the water oxidised instead of the Chlorine ions? Because in the concentrated NaCl solution, the chlorine ions do get oxidised, however in the very dilute solution they do not. Just curious as to why this is.
Cheers.
Post by: marynguyen18 on July 25, 2016, 09:11:07 pm
Post by: RuiAce on July 25, 2016, 09:23:15 pm
Industrial Chemistry Question:Compare the Eo values of both oxidations:
During the electrolysis of dilute NaCl, why is the water oxidised instead of the Chlorine ions? Because in the concentrated NaCl solution, the chlorine ions do get oxidised, however in the very dilute solution they do not. Just curious as to why this is.
Cheers.
2 Cl- ⇌ Cl2(g) + 2 e- Eo=-1.36 V
2 H2O(l) ⇌ O2(g) + 4 H+ + 4 e- Eo=-1.23 V
Notice how the two values are very similar. This is opposed to the reduction, where the voltage required to cause sodium to reduce is significantly larger to water reducing.
Because the voltage is so similar, the EMF supplied will seek out the more abundant substance as about a similar amount of voltage is consumed anyway for either reaction. In concentrated sodium chloride solution, there is actually a sufficient amount of chloride ions for their oxidation to be favoured. In dilute solution, there really isn't that many, and water will be oxidised anyway.
Hey i was wondering if anyone has studied/currently studying shipwrecks that could help me understand the difference between a galvanic cell and an electrolytic cell, because I'm getting confused with the charge of the cathode and anode are.Fortunately this overlaps into industrial chemistry.
Galvanic cells and electrolytic cells function oppositely.
In a galvanic cell, a chemical reaction takes place producing a voltage, thereby chemical energy is converted to electrical energy
In an electrolytic cell, electricity is passed through a cell to reverse the effect of the galvanic cell, thereby electrical energy is converted to galvanic energy.
(The EMF value for a galvanic cell is positive, whereas that of an electrolytic cell is negative.)
Effectively, electrolytic cells can be used as battery rechargers.
The mnemonic RED-CAT AN-OX still takes play. Reduction happens at the cathode and oxidation happens at the anode.
However, the polarity reverses in an electrolytic cell.
For a galvanic cell, the anode is negative and the cathode is positive.
Cathode: Cu2+ + 2 e- -> Cu(s)
Anode: Zn(s) -> Zn2+ + 2 e-
For an electrolytic cell, the anode is positive and the cathode is negative.
Cathode: Zn2+ + 2 e- -> Zn(s)
Anode: Cu(s) -> Cu2+ + 2 e-
Deleted last bit. Forgot the why. JAKE!
Post by: marynguyen18 on July 25, 2016, 09:37:25 pm
Post by: RuiAce on July 25, 2016, 09:39:01 pm
thank you so much! but with the half equations why is that some of them are either producing H+ or OH- if that makes sense?Not quite
All I can respond to out of that is that one of them is an oxidation whilst the other is a reduction
Post by: onepunchboy on July 25, 2016, 09:56:57 pm
Hey guys im not sure to do q 15 hehe thanks
Post by: MysteryMarker on July 25, 2016, 10:03:00 pm
Compare the Eo values of both oxidations:
2 Cl- ⇌ Cl2(g) + 2 e- Eo=-1.36 V
2 H2O(l) ⇌ O2(g) + 4 H+ + 4 e- Eo=-1.23 V
Notice how the two values are very similar. This is opposed to the reduction, where the voltage required to cause sodium to reduce is significantly larger to water reducing.
Because the voltage is so similar, the EMF supplied will seek out the more abundant substance as about a similar amount of voltage is consumed anyway for either reaction. In concentrated sodium chloride solution, there is actually a sufficient amount of chloride ions for their oxidation to be favoured. In dilute solution, there really isn't that many, and water will be oxidised anyway.Fortunately this overlaps into industrial chemistry.
Cheers man, another question for industrial chemistry:
Within a diaphragm cell, why is it more important to stop the migration of hydroxide ions into the anode compartment then it is to stop the migration of chloride ions into the cathode compartment?
Post by: anotherworld2b on July 25, 2016, 10:51:34 pm
I didn't. It appeared on both the products AND the reactants once I did simultaneous equations, so that's when I knew to cancel them out.
c) seems to have two correct answers. I'm not sure which direction to head either, actually. Can we please have the answers?
(It's because I'm not sure whether we want Cl to cancel out, or ClO to cancel out.)
Post by: Happy Physics Land on July 25, 2016, 11:30:10 pm
Cheers man, another question for industrial chemistry:
Within a diaphragm cell, why is it more important to stop the migration of hydroxide ions into the anode compartment then it is to stop the migration of chloride ions into the cathode compartment?
Hey Mystery Marker!
Both are equally just as important as each other. By preventing OH- from entering the anode we are ensuring that all of the Na+ from the brine are reacting with OH-. This is important because after all, it takes energy to reduce water to produce hydroxide and therefore any waste of OH- (i.e. those that dont end up participating in the formation of NaOH) is undesirable as it is wasting the energy input. We also dont want Cl- to migrate into cathode compartment because we dont like the formation of NaCl in our final product, all we want is NaOH. A major concern that may answer your question is that we can get Na+ and Cl- very easily, just from natural sea water (brine) but to get OH- we need quite an intensive amount of energy (diaphragm cell needs about 2750kwh energy). So it's less of a concern to form NaCl than to not form NaOH.
Best regards
Happy Physics Land
Post by: RuiAce on July 25, 2016, 11:40:03 pm
Lol i made a dumb. Of course the final products must stay there.
In (1) and (2) there is exactly 1 mol of Cl(g) there. Thus, we can immediately perform (1) + (2)
CFCl3(g) + Cl(g) + O3(g) -> CFCl2(g) + Cl(g) + ClO(g) + O2(g)
Cancel out the Cl on both sides:
CFCl3(g) + O3(g) -> CFCl2(g) + ClO(g) + O2(g)
This new equation, and equation 3, both have exactly 1 mol of ClO present. Thus we can perform (3) + the above
CFCl3(g) + O3(g) + ClO(g) + O(g) -> CFCl2(g) + ClO(g) + O2(g) + Cl(g) + O2(g)
Cancel out the ClO:
CFCl3(g) + O3(g) + O(g) -> CFCl2(g) + 2 O2(g) + Cl(g)
(http://uploads.tapatalk-cdn.com/20160725/7b2bc71fff701243b9595ec7e7dd1c0b.jpg)This is just your ordinary rigorous calculation.
Hey guys im not sure to do q 15 hehe thanks
Write out relevant equations:
A) K2CO3 + 2 HCl -> CO2 + H2O + 2 KCl
B) KHCO3 + HCl -> CO2 + H2O + KCl
C) Na2CO3 + 2 HCl -> CO2 + H2O + 2 KCl
D) NaHCO3 + HCl -> CO2 + H2O + KCl
Clearly, 1 mol of reactant _ yields 1 mol of CO2 in all four equations.
Now, determine the moles of each substance present using n=m/MM:
A) nCO2 = nK2CO3 = 7.23 * 10-3 mol
B) nCO2 = nKHCO3 = 1.05 * 10-2 mol
C) nCO2 = nNa2CO3 = 9.43 * 10-3 mol
D) nCO2 = nNaHCO3 = 1.19 * 10-2 mol
So since n = V/VM, clearly the answer must be D.
And now for the smart way
However obvious it was that the moles of the reactant and the moles of CO2 were the same varies for each person. But without even writing out the full equation you
Then, because m=1, we have that the moles is INVERSELY proportional to the molar mass. Thus the one with the smallest molar mass (NaHCO3) will yield the largest moles.
Post by: Aliceyyy98 on July 26, 2016, 12:57:50 am
Hey human
Hopefully, I can help a bit...
Buffers are solutions with the ability to resist pH change when small quantities of acid/base are added to them. Buffers are generally a solution which contains a weak acid-base conjugate pair (a weak acid and its conjugate base or vice-versa). These acids and bases and co-exist without neutralising each other, as neutralisation generally occurs in an acid-base reaction, yet they can still react to neutralise any strong acid or strong base added to the buffer.
An example of a buffer solution could include a 2mol ethanoic acid solution, and 2 mol of sodium ethanoate; then making the solution u to 1L. The buffer contains a high concentration of both weak acid CH3COOH and its conjugate weak base, CH3COO-.
The equilibrium reaction is shown below:
CH3COOH (aq) + H20 (l) <----> CH3COO- +H3O+(aq)
Buffer behaviour can be predicted via Le Chateliers Principle (LCP). Eg, if you add a base to the buffer will neutralise some of the H3O+ present, causing its concentration to fall. This falling concentration causes the equilibrium reaction to shift right, as explained via LCP, (forward reaction favoured) o replace some of the lost hydronium concentration, and preventing too much of a significant fall. This is the property of a buffer being able to resist some changes in pH. If you flood the buffer with acid/base this will not work, and a pH change will result (each buffer has a buffer capacity)
Thanx a bunch!
Post by: Alexander23 on July 26, 2016, 08:56:38 am
Also, I was hoping for a clearer explanation as to why temperature is the only factor which effects the K expression of an equilibrium reaction.
Thanks. :)
Post by: RuiAce on July 26, 2016, 09:14:02 am
I've been told that the addition of an inert gas has no effect on a reaction at equilibrium. I was wondering why it has no effect on equilibrium direction, even if you are adding in this non reaction gas in a closed system which seems to decrease volume and hence increase pressure.Adding an inert gas is essentially doing the opposite of the effect of a catalyst - it slows down the rate equilibrium is attained however does nothing to the equilibrium itself.
Also, I was hoping for a clearer explanation as to why temperature is the only factor which effects the K expression of an equilibrium reaction.
Thanks. :)
This is because by adding a catalyst, you are not hindering just ONE of the forward/reverse reactions. You are actually hindering BOTH of the forward/reverse reactions.
If you change the pressure by pumping in more equilibrium mixture you are causing an imbalance in the concentrations of products and reactants. This is the same as varying the volume of the vessel, because the amount of space you have will promote either the forward or reverse reaction to allow the moles of gas to be reasonable. (Recall: Avogadro's law states that at equal temperatures and pressures, systems will rather having an equal number of molecules of gases.)
Now, temperature is an extrinsic factor to the position of the equilibrium. All other factors such as pressure and varying concentrations are intrinsic. When you alter the pressure by pumping equilibrium mixture, change the volume of the vessel or increase/decrease concentrations of just one substance, as a consequence of Le Chatelier's Principle the system will try to bring back the equilibrium it started with.
Note that K is just a rough indicator of where the equilibrium actually is to begin with! The concentrations of your reactants/products readjust so that the reaction quotient Q returns to K.
When you alter the temperature, however, you didn't actually alter the concentrations of the reactants and products. Instead, you manipulated the fact that the reaction is exothermic or endothermic. From LCP we do know that the equilibrium does indeed adjust if you supply/take out heat, however in doing so you're forcing the ratio of products and reactants to change themselves WITHOUT actually having added/taken away any in advance! Hence, K will actually change for different temperatures
Post by: liiz on July 26, 2016, 02:55:45 pm
Post by: sweetcheeks on July 26, 2016, 03:52:49 pm
Hey there, just wondering if someone would be please able to help me with this question. I find calculation questions super hard so any explanation would be awesome, thankyou!! ;D ;D"Phosphorus pentoxide reacts with water to form phosphoric acid according to the following equation P2O5(s) + 3H2O(l) → 2H3PO4 (aq) Phosphoric acid reacts with sodium hydroxide according to the following equation. H3PO4 (aq) + 3NaOH(aq) → Na3PO4 (aq) + 3H2O(l). A student reacted 1.42 g of phosphorus pentoxide with excess water. What volume of 0.30 mol L-1 sodium hydroxide would be required to neutralise all the phosphoric acid produced?"
Initially we start with 1.42g of phosphorus pentoxide. First we must find the amount of moles of phosphorus pentoxide. Each molecule of phosphorus pentoxide produces two mole of phosphoric acid. The phosphoric acid produced will then react with Sodium hydroxide in a 1:3 ratio, that is you need 3 molecules of NaOH per molecule of phosphoric acid.
Here are the steps
1. Find moles of phosphorus pentoxide, using the formula n=m/M where n=number moles m=grams M=molecular weight
2. Ratio this to moles of Phosphoric acid produced (2:1)
3. Substitute the mole of phosphoric acid into the second equation and ratio to NaOH to find the amount (in mole) of NaOH required. Using the formula n/C=v (rearranged C=n/v), where n=moles of NaOH, C= concentration of the NaOH solution and V is the volume in litres.
It helps to draw a diagram when doing questions like this. Also, write out both equations and work in columns under the substances of interest (i.e. when finding moles of phosphorus pentoxide, work underneath it on the formula.)
Is there anything in particular that you struggle with when it comes to these type of questions?
Post by: RuiAce on July 26, 2016, 05:01:10 pm
Hey there, just wondering if someone would be please able to help me with this question. I find calculation questions super hard so any explanation would be awesome, thankyou!! ;D ;D"Phosphorus pentoxide reacts with water to form phosphoric acid according to the following equation P2O5(s) + 3H2O(l) → 2H3PO4 (aq) Phosphoric acid reacts with sodium hydroxide according to the following equation. H3PO4 (aq) + 3NaOH(aq) → Na3PO4 (aq) + 3H2O(l). A student reacted 1.42 g of phosphorus pentoxide with excess water. What volume of 0.30 mol L-1 sodium hydroxide would be required to neutralise all the phosphoric acid produced?"Because the equations are given to you, there's one less thing to do. Because normally when you work between reactants/products and other reactants/products you always need a chemical equation.
You also always work in moles. According to n=m/MM
nP2O5 = 1.42/(2*30.97 + 5*16.00) = 0.010004227... mol
Look at the equation and compare the mole ratio. 1 mol of diphosphorus pentoxide produces 2 moles of phosphoric acid
nH3PO4 = 2 nP2O5 = 0.02000845427... mol
So we now know the moles of phosphoric acid we must neutralise.
Look at the second equation for the neutralisation. Clearly three moles of NaOH is required to neutralise one mole of H3PO4
nNaOH = 3 nH3PO4 = 0.06002536282... mol
We are given the concentration of NaOH. Because we know that C=n/V
V = n/C
VNaOH = 0.06002536282... / 0.30
= 0.06002536282... L
The lowest amount of significant figures used in this question is three (from the mass of diphosphorus pentoxide. So rounding to 3 s.f.
V ≈ 6.0 * 10-2 L
Key strategy: Break the question apart and do not try to do multiple things at once. Chemistry calculations are always one-way.
It helps to draw a diagram when doing questions like this. Also, write out both equations and work in columns under the substances of interest (i.e. when finding moles of phosphorus pentoxide, work underneath it on the formula.)May I ask what diagram?
Post by: sweetcheeks on July 26, 2016, 05:22:31 pm
Post by: RuiAce on July 26, 2016, 05:30:43 pm
@RuiAce a diagram that shows the steps of the process. It helps visualize what is going on. for example, you draw the phosphorus pentoxide being weighed out, being added to water, the solution being reacted with NaOH. Add the information underneath to help keep track of what is going on and what where calculations need to be made. It works especially great for things like titrations involving samples and dilutions. My teacher swears by this and always did it when he worked in the field.A flow chart?
Post by: MysteryMarker on July 26, 2016, 05:35:04 pm
During the production of soap in a school laboratory, why is saturated salt solution added to aid in the precipitation of soap from the mixture?
Thanks guys.
Post by: RuiAce on July 26, 2016, 05:46:48 pm
Industrial Chemistry Question:Haven't done this in ages so I don't remember much at all but I'm pretty sure you do this both in the laboratory and in industrial preparation.
During the production of soap in a school laboratory, why is saturated salt solution added to aid in the precipitation of soap from the mixture?
Thanks guys.
The idea is to manipulate a precipitation procedure known as 'salting out'. The solubility of the soap is forced down because the other ions (Na+ and Cl-), which are more soluble, dissolve in their place. It makes it easier for us to collect the soap by using this method.
Post by: leila_ameli on July 26, 2016, 08:20:30 pm
Hey!
Essentially, all I would be talking about to answer this question is the reactivity of each alloptrope. As Ozone has a lower bond energy, it will be easier to break/react. Oxygen will be more difficult to break/react. Clearly, Ozone is more reactive than Oxygen. We can explain this by saying that coordinate covalent bonds are easier to break than normal covalent bonds. This explains why Ozone only really forms in higher levels of the atmosphere, where even more reactive Oxygen free radicals are present to react. I think this is as much as an answer would require in the HSC!
Jake
thankyouu!!
Post by: liiz on July 26, 2016, 08:47:03 pm
Initially we start with 1.42g of phosphorus pentoxide. First we must find the amount of moles of phosphorus pentoxide. Each molecule of phosphorus pentoxide produces two mole of phosphoric acid. The phosphoric acid produced will then react with Sodium hydroxide in a 1:3 ratio, that is you need 3 molecules of NaOH per molecule of phosphoric acid.Ah, awesome I get you. Those steps really helped to guide me through working it out :) And the diagram suggestion is really awesome too!! I've never thought to do diagrams like that before, and as a more visual learner it's something I can definitely incorporate now!! Thanks so much for your help :)) I think I just struggle to simply comprehend what the question is actually asking me and relate it to the formulas I know and what info they've given. I guess just doing heaps of practice, I'll (hopefully) get there haha
Here are the steps
1. Find moles of phosphorus pentoxide, using the formula n=m/M where n=number moles m=grams M=molecular weight
2. Ratio this to moles of Phosphoric acid produced (2:1)
3. Substitute the mole of phosphoric acid into the second equation and ratio to NaOH to find the amount (in mole) of NaOH required. Using the formula n/C=v (rearranged C=n/v), where n=moles of NaOH, C= concentration of the NaOH solution and V is the volume in litres.
It helps to draw a diagram when doing questions like this. Also, write out both equations and work in columns under the substances of interest (i.e. when finding moles of phosphorus pentoxide, work underneath it on the formula.)
Is there anything in particular that you struggle with when it comes to these type of questions?
Post by: liiz on July 26, 2016, 08:49:14 pm
Because the equations are given to you, there's one less thing to do. Because normally when you work between reactants/products and other reactants/products you always need a chemical equation.Thanks Ruiace for your input too!! I was able to do it from Sweetcheeks and then check my working with yours which was really helpful!! Awesome tip as well to not do multiple things at once - will stick by it a bit more now!! Thanks again :))
You also always work in moles. According to n=m/MM
nP2O5 = 1.42/(2*30.97 + 5*16.00) = 0.010004227... mol
Look at the equation and compare the mole ratio. 1 mol of diphosphorus pentoxide produces 2 moles of phosphoric acid
nH3PO4 = 2 nP2O5 = 0.02000845427... mol
So we now know the moles of phosphoric acid we must neutralise.
Look at the second equation for the neutralisation. Clearly three moles of NaOH is required to neutralise one mole of H3PO4
nNaOH = 3 nH3PO4 = 0.06002536282... mol
We are given the concentration of NaOH. Because we know that C=n/V
V = n/C
VNaOH = 0.06002536282... / 0.30
= 0.06002536282... L
The lowest amount of significant figures used in this question is three (from the mass of diphosphorus pentoxide. So rounding to 3 s.f.
V ≈ 6.0 * 10-2 L
Key strategy: Break the question apart and do not try to do multiple things at once. Chemistry calculations are always one-way.
May I ask what diagram?
Post by: MysteryMarker on July 27, 2016, 04:22:56 pm
Hey guys, just wondering what would the recommended structure be for this/ some main points i would need to discuss to get the 6 marks.
Thanks
Post by: jakesilove on July 27, 2016, 11:13:52 pm
Assess the significance of the industrial development of the Haber process to society at the beginning on the 1900s. Include a relevant chemical equation in your answer. 6 marks.
Hey guys, just wondering what would the recommended structure be for this/ some main points i would need to discuss to get the 6 marks.
Thanks
Hey!
Firstly, holy crap. What a terrifying question. Still, definitely something that you can work through, and get 6 marks for. Let's look at the subheadings that I would recommend you used.
History
You need to explain WHY the Haber process was developed, by putting it in context. Talk about the importance of Ammonia is soil, which was naturally sourced from Chile. The British blockaded Germany following the outbreak of WWI (1914-8), and they could not longer receive Ammonia. Their crops perished, and hundreds of thousands of Germans died on the homefront. Haber was a chemist who developed a process to produce Ammonia.
Chemistry
Here, put the Chemical equation (Which I can't be bothered typing out), and the reaction conditions that yield the highest result (plus the catalyst). YOU WILL LOSE A MARK FOR LEAVING OUT STATES!
Significance
Here, you need to make an assessment. The Haber process allowed for Ammonia-rich soil to grow crops, saving thousands of starving Germans. It was also used to produce bombs. Both of these, potentially, INCREASED the length of WWI, in which over 17 million people were killed. Thus, the development of the process at this specific period of time could be conceived as vastly negative.
You're assessment doesn't need to be exactly what I said above. However, notice the specific details I've thrown in (dates of WWI, people killed, Chile etc.). I've done that to impress the marker, because this is a beast of a question, and figuring out where to get 6 marks is tough. Make sure to remember to impress the marker!
That's what I think you should include; feel free to throw in some more suggestions! You need to be ready for shitty questions like this, so know each dot point in depth. Hopefully, however, you don't come across a beast like this in your Trials :)
Jake
Post by: Maz on July 28, 2016, 12:58:04 pm
My class recently did an experiment on the reactivity of alcohols. We reacted primary, secondary and tertiary alcohols with dichromate, permanganate, and sodium...I have formed the reaction redox equations. We have a test on it on Friday and I was wandering if you had any idea about some of the questions that may come up/any questions that I should really study?
Also one more thing please...For household cleaning would it be better to have a primary alchohol, or secondary? In a primary one there is more solubility, but less reactivity. In a secondary one there would be slightly less solubility, and a bit more reactivity. I'm thinking secondary...but I'm not really sure?
Any help would be really appreciated :)
Thankyou,
Maryam
Post by: jakesilove on July 28, 2016, 02:39:58 pm
Hey guys, :)
My class recently did an experiment on the reactivity of alcohols. We reacted primary, secondary and tertiary alcohols with dichromate, permanganate, and sodium...I have formed the reaction redox equations. We have a test on it on Friday and I was wandering if you had any idea about some of the questions that may come up/any questions that I should really study?
Also one more thing please...For household cleaning would it be better to have a primary alchohol, or secondary? In a primary one there is more solubility, but less reactivity. In a secondary one there would be slightly less solubility, and a bit more reactivity. I'm thinking secondary...but I'm not really sure?
Any help would be really appreciated :)
Thankyou,
Maryam
Hey MQ,
Unfortunately, none of that is part of the HSC curriculum. I've done some Chemistry at Uni, but never heard of that terminology. I'm afraid I'm not going to be able to help you out, I'm really sorry! I hope you can do some research and find the answers you seek :)
Jake
Post by: HighTide on July 28, 2016, 04:07:56 pm
Hey guys, :)Interesting question. I was given to think that primary alcohols were also more reactive due to their exposure? And secondary, and tertiary alcohols were progressively more stable. From what we've learned at uni, detergents and all that rely on the ability to form micelles. So one side would be hydrophobic and this would attach to the dirt, the other part would be hydrophilic and form hydrogen bonds, so technically more soluble would be preferred? This is just an idea and this is for detergents and stuff like that. I'm not 100% sure about household cleaning, but perhaps it's a similar idea.
My class recently did an experiment on the reactivity of alcohols. We reacted primary, secondary and tertiary alcohols with dichromate, permanganate, and sodium...I have formed the reaction redox equations. We have a test on it on Friday and I was wandering if you had any idea about some of the questions that may come up/any questions that I should really study?
Also one more thing please...For household cleaning would it be better to have a primary alchohol, or secondary? In a primary one there is more solubility, but less reactivity. In a secondary one there would be slightly less solubility, and a bit more reactivity. I'm thinking secondary...but I'm not really sure?
Any help would be really appreciated :)
Thankyou,
Maryam
Hopefully |zxn| or jyce may be able to shed light on this.
Post by: anotherworld2b on July 29, 2016, 01:30:13 am
Post by: wesadora on July 29, 2016, 01:52:14 pm
Post by: jakesilove on July 29, 2016, 02:54:49 pm
I was wondering if i could get help in understandong dipole dipole forces, dispersion forces and hydrogen bonding. We went through these forces extremely briefly and quickly at school so i am bit confused about what there are and their purpose
(http://images.slideplayer.com/1/261608/slides/slide_22.jpg)
Hey! I'll just give a brief summary of these forces.
Hydrogen Bonding
Hydrogen is attracted to very electronegative elements. So, if there is hydrogen present in the chemical structure of a compound, and there are also atoms of Oxygen, Flourine or Nitrogen (FON!), there will be strong intermolecular forces between them. See the image above, it makes it pretty clear.
Dipole-Dipole
This is similar to Hydrogen bonding, but less strong. Some elements have positive charges (eg. metals) and some have negative charges (eg. halons). These will naturally be attracted, and that's what dipole-dipole forces are. If a compound has a 'negative' side, it will be attracted to the 'positive' side of another compound. This is only the case if the compound is polar (ie. has a positive and negative side).
Dispersion forces
These forces occur is all compounds. As electrons move around the nucleus, sometimes they will be in certain areas with a higher density than elsewhere. As such, there will be a greater negative charge in that region. This essentially creates a 'more positive' area, and a 'more negative' area, which line up and are attracted to each other.
Hope this helps! Let me know if I can expand; they aren't too difficult as concepts, hopefully the image helps.
Jake
Post by: jakesilove on July 29, 2016, 03:02:56 pm
Two questions are attached. I don't even understand what w/v is, and I'm just very confused on the whole dissolved oxygen calculations in general. I suppose I understand the theory behind it (i.e. more dissolved oxygen in a waterway = good, as it is plentiful for aquatic life to live healthily etc. etc.) but don't understand these calcs. Thanks :)
Two questions are attached. I don't even understand what w/v is, and I'm just very confused on the whole dissolved oxygen calculations in general. I suppose I understand the theory behind it (i.e. more dissolved oxygen in a waterway = good, as it is plentiful for aquatic life to live healthily etc. etc.) but don't understand these calcs. Thanks :)
Cool question!
So, we know that at 25 degrees, 8mg/L of Oxygen will be dissolved. This can be read straight from the graph. However, there are 10.0L of water, therefore we know that 10*8 = 80mg of Oxygen is dissolved in the liquid.
Now, we need a volume of Oxygen, not a weight. First, let's convert from mg to g.
We can now convert this into moles.
We know that moles of gas occupy a fixed volume at a certain temperature: specifically, 24.79L per mole.
So the answer is A, 62mL. I think I may have screwed up the number of zeros somewhere (there should be one less), but that is the general structure of the answer. Hope that helps!
Jake
Post by: wesadora on July 29, 2016, 03:36:19 pm
here's the other one i was confused about, sorry thought I attached two.
and a quick question while my brain doesn't forget to ask: Does ozone have a higher MP/BP than Oxygen because of the stronger IMFs (i.e. dipole dipole AND two sites for hydrogen bonding? or is it just the dipole dipole?
CHEEEEEEEEEEEEEEEEERS m8 :)
Post by: conic curve on July 29, 2016, 03:47:43 pm
A student performed an experiment to determine the molar heat of solution of ammonium chloride. However, their experimental value was observed to be lower than the standard theoretical value. Which of the following could best explain the observation?
a)Wind gusts reducing the heat transfer from the flame to the water
b)Inclusion of the mass of salt in the caliorimetry experiment
c)Incomplete combustion of the salt, leading to less heat transfer
d) Over stirring of the solution leading to heat generation
Also how do you prepare for prac exams at school?
Post by: RuiAce on July 29, 2016, 03:59:05 pm
Thanks Jake! I getcha! :) not bad actually hahaThere's no hydrogen in ozone for there to be hydrogen bonding. It's the bent structure that promotes dipole-dipole interactions.
here's the other one i was confused about, sorry thought I attached two.
and a quick question while my brain doesn't forget to ask: Does ozone have a higher MP/BP than Oxygen because of the stronger IMFs (i.e. dipole dipole AND two sites for hydrogen bonding? or is it just the dipole dipole?
CHEEEEEEEEEEEEEEEEERS m8 :)
I did that question recently for my student. Analyse it systematically as always through determining moles.
nNa2SO3 = CV = 8.00*10-3 L * 0.0100 mol L-1 = 8 * 10-5 mol.
Observe the mole ratio: 1/4 mol of O2 reacts with 1 mol of Na2SO3
nO2 = 2 * 10-5 mol
Utilise your units. The question asks for weight/volume. Convert the moles of O2 into a mass:
mO2 = nMM = 2*10-5 mol * 32.00 g mol-1 = 6.4*10-4 g
(w/v) is a measure of how many grams there are of a substance, in how many millilitres of solution. As a percentage, %(w/v) therefore measures how many grams are present in how many one hundred millilitres of a solution.
%(w/v) = 6.4*10-4 g / 50.00mL * 100% = 1.28*10-3%
Further examples can be found here http://www.ausetute.com.au/wtvol.html
I need help with a practical question (since I suck at them)Which one would be most logical?
A student performed an experiment to determine the molar heat of solution of ammonium chloride. However, their experimental value was observed to be lower than the standard theoretical value. Which of the following could best explain the observation?
a)Wind gusts reducing the heat transfer from the flame to the water
b)Inclusion of the mass of salt in the caliorimetry experiment
c)Incomplete combustion of the salt, leading to less heat transfer
d) Over stirring of the solution leading to heat generation
Also how do you prepare for prac exams at school?
If you ask me, complete combustion will be quite easy for a molecule with a relatively low molar mass so C is out. D cannot even contribute because the temperature change in q=mc∆T and moles reacted will cancel out in finding a value for ∆H (do you see why?). And if anything, the specific heat capacity will be what changes the value calculated, not the mass of the salt in the experiment.
How to prepare for practical experiments at school? If you're lucky your teacher will give up time to allow you to do the experiment again at lunch. Otherwise, you should've already done the experiment once in class and be prepared to utilise the same techniques. Go over the methods, go over any possible discussion questions, remember what you require and get into it.
Post by: conic curve on July 29, 2016, 04:20:34 pm
There's no hydrogen in ozone for there to be hydrogen bonding. It's the bent structure that promotes dipole-dipole interactions.
I did that question recently for my student. Analyse it systematically as always through determining moles.
nNa2SO3 = CV = 8.00*10-3 L * 0.0100 mol L-1 = 8 * 10-5 mol.
Observe the mole ratio: 1/4 mol of O2 reacts with 1 mol of Na2SO3
nO2 = 2 * 10-5 mol
Utilise your units. The question asks for weight/volume. Convert the moles of O2 into a mass:
mO2 = nMM = 2*10-5 mol * 32.00 g mol-1 = 6.4*10-4 g
(w/v) is a measure of how many grams there are of a substance, in how many millilitres of solution. As a percentage, %(w/v) therefore measures how many grams are present in how many one hundred millilitres of a solution.
%(w/v) = 6.4*10-4 g / 50.00mL * 100% = 1.28*10-3%
Further examples can be found here http://www.ausetute.com.au/wtvol.htmlWhich one would be most logical?
If you ask me, complete combustion will be quite easy for a molecule with a relatively low molar mass so C is out. D cannot even contribute because the temperature change in q=mc∆T and moles reacted will cancel out in finding a value for ∆H (do you see why?). And if anything, the specific heat capacity will be what changes the value calculated, not the mass of the salt in the experiment.
How to prepare for practical experiments at school? If you're lucky your teacher will give up time to allow you to do the experiment again at lunch. Otherwise, you should've already done the experiment once in class and be prepared to utilise the same techniques. Go over the methods, go over any possible discussion questions, remember what you require and get into it.
No not quite. It would be great if you could explain further :D
I don't think my teacher would allow that. I wish she could though :'(
Post by: RuiAce on July 29, 2016, 04:39:15 pm
No not quite. It would be great if you could explain further :DRegarding that part.
I don't think my teacher would allow that. I wish she could though :'(
∆H = q/n = q*MM/msalt = mH2OC ∆T * MM / msalt
If we over stir, the change in temperature deltaT could be increased, however the mass used on the bottom also goes up because we combust more of the substance. The overall effects cancel out and do nothing to the value of DeltaH.
Post by: conic curve on July 29, 2016, 05:27:25 pm
Regarding that part.
∆H = q/n = q*MM/msalt = mH2OC ∆T * MM / msalt
If we over stir, the change in temperature deltaT could be increased, however the mass used on the bottom also goes up because we combust more of the substance. The overall effects cancel out and do nothing to the value of DeltaH.
I'm confused with this:
Nacl(aq) -->na+(aq)+cl-(aq)
K2so4(aq)-->2k+(aq)+so4 2-(aq)
For the second one does it have to do with the charges or what?
For the first one I don't get how they are in aqueous state and that when they dissolve, they are still in aqueous state
H2so4(aq)-->2h+(aq)+so4 2-(aq)
Des this have to do with the charges or what?
Post by: RuiAce on July 29, 2016, 05:29:30 pm
I'm confused with this:Your question is not making sense here. If they're dissolved their already in aqueous state. Aqueous means dissolved in solution.
Nacl(aq) -->na+(aq)+cl-(aq)
K2so4(aq)-->2k+(aq)+so4 2-(aq)
For the second one does it have to do with the charges or what?
For the first one I don't get how they are in aqueous state and that when they dissolve, they are still in aqueous state
H2so4(aq)-->2h+(aq)+so4 2-(aq)
Again does this have to do with the charges or what?
(Splitting NaCl(aq) into Na+ and Cl- does nothing. They were always dissolved)
Post by: conic curve on July 29, 2016, 05:38:05 pm
Your question is not making sense here. If they're dissolved their already in aqueous state. Aqueous means dissolved in solution.
(Splitting NaCl(aq) into Na+ and Cl- does nothing. They were always dissolved)
I meant why is it represented that way, with the plus and minus charges?
Post by: RuiAce on July 29, 2016, 05:49:45 pm
I meant why is it represented that way, with the plus and minus charges?NaCl(s) is just solid table salt.
NaCl(aq) is a shorthand for Na+(aq) + Cl-(aq).
If a substance featuring an ionic bond is not dissolved, the fact that the sodium ions and chloride ions are attracted to each other will allow positive and negative (opposite) charges to attract. This forms a sturdy lattice of alternating NaCl molecules.
If a substance featuring an ionic bond is dissolved, a ton of polar water molecules will be attracted to the charged ions. The interaction between the sodium and chloride ions breaks apart due to the interference of the water. There is no longer a true NaCl left being held together; instead a ton of Na+ and Cl- ions are present in the water.
Post by: conic curve on July 29, 2016, 07:33:41 pm
Zn(s)-->Zn2+(aq)+Zn-. Why is there a "2" there?
Cu2+ + 2e- --->Cu(s) Is this based on charges in the periodic table?
How would you find the net ionic equation of this as an example?
Ca(s)+ZnSO4(aq)--->CaSO4(aq)+Zn(s)
Post by: RuiAce on July 29, 2016, 07:49:53 pm
I'm stuck with the following chemical equationsYou don't use the periodic table to determine charges on transitional elements.
Zn(s)-->Zn2+(aq)+Zn-. Why is there a "2" there?
Cu2+ + 2e- --->Cu(s) Is this based on charges in the periodic table?
How would you find the net ionic equation of this as an example?
Ca(s)+ZnSO4(aq)--->CaSO4(aq)+Zn(s)
They will always specify if the iron ion has a 2+ or 3+ charge. Copper, lead and tin generally have a 2+ charge when unstated. If they say copper has a 1+ charge, or lead/tin has a 4+ charge, then go with that.
The zinc ion has a 2+ charge
The silver ion has a 1+ charge
These are common ones that you need to just know. Just like how you need to know sulfates have a 2- charge.
__________________________
Analyse the species separately watching out for where an electron transfer has occurred:
Ca: It went from solid metal to becoming a calcium ion in calcium sulfate
Zn: It went from being an ion in zinc sulfate to just being a solid metal
SO4: Nothing happened to sulfate
So you have Ca(s) + Zn2+ -> Ca2+ + Zn(s)
Post by: onepunchboy on July 30, 2016, 11:04:40 am
Hi guz , i dont really get this graph question
Post by: RuiAce on July 30, 2016, 12:22:59 pm
(http://uploads.tapatalk-cdn.com/20160730/9fe2ff85e060ef25f439e012c9eafa76.jpg)Note how we're talking about rates here.
Hi guz , i dont really get this graph question
We know that at equilibrium, the rate of the forward reaction equals the rate of the reverse reaction. I'll assumed you deciphered this part out yourself since you picked A.
We are saying that the equilibrium shifts to the right. This means that the forward reaction is being favoured. Thus, the rate A+B->C is happening is going to be much greater than the rate C->A+B is happening. This is why the answer is B.
Of course, the two graphs collide once the equilibrium has been established.
Post by: conic curve on July 31, 2016, 09:26:54 am
Post by: jakesilove on July 31, 2016, 09:50:53 am
In prelim chem, is it possible that they'll assess you on groundwater, even though it's a minor part of the syllabus? (I've never seen a question on groundwater before)
I don't remember anything about the prelim syllabus, but if groundwater is in there, then yes they can assess you on it. If you think that it is unlikely to be tested, just learn it in vague detail, in the event that a question does come up.
Post by: senara on July 31, 2016, 01:08:39 pm
Post by: RuiAce on July 31, 2016, 01:15:26 pm
this is a really generic question but in the case we have to describe an experiment, what exactly do we need to include? because different papers have different marking guidelines :/The question gives you the aim, so the method that you present them has to address the aim. (Diagram is optional - situationally preferrable)
What do you mean different papers have different marking guidelines? The method is different for every individual experiment. Unless you can post two questions the exact same and their marking guidelines for comparison.
Post by: senara on July 31, 2016, 01:43:36 pm
The question gives you the aim, so the method that you present them has to address the aim. (Diagram is optional - situationally preferrable)
What do you mean different papers have different marking guidelines? The method is different for every individual experiment. Unless you can post two questions the exact same and their marking guidelines for comparison.
No I mean, some requires safety precautions others variables, discussion etc. and for example a 5 mark question do we need to include everything, for e.g. aim, hypothesis, variables, method, safety...... and so and how would that differ from a 7 mark question (if that makes sense...)
Post by: RuiAce on July 31, 2016, 01:46:53 pm
No I mean, some requires safety precautions others variables, discussion etc. and for example a 5 mark question do we need to include everything, for e.g. aim, hypothesis, variables, method, safety...... and so and how would that differ from a 7 mark question (if that makes sense...)Look at the question. If it just says to outline then all they want is a method.
Discussion is almost always asked as a separate question, e.g. assess the validity of your experiment, or e.g. comment on why the calculated Eo value is less than the theoretical.
I do not recall having to include the safety in an experiment unless it actually says to outline the safety. You'll have to provide examples of this. (In fact, I've never heard of a question about an experiment being a 7 marker)
Generally, there's not enough marks for the examiners to give away for you to write a full scientific report on an experiment. Most of the time they just want to see the method or a discussion question as a follow-up.
Post by: senara on July 31, 2016, 01:59:22 pm
Look at the question. If it just says to outline then all they want is a method.
Discussion is almost always asked as a separate question, e.g. assess the validity of your experiment, or e.g. comment on why the calculated Eo value is less than the theoretical.
I do not recall having to include the safety in an experiment unless it actually says to outline the safety. You'll have to provide examples of this. (In fact, I've never heard of a question about an experiment being a 7 marker)
Generally, there's not enough marks for the examiners to give away for you to write a full scientific report on an experiment. Most of the time they just want to see the method or a discussion question as a follow-up.
Ahh okay.. Thank you :)
Post by: jakesilove on July 31, 2016, 03:06:18 pm
Look at the question. If it just says to outline then all they want is a method.
Discussion is almost always asked as a separate question, e.g. assess the validity of your experiment, or e.g. comment on why the calculated Eo value is less than the theoretical.
I do not recall having to include the safety in an experiment unless it actually says to outline the safety. You'll have to provide examples of this. (In fact, I've never heard of a question about an experiment being a 7 marker)
Generally, there's not enough marks for the examiners to give away for you to write a full scientific report on an experiment. Most of the time they just want to see the method or a discussion question as a follow-up.
100% agree with Rui!
Post by: bethjomay on July 31, 2016, 03:47:32 pm
Post by: conic curve on July 31, 2016, 04:36:14 pm
Post by: Sssssrr on August 01, 2016, 09:58:29 pm
many thanks
Post by: RuiAce on August 01, 2016, 10:00:43 pm
hey, could some please explain what equilibrium constants actually areWhat it actually is? Well it is a measure of just where exactly does the equilibrium lie for a reversible reaction. I.e. by how much the products or reactants dominate at equilibrium.
many thanks
Post by: conic curve on August 01, 2016, 10:03:16 pm
hey, could some please explain what equilibrium constants actually are
many thanks
The equilibrium constant of a chemical reaction is the value of the reaction quotient when the reaction has reached equilibrium. An equilibrium constant value is independent of the analytical concentrations of the reactant and product species in a mixture, but depends on temperature and on ionic strength. Known equilibrium constant values can be used to determine the composition of a system at equilibrium.
Post by: RuiAce on August 01, 2016, 10:04:22 pm
Equilibrium constants are separated into a homogenous equilibrium and heterogenous equilibriumAll of this isn't actually a part of the syllabus, fyi.
A homogeneous equilibrium has everything present in the same phase. The usual examples include reactions where everything is a gas, or everything is present in the same solution
A heterogeneous equilibrium has things present in more than one phase. The usual examples include reactions involving solids and gases, or solids and liquids.
Post by: jakesilove on August 01, 2016, 10:13:04 pm
All of this isn't actually a part of the syllabus, fyi.
I think maybe it's part of Industrial Chem? Not entirely sure
Post by: RuiAce on August 01, 2016, 10:15:36 pm
I think maybe it's part of Industrial Chem? Not entirely sureIt's not. Relating heterogeneous/homogeneous isn't.
Source: Personal experience :P
Post by: jakesilove on August 01, 2016, 10:17:02 pm
It's not. Relating heterogeneous/homogeneous isn't.
Source: Personal experience :P
Ah sorry I see I see, I didn't actually see your definition post :)
Post by: onepunchboy on August 02, 2016, 07:48:08 pm
Im not sure how to convert into ppm. Can someone crash course this q for me and maybe ppm in general? Thanks
Post by: RuiAce on August 02, 2016, 07:52:03 pm
(http://uploads.tapatalk-cdn.com/20160802/ecf1e4f0d7c79de918e12807b9b0b7cd.jpg)To say the least
Im not sure how to convert into ppm. Can someone crash course this q for me and maybe ppm in general? Thanks
The units "ppm" is decomposed to mg/kg or mg/L
I.e. 1 ppm = 1mg/1kg
OR 1 ppm = 1mg/1L
To determine concentration in ppm, convert the numerator into a mass in mills. Also convert the bottom to a mass in kilograms, or a mass in litres.
To go from molarity (moles per litre) you would leave the denominator the same (possibly equal to 1), and use m=nMM on the numerator and convert the grams to mills.
(Your question can't be done yet as I don't know what the actual precipitate is.)
Post by: onepunchboy on August 02, 2016, 07:58:45 pm
To say the leastSilver chloride hehe
The units "ppm" is decomposed to mg/kg or mg/L
I.e. 1 ppm = 1mg/1kg
OR 1 ppm = 1mg/1L
To determine concentration in ppm, convert the numerator into a mass in mills. Also convert the bottom to a mass in kilograms, or a mass in litres.
To go from molarity (moles per litre) you would leave the denominator the same (possibly equal to 1), and use m=nMM on the numerator and convert the grams to mills.
(Your question can't be done yet as I don't know what the actual precipitate is.)
Post by: RuiAce on August 02, 2016, 09:33:23 pm
Silver chloride heheThe percentage composition of chloride ions in silver chloride (by molar masses) is
35.45/(107.9+35.45) * 100% = 24.7296826...%
Thus, the mass of chloride ions present in the sample is
mCl- = 3.65g * 24.7296826.../100 = 0.9026334147...g
Convert this mass into milligrams: 902.6334147... mg
The mass of the water is in mL. We want to convert this to L: 0.05 L
Concentration in ppm = 902.6334147...mg / (0.05 L) = 18052.66829... ppm = 1.80*104 ppm (3 s.f.)
That's quite a large concentration. Unsurprising because 3.65g precipitate out of 50mL is massive in the context of ppm but still dubious. Will let someone else check my working here.
Post by: jakesilove on August 02, 2016, 09:39:02 pm
The percentage composition of chloride ions in silver chloride (by molar masses) is
35.45/(107.9+35.45) * 100% = 24.7296826...%
Thus, the mass of chloride ions present in the sample is
mCl- = 3.65g * 24.7296826.../100 = 0.9026334147...g
Convert this mass into milligrams: 902.6334147... mg
The mass of the water is in mL. We want to convert this to L: 0.05 L
Concentration in ppm = 902.6334147...mg / (0.05 L) = 18052.66829... ppm = 1.80*104 ppm (3 s.f.)
That's quite a large concentration. Unsurprising because 3.65g precipitate out of 50mL is massive in the context of ppm but still dubious. Will let someone else check my working here.
Was just about to post my working, and got the same solution. Unless we're both doing the same thing wrong, pretty sure that checks out!
Post by: conic curve on August 02, 2016, 09:47:39 pm
The concentration of mercury in a solution is 0.005g (% w/w). Express this concentration in ppm
Post by: jakesilove on August 02, 2016, 09:53:59 pm
I'm not getting the idea of this question:
The concentration of mercury in a solution is 0.005g (% w/w). Express this concentration in ppm
Assuming that this just means 0.005% of the solution was Mercury (I can't think of another way to answer this question), we can convert it straight to ppm. Parts per million are exactly that; if there were a million 'parts', how many would be Mercury? Well, 0.005% of them. You can plug this straight into a calculator, and find that the answer is 50ppm
Post by: conic curve on August 03, 2016, 07:03:30 am
Assuming that this just means 0.005% of the solution was Mercury (I can't think of another way to answer this question), we can convert it straight to ppm. Parts per million are exactly that; if there were a million 'parts', how many would be Mercury? Well, 0.005% of them. You can plug this straight into a calculator, and find that the answer is 50ppm
Thanks Jake
Need help:
The haber process is an industrial process used to produce ammonia (NH3) from hydrogen and nitrogen gas in the air. The reaction shown below details this process
(it is attached)
Identify three factors that can increase the forward reaction rate and explain how this occurs (6 marks)
Post by: HighTide on August 03, 2016, 07:46:22 am
Thanks JakeYou should have a go at these. Read the principles here: http://www.chemguide.co.uk/physical/equilibria/lechatelier.html
Need help:
The haber process is an industrial process used to produce ammonia (NH3) from hydrogen and nitrogen gas in the air. The reaction shown below details this process
(it is attached)
Identify three factors that can increase the forward reaction rate and explain how this occurs (6 marks)
Hopefully you have a go at it first, then read the spoiler.
Spoiler
Increase reaction rate by:
1. Increase temperature--> increase kinetic energy = more collisions at right orientation in a given time
2. Increase pressure --> Decrease volume= less room= more collisions at right orientation in a given time
3. Add a catalyst--> Decrease activation energy = increase proportion of particles which can surpass activation energy = faster
1. Increase temperature--> increase kinetic energy = more collisions at right orientation in a given time
2. Increase pressure --> Decrease volume= less room= more collisions at right orientation in a given time
3. Add a catalyst--> Decrease activation energy = increase proportion of particles which can surpass activation energy = faster
Post by: conic curve on August 03, 2016, 08:02:36 am
You should have a go at these. Read the principles here: http://www.chemguide.co.uk/physical/equilibria/lechatelier.html
Hopefully you have a go at it first, then read the spoiler.SpoilerIncrease reaction rate by:
1. Increase temperature--> increase kinetic energy = more collisions at right orientation in a given time
2. Increase pressure --> Decrease volume= less room= more collisions at right orientation in a given time
3. Add a catalyst--> Decrease activation energy = increase proportion of particles which can surpass activation energy = faster
Le chaletier (if that;s how you spell it) is not part of the year 11 syllabus
In order to compare the reactivity of three different metals, Macy conducted an experiment where in which she placed the metals in different electrolyte solutions and recorded observations that occurred over time.
1. When metal A was placed in a 0.1M nitrate solution of Metal B there was no reaction observed
2. When metal B was placed in a 0.1M nitrate solution of Metal C a precipitate was seen to deposit on the surface of metal A
3. Metal B was then placed in a 0.1M nitrate solution of Metal C. Macy noticed that a precipitate was deposited on the surface of metal B
Arrange the metals in order of decreasing reactivity and explain the reasoning behind your arrangement (4 marks)
Post by: RuiAce on August 03, 2016, 08:45:14 am
Le chaletier (if that;s how you spell it) is not part of the year 11 syllabusIf I remember correctly whilst you don't explicitly learn LCP in prelim the same rules are applied and you still need to know what he said.
In order to compare the reactivity of three different metals, Macy conducted an experiment where in which she placed the metals in different electrolyte solutions and recorded observations that occurred over time.
1. When metal A was placed in a 0.1M nitrate solution of Metal B there was no reaction observed
2. When metal B was placed in a 0.1M nitrate solution of Metal C a precipitate was seen to deposit on the surface of metal A
3. Metal B was then placed in a 0.1M nitrate solution of Metal C. Macy noticed that a precipitate was deposited on the surface of metal B
Arrange the metals in order of decreasing reactivity and explain the reasoning behind your arrangement (4 marks)
Now, precipitation reactions in this context are intended to demonstrate metal displacement reactions.
The more reactive metal will displace the less reactive metal and become ionised.
E.g. Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s)
Zinc being more reactive than copper displaces it.
In #1, metal A did not displace metal B. So metal A is not as reactive as metal B
#2 I think you had a typo. You started talking about metal A out of nowhere
In #3, metal B did displace metal C. So metal B is more reactive than metal C
Post by: wesadora on August 03, 2016, 05:16:39 pm
I am aware rusting does not occur when pH is above 9, but why does a more acidic environment (lower pH) accelerate rusting?
The explanation given to me was that the reduction reaction O2 + 2H2O + 4e-<--> 4OH- exists in equilibrium - so, when H+ ions (from acid) are present it reacts with the OH- produced, and thus by LCP removing the product (OH) will cause the equilibrium to shift to the right and consequently accelerating rusting.
HOWEVER, my teacher and many other sources say that the reaction at the cathode is not reversible and i'm....confused.
cheers :)
Post by: onepunchboy on August 03, 2016, 05:31:15 pm
Can someone explain why the answer is c. Shouldnt the h2o be cancelled out in the middle? Where did the oxygen cone from?
Post by: wesadora on August 03, 2016, 05:48:35 pm
apparently it's 2. I'm getting 2.3 from -log10[H+], but apparently you're supposed to do -log10[2 x 5 x 10-3]. why? :/
Post by: jakesilove on August 03, 2016, 06:07:47 pm
For anyone that does Shipwrecks...I HAVE A QUESTION.
I am aware rusting does not occur when pH is above 9, but why does a more acidic environment (lower pH) accelerate rusting?
The explanation given to me was that the reduction reaction O2 + 2H2O + 4e-<--> 4OH- exists in equilibrium - so, when H+ ions (from acid) are present it reacts with the OH- produced, and thus by LCP removing the product (OH) will cause the equilibrium to shift to the right and consequently accelerating rusting.
HOWEVER, my teacher and many other sources say that the reaction at the cathode is not reversible and i'm....confused.
cheers :)
I'm sorry I can't answer your question in any more detail, because the explanation you were given is how I understood the process to work! Personally, I would just use that LCP explanation in an exam. Hopefully someone doing Shipwrecks now can help us out here!
Post by: jakesilove on August 03, 2016, 06:11:29 pm
'The pH of 5 x 10-3M H2SO4 is'?
apparently it's 2. I'm getting 2.3 from -log10[H+], but apparently you're supposed to do -log10[2 x 5 x 10-3]. why? :/
Hey! Sulfuric acid is DIPROTIC. That means it donates 2 hydrogen ions (two protons), instead of just one (like HCl does). This means that, for one more of Sulfuric acid, two moles of hydrogen ions are donated. Therefore, you DOUBLE the concentration before plugging it into the formula. If it was triprotic (eg. Citric acid), you would TRIPLE the concentration! Hope that makes sense!
Post by: jakesilove on August 03, 2016, 06:17:41 pm
(http://uploads.tapatalk-cdn.com/20160803/50d7c459c904812b3094294ebf4de1f5.jpg)
Can someone explain why the answer is c. Shouldnt the h2o be cancelled out in the middle? Where did the oxygen cone from?
To be completely honest, whilst I can't really answer your question directly, I can answer the question by elimination. I think that's the best way to deal with this question anyway.
Firstly, by looking to the Polymer, we know that one monomer should be 5 carbon lengths long, and one should be 4 carbon lengths long. This immediately rules out A or D, leaving us with B or C.
Now, let's get of an OH from one monomer, and a H from another monomer. We know that a H2O is condensed out of the monomers somewhere, so this is a fair thing to do. According to B, this would result in one of two things: either BOTH monomers still have an OH group, or one doesn't an an OH group, and one has two. Looking to the Polymer, we can't see a single OH in sight. As such, B doesn't make any sense; we should see some left over.
Well damn. Actually, C has two OH in each monomer. Maybe that means two Water molecules are eliminated? I'm stumped, sorry I'm not great at visualising Chemistry stuff like this. Maybe someone can come help us out?
Post by: wesadora on August 03, 2016, 06:27:58 pm
Can someone explain why the answer is c. Shouldnt the h2o be cancelled out in the middle? Where did the oxygen cone from?
Pretty much, you NEED two OH groups to form the (C-O-C carboxylic bond AND have excess water (H2O) on both sides of the monomer.
By process of elimination, you first need two monomers with 4 and 5 carbons, and thus the answer is either B or C.
The reason the answer is C, is because each monomer given has two OH groups. The simplified structural formula has a [CH2CH3], which on the 5 carbon monomer in question C is the two left-most most carbons leftover, after the OH groups connect.
This is really hard to explain just using words and it would be so much easier for you to visualise if I could physically point things out with my finger but....yeah ._.
Post by: RuiAce on August 03, 2016, 06:34:14 pm
(http://uploads.tapatalk-cdn.com/20160803/50d7c459c904812b3094294ebf4de1f5.jpg)The question's answered so I'm going to give a piece of advice here.
Can someone explain why the answer is c. Shouldnt the h2o be cancelled out in the middle? Where did the oxygen cone from?
Try to work backwards. I always figured out which two MONOMERS would give the POLYMER in question
Post by: Loki98 on August 04, 2016, 12:55:17 pm
The molar heat of combustion of 1-propanol is 2021kJmol-1. What mass of 1-propanol must be combusted to boil 300mL of water if the water has an initial temperature of 20°C and only absorbs 75% of the heat released?
Post by: RuiAce on August 04, 2016, 01:22:37 pm
How would i do this question?Because the density of water is just 1 g mL-1, we know that the mass of water present is 300g.
The molar heat of combustion of 1-propanol is 2021kJmol-1. What mass of 1-propanol must be combusted to boil 300mL of water if the water has an initial temperature of 20°C and only absorbs 75% of the heat released?
Because only 75% of the heat is released, our enthalpy is
q = 75% * mc∆T = 0.75 * 300g * 4.18 J g-1 K-1 * 20 K = 18810 J
So q = 18.81 kJ
Using the fact that ∆H * n = -q
-2021 kJ mol-1 * n = -18.81 kJ
n = 0.00930727362... mol
Lastly, using the fact that m = n*MM
m = (0.00930727362...mol) * (60.094 g mol-1) = 0.5590320831... g
Then round to 2 s.f.
Post by: Loki98 on August 04, 2016, 01:42:01 pm
Post by: wesadora on August 04, 2016, 02:31:09 pm
plz explain :'(
my working was just 0.45mg/25mL * 40/40 (to get it into micrograms/L) but I feel there's more to it xD
Post by: jakesilove on August 04, 2016, 02:53:14 pm
For part b), i keep getting 18ppm as my answer, but apparently the answer is 4.5 x 10-3 ppm.
plz explain :'(
my working was just 0.45mg/25mL * 40/40 (to get it into micrograms/L) but I feel there's more to it xD
Hey!
I hope the below answer makes enough sense. Let me know if you need more details!
(http://i.imgur.com/9q8Xfjq.jpg?1)
Jake
Post by: wesadora on August 04, 2016, 03:01:18 pm
Hey!
I hope the below answer makes enough sense. Let me know if you need more details!
Jake
Legend. :)
Also- in a chemistry exam, when do you have to/don't have to define LCP? My logic says do it the very first time it's required to state it, and then i can just leave it and say 'by LCP' for the rest of the exam later in the paper whenever i need to...but have chem trial tomorrow and wanna make sure HAHA!
Cheers:)
Post by: wesadora on August 04, 2016, 03:04:14 pm
Post by: jakesilove on August 04, 2016, 03:06:43 pm
Legend. :)
Also- in a chemistry exam, when do you have to/don't have to define LCP? My logic says do it the very first time it's required to state it, and then i can just leave it and say 'by LCP' for the rest of the exam later in the paper whenever i need to...but have chem trial tomorrow and wanna make sure HAHA!
Cheers:)
You need to define it in every single question to do with LCP. It's not enough to prove to the marker once that you do actually know it; it is vital to any question about equilibrium! Just because you show the marker you can do concentration calculations in one question, doesn't mean you can skip the next calculation question :P For the same reason, ALWAYS define LCP (as painful as that may be)
Post by: Loki98 on August 04, 2016, 04:36:45 pm
Post by: RuiAce on August 04, 2016, 04:41:33 pm
Could someone please help me out with this dot point, "Identify a range of salts which form acidic, basic or neutral solutions and explain their acidic, neutral or basic nature." How to i determine which salts dissolved in water, form acidic, basic or neutral solutions?By comparing what ions built up to give that salt.
Ions such as Na+, K+, Cl- and NO3- are all neutral cations. Their conjugate acid/base is so strong that they are only a tiny bit basic/acidic and don't impact.
As opposed to something like NH4+, which is slightly acidic because NH3 is just decently basic.
Or something like CH3COO- which is slightly basic because CH3COOH is just somewhat acidic.
hold on....why is the treated water sample "0.450 x 10-3"mg/25mL in the first line of your working? I thought that frmo the previous question before and just reading off the graph it's .450 mg/25mL (450 x -3)Milli does not mean micro
1000 micrograms in a milligram
Post by: conic curve on August 04, 2016, 05:20:33 pm
I don't understand the complete ionic equation and net ionic equation behind this?
AgNO3(aq)+cl-(aq)-->AgCl(s)
I don't get the complete ionic equation in this
Post by: RuiAce on August 04, 2016, 05:54:46 pm
NiCl2(aq)+NaOH(aq)-->Ni(OH)2(s)+NaCl(aq)No tidy typing cause I'm on my phone
I don't understand the complete ionic equation and net ionic equation behind this?
AgNO3(aq)+cl-(aq)-->AgCl(s)
I don't get the complete ionic equation in this
With your complete ionic equation all you do is separate the aqueous substances into their components really. It's only for the net ionic equation where you have to throw out your spectator ions.
However, one very important thing to note is that both of your given equations are unbalanced.
First one:
NiCl2(aq) + 2 NaOH(aq) -> Ni(OH)2(s) + 2 NaCl(aq)
In complete ionic:
Ni(2+) + 2 Cl(-) + 2 Na(+) + 2 OH(-) -> Ni(OH)2(s) + 2 Na(+) +2 Cl(-)
Cancel out all the spectators for net ionic
Ni(2+) + 2 OH(-) -> Ni(OH)2(s)
Post by: anotherworld2b on August 04, 2016, 10:02:09 pm
Post by: Skidous on August 04, 2016, 10:10:09 pm
So for Cl it has a valency of 7, so it needs an extra electron to become stable (like the noble gases) in which case it forms a covalent bond with another Cl atom, they share that one covalent bond and thus share a valence electron to become stable.
Then you go through and figure out the number of covalent bonds needed to make be valence shells of each element full
For example
O has 6 valence, therefore 2 covalent bonds are needed to fill the valence shell
H has 1 valence and thus forms one covalent bond to fill the valence shell (it only occupies the first shell)
Hope this helps
Skidous
Post by: RuiAce on August 04, 2016, 10:13:05 pm
It's talking about the number of covalent bonds each element can make, it relates to valency in that it involves the sharing of electronsThe question asks for the maximum amount. This makes me question as to whether they mean maximum amount so that the atoms are "happy", or the actual maximum amount.
So for Cl it has a valency of 7, so it needs an extra electron to become stable (like the noble gases) in which case it forms a covalent bond with another Cl atom, they share that one covalent bond and thus share a valence electron to become stable.
Then you go through and figure out the number of covalent bonds needed to make be valence shells of each element full
For example
O has 6 valence, therefore 2 covalent bonds are needed to fill the valence shell
H has 1 valence and thus forms one covalent bond to fill the valence shell (it only occupies the first shell)
Hope this helps
Skidous
Why do I say this? Oxygen technically only needs (and ever wants) 2 covalent bonds to be happy. Sulfur, on the other hand, can take 2, 4 or even 6.
I feel that this was a badly worded question due to its lack of clarity.
Post by: wesadora on August 04, 2016, 10:13:54 pm
Post by: RuiAce on August 04, 2016, 10:16:36 pm
is the answer C because citric acid only has a 1% ionisation strength compared to hydrochloric acid's 100%? What's the reasoning/how do you work this out?pH is 2? Shouldn't the answer then be B because [H3O+] = 10-pH = 10-2 = 0.010
Calling in Jake...
However yes, ionisation strength definitely plays a part.
Post by: jakesilove on August 04, 2016, 10:27:18 pm
pH is 2? Shouldn't the answer then be B because [H3O+] = 10-pH = 10-2 = 0.010
Calling in Jake...
However yes, ionisation strength definitely plays a part.
Yes, the answer is definitely B, not C. Like Rui shows, we know that the concentration of HCl to be 0.010 moles per liter. That equation works directly for all strong acids/bases, so it can't be wrong. This means that the only possible answer is B. We also happen to know that Citric acid is much, much weaker, and so requires a concentration that is much stronger (eg. double) that HCl to produce the same pH. B therefore continues to make sense, and so is the only possible answer.
Jake
Post by: anotherworld2b on August 04, 2016, 10:29:45 pm
The question asks for the maximum amount. This makes me question as to whether they mean maximum amount so that the atoms are "happy", or the actual maximum amount.
Why do I say this? Oxygen technically only needs (and ever wants) 2 covalent bonds to be happy. Sulfur, on the other hand, can take 2, 4 or even 6.
I feel that this was a badly worded question due to its lack of clarity.
For part c wouldnt N have a valence of 5 eletrons? So it needs 3 more electrons? But the answer says 5?
Post by: RuiAce on August 04, 2016, 10:31:42 pm
For part c wouldnt N have a valence of 5 eletrons? So it needs 3 more electrons? But the answer says 5?Just as I suspected.
They want to know what is the maximum amount of covalent bonds that can form. That is always the amount of electrons in the valence shell.
For nitrogen, this is 5.
(In reality will it form 5? Never. But that's not the point of the question.)
Post by: anotherworld2b on August 04, 2016, 10:35:55 pm
Just as I suspected.
They want to know what is the maximum amount of covalent bonds that can form. That is always the amount of electrons in the valence shell.
For nitrogen, this is 5.
(In reality will it form 5? Never. But that's not the point of the question.)
Oh okay ;D
Thank you i think it get it now :D
Post by: anotherworld2b on August 04, 2016, 10:51:42 pm
An example im struggling with is
SO4 ^2-
Post by: Jakeybaby on August 04, 2016, 11:18:20 pm
I struggle with drawing lewis structures and was wondering if i could get some tips kn how to draw themInitially, we will count the number of valence electrons:
An example im struggling with is
SO4 ^2-
We know that Sulfur has 6, as does Oxygen.
Therefore:
Number of valence electrons = Valence Electrons(sulfur) + 4*Valence Electrons(oxygen) + 2(negative 2 charge shows 2 electrons are present)
Number of valence electrons = 6 + 6(4) + 2
= 32
Number of valence electrons = 6 + 6(4) + 2
= 32
Drawing the ion:
Spoiler
(https://i.gyazo.com/2136bc0cf3fafac2cf5727c7fd3ce3f8.png)
We need to ensure that the outer atoms have octets, therefore, we need to draw 6 dots around each of the oxygen atoms, and counting the number of valence electrons present for each atom: it can be seen that all the oxygens have 8, as does the centre sulfur atom.
Spoiler
(https://i.gyazo.com/4ccd9d9dcc7ea3599cfead95d0b196a7.png)
But hang on, we're not done:
The formal charges need to be looked at:
Formal Charge = Valence Electrons - Non-Bonding Valence Electrons - Bonding Electrons/2
Therefore:
Formal ChargeOxygen = 6 - 6 - 2/2 = -1
Formal ChargeSulfur = 6 - 0 - 8/2 = +2
We want the formal charges to be as close to 0 as possible on each atom, hence, to do this, we can take 2 valence electrons from 2 seperate oxygen atoms, to make some double bonds:
Spoiler
(https://i.gyazo.com/c0b736b4e5e2d6b55b21a4a1dddd3f5b.png)
Formal ChargeSulfur = 6 - 0 - 12/2 = 0
Formal ChargeBlack Oxygen = 6 - 6 - 2/2 = -1
Formal ChargeBlue Oxygen = 6 - 4 - 4/2 = 0
Therefore, as we cannot get the formal charges any closer to 0, we have the lewis structure of the sulfate ion.
Spoiler
(https://i.gyazo.com/ccb02b725768741f26fba536685f1825.png)
Post by: RuiAce on August 04, 2016, 11:41:33 pm
The concept of "formal charges" is not taught in the HSC nor preliminary courses. At most, the octet rule is.
Post by: sire123 on August 06, 2016, 03:06:01 am
Post by: RuiAce on August 06, 2016, 08:34:39 am
Can you guys explain to me why for the bromine water prac, it's either Br2 or HOBr? Can you show both word and structural equations for the reactions that take place with cyclohexene + bromine water, and cyclohexene + bromine water?For your first question, it does not matter whether we use Br2 or HOBr. However, for reference sake, this is what actually happens.
Generally speaking, rather than using liquid bromine we mix the bromine into the water. A reaction takes place:
Br2(l) + H2O(l) -> HBr(aq) + HOBr(aq)
Note that the hydrobromic acid does nothing.
(Not too sure but I think bromine water is preferred over actual bromine because it oxidises more easily.)
____________________
To understand the reactions that take place you need to consider what exactly are substitution and addition reactions. Substitution reactions are exactly that - certain atoms effectively displace other atoms. Addition reactions are when bonds are broken and the atoms form new bonds.
So say you went from ethane to ethanol through reaction with water. Then two of the hydrogens are going to have to be substituted for the H and OH from water.
If you went form ethene to ethanol though, the double bond breaks and the water molecule just adds on. The H goes to one carbon whereas the OH goes to the other.
Now, cyclohexene has formula C6H10. We can't really assign it a number as to where the double bond is (unlike say, 1-hexene), but we know that there is a double bond there. It has a high electron density and is very reactive, so when the HOBr is introduced it will readily add on.
(Note that HOBr is what has the brown or whatever colour. The bromine water is what gets decolourised, not the cyclohexene.)
C6H10(l) + HOBr(aq) -> C6H10BrOH(l)
On the contrary, cyclohexane only contains single bonds. For a substitution reaction to take place on top of any of those single bonds we need to provide much more energy. In fact, this can only be done so through adding UV radiation.
C6H12(l) + HOBr(aq) -(UV)-> C6H10BrOH(l) + H2(g)
Under laboratory conditions, you're not going to find that much UV at all. So you will see the cyclohexene decolourise it, and not the cyclohexane. That is all.
Structural diagrams can be found at EasyChem
Post by: sire123 on August 06, 2016, 10:14:51 am
Well first one is a 6 marker "Discuss the process of fermenting glucose into ethanol. Include a chemical equation and a chemical structure of ethanol." How would you tackle and structure this q? Is it asking for the prac, and all the conditions needed to make this work...
And what is the difference between Biopol, polydroxybutanoate, and polyhydroxybutyrate? Are they all the same?
Post by: jakesilove on August 06, 2016, 01:14:53 pm
Wow cheers Rui! I have a couple more questions.
Well first one is a 6 marker "Discuss the process of fermenting glucose into ethanol. Include a chemical equation and a chemical structure of ethanol." How would you tackle and structure this q? Is it asking for the prac, and all the conditions needed to make this work...
And what is the difference between Biopol, polydroxybutanoate, and polyhydroxybutyrate? Are they all the same?
Hey Sire!
Let's start with that 6 marker: "Discuss the process of fermenting glucose into ethanol. Include a chemical equation and a chemical structure of ethanol."
Okay, clearly we need a chemical equation and the chemical structure of ethanol. I would probably smash that out right at the start, so that you don't forget!
(http://i.imgur.com/FAmGeIU.gif?1)
Make sure to include Yeast as a catalyst as well! As for the chemical structure of Ethanol:
(http://www.bbc.co.uk/staticarchive/728c1c64297ffeacfb59d01b4f2d71ec3a6e4b98.gif)
Easy! Okay, let's try to answer the actual question. We don't really need to be talking about the prac specifically (ie. you don't need to write a method). Instead, I would be listing as much information as possible. I would be writing something like this.
The fermentation of Glucose to Ethanol can be described chemically by the equation given above. However, to achieve the greatest yield of Ethanol, certain conditions must be satisfied.
- Enzymes to convert the Glucose to Ethanol (specifically, Yeast)
- Moderate temperatures (about 37 degrees Celsius)
- A mixture of Glucose in solution
- Anaerobic conditions (ie. low oxygen)
- An Ethanol concentration <15% (or else the Yeast will die)
Great! So we've identified a whole bunch of important factors, we've explained how the process works, we're nearly done! Finally, I would talk about the fact that if we want to USE the ethanol, we need to use multiple iterations of fractional distillation. This is because the Ethanol/Glucose solution will only have a concentration of about 15%. As such, we need to purify it! This allows it to be used.
That should be enough to satisfy the 6 marks! You can always add some extra info, but I don't think you need more than that :)
As for your Biopol/polydroxybutanoate/polyhydroxybutyrate conundrum, basically Biopol is the brand name of PHB (like Panadol is the brand name, but Paracetamol is the drug in it!). I don't think that polydroxybutanoate exists, but polyhydroxybutyrate is the full chemical name of PHB (I would use this last term).
Let me know if you need any more help! Great questions :)
Jake
Post by: jakesilove on August 06, 2016, 07:23:57 pm
(http://i.imgur.com/kW0MFep.jpg)
(http://i.imgur.com/aoAn3Dx.jpg)
(http://i.imgur.com/fAjyAVa.jpg)
(http://i.imgur.com/qGYKHAU.jpg)
Jake
Post by: jakesilove on August 06, 2016, 07:31:49 pm
I'm just posting a couple of questions for another user; feel free to write up some solutions, otherwise I'll get to an answer shortly!
(http://i.imgur.com/kW0MFep.jpg)
(http://i.imgur.com/aoAn3Dx.jpg)
(http://i.imgur.com/fAjyAVa.jpg)
(http://i.imgur.com/qGYKHAU.jpg)
Jake
For the first question, you need to start by stating Le Chatelier's Principle. Something along the lines of "When a system undergoes some kind of change, the equilibrium will shift to try to minimise that change". Then, identify that the equilibrium is exothermic (as delta H is negative). As such, an increase in temperature will shift the equilibrium to the left (towards the reactants). Elaborate on this a little, and this will explain the results observed!
Post by: jakesilove on August 06, 2016, 07:35:03 pm
I'm just posting a couple of questions for another user; feel free to write up some solutions, otherwise I'll get to an answer shortly!
(http://i.imgur.com/kW0MFep.jpg)
(http://i.imgur.com/aoAn3Dx.jpg)
(http://i.imgur.com/fAjyAVa.jpg)
(http://i.imgur.com/qGYKHAU.jpg)
Jake
The third question is to do with Hydrogen bonds and dipole-dipole interaction. Identify that Ethane/Butane etc. will only have dispersion inter-molecular forces. These are the weakest inter molecular forces. However, the identified compounds will have dipole-dipole interactions (as the hydroxide groups are electronegative) and hydrogen bonding (between hydrogen and Oxygen). Dipole-dipole interactions are stronger than dispersion, and Hydrogen bonding is stronger still. Therefore, as the intermolecular bonding is much stronger, the boiling point will be much higher (more energy required to break the bonds). This explains the boiling points!
Post by: jakesilove on August 06, 2016, 07:38:55 pm
I'm just posting a couple of questions for another user; feel free to write up some solutions, otherwise I'll get to an answer shortly!
(http://i.imgur.com/kW0MFep.jpg)
(http://i.imgur.com/aoAn3Dx.jpg)
(http://i.imgur.com/fAjyAVa.jpg)
(http://i.imgur.com/qGYKHAU.jpg)
Jake
For the final question, let's break down the two marks into 1) Improving validity and 2) Improving reliability.
Validity
This method does not take into account the fact that water is evaporating as the carbon dioxide is being released. A more valid experiment would be filling a similar can with water, and weighing the water evaporation over time simultaneously. Then, the water evaporation can be subtracted from the changes over time, and the carbon dioxide can be isolated.
Reliability
This one is easy; just do it multiple times! Potentially, take more measurements as well. Nothing too tricky here
Post by: sire123 on August 06, 2016, 08:09:49 pm
Post by: RuiAce on August 06, 2016, 08:42:42 pm
Can you do the second image as well? And continuing that q it says "Justify the conditions used during he Haber synthesis of ammonia - High temperature (reactor) and High pressure". But isn't 400 degrees celsius and 200 atm not that high?Why not? If you ask me 400 degrees would burn me and 200 atm would easily crush me in a matter of seconds as well. (Keep in mind 1atm is everyday pressure)
The temperature of 400deg C is a moderately high temperature chosen as a compromise to balance out between the need to increase the rate of reaction (high temperatures lead to increased kinetic energy among particles) and forcing the equilibrium to the right (Because the production of ammonia is exothermic, by LCP lower temperatures favour the yield)
Pressure is self explanatory: High pressures favour the yield by LCP since there are fewer moles of gas in the products. However if we excessively increase the pressure though, we risk the glassware breaking and losing everything we have.
Post by: sire123 on August 06, 2016, 08:57:06 pm
Post by: RuiAce on August 06, 2016, 09:02:40 pm
Post by: amandali on August 07, 2016, 04:04:30 pm
(http://uploads.tapatalk-cdn.com/20160806/337b423c85717c294e61b33e38c80c01.jpg)
how to know which one is oxidant and reductant
Post by: RuiAce on August 07, 2016, 04:42:40 pm
[(http://uploads.tapatalk-cdn.com/20160806/2f51c1b7f610d5663feaa90f5f0cd47b.jpg)Consult your table of standard reduction potentials and observe the relevant Eo values to determine if a reaction will occur.
(http://uploads.tapatalk-cdn.com/20160806/337b423c85717c294e61b33e38c80c01.jpg)
how to know which one is oxidant and reductant
In beaker 1, zinc is higher on the potentials sheet than acid. In fact, this is just your ordinary metal + acid -> salt + hydrogen gas reaction.
Zn(s) + H2SO4(aq) -> ZnSO4(aq) + H2(g)
Remember that the oxidant (aka oxidising agent) is the one that gets reduced. Using OIL-RIG, we know that reduction is gain, so we identify the species that gained electrons.
The correct answer is H+, because two hydrogen ions gained an electron in the formation of hydrogen gas.
Equation 2 works similarly, however is an example of metal displacement:
Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s)
Post by: Skidous on August 07, 2016, 08:06:24 pm
[(http://uploads.tapatalk-cdn.com/20160806/2f51c1b7f610d5663feaa90f5f0cd47b.jpg)
(http://uploads.tapatalk-cdn.com/20160806/337b423c85717c294e61b33e38c80c01.jpg)
how to know which one is oxidant and reductant
I know this isn't an answer but I want to know how you got your picture in the post like that
Post by: RuiAce on August 07, 2016, 08:13:14 pm
I know this isn't an answer but I want to know how you got your picture in the post like that
You either upload it to imgur or something and copy/paste the [img] code, or you upload it where you write the post - attachments are uploaded below the box you type your messageIn her instance, however, she uploaded through Tapatalk.
Post by: anotherworld2b on August 08, 2016, 12:43:05 am
I have some more chem questions if its okay :)
I tried q3b but i was unable to get the right answer
For q13 i am not sure how to approach the question
Post by: RuiAce on August 08, 2016, 09:19:37 am
Hi :D
I have some more chem questions if its okay :)
I tried q3b but i was unable to get the right answer
For q13 i am not sure how to approach the question
Q13 has gone way outside what could possibly be asked in the HSC. However, it still seems like nothing more than gravimetric analysis so this is my attempt.
a) Weight loss - According to the data given the hydrated copper sulfate definitely decomposed, but because we're talking about 200oC here, the water most likely got evaporated off.
b) 1.67/4.77 * 100% = 35%(approx) - use mass, not molar mass to find percentage composition here.
c) Error is not in the HSC course.
Post by: sire123 on August 09, 2016, 01:26:29 pm
- principle shell notation
- sub-shell notation
- orbital notation
How do u do it?
Post by: senara on August 11, 2016, 08:58:35 pm
Post by: jakesilove on August 11, 2016, 10:30:34 pm
Hi Just wondering do we need to know how to draw dry cell and vanadium redox?
Hey! It is definitely knowing how to draw your battery, if it is at all simple. Specifically, the Dry Cell is quite easy to draw, so I would recommend having that up your sleeve. Vanadium Redox is slightly more complicated, so you probably don't need to have that down. In reality, you might never use either, so it's like an additional "LOOK HOW MUCH I KNOW!" point that you can use to score some marks
Post by: RuiAce on August 11, 2016, 10:37:46 pm
Hi Just wondering do we need to know how to draw dry cell and vanadium redox?Food for thought: I literally drew out the Gratzell cell in the exam last year to help get my marks. (Dry cell wasn't examined for us.)
Post by: Loki98 on August 13, 2016, 05:23:55 pm
The colourless aqueous solution of a white solid sample formed a precipitate with an acidified aqueous solution of lead (II) nitrate. what is the possible identity of the original solution of this sample?
a) copper (II) sulfate
b) Barium chloride
c) Sodium sulfate
d) silver chloride
Could you please explain how you disprove the incorrect options.
Thx =]
Post by: RuiAce on August 13, 2016, 05:45:35 pm
Could someone please explain this question to me please.This answer is incomplete. I'll ask that someone else finish it off because I'm not sure how to incorporate the acidification. I've started it off in the meantime
The colourless aqueous solution of a white solid sample formed a precipitate with an acidified aqueous solution of lead (II) nitrate. what is the possible identity of the original solution of this sample?
a) copper (II) sulfate
b) Barium chloride
c) Sodium sulfate
d) silver chloride
Could you please explain how you disprove the incorrect options.
Thx =]
Since we know that all nitrates are soluble, there is no way that the precipitate formed could've involved the nitrate ion.
Hence, we consider substances involving the lead ion Pb2+ which are insoluble. Problem is that both PbSO4 AND PbCl2 are insoluble!
Begin a process of elimination.
D) is the first one to go. This is because AgCl is also insoluble. Since silver chloride is insoluble, it cannot have been the aqueous solution.
A) is the second one to go. The presence of the Cu2+ ion means that the solid would've been blue, when clearly it is white.
The last bit is the hard one.
Post by: MysteryMarker on August 13, 2016, 06:27:30 pm
CH3NH2(aq) + H2O <----> OH- + CH3NH3+\
If the methyl amine (CH3NH2(aq)) reacts with HCl to produce the salt methyl ammonium chloride. Write the equation for this neutralisation reaction. Explain, with the aid of an equation, why this salt does no produce a neutral solution when dissolved in water.
Cheers.
Post by: RuiAce on August 13, 2016, 08:21:55 pm
Kinda stuck on this question,Given that the equation is reversible due to the presence of ⇌, we know that methyl amine is weakly basic and its conjugate; the methyl ammonium ion is weakly acidic.
CH3NH2(aq) + H2O <----> OH- + CH3NH3+\
If the methyl amine (CH3NH2(aq)) reacts with HCl to produce the salt methyl ammonium chloride. Write the equation for this neutralisation reaction. Explain, with the aid of an equation, why this salt does no produce a neutral solution when dissolved in water.
Cheers.
Suppose we react with hydrochloric acid. Because we have a strong acid, the equation loses its nature as reversible. So it just becomes this.
CH3NH2(aq) + HCl(aq) -> CH3NH3Cl(aq)
If we dissolve this salt in water, it will dissociate.
CH3NH3Cl(s) -> CH3NH3+ + Cl-
The chloride ion is neutral. Because HCl is a strong acid, Cl- is so basic to the point it basically doesn't react.
However, the weakly acidic methyl ammonium ion will just do the original equation's thing in reverse (i.e. donate a proton):
CH3NH3+ + H2O(l) -> H3O+ + CH3NH2(aq)
So in fact, the salt formed is slightly acidic. Not neutral.
Post by: MysteryMarker on August 13, 2016, 09:46:22 pm
Given that the equation is reversible due to the presence of ⇌, we know that methyl amine is weakly basic and its conjugate; the methyl ammonium ion is weakly acidic.
Suppose we react with hydrochloric acid. Because we have a strong acid, the equation loses its nature as reversible. So it just becomes this.
CH3NH2(aq) + HCl(aq) -> CH3NH3Cl(aq)
If we dissolve this salt in water, it will dissociate.
CH3NH3Cl(s) -> CH3NH3+ + Cl-
The chloride ion is neutral. Because HCl is a strong acid, Cl- is so basic to the point it basically doesn't react.
However, the weakly acidic methyl ammonium ion will just do the original equation's thing in reverse (i.e. donate a proton):
CH3NH3+ + H2O(l) -> H3O+ + CH3NH2(aq)
So in fact, the salt formed is slightly acidic. Not neutral.
For the reaction between methyl amine and hydrochloric acid, shouldn't this be a neutralisation reaction as we established that methyl amine is a weak base and we know that hydrochloric acid is a strong acid? So shouldn't water also be a product in this reaction?
Post by: RuiAce on August 13, 2016, 09:51:03 pm
For the reaction between methyl amine and hydrochloric acid, shouldn't this be a neutralisation reaction as we established that methyl amine is a weak base and we know that hydrochloric acid is a strong acid? So shouldn't water also be a product in this reaction?I didn't know you always had to produce water in neutralisation:
NH3 + HCl -> NH4Cl
Post by: MysteryMarker on August 14, 2016, 10:27:21 am
I didn't know you always had to produce water in neutralisation:
NH3 + HCl -> NH4Cl
OHHHHHHH rekt. Thanks man, i understand it now :P
Got another question:
The correct statement about biopolymers if:
A) biopolymers are made by living things but are not biodegradeable.
B)All biopolymers can be manufactured synthetically in the laboratory by condensation reactions.
C)Synthetic biopolymers are being produced from living organisms and are replacing polymers made from petrochemicals
D)All natural biopolymers are made by condensation reactions involving glucose monomers.
Cheers.
Post by: jakesilove on August 14, 2016, 11:45:30 am
OHHHHHHH rekt. Thanks man, i understand it now :P
Got another question:
The correct statement about biopolymers if:
A) biopolymers are made by living things but are not biodegradeable.
B)All biopolymers can be manufactured synthetically in the laboratory by condensation reactions.
C)Synthetic biopolymers are being produced from living organisms and are replacing polymers made from petrochemicals
D)All natural biopolymers are made by condensation reactions involving glucose monomers.
Cheers.
Hey,
The answer here is definitely C. Just do this by elimination, in terms of what you have actually studied. Biopolymers may be biodegradable, they may not be, however you aren't supposed to know EVERY biopolymer out there! So, the answer can't be A. Again, you would need to know the reaction of EVERY biopolymer, some of which may be condensation reactions, most of which would be addition reactions, for the answer to be B. So, it isn't. Same goes for D. However, the REASON you learn about biopolymers is that we need to find alternatives to petrochemicals (the focus of the whole of Production of Materials). Plus, BIOpolymers are obviously produced by living organisms. So, the answer is C.
Jake
Post by: RuiAce on August 14, 2016, 11:58:35 am
Post by: jakesilove on August 14, 2016, 12:10:01 pm
Hey Jake, please finish off the question that I only half completed at the bottom of the last page. I still can't show that sodium sulfate (prediction) would be the correct one
On it!
Post by: jakesilove on August 14, 2016, 12:17:40 pm
This answer is incomplete. I'll ask that someone else finish it off because I'm not sure how to incorporate the acidification. I've started it off in the meantime
Since we know that all nitrates are soluble, there is no way that the precipitate formed could've involved the nitrate ion.
Hence, we consider substances involving the lead ion Pb2+ which are insoluble. Problem is that both PbSO4 AND PbCl2 are insoluble!
Begin a process of elimination.
D) is the first one to go. This is because AgCl is also insoluble. Since silver chloride is insoluble, it cannot have been the aqueous solution.
A) is the second one to go. The presence of the Cu2+ ion means that the solid would've been blue, when clearly it is white.
The last bit is the hard one.
So, for reasons unknown to me, and irrelevant to the HSC curriculum, Lead (II) Chloride is slightly soluble in Acidic conditions. It is also slightly soluble in water (but only a tiny bit!). Either way, the BETTER answer is going to be C, because Lead Sulfate is not soluble at all. This isn't something that you're supposed to know. Typical James Ruse going wayyy beyond the curriculum.
Jake
Post by: MysteryMarker on August 14, 2016, 12:57:38 pm
Hey,
The answer here is definitely C. Just do this by elimination, in terms of what you have actually studied. Biopolymers may be biodegradable, they may not be, however you aren't supposed to know EVERY biopolymer out there! So, the answer can't be A. Again, you would need to know the reaction of EVERY biopolymer, some of which may be condensation reactions, most of which would be addition reactions, for the answer to be B. So, it isn't. Same goes for D. However, the REASON you learn about biopolymers is that we need to find alternatives to petrochemicals (the focus of the whole of Production of Materials). Plus, BIOpolymers are obviously produced by living organisms. So, the answer is C.
Jake
Thanks man, totally understand it now. Just got a few more questions if thats alright.
1. Sodium hydrogen carbonate is a common laboratory chemical. Explain why arrhenius acid/base definition is unable to account for the acid/base properties of this species whereas the BL theory can. Include chemical equations. (5marks)
For this question would i just state Arrhenius's definition + BL definition, and then show that HCO3- is amphiprotic. By its reaction with water, showing it accepting a hydrogen ion and donating a hydrogen ion, and as it can do both Arhennius would be unable to define this as a base/acid?
2. What is meant by the Eo of an electrode in an electrochemical cell. (2marks)
3. For the complete combustion of a fuel, is the water produced in the reaction gaseous or liquid?
Cheers.
Post by: RuiAce on August 14, 2016, 01:07:38 pm
Thanks man, totally understand it now. Just got a few more questions if thats alright.1. That does show how it's a B-L acid because according to B-L theory, acids are proton donors.
1. Sodium hydrogen carbonate is a common laboratory chemical. Explain why arrhenius acid/base definition is unable to account for the acid/base properties of this species whereas the BL theory can. Include chemical equations. (5marks)
For this question would i just state Arrhenius's definition + BL definition, and then show that HCO3- is amphiprotic. By its reaction with water, showing it accepting a hydrogen ion and donating a hydrogen ion, and as it can do both Arrhenius would be unable to define this as a base/acid?
Also, note that Arrhenius thought bases had to ionise to produce OH-. You can't do that with NaHCO3 either.
2. What is meant by the Eo of an electrode in an electrochemical cell. (2marks)
3. For the complete combustion of a fuel, is the water produced in the reaction gaseous or liquid?
Cheers.
Svante Arrhenius' definition stated that acids ionised in solution (water) to produce H+. When you dissolve your sample of sodium hydrogen carbonate you don't produce that; you just produce the sodium ion and the hydrogen carbonate ion.
NaHCO3(s) -> Na+ + HCO3-
So it fails to be an Arrhenius acid.
2. Your table of standard reduction potentials lists several Eo values for individual half equations.
The EMF produced in an electrochemical cell is just the sum of the two relevant half equations. It is thus, a measure of the voltage produced, from you making that electrochemical cell. (Remember: In a Galvanic cell, chemical energy is converted into electrical energy.)
3. Gas. The temperatures in combustion are extremely high.
Post by: MysteryMarker on August 14, 2016, 04:25:59 pm
Kinda confused for this question, cause i know that HCO3- is amphiprotic so wouldn't its acidity/basicity depend on the solvent? Or would the salt just be neutral because assuming the solvent is water, it will produce an equal amount of hydrogen and hydroxide ions?
Cheers.
Post by: RuiAce on August 14, 2016, 04:33:02 pm
Classify the salt formed from HCO3- as acidic, basic or neutral and justify its classification. Include a chemical equation to illustrate your answer. (2marks)Actually, for the hydrogen carbonate ion it exhibits basic properties with water.
Kinda confused for this question, cause i know that HCO3- is amphiprotic so wouldn't its acidity/basicity depend on the solvent? Or would the salt just be neutral because assuming the solvent is water, it will produce an equal amount of hydrogen and hydroxide ions?
Cheers.
HCO3- + H2O(l) ⇌ H2CO3(aq) + OH-
I think this is because water is a stronger acid than hydrogen carbonate. Or alternatively hydrogen carbonate is a stronger base than water. It's also found in the human body as a buffer system in this way.
I'm not entirely sure how this works though.
Post by: MysteryMarker on August 14, 2016, 06:15:07 pm
Another question i have is justify the use of AAS in the mining industry, and in safeguarding the environment.
Just curious as to what points i should bring up for 'mining industry'.
Cheers.
Post by: Happy Physics Land on August 14, 2016, 08:18:54 pm
Ahh, so HCO3- acts as a base. Would this be because it is the strong conjugate base of the weak acid H2CO3?
Another question i have is justify the use of AAS in the mining industry, and in safeguarding the environment.
Just curious as to what points i should bring up for 'mining industry'.
Cheers.
Hey MysteryMarker!
To your enquiry about the conjugate base/acid pair, yes and no. HCO3- is a strong base since H2CO3. However, it is only strong compared to water, meaning that when reacting with water, it is able to accept a hydrogen proton from water. But compared to NaOH which is a very strong base, HCO3- is less strong as a base. In our blood stream, HCO3- and H2CO3 act as buffer pairs and if HCO3- is a strong base then it wouldnt have the buffer effect since strong bases result in irreversible reactions. So when you talk about HCO3-, say its a strong base RELATIVE TO WATER.
In regards to AAS, I will just jot down a few dotpoints here for you to consider:
- Before AAS there was no way to detect the existence of trace elements in waterways (trace elements = extremely small quantities)
- AAS has helped industries to monitor heavy metal pollution into nearby waterways or atmosphere
- For example, if you do industrial chemistry, you can talk about the mercury cell which leeches the neurotoxin mercury into waterways and this can bioaccumulate and affect the health of animals going up the food chain
- If we wanna talk about mining industry, we can say that AAS helps to detect lead or mercury vapours that may rise from underground deposit caverns and these can really threaten workers' health
- And almost if you want to see the purity of the metal ore that is dug up from the ground, AAS would be useful for detecting small amounts of impurities
I think its quite hard to talk about the use of AAS in mining industry without being a mining engineer but generally AAS would just be used for detecting heavy metal existences and purity of materials. In HSC, they would ask for applications of AAS, but very unlikely to ask about its relation with mining industry. More likely they would be asking about AAS' use in water quality examination or just its general uses.
Best Regards
Happy Physics Land
Post by: Happy Physics Land on August 14, 2016, 08:22:04 pm
I AM WELL AND TRULY BACK BOYS
Post by: jakesilove on August 15, 2016, 08:30:10 am
Let's just proclaim the good news:
I AM WELL AND TRULY BACK BOYS
The prodigal son has returned!
Post by: anotherworld2b on August 15, 2016, 09:53:58 am
•Justify the use of a particular chromatography technique by considering
•the properties of the substances being separated,
•the amount of substance available for analysis
•the sensitivity of the equipment
Post by: Loki98 on August 15, 2016, 01:18:56 pm
What kind of molecule would a micelle be classified as?
How do you explain soap acting as a emulsifier with reference to the formation of micelles?
Post by: Happy Physics Land on August 15, 2016, 02:00:41 pm
Industrial Chemistry Questions:
What kind of molecule would a micelle be classified as?
How do you explain soap acting as a emulsifier with reference to the formation of micelles?
Hey Loki!!!
Ohhhh okay we don't usually classify micelle as a molecule, rather I would say its just a mixture. A micelle is essentially a spherical structure where the hydrophobic hydrocarbon chain from soap is adsorbed (i.e. dissolved, attached) into grease/oil through dispersion force and the negatively charged hydrophilic carboxylate head interacts with water molecules through ion-dipole forces. This is how I would define as a micelle, and I attached an image below to show you what a micelle look like. Now why is it a mixture? Because nothing is reacted, nothing is consumed and its just simply soap molecules attached onto grease.
(http://i.imgur.com/CHXaBMK.png)
Ok so I will kinda explain emulsification to you in numbered steps so its clearer:
1. Soap molecules are dissolved in water
2. Hydrophilic head of soap ion interacts with water molecule through ion-dipole, hydrocarbon chain interacts with and dissolves into grease through dispersion forces
3. Surfactant molecules continue to adsorb into the grease, decreasing the surface tension between grease and water
4. Ion-dipole force, being a stronger intermolecular force than dispersion force, pulls grease off the surface and this forms spherical droplets known as micelles
5. The negatively charged heads on the soap repel each other, preventing grease and dirt from joining back together and this keeps micelles dispersed throughout the solution and therefore achieving the emulsifying action
Post by: MysteryMarker on August 15, 2016, 09:18:54 pm
(n-1)H2O + n(-C6H10O6-) --> n(-C6H12O6)
or just the same reaction but instead of (n-1) H2O its just H2O
Just a bit confused ;P
Post by: RuiAce on August 15, 2016, 10:02:59 pm
Would the general equation for cellulose to glucose be(n-1) is correct.
(n-1)H2O + n(-C6H10O6-) --> n(-C6H12O6)
or just the same reaction but instead of (n-1) H2O its just H2O
Just a bit confused ;P
Just remember: For your cellulose, the n is a subscript under the dash (a bit hard to demonstrate on forums):
-(C6H10O5)n-
And when it's glucose there's no need for a dash obviously
Post by: Maz on August 15, 2016, 11:40:13 pm
I have a test coming up on soap and detergents, would you know any common questions that come up on a topic like that? or any things I should watch out for?
Thankyou :)
Post by: Happy Physics Land on August 16, 2016, 08:15:26 pm
Hey,
I have a test coming up on soap and detergents, would you know any common questions that come up on a topic like that? or any things I should watch out for?
Thankyou :)
Hey mq123!
They can ask you various forms of questions about soaps and detergents that can largely be summed up in six categories:
- Explain how soaps are produced through saponification
- Explain the cleaning action of soaps/detergents
- Explain why detergents have been developed to replace soaps
- Explain the uses of soaps and detergents, in relation to their properties
- Explain how soaps/detergents act as emulsifiers
- Assess/Discuss the environmental and social impacts of soaps and detergents
What I recommend, along with knowing how to verbally address these questions, is to know how to draw the chemical composition of soaps/detergents (anionic cationic and non-ionic), or at least, their skeletal structure. With soaps you need to ensure that you include your carboxylate head group. With anionic detergents, make sure you include the sulfonate head group. With cationic detergents, make sure you include the quarternary ammonium group. Finally with non-ionic detergents you may like to show the hydroxyl head group. These are what distinguishes the types of detergents and their different uses and properties would rely on the function of these head groups.
Post by: conic curve on August 17, 2016, 01:01:40 pm
1. A student dissolves 5.3g of anhydrous sodium carbonate (Na2CO3) in water and then adds more water to make the volume up to 125 ml
a)why did the student first dissolve it on water and then make up the final volume by adding water
b)Why did the student use Anhydrous Na2CO3?
2. Calculate how many grams of solute are in 500mL of 0.5mol L^-1 sodium cholride (nacl) solution
Thanks Guys ;D
Post by: anotherworld2b on August 18, 2016, 06:24:37 pm
I am confused on how cations and anions are involved in the reaction?
I am overall confused in general ???
Post by: RuiAce on August 18, 2016, 06:33:53 pm
Hi I was wondering how do you write precipitation reactions?What do you mean by how to "write" precipitation reactions? You write them the same way you write everything else.
I am confused on how cations and anions are involved in the reaction?
I am overall confused in general ???
Also a cation is just an ion that has a positive charge (e.g. Na+) whereas an anion is an ion with a negative charge (e.g. Cl-)
They're not really important either unless you meant "ionic" equations.
Post by: conic curve on August 18, 2016, 08:12:07 pm
Hi I was wondering how do you write precipitation reactions?
I am confused on how cations and anions are involved in the reaction?
I am overall confused in general ???
I'm not exactly sure how cations and anions are involved in the reaction but all I know is that Cations and anions are formed when a metal loses electrons and a non metal gains those electrons (I don't know why that is the case). This then allows the electrostatic attraction between the positive charge and the negative charge to create an ionic compound
Read here for more: http://www.dummies.com/education/science/chemistry/positive-and-negative-ions-cations-and-anions/
Moderation edit: Removed tutoring service website link
Post by: jyce on August 18, 2016, 08:30:41 pm
Hi I was wondering how do you write precipitation reactions?
I am confused on how cations and anions are involved in the reaction?
I am overall confused in general ???
Cations and anions are indeed involved in precipitation reactions. A precipitation reaction occurs when a positive ion (i.e., a cation) and a negative ion (i.e., an anion) combine to form an insoluble salt (the precipitate).
e.g., Ag+(aq) + Cl-(aq) -> AgCl(s)
Post by: RuiAce on August 18, 2016, 10:43:07 pm
Cations and anions are indeed involved in precipitation reactions. A precipitation reaction occurs when a positive ion (i.e., a cation) and a negative ion (i.e., an anion) combine to form an insoluble salt (the precipitate).That being said, this is an example of an ionic equation.
e.g., Ag+(aq) + Cl-(aq) -> AgCl(s)
Post by: anotherworld2b on August 19, 2016, 01:05:18 am
That being said, this is an example of an ionic equation.
Then how would you do this question? Im confused on what anions and cations to join together? :-\
Post by: RuiAce on August 19, 2016, 08:03:13 am
Then how would you do this question? Im confused on what anions and cations to join together? :-\This requires you to know what's called your solubility rules.
You need to identify which combination of ions will not dissolve in water. That is, those that are insoluble in water.
Take the first one. According to your solubility rules, Na+, NH4+ and K+ are always soluble. Leaving behind the option of Mg2+
CH3COO- and NO3- are both soluble always. This leaves Cl- and CO32-
But MgCl2 is soluble. In fact, CO32- is ALWAYS INSOLUBLE unless paired with a group 1 metal ion or the ammonium ion.
Hence the precipitate is MgCO3
So we can build our equation by nothing more than looking at what we have:
MgCl2(aq) + (NH4)2CO3(aq) -> 2 NH4Cl(aq) + MgCO3(s)
Post by: anotherworld2b on August 20, 2016, 02:08:08 am
That link is really helpful
This requires you to know what's called your solubility rules.
You need to identify which combination of ions will not dissolve in water. That is, those that are insoluble in water.
Take the first one. According to your solubility rules, Na+, NH4+ and K+ are always soluble. Leaving behind the option of Mg2+
CH3COO- and NO3- are both soluble always. This leaves Cl- and CO32-
But MgCl2 is soluble. In fact, CO32- is ALWAYS INSOLUBLE unless paired with a group 1 metal ion or the ammonium ion.
Hence the precipitate is MgCO3
So we can build our equation by nothing more than looking at what we have:
MgCl2(aq) + (NH4)2CO3(aq) -> 2 NH4Cl(aq) + MgCO3(s)
Post by: onepunchboy on August 20, 2016, 12:59:21 pm
What are the tests for the products?
Halp
Post by: RuiAce on August 20, 2016, 01:21:22 pm
(http://uploads.tapatalk-cdn.com/20160820/71095ed21d9025a02d97326694301837.jpg)In the first-hand investigation, the most convenient way to demonstrate electrolysis is by means of a Hoffman Voltameter.
What are the tests for the products?
Halp
We need to keep in mind the half equations that occur during the electrolysis.
For concentrated NaCl solution, the following half equations occur:
Anode: 2 Cl- -> Cl2(g) + 2e-
Cathode: 2 H2O(l) + 2e- -> H2(g) + 2 OH-(aq)
The presence of the chlorine gas at the anode can be tested using litmus paper. The chlorine gas will bleach the litmus, causing it to turn pale.
The presence of the hydrogen gas at the cathode can be tested using the pop test. Set the hydrogen on fire and anticipate a pop.
The presence of the hydroxide ions at the cathode can be tested using an appropriate indicator. Pour an appropriate indicator into a portion of the liquid produced at the cathode, and observe colour changes.
For dilute NaCl solution, we have the electrolysis of water. Whilst the cathode equation is the same, the anode equation is changed.
Anode: 2 H2O(l) -> O2(g) + 4 H+ + 4e-
The presence of the hydrogen ions at the anode can also be tested using an appropriate indicator.
The presence of the abundance of oxygen can be tested using the burning splint test. Raise a burning splint over the region with excess oxygen. The splint lies on fire itself.
Further info on the burning splint test can be researched on Google.
Post by: onepunchboy on August 20, 2016, 01:42:51 pm
I cant think of any other reasons for 6 marks other than conc. Nacl requires less voltage and produces more favourable products..
Halp
Post by: anotherworld2b on August 20, 2016, 02:48:07 pm
Post by: anotherworld2b on August 20, 2016, 07:30:33 pm
Post by: Happy Physics Land on August 20, 2016, 11:02:01 pm
Thanks rui !, just another one (http://uploads.tapatalk-cdn.com/20160820/0172b99b7a155eef262038829afb9f66.jpg)
I cant think of any other reasons for 6 marks other than conc. Nacl requires less voltage and produces more favourable products..
Halp
Hey Onepunchboy!
The chlor-alkali industry favours the use of concentrated NaCl(aq) over molten NaCl(l) because NaOH is a favourable product that can only be produced using concentrated brine and it has a major role in industries that involve the manufacturing of soaps, papers and synthetic fibres. In the electrolysis of Molten NaCl, the products are Na(l) and Cl(g). Whilst Na(l) does have some importance in the manufacture of indigo, pharmaceuticals and petrol additives, the use of NaOH in industry is much more diverse and dominant. In addition, like what you have correctly justified, it takes an energy input of 4.07V for the electrolysis of molten NaCl to take place (2Cl- ——> Cl2(g) + 2e-, Eox= -1.36V. Na+ + e- ---> Na, Ered = -2.71V) whilst it only takes an energy input of 2.19V for the electrolysis of aqueous NaCl. Consequently, chlor-alkali companies can effective reduce energy consumptions, reduce the cost involved and maximise their profits.
Post by: Happy Physics Land on August 20, 2016, 11:09:57 pm
I am able to do simple precipiation reactions with 2 solutions but how you do this question?
Hey Anotherworld2b!
This question requires some familiarity with the precipitation rules and observation of the compounds. If we look carefully, amongst all the compounds, potassium, sodium, ammonium and nitrate compounds would all be soluble according to our solubility rules. Sulphide is not included as a part of our solubility rules and in fact, sulphide is unable to form a precipitate compound with any of those cations. So this leaves us with lead and chloride. If you are experienced with these sorts of questions and familiar with solubility rules, you should immediately recognise that lead (II) chloride would produce a white coloured precipitate. So the ionic equation for the formation of this precipitate would be as follow:
Pb2+(aq) + 2Cl-(aq) -----> PbCl2(s)
Post by: Happy Physics Land on August 20, 2016, 11:27:38 pm
I have a prac test that is about using precipiation reactions to identify 4 unknown solutions. We will be given the names of the solutions but we need to allocate the right name to the correct solution. I was wondeing what would be the best way to approach doing this prac?
Hey Anotherworld2b!
You definitely asked the right person, because I did the same prac exam as you in term 1 with 5 unknown solutions. I scored 25/25 in that exam and this one simple table trick I will show you below has really helped me! So essentially how it works is you construct two tables. You will be given on the day what all the four chemicals are, except you dont really know what each individual chemical is. So on one table, you will be constructing a table like what I have shown below, with your different unknown chemicals and the outcomes of the reactions (i.e. whether there are precipitates or not). This is your experimental data.
(http://imgur.com/K9SkvMS.png)
On another table, you will be constructing a table like what I have also shown below, with your different KNOWN chemicals and the outcomes of the reactions (i.e. whether there are precipitates or not). This is your theoretical data obtained through your solubility rules.
(http://imgur.com/Yb9LQIS.png)
So now what we do is we match the outcomes. For instance, whichever chemical in the first table that is able to form 2 precipitates should be matched to whichever chemical in the second table that is also able to form 2 precipitates. This way we can identify that the two chemicals are the same and hence identify the unknown chemical as I have shown below.
(http://imgur.com/h8aaklC.png)
But now, sometimes you can get situations where you have 2 chemicals from the first table that can form equal amounts of precipitates. Teachers shouldnt give you this sort of situations but if this situation does happen, normally it would not matter which one you would identify as the correct chemical. But, there can be special cases, like what I have shown below, that can help you to determine the correct chemical when two particular chemicals can form equal amounts of precipitates.
(http://imgur.com/X716dG7.png)
I hope my guide helps, and I certainly really enjoyed writing this explanation and I really hope you can achieve a high mark by following what I suggested above!
Best Regards
Happy Physics Land
Post by: anotherworld2b on August 21, 2016, 01:02:41 am
I had no idea how to even start preparing for this practical but now I feel I have gathered a wealth of confidence :D
I have some questions to ask if that's okay to make sure I thoroughly understand
So basically for the first table you physically add the chemicals together and observe whether there is a precipitate or not.
eg. A+B = NR. Where A is one unknown and B is another unknown chemical. Then repeat for the other parts of the table?
Then for the parts of table where its AA, BB and ect do you leave that empty because your testing for example chemical A with chemical A? Which is done for the second table as well?
Would making a table of some kind to write down the appearance of the reaction such as the colour of the precipitate be necessary as well? How would you know if a precipitate is slightly soluble?
I am also a bit confused about the last part about identifying the chemical when there are two particular chemicals can form equal amounts of precipitates.
I was wondering what other things did you have to do in your prac exam apart from identifying the solutions?
I apologise for asking so many questions and would really like to say that I really appreciate your help
Hey Anotherworld2b!
You definitely asked the right person, because I did the same prac exam as you in term 1 with 5 unknown solutions. I scored 25/25 in that exam and this one simple table trick I will show you below has really helped me! So essentially how it works is you construct two tables. You will be given on the day what all the four chemicals are, except you dont really know what each individual chemical is. So on one table, you will be constructing a table like what I have shown below, with your different unknown chemicals and the outcomes of the reactions (i.e. whether there are precipitates or not). This is your experimental data.
(http://imgur.com/K9SkvMS.png)
On another table, you will be constructing a table like what I have also shown below, with your different KNOWN chemicals and the outcomes of the reactions (i.e. whether there are precipitates or not). This is your theoretical data obtained through your solubility rules.
(http://imgur.com/Yb9LQIS.png)
So now what we do is we match the outcomes. For instance, whichever chemical in the first table that is able to form 2 precipitates should be matched to whichever chemical in the second table that is also able to form 2 precipitates. This way we can identify that the two chemicals are the same and hence identify the unknown chemical as I have shown below.
(http://imgur.com/h8aaklC.png)
But now, sometimes you can get situations where you have 2 chemicals from the first table that can form equal amounts of precipitates. Teachers shouldnt give you this sort of situations but if this situation does happen, normally it would not matter which one you would identify as the correct chemical. But, there can be special cases, like what I have shown below, that can help you to determine the correct chemical when two particular chemicals can form equal amounts of precipitates.
(http://imgur.com/X716dG7.png)
I hope my guide helps, and I certainly really enjoyed writing this explanation and I really hope you can achieve a high mark by following what I suggested above!
Best Regards
Happy Physics Land
Post by: cajama on August 21, 2016, 09:33:07 pm
Post by: jakesilove on August 21, 2016, 09:54:38 pm
Hi I feel like these kinds of questions should be simple since its a multiple choice q. but I still struggle to answer them. Please help! (Question 19 - 2015 HSC Chem. Exam | Screenshots of the question attached)
Hey!
Firstly, we know that the molar mass of Lead Chloride is 278.1g/mol. Therefore, we have
Since there is one mole of Lead in every mole of Lead Chloride, we have equal as many moles of Lead! We can therefore calculate the concentration in the original sample
And that's that! Somewhere there my order of magnitude is a bit off, but that doesn't matter; clearly, the answer is C. Hope this helps!
Post by: cajama on August 21, 2016, 10:49:16 pm
Hey!
Firstly, we know that the molar mass of Lead Chloride is 278.1g/mol. Therefore, we have
Since there is one mole of Lead in every mole of Lead Chloride, we have equal as many moles of Lead! We can therefore calculate the concentration in the original sample
And that's that! Somewhere there my order of magnitude is a bit off, but that doesn't matter; clearly, the answer is C. Hope this helps!
Ahh I see now but how do we know how many moles of lead there are for every lead chloride. Is it because in PbCl2 there is only one mole of lead in the molecule?
Where as, say if the precipitate was for whatever reason Pb3(PO4)2, there would be 1/3 moles for every mole of Pb3(PO4)2?
Post by: jakesilove on August 21, 2016, 10:54:36 pm
Ahh I see now but how do we know how many moles of lead there are for every lead chloride. Is it because in PbCl2 there is only one mole of lead in the molecule?
Where as, say if the precipitate was for whatever reason Pb3(PO4)2, there would be 1/3 moles for every mole of Pb3(PO4)2?
Almost! For every mole of Pb3(PO4)2, there are actually 3 mole-s of Pb! The little number is the number of 'moles' within the compound. So, for PbCl2, there is two moles of Cl and one mole of Pb. Makes sense?
Post by: cajama on August 21, 2016, 11:03:17 pm
Almost! For every mole of Pb3(PO4)2, there are actually 3 mole-s of Pb! The little number is the number of 'moles' within the compound. So, for PbCl2, there is two moles of Cl and one mole of Pb. Makes sense?
Oh yeah I get it completely now. Thank you so much!
Post by: conic curve on August 24, 2016, 05:25:00 am
Post by: RuiAce on August 24, 2016, 08:21:54 am
Does the pressure a substance exerts on the container increase as it goes from solid to liquid or gas? Or is it the other way around?That way is correct.
Pressure only ever relates to the quantity of gaseous substances present in a (generally speaking, closed) system. Liquids and solids are not counted, so if gas is converted into liquid/solid we go the other way and decrease pressure.
Post by: massive on August 24, 2016, 04:46:31 pm
Post by: RuiAce on August 24, 2016, 05:39:22 pm
hey guys, how do you do this :S (answer is d btw)Because our system is constantly at 25oC, 100kPa, observe that VM = 24.79 L mol-1 is constant.
Since n = V/VM, because the molar volume is fixed the reaction that produces the greatest moles of gas will also produce the largest volume.
Write out the relevant chemical equation for each and every one of your reaction to determine the relevant moles. Keep in mind that as HCl is the excess reagent, it does not matter how many moles of it we use.
A) CaCO3(s) + 2 HCl(aq) -> CaCl2(aq) + CO2(g) + H2O(l)
nCaCO3 = 50/100.9 = 0.49554... mol
nCO2 = nCaCO3 = 0.4955... mol
B) Zn(s) + 2 HCl(aq) -> ZnCl2(aq) + H2(g)
nZn = 50/65.38 = 0.7647... mol
nH2 = nZn = 0.7647... mol
C) Na2CO3(s) + 2 HCl(aq) -> 2 NaCl(aq) + CO2(g) + H2O(l)
nNa2CO3 = 50/105.99 = 0.4717... mol
nCO2 = nNa2CO3 = 0.4717... mol
D) 2 Na(s) + 2 HCl(aq) -> 2 NaCl(aq) + H2(g)
nNa = 50/22.99 = 2.1748... mol
nH2 = 1/2 nNa = 1.0874... mol
By comparison, the greatest moles of gas produced occurs in reaction D.
Note that the purpose of writing all the equations is to identify the mole ratio. The gas produced itself doesn't matter because we are talking about VOLUME, not mass.
Post by: massive on August 24, 2016, 05:46:37 pm
Theres also one other thing i don't get- its about equilibrium. I understand the whole lcp stuff about temperature and pressure but i don't get everything about concentration, i.e. when you add things to the system.
For q9 i know that the answer is b, i was just wondering if part c and d have any effect on the equilibrium.
Also for 10, this is what i was talking about the concentration stuff, how do you do it
Post by: massive on August 24, 2016, 06:21:20 pm
Hey Jake why do you use 50ml for the volume? they also say that 25ml of sodium chloride was used, don't you add the volumes together :S
Post by: RuiAce on August 24, 2016, 07:03:13 pm
OOOHH i get it now thanks!With Q9, C and D would end up shifting the equilibrium to the left because you introduce more product, not reactant.
Theres also one other thing i don't get- its about equilibrium. I understand the whole lcp stuff about temperature and pressure but i don't get everything about concentration, i.e. when you add things to the system.
For q9 i know that the answer is b, i was just wondering if part c and d have any effect on the equilibrium.
Also for 10, this is what i was talking about the concentration stuff, how do you do it
So.
If there is an excess of either reactant or product, then the equilibrium has been disturbed (or it was never at equilibrium). Therefore, according to LCP, the system wants to get rid of that EXCESS.
Conversely, if there is a shortage of either reactant or product, then LCP predicts that the system wants to bring in more of what's lacked to address the shortage. This is how equilibrium is (re-)established.
So for Q10, where the effect is not immediately obvious, you must analyse what happens.
You introduced hydrochloric acid. Obviously it's not going to react with water or the ammonium ion here. Does it react with ammonia?
No. The HCl will have a tendency to react with that hydroxide ion, which is clearly a base.
HCl + OH- -> Cl- + H2O
So since we take out OH- through the introduction of the acid, the equilibrium will shift to the right.
Now, immediately I can tell this question is beyond the HSC's scope, however it's not unreasonable. This is because of unnecessarily introducing option C: Equation is driven to completion.
A few drops of HCl is probably insufficient to do so. In practicality? Yes, this is possible if you had an unlimited amount of HCl. This is because you end up introducing so much acid that all the base ends up neutralised, and you have excess acid instead.
So for this question? It should be A.
Post by: massive on August 24, 2016, 07:15:32 pm
So for Q10, where the effect is not immediately obvious, you must analyse what happens.
You introduced hydrochloric acid. Obviously it's not going to react with water or the ammonium ion here. Does it react with ammonia?
No. The HCl will have a tendency to react with that hydroxide ion, which is clearly a base.
HCl + OH- -> Cl- + H2O
So since we take out OH- through the introduction of the acid, the equilibrium will shift to the right.
wait so are we just meant to learn this? is this part of module 2? and yeah the answer is A
Post by: RuiAce on August 24, 2016, 07:28:53 pm
wait so are we just meant to learn this? is this part of module 2? and yeah the answer is AYou were already taught it. It's just a process of deduction. And yes of course it counts under The Acidic Environment
Post by: massive on August 24, 2016, 11:15:30 pm
Post by: jakesilove on August 25, 2016, 10:57:20 am
Guys how do you do part b for the question attached. I was just confused because there is no equation, so how do you use mole ratio comparison like you do if you want to figure out the percent of sulfate in fertilisers.
You don't need any sort of equation; what you're given is plenty. First, using the molar mass of the substance and the mass you've been given, find the moles. Then, recall that for a substance
that this means that for every one mole of the substance, there are n moles of X and m moles of Y. So, for one mole of the substance in this question, there will be 2 moles of phosphorus. I'll let you do the maths
Post by: anotherworld2b on August 25, 2016, 11:14:33 am
PF5 abd PH3 have different shapes because of the structure and cimposition of molecukes each one contains. This determines which intermolecular forces are present, and these forces determine the physical properties of the material
I also wanted to ask what is the difference between a dipole, polar bond and net dipole?
Post by: Happy Physics Land on August 26, 2016, 06:05:19 pm
I was wondering would my answer to this question be correct?
PF5 abd PH3 have different shapes because of the structure and cimposition of molecukes each one contains. This determines which intermolecular forces are present, and these forces determine the physical properties of the material
I also wanted to ask what is the difference between a dipole, polar bond and net dipole?
Hey Anotherworld2b!
You are certainly right! PF5 and PH3 do have different shapes because of the structure and composition of the molecules! But this would be a little too general as an answer. When you are asked questions of a similar kind, what you need to include in your answer in "VSEPR principles" and "electronegativity". So according to VESPR rules, molecules are arranged in particular 3D shapes that aim to keep the valence electrons on elements as far apart as possible (since electrons repel each other) in order to maintain a stable structure. The main differences between PF5 and PH3 here are the number of elements involved and the contrast between fluorine's electronegativity and hydrogen's electronegativity. Because there are 5 fluorine elements, the molecule would take on a bitrigonal pyramidal structure with bond angles of 120 degrees and 90 degrees in order to fit all these fluorine atoms in a stable arrangement (more closely packed). In PH3, because there is only 3 hydrogen atoms, it takes on a trigonal pyramidal shape of 120 degrees between all hydrogen atoms (more loosely packed). Because fluorine has a high electronegativity, or a high affinity towards electrons, they must be kept considerably far away from each other to prevent the decomposition of the structure as fluorine tries to gain other electrons in the molecule. These factors all influence the shape and composition of PF5 and PH3.
Post by: Happy Physics Land on August 26, 2016, 06:16:47 pm
I was wondering would my answer to this question be correct?
PF5 abd PH3 have different shapes because of the structure and cimposition of molecukes each one contains. This determines which intermolecular forces are present, and these forces determine the physical properties of the material
I also wanted to ask what is the difference between a dipole, polar bond and net dipole?
A dipole is where there is a pair of temporary opposite partial charges on opposite sides of a molecule.
A polar bond is the dipole-dipole interaction (for instance, the partial negative charge on one molecule and the partial positive charge on another molecule) between polar molecules.
A net dipole is essentially just a net partial charge. A substance without a net dipole would often be symmetrical in shape and bondings. A substance with a net dipole would have different electronegativities at the head and tail of the molecule.
Post by: massive on August 26, 2016, 11:46:36 pm
For part iii of the q attached, why is the answer just 0.16g/L. Isn't that meant to be 0.16g/100ml and then it becomes 1.6g/L???
thnx
Post by: anotherworld2b on August 27, 2016, 12:58:27 am
Also whats the difference between ionisation and dissociation? They seem to be the same thing to me :-\
Post by: anotherworld2b on August 27, 2016, 05:43:04 pm
Post by: Happy Physics Land on August 27, 2016, 10:20:00 pm
Hi i was wondering if i could get my current answers checked and get help with the ones left blank.
Also whats the difference between ionisation and dissociation? They seem to be the same thing to me :-\
Hey anotherworld2b!
Your colour identification for CuSO4, CuCO3 and potassium manganate are incorrect. CuSO4 should have a blue colour and CuCO3 under normal conditions should have a green colour. KMnO4 is a weird one but it should have a purple colour.
In the context of chemistry, ionisation and dissociation can be used interchangeably, they both mean the splitting up of a compound into its component ions when its in an aqueous form.
Post by: Happy Physics Land on August 27, 2016, 10:33:01 pm
The other part because the attachment was too large
In your exams I'm sure you wont be asked about most of these chemicals listed in your second post - you are more likely to be asked about the colour of the common types of precipitates. Cr(NO3)3 (chromium nitrate) is usually dark-violet in colour. The mixture of CrCl3 and Mn(NO3)2 is a little more tricky. Basically all it asks you to do is to find the colour of CrCl3 solution and the colour of Mn(NO3)2 solution and find the resultant colour of the mixture of two colours. The colour of CrCl3 is violet and the colour of Mn(NO3)2 is white. So mixing the two colours together, the resulting mixture should be a light purple colour.
Post by: anotherworld2b on August 28, 2016, 01:10:00 pm
I have another question if that's okay
I am really confused on how to know when to use which concentration unit and how to convert from one to the other?
- %v/v %w/w and % v/w
Could I get advice before tomorrow? I have a chem test on this tomorrow :-\
Post by: RuiAce on August 28, 2016, 01:11:53 pm
thank you very much for your helpThat's odd. You're going to have to provide questions that interrelate between %v/v and %w/w
I have another question if that's okay
I am really confused on how to know when to use which concentration unit and how to convert from one to the other?
- %v/v %w/w and % v/w
Could I get advice before tomorrow? I have a chem test on this tomorrow :-\
Also dissolution is actually dissolving something. If you dissolve Na2CO3 you're not exactly "ionising" because the transfer of electrons has already happened. You're just splitting the molecule apart.
As opposed to reacting sodium with chlorine gas: 2 Na + Cl2 -> 2 NaCl. This is ionising
Post by: RuiAce on August 28, 2016, 01:17:44 pm
guys just a quick question, i don't really understand this working out:Yeah. They should've just considered concentration in g/L instead of be like mass per litre = ...
For part iii of the q attached, why is the answer just 0.16g/L. Isn't that meant to be 0.16g/100ml and then it becomes 1.6g/L???
thnx
Post by: anotherworld2b on August 28, 2016, 02:53:25 pm
How would you tell when to use a particular concentration unit?
Post by: jakesilove on August 28, 2016, 03:18:13 pm
I believe b interrelates %v/v and %w/w but im not sure
How would you tell when to use a particular concentration unit?
Parts per million is the equivalent of
So, you can relate ppm and %w/w (as mg/kg is w/w, you may just need to convert units!)
Post by: massive on August 29, 2016, 03:31:11 pm
(the answer is A btw)
Post by: massive on August 29, 2016, 05:04:21 pm
Post by: jakesilove on August 29, 2016, 05:43:12 pm
Guys how do you do this multiple choice question. And also in general how do you decide what electrolyte you want to use??
(the answer is A btw)
Hey! For the first question, you just sort of need to know that when you use a platinum electrode (ie. an inert electrode), we generally use a substance like HCl that contains Hydrogen (sometimes, we just use water) which get reduced to form Hydrogen gas. I'm not sure of the best explanation here, except to say that none of the other answers make sense, and we don't have the required metal in the cathode itself.
Post by: jakesilove on August 29, 2016, 05:46:40 pm
Guys the answer is D for this question, but couldn't it be A as well???
As far as I can tell, the answer could also be A. The equilibrium should shift away from any increase in concentration. Not sure why the answer is D, and not A, although D is definitely correct
Post by: RuiAce on August 29, 2016, 06:00:18 pm
Guys the answer is D for this question, but couldn't it be A as well???
As far as I can tell, the answer could also be A. The equilibrium should shift away from any increase in concentration. Not sure why the answer is D, and not A, although D is definitely correctDoes the fact that the reactant is a solid play a role?
Post by: jyce on August 31, 2016, 12:26:32 am
Does the fact that the reactant is a solid play a role?
Yes, I believe it does. Adding more CaCO3(s) does not change its concentration, as it's a solid. Therefore, adding more CaCO3 does not cause a shift in the equilibrium position, leaving D as the correct option.
Post by: massive on August 31, 2016, 12:40:41 am
Yes, I believe it does. Adding more CaCO3(s) does not change its concentration, as it's a solid. Therefore, adding more CaCO3 does not cause a shift in the equilibrium position, leaving D as the correct option.
Wait what, isn't the concentration still increasing because technically there's more reactants and the system wants to counteract this by decreasing the reactants, thus it shifts right ? :S
Post by: Jakeybaby on August 31, 2016, 12:55:23 am
Wait what, isn't the concentration still increasing because technically there's more reactants and the system wants to counteract this by decreasing the reactants, thus it shifts right ? :SThe concentration of the solids are constant, only (aq) and (g) are in the equilibrium expressions. You can exclude pure solids and pure liquids from the equilibrium expressions.
Post by: RuiAce on August 31, 2016, 07:38:36 am
Yes, I believe it does. Adding more CaCO3(s) does not change its concentration, as it's a solid. Therefore, adding more CaCO3 does not cause a shift in the equilibrium position, leaving D as the correct option.What I thought. I remember being taught this but it was a blur.
Wait what, isn't the concentration still increasing because technically there's more reactants and the system wants to counteract this by decreasing the reactants, thus it shifts right ? :SWhen you add more moles of solid, you add the same amount of volume of the solid to counteract a potential increase in concentration. Given C=n/V, if n and V go up in the exact same proportion, the concentration is not affected.
Note that it is the CONCENTRATION of the substance which shifts the equilibrium, not the actual quantity (moles)
Post by: massive on September 01, 2016, 02:29:57 am
Thanks heaps!
Post by: conic curve on September 01, 2016, 10:43:26 am
Hey guys, could someone explain how to do hydrolysis of salts to me, i still don't get how to do them :/ (like determining whether the salt is acid or base)
Thanks heaps!
I don't know whether or not this is useful, but maybe you could take a look at this: http://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Equilibria/Solubilty/Hydrolysis
Hopes that helps :D
Post by: RuiAce on September 01, 2016, 01:35:29 pm
Hey guys, could someone explain how to do hydrolysis of salts to me, i still don't get how to do them :/ (like determining whether the salt is acid or base)When something is dissolved in water, the ions that form the compound are dissociated (e.g. Na2CO3(s) -> 2 Na+(aq) + CO32-(aq)). To determine the nature of the dissolved substance as acidic or basic, we need to look at the ions that are formed.
Thanks heaps!
Take this example first. CO32- is basic. However Na+ is a neutral cation. Therefore the resulting solution is basic and the original salt was also basic.
Take NH4Cl. NH4+ is acidic, however Cl- is a neutral cation. Therefore we have something acidic.
If we take NaCl, then both Na+ and Cl- are neutral so the result is neutral.
Simply put, analyse the cation and anion separately.
Post by: anotherworld2b on September 02, 2016, 01:29:33 am
Post by: jakesilove on September 02, 2016, 10:36:48 am
How would you solve question b ii and c?
Hey! The formula we need for these questions is
For i)
For ii), we WANT a certain volume and concentration out, and we HAVE an initial concentration. The only thing we don't have is the initial volume, which is perfect, because that's exactly how the formula works!
For c), again it's just a simply application of the formula. Remember than C1 is your initial concentration, V1 is your initial volume, C2 is your final concentration, and V2 is your final volume.
The trick to this solution is that the TOTAL final volume is going to be 10ml+55mL=65mL
Post by: anotherworld2b on September 02, 2016, 07:05:09 pm
Hey! The formula we need for these questions is
For i)
For ii), we WANT a certain volume and concentration out, and we HAVE an initial concentration. The only thing we don't have is the initial volume, which is perfect, because that's exactly how the formula works!
For c), again it's just a simply application of the formula. Remember than C1 is your initial concentration, V1 is your initial volume, C2 is your final concentration, and V2 is your final volume.
The trick to this solution is that the TOTAL final volume is going to be 10ml+55mL=65mL
Post by: onepunchboy on September 03, 2016, 07:33:10 pm
Yo can anyone help me out on q 16 and 17 ?
Thankss
Post by: jakesilove on September 03, 2016, 10:12:39 pm
(http://uploads.tapatalk-cdn.com/20160903/fb44814da4623c3fd3366c365f19008a.jpg)
Yo can anyone help me out on q 16 and 17 ?
Thankss
Not entirely sure for Q16; is the answer supposed to be A?
For Q17, let's think about what's actually going on. If we had chosen the correct indicator, the endpoint would have been somewhere in the 8-10 range (strong base, weak acid). Therefore, the pH should start on the lower end (ie. pH of 2-3, due to the weak acid) and strong base should be added until the pH is 8-10. However, if the indicator will change colors when the pH reaches 3-4, then much LESS of the base will be required! The greater the pH change, the more base would be required. Since we've used LESS base than expected to neutralise the acid, it will be expected that the acid is WEAKER. Therefore, I believe that the answer is B
Post by: jakesilove on September 03, 2016, 10:15:18 pm
(http://uploads.tapatalk-cdn.com/20160903/fb44814da4623c3fd3366c365f19008a.jpg)
Yo can anyone help me out on q 16 and 17 ?
Thankss
As far as I can tell, the Cathode should be positive as electrons flow TOWARDS it (remember, Reduction It Gains (RIG)). This won't come 'from' the electrolyte, but rather from the anode. If you consider the anode to be the external circuit, I imagine the answer is D
Post by: massive on September 04, 2016, 02:52:11 pm
Post by: jakesilove on September 04, 2016, 03:14:03 pm
Hey guys, i was doing a question for my option topic (industrial chem) and the question was to find the equilibrium constant (K). However the question didn't give the concentrations of the reactants and products in mol/L but rather gave it in kPa. When instructed to find K, the answers just substituted these kPa values into the formula. I was just confused, i thought you could only sub the concentration that are in mol/L. :S
Since the formula relies on ratios, if everything is in a gaseous form, the relative pressures will be equivalent to concentration. This assumes ideal gas etc etc. but we can assume all of that in a question like this. For instance, if I combined 1kPa of Nitrogen and 2kPa of Hydrogen, because the pressure is across a specific unit area (which is the same in both measurements), and because Gas will occupy a constant space per mole regardless of the chemical composition, we can assume that Hydrogen is 'twice as concentrated'. I didn't do Industrial, and I'm sure there are way more complicated reasons why this trick works, but basically if you ever see another question like this just use the kPa measurements! May not make 100% sense, just turns out to be what you need to do.
Post by: RuiAce on September 04, 2016, 03:19:45 pm
Since the formula relies on ratios, if everything is in a gaseous form, the relative pressures will be equivalent to concentration. This assumes ideal gas etc etc. but we can assume all of that in a question like this. For instance, if I combined 1kPa of Nitrogen and 2kPa of Hydrogen, because the pressure is across a specific unit area (which is the same in both measurements), and because Gas will occupy a constant space per mole regardless of the chemical composition, we can assume that Hydrogen is 'twice as concentrated'. I didn't do Industrial, and I'm sure there are way more complicated reasons why this trick works, but basically if you ever see another question like this just use the kPa measurements! May not make 100% sense, just turns out to be what you need to do.Oh dear. Seems unfair to throw that on them. Yeah that's abnormal for the HSC
Post by: massive on September 04, 2016, 04:03:47 pm
Post by: RuiAce on September 04, 2016, 04:09:27 pm
Thanks Jake, just one more q, Why is it when pressure is increased the concentrations of everything increases instantaneously but then equilibrium shifts to side with fewer moles of gases. Also does this instant increase happen with any of the other variables (e.g. temperature or concentration). What i mean is, if the temperature is increased, does everything else increase suddenly then equilibrium shifts right or left (depending if exo/endo) and everything decreases or increases according to this new equilibrium position?We had this discussion on pressure already.
The concentration of the solids are constant, only (aq) and (g) are in the equilibrium expressions. You can exclude pure solids and pure liquids from the equilibrium expressions.
What I thought. I remember being taught this but it was a blur.When you add more moles of solid, you add the same amount of volume of the solid to counteract a potential increase in concentration. Given C=n/V, if n and V go up in the exact same proportion, the concentration is not affected.Temperature is different. Temperature depends on your enthalpy change value, that is, your value for ΔH. The value for ΔH has nothing to do with the state your substances are in, be it solid, liquid or gaseous.
Note that it is the CONCENTRATION of the substance which shifts the equilibrium, not the actual quantity (moles)
If you mean the actual concentrations, which are best depicted on a graph, then no, only changes in concentrations spike the graph up. This is because a change in temperature is nothing but a change in temperature - we are not introducing or extracting a substance relative to the volume of the vessel. Pressure or increases in concentration disturbs the ratio of substances (moles) to the volume of the vessel (volume, and note C=n/V), whereas temperature does not.
Note that the more gradual changes later are just a consequence of Le Chatelier's principle. The spikes are when we suddenly change concentrations in a matter of a second, whereas what comes next is the system trying to shift it's equilibrium to counter the change. (Once again - temperature does NOT forcibly disturb what's in there immediately, as opposed to e.g. taking out 2L of hydrogen.)
Post by: massive on September 04, 2016, 04:15:16 pm
We had this discussion on pressure already.Temperature is different. Temperature depends on your enthalpy change value, that is, your value for ΔH. The value for ΔH has nothing to do with the state your substances are in, be it solid, liquid or gaseous.OHHH thanks man i get it now! btw, if the question gives the concentration of one substance we can't do mole ratio comparison using the balanced equation to get the concentration of the other species right?
If you mean the actual concentrations, which are best depicted on a graph, then no, only changes in concentrations spike the graph up. This is because a change in temperature is nothing but a change in temperature - we are not introducing or extracting a substance relative to the volume of the vessel. Pressure or increases in concentration disturbs the ratio of substances (moles) to the volume of the vessel (volume, and note C=n/V), whereas temperature does not.
Note that the more gradual changes later are just a consequence of Le Chatelier's principle. The spikes are when we suddenly change concentrations in a matter of a second, whereas what comes next is the system trying to shift it's equilibrium to counter the change. (Once again - temperature does NOT forcibly disturb what's in there immediately, as opposed to e.g. taking out 2L of hydrogen.)
Post by: RuiAce on September 04, 2016, 04:17:40 pm
OHHH thanks man i get it now! btw, if the question gives the concentration of one substance we can't do mole ratio comparison using the balanced equation to get the concentration of the other species right?Well technically since n=CV, if you just leave V as something you don't know (like x in maths) you could carry from there.
But in general, no. Because such a method is asking too much for HSC chemistry students.
Post by: massive on September 04, 2016, 04:27:16 pm
For the equilibrium reaction below, would adding acid or base affect the equilibrium position at all?
N2O4 <---> 2NO2
Post by: RuiAce on September 04, 2016, 04:31:58 pm
ohk nws, thankyou. I just have one last question (hopefully)That equilibrium is between two gases. Highly doubt an acid or base will do anything to it
For the equilibrium reaction below, would adding acid or base affect the equilibrium position at all?
N2O4 <---> 2NO2
Post by: jakesilove on September 04, 2016, 04:46:10 pm
That equilibrium is between two gases. Highly doubt an acid or base will do anything to it
Ditto. It would only have an effect if the substance (namely, Nitrogen Dioxide) was in aqueous form, which would have a different equilibrium anyway.
Post by: massive on September 04, 2016, 11:45:08 pm
Post by: Sine on September 04, 2016, 11:49:35 pm
GUYS, for dehydration of ethanol and hydration of ethylene what are the states for each species. There are some notes that say that water is gas and others that say it's liquid...so confused D:water is a gas as the reaction requires such a high temperature. In VCE examiners still accept liquid though. i'm not aware of how it's marked in HSC.
EDIT: minor fix
Post by: jakesilove on September 05, 2016, 12:00:04 am
water is a gas as the reaction requires such a high temperature. In VCE examiners still accept liquid though. i'm not aware of how it's mark in HSC.
Same with the HSC. Basically, whether you put gas or liquid doesn't matter (although generally liquid is safer). Just as long as you include a state!
Post by: massive on September 05, 2016, 05:15:30 pm
Post by: RuiAce on September 05, 2016, 05:38:25 pm
Is HPO-4 and acid? Shouldn't it be a base, becuase H2SO4 is a strong acid and HPO-4 is its conjugate base ??Phosphoric acid and sulfuric acid are two different things. Sulfuric acid being acidic doesn't have anything to do with the nature of the hydrogen phosphate ion.
Presumably you meant HSO4 or something. Please clarify and then we'll get back to you
Post by: massive on September 05, 2016, 06:09:48 pm
Phosphoric acid and sulfuric acid are two different things. Sulfuric acid being acidic doesn't have anything to do with the nature of the hydrogen phosphate ion.
Presumably you meant HSO4 or something. Please clarify and then we'll get back to you
Yeah that's what i meant, my bad!
Post by: RuiAce on September 05, 2016, 08:56:04 pm
Consider the massively long sequence something like phosphoric acid goes through:
H3PO4 <-> H2PO4- + H+ <-> HPO42- + 2 H+ + PO43- + 3 H+
You can say that dihydrogen phosphate is the conjugate base of phosphoric acid. But at the same time, dihydrogen is the conjugate acid of hydrogen phosphate. So what do you reckon? Is dihydrogen phosphate an acid or a base?
The conclusion is that conjugate bases and acids are not enough to simply determine whether your substance is acidic or basic. Conjugate acids only relate B-L theory between a specific acid and a base, not on a generic scale. An amphiprotic substance can easily enough be either.
Your example is, however, unique. HSO4- exerts acidic properties in water. This is one of which you must know.
Post by: katherine123 on September 07, 2016, 04:55:41 pm
do i need to mention the different temperature conditions and % conversion ie initially 1000 degrees, 550 degrees (97% conversion), 400 degrees (99.7% conversion) or is there a specific compromise temperature
Post by: RuiAce on September 07, 2016, 06:16:57 pm
for questions involving the production of sulfuric acid processWell if you want to maximise your marks you would try to remember all three parts to as accurate as you can.
do i need to mention the different temperature conditions and % conversion ie initially 1000 degrees, 550 degrees (97% conversion), 400 degrees (99.7% conversion) or is there a specific compromise temperature
With temperature you'd want to remember that they're different for each of the three parts. Initially 1000 degrees yes but that's just for part 1.
Part 2 is where all the equilibrium principles come in - catalytic oxidation of sulfur dioxide into sulfur trioxide. I just memorised somewhere around 450 degrees C though I think. (But I did mention the first bed giving a 70% yield and the last giving a 99.7%)
It's not specific either I don't think. But it shouldn't be too far out from 450. 400 and 550 look acceptable.
Post by: zoeh on September 08, 2016, 04:07:16 pm
Why are cyclohexene and cyclohexane used instead of just hexane and hexene when comparing reactivity of alkenes and alkanes?
Thankyouu :)
Post by: jakesilove on September 08, 2016, 04:18:47 pm
Hi,
Why are cyclohexene and cyclohexane used instead of just hexane and hexene when comparing reactivity of alkenes and alkanes?
Thankyouu :)
Hey!
This is a really good question. It's important to understand this, because I've seen multiple HSC questions asking about it!
Basically, we just need an Alkane and an Alkene to test, right? Unfortunately, most Alkenes have EXTREMELY low boiling points. Therefore, it is different to directly compare an Alkane and corresponding Alkene (because it's fairly difficult to test a gas. We want two liquids)! Cyclohexane/ene, due to its cyclical structure, 'sticks' together and remains a liquid much more easily. Additionally, it is non-reactive (won't explode upon testing) and fairly non-toxic.
I hope that answers your question!
Post by: zoeh on September 08, 2016, 05:24:02 pm
In the shipwrecks option, why do acidic environments accelerate corrosion?
Post by: RuiAce on September 08, 2016, 05:37:22 pm
Hi,May be of interest - I actually did the experiment with BOTH cyclohexene/cyclohexane and hex-1-ene/hexane. BOTH produced the same results you'd anticipate.
Why are cyclohexene and cyclohexane used instead of just hexane and hexene when comparing reactivity of alkenes and alkanes?
Thankyouu :)
In fact, I don't know if Jake will disagree but I reckon BOTH are acceptable from my first-hand experience.
What we CANNOT use is something like ethane/ethylene. For reasons he pointed out.
Post by: jakesilove on September 08, 2016, 08:18:51 pm
May be of interest - I actually did the experiment with BOTH cyclohexene/cyclohexane and hex-1-ene/hexane. BOTH produced the same results you'd anticipate.
In fact, I don't know if Jake will disagree but I reckon BOTH are acceptable from my first-hand experience.
What we CANNOT use is something like ethane/ethylene. For reasons he pointed out.
I would generally stick to Cyclohexane/ene, because I've seen questions outlining the specific benefit of using that chemical.
Post by: jakesilove on September 08, 2016, 08:20:28 pm
Hey just another question,
In the shipwrecks option, why do acidic environments accelerate corrosion?
Whilst I don't have the specifics right now, take a look at the Chemical equations for the formation of rust. On the right hand side (the products, including rust), there is also a Hydroxide group. If you add acid, by Le Chatelier's principle, the equilibrium will shift towards the side with the base (the products) to neutralise the added acid. This also increases the yield of rust!
Additionally, I think there are factors to do with bacteria propagation, having an area for the reaction to move forward etc. Sorry, I don't remember much from this topic!
Jake
Post by: zoeh on September 10, 2016, 11:27:51 am
Whilst I don't have the specifics right now, take a look at the Chemical equations for the formation of rust. On the right hand side (the products, including rust), there is also a Hydroxide group. If you add acid, by Le Chatelier's principle, the equilibrium will shift towards the side with the base (the products) to neutralise the added acid. This also increases the yield of rust!Thankyou!
Additionally, I think there are factors to do with bacteria propagation, having an area for the reaction to move forward etc. Sorry, I don't remember much from this topic!
Jake
Post by: amandali on September 11, 2016, 11:59:07 pm
a)HCl
b)Mg(NO3)2
c)PtI2 platinum iodide
d) KNO3
not sure why its A
Post by: RuiAce on September 12, 2016, 08:29:51 am
queS: an electrochemical cell was constructed using Mg anode and Pt cathode. the cell potential was measured to be 3.8V. Which would be the suitable electrolyte for cathode half cell?If the anode was Mg metal then the anolyte must have been Mg2+.
a)HCl
b)Mg(NO3)2
c)PtI2 platinum iodide
d) KNO3
not sure why its A
Oxidation occurs at the anode. Magnesium metal is converted into magnesium ions. According to the data sheet, this has an Eo value of 2.36V
So all we have to do is subtraction.
3.8-2.36=1.44
Which is approximately equal to 1.36V which is the value of the chlorine gas/chloride ion reduction's Eo value.
______________
Alternatively, this can be done through process of elimination.
The cathode is Pt. Pt is an inert metal, and used whenever there are gases involved in the equation. So recalling that chlorine gas gets reduced into chloride ions, A has to be correct.
B is wrong because Mg2+ is the anolyte, not the catholyte.
C is the weird one. If the cathode is Pt and the catholyte is PtI2, then platinum must be taking part in the reaction. Logically, this is fine because magnesium is going to be heaps more reactive than platinum. However that's the thing; platinum is so unreactive to the point this probably just won't occur at all. Also, it was probably intended to be a dud option as Pt does not appear on the standard reduction potentials on the data sheet
D is wrong because K is more reactive than Mg. If the cell was constructed with potassium ions then the magnesium metal would have to be the cathode.
Post by: conic curve on September 12, 2016, 05:21:11 pm
Post by: RuiAce on September 12, 2016, 06:22:40 pm
Could someone help me with this question I attached belowSaturated means that no more of a substance can dissolve. What happens when it doesn't dissolve? It precipitates.
What is the precipitate.
Post by: katherine123 on September 14, 2016, 08:07:11 am
If the anode was Mg metal then the anolyte must have been Mg2+.
Oxidation occurs at the anode. Magnesium metal is converted into magnesium ions. According to the data sheet, this has an Eo value of 2.36V
So all we have to do is subtraction.
3.8-2.36=1.44
Which is approximately equal to 1.36V which is the value of the chlorine gas/chloride ion reduction's Eo value.
______________
Alternatively, this can be done through process of elimination.
The cathode is Pt. Pt is an inert metal, and used whenever there are gases involved in the equation. So recalling that chlorine gas gets reduced into chloride ions, A has to be correct.
B is wrong because Mg2+ is the anolyte, not the catholyte.
C is the weird one. If the cathode is Pt and the catholyte is PtI2, then platinum must be taking part in the reaction. Logically, this is fine because magnesium is going to be heaps more reactive than platinum. However that's the thing; platinum is so unreactive to the point this probably just won't occur at all. Also, it was probably intended to be a dud option as Pt does not appear on the standard reduction potentials on the data sheet
D is wrong because K is more reactive than Mg. If the cell was constructed with potassium ions then the magnesium metal would have to be the cathode.
so any inert metal can have any type of gas but it depends on what the electrolyte is? eg. Pt metal reducing Hydrogen ions to hydrogen gas
Post by: RuiAce on September 14, 2016, 08:29:52 am
Yeah that's right. It depends on what the electrolyte is.
so any inert metal can have any type of gas but it depends on what the electrolyte is? eg. Pt metal reducing Hydrogen ions to hydrogen gas
Post by: amandali on September 14, 2016, 08:32:08 am
im still not 100% sure in terms of rounding
so for b) is it necessary to write the actual number of moles (line 2) or do i just need to write number of moles rounded to 3 sig fig only?
i was taught to use variable like x or y to show examiner that im using the actual numbers not the rounded one. So for part c) will it be wrong if i write in the rounded number (but using the actual number if im not using variables)?
And should the value for n(HCl) initial and n(HCl remaining) (not the answer but part of the working) be in 3 sig fig?
Post by: RuiAce on September 14, 2016, 08:39:08 am
(http://uploads.tapatalk-cdn.com/20160913/8b8c1bc19ae3bf1cf819bd22e36b9147.jpg)In your working out, you are meant to use the maximum amount of sig figs possible.
im still not 100% sure in terms of rounding
so for b) is it necessary to write the actual number of moles (line 2) or do i just need to write number of moles rounded to 3 sig fig only?
i was taught to use variable like x or y to show examiner that im using the actual numbers not the rounded one. So for part c) will it be wrong if i write in the rounded number (but using the actual number if im not using variables)?
And should the value for n(HCl) initial and n(HCl remaining) (not the answer but part of the working) be in 3 sig fig?
When it comes to answer, you look at the data you used in that question or in advance to arrive at your answer. The lowest number is your s.f. value.
For b), you need to write both. This is what you do.
1. Punch the numbers into the calculator
2. Write down your unrounded answer (don't forget units - I know you didn't here)
3. KEEP that unrounded answer in your calculator if you need it for later
4. Write down the rounded answer (3 s.f.) remembering the units again
5. For the next part, use your unrounded answer.
That let = x thing is up to you. Writing an unrounded answer can just take a lot of time, which is the benefit of introducing an 'x'. I never did though.
Post by: amandali on September 14, 2016, 09:41:34 am
ques: explain nature of X^(-2) ion using the graph
i dont understand why the answer says it s a moderately weak conjugate base
(http://uploads.tapatalk-cdn.com/20160913/af378a3f9d864f1d7d417a00ab347d6d.jpg)
shouldnt this be A since bromo needs to be mentioned before chloro (alphabetical order) or is it by arragrment from carbon 1
Post by: jakesilove on September 14, 2016, 12:01:30 pm
(http://uploads.tapatalk-cdn.com/20160913/4c145fb6e5e624d4eb8f2bf605174dc9.jpg)
ques: explain nature of X^(-2) ion using the graph
i dont understand why the answer says it s a moderately weak conjugate base
(http://uploads.tapatalk-cdn.com/20160913/af378a3f9d864f1d7d417a00ab347d6d.jpg)
shouldnt this be A since bromo needs to be mentioned before chloro (alphabetical order) or is it by arragrment from carbon 1
For your first question, I wouldn't have said it was a conjugate base either; but we can tell that it's a weak base. If we titrate a strong acid with a strong base, we expect the equivalence point to sit at about a pH of 7. Strong acid + weak base = pH of 4-5. Strong base + weak acid = pH of 8-10. Therefore, based on the graph (and since NaOH is a strong base), H2X is going to be a weak acid!'
As for your second question, in my mind anyway, it should definitely be A based on HSC rules. I think the answer may be wrong!
Post by: RuiAce on September 14, 2016, 12:29:14 pm
Post by: conic curve on September 15, 2016, 05:18:38 pm
Post by: RuiAce on September 15, 2016, 05:45:20 pm
A compound is found to contain 23.3% magnesium, 30.7% sulfur and 46.0% oxygen. What is the empirical formula for this compound?Compare the percentage composition to the molar masses of the relevant substances for the sake of this question. The comparison should be done so using division, because that's the only way we can get something useful.
Note that molar masses tell us which atoms have more mass on a broader scale.
Mg: 23.3%/24.31 = 0.958% = approx 1%
S: 30.7%/32.07 = 0.957% = approx 1%
O: 46%/16.00 = 2.875% = approx 3%
Comparing ratios - Ratio of Mg to S to O is
Mg:O:S
1:1:3 (approximately)
Therefore i predict it to be MgSO3.
A bit poorly worded. Feel free to point out confusion.
Post by: jakesilove on September 15, 2016, 05:55:31 pm
A compound is found to contain 23.3% magnesium, 30.7% sulfur and 46.0% oxygen. What is the empirical formula for this compound?
Hey! So, essentially we just need to compare their masses and find some values that make approximate sense. Let's say that the formula is
We know that mm(Mg)=24.3g, mm(S)=32.1g, mm(O)=16g. So, what can we tell from all this?
This is just the maths that pops out of the info we know. Whilst is COULD be helpful, there's an easier way. Notice that there is twice as much Oxygen as there is Magnesium (approximately). How can we get a ratio of Mg:O that is 1:2 by mass? Well, if we take three atoms of Oxygen, that will weight 48g. Half of 48g is 24g, which is basically the 24.3g we need! So, the empirical formula LOOKS LIKE it will take three moles of Oxygen for one more of Magnesium.
Now, to work in the sulfur. The mass needs to be between the 24.3g of Magnesium, and the 48g of Oxygen. Well, looks like it already is! If we double the moles of Sulfur, the mass will be GREATER than that of Oxygen, which we don't want. Okay, so let's check out what we've discovered and see if it makes sense.
The total mass of the above is 104.4g. For Mg, 24.3/104.4=0.23, exactly as much as desired. For Sulfur, 32.1/104.4=0.307, exactly as expected. Finally, for Oxygen, 3*16/104.4=0.46, exactly as expected.
You'll note that there was a level of intuition in the above answer. I remember there being a move definitive way; let me think for a bit, and I'll try remember it!
Jake
Post by: jakesilove on September 15, 2016, 05:56:29 pm
Compare the percentage composition to the molar masses of the relevant substances for the sake of this question. The comparison should be done so using division, because that's the only way we can get something useful.
Note that molar masses tell us which atoms have more mass on a broader scale.
Mg: 23.3%/24.31 = 0.958% = approx 1%
S: 30.7%/32.07 = 0.957% = approx 1%
O: 46%/16.00 = 2.875% = approx 3%
Comparing ratios - Ratio of Mg to S to O is
Mg:O:S
1:1:3 (approximately)
Therefore i predict it to be MgSO3.
A bit poorly worded. Feel free to point out confusion.
And that would be the more definitive way
Post by: conic curve on September 16, 2016, 11:21:09 am
I know english is all about writing practice essays, memorising quotes, practicing adapting, reediting your essay/creative etc. I know maths is all about practice and learning from your mistakes through past papers but what about Chemistry and Physics?
Post by: RuiAce on September 16, 2016, 11:22:12 am
Sorry to get a bit off track but how do you study for sciences like Phys and chem?Same as maths.
I know english is all about writing practice essays, memorising quotes, practicing adapting, reediting your essay/creative etc. I know maths is all about practice and learning from your mistakes through past papers but what about Chemistry and Physics?
Just that you're practicing how to write 6 mark responses and mark maximisation instead of a bunch of equations
Post by: conic curve on September 16, 2016, 11:23:46 am
Same as maths.
Just that you're practicing how to write 6 mark responses and mark maximisation instead of a bunch of equations
Oh...what about looking through notes?
Post by: RuiAce on September 16, 2016, 11:29:09 am
Oh...what about looking through notes?Oh right. Notes actually do contribute in some way or another in physics/chemistry.
Whereas maths you just need to jump into past papers, once you get into HSC you'll find you may spend some time revising notes and gradually shift into past papers.
Post by: conic curve on September 16, 2016, 11:31:36 am
Oh right. Notes actually do contribute in some way or another in physics/chemistry.
Whereas maths you just need to jump into past papers, once you get into HSC you'll find you may spend some time revising notes and gradually shift into past papers.
I thought for maths, notes are helpful at first when you're getting introduced to new abstract ideas and then over time you don't need it as much
Notes, I thought were only good for basic understanding for physics and chemistry and over time, it wouldn't be needed as much
Post by: RuiAce on September 16, 2016, 11:46:03 am
I thought for maths, notes are helpful at first when you're getting introduced to new abstract ideas and then over time you don't need it as muchSome people use it. I never had notes for maths personally.
Notes, I thought were only good for basic understanding for physics and chemistry and over time, it wouldn't be needed as much
Yeah that's the point. That's why I say "gradually" shift to past papers. But at least notes helps me check up on what I don't know when I get stumped on a past paper qn.
Post by: karenc. on September 18, 2016, 04:24:42 pm
Post by: jakesilove on September 18, 2016, 04:26:07 pm
Hi everyone, I am just wondering if you guys have any questions for ppm calculations because I sometimes get them wrong in exams. Many thanks Karen.
Maybe you could find some questions that you've gotten wrong, post them up and show us your solutions? That way, we can figure out what the problem is!
Jake
Post by: ssarahj on September 18, 2016, 04:28:05 pm
Hi everyone, I am just wondering if you guys have any questions for ppm calculations because I sometimes get them wrong in exams. Many thanks Karen.
I'm with you on this Karen, ppm questions are the death of me
Post by: karenc. on September 18, 2016, 04:31:23 pm
I'm with you on this Karen, ppm questions are the death of me
me too, there were too many questions on this for my trial!
Post by: jakesilove on September 18, 2016, 04:40:25 pm
me too, there were too many questions on this for my trial!
A good question was from the 2007 HSC Chemistry exam. It tells you that 'Sulfur in diesel is 50 parts per million (ppm)'. Then, it asks you to calculate the volume of sulfur dioxide produced when a full tank (capacity 60 kg) of diesel is consumed. So, how do we approach this question?
We know that parts per million is a weight/weight or v/v measurement. This means that for every gram/kg of a substance, there will be 50 ppm of sulfur in it. So, it's sort of like a percentage! This is the way I think about the question anyway. First, we write ppm as a percentage.
Then, we multiply 60kg by that 'percentage'
Now, we can answer the question! Just combust the Sulfur (using an appropriate chemical formula), figure out the moles of Sulfur dioxide and use the molar volume to find the answer!
Let me know if any of this didn't make sense.
Jake
Post by: RuiAce on September 18, 2016, 04:47:55 pm
However, you are much better off posting questions and just asking us to do them if you wish for demonstrations from us. They are tough - probably the second hardest type of calculation in the course.
Post by: karenc. on September 18, 2016, 04:51:40 pm
A good question was from the 2007 HSC Chemistry exam. It tells you that 'Sulfur in diesel is 50 parts per million (ppm)'. Then, it asks you to calculate the volume of sulfur dioxide produced when a full tank (capacity 60 kg) of diesel is consumed. So, how do we approach this question?
We know that parts per million is a weight/weight or v/v measurement. This means that for every gram/kg of a substance, there will be 50 ppm of sulfur in it. So, it's sort of like a percentage! This is the way I think about the question anyway. First, we write ppm as a percentage.
Then, we multiply 60kg by that 'percentage'
Now, we can answer the question! Just combust the Sulfur (using an appropriate chemical formula), figure out the moles of Sulfur dioxide and use the molar volume to find the answer!
Let me know if any of this didn't make sense.
Jake
Thanks jake your answer made sense :) Anyways are these formulas correct? 1g=1000mg, 1mg/kg=1mg/L=1ppm
Post by: RuiAce on September 18, 2016, 04:53:26 pm
Thanks jake your answer made sense :) Anyways are these formulas correct? 1g=1000mg, 1mg/kg=1mg/L=1ppmYes those are correct
Post by: karenc. on September 18, 2016, 04:54:42 pm
There was a noticeable amount of ppm calculations in James Ruse papers from memoryThanks for the advice RuiAce :) I have another question is a raw mark of 85 good enough to get a band 6 when it is scaled?
However, you are much better off posting questions and just asking us to do them if you wish for demonstrations from us. They are tough - probably the second hardest type of calculation in the course.
Post by: jakesilove on September 18, 2016, 05:02:58 pm
Thanks for the advice RuiAce :) I have another question is a raw mark of 85 good enough to get a band 6 when it is scaled?
Absolutely! There's no way to figure out how your scaled marks will look, but if you study hard and smash out your HSC, a band 6 is definitely achievable :)
Post by: RuiAce on September 18, 2016, 05:03:48 pm
Thanks for the advice RuiAce :) I have another question is a raw mark of 85 good enough to get a band 6 when it is scaled?Oh yeah. General trend is that if you're getting at least 85 raw you're safe.
I got a raw mark of 87 and it got aligned up to 93
Post by: conic curve on September 18, 2016, 05:42:10 pm
Oh yeah. General trend is that if you're getting at least 85 raw you're safe.
I got a raw mark of 87 and it got aligned up to 93
Is raw mark, the internal mark you get in school?
How do you get a raw mark of 90 (I'm trying to aim for 90 but I don't think I'm on that level yet)?
Post by: RuiAce on September 18, 2016, 05:43:16 pm
Is raw mark, the internal mark you get in school?I presume she meant her raw mark for the final exam. Not considering internals at all here, only externals.
How do you get a raw mark of 90 (I'm trying to aim for 90 but I don't think I'm on that level yet)?
Post by: jakesilove on September 18, 2016, 05:45:14 pm
How do you get a raw mark of 90
For every 10 marks, you need to successfully gain 9 of them.
This would require constant study, to the detriment of most of your other subjects, and discipline beyond that of most human beings. I would never be aiming for a raw mark of 90; it's not unachievable, but it's pretty damn tough.
Basically, to get the absolute best atar you can, you need to try consistently hard throughout the year, and do the best you can. Don't aim for a particular mark, except maybe to match/beat your previous mark. You could get raw marks of 60 throughout the year and still get a 99.95 Atar. So don't worry about marks; just try, try and try again.
Jake
Post by: conic curve on September 18, 2016, 06:06:33 pm
For every 10 marks, you need to successfully gain 9 of them.
This would require constant study, to the detriment of most of your other subjects, and discipline beyond that of most human beings. I would never be aiming for a raw mark of 90; it's not unachievable, but it's pretty damn tough.
Basically, to get the absolute best atar you can, you need to try consistently hard throughout the year, and do the best you can. Don't aim for a particular mark, except maybe to match/beat your previous mark. You could get raw marks of 60 throughout the year and still get a 99.95 Atar. So don't worry about marks; just try, try and try again.
Jake
Thanks for inspiration Jake :D
Post by: anotherworld2b on September 18, 2016, 09:44:28 pm
Post by: jakesilove on September 19, 2016, 10:29:28 am
Hi i was wondering if i could get help with q7
Hey! So, at each of the times specified, you need to draw a straight line tangential to the graph. Tangential just means with the same gradient, so it should just touch the graph once. Then, you need to find the gradient of that graph; to do this, choose two points on the line, and use the gradient formula!
Jake
Post by: conic curve on September 19, 2016, 03:48:28 pm
BTW Jake, I think I asked you this before but do you study chemistry at uni?
Post by: jakesilove on September 19, 2016, 03:49:59 pm
In year 12, do you do a chemistry prac on dilution? If so which module is it in?
BTW Jake, I think I asked you this before but do you study chemistry at uni?
No, you don't. However, maths questions can ask you to utilise
I studied Chem in the first semester of my first year, but not beyond that.
Post by: conic curve on September 19, 2016, 03:56:07 pm
No, you don't. However, maths questions can ask you to utilise
I studied Chem in the first semester of my first year, but not beyond that.
Was it chem that was related to the HSC (i.e. they teach you concepts from the HSC which builds up in uni)
Post by: karenc. on September 19, 2016, 03:59:33 pm
In year 12, do you do a chemistry prac on dilution? If so which module is it in?
BTW Jake, I think I asked you this before but do you study chemistry at uni?
no it's part of the year 11 water topic
Post by: jakesilove on September 19, 2016, 05:00:47 pm
Was it chem that was related to the HSC (i.e. they teach you concepts from the HSC which builds up in uni)
No, university Chemistry is nothing at all like HSC chem.
Post by: massive on September 19, 2016, 10:58:19 pm
What is the pH if 1 litre of hydrogen chloride gas at STP is dissolved in 1.2 litres of pure water?
Post by: RuiAce on September 19, 2016, 11:05:00 pm
how do you do this?Try converting the volume of gas into moles of gas using n = V/Vm
What is the pH if 1 litre of hydrogen chloride gas at STP is dissolved in 1.2 litres of pure water?
Vm = 24.79 L mol-1 at STP from the data sheet
Then you can just C=n/V it to get your required concentration(s)
Post by: wesadora on September 20, 2016, 08:10:54 am
Try converting the volume of gas into moles of gas using n = V/Vm
Vm = 24.79 L mol-1 at STP from the data sheet
Then you can just C=n/V it to get your required concentration(s)
is final V in c=n/v 2.2L or 1.2L?
Post by: RuiAce on September 20, 2016, 08:15:30 am
is final V in c=n/v 2.2L or 1.2L?You don't use final V.
You use V = volume of water. Not V = volume of solution.
Because concentration is a measure of quantity of a substance with respect to an environment. We only consider the "changed" environment for FURTHER disturbances.
Post by: anotherworld2b on September 24, 2016, 01:40:41 am
Post by: jakesilove on September 24, 2016, 11:08:57 am
For these two questions what would be necessary to include to get full marks?
Hey!
For your first question, I would say something like this. Neutralization will occur when equal amounts of Hydroxide and Hydrogen ions are present in solution. If you are titrating a strong acid/strong base, this will be around a pH of 7. If you are titrating a weak acid/strong base etc. etc. etc. A pH indicator can be selected so that it's end point is near the desired pH. When the colour changes, the desired pH has been reached, and thus the equivalence point is at approximately this point.
For your second question, note that the equivalence point is the point at which there are equal moles of -OH and H in the solution, whilst the end point is the point at which the indicator changes colour. These might not be precisely the same, leading to errors in the experiment.
Jake
Post by: massive on September 24, 2016, 02:27:11 pm
Which of the following does NOT usually occur during the cracking of high boiling fractions of crude oil?
A) a decrease in the pressure of the reaction vessel
B) the formation of products with a higher total chemical potential energy than the reactants
C) the production of smaller saturated hydrocarbons
D) the formation of unsaturated hydrocarbons
Post by: jakesilove on September 24, 2016, 02:36:32 pm
What would be the answer to the following question:
Which of the following does NOT usually occur during the cracking of high boiling fractions of crude oil?
A) a decrease in the pressure of the reaction vessel
B) the formation of products with a higher total chemical potential energy than the reactants
C) the production of smaller saturated hydrocarbons
D) the formation of unsaturated hydrocarbons
Hey! First, for anyone reading this, this question is outside the HSC curriculum. As such, I'm not really confident with my response. However, since we're boiling stuff, I would imagine that liquids are turning into gases. Therefore, the pressure of the vessel should really INCREASE almost every time, not decrease. Thus, I think the answer should be A.
Jake
Post by: lha on September 25, 2016, 11:11:53 am
Does anyone have an easy method of memorising solvay process?
Moderator action: Please do not double post unnecessarily.
Post by: RuiAce on September 25, 2016, 11:13:57 am
What mark do you need in chemistry to get a decent atar of 90+?Your ATAR is dependent on all of your subjects. If you got 100 in chemistry but only 50 in everything else then it was all for nothing (except maybe a glorious state rank).
What mark do you need in chemistry to get a 90+ ATAR? Think about all of your subjects all at once, not just chemistry.
Does anyone have an easy method of memorising solvay process?This is probably one of the most rigorous things to memorise. The reason is that there's heaps of processes that go on in the Solvay process.
I would recommend the following three:
- A flow chart
- A list of chemical equations
- The steps, in your notes
You should read over your notes ample times so that the steps are actually presentable, however it may be too hard to process. Hence, utilise the flow chart and compare your notes to the chart. The flow chart gives a much more visual instead of mental approach to the things at hand.
As for the chemical equations, you may choose to put the equations ON the flow chart as well, but you may choose to just write it out under a subheading
E.g. Brine purification: ...
Decomposition of sodium carbonate into sodium hydrogen carbonate: ...
(I'm fairly sure there are 6 subheadings if you take this approach)
Post by: zoeh on September 25, 2016, 01:35:23 pm
Post by: marynguyen18 on September 25, 2016, 03:57:51 pm
Post by: RuiAce on September 25, 2016, 04:07:46 pm
does dry cells and fuel cells have a chemical equation that you have to remember?Yes
For example, for the dry cell we have
Anode: Zn(s) → Zn2+ + 2 e-
Cathode: 2 MnO2(s) + 2 NH4+ + 2 e- → Mn2O3(s) + H2O(l) + 2 NH3(aq)
Post by: wesadora on September 26, 2016, 10:36:25 pm
I mean, using a dissolved oxygen sensor is also a quantitative method of measuring DO, would give a more accurate reading and is way easier to explain. I guess is it only important to be aware it's a feasible method for DO testing, maybe in case it comes up in MCs or something? Have there been any past HSC/trial exam questions you're aware of that have required a thorough explanation of the Winkler titration.
Same with the EDTA titration. Like, do we need to know Ca2+ + EDTA4- --> Ca(EDTA)2- and the water sample should turn from red to blue with Eriochrome Black T indicator blahblahlah.....
lmao.
Thanks :)
Post by: RuiAce on September 26, 2016, 10:38:04 pm
for the Winkler titration...how important is it to know it o.OI did memorise that EDTA equation just in case, but in all honesty I just memorised what they were used for. I knew nothing about the methods themselves to EDTA or Winkler at all.
I mean, using a dissolved oxygen sensor is also a quantitative method of measuring DO, would give a more accurate reading and is way easier to explain. I guess is it only important to be aware it's a feasible method for DO testing, maybe in case it comes up in MCs or something? Have there been any past HSC/trial exam questions you're aware of that have required a thorough explanation of the Winkler titration.
Same with the EDTA titration. Like, do we need to know Ca2+ + EDTA4- --> Ca(EDTA)2- and the water sample should turn from red to blue with Eriochrome Black T indicator blahblahlah.....
lmao.
Thanks :)
Jake might dispute me on this one though.
Post by: ml125 on September 27, 2016, 01:41:38 am
In the shipwrecks option, there was a past paper question (from 2005 hsc) showing an electrolytic cell with 2 inert electrodes, with Cl- coating the anode and H20 around the cathode, in a solution of aqueous NaCl. The question asked to indicate one possible anode and cathode reaction that may occur (it was only a 1 mark question). How do you do this? Like how do you know which ones can act as an anode or cathode?There are two possible cathode reactions which can occur:
The standard potential of the reduction of water is lower than that of Na+, therefore the second reaction is more likely to occur.
There are two possible anode reactions which can occur:
Notice that the two standard potentials are relatively close to each other. The reaction to take place will depend on the concentration of Cl- in solution. It would be fine to state either of the reactions, given that you also state conditions eg. if [Cl-]<1M, water is oxidised. However without stating conditions, I'd say that it would be safe to state that the chlorine reaction would occur.
Hi i was wonder if i could my answers to these questions checked please.In your last line of working, you divide by 0.001 instead of 0.01! The answer should be 0.423M instead of 4.23M. Other than that, the rest should be fine :)
There are answers so im not sure whether i got them right or not
Post by: jakesilove on September 27, 2016, 08:45:06 am
I did memorise that EDTA equation just in case, but in all honesty I just memorised what they were used for. I knew nothing about the methods themselves to EDTA or Winkler at all.
Jake might dispute me on this one though.
I just knew the methods, and didn't remember the formulas. So, basically, you just need enough specific information SOMEWHERE (what it is is up to you!)
Post by: anotherworld2b on September 27, 2016, 09:48:29 pm
Post by: RuiAce on September 27, 2016, 09:55:33 pm
Could i please get help with these questions? I am not sure what to do :-\Hints
Q9: Limewater is calcium hydroxide (CaOH). You should have heard about it, because limewater is the most commonly used chemical used to test for presence of CO2. But the equation may not be too obvious. Giveaway: One of the products is CaCO3.
Limewater turns white ("milky") in the presence of carbon dioxide. This is essentially the insoluble calcium carbonate forming
Q7: This is just a metal displacement reaction.
Post by: anotherworld2b on September 28, 2016, 10:35:47 am
I was wondering would my answers yo these two questions be correct?
Hints
Q9: Limewater is calcium hydroxide (CaOH). You should have heard about it, because limewater is the most commonly used chemical used to test for presence of CO2. But the equation may not be too obvious. Giveaway: One of the products is CaCO3.
Limewater turns white ("milky") in the presence of carbon dioxide. This is essentially the insoluble calcium carbonate forming
Q7: This is just a metal displacement reaction.
Post by: RuiAce on September 28, 2016, 11:10:29 am
Thank you very much for your help ;)Q6 is definitely fine
I was wondering would my answers yo these two questions be correct?
Your working for Q7 is a bit over the place but I saw nLiOH = 0.253 * 0.01673 which is good. And it looks like you divided back out by 0.01 to get the concentration of HBr so it should be fine
Post by: Sssssrr on September 28, 2016, 02:57:07 pm
Post by: jakesilove on September 28, 2016, 03:10:42 pm
could someone please explain the difference between endpoint and equivalence point in a titration?
Hey! See my response to a previous forum question here! Let me know if you need further clarification :)
Post by: Sssssrr on September 28, 2016, 04:19:13 pm
Hey! See my response to a previous forum question here! Let me know if you need further clarification :)
Hey, that did help, but i'm still slightly unsure. To clarify, equivalence point is when the number of moles of the acid equals he number of moles of the base. But then, what its the defitnion of the endpoint, how are they different?
Thanks
Post by: anotherworld2b on September 28, 2016, 04:50:42 pm
Post by: ml125 on September 28, 2016, 06:40:53 pm
Hi could i please get help with q4, 5 and q9b?
4. Here, you would first start by determining the moles of CO2 used in the reaction. As the molar ratio of each of the components is 1:1, this will equal the moles of limestone used in the reaction. You can then calculate a percentage using the mass of limestone used and the given value.
5. This is similar to Q4 in that you first need to find a molar value given the information you have – in this case, the moles of H2. You then follow the same process as the earlier question.
9. As I assume you have already done part a, you should have a molar value for any of the compounds within the equation. The ratio between each of the components is 1:1, therefore you can find the mass of CO2 from the moles.
Another way you could do this is that from part a, you would have had to find the initial mass of limestone used. From this, you can subtract the answer you get from part a (mass of quicklime). This will be the mass of CO2. Mass of reactants = mass of products.
Post by: anniez on September 28, 2016, 07:53:04 pm
Could someone pls help me with the attached question? I thought the answer would be D but online it says A and I was like 'what?'. If the answer is A could you pls help explain why? Thanks first! :)
Post by: RuiAce on September 28, 2016, 07:54:28 pm
Hi everyone!D is wrong because the equation isn't balanced
Could someone pls help me with the attached question? I thought the answer would be D but online it says A and I was like 'what?'. If the answer is A could you pls help explain why? Thanks first! :)
Incomplete combustion produces C, CO, or both.
Post by: anniez on September 28, 2016, 07:58:19 pm
Post by: anotherworld2b on September 29, 2016, 12:37:45 am
I got the wrong answer still for q9b but i am not sure why
I was also wondering how to do these other questions
4. Here, you would first start by determining the moles of CO2 used in the reaction. As the molar ratio of each of the components is 1:1, this will equal the moles of limestone used in the reaction. You can then calculate a percentage using the mass of limestone used and the given value.
5. This is similar to Q4 in that you first need to find a molar value given the information you have – in this case, the moles of H2. You then follow the same process as the earlier question.
9. As I assume you have already done part a, you should have a molar value for any of the compounds within the equation. The ratio between each of the components is 1:1, therefore you can find the mass of CO2 from the moles.
Another way you could do this is that from part a, you would have had to find the initial mass of limestone used. From this, you can subtract the answer you get from part a (mass of quicklime). This will be the mass of CO2. Mass of reactants = mass of products.
Post by: anotherworld2b on September 29, 2016, 12:40:58 am
Post by: ml125 on September 29, 2016, 03:07:38 am
Thank you very much for your help :DIn your working you used the wrong molar mass for carbon - it should be 12.01 instead of 10 ^^ The rest of your working is correct, though. :)
I got the wrong answer still for q9b but i am not sure why
I was also wondering how to do these other questions
Q5. At STP, one mole of any gas takes up 24.79L. Therefore, if you divide the current volume of gas by this number, you will get the moles of gas. As you have a value for moles and mass, you can find molar mass.
Q9. As Rui stated earlier, this is a reaction between calcium hydroxide (Ca(OH)2) and CO2. The solution turns cloudy due to the formation of insoluble calcium carbonate (CaCO3). Try writing out the equation with the information you have now - see what components are missing from the reactants side and you should be able to determine what the other product is!
I have asked about q9 before but iam still confused how to do it as well as this quesitonAs sodium is a very reactive metal, the reaction it undergoes with water is rapid.The reaction is exothermic, and as a result the sodium metal heats up and burns with an orange flame. One of the products of this reaction is a very commonly used base. The other is a gas.
Post by: RuiAce on September 29, 2016, 08:19:02 am
Thank you very much for your help :D
I got the wrong answer still for q9b but i am not sure why
I was also wondering how to do these other questions
For the ionic equation, make sure you know what exactly an ionic equation is first. And read off the complete balanced equation to get an ionic equation. I know you have a complete equation because you asked for it previously.
Complete equation: Ca(OH)2 + CO2 -> CaCO3 + H2O
(I specifically asked you to figure out the water by yourself for a reason. You need to get used to identifying what's MISSING in equations. The CaCO3 was not obvious, which is why I gave it away, but this one should've been obvious given the hint.)
Full ionic equation: Ca(2+) + 2 OH(-) + CO2 -> Ca(2+) + CO3(2-) + H2O
You need to realise that Ca(OH)2 and CaCO3 are ionic compounds, thus for an ionic equation they can be broken up
Now write down the net ionic equation the question asks you to.
Also, sodium in the water is a very famous experiment. You should well be aware of the immensely high reactivity of group I metals such as sodium.
Post by: MysteryMarker on September 29, 2016, 06:11:39 pm
There are two beakers, one labelled A and one labelled B. Each beaker has 200ml of either a weak acid or strong acid. The concentration of each acid is unknown. What procedure would you use to determine which beaker contains the strong acid and which contains the weak acid?
Thanks guys.
Post by: conic curve on September 29, 2016, 06:43:29 pm
Hey guys, just a basic question on acids. (Ha the irony)
There are two beakers, one labelled A and one labelled B. Each beaker has 200ml of either a weak acid or strong acid. The concentration of each acid is unknown. What procedure would you use to determine which beaker contains the strong acid and which contains the weak acid?
Thanks guys.
I believe it's dilution
Post by: anotherworld2b on September 29, 2016, 06:50:31 pm
I was wondering how would you know the expected observations from a reaction? I know general base and acid reactions
Post by: RuiAce on September 29, 2016, 06:57:08 pm
Hey guys, just a basic question on acids. (Ha the irony)I find it quite unfair that they'd ask you something like that in the HSC. I do not know of any practical in the syllabus dot points that relate to this one.
There are two beakers, one labelled A and one labelled B. Each beaker has 200ml of either a weak acid or strong acid. The concentration of each acid is unknown. What procedure would you use to determine which beaker contains the strong acid and which contains the weak acid?
Thanks guys.
Recommendation: First, perform a titration to determine concentrations.
IF: Concentrations are around the same - Use an indicator (or a pH probe) to figure it out.
IF: Concentrations are noticeably different
React the substances with a weak base. I suspect that something will happen to distinguish between the two. Best let Jake take over here though.
I believe it's dilutionHow does dilution tell you anything about whether an acid is weak or strong?
Post by: RuiAce on September 29, 2016, 07:01:43 pm
Thank you RuiAce and ml125 for your help ;DThe image didn't appear the first time.
I was wondering how would you know the expected observations from a reaction? I know general base and acid reactions
For that question, unless you have the skills to combine a ton of equations together you need to keep working backwards with the calculations. Easy but EXTREMELY tedious.
If you want me to show you how to combine equations here, feel free to ask.
___________________________________
And yeah. With identifying reactions, you need to know how to deduce what makes sense. Know the type of reactions and thus anticipate the products.
Post by: anotherworld2b on September 29, 2016, 07:06:51 pm
The image didn't appear the first time.
For that question, unless you have the skills to combine a ton of equations together you need to keep working backwards with the calculations. Easy but EXTREMELY tedious.
If you want me to show you how to combine equations here, feel free to ask.
___________________________________
And yeah. With identifying reactions, you need to know how to deduce what makes sense. Know the type of reactions and thus anticipate the products.
Post by: RuiAce on September 29, 2016, 07:26:15 pm
Yes could you please show me?I'm sorry, I take it back. Combining for that one can be done in too many different ways and I don't know which one is correct.
Yeah, it's just an extremely tedious question. You'll have to work through each individual thing step by step through every single equation.
Using n=m/M you have the moles of HNO3 being 396.71 or something
Then 1 mol of HNO3 is formed from 1 mol of NO2 so moles of NO2 also equals 396.71
Then 1 mol of NO2 is formed from 1 mol of NO so moles of NO is also 396.71 and just keep going.
Post by: anotherworld2b on September 29, 2016, 07:42:49 pm
I'm sorry, I take it back. Combining for that one can be done in too many different ways and I don't know which one is correct.
Yeah, it's just an extremely tedious question. You'll have to work through each individual thing step by step through every single equation.
Using n=m/M you have the moles of HNO3 being 396.71 or something
Then 1 mol of HNO3 is formed from 1 mol of NO2 so moles of NO2 also equals 396.71
Then 1 mol of NO2 is formed from 1 mol of NO so moles of NO is also 396.71 and just keep going.
Post by: RuiAce on September 29, 2016, 08:10:30 pm
Ok okay. Which question are u referring to because I posted two images for two questions :oIf there was a second one I didn't see it because I only saw one.
Post by: anotherworld2b on September 29, 2016, 08:55:59 pm
Post by: RuiAce on September 29, 2016, 08:57:43 pm
How accurate do the answets have to be? I got 144ml for this question but the answer is 146ml in the bookNever round halfway through. Only ever round at the end.
Post by: jakesilove on September 29, 2016, 09:30:20 pm
How accurate do the answets have to be? I got 144ml for this question but the answer is 146ml in the book
Make sure to ONLY round to sig figs at the END of the question (ie. when you get the final answer). That being said, many HSC questions will allow for some small variation in answer (generally less than 2mL though!). I used to just always write down at least 5-8 decimal places, if I couldn't keep all the values stored on my calculator, so that I had values to come back to with enough accuracy. No point losing silly marks when you've done all the hard work!
Post by: anotherworld2b on September 29, 2016, 10:00:47 pm
I was also wondering for this question i got the wrong answer but im not sure why
Post by: jakesilove on September 29, 2016, 10:05:23 pm
Oh ok. Would my working out be right Though?
I was also wondering for this question i got the wrong answer but im not sure why
Yep, if you got an answer that is 2mL off the actual answer, your working would be right!
The problem with your working out in the last question is that you used 5.09g, where you should have used the CHANGE in mass. So, a certain amount has reacted, and THAT is the number of moles that you care about. So, subtract the final mass from the initial mass, and find how many moles of Calcite that is :)
Post by: anotherworld2b on September 29, 2016, 10:45:28 pm
For these two questions I was confused on how to do them as well.
I appreicate all the help I am recieving. It is really helping me practise using stoich which is currently my weakest point in chem.
Yep, if you got an answer that is 2mL off the actual answer, your working would be right!
The problem with your working out in the last question is that you used 5.09g, where you should have used the CHANGE in mass. So, a certain amount has reacted, and THAT is the number of moles that you care about. So, subtract the final mass from the initial mass, and find how many moles of Calcite that is :)
Post by: jakesilove on September 30, 2016, 10:41:46 am
Thank you very much for your help :)
For these two questions I was confused on how to do them as well.
I appreicate all the help I am recieving. It is really helping me practise using stoich which is currently my weakest point in chem.
That's really no problem! It's clear you're getting heaps better at these questions, so I have no doubt that you'll be confident come exam time.
For the first question (which is definitely a weird one), you need to start by interpreting the whole 'driving for a year' bit correctly. If it releases 3g per kilometer, and it travels for 1.6*10^4 km, then we can find the TOTAL grams emitted by multiplying the two numbers together! Then, we can find the moles of NO released based on the mass just found and the molar mass. Finally, we use the 4:6 molar ratio to find the moles of Ammonia, and convert that into litres using the molar volume formula.
For the second question, you need to oxidise liquid Hydrogen. This means 'inventing' a formula; I'd go with something like this
2kg of Hydrogen was used up; you can figure out the rest!
Jake
Post by: angiezhang9 on September 30, 2016, 10:42:26 pm
I am stuck on Q18 of the 2012 HSC Paper.
Which of the following changes take place when 50 ml of water is added to 50ml of 0.1 mol/L acetic acid? The answer is pH increases and degree of ionisation increases.
I don't understand why ionisation increases. Wouldn't this then mean that ph will decrease?
Thanks
Post by: jakesilove on September 30, 2016, 11:11:21 pm
Hey guys,
I am stuck on Q18 of the 2012 HSC Paper.
Which of the following changes take place when 50 ml of water is added to 50ml of 0.1 mol/L acetic acid? The answer is pH increases and degree of ionisation increases.
I don't understand why ionisation increases. Wouldn't this then mean that ph will decrease?
Thanks
Hey! It's a really good question, and quite a tough one too. Firstly, we need to remember that ionisation is the removal of a hydrogen ion. For weak acids, like acetic acid, this will only occur a small percentage of the time. If we add more water, more acid ions/hydrogen ions will dissociate. This means that the ionisation increases, just because it has more water to react with!
So, we know that there are MORE hydrogen ions in the solution after we add water than before. Why doesn't the pH change? Remember that the formula for pH is -log[H], where [H] is the CONCENTRATION of Hydrogen ions. Whilst the AMOUNT of Hydrogen ions have increased, the amount of liquid has doubled too. Ionisation will only barely increase, so the relative concentration of ions will actually decrease. As such, the concentration of Hydrogen ions will decrease, and thus the pH will increase.
Let me know if this explanation makes sense!
Jake
Post by: lha on October 01, 2016, 10:57:17 am
Alright thank you
Post by: RuiAce on October 01, 2016, 11:14:58 am
Should i only be practicing long response questions rather than short responses?If you ask me, neglecting short responses will mean you end up being bad at those and over complicating it. I always treated short and long responses differently - short just requires a suitable answer; long requires structure.
(I'll assume short responses excludes calculations here)
But you need to figure out YOUR best studying techniques.
Post by: angiezhang9 on October 01, 2016, 09:58:11 pm
Hey! It's a really good question, and quite a tough one too. Firstly, we need to remember that ionisation is the removal of a hydrogen ion. For weak acids, like acetic acid, this will only occur a small percentage of the time. If we add more water, more acid ions/hydrogen ions will dissociate. This means that the ionisation increases, just because it has more water to react with!
So, we know that there are MORE hydrogen ions in the solution after we add water than before. Why doesn't the pH change? Remember that the formula for pH is -log[H], where [H] is the CONCENTRATION of Hydrogen ions. Whilst the AMOUNT of Hydrogen ions have increased, the amount of liquid has doubled too. Ionisation will only barely increase, so the relative concentration of ions will actually decrease. As such, the concentration of Hydrogen ions will decrease, and thus the pH will increase.
Let me know if this explanation makes sense!
Jake
That was a perfect explanation! Thanks so much Jake :)
Post by: milkteaOO on October 01, 2016, 10:15:41 pm
Post by: RuiAce on October 01, 2016, 10:22:11 pm
Hi! I was just wondering if anyone could tell me the rules for calculating how many significant figures in something in chemistry? Thank you! :)This has most likely been answered already in this thread but I can't be bothered digging it up.
Of all of the data (numbers) you use, whichever number has the least amount of significant figures is the amount that you use.
E.g. What volume of O2 would be occupied by 3.49mol at 25oC and 100kPa
You would us the formula n=V/VM with n=3.49 and VM=24.79L mol-1
Hence 3 s.f.
Post by: anotherworld2b on October 02, 2016, 01:56:06 am
Post by: ml125 on October 02, 2016, 03:11:00 am
Hi i just wanted ask for some advice my exams are approximately 3 weeks from now but there is a lot of content to recall. Is there a way to effectively and quickly revise for chemistry? How did high achievers like yourselves prep for exams? I just wanted to see how others approach studying for exams because ive benn spending a lot of time just on stoichimetry.I'm sure that by now you're tired of hearing this - but it's always best to go through whole past papers or even just single past exam questions. In between practice exams, particularly for chemistry, I will usually go through specific questions on topics I am less confident in until I feel I can do them properly. What has helped me a lot is that within my notes, I include questions and examples to address any major concepts so I have an easy resource to look back on if I get stuck.
Post by: jakesilove on October 02, 2016, 09:36:23 am
Hi i just wanted ask for some advice my exams are approximately 3 weeks from now but there is a lot of content to recall. Is there a way to effectively and quickly revise for chemistry? How did high achievers like yourselves prep for exams? I just wanted to see how others approach studying for exams because ive benn spending a lot of time just on stoichimetry.
Hey! I totally agree with the above reply. The other thing I would be doing is going through a comprehensive set of notes, and just writing out all of the information you're not comfortable with yet. Be super succinct in your summary, and use colours to structure your notes. Learn from these summary sheets, and cut down on them as you learn the content. Ideally, you want to get these sheets (which you should rewrite everytime you understand something, because then you can take it OFF your sheet) down to like one or two pages by the time the exam comes around.
Other then that, just focus on past papers!
Jake
Post by: anotherworld2b on October 02, 2016, 03:33:35 pm
Spoiler
In your last line of working, you divide by 0.001 instead of 0.01! The answer should be 0.423M instead of 4.23M. Other than that, the rest should be fine :)
Post by: anotherworld2b on October 02, 2016, 04:55:37 pm
Post by: RuiAce on October 02, 2016, 05:05:32 pm
I was also wondering is my answer right for these questions ?It's hard to say whether or not you're doing it right for a question like this without seeing all the previous parts.
Also, it's an ask but if you're going to use pencil can you please use a darker one? It's a bit tiring to read at times
Post by: anotherworld2b on October 02, 2016, 07:36:13 pm
Post by: MysteryMarker on October 03, 2016, 11:35:30 am
Just curious as to what the syllabus is actually asking for this dot point, 'Compare the properties of the gaseous forms of oxygen and the oxygen free radical.
Like what would I write if this were to come as a short answer question in my exam?
Cheers guys.
Post by: RuiAce on October 03, 2016, 11:48:53 am
Hey guys,In terms of reactivity, everyone knows that oxygen is moderately reactive as whilst it doesn't react aggressively by itself, it can be made aggressive through means of burning/combustion and etc.
Just curious as to what the syllabus is actually asking for this dot point, 'Compare the properties of the gaseous forms of oxygen and the oxygen free radical.
Like what would I write if this were to come as a short answer question in my exam?
Cheers guys.
However, ozone is appreciably more reactive due to the instability of the coordinate bond. It is essentially a site of higher reactivity.
The oxygen free radical is the most reactive due to the fact that it's a single atom. It doesn't even have a full outer shell and wants to fill it up.
Then in terms of melting/boiling points, oxygen is a linear molecule and its melting/boiling points are extremely low
However, the melting/boiling points of ozone are slightly more elevated due to the fact that it is bent. The central oxygen has a lone, unbonded pair of electrons which is a site for dipole-dipole interactions.
Post by: MysteryMarker on October 03, 2016, 12:11:01 pm
In terms of reactivity, everyone knows that oxygen is moderately reactive as whilst it doesn't react aggressively by itself, it can be made aggressive through means of burning/combustion and etc.
However, ozone is appreciably more reactive due to the instability of the coordinate bond. It is essentially a site of higher reactivity.
The oxygen free radical is the most reactive due to the fact that it's a single atom. It doesn't even have a full outer shell and wants to fill it up.
Then in terms of melting/boiling points, oxygen is a linear molecule and its melting/boiling points are extremely low
However, the melting/boiling points of ozone are slightly more elevated due to the fact that it is bent. The central oxygen has a lone, unbonded pair of electrons which is a site for dipole-dipole interactions.
Oh thanks, but I was asking for the differences between oxygen and the oxygen free radical, not oxygen and ozone haha.
Cheers.
Post by: jakesilove on October 03, 2016, 12:16:31 pm
Oh thanks, but I was asking for the differences between oxygen and the oxygen free radical, not oxygen and ozone haha.
Cheers.
Really, the main points I would know are where they are found (ie. radicals are largely in upper atmosphere, as they are very unstable etc.), their comparative reactivities and their chemical structures. You'll never be asked more than three marks on it (tbh you'll never be asked on this at all). Potentially, you can discuss each of their importance.
Jake
Post by: RuiAce on October 03, 2016, 12:28:43 pm
Oh thanks, but I was asking for the differences between oxygen and the oxygen free radical, not oxygen and ozone haha.I've mentioned the reactivity of the free radical.
Cheers.
That's it. The chemical structure is technically a given, and due to how extreme the reactivity is there is no way to really evaluate the "boiling point" or "melting point" of the radical.
And as Jake said, this can't be more than theee marks due to the complete lack of information examinable
Post by: anotherworld2b on October 03, 2016, 07:33:38 pm
Post by: ProfLayton2000 on October 03, 2016, 10:11:55 pm
I'm not sure how to attach a picture so I'll try describe my question
The question has a map containing a farm, logged native forest, undisturbed pine forest and undisturbed native forest, all next to a lake leading to a town's water supply.
Part of the question asks for two sources of contamination, but I only find one (phosphates from farm). Whats another?
Post by: RuiAce on October 03, 2016, 10:21:35 pm
Q26 2005 HSCHave you considered the logging? Think about some issues related to soil erosion and come back if you're still confused
I'm not sure how to attach a picture so I'll try describe my question
The question has a map containing a farm, logged native forest, undisturbed pine forest and undisturbed native forest, all next to a lake leading to a town's water supply.
Part of the question asks for two sources of contamination, but I only find one (phosphates from farm). Whats another?
Post by: ProfLayton2000 on October 04, 2016, 11:31:16 am
Have you considered the logging? Think about some issues related to soil erosion and come back if you're still confused
Sorry still lost (would it have to do with the introduction of common ions in soil)?
Post by: RuiAce on October 04, 2016, 12:09:28 pm
Sorry still lost (would it have to do with the introduction of common ions in soil)?It can cause that.
It also just causes an accumulation of sediment in the water that's not necessary.
Jake. Appear already. I'm not good with water quality.
Post by: jakesilove on October 04, 2016, 12:58:51 pm
Jake. Appear already. I'm not good with water quality.
Say my name and I appear. When it comes to questions like this, you can genuinely choose ANY two features of the catchment area, and basically make something up related to that area. You've clearly chosen the most obvious one, which is great, and explained it well. For the rest, you just need to make up something that makes sense. For instance, if the river runs through a forest, it is very possible that a tree will at some point fall into the river. This will decay, causing bacteria and other microbes to grow around it. This will increase the biochemical oxygen demand. Similarly, as you've both alluded to, soil erosion is important as it may cause riverbanks to fall into the river. This increases the quantity of debris in the water, as well as total dissolved solids. As a last resort, you could always choose two sources of contamination FROM the farm itself; ie. Phosphates, but also perhaps general sewage from the home, acids from pesticides, etc. etc. etc.
Hope this helps! It's a weird type of question, but very common, so get used to making stuff up on the fly.
Jake
Post by: ProfLayton2000 on October 04, 2016, 02:49:01 pm
Say my name and I appear. When it comes to questions like this, you can genuinely choose ANY two features of the catchment area, and basically make something up related to that area. You've clearly chosen the most obvious one, which is great, and explained it well. For the rest, you just need to make up something that makes sense. For instance, if the river runs through a forest, it is very possible that a tree will at some point fall into the river. This will decay, causing bacteria and other microbes to grow around it. This will increase the biochemical oxygen demand. Similarly, as you've both alluded to, soil erosion is important as it may cause riverbanks to fall into the river. This increases the quantity of debris in the water, as well as total dissolved solids. As a last resort, you could always choose two sources of contamination FROM the farm itself; ie. Phosphates, but also perhaps general sewage from the home, acids from pesticides, etc. etc. etc.
Hope this helps! It's a weird type of question, but very common, so get used to making stuff up on the fly.
Jake
Many thanks Rui and Jake
Attended your chem lecture, must say it was good stuff
Post by: jakesilove on October 04, 2016, 02:50:43 pm
Many thanks Rui and Jake
Attended your chem lecture, must say it was good stuff
I'm glad you found it useful!
Jake
Post by: zoeh on October 04, 2016, 03:30:47 pm
Just a quick question,
are all buffer solutions made from weak acids and their conjugate bases? or are there other types of buffer solutions? How do you know if something forms a buffer solution?
Post by: jakesilove on October 04, 2016, 03:34:49 pm
Hey!
Just a quick question,
are all buffer solutions made from weak acids and their conjugate bases? or are there other types of buffer solutions? How do you know if something forms a buffer solution?
Hey! Whilst that is common when it comes to buffers, it is not always necessary for buffers to be composed of a weak acid and their conjugate base. Whilst this is a GOOD indicator that the solution is a buffer, it isn't definitive. I assume you're asking with regards to multiple choice questions asking you to identify a buffer; the way decide is by pretending I add -OH molecules, and -H molecules and check that the buffer will shift right/left as required!
Post by: RuiAce on October 04, 2016, 04:02:03 pm
Also, keep in mind you can't have a buffer with something strong, such as HCl and Cl-
Post by: anotherworld2b on October 05, 2016, 10:20:51 am
I also wanted to ask how can you idenity a subtance as acidic or basic?
Post by: jakesilove on October 05, 2016, 11:10:06 am
Could i llease get help with these two questions?
I also wanted to ask how can you idenity a subtance as acidic or basic?
For the first question, you can easily find the initial Hydrogen concentration, and thus the moles of Hydrogen present in solution. Then, find the number of Hydrogen ions added (again, easy given the concentration). Add up the total moles of Hydrogen, and use C=n/V to find the concentration!
For the second one, figure out the initial concentration of Hydrogen ions in solution. Again, this is a simple manipulation of the -Log(H) formula. Finally, use c1v1=c2v2 to find the required volume in the second solution (after figuring out what your desired concentrated is), and thus find the amount of water needing to be added.
When distinguishing between an acid and a base, I usually just add water to the equation and see what happens. If it looks like the substance will GIVE a hydrogen ion to the water (ie. proton donor), it is an acid. If it looks like it will TAKE a hydrogen ion from the water (ie. proton acceptor), it is a base. Hope this helps!
Post by: anotherworld2b on October 05, 2016, 01:12:55 pm
Could i get some help with disocisation? Im confused on how H+ ions and OH- ions are produced when water is added to acid and base respectively. ???
For the first question, you can easily find the initial Hydrogen concentration, and thus the moles of Hydrogen present in solution. Then, find the number of Hydrogen ions added (again, easy given the concentration). Add up the total moles of Hydrogen, and use C=n/V to find the concentration!
For the second one, figure out the initial concentration of Hydrogen ions in solution. Again, this is a simple manipulation of the -Log(H) formula. Finally, use c1v1=c2v2 to find the required volume in the second solution (after figuring out what your desired concentrated is), and thus find the amount of water needing to be added.
When distinguishing between an acid and a base, I usually just add water to the equation and see what happens. If it looks like the substance will GIVE a hydrogen ion to the water (ie. proton donor), it is an acid. If it looks like it will TAKE a hydrogen ion from the water (ie. proton acceptor), it is a base. Hope this helps!
Post by: jakesilove on October 05, 2016, 01:16:25 pm
Thank you for your help :)
Could i get some help with disocisation? Im confused on how H+ ions and OH- ions are produced when water is added to acid and base respectively. ???
Absolutely! For example, let's look at something like Sulfuric acid, and see what happens. Note: I don't know how to do the equilibrium symbol, so I'm using a right arrow instead.
Let's add water to Sulfuric acid
Clearly, the Acid has DONATED a proton here, and the water has accepted it. The water has therefore acted as a base, by the B-L definition. You sort of just need to know if a substance is an acid or a base (based on your studies this far), and if you don't just make up an equation that makes sense. If there are lots of Hydrogen atoms, it's usually easier to lose a hydrogen than to gain one. If there is an OH in the chemical structure, it is generally a base. It's also a little bit intuitive; just to past questions, and if you're really not sure, guess and write out an equation that makes sense!
Post by: anotherworld2b on October 05, 2016, 01:38:08 pm
Absolutely! For example, let's look at something like Sulfuric acid, and see what happens. Note: I don't know how to do the equilibrium symbol, so I'm using a right arrow instead.
Let's add water to Sulfuric acid
Clearly, the Acid has DONATED a proton here, and the water has accepted it. The water has therefore acted as a base, by the B-L definition. You sort of just need to know if a substance is an acid or a base (based on your studies this far), and if you don't just make up an equation that makes sense. If there are lots of Hydrogen atoms, it's usually easier to lose a hydrogen than to gain one. If there is an OH in the chemical structure, it is generally a base. It's also a little bit intuitive; just to past questions, and if you're really not sure, guess and write out an equation that makes sense!
Post by: anotherworld2b on October 05, 2016, 01:48:41 pm
I am really sturggling with pH calculations ???
Post by: RuiAce on October 05, 2016, 02:31:26 pm
Could i get help with these two questions as well?
I am really sturggling with pH calculations ???
__________________________________
I don't have the time to check the values.
Post by: RuiAce on October 05, 2016, 02:32:39 pm
Do you have the answers? (I'm guessing not).Let the unknown volume be v.
If you do: For the first question I got a pH of 13.4 as my final answer. I don't want to share my working out if its wrong :3
Also what is that second question?
Then the question is just a question involving neutralisation asking you to work both backwards (and maybe forwards) to find a required volume.
Post by: anotherworld2b on October 05, 2016, 05:18:08 pm
Post by: marynguyen18 on October 06, 2016, 11:59:59 am
Post by: RuiAce on October 06, 2016, 12:07:38 pm
I'm still a bit unsure about the nuclear chemistry, whats the difference between alpha, beta and gamma decay?The difference is that what's emitted is completely different.
In alpha decay, what's emitted is an entire helium nucleus. This means that 2 protons and neutrons are both emitted together.
Alpha decay occurs when the nucleus is too large. It has to do with the weak nuclear force but all you care is that it's too large and thus the nucleus needs to be made smaller.
In beta decay, what's emitted is an electron from the nucleus. The neutron emits the electron, and turns into a proton. This happens when the n:p ratio is too large so that we can rebalance it out.
In gamma decay, what's emitted is gamma radiation. This is EMR. This simply occurs because the nucleus has too much energy in it and needs to be radiated away.
Post by: marynguyen18 on October 06, 2016, 01:49:48 pm
Post by: ml125 on October 06, 2016, 10:58:40 pm
Can i please get help with these questions17. This is a double replacement reaction. To start off, write out the equation. You will then need to find the moles of the precipitate (BaSO4) using the given mass. From the equation, you can then find the moles of sulfuric acid using the molar ratio, and thus find [H2SO4]. You will then need to consider the proticity of the acid and base to determine the value of [H+] and [OH-] to be used in neutralisation. Then, sub in these values and the given volume of acid into C1V1=C2V2 to solve for the volume of NaOH.
1. With the information you have, you can find the moles of aluminium hydroxide (Al(OH)3). Looking at the composition of the compound, the moles of OH- will be triple the moles of Al(OH)3. From this, you can calculate a value for [OH-] which will be equivalent to both [H+] and [HCl](as HCl is monoprotic). You will then be able to find the moles and thus the mass of HCl.
2. First, you will need to find the moles of H+ using the given value for [H+] and the volume required. This will be equal to the moles of OH- required for neutralisation. Observe the formula for calcium hydroxide: Ca(OH)2. The moles of Ca(OH)2 will be half the moles of OH-. Using the attained molar value, you will be able to find the mass of Ca(OH)2 required for neutralisation.
Post by: anotherworld2b on October 07, 2016, 10:44:09 am
Post by: :3 on October 07, 2016, 11:37:40 am
Can i get help with these questions please? Also i am kind of confused about how to do equilibrium equations?
1.
A) Basically remember these formulas as they're commonly asked in exams:
- SO2 + H2O --> H2SO3 (sulfurous acid)
- 2H2SO3 + O2 --> 2H2SO4 (sulfuric acid).
B) Like above, remember this:
- H2SO4 + CaCO3 (marble) --> CaSO4 (crumbly mineral) + H2O + CO2.
2.
A) Find the limiting reagent (i.e. find both of the moles for each compound, then the one with least moles/insufficent moles to react with the other compound = used up one).
B) Find excess moles (i.e. you minus the one with the least moles from above from the one with the most moles), and then convert it into grams using the formula: g = n x mm.
C) Use mole to mole ratios (using the compound that is completely used up) to find the moles of the nitrogen dioxide, then times it by the STP number to get the volume in litres.
Hopefully this isn't too confusing.
Post by: nibblez16 on October 07, 2016, 11:53:53 am
Post by: RuiAce on October 07, 2016, 12:19:02 pm
Hello. Since we have other subjects coming up for chem, how can we stay up to date with it?What do you mean by other subjects coming up for chem? Did you mean a ton of exams before chemistry?
Post by: Gregs on October 07, 2016, 01:30:43 pm
p.s. not a chemical equation list with all of the equations we need to know
Post by: jakesilove on October 07, 2016, 01:38:40 pm
Yup! I meant other exams before chem. Typo lol
If you have a tonne of exams before Chemistry, definitely focus on those. I would take the bulk of you study time as focussed towards imminent subjects, making sure to give yourself a break wherever possible. However, as Chem gets closer (so maybe after Eng Paper 2) start to filter in a past papers or two. Maybe do a Chem past paper every couple of days, and go over your notes etc. etc. just so you don't lose track. When Chem is about a week away, focus on it must, much more. However, you have to take exams one at a time!
Post by: RuiAce on October 07, 2016, 01:52:30 pm
Yup! I meant other exams before chem. Typo lolI think Jake has you all covered.
For someone like me I actually gunned the first few weeks of stuvac on the courses to be examined later, in an attempt to store it in my long term memory. When I went back to it, it worked
Post by: RuiAce on October 07, 2016, 01:53:24 pm
Hi, I was just wondering if anyone had any word documents or something of the like with heaps of the Chemistry-maths equation questions that we can be given. ThanksI have a small table. I'll upload it later.
p.s. not a chemical equation list with all of the equations we need to know
It has all of what's needed except %(w/w) stuff.
Post by: Hua Fei on October 07, 2016, 02:18:31 pm
I think this might be a stupid question but I really don't understand if ozone being a polar molecule has any effect on its boiling point (-111 degrees Celsius) being higher than oxygen's boiling point (-183 degrees Celsius). If the polarity does have an effect, then how and why?
Thank you very much!
Post by: jakesilove on October 07, 2016, 02:20:23 pm
Hey there!
I think this might be a stupid question but I really don't understand if ozone being a polar molecule has any effect on its boiling point (-111 degrees Celsius) being higher than oxygen's boiling point (-183 degrees Celsius). If the polarity does have an effect, then how and why?
Thank you very much!
Hey!
By my understanding, the main difference in boiling point comes about because of their relative size. Ozone is bigger, and so the boiling point is higher (ie. more energy is required to vapourize larger molecules)!
However, polarity may come into play as well. Polar molecules "stick together" better, as dipole-dipole action is stronger than dispersion forces. So, MORE energy is required to break intermolecular bonds for polar molecules than non-polar molecules! Still, I'd stick with the first explanation :)
Jake
Post by: Hua Fei on October 07, 2016, 02:29:22 pm
Hey!
By my understanding, the main difference in boiling point comes about because of their relative size. Ozone is bigger, and so the boiling point is higher (ie. more energy is required to vapourize larger molecules)!
However, polarity may come into play as well. Polar molecules "stick together" better, as dipole-dipole action is stronger than dispersion forces. So, MORE energy is required to break intermolecular bonds for polar molecules than non-polar molecules! Still, I'd stick with the first explanation :)
Jake
Thank you Jake! I understand it now =D
Post by: jakesilove on October 07, 2016, 02:30:59 pm
Thank you Jake! I understand it now =D
Great! Let me know if you have any other questions!
Post by: Gregs on October 07, 2016, 02:35:20 pm
I have a small table. I'll upload it later.
It has all of what's needed except %(w/w) stuff.
Thanks Rui
Post by: RuiAce on October 07, 2016, 03:00:42 pm
Hey there!
I think this might be a stupid question but I really don't understand if ozone being a polar molecule has any effect on its boiling point (-111 degrees Celsius) being higher than oxygen's boiling point (-183 degrees Celsius). If the polarity does have an effect, then how and why?
Thank you very much!
Hey!I am fairly positive it does. Ozone is polar whereas oxygen is not. Dipole-dipole interactions are stronger as you said there Jake
By my understanding, the main difference in boiling point comes about because of their relative size. Ozone is bigger, and so the boiling point is higher (ie. more energy is required to vapourize larger molecules)!
However, polarity may come into play as well. Polar molecules "stick together" better, as dipole-dipole action is stronger than dispersion forces. So, MORE energy is required to break intermolecular bonds for polar molecules than non-polar molecules! Still, I'd stick with the first explanation :)
Jake
Post by: Hua Fei on October 07, 2016, 07:58:51 pm
Question: An experiment was performed where equal volumes of 0.1 mol/L Pb(NO3)2 and 0.1 mol/L KI solutions were mixed. Soon after mixing a precipitate was observed.
Complete the table below to show the concentration of all ions present in solution at each stage of the experiment.
Note: the table & the answers are attached.
What I understand: the first row of answers and how you get from the 2nd row to the third row...don't understand the other parts (i.e. how you get from the 1st to the 2nd row) :'(
Post by: sweetcheeks on October 07, 2016, 08:14:52 pm
Hey there!
Question: An experiment was performed where equal volumes of 0.1 mol/L Pb(NO3)2 and 0.1 mol/L KI solutions were mixed. Soon after mixing a precipitate was observed.
Complete the table below to show the concentration of all ions present in solution at each stage of the experiment.
Note: the table & the answers are attached.
What I understand: the first row of answers and how you get from the 2nd row to the third row...don't understand the other parts (i.e. how you get from the 1st to the 2nd row) :'(
There are equal volumes of the lead (ii) nitrate and potassium iodide solutions. When you add them together, the concentration will be halved, as the volume is doubled. Say you have 100mL of each solution, adding them together will result in a combined volume of 200mL, double the initial volume, resulting in half the original concentration.
Post by: Hua Fei on October 07, 2016, 08:21:07 pm
There are equal volumes of the lead (ii) nitrate and potassium iodide solutions. When you add them together, the concentration will be halved, as the volume is doubled. Say you have 100mL of each solution, adding them together will result in a combined volume of 200mL, double the initial volume, resulting in half the original concentration.
Oh. My. God. I can't believe I forgot that! - Thank you!!!
Post by: marynguyen18 on October 07, 2016, 09:05:23 pm
Post by: jakesilove on October 07, 2016, 09:10:14 pm
Why is ethanol a good source for alternative fuel in terms of boiling point and heat of combustion?
Well, when we combust Ethanol (make sure you know the equation for this!) it releases a decent amount of energy. It also has a relatively low boiling point, which makes it easier to combust. So, when we burn Ethanol, we can use the energy released to run machines/cars/whatever we feel like!
Post by: marynguyen18 on October 07, 2016, 09:14:30 pm
Post by: RuiAce on October 07, 2016, 09:14:43 pm
Why is ethanol a good source for alternative fuel in terms of boiling point and heat of combustion?Note that it's only truly good relative to octane with B.P.
With heat of combustion, I'd argue ethanol is worse than octane because it releases a lower amount of energy.
Post by: jakesilove on October 07, 2016, 09:17:24 pm
Note that it's only truly good relative to octane with B.P.
With heat of combustion, I'd argue ethanol is worse than octane because it releases a lower amount of energy.
Yes, it's definitely worse! Just trying to answer the question of why it can be considered good.
Post by: marynguyen18 on October 07, 2016, 09:22:21 pm
Post by: marynguyen18 on October 07, 2016, 09:25:00 pm
Post by: jakesilove on October 07, 2016, 09:57:03 pm
with the layers of the atmosphere what do each of them do? I'm really confused on which layer is ozone good and bad in? (if that questions makes any sense)
Ozone is a pollutant in the Troposphere (as it is toxic) and great in the higher atmosphere (stratosphere) as it absorbs UV light! Definitely do some research and figure out what happens in each layer of the atmosphere, as that's really important. As for your next question, that's way too big a question to answer here. Try find some online notes
Post by: bethjomay on October 08, 2016, 01:42:01 pm
The answers for this question say that the change from 4-10 seconds is due to an increase in temperature. Could it also be a decrease in pressure as there are more moles of gas on the right hand side? Would this be accepted as an answer?
Post by: RuiAce on October 08, 2016, 01:43:31 pm
(http://uploads.tapatalk-cdn.com/20161007/1adfc4e2d0b0a47562c858a942ac21d3.jpg)Nope. Note that concentration is the y-axis. Not moles.
The answers for this question say that the change from 4-10 seconds is due to an increase in temperature. Could it also be a decrease in pressure as there are more moles of gas on the right hand side? Would this be accepted as an answer?
If those were moles then that would be fine. However concentration is different. Recall that C=n/V
Concentration depends on volume as well. If we change the pressure, we change the volume. Thus, for a pressure change, there would be a spike in the graph, similar to at 14 seconds.
Post by: bethjomay on October 08, 2016, 02:58:25 pm
Post by: DaCoon on October 08, 2016, 03:03:13 pm
Post by: RuiAce on October 08, 2016, 03:18:22 pm
What is the molecular formula for 2-butanol and the structural formula? I tried to search it up and it wasn't there on the internet. What is the systematic name for c5h11OH?The prefix but- states that we have FOUR carbons. Using the formula CnH2n+1OH we have
Butanol: C4H9OH
And similarly, C5H11OH is pentanol.
Observe that we do not know if this is 1-pentanol, 2-pentanol or 3-pentanol. This isn't given to us.
On the other hand, because we know that it's 2-butanol, we know what its structural formula is.
The 2 means that the -OH group (for alkanols) lies on carbon #2.(http://uploads.tapatalk-cdn.com/20161008/4e2ccd92abd023e6fcc8532b16d18a38.jpg)
Post by: DaCoon on October 08, 2016, 03:24:52 pm
When it says "Outline the procedure" are you supposed to retell the method word for word or what?
Post by: RuiAce on October 08, 2016, 03:27:42 pm
I was wondering if anyone could help me with the dot point in Industrial Chem - "Perform a first-hand investigation to gather information and describe the properties of a named emulsion and relate these properties to its uses." We didn't cover it in class except for actually making the emulsion (butter) and my teacher isn't getting back to me. :POne example could be mayonnaise, which is an emulsion of oil and water. Egg is the emulsifying agent.
You would most likely know what mayonnaise is used for.
ThanksYeah. A procedure is just another way of saying a method.
When it says "Outline the procedure" are you supposed to retell the method word for word or what?
Post by: DaCoon on October 08, 2016, 04:09:16 pm
https://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-hsc-chemistry.pdf
Post by: RuiAce on October 08, 2016, 04:13:02 pm
Can someone help me on question 18 in the 2014 paperClosely inspect the polymer given. It repeats itself once. Note the alternating (CH2)4 and (CH2)6
https://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-hsc-chemistry.pdf
Thus, ignore the repeat and only consider one of it.
Clearly, B is wrong as it has no (CH2)6
The (CH2)6 is connected to N, not O. Suddenly we just realised that C and A are also wrong due to this problem.
So the correct answer is D.
It's all about knowing what you're looking for. You have to break the molecule apart.
Post by: nimasha.w on October 08, 2016, 11:54:08 pm
Post by: RuiAce on October 08, 2016, 11:59:23 pm
hi! i don't know where to begin with this question, can someone please explain it to me :-)Because we are talking about heptanol, we are clearly talking about the one with 7 carbons. So ∆H = -4638kJ mol-1 from that table.
Then it's our very classic q=mC∆T calculation. We start by determining the enthalpy - note that the molar heat of combustion is the enthalpy change.
nC7H15OH = m/M = 1/(12.01*7+1.008*16+16.00) = 0.0086060001 mol
So using ∆H = -q/n
4638 = q/0.0086060001
q = 39.914628479 kJ
Lastly just plug everything into q=mC∆T. Note that here m = mass of water, and ∆T = 25
(C is on your data sheet.)
Take care with units
Post by: anotherworld2b on October 09, 2016, 04:21:48 pm
Could I please get some help?
17. This is a double replacement reaction. To start off, write out the equation. You will then need to find the moles of the precipitate (BaSO4) using the given mass. From the equation, you can then find the moles of sulfuric acid using the molar ratio, and thus find [H2SO4]. You will then need to consider the proticity of the acid and base to determine the value of [H+] and [OH-] to be used in neutralisation. Then, sub in these values and the given volume of acid into C1V1=C2V2 to solve for the volume of NaOH.
1. With the information you have, you can find the moles of aluminium hydroxide (Al(OH)3). Looking at the composition of the compound, the moles of OH- will be triple the moles of Al(OH)3. From this, you can calculate a value for [OH-] which will be equivalent to both [H+] and [HCl](as HCl is monoprotic). You will then be able to find the moles and thus the mass of HCl.
2. First, you will need to find the moles of H+ using the given value for [H+] and the volume required. This will be equal to the moles of OH- required for neutralisation. Observe the formula for calcium hydroxide: Ca(OH)2. The moles of Ca(OH)2 will be half the moles of OH-. Using the attained molar value, you will be able to find the mass of Ca(OH)2 required for neutralisation.
Post by: jakesilove on October 09, 2016, 05:22:11 pm
Im having trouble with writing Q1 and 2 out
Could I please get some help?
How about you show us some working, and we can point you in the right direction :)
Post by: anotherworld2b on October 09, 2016, 06:57:23 pm
Post by: ml125 on October 09, 2016, 07:59:02 pm
This is all i understand so far ???For Q1 - the number of moles for OH- you have found will equal the number of moles of H+ that is used in neutralisation. This equals the number of moles of HCl as the acid is monoprotic. You can thus find [HCl] since you have this amount of moles for a given volume. You can then apply this to the new volume to find the moles present - and thus find the mass of acid.
With Q2, take note of the units used for concentration: gL-1 as opposed to the usual M or molL-1. You need to perform a conversion in order to get a molar value for H+. You can do this by either finding the mass of H+ then converting this to moles or converting the concentration to molL-1 then finding the moles of H+ from there - either method will work. Once you obtain this value, the question should be as I have explained earlier.
Post by: marynguyen18 on October 09, 2016, 09:40:39 pm
Post by: RuiAce on October 09, 2016, 09:45:53 pm
this might be a silly question but why do we need to know two different batteries like dry cell and button cell?According to this dot point:
gather and present information on the structure and chemistry of a dry cell or lead-acid cell and evaluate it in comparison to one of the following:
button cell
fuel cell
vanadium redox cell
lithium cell
liquid junction photovoltaic device (eg the Gratzel cell)
You need to know one of dry cell and lead-acid battery cell, as well as one of those in that list
Post by: marynguyen18 on October 09, 2016, 09:48:09 pm
According to this dot point:
gather and present information on the structure and chemistry of a dry cell or lead-acid cell and evaluate it in comparison to one of the following:
button cell
fuel cell
vanadium redox cell
lithium cell
liquid junction photovoltaic device (eg the Gratzel cell)
You need to know one of dry cell and lead-acid battery cell, as well as one of those in that list
thank you so much RuiAce
Post by: Justina Shehata on October 09, 2016, 11:08:22 pm
Any advice would be great
Post by: RuiAce on October 09, 2016, 11:11:31 pm
I alwayys lose marks in questions with 'assess' in it because I do not make a judgement but I'm just not sure what to say. For example, when we assessment the sustainability of ethanol as a fuel, I write the advantages and disadvantages but im just not sure how to end itThere is no such judgment that is a "correct answer" for starters. Your assessment just needs to make sense based off what you written.
Any advice would be great
For example, for ethanol, I typically conclude in a manner similar to
"Ultimately, the fact that ethanol can be considered as a renewable resource implies that it is suitable as a potential fuel source in the future."
Make sure the assessment answers the question. The whole point of making an evaluation is to consider what you've got, and state some conclusion that will, adequately, match up with what's there.
Post by: Justina Shehata on October 09, 2016, 11:21:58 pm
There is no such judgment that is a "correct answer" for starters. Your assessment just needs to make sense based off what you written.
For example, for ethanol, I typically conclude in a manner similar to
"Ultimately, the fact that ethanol can be considered as a renewable resource implies that it is suitable as a potential fuel source in the future."
Make sure the assessment answers the question. The whole point of making an evaluation is to consider what you've got, and state some conclusion that will, adequately, match up with what's there.
Makes sense! Thank you!
Post by: amandali on October 11, 2016, 12:45:18 am
(http://uploads.tapatalk-cdn.com/20161010/a6f5713b1c2428e363f133cd996c83e4.jpg)
need help with these ques thanks
Post by: katherine123 on October 11, 2016, 12:51:52 am
Using indicator paper, 0.1 mol/L solutions of NaCH3COO and NH4Cl are found to have pH values of 9 and 5, respectively. NaCl solution is neutral. Account for the observed pH values of NaCH3COO and NH4Cl in solutions. 3 marks
Post by: ml125 on October 11, 2016, 05:41:53 am
need help with these ques thanksFor the first question, the first thing you need to do is find the moles of NaOH, which I can see you have already done. This will equal the moles of OH-, and thus the moles of H+ used in neutralisation. To find the moles of citric acid, we first need to consider the proticity of the acid. As the acid is triprotic, this value will be a third of the moles of H+. Once you have the molar value for the acid, you can find the concentration of it by using the given volume. There is a dilution factor of 10, thus you can obtain the original concentration of the undiluted acid by multiplying the concentration you get by 10 :D This question provides way more information than is required to answer it!
For the second question, the first step is to find the moles of H+ remaining after neutralisation - this is where you got up to. From here, what you need to do is find [H+] for the final volume of the solution - 100mL. Then you can find the pH of solution (pH=-log[H+]), which works out to be B :)
how to answer this ques:As the concentration of both solutions is the same, and there is a variation in pH, what you will need to discuss is the relative strengths of each of the solutions. To do this, it is easiest to observe each compound's behaviour when dissolved in water.
Using indicator paper, 0.1 mol/L solutions of NaCH3COO and NH4Cl are found to have pH values of 9 and 5, respectively. NaCl solution is neutral. Account for the observed pH values of NaCH3COO and NH4Cl in solutions. 3 marks
Sodium acetate(NaCH3COO), when dissolved, dissociates into sodium(Na+) and acetate (CH3COO-) ions.
As we are aware that NaCl solution is neutral, the Na+ ion can be ignored. When reacted with water, the acetate ion forms acetic acid (CH3COOH) and a free hydroxide ion (OH-)
Due to the free hydroxide ion, and the weak nature of acetic acid(as it only partly dissociates in water), it is safe to assume that the solution will be basic. (pH = 9)
Ammonium chloride (NH4Cl), when dissolved, dissociates into ammonium(NH4+) and chloride(Cl-) ions.
Like earlier, as we are aware that NaCl solution is neutral, the Cl- ion can be ignored. When reacted with water, the ammonium ion forms ammonia(NH3) and a hydronium(H3O+) ion.
Due to the free hydronium ion, and the weak nature of ammonia as a base(as it only partly dissociates in water), it is safe to assume that the solution will be acidic. (pH = 5)
Post by: xorichard on October 18, 2016, 04:50:09 pm
Post by: jakesilove on October 18, 2016, 04:51:33 pm
Hello all! Just wondering when you would use NAGSAG? is it genreally in identifying stuff in formulas or is that a syllabus dot point thingy?
I would use NAGSAG whereever it's relevant! Not entirely sure what you mean by the question; if it's something to do with 'will this substance be soluble', or 'how do you separate these substances', NAGSAG is your best friend
Post by: jakesilove on October 18, 2016, 04:53:07 pm
Post by: RuiAce on October 18, 2016, 05:02:14 pm
Hello all! Just wondering when you would use NAGSAG? is it genreally in identifying stuff in formulas or is that a syllabus dot point thingy?Basically as Jake has said, whenever it is relevant. Some situations where it is relevant include precipitation tests and just general writing correct states on chemical equations.
There is no specific time to use it. You use it when they're appropriate.
They are just mnemonics to solubility rules.
Post by: xorichard on October 18, 2016, 05:03:58 pm
Post by: RuiAce on October 18, 2016, 05:05:44 pm
So is NAGSAG a base to understanding which compounds can be broken up into its ions? Thanks!If by base you meant basis, sure, certainly
Base is a bit misleading in chemistry for... you know... acid/base stuff lol
Technicality note though - make sure you realise that when you say "broken up", it's as a part of dissolution. If you don't dissolve it then it can't be broken up.
Post by: tennis1 on October 19, 2016, 07:06:36 pm
I just have a couple of random Q's:
(my teacher barely uses email so I thought I'd just post them here)
*Why does a decrease in pH, increase the rate of corrosion
(c) Before performing the experiment, the glassware was washed with the solutions given in the table. (4 marks)
Washing procedure
The conical flask was washed with distilled water.
Effect on the volume of hydrochloric acid used
Effect on the % of aspirin calculated
The burette was washed with distilled water.
Effect on the volume of hydrochloric acid used
Effect on the % of aspirin calculated
Co(H2O)62+(aq) + 4 Cl–(aq) ⇌ CoCl42–(aq) + 6 H2O(l)
If H20 is added:
does it favour the side with the H2O (reverse(t,
OR
not affect equilibrium at all since it is a liquid & has no concentration,
OR decrease the concentration of all the species and so favour the side with the least moles? (forward)
and
also if a solid is added does it only not affect equilibrium if it is insoluble
thanks!
Post by: RuiAce on October 19, 2016, 07:09:26 pm
Hey guys,I didn't do shipwrecks but Jake did so he can have those.
I just have a couple of random Q's:
(my teacher barely uses email so I thought I'd just post them here)
*Why does a decrease in pH, increase the rate of corrosion
(c) Before performing the experiment, the glassware was washed with the solutions given in the table. (4 marks)
Washing procedure
The conical flask was washed with distilled water.
Effect on the volume of hydrochloric acid used
Effect on the % of aspirin calculated
The burette was washed with distilled water.
Effect on the volume of hydrochloric acid used
Effect on the % of aspirin calculated
Co(H2O)62+(aq) + 4 Cl–(aq) ⇌ CoCl42–(aq) + 6 H2O(l)
If H20 is added:
does it favour the side with the H2O (reverse(t,
OR
not affect equilibrium at all since it is a liquid & has no concentration,
OR decrease the concentration of all the species and so favour the side with the least moles? (forward)
and
also if a solid is added does it only not affect equilibrium if it is insoluble
thanks!
H2O does nothing because like said, it is a liquid and hence no concentration change.
And for a similar reasons - solids have no concentration.
If a solid dissolves, note that it becomes aqueous and thus WILL have a concentration
With that part c), post up the rest of the question please or we cannot be sure of what we're doing
Post by: jakesilove on October 19, 2016, 08:14:37 pm
Hey guys,
*Why does a decrease in pH, increase the rate of corrosion
Hey! There are a whole bunch of ways to think about this; I think the quantitative method is easiest. Remember, corrosion is essentially when an electrolytic cell is set up between Iron and water, or iron and something else, in which Iron LOSES electrons (ie. oxidises). The cell between Iron and water has a specific voltage, that you can calculate using the table of standard potentials. This is a 'path' of corrosion. However, with excess H+ ions in the water, Iron may 'prefer' to set up a cell with the H+ instead! This cell will have a lower voltage (again, see the table of standard potentials) and thus will be EASIER to move forward. So, the overall effect is that it becomes easier for Iron to corrode; the same amount of energy will corrode MORE iron! I hope that makes sense, let me know if I can clarify anything!
Jake
Post by: tennis1 on October 20, 2016, 02:17:33 am
I didn't do shipwrecks but Jake did so he can have those.
H2O does nothing because like said, it is a liquid and hence no concentration change.
And for a similar reasons - solids have no concentration.
If a solid dissolves, note that it becomes aqueous and thus WILL have a concentration
With that part c), post up the rest of the question please or we cannot be sure of what we're doing
ok thanks :) Also here is the first part of part c)
In one commercial brand of aspirin, each ‘300 mg tablet’ is claimed to contain 100% aspirin. To determine the actual percentage by mass of aspirin in an aspirin tablet, the following procedure, involving a back titration, was used.
Step 1: Step 2:
Three aspirin tablets, each with a mass of 300.0 mg, were crushed and dissolved in excess sodium hydroxide solution. Exactly 100.0 mL of 0.204 mol L–1 solution of sodium hydroxide was used. The mixture was boiled to ensure complete reaction.
The excess sodium hydroxide solution was titrated with hydrochloric acid as follows: 20.0 mL of the solution from step 1 was pipetted into a conical flask and 0.125 mol L–1 hydrochloric acid was placed in the burette. The indicator, phenolphthalein, was used and an average titre of 17.89 mL of hydrochloric acid was required to reach the end-point.
Post by: tennis1 on October 20, 2016, 02:20:08 am
Hey! There are a whole bunch of ways to think about this; I think the quantitative method is easiest. Remember, corrosion is essentially when an electrolytic cell is set up between Iron and water, or iron and something else, in which Iron LOSES electrons (ie. oxidises). The cell between Iron and water has a specific voltage, that you can calculate using the table of standard potentials. This is a 'path' of corrosion. However, with excess H+ ions in the water, Iron may 'prefer' to set up a cell with the H+ instead! This cell will have a lower voltage (again, see the table of standard potentials) and thus will be EASIER to move forward. So, the overall effect is that it becomes easier for Iron to corrode; the same amount of energy will corrode MORE iron! I hope that makes sense, let me know if I can clarify anything!
Jake
Yeah that makes sense! thank you
Post by: albert_c on October 21, 2016, 07:51:47 pm
"There has been an increase in the concentration of the oxides of nitrogen in the atmosphere as a result of combustion.
Assess both the evidence to support this statement and the need to monitor these oxides"
What do I need to talk about to ensure 5 marks in this response?
Thanks in advance
Post by: onepunchboy on October 22, 2016, 08:55:11 am
After calculating -mcAt, why do you divide by 0.45 instead of times 0.45?
Thanks in adv
Post by: RuiAce on October 22, 2016, 09:24:32 am
Hi, I'm not sure if this has been asked or not but in HSC 2009 Question 18 (5 Marker)Look at the marking criteria first.
"There has been an increase in the concentration of the oxides of nitrogen in the atmosphere as a result of combustion.
Assess both the evidence to support this statement and the need to monitor these oxides"
What do I need to talk about to ensure 5 marks in this response?
Thanks in advance
And also plan your response. Two examples to start you off, however
- Incidents of photochemical smog
- Acid rain
Hey guys for the Q : The heat of combustion of ethanol is 1360 KJ mol^-1. What mass of ethanol needs to be burnt to raise the temp of 250g of water through 40 degrees, if 55% of the heat released by ethanol is lost to the surroundings?
After calculating -mcAt, why do you divide by 0.45 instead of times 0.45?
Thanks in adv
Post by: onepunchboy on October 22, 2016, 09:32:42 am
Post by: RuiAce on October 22, 2016, 09:39:23 am
wait so when do you times the heat loss with the enthalpy and when do you divide, because alot of questions you need to times... i get confusedBreak down the question. Every question is a different case.
Although here I apologise, since I think I stuffed it up at the start.
If we want enough heat to COUNTERACT the change, we need mass to be greater. Hence it should be what I am about to edit above.
In other words, the mass must be LARGE enough (higher than usual) to counteract the heat loss
I treat these questions the same way I treat relativity in physics. I just use logic and common sense to figure out what's right.
Post by: onepunchboy on October 22, 2016, 09:56:31 am
Post by: Neutron on October 22, 2016, 08:15:43 pm
"Compare the process of polymerisation of ethylene and glucose. Include relevant chemical equations in your answer." (3 marks)
What more do they want us to say apart from the fact that ethylene is addition polymerisation and glucose is condensation? :o like did we ever learn/need to know the specifics to condensation polymerisation? All I know is that it's when two difunctional monomers join together and eliminate a small molecule, I don't really know much about the 'process' :/ And with ethylene do we need to go into zieggler natta and free radical polymerisation (for HDPE and LDPE) ? And what equations do they even want omg (do they want the n(glucose)--> (cellulose)n + (n-1) water? With the actual equations ofc i just cbb to type it out) this whole question just confuses me hahahaha cheers
Neutron
Post by: RuiAce on October 22, 2016, 08:22:27 pm
Hey hey! Bit rusty with POM but just with this question:You do actually need to know the whole process. That's what you're comparing.
"Compare the process of polymerisation of ethylene and glucose. Include relevant chemical equations in your answer." (3 marks)
What more do they want us to say apart from the fact that ethylene is addition polymerisation and glucose is condensation? :o like did we ever learn/need to know the specifics to condensation polymerisation? All I know is that it's when two difunctional monomers join together and eliminate a small molecule, I don't really know much about the 'process' :/ And with ethylene do we need to go into zieggler natta and free radical polymerisation (for HDPE and LDPE) ? And what equations do they even want omg (do they want the n(glucose)--> (cellulose)n + (n-1) water? With the actual equations ofc i just cbb to type it out) this whole question just confuses me hahahaha cheers
Neutron
Only similarity is probably that they're both polymerisation processes. The differences are in the processes themselves.
Ethylene undergoes addition polymerisation, which MUST have an initiator or Ziegler-Natta catalyst (can't be spontaneous). The three phases are initiation+activation, propagation and termination.
Glucose undergoes condenstion polymerisation, in which beta-glucose monomers join together flipping, and also always expels the water molecule. Two monomers join to form a dimer, and then these dimers undergo the polymerisation process.
But no great depth is needed here though.
Post by: onepunchboy on October 23, 2016, 03:49:42 pm
this is for a multiple choice questions
thanks in adv :)
Post by: jakesilove on October 23, 2016, 04:11:15 pm
Hello! can anyone explain why propane is more volatile than propan-1-ol, propanoic acid and methyl propanoate?
this is for a multiple choice questions
thanks in adv :)
Hey! Volatility is about how easily something evaporates. Unlike the other substances listed, Propane doesn't have strong hydrogen/dipole-dipole bonds. It's also much smaller than the other molecules. As such, it doesn't 'bind' to itself very strongly (ie. weak intermolecular bonds), and so less energy is required to break them apart/evaporate Propane!
Does that make sense?
Post by: onepunchboy on October 23, 2016, 04:30:31 pm
Post by: onepunchboy on October 24, 2016, 11:07:02 am
Can anyone help me out with this Q : " explain , in terms of intermolecular bonding forces involved, why esters are only slightly soluble in water, even though the chemicals used in its manufacture are soluble"?
i thought esters were actually quite soluble because of (-OH) but idk
Thanks!
Post by: sweetcheeks on October 24, 2016, 11:20:57 am
Hello!The chemicals used to make the ester (acid and alcohol) both have -OH groups allowing them to form hydrogen bonds with the water molecules. When the reaction occurs, the ester does not contain any -OH groups and can not form hydrogen bonds. The ester only has a COO group which has slight dipole-dipole forces, making the molecule less polar than its constituents.
Can anyone help me out with this Q : " explain , in terms of intermolecular bonding forces involved, why esters are only slightly soluble in water, even though the chemicals used in its manufacture are soluble"?
i thought esters were actually quite soluble because of (-OH) but idk
Thanks!
Post by: jakesilove on October 24, 2016, 11:25:27 am
The chemicals used to make the ester (acid and alcohol) both have -OH groups allowing them to form hydrogen bonds with the water molecules. When the reaction occurs, the ester does not contain any -OH groups and can not form hydrogen bonds. The ester only has a COO group which has slight dipole-dipole forces, making the molecule less polar than its constituents.
Exactly right! I don't really have anything to add; clearly identify WHY substances are soluble in certain solutions, and then outline the factors above to come to the required conclusion. Let us know if we can clarify anything!
Post by: noonedoesnt on October 24, 2016, 08:44:48 pm
Post by: RuiAce on October 24, 2016, 08:47:23 pm
Confusing questionUse process of elimination
A) Does ethylene become cellulose? Lol no wtf
B) Does ethylene become ethanol? Yes - use hydration. Does ethanol get polymerised? Lol no wtf
C) Does ethylene become glucose? Lol no wtf
D) Does ethylene become styrene? Yes - use addition reaction. Does styrene get polymerised? Yes it becomes polystyrene.
Therefore D
Post by: noonedoesnt on October 24, 2016, 08:48:47 pm
Post by: noonedoesnt on October 25, 2016, 12:59:20 pm
Post by: Biology24123 on October 25, 2016, 01:10:18 pm
Another question, can i get help with part C. Regards.
Divide the energy found in part b by the heat of combustion of hydrogen (convert to KJ)
Then use PV=nRT formula to find the volume of hydrogen
Post by: noonedoesnt on October 25, 2016, 01:31:14 pm
Post by: RuiAce on October 25, 2016, 02:10:35 pm
Divide the energy found in part b by the heat of combustion of hydrogen (convert to KJ)This is not in the HSC chemistry course. Help is always appreciated but unfortunately you need to consider the fact that the HSC chemistry course is dumbed down
Then use PV=nRT formula to find the volume of hydrogen
Post by: jakesilove on October 25, 2016, 02:43:15 pm
Another question, can i get help with part C. Regards.
Hey! So, we want to combust Hydrogen gas to produce the same amount of energy. Thus,
Where n is the number of moles. I've also converted the energy into kJ. We can easily solve for n, giving us the required moles of Hydrogen gas. Then, we multiply by the molar volume to get the volume of the gas!
Post by: Cindy2k16 on October 25, 2016, 06:24:12 pm
Post by: Rikahs on October 25, 2016, 06:37:49 pm
Post by: RuiAce on October 25, 2016, 06:38:24 pm
When a question asks how many isomers a compound has and the structure of the compound is provided, is the number of isomers including the one pictured?? ( and I'm not sure if this has happened but what if only the formula is provided?
yeah im pretty sure it's including the picture given.Can confirm - definite yes to that one
Post by: Cindy2k16 on October 25, 2016, 07:24:42 pm
yeah im pretty sure it's including the picture given.
Can confirm - definite yes to that one
Thanks :) Turns out the past paper book I have (the yellow book) actually printed the wrong answer and wrongly explained that you don't count the one pictured.
Post by: RuiAce on October 25, 2016, 07:26:07 pm
Thanks :) Turns out the past paper book I have (the yellow book) actually printed the wrong answer and wrongly explained that you don't count the one pictured.Maybe a bit late now (give or take, you can always borrow it from the library) but have you considered Excel Success One?
Post by: Cindy2k16 on October 25, 2016, 07:33:24 pm
Maybe a bit late now (give or take, you can always borrow it from the library) but have you considered Excel Success One?
My chemistry teacher recommended the yellow book so I borrowed it from the library. I'd say that one question is the only issue I've had with it so it's probably fine overall. Besides, I currently have a lot of overdue books from hoarding them for HSC ;D so borrowing another would be a hassle
Post by: tennis1 on October 25, 2016, 09:08:45 pm
does anyone know how to draw the ester from a reaction with a TERTIARY or secondary alcohol and a carboxylic acid?
And why does a redox primary standard have to be acidified?
Thanks :)
Post by: jakesilove on October 26, 2016, 09:58:15 am
hey guys
does anyone know how to draw the ester from a reaction with a TERTIARY or secondary alcohol and a carboxylic acid?
And why does a redox primary standard have to be acidified?
Thanks :)
I have no idea what you even mean by your first question, which just means that it isn't part of the HSC curriculum! So don't worry about it :)
The redox reaction is acidified with Sulfuric acid, as this acts as a catalyst. Sulfuric acid is a dehydrating agent, removing products in the esterification reaction. By LCP, the equilibrium will shift to the right to make up for that loss, increasing the yield of Ester!
Jake
Post by: tennis1 on October 26, 2016, 11:02:36 am
I have no idea what you even mean by your first question, which just means that it isn't part of the HSC curriculum! So don't worry about it :)
The redox reaction is acidified with Sulfuric acid, as this acts as a catalyst. Sulfuric acid is a dehydrating agent, removing products in the esterification reaction. By LCP, the equilibrium will shift to the right to make up for that loss, increasing the yield of Ester!
Jake
thanks Jake :)
My first q must just be part of the WA curriculum then
Post by: jakesilove on October 26, 2016, 11:15:02 am
thanks Jake :)
My first q must just be part of the WA curriculum then
Ahhh you're a WAer! Sorry, not gonna be able to help you out with that, hope you work out the answer!
Post by: onepunchboy on October 26, 2016, 12:16:44 pm
thanks :))
Post by: RuiAce on October 26, 2016, 12:19:31 pm
hey guys anyone know how to do this q? does the strongest acid mean highest conductivity? im not sureThe strongest acid has the highest degree of ionisation.
thanks :))
Weak acids do not have a degree of ionisation of 100% so CH3COOH is automatically disqualified.
HNO3 and HCl both have the same degree of ionisation of roughly 100%.
But H2SO4 has a degree of ionisation of somewhere around 160%. This is because it is diprotic - the first hydrogen ion fully ionises, and the second is also quite acidic as well.
Hence sulfuric acid is the answer.
Post by: onepunchboy on October 26, 2016, 02:54:57 pm
Hey another q , just asking to confirm. In q 2 in the multi choice the weak acid required the SAME volume of base to reach equivalence as the strong acid because the student titrated equal conc of 0.1M of acid? However in the graph , because of different concentrations a LARGER volume of base is needed to reach equivalence.
Is this right?
Thanks again
Post by: RuiAce on October 26, 2016, 03:01:05 pm
(http://uploads.tapatalk-cdn.com/20161026/5294d05691373c9b4dad498cca88c643.jpg)(http://uploads.tapatalk-cdn.com/20161026/8bc31bf9f28139b1dc187ad776e7c89a.jpg)The answer to Q2 is that they are the same because you titrated with a strong base. So long as one of the acid or base is strong, the neutralisation goes to completion. Hence the degree of ionisation of the weak base is useless because the same volume (so this part you are right) of the same STRONG base was used
Hey another q , just asking to confirm. In q 2 in the multi choice the weak acid required the SAME volume of base to reach equivalence as the strong acid because the student titrated equal conc of 0.1M of acid? However in the graph , because of different concentrations a LARGER volume of base is needed to reach equivalence.
Is this right?
Thanks again
Post by: RuiAce on October 26, 2016, 03:11:51 pm
In every instance we titrate with a strong base, so we need to consider what happens using the equivalence point.
Note that:
Strong acid + Strong base produces a neutral salt
Strong acid + Weak base produces an acidic salt
Weak acid + Strong base produces a basic salt.
The equivalence point tells us which is what.
For the first acid, the pH jumping from 3 to 10 hints that we have an equivalence point at around pH 7, I.e. Neutral. So working backwards that should be a strong acid strong base reaction.
For the second, it goes from 6 to 10, which hints an equivalence point of 8. This is suggestive of a strong base weak acid reaction.
The graphs are characteristic. Frequently appears in acids and isn't hard to memorise
Post by: onepunchboy on October 26, 2016, 03:41:16 pm
Whereas for the graph, whether or not the acid is strong or weak will matter.
In every instance we titrate with a strong base, so we need to consider what happens using the equivalence point.
Note that:
Strong acid + Strong base produces a neutral salt
Strong acid + Weak base produces an acidic salt
Weak acid + Strong base produces a basic salt.
The equivalence point tells us which is what.
For the first acid, the pH jumping from 3 to 10 hints that we have an equivalence point at around pH 7, I.e. Neutral. So working backwards that should be a strong acid strong base reaction.
For the second, it goes from 6 to 10, which hints an equivalence point of 8. This is suggestive of a strong base weak acid reaction.
The graphs are characteristic. Frequently appears in acids and isn't hard to memorise
wait so that means both acid 1 and 2 will require to same volume of base to reach equivalence because there is a strong base reacting?
Post by: RuiAce on October 26, 2016, 03:50:45 pm
Post by: Neutron on October 26, 2016, 10:37:38 pm
Hi! I think I'm just a wreck right now but if someone could explain this to me I would really appreciate it! Like I thought because strong acids ionise more H+ ions, that means they would need more base (i.e more OH ions) to neutralise but I guess not
Post by: massive on October 27, 2016, 07:56:46 am
Post by: RuiAce on October 27, 2016, 08:02:08 am
(http://uploads.tapatalk-cdn.com/20161026/73ea546db3c21441f9947e2f4dcdfd40.jpg)The NaOH drives the reaction to completion.
Hi! I think I'm just a wreck right now but if someone could explain this to me I would really appreciate it! Like I thought because strong acids ionise more H+ ions, that means they would need more base (i.e more OH ions) to neutralise but I guess not
Recall that only the reaction between a weak base and an weak acid is reversible, i.e. it goes into equilibrium. The presence of at least one of a strong acid, or a strong base, will force the reaction to go to the end.
This is because, in the case that both substances are partially ionised (weak), there's only so many H+ and OH- to react. So this system will stabilise out. Whereas if you have something strong, the NaOH here, the presence of the OH- ions will continuously drive the equilibrium (by LCP) so that the reaction ultimately goes to completion.
We note that the concentration of NaOH and both the volume AND concentration of the acids have been fixed. So the fact that we have all those OH- there from the NaOH will drive the reaction regardless of the acids. Hence, if the same outcome is to be achieved, the last thing to be fixed is the volume of NaOH.
is petroleum an example of a fossil fuel?Yes
Post by: wesadora on October 27, 2016, 08:42:18 am
For question b)(ii): In this reaction - I'm a bit confused. Why do both metals undergo their own seperate chemical reactions (i.e. two different systems). My first impression was that magnesium would just rust, whereas the iron wouldn't - acting as a sacrificial anode. BUT apparently, the Mg will oxidise, its electrons will react with Fe2+ from the electrolyte solution and thus a solid iron coating will form on the Mg, with the electrolyte solution turning less green (less Fe2+ ions). Thus, the iron nail is left to just rust itself, and do its own little thing (standard process of rusting: Fe --> Fe2+ --> Fe3+...).
What I don't understand, is why they don't effect each other even if they're in such close proximity? I think im being misled its all one system because it's in one single beaker...do sacrificial anodes need to PHYSICALLY TOUCH? I don't even know how it works. ._.
Post by: nimasha.w on October 27, 2016, 08:49:34 am
Post by: RuiAce on October 27, 2016, 08:54:24 am
hey! in the 2013 HSC paper, question 24b, it says that adding a catalyst with have no visible effect on the reaction mixture because it will increase the reaction rate of both the forward and reverse reactions. Does this mean with such things like the Haber process, the adding of the magnetite catalyst does not increase the reaction rate, it only lowers the activation energy?Lowering the activation energy DOES increase the rate.
What they mean that it has no visible effect on the reaction mixture is that the final equilibrium CONCENTRATIONS are the same. i.e. The role of the catalyst speeds up the rate equilibrium is achieved, however does nothing on the equilibrium ITSELF.
Post by: massive on October 27, 2016, 09:56:34 am
Post by: jakesilove on October 27, 2016, 09:59:08 am
SHIPWRECKS
For question b)(ii): In this reaction - I'm a bit confused. Why do both metals undergo their own seperate chemical reactions (i.e. two different systems). My first impression was that magnesium would just rust, whereas the iron wouldn't - acting as a sacrificial anode. BUT apparently, the Mg will oxidise, its electrons will react with Fe2+ from the electrolyte solution and thus a solid iron coating will form on the Mg, with the electrolyte solution turning less green (less Fe2+ ions). Thus, the iron nail is left to just rust itself, and do its own little thing (standard process of rusting: Fe --> Fe2+ --> Fe3+...).
What I don't understand, is why they don't effect each other even if they're in such close proximity? I think im being misled its all one system because it's in one single beaker...do sacrificial anodes need to PHYSICALLY TOUCH? I don't even know how it works. ._.
Hey! This is a beyond stupid question. Normally, you'd be 100% right; there are two dissimilar metals, and there is seemingly an electrolyte between the two. The trick is that there are TWO different solutions combined together; one relevant to the Iron, one relevant to the Magnesium. This essentially 'requires' each metal to do their own thing, as the solutions won't interact with each other. Extremely stupid question, and it won't teach you anything unless exactly the same question, so learn from your mistake and move on because it is 99% unlikely to come up again.
Post by: jakesilove on October 27, 2016, 10:02:17 am
what would be the answer to part b?
Hey! If a reaction is 'endothermic', it requires energy for the reaction to move forward. It ABSORBS energy from it's surroundings. Therefore, if we add heaps of energy (through heat), the reaction will more forward at a greater rate! The answer here is basically just that the reaction is endothermic :)
Post by: wesadora on October 27, 2016, 10:19:27 am
Hey! This is a beyond stupid question. Normally, you'd be 100% right; there are two dissimilar metals, and there is seemingly an electrolyte between the two. The trick is that there are TWO different solutions combined together; one relevant to the Iron, one relevant to the Magnesium. This essentially 'requires' each metal to do their own thing, as the solutions won't interact with each other. Extremely stupid question, and it won't teach you anything unless exactly the same question, so learn from your mistake and move on because it is 99% unlikely to come up again.
I see. Hold on...so do sacrificial anodes have to be in physical contact? (e.g. magnesium wrapped around an iron nail)
Or say, could a magnesium and an iron strip be next to each other (on opposite sides) of a petri dish with like...an NaCl electrolyte - and the magnesium would still rust preferentially instead of the iron (forcing iron to be a cathode), or would both just do their own things? O.O
Post by: Sssssrr on October 27, 2016, 10:29:01 am
Post by: RuiAce on October 27, 2016, 10:34:11 am
hi, does changing concentrations of products/reactants have any effect on reaction rate in an equilibrium reaction, or does it only impact yield?Whether or not it impacts on the rate is outside the scope of the HSC. We only care about yield.
You could logically infer what happens; in every typical equilibrium graph if a substance is made more concentrated then initially it gets used up more rapidly. And in fact, the more you add in, the more rapid it is at the start. But then you just can't say for sure what happens.
Post by: nimasha.w on October 27, 2016, 11:48:03 am
Post by: RuiAce on October 27, 2016, 11:51:59 am
hey! i don't understand this questionHint: Look at the table of standard reduction potentials, and decide if the equation was written correct or possibly mistakenly written backwards for a galvanic cell.
Only the correct galvanic cell equation will occur spontaneously. The wrong equation only occurs in an electrolytic cell and that's NOT spontaneous.
Post by: angiezhang9 on October 27, 2016, 01:22:04 pm
Post by: RuiAce on October 27, 2016, 01:32:25 pm
Is there a difference between how a transuranic element and a commercial radioisotope is produced? My understanding is that they're both produced by firing neutrons in a nuclear reactor or by bombarding with a charged particle in an accelerator.Yeah not really, they're pretty much produced the same way. They're just two dot points because you need to be prepared to give examples for each.
Post by: wesadora on October 27, 2016, 03:36:35 pm
Is there a difference between how a transuranic element and a commercial radioisotope is produced? My understanding is that they're both produced by firing neutrons in a nuclear reactor or by bombarding with a charged particle in an accelerator.
Yeah nerd you gotta know examples for every one :'(
Post by: wesadora on October 27, 2016, 05:07:16 pm
I don't understand how this equation is valid, ...I mean, the H30+ does donate a proton, but another acid (H2CO3) is created anyway? How is this 'neutralisation', or is this equation even good to use in an answer of why NaHCO3 is suitable for cleaning up acid/base spills? I'm pretty sure it is though, seen it in HSC suggested answers. Just don't understand what's up with h2co3...(carbonic acid) :P
Post by: RuiAce on October 27, 2016, 05:33:19 pm
When sodium hydrogen carbonate acts to neutralise an acid (and considering the amphiprotic nature of HCO3-,Neutralisation just means acid + base. It doesn't always give you something with a pH of 7.
I don't understand how this equation is valid, ...I mean, the H30+ does donate a proton, but another acid (H2CO3) is created anyway? How is this 'neutralisation', or is this equation even good to use in an answer of why NaHCO3 is suitable for cleaning up acid/base spills? I'm pretty sure it is though, seen it in HSC suggested answers. Just don't understand what's up with h2co3...(carbonic acid) :P
Note that BY DEFINITION, according to B-L theory, an acid is a proton DONOR and a base is a proton ACCEPTOR.
The whole point of NaHCO3 in neutralisation of spills is that we have a WEAK acid/base remaining, not a STRONG one.
Post by: wesadora on October 27, 2016, 07:06:08 pm
Neutralisation just means acid + base. It doesn't always give you something with a pH of 7.
Note that BY DEFINITION, according to B-L theory, an acid is a proton DONOR and a vase is a proton ACCEPTOR.
The whole point of NaHCO3 in neutralisation of spills is that we have a WEAK acid/base remaining, not a STRONG one.
OHHHHHH that makes so much sense. Thanks Rui.
Post by: massive on October 27, 2016, 11:33:53 pm
Post by: RuiAce on October 27, 2016, 11:36:07 pm
is there a difference between petroleum and crude oil?Crude oil is the rest of the mixture. Petroleum is the actual hydrocarbons we use from the mixture.
But these terms get used interchangeably too often and nobody cares if you get them confused.
Post by: Neutron on October 28, 2016, 12:30:58 am
The NaOH drives the reaction to completion.
Recall that only the reaction between a weak base and an weak acid is reversible, i.e. it goes into equilibrium. The presence of at least one of a strong acid, or a strong base, will force the reaction to go to the end.
This is because, in the case that both substances are partially ionised (weak), there's only so many H+ and OH- to react. So this system will stabilise out. Whereas if you have something strong, the NaOH here, the presence of the OH- ions will continuously drive the equilibrium (by LCP) so that the reaction ultimately goes to completion.
We note that the concentration of NaOH and both the volume AND concentration of the acids have been fixed. So the fact that we have all those OH- there from the NaOH will drive the reaction regardless of the acids. Hence, if the same outcome is to be achieved, the last thing to be fixed is the volume of NaOH. Yes
But even if it 'drives it to completion' wouldn't you still only need less OH- to completely neutralise the weak acid?
Post by: onepunchboy on October 28, 2016, 02:29:41 am
Post by: onepunchboy on October 28, 2016, 05:00:45 am
Also im not sure how to do this q, sry about asking so many q haha... thanks
Post by: nimasha.w on October 28, 2016, 09:24:55 am
Post by: RuiAce on October 28, 2016, 09:53:50 am
But even if it 'drives it to completion' wouldn't you still only need less OH- to completely neutralise the weak acid?No. Because since it is driven to completion, the weak acid will "ultimately have the same amount of H+ to offer", requiring the same volume.
Note that as the NaOH reacts with the weak acid, H+ is taken out of the system. But the weak acid still has its own degree of ionisation, which forms a relationship between the concentration of the acid, and the concentration of H+.
With H+ gone, this degree of ionisation is disturbed. More H will ionise into H+ allowing more of it to react with NaOH.
This process repeats itself and ultimately the same amount of H+ is needed to be neturalised with the OH-
Post by: RuiAce on October 28, 2016, 09:56:19 am
Can anyone explain why water samples taken from a fast-running stream and from a swap differ in oxygen concentrations? thanks :))))A swamp is full of useless stuff. Oxygen isn't going to dissolve into that water as easily as a stream which just casually flows. We need common sense here.
Post by: RuiAce on October 28, 2016, 10:01:19 am
hey! can i please have some help with these questions :-)Q15 discussed recently in post #966
(http://uploads.tapatalk-cdn.com/20161027/7071893bf5879af9272b3c9dcc0a4f8c.jpg)I only have ideas on how to do this one. It's clearly stretching the boundaries of the HSC course but if you could provide the answer then I can explain it. Cause I cannot guarantee an answer yet
Also im not sure how to do this q, sry about asking so many q haha... thanks
Post by: RuiAce on October 28, 2016, 10:09:32 am
hey! can i please have some help with these questions :-)Observe that in Q18, the molar mass is what we would use at the start.
But instead it is what we're trying to find. Hence, we must work backwards.
NaX + HCl -> NaCl + ?
We don't care about the products. We only care about the reactants.
Note that the 1:1 mole ratio between the reactants is correct. This is because whilst it's obvious that HCl only has one proton, the Na+ ion has a +1 charge, hinting that the X ion has a -1 charge to balance it out. So if the mole ratio was not 1:1 there would be an imbalance.
nHCl = 0.1 * (24.4*10-3) = 0.00244 mol
Therefore nNaX in reaction = 0.00244 mol
i.e. [NaX] = 0.00244/(20*10-3) = 0.122 mol L-1
Note that the concentration of the base must be what's fixed. The same base that was prepared was used in the titration, so there's no dilution factor here.
So since the volume at the start was 100 mL
nNaX prepared = 0.122 * 0.1 = 0.0122 mol
Finally, use n = m/M, where m = 1
Post by: RuiAce on October 28, 2016, 10:15:08 am
hey! can i please have some help with these questions :-)The actual calculations for Q20 are straightforward so I'll briefly explain the weird part.
The equal amounts are consumed in each reaction means that 200g went into equation 1 (to produce CO), whereas the other 200g went into equation 2 (to produce CO2)
So break down the formula, and figure out what moles of what you need.
The last step is just to add your final masses of CO and CO2
Post by: wesadora on October 28, 2016, 10:32:06 am
1. How can you show NaOH acts as a base by reacting it with water, because it just dissociates into Na+ and OH-... but like, you can react HCl with water to show it's an acid (by donating hydrogen): HCl + H20 --> H3O+ + Cl-....
but NaOH is just NaOH --> Na+ + OH- ?
2. See attached. For monitoring ozone concentrations: how is UV 'backscattered' (i.e. reflected off ozone to be detected by the TOMS). I understand how it detects the differences between UV-B (absorbed by ozone) and UV-A/UV-C (passes through ozone, not absorbed & harmless) but...backscattering? o.O
thx :)
Post by: RuiAce on October 28, 2016, 10:37:15 am
Two questions:1. Technically, if you react something with a base it also suffices to be an acid. As if you react with an acid it suffices to be a base. Which is usually preferred: e.g. NaOH is a base as you can react it with an acid like HCl.
1. How can you show NaOH acts as a base by reacting it with water, because it just dissociates into Na+ and OH-... but like, you can react HCl with water to show it's an acid (by donating hydrogen): HCl + H20 --> H3O+ + Cl-....
but NaOH is just NaOH --> Na+ + OH- ?
2. See attached. For monitoring ozone concentrations: how is UV 'backscattered' (i.e. reflected off ozone to be detected by the TOMS). I understand how it detects the differences between UV-B (absorbed by ozone) and UV-A/UV-C (passes through ozone, not absorbed & harmless) but...backscattering? o.O
thx :)
But the reaction with water method is also acceptable and you would show that solid NaOH dissociates into its ions. i.e. You must include the state change from (s) to (aq)
And there is no attachment.
Post by: massive on October 28, 2016, 11:16:37 am
Post by: RuiAce on October 28, 2016, 11:18:20 am
for the cyclohexane/cyclohexene practical, are we meant to use liquid bromine (Br2) or bromine water (BrOH). Also is liquid bromine aq?Liquid bromine is liquid. That's why it's called liquid bromne.
Technically you're using bromine water, which is prepared by reacting, guess what, bromine and water.
But when I asked my chemistry teacher, for the chemical equations you're allowed to write it with either bromine OR bromine water.
Post by: wesadora on October 28, 2016, 11:57:35 am
1. Technically, if you react something with a base it also suffices to be an acid. As if you react with an acid it suffices to be a base. Which is usually preferred: e.g. NaOH is a base as you can react it with an acid like HCl.
But the reaction with water method is also acceptable and you would show that solid NaOH dissociates into its ions. i.e. You must include the state change from (s) to (aq)
And there is no attachment.
I see. Also, edited the post now - should be attached :)
Post by: bethjomay on October 28, 2016, 01:20:43 pm
Post by: RuiAce on October 28, 2016, 01:37:32 pm
For the batteries dot point, do we need to know the anode and cathode as well as the overall?Well if you know the half equations for the anode and cathode you can just derive the overall equation on the spot.
But you definitely need to know the half equations at the least, because the dot point mentions the chemistry of the cell.
Post by: jakesilove on October 28, 2016, 01:49:02 pm
Two questions:
1. How can you show NaOH acts as a base by reacting it with water, because it just dissociates into Na+ and OH-... but like, you can react HCl with water to show it's an acid (by donating hydrogen): HCl + H20 --> H3O+ + Cl-....
but NaOH is just NaOH --> Na+ + OH- ?
2. See attached. For monitoring ozone concentrations: how is UV 'backscattered' (i.e. reflected off ozone to be detected by the TOMS). I understand how it detects the differences between UV-B (absorbed by ozone) and UV-A/UV-C (passes through ozone, not absorbed & harmless) but...backscattering? o.O
thx :)
Hey! I'm not going to give you a proper answer, because at this stage you don't need one. It isn't important, at all, to understand how TOMS works. You just need to cite off some fancy words that sound right (exactly as you've shown us in the attachment), and that will get you full marks 100% of the time. It's always an 'outline' question, not 'explain', as you're not supposed to understand how the mapping works. So, learn what you've shown us, and move on!
Post by: captainclank on October 28, 2016, 02:15:27 pm
8. When dilute sulfuric acid was added to an unknown solution, L, a white precipitate was formed.
The possible ions in the unknown are: sodium, calcium, barium and lead.
What is an additional test that can be done to a fresh sample to determine the exact identity of the
ion present?
(A) add silver nitrate
(B) do the flame test
(C) add sodium carbonate
(D) test the pH
*Correct answer was B* (James Ruse 2014 Trial)
Post by: jakesilove on October 28, 2016, 02:17:43 pm
Wouldn't it be inappropriate to do a flame test when there is a possibility of toxic chemicals (lead) in the solution?
8. When dilute sulfuric acid was added to an unknown solution, L, a white precipitate was formed.
The possible ions in the unknown are: sodium, calcium, barium and lead.
What is an additional test that can be done to a fresh sample to determine the exact identity of the
ion present?
(A) add silver nitrate
(B) do the flame test
(C) add sodium carbonate
(D) test the pH
*Correct answer was B* (James Ruse 2014 Trial)
You're absolutely right, and you'll never be given an HSC question where you need to burn Lead! Trial papers are never as good/accurate as HSCs, so I would be focusing on HSC papers from now on :)
Post by: RuiAce on October 28, 2016, 02:20:49 pm
Wouldn't it be inappropriate to do a flame test when there is a possibility of toxic chemicals (lead) in the solution?Obviously sodium is wrong as otherwise there wouldn't be a precipitate.
8. When dilute sulfuric acid was added to an unknown solution, L, a white precipitate was formed.
The possible ions in the unknown are: sodium, calcium, barium and lead.
What is an additional test that can be done to a fresh sample to determine the exact identity of the
ion present?
(A) add silver nitrate
(B) do the flame test
(C) add sodium carbonate
(D) test the pH
*Correct answer was B* (James Ruse 2014 Trial)
Leaving us to distinguish between calcium, barium and lead.
Because we're trying to find a cation, adding nitrate does nothing and so A is wrong.
Adding carbonate will cause all three of them to precipitate, so C is wrong
pH is clearly a dud option; what does that do.
B is the correct answer. Note that firstly, lead is something you're testing for, thus if it were a toxic chemical it would be in such high concentrations to the point the blue/white flame is just obvious.
Note that each of the three remaining metals all have distinct flame test colours.
But at the same time, this is a James Ruse paper and the questions are just weird. Factoring in technicalities, flame tests should be a last resort.
Post by: wesadora on October 28, 2016, 02:46:11 pm
I see. Hold on...so do sacrificial anodes have to be in physical contact? (e.g. magnesium wrapped around an iron nail)
Or say, could a magnesium and an iron strip be next to each other (on opposite sides) of a petri dish with like...an NaCl electrolyte - and the magnesium would still rust preferentially instead of the iron (forcing iron to be a cathode), or would both just do their own things? O.O
hey - sorry jake i think you missed this question :) On page 64.
Post by: jakesilove on October 28, 2016, 02:50:24 pm
hey - sorry jake i think you missed this question :) On page 64.
Sorry! I definitely missed that :) No, they don't need to be in physical contact, there just needs to be some sort of electrolyte between them. Physical contact will work, but any sort of method of transfer of ions is enough. So, sacrificial anodes to protect piping underground can use the damp soil as an electrolyte? Does that make sense?
Post by: wesadora on October 28, 2016, 03:10:51 pm
Sorry! I definitely missed that :) No, they don't need to be in physical contact, there just needs to be some sort of electrolyte between them. Physical contact will work, but any sort of method of transfer of ions is enough. So, sacrificial anodes to protect piping underground can use the damp soil as an electrolyte? Does that make sense?
got it. :)
Post by: massive on October 28, 2016, 07:09:19 pm
Post by: RuiAce on October 28, 2016, 07:12:52 pm
whats the difference between hydroxy group and hydroxyl group??Different names for the same thing.
Post by: massive on October 28, 2016, 10:33:29 pm
Post by: jakesilove on October 28, 2016, 10:43:17 pm
Guys you know how ethanol has a non-polar part and a polar end, that's the same for all alcohols right? And is that also the reason why alcohols have a higher melting and boiling point than their corresponding alkanes/alkenes because of the hydrogen bonding that they inherently contain??
Well, Methanol probably doesn't quite have a polar/non-polar end.
Plus, I think the important part of Ethanol is that it is EQUALLY polar and non-polar, due to its size. I would hazard a guess that larger molecules, assuming 1 -OH group, go back to being essentially non-polar.
Post by: sharpeye on October 29, 2016, 08:58:51 am
Just to clarify about the batteries part of POM - you only need to know EITHER dry cell/lead-acid cell and one of the other batteries listed, right? ie dry cell + button cell? Could they potentially ask for more than that? Thanks!
Post by: RuiAce on October 29, 2016, 09:09:06 am
Hey there,Yep that's correct. One of the two, and one of the list.
Just to clarify about the batteries part of POM - you only need to know EITHER dry cell/lead-acid cell and one of the other batteries listed, right? ie dry cell + button cell? Could they potentially ask for more than that? Thanks!
Post by: angiezhang9 on October 29, 2016, 09:30:27 am
Yeah not really, they're pretty much produced the same way. They're just two dot points because you need to be prepared to give examples for each.
Thanks RuiAce
Post by: MysteryMarker on October 29, 2016, 10:12:10 am
For this reaction, I remember reading that the acid and alkanol are used in a 1:3 ratio even if the mol ratio is 1:1. (e.g. ethanoic acid and ethanol) So is this excess used so that as per LCP the equilibrium will shift to the products side as well as to make the acid a limiting reagent?
Also, does it matter whether the acid/alkanol is used in excess, like is one more expensive than the other?
Cheers.
Post by: RuiAce on October 29, 2016, 11:11:08 am
Hey guys, this is just more of a confirmation for one of the conditions implemented in esterification.I believe that's right; might wanna get Jake to double check. I do recall a mole ratio favouring the alkanoic acid for the sake of shifting the equilibrium.
For this reaction, I remember reading that the acid and alkanol are used in a 1:3 ratio even if the mol ratio is 1:1. (e.g. ethanoic acid and ethanol) So is this excess used so that as per LCP the equilibrium will shift to the products side as well as to make the acid a limiting reagent?
Also, does it matter whether the acid/alkanol is used in excess, like is one more expensive than the other?
Cheers.
I think the acid is chosen because it can be neutralised. The alkanol is harder to eliminate in the final mixture when it's used in excess. So we want the alkanoic acid to be in excess and the alkanol to be limiting.
Post by: nimasha.w on October 29, 2016, 12:38:21 pm
Post by: RuiAce on October 29, 2016, 12:47:22 pm
hi! i don't really understand this questionRecall that the relationship between absorbance and concentration is linear. When you plot the absorbance on the y-axis and the concentration on the x-axis, you basically get the line y=mx. (In y=mx+b, b=0 because it's obvious that at 0 concentration there is no absorbance.)
The standard solution has concentration 10ppm and absorbance 400. So if you have a substance with absorbance 500 (=5/4 * 400), then your concentration (of copper ions) will be 12.5ppm.
Then we must identify the reaction that takes place
Cu2+ + Na2CO3(aq) -> CuCO3(s) + 2 Na+
Now, recall that 1 ppm is really just a fancy way of saying 1mg/L
So here, our concentration of Cu is 12.5mg L-1
i.e. 0.0125 g L-1
Since our volume was 100mL = 0.1L, we have our mass of Cu2+ being 0.00125g
You should be able to take over here. The rest of the calculations are just typical standard stuff: convert to moles and etc.
Post by: jakesilove on October 29, 2016, 02:13:51 pm
Hey guys, this is just more of a confirmation for one of the conditions implemented in esterification.
For this reaction, I remember reading that the acid and alkanol are used in a 1:3 ratio even if the mol ratio is 1:1. (e.g. ethanoic acid and ethanol) So is this excess used so that as per LCP the equilibrium will shift to the products side as well as to make the acid a limiting reagent?
Also, does it matter whether the acid/alkanol is used in excess, like is one more expensive than the other?
Cheers.
The things you've discussed are WAYYYY beyond the curriculum, and I have no idea if they're true. I really wouldn't worry about it!
Post by: RuiAce on October 29, 2016, 02:16:14 pm
The things you've discussed are WAYYYY beyond the curriculum, and I have no idea if they're true. I really wouldn't worry about it!I think he's right though. I remember having that sort of scenario when I did the experiment.
Post by: ProfLayton2000 on October 29, 2016, 02:17:19 pm
Post by: RuiAce on October 29, 2016, 02:21:31 pm
When doing calculations where are your given info on two products, how would formally show which one is the limiting reagent?Assuming you meant info on two reactants, not products.
Figure out the number of moles of each reagent first, then write out the balanced equation to determine your mole ratio.
Using your mole ratio, try to figure out which one would get used up. Then you don't have to be too formal, you can just say since 1 mol of X reacts with 2 mol of Y, X is the excess reagent and Y is the limiting reagent (or something even more simpler than that)
Post by: angiezhang9 on October 29, 2016, 02:38:34 pm
"Carbon-12, carbon-13 and carbon -14 are isotopes of carbon. Atoms of these isotopes will differ in:
a) the number of protons in their nuclei
b) their electron configuration
c) their chemical properties
d) their physical properties
I thought the answer was c but the book says that the answer is d.
Post by: Yasminpotts1105 on October 29, 2016, 03:37:41 pm
Post by: RuiAce on October 29, 2016, 03:46:29 pm
there's a multiple choice question in the cambridge checkpoints book which i don't understand.Bit dodgy and pushing the boundaries of the HSC course.
"Carbon-12, carbon-13 and carbon -14 are isotopes of carbon. Atoms of these isotopes will differ in:
a) the number of protons in their nuclei
b) their electron configuration
c) their chemical properties
d) their physical properties
I thought the answer was c but the book says that the answer is d.
Obviously A and B are wrong, but isotopic behaviour is considered to be physics.
Carbon 14 will react the same way as carbon 12 will in any chemical reaction. Their reactivities and etc. do not change. Carbon 14 just undergoes radioactive decay.
Radioactive decay isn't considered chemical as it doesn't involve any real chemistry. It's just what goes on inside the nucleus of the atom.
Post by: Yasminpotts1105 on October 29, 2016, 03:48:10 pm
Because I am so passionate about science and willing to put in effort but my teacher just keeps explaining things in ways I cannot understand and anything I learn is outside of his classes.
Or any advice?
Post by: RuiAce on October 29, 2016, 03:49:46 pm
Does anyone have a good link or way of knowing how to know which numbers to put out the front or in the middle of alkanes and alkenes?I don't know what number you're putting in front of an alkane.
For an alkene, the number depends on where the double bond is. The number corresponds to the first possible carbon atom with a double bond attached.
E.g. Pretending that the H's are all there (not typed due to inconvenience), consider
C-C=C-C-C-C-C-C
and
C-C-C-C=C-C-C-C
In the first one, the double bond starts on carbon #2.
In the second one, the double bond starts on carbon #4.
So the first one is 2-octene and the second one is 4-octene
Trap:
C-C-C-C-C-C=C-C
is also 2-octene. Because we are allowed to count from left to right, or right to left. We count, in a way, so that the number is always as small as possible.
Otherwise, you need to provide a question.
I was just wondering what people think about if how well you do in chemistry depends on your teacher or not?No. Unless you have a tendency to rely on teachers they aren't important if you can put in the effort to get it yourself. That's with every subject and not just chemistry.
Because I am so passionate about science and willing to put in effort but my teacher just keeps explaining things in ways I cannot understand and anything I learn is outside of his classes.
Or any advice?
Post by: angiezhang9 on October 29, 2016, 04:28:14 pm
Bit dodgy and pushing the boundaries of the HSC course.
Obviously A and B are wrong, but isotopic behaviour is considered to be physics.
Carbon 14 will react the same way as carbon 12 will in any chemical reaction. Their reactivities and etc. do not change. Carbon 14 just undergoes radioactive decay.
Radioactive decay isn't considered chemical as it doesn't involve any real chemistry. It's just what goes on inside the nucleus of the atom.
THANK YOU!
Post by: medicinestudent on October 29, 2016, 07:08:30 pm
A sample of pond water from a contaminated site was analysed to determine the
concentration of lead ions using the following procedure.
• A measuring cylinder was used to collect a 50 mL sample from the pond.
• The sample was placed in a clean dry beaker.
• 25.0 mL of 0.200 mol L−1 sodium chloride solution was added to the sample.
• The precipitate of lead(II) chloride that formed was filtered, dried and weighed. It had
a mass of 0.13 g.
2015 hsc mc 19
What was the concentration of lead ions in the sample?
(A) 5.0 × 10−3 mol L−1
(B) 5.8 × 10−3 mol L−1
(C) 9.3 × 10−3 mol L−1
(D) 10.7 × 10−3 mol L−1
i can't do ppm or concentration of ions in solution!!! no idea where to start? moles? volume? c1v1?
Post by: RuiAce on October 29, 2016, 07:15:23 pm
not sure if this question has been asked yet?Write out the relevant equation to determine the species and mole ratio
A sample of pond water from a contaminated site was analysed to determine the
concentration of lead ions using the following procedure.
• A measuring cylinder was used to collect a 50 mL sample from the pond.
• The sample was placed in a clean dry beaker.
• 25.0 mL of 0.200 mol L−1 sodium chloride solution was added to the sample.
• The precipitate of lead(II) chloride that formed was filtered, dried and weighed. It had
a mass of 0.13 g.
2015 hsc mc 19
What was the concentration of lead ions in the sample?
(A) 5.0 × 10−3 mol L−1
(B) 5.8 × 10−3 mol L−1
(C) 9.3 × 10−3 mol L−1
(D) 10.7 × 10−3 mol L−1
i can't do ppm or concentration of ions in solution!!! no idea where to start? moles? volume? c1v1?
Pb2+ + 2 NaCl -> PbCl2(s) + 2 Na+
We should work backwards since they give us information on the product
nPbCl2 = 0.13/(207.2 + 35.45*2) = 4.674... * 10-4 mol
Briefly check the moles of NaCl we had added:
nNaCl added=0.025*0.2 = 5*10-3 mol
Implying (despite the mole ratio) that NaCl was the excess reagent after all, and Pb2+ was the limiting reagent.
Due to the mole ratio, we have nPb2+ = 4.674... * 10-4 mol
Hence, via C=n/V
C = 4.674... * 10-4/0.05 = 9,349... * 10-3 mol L-1
Answer: C
Post by: Sssssrr on October 29, 2016, 08:21:16 pm
Post by: RuiAce on October 29, 2016, 08:22:44 pm
for the dot point in industrial chemistry - "identify, given a flow chart, the sequence of steps used in the solvay process to describe the chemistry involved in: brine purification, hydrogen carbonate formation, ammonia recovery", does this mean if they ask a question on it in the hsc they;; put in a flowchart, or do we have to know the flowchart?Every instance (not even necessarily this dot point) in the chemistry course where they asked you to draw a flow chart, you had to draw it yourself.
Yet I've never seen that dot point examined like that either. I recall writing a 7 marker on the Solvay process and its procedures but never the need to actually draw it out.
Post by: nibblez16 on October 29, 2016, 10:30:28 pm
Post by: Sssssrr on October 29, 2016, 11:59:12 pm
Every instance (not even necessarily this dot point) in the chemistry course where they asked you to draw a flow chart, you had to draw it yourself.
Yet I've never seen that dot point examined like that either. I recall writing a 7 marker on the Solvay process and its procedures but never the need to actually draw it out.
okay thanks!
Post by: Sssssrr on October 30, 2016, 12:01:10 am
Hello, for this question can we identify (A) as starch?out of curiosity, if this from production of materials or is it from an option topic?
Post by: RuiAce on October 30, 2016, 12:05:23 am
Post by: massive on October 30, 2016, 12:56:11 am
Hello, for this question can we identify (A) as starch?
Hey i'm pretty sure A is cellulose since the alternating glucose monomers that compose the polymer chain are inverted and this maintains its linear structure, that's why B is starch.
Not sure how correct I am, this isn't an option topic is it?
Post by: nibblez16 on October 30, 2016, 01:07:43 am
out of curiosity, if this from production of materials or is it from an option topic?
out of curiosity, if this from production of materials or is it from an option topic?
Its from Forensic chemistry :)
The answer was amylose thats y i got confused...
Post by: nibblez16 on October 30, 2016, 01:05:13 pm
Post by: WLalex on October 30, 2016, 01:24:08 pm
Thanks :)
Post by: massive on October 30, 2016, 01:28:17 pm
Post by: jakesilove on October 30, 2016, 01:35:48 pm
When a question says... "outline the procedure you would use to..." should we simply do sentences or numbered steps?
Thanks :)
You don't need to use numbered steps, but I think it is always easier to. Basically, you can do whatever you feel most comfortable with. Personally, I would use numbered steps
Post by: jakesilove on October 30, 2016, 01:36:35 pm
guys for galvanic cells how do you figure out what electrolyte to use? Also how can you determine the colour of the electrolyte solution??
You just always use the Metal Nitrate as the electrolyte in that half cell! Nitrates are always soluble, so are perfect for this use :) You don't actually need to know colours of electrolyte solutions, they will always give it to you. It can't hurt to know that Copper nitrate is Blue though :)
Post by: kimmie on October 30, 2016, 02:03:21 pm
Post by: Sanaz on October 30, 2016, 02:05:27 pm
Post by: wesadora on October 30, 2016, 02:15:34 pm
Hey :) Do you need to know all the batteries; lead-acid, button, dry cell, hydrogen fuel cell, lithium cell etc or do you only need to know two for comparison? What if they are specific?
You just need to know EITHER lead-acid or dry, and then another one out of that list: -fuel cell, silver button cell, lithium ion battery, etc...
so two in total. :)
I'm doing lead-acid and silver button and you have to know chemistry (anode, cathode, electrolyte, equations), cost/practicality (e.g. lead acid cell is cost effective supplying high voltages needed for cars, with a long life span + is rechargeable, etc.) and impacts on society/environment for each.
Post by: WLalex on October 30, 2016, 02:28:02 pm
Hey can someone help me I don't understand why the answer is part (c)
Hey! as it is a solid it will have 0 affect - that is just a rule you should know which now you do :)
Post by: RuiAce on October 30, 2016, 02:39:10 pm
Hey! as it is a solid it will have 0 affect - that is just a rule you should know which now you do :)Not quite the answer here though
In this scenario, it is because sulfate appears:
a) in a ONE-sided equation, not reversible
b) as a PRODUCT
Sodium sulfate is guaranteed to dissolve, so it goes from solid to aqueous. It is the fact that we have a one-way reaction and not an equilibrium reaction that justifies the answer here.
Post by: wesadora on October 30, 2016, 02:40:05 pm
--> and thus alkanoic acids have a higher BP/MP than alkanols/alkanes due to these stronger IMF
Post by: RuiAce on October 30, 2016, 02:42:39 pm
What is meant by a carboxylic acid functional group (COOH) having 'two sites for hydrogen bonding'. Is it literally just because there's the hydrogen bonding between the O and H in the OH, and another possible site for hydrogen bonding due to the double bonded H?Recall that -COOH looks like
--> and thus alkanoic acids have a higher BP/MP than alkanols/alkanes due to these stronger IMF
O
C<
OH
The -OH site is one site for hydrogen bonding as it is clear that the -OH site is similar to the hydroxyl functional group.
The =O site is yet another site for hydrogen bonding. It is clear that this oxygen with its double bond is going to be extremely polar, allowing for very strong intermolecular forces to form
I.e. One of the sites is the OH site, whereas the other site is the extra O's site
Edit: Upon rechecking I noticed you said double bonded H. If this was a typo for double bonded O, then yes.
Post by: wesadora on October 30, 2016, 02:43:56 pm
Recall that -COOH looks like
O
C<
OH
The -OH site is one site for hydrogen bonding as it is clear that the -OH site is similar to the hydroxyl functional group.
The =O site is yet another site for hydrogen bonding. It is clear that this oxygen with its double bond is going to be extremely polar, allowing for very strong intermolecular forces to form
I.e. One of the sites is the OH site, whereas the other site is the extra O's site
simple! thanks rui :)
Post by: wesadora on October 30, 2016, 03:43:11 pm
Post by: massive on October 30, 2016, 03:49:27 pm
Post by: massive on October 30, 2016, 03:51:05 pm
do you write states for radicals? e.g. O. or O.(g) ?I'm pretty sure you don't write states for radicals
Post by: wesadora on October 30, 2016, 04:08:35 pm
I'm pretty sure you don't write states for radicals
yeah that's what i thought :) some ppl do though so i got confused :|
Post by: kimmie on October 30, 2016, 04:26:49 pm
Post by: Cindy2k16 on October 30, 2016, 04:40:13 pm
Hey for Qs like these, how do we know that which is the positive or negative terminal? And also which is the cathode and anode?
I'm not sure, but you might be able to use the standard potentials table to figure out which material in the lead acid cell is oxidised/reduced and therefore the cathode anode.
The material that is reduced is the cathode-
The material that is oxidised is the anode-
I remember it as RED CAT, AN OX. RED(uction) CAT(hode). AN(ode) OX(idation)
Otherwise, you should know one of the two batteries pictured since you are required to memorise information about one of them for one of the syllabus dotpoints. Using your knowledge of just one of them you can establish what the anode and cathode is an thus what 1 and 2 is.
Also, in a galvanic cell cathode is positive and anode is negative. I remember this by thinking of how cats are great haha. Cats --> cathode. Good -->positive.
Hope this helps :)
Post by: RuiAce on October 30, 2016, 04:44:28 pm
do you write states for radicals? e.g. O. or O.(g) ?
I'm pretty sure you don't write states for radicals
yeah that's what i thought :) some ppl do though so i got confused :|Technically that one is a gas but they care less most of the time for radicals because, being honest radicals are things without a full outer shell so they're extremely reactive and they're gonna become something else soon enough.
Post by: RuiAce on October 30, 2016, 04:48:49 pm
guy for part b of the question attached. the order of reactivity (increasing) is X<Z<Y. I was just confused tho, ik that X produces the least voltage however compared to copper it undergoes oxidation (as its negative electrode) and Y and Z undergo reduction (positive electrodes), doesn't that mean that X is more reactive than Y and Z??The polarity of copper tells us which electrode copper is. Here, copper is the negative electrode (i.e. anode) in the cell with X, and the positive electrode (i.e. cathode) in the cell with Y.
Recall RED-CAT AN-OX, i.e. copper is oxidised with X but reduced with Y and Z. Copper is the one undergoing these reactions, not the other three.
So since copper oxidises, X gets reduced, and hence X is less reactive.
and etc.
Post by: RuiAce on October 30, 2016, 04:50:43 pm
Hey for Qs like these, how do we know that which is the positive or negative terminal? And also which is the cathode and anode?These are just the dry cell and the lead-acid cell. They are the cells you studied.
Whilst you aren't expected to replicate that diagram, if you knew your cell well then you would know which is which and thus be able to infer it from your own knowledge.
Or you can use common sense, because a dry cell is literally a BATTERY and we all know what a battery looks like (cylinder with a head poking out on the +'ve terminal)
Also, by definition the positive terminal represents the cathode, and the negative represents the anode.
Post by: massive on October 30, 2016, 05:02:00 pm
The polarity of copper tells us which electrode copper is. Here, copper is the negative electrode (i.e. anode) in the cell with X, and the positive electrode (i.e. cathode) in the cell with Y.
Recall RED-CAT AN-OX, i.e. copper is oxidised with X but reduced with Y and Z. Copper is the one undergoing these reactions, not the other three.
So since copper oxidises, X gets reduced, and hence X is less reactive.
and etc.
wait so you're saying a positive polarity corresponds to a negative terminal ? :S
Post by: RuiAce on October 30, 2016, 05:08:47 pm
wait so you're saying a positive polarity corresponds to a negative terminal ? :SWhere exactly did I say that?
Post by: massive on October 30, 2016, 05:12:23 pm
Here, copper is the negative electrode (i.e. anode) in the cell with X, and the positive electrode (i.e. cathode) in the cell with Y.
over here :S
Post by: kimmie on October 30, 2016, 05:16:45 pm
These are just the dry cell and the lead-acid cell. They are the cells you studied.
Whilst you aren't expected to replicate that diagram, if you knew your cell well then you would know which is which and thus be able to infer it from your own knowledge.
Or you can use common sense, because a dry cell is literally a BATTERY and we all know what a battery looks like (cylinder with a head poking out on the +'ve terminal)
Also, by definition the positive terminal represents the cathode, and the negative represents the anode.
Post by: RuiAce on October 30, 2016, 05:20:11 pm
over here :SOh wow. Sorry it must've been that I got confused in what you said at the start. For some reason I saw things in reverse.
In that case I'm convinced you're right. What is the source of this question?
Post by: kimmie on October 30, 2016, 05:20:35 pm
Post by: RuiAce on October 30, 2016, 05:21:37 pm
Hey me again 😛 I don't understand why there can't be 6?Can you send a picture of the isomers you drew?
Post by: massive on October 30, 2016, 05:38:49 pm
Oh wow. Sorry it must've been that I got confused in what you said at the start. For some reason I saw things in reverse.
In that case I'm convinced you're right. What is the source of this question?
I actually have no idea where this q came from tbh, It was just in my notes. sorry!
Post by: massive on October 30, 2016, 05:40:07 pm
Post by: Cindy2k16 on October 30, 2016, 05:41:08 pm
guys what happens if there is no salt bridge in a galvanic cell?
The circuit is incomplete so no current can flow
Post by: kimmie on October 30, 2016, 05:41:26 pm
Can you send a picture of the isomers you drew?
Post by: RuiAce on October 30, 2016, 05:41:58 pm
I actually have no idea where this q came from tbh, It was just in my notes. sorry!Ah. In that case yeah I'm just convinced they're wrong. Jake's going to have his hands full with physics tonight so I won't bother bringing it up for now, but if he sees this he may be able to input.
guys what happens if there is no salt bridge in a galvanic cell?In general, it won't work because you can't allow migration of ions. An exception is made when they're just one compartment (and not separated), which needs a bit more thought than the norm (see Jake's lecture slides I think it's there)
The circuit is incomplete so no current can flowNote - Only true in GENERAL.
Post by: kimmie on October 30, 2016, 05:44:12 pm
I actually have no idea where this q came from tbh, It was just in my notes. sorry!
Post by: RuiAce on October 30, 2016, 05:45:00 pm
Sorry, picture's too small.
Maybe download the ATARnotes app and then just directly upload the photo in the future
But these are the only ones I've found:
1,1-dichloro
1,2-dichloro
2,2-dichloro
1,3-dichloro
Post by: RuiAce on October 30, 2016, 05:47:13 pm
This Q seems pretty similarAt least that past HSC question wasn't ambiguous
Post by: massive on October 30, 2016, 05:48:02 pm
This Q seems pretty similarwow thanks for this, is the answer D ??
Post by: RuiAce on October 30, 2016, 05:48:52 pm
wow thanks for this, is the answer D ??From memory (and just double checked), yes
Post by: massive on October 30, 2016, 06:00:12 pm
From memory (and just double checked), yesthanks!
Post by: starkidshani on October 30, 2016, 06:59:33 pm
Post by: jakesilove on October 30, 2016, 07:08:17 pm
Hello! Could someone please help me out with summarising the chemistry of a Dry Cell? :)
Hey! This is the information I had going into my HSC
(http://i.imgur.com/raKYt0c.png)
Post by: starkidshani on October 30, 2016, 07:23:44 pm
Hey! This is the information I had going into my HSC
(http://i.imgur.com/raKYt0c.png)
Awesome, thank you! What about the overall equation?
Post by: jarin_ on October 30, 2016, 07:39:47 pm
Post by: RuiAce on October 30, 2016, 07:42:49 pm
hey, what pracs are the most common for the core topics & industrial and how well should we know them?Any prac can be asked whenever BOSTES feels like it and you should know all of them really well. As for how well, you can decide which pracs knowing the method is enough, whereas which pracs you need also some expected results. Discussion questions (especially the classic 3 reliability, accuracy and validity) just need to be known.
Though I only remember these pracs in industrial
- Modelling equilibriums
- Laboratory electrolysis of NaOH
- Laboratory saponification
Post by: jarin_ on October 30, 2016, 08:04:52 pm
Any prac can be asked whenever BOSTES feels like it and you should know all of them really well. As for how well, you can decide which pracs knowing the method is enough, whereas which pracs you need also some expected results. Discussion questions (especially the classic 3 reliability, accuracy and validity) just need to be known.
Though I only remember these pracs in industrial
- Modelling equilibriums
- Laboratory electrolysis of NaOH
- Laboratory saponification
thanks!!
also i'm having trouble with pH calculations
for eg;
A solution was made of mixing 75.00 mL of 0.120M HCL with 25mL of 0.2M NaOH.
What is the pH of the solution?
Is there a particular set of steps in doing these types of questions?
Post by: jakesilove on October 30, 2016, 08:08:14 pm
thanks!!
also i'm having trouble with pH calculations
for eg;
A solution was made of mixing 75.00 mL of 0.120M HCL with 25mL of 0.2M NaOH.
What is the pH of the solution?
Is there a particular set of steps in doing these types of questions?
There certainly is! Calculate the moles of H+, and the moles of OH-. Figure out the limiting reagent, and then how many left-over moles of acid there are. Then, you can figure out the overall concentration of the left over hydroxide (Left over H+/0.1L), and sub that into the -log(H) formula!
Post by: Potato101 on October 30, 2016, 09:29:22 pm
thanks!!
also i'm having trouble with pH calculations
for eg;
A solution was made of mixing 75.00 mL of 0.120M HCL with 25mL of 0.2M NaOH.
What is the pH of the solution?
Is there a particular set of steps in doing these types of questions?
Writing out the equation and writing the volume and conc. given, underneath the respective acid/base makes it easier
Post by: wesadora on October 30, 2016, 10:47:47 pm
However, the HSC answers in the past papers book (this is 2008) says that iron acts as a cathodic site and thus isn't reduced??!?! But water /oxygen (presumably from surrounding soil) is instead? I understand the conquering chem answer but idk what the hell the HSC suggested answers are talking about.
thanks!
- probs a Q for Jake. :)
Post by: jarin_ on October 31, 2016, 08:37:24 am
There certainly is! Calculate the moles of H+, and the moles of OH-. Figure out the limiting reagent, and then how many left-over moles of acid there are. Then, you can figure out the overall concentration of the left over hydroxide (Left over H+/0.1L), and sub that into the -log(H) formula!
Writing out the equation and writing the volume and conc. given, underneath the respective acid/base makes it easier
thank u so much guys!!
Post by: WLalex on October 31, 2016, 08:38:40 am
I got 7.6mg but I always struggle with these type of Q's so not sure if it us right
Post by: wesadora on October 31, 2016, 08:59:37 am
Hey! Was wondering if someone could please confirm the correct answers to this question..the paper is too old so I can't find answers
I got 7.6mg but I always struggle with these type of Q's so not sure if it us right
You're right :) just the n=cv, then m=nM and conv g to mg. yay
Post by: IkeaandOfficeworks on October 31, 2016, 09:32:07 am
Post by: jakesilove on October 31, 2016, 09:53:27 am
Hey - for this question, I'm getting two different answers. The conquering chemistry answer correlates to my response when i first answered this question, saying that iron would be forced to act as a cathode and thus reduce.
However, the HSC answers in the past papers book (this is 2008) says that iron acts as a cathodic site and thus isn't reduced??!?! But water /oxygen (presumably from surrounding soil) is instead? I understand the conquering chem answer but idk what the hell the HSC suggested answers are talking about.
thanks!
- probs a Q for Jake. :)
Definitely stick with the answer you gave, and the answer in the Chem book. Both answers are technically correct (all it's saying is that it would take less energy to reduce water, rather than reduce iron, so that is more likely to occur and produce some sort of oxide layer). However, the reduction of iron is really what we care about it. Sounds like you understand the topic well, just ignore the HSC answer that has confused you!
Post by: jakesilove on October 31, 2016, 09:56:14 am
Hi guys, how do you do this question? Thank you. (Btw the answer is C)
Hey! So we know that
Because the LHS is how ppm works, and the RHS is how fractions work. This gets us an easy answer of
And thus C! Definitely a difficult question conceptually, but once you've seen the method once, you shouldn't have any trouble replicating it :)
Post by: IkeaandOfficeworks on October 31, 2016, 10:38:59 am
Post by: RuiAce on October 31, 2016, 10:59:03 am
Hi again! Can you explain why the answer is D? Thank you. :DThat just looks wrong. I would've said C. Going to get Jake's opinion later.
Post by: jakesilove on October 31, 2016, 11:01:38 am
Hi again! Can you explain why the answer is D? Thank you. :D
Definitely C
Post by: IkeaandOfficeworks on October 31, 2016, 11:08:33 am
Post by: jakesilove on October 31, 2016, 11:34:47 am
Hi guys! how do you do this question? Thank you. :D
Hey! The molar heat of combustion of a substance is, by definition, the heat liberated when one mole of the substance is burned. So, you just find the molar mass (44g), and that's your answer! This is kind of just a 'by definition' answer :)
Post by: Jpw1627 on October 31, 2016, 11:41:57 am
Post by: jakesilove on October 31, 2016, 11:50:24 am
Hey guys, I have a quick question in regards to calculating the PH of a mixture with a dilution. I have attached the question as a photo if you could have a look. It is from a Ruse Trial. Thanks heaps
Hey! So, we need to figure out the moles of Hydrogen and Hydroxide ions. To do this, we use C=n/V
So, we know we have more moles of Hydrogen than Hydroxide. In fact, we have
of excess hydrogen! We can use C=n/V to thus find the final concentration of Hydrogen ions (the rest have reacted with the base to form Water
Plug that into -Log(H) and you get a pH of 2.85... maybe I messed up my calculations somewhere, but that is the general structure of getting an answer out!
Post by: Jpw1627 on October 31, 2016, 12:00:24 pm
Post by: jakesilove on October 31, 2016, 12:12:53 pm
Hey the Answer is B. I think there is something to do with the stoichiometry, as it is a 1:2 ratio.
Absolutely right, I totally ignored that
Post by: karenc. on October 31, 2016, 12:22:19 pm
1) How does one answer a 'justify' question? is it similar to evaluate?
2) Why do we not use an indicator for a titration with a weak acid and weak base?
Many thanks
Post by: itsAnuu on October 31, 2016, 12:25:48 pm
Hi I just have 2 questions:
1) How does one answer a 'justify' question? is it similar to evaluate?
2) Why do we not use an indicator for a titration with a weak acid and weak base?
Many thanks
Justify is supporting an argument or conclusion, whereas evaluate is making a judgment based on criteria
Weak acid and weak base titrations are generally avoided due to no great change in pH, a gradual rise, and the titration curve not being very steep
Post by: karenc. on October 31, 2016, 12:29:55 pm
Justify is supporting an argument or conclusion, whereas evaluate is making a judgment based on criteria
Weak acid and weak base titrations are generally avoided due to no great change in pH, a gradual rise, and the titration curve not being very steep
Thanks for your help :)
so just clarifying on the justify question, would someone just write points supporting the statement?
i really hope they ask something similiar to the 2010 8 marker question
Post by: itsAnuu on October 31, 2016, 12:39:16 pm
Thanks for your help :)
so just clarifying on the justify question, would someone just write points supporting the statement?
i really hope they ask something similiar to the 2010 8 marker question
Yeah suppose there was a question like, justify the conditions used in the Haber process. You would list the conditions of e.g. moderate temperatures and then justify why it is moderate, as high temps shift equilibrium to the left, producing more reactants, and low temperatures shift to right but is extremely slow and not economically viable. <- in more detail of course :D
Post by: wesadora on October 31, 2016, 12:50:11 pm
also - for qualitative/quantitative tests regarding eutrophication, in my notes i have the fricken randomest things called the 'Kjeldahl method' and 'colorimetry' (just AAS with colours, as far as i understand) that is kinda complex and ceebs remembering. What do other people have for eutrophication tests.... :|
Post by: WLalex on October 31, 2016, 12:52:44 pm
On my notes I have written that TDS = dissolved solids + suspended solids
SO for the questions I added the mass difference from both evaporating and filtering but on the answers they one did the mass of the dissolved solids (1.14g instead of 1.38 as I did). Is this right?
Post by: WLalex on October 31, 2016, 12:55:30 pm
This is from 2009 HSC Chem paper. I thought AAS could only be used for metal ions that can be dissolved in solution? Not like...phosphates (which is phosphorous and oxygen anyway, using AAS wouldn't make sense) or are they just playin wit us :(
also - for qualitative/quantitative tests regarding eutrophication, in my notes i have the fricken randomest things called the 'Kjeldahl method' and 'colorimetry' (just AAS with colours, as far as i understand) that is kinda complex and ceebs remembering. What do other people have for eutrophication tests.... :|
for eutrophication i only have testing phosphate using AAS or photometry because phosphate is the growth-limiting so its better to test than nitrate..if that helps at all haha
Post by: jakesilove on October 31, 2016, 12:56:15 pm
This is from 2009 HSC Chem paper. I thought AAS could only be used for metal ions that can be dissolved in solution? Not like...phosphates (which is phosphorous and oxygen anyway, using AAS wouldn't make sense) or are they just playin wit us :(
also - for qualitative/quantitative tests regarding eutrophication, in my notes i have the fricken randomest things called the 'Kjeldahl method' and 'colorimetry' (just AAS with colours, as far as i understand) that is kinda complex and ceebs remembering. What do other people have for eutrophication tests.... :|
Don't worry about that, just accept that the question gives you AAS with phosphates! As for your second question, that's definitely way more information than you need, so don't worry about that either
Post by: jakesilove on October 31, 2016, 12:58:18 pm
Hey, just wanted some clarity regarding this questions.
On my notes I have written that TDS = dissolved solids + suspended solids
SO for the questions I added the mass difference from both evaporating and filtering but on the answers they one did the mass of the dissolved solids (1.14g instead of 1.38 as I did). Is this right?
Mass of dissolved solids is different to total dissolved solids. The question only asks for dissolved solids, so only do the dissolved solids for this question!
Post by: WLalex on October 31, 2016, 01:01:17 pm
Mass of dissolved solids is different to total dissolved solids. The question only asks for dissolved solids, so only do the dissolved solids for this question!
but it says percentage of the total dissolved solids...? haha sorry I'm confused
Post by: jakesilove on October 31, 2016, 01:05:21 pm
but it says percentage of the total dissolved solids...? haha sorry I'm confused
Ah sorry I see! Basically, TDS doesn't include suspended solids, only dissolved solids, so I think you've just written that down wrong in your notes :) I didn't really know that either, but according to Google dissolved solids shouldn't be able to be removed by filtration through a filter funnel. Basically, if you get exactly the same question in your exam, follow the method the answers gave!
Post by: wesadora on October 31, 2016, 01:15:22 pm
Ah sorry I see! Basically, TDS doesn't include suspended solids, only dissolved solids, so I think you've just written that down wrong in your notes :) I didn't really know that either, but according to Google dissolved solids shouldn't be able to be removed by filtration through a filter funnel. Basically, if you get exactly the same question in your exam, follow the method the answers gave!yah, Jake's right. :) Suspended solids are more for like turbidity - the .03g of solid left in the filter paper is the suspended solids. The DISSOLVED solids are what couldn't be trapped by filter paper, and thus can only be obtained after evaporating to dryness in an evap basin. (so TDS is 0.26g and go calc from there)
also.... soz continuing with the eutrophication questions: why do algal blooms consume dissolved oxygen anyway? Aren't they photosynthetic o.O (Creating oxygen), but I understand they block sunlight from getting to other photosynthetic organisms and thus lowering DO.
thanks.
Post by: jakesilove on October 31, 2016, 01:19:59 pm
yah, Jake's right. :) Suspended solids are more for like turbidity - the .03g of solid left in the filter paper is the suspended solids. The DISSOLVED solids are what couldn't be trapped by filter paper, and thus can only be obtained after evaporating to dryness in an evap basin. (so TDS is 0.26g and go calc from there)
also.... soz continuing with the eutrophication questions: why do algal blooms consume dissolved oxygen anyway? Aren't they photosynthetic o.O (Creating oxygen), but I understand they block sunlight from getting to other photosynthetic organisms and thus lowering DO.
thanks.
It's actually the DECAYING of plant matter, grown in eutrophication, that consumes oxygen. Why? No clue, it just does.
Post by: jarin_ on October 31, 2016, 01:28:21 pm
Why does the neutralisation of any strong acid in an aqueous solution by any strong base always result in a heat of reaction of approx. -57 kJ/mol ?
Post by: jakesilove on October 31, 2016, 01:31:41 pm
hey guys I don't understand the sample answer for this so please could explain it:
Why does the neutralisation of any strong acid in an aqueous solution by any strong base always result in a heat of reaction of approx. -57 kJ/mol ?
So, you just need to think about the actual reaction. Every strong acid and strong base will be literally the same reaction! It's just H+ and OH-. Every reaction has a specific heat that is absorbed/released, which is -57 kJ/mol when it comes to neutralisation. So, identify that every strong acid/strong base reaction is the same, regardless of the actual acid or base. Then, discuss the above! Does that make sense!
Post by: IkeaandOfficeworks on October 31, 2016, 01:47:37 pm
Post by: jarin_ on October 31, 2016, 01:52:16 pm
So, you just need to think about the actual reaction. Every strong acid and strong base will be literally the same reaction! It's just H+ and OH-. Every reaction has a specific heat that is absorbed/released, which is -57 kJ/mol when it comes to neutralisation. So, identify that every strong acid/strong base reaction is the same, regardless of the actual acid or base. Then, discuss the above! Does that make sense!
ohh ok thanks that makes sense
but are we supposed to know the specific heat for all types of reactions?
Post by: g98 on October 31, 2016, 01:56:11 pm
What kind of info do we need to know about our chosen scientist?
Thanks :)
Post by: WLalex on October 31, 2016, 03:09:37 pm
However, I just came across this question from 2005 and it is virtually an exact repeat however the working to does not make sense to me.
In the success one books they misused the mass of the filtered solid (0.03g) from the sample (500) so it was 499.97 and divided the mass from evaporating (0.26) but that. Why did they do that in this question and not the other? and why would they even minus the suspended solids from the mass of the water in the first place..
thanks
Post by: jakesilove on October 31, 2016, 03:15:05 pm
Hey thanks for the answers to that previous question. Much appreciated and makes sense now.
However, I just came across this question from 2005 and it is virtually an exact repeat however the working to does not make sense to me.
In the success one books they misused the mass of the filtered solid (0.03g) from the sample (500) so it was 499.97 and divided the mass from evaporating (0.26) but that. Why did they do that in this question and not the other? and why would they even minus the suspended solids from the mass of the water in the first place..
thanks
Hey! I completely disagree with this answer; what the answers are suggesting is that not all 500mL of solution was water (some of it was dissolved solids), so when they divided by volume, they subtracted the weight of the suspended solids. That is wrong. Firstly, 500mL is a volume measurement, and 0.3 is a mass measurement. Whilst 500mL=500g, we don't actually care about the mass of the water, we care about the volume. Which brings me to the second point; concentration includes the entire volume. So, using 500mL would be correct. I'd say that the answer is just wrong.
Post by: youknowwho on October 31, 2016, 04:26:42 pm
would someone please be able to help me solve this question:
what is the pH of a mixture of 20.0 mL of 0.102 mol L^-1 barium hydroxide solution and 40.0 mL of 0.150 mol L^-1 hydrochloric acid diluted to a final volume of 100 mL
Post by: jakesilove on October 31, 2016, 04:32:11 pm
Hey,
would someone please be able to help me solve this question:
what is the pH of a mixture of 20.0 mL of 0.102 mol L^-1 barium hydroxide solution and 40.0 mL of 0.150 mol L^-1 hydrochloric acid diluted to a final volume of 100 mL
Check out my answer here, which is the same question but with different numbers. Read the next few posts as well, because I forgot to include the 1:2 ratio initially :)
Post by: ProfLayton2000 on October 31, 2016, 04:37:48 pm
Hey,
would someone please be able to help me solve this question:
what is the pH of a mixture of 20.0 mL of 0.102 mol L^-1 barium hydroxide solution and 40.0 mL of 0.150 mol L^-1 hydrochloric acid diluted to a final volume of 100 mL
The reaction for this reaction is Ba(OH)2(aq) + 2HCl(aq) --> BaCl2(aq) + H2O(l)
n(Ba(OH)2) = c*v = 0.102 * 0.02 = 0.00204mol
n(HCl) = c*v = 0.150*0.4 = 0.006mol
Because Ba(OH)2 and HCl are supposed to react in a 1:2 ratio, we can see that Ba(OH)2 is the limiting reagent.
Moreover, we can deduce that because all of the Ba(OH)2 will be used (0.00204mol), it can calculated that n(HCl used) = n(Ba(OH)2) * 2 = 0.00408mol.
Next, we show that: n(HCl) = 0.006 - 0.00408 = 0.00192mol, hence there are 0.00192 moles of HCl in excess.
Next, we find the concentration of HCl in the final mixture: n/v = 0.00192/(0.1) = 0.0192 molL-1
HCl is monoprotic so we can say that therefore [H+] = 0.0192molL-1
And pH = -log[H+] = log(0.032) = 1.72 (3sf)
Can someone check answer including sig figs are right
Post by: jakesilove on October 31, 2016, 04:41:26 pm
The reaction for this reaction is Ba(OH)2(aq) + 2HCl(aq) --> BaCl2(aq) + H2O(l)
n(Ba(OH)2) = c*v = 0.102 * 0.02 = 0.00204mol
n(HCl) = c*v = 0.150*0.4 = 0.006mol
Because Ba(OH)2 and HCl are supposed to react in a 1:2 ratio, we can see that Ba(OH)2 is the limiting reagent.
Moreover, we can deduce that because all of the Ba(OH)2 will be used (0.00204mol), it can calculated that n(HCl used) = n(Ba(OH)2) * 2 = 0.00408mol.
Next, we show that: n(HCl) = 0.006 - 0.00408 = 0.00192mol, hence there are 0.00192 moles of HCl in excess.
Next, we find the concentration of HCl in the final mixture: n/v = 0.00192/(0.1) = 0.0192 molL-1
HCl is monoprotic so we can say that therefore [H+] = 0.0192molL-1
And pH = -log[H+] = log(0.032) = 1.72 (3sf)
Can someone check answer including sig figs are right
Looks perfect!
Post by: ProfLayton2000 on October 31, 2016, 04:57:28 pm
Post by: wesadora on October 31, 2016, 05:08:31 pm
Does it matter whether we take bromine water to be Br2 or HOBr?
Nah, both are fine (HOBr is 'more right' but they don't care cuz we're "just" HSC chem bahahaha). I just do Br2 for simplicity :) (but aqueous ofc)
Post by: Cindy2k16 on October 31, 2016, 07:16:15 pm
TIA
Post by: jakesilove on October 31, 2016, 07:18:00 pm
Could they ask us to write the chemical formula/ draw the structural formula for the polymer and/or monomer of our chosen biopolymer?
TIA
I'm fairly sure you don't need to draw it, or know the empirical formulas, but you need to be able to state the name of the polymer and associated monomer :)
Post by: Cindy2k16 on October 31, 2016, 07:20:35 pm
I'm fairly sure you don't need to draw it, or know the empirical formulas, but you need to be able to state the name of the polymer and associated monomer :)
oh thank goodness. Thanks!
Post by: noonedoesnt on October 31, 2016, 08:38:23 pm
Post by: noonedoesnt on October 31, 2016, 08:39:57 pm
oh thank goodness. Thanks!
I can't recall the question, maybe from a trial paper, but it asked to draw our biopolymer. Mine was PHBV, which is difficult to draw and I didn't know what to do at the time lol
Post by: jakesilove on October 31, 2016, 08:40:53 pm
I can't recall the question, maybe from a trial paper, but it asked to draw our biopolymer. Mine was PHBV, which is difficult to draw and I didn't know what to do at the time lol
I seriously doubt that it is even assessable in an HSC exam
Post by: noonedoesnt on October 31, 2016, 08:41:29 pm
I seriously doubt that it is even assessable in an HSC exam
I would really hope not!
Post by: jakesilove on October 31, 2016, 08:44:09 pm
I got 4.65g, which is closest to B, which is the correct answer. Anyone have an idea why its 5.30g?
If you take the equation to be 1:1, then you get 0.05 moles of each substance, right? The molar mass of sodium carbonate is 106, so 0.05*106=5.3g
Post by: noonedoesnt on October 31, 2016, 08:46:46 pm
If you take the equation to be 1:1, then you get 0.05 moles of each substance, right? The molar mass of sodium carbonate is 106, so 0.05*106=5.3g
Oh thanks, stupid error. Calculated wrong molar mass, used molar mass of NaHCO3... thanks
Post by: jakesilove on October 31, 2016, 08:47:13 pm
Oh thanks, stupid error. Calculated wrong molar mass, used molar mass of NaHCO3... thanks
Fair enough! Shit happens, and you got the right answer anyway :)
Post by: Cindy2k16 on October 31, 2016, 08:51:48 pm
I got 4.65g, which is closest to B, which is the correct answer. Anyone have an idea why its 5.30g?
dumb question but whats the chemical equation for this reaction? i can't seem to balance it- my mind is still in a post-physics haze lmao
Edit: oh haha nevermind ive got it
another edit: The chemical equation I wrote doesnt work to get the correct answer. I get A instead :/ the mole ratio for Na2CO3, CH3COOH, CH3COONa, CO3, H2O in the equation I wrote is 1:2:2:1:1. Where did I stuff up?
Post by: jakesilove on October 31, 2016, 09:18:08 pm
dumb question but whats the chemical equation for this reaction? i can't seem to balance it- my mind is still in a post-physics haze lmao
Edit: oh haha nevermind ive got it
another edit: The chemical equation I wrote doesnt work to get the correct answer. I get A instead :/ the mole ratio for Na2CO3, CH3COOH, CH3COONa, CO3, H2O in the equation I wrote is 1:2:2:1:1. Where did I stuff up?
You're totally right, and you're totally wrong; based on research, it looks like the reaction could have been 1:1 OR 1:2. Initially, I actually got what you got, but changed my mind because I knew the answer. Basically, if a reaction could be EITHER 1:1 or 1:2, the HSC just won't assess you on it, because that's totally unfair.
Post by: Cindy2k16 on October 31, 2016, 09:20:58 pm
You're totally right, and you're totally wrong; based on research, it looks like the reaction could have been 1:1 OR 1:2. Initially, I actually got what you got, but changed my mind because I knew the answer. Basically, if a reaction could be EITHER 1:1 or 1:2, the HSC just won't assess you on it, because that's totally unfair.
I don't quite understand what you mean by it can be 1:1 or 1:2.. could u expand on that? TIA
Post by: jakesilove on October 31, 2016, 09:24:42 pm
I don't quite understand what you mean by it can be 1:1 or 1:2.. could u expand on that? TIA
Only that you can come up with an equation stating that
Acetic Acid + Sodium Carbonate --> Some products
The above reaction is 1:1
2*Acetic Acid + Sodium Carbon --> Different products
The above reaction is 2:1
Both of these equations seem to work, and so it's impossible to tell which one to use! In the HSC, you don't need to worry about that, because only one equation will work.
Post by: noonedoesnt on October 31, 2016, 09:26:03 pm
Post by: noonedoesnt on October 31, 2016, 09:29:34 pm
Post by: grace_joseph on October 31, 2016, 09:31:04 pm
Just wondering if someone could explain to me why ozone is polar? I know it's normally to do with electron distribution but I can't seem to figure this one out. Thanks :)
Post by: jakesilove on October 31, 2016, 09:32:29 pm
always get 164 g/mol. Can someone help pls
2*Sodium + 1*Carbon + 3*Oxygen = 2(22.99)+12+3(16)=106g
Post by: Cindy2k16 on October 31, 2016, 09:34:42 pm
Only that you can come up with an equation stating that
Acetic Acid + Sodium Carbonate --> Some products
The above reaction is 1:1
2*Acetic Acid + Sodium Carbon --> Different products
The above reaction is 2:1
Both of these equations seem to work, and so it's impossible to tell which one to use! In the HSC, you don't need to worry about that, because only one equation will work.
Oh I see. Thanks! And what do you mean that only one equation will work? If this was in a long answer question how would you know which one to use?
Actually I believe there was actually a titration question in the HSC in the past that had something similar to this and due to the production of CO2 the titration had 2 equivalence points...Though I think the HSC answer didn't require you to know that lol
Post by: jakesilove on October 31, 2016, 09:35:11 pm
Hi Jake,
Just wondering if someone could explain to me why ozone is polar? I know it's normally to do with electron distribution but I can't seem to figure this one out. Thanks :)
Basically, there's a weird number of electrons around a weird shaped object, right? It's configuration will be such that the electrons are as spread out as possible, as they are repulsive. Is it more likely for the 3-Oxygen molecule to be perfectly straight (linear), or just anything else? Obviously, it's just more likely for it to be in some other shape (probably bent, but who knows!). As it isn't linear, it is going to have to be polar (as somewhere there are going to be more electrons etc.). You don't really need to know why it's polar, just that it is :)
Post by: jakesilove on October 31, 2016, 09:36:28 pm
Oh I see. Thanks! And what do you mean that only one equation will work? If this was in a long answer question how would you know which one to use?
Actually I believe there was actually a titration question in the HSC in the past that had something similar to this and due to the production of CO2 the titration had 2 equivalence points...Though I think the HSC answer didn't require you to know that lol
All I'm saying is that this particular question wouldn't be asked, because there was more than one way of working it out, and it gives you two different answers. Formulas like Sodium Hydroxide and HCl are much more likely, as there is a clear answer. The HSC won't give you anything vague, or anything that you have a 50% chance of getting right! Don't worry about it :)
Post by: noonedoesnt on October 31, 2016, 09:43:13 pm
Post by: Cindy2k16 on October 31, 2016, 09:48:04 pm
All I'm saying is that this particular question wouldn't be asked, because there was more than one way of working it out, and it gives you two different answers. Formulas like Sodium Hydroxide and HCl are much more likely, as there is a clear answer. The HSC won't give you anything vague, or anything that you have a 50% chance of getting right! Don't worry about it :)
Oh ok thank you!
Also some other questions..
1. Is there a simple equation for a radioisotope produced in a cyclotron? The one I have (Iodine 123) is quite complicated so it's hard to memorise and I was wondering if there were any more basic ones?
2. Do we need to know how our chosen radioisotopes (medical, industrial) are produced?
TIA
Post by: jakesilove on October 31, 2016, 09:49:23 pm
sorry, uploaded wrong question earlier.
We know that
After realising it's a 1:1 reaction, and noting that the moles of each substance must be the same. Therefore
Therefore, in 100mL, there would be
If 1 gram is that many moles, then
Is that the answer? I'm seriously tired and I may have made a calculation error, but that's the method :)
Post by: jakesilove on October 31, 2016, 09:51:19 pm
Oh ok thank you!
Also some other questions..
1. Is there a simple equation for a radioisotope produced in a cyclotron? The one I have (Iodine 123) is quite complicated so it's hard to memorise and I was wondering if there were any more basic ones?
2. Do we need to know how our chosen radioisotopes (medical, industrial) are produced?
TIA
I definitely didn't have a cyclotron reaction up my sleeve when I went into the HSC! That's the sort of thing I would only try to learn if you've done absolutely everything else. As for the production of your radioisotope, maybe have a general idea (ie. cyclotron or, more likely, small-scale nuclear reactor and nuclear decay) so you can write half a sentence on it to sound smart, but you really don't need anything in depth.
Post by: noonedoesnt on October 31, 2016, 09:54:03 pm
We know that
After realising it's a 1:1 reaction, and noting that the moles of each substance must be the same. Therefore
Therefore, in 100mL, there would be
If 1 gram is that many moles, then
Is that the answer? I'm seriously tired and I may have made a calculation error, but that's the method :)
yeah thats the answer, thanks heaps
Post by: jakesilove on October 31, 2016, 09:56:50 pm
yeah thats the answer, thanks heaps
No problem! Aha I'm super glad I got that right.
Post by: massive on October 31, 2016, 10:17:42 pm
Post by: wesadora on October 31, 2016, 10:18:20 pm
Oh ok thank you!
Also some other questions..
1. Is there a simple equation for a radioisotope produced in a cyclotron? The one I have (Iodine 123) is quite complicated so it's hard to memorise and I was wondering if there were any more basic ones?
2. Do we need to know how our chosen radioisotopes (medical, industrial) are produced?
TIA
if you're just referring to a 'radioisotope' and not a transuranic (i have one for this - californium, but its not as simple), I just use one for Fluorine-18.
Bombardment of nitrogen-14 (which is just how it exists normally, check periodic table) with a normal helium nuclei, to make Fluorine-18. Adds up nicely and pretty simple, just adding standard nitrogen and helium! :)
Note - fluorine-18 is a neutron deficient radioisotope, which you'll find is what happens when you do this kind of light nuclei bombardment in cyclotrons (as opposed to neutron bombardment, which creates neutron rich radioisotopes) because the neutron to proton ratio for a stable atom increases the higher you go up the periodic table (e.g. helium is 1:1, while platinnum is exactly 1.5)
Post by: wesadora on October 31, 2016, 10:31:35 pm
guyyys how do you do the q attached, the answer is c
all you're doing, is dividing the molar mass of propylene (given) by the molar mass of propene - in order to find out how many propenes there are in the entire propylene chain (molecule). You know there are 3 carbons per propene, and so times the number of propenes by 3, to find the total number of carbons. lel
Post by: jamtimsal on October 31, 2016, 10:41:41 pm
Quote
Petroleum and sugar cane are both raw materials used for the production of ethanol. Construct separate flow diagrams for the production of ethanol from each raw material. (5 marks)
Petroleum
Sugar Cane
(Required to draw separate flow charts -->)
Pretty sure this was a past HSC question but can't find it at the moment.
Just wondering what level of detail is required for the 5 marks. I remember the sample answer being almost unnecessarily specific (like ratio's of chemicals and fully explaining the fermentation process)
Post by: Cindy2k16 on October 31, 2016, 11:18:59 pm
eg soap is RCOO-Na+
Also when describing experiments is it necessary to have quantities eg. 10mL of water or is describing qualitatively sufficient?
TIA
Post by: RuiAce on November 01, 2016, 06:05:08 am
Hi for industrial chemistry are we required to be able to write the chemical formula of a fat/oil? Or can we write the process of saponification with a general equation?Umm I think RCOO- Na+ will suffice. I haven't ever come across a question that needed me to specifically write out a correct chemical formula for one of them.
eg soap is RCOO-Na+
Also when describing experiments is it necessary to have quantities eg. 10mL of water or is describing qualitatively sufficient?
TIA
Though I did learn the "general" structure of it so that I could draw them. I found describing the structure of soaps and etc. hard so I drew a diagram to aid my description.
Unfortunately you will have to make a judgment on that one as for things such as esterification you should be writing quantitatively. Think about where quantities actually won't matter (potentially, experiments without calculations), then you can write something qualitative (e.g. 3 drops)
Post by: jakesilove on November 01, 2016, 09:36:56 am
Pretty sure this was a past HSC question but can't find it at the moment.
Just wondering what level of detail is required for the 5 marks. I remember the sample answer being almost unnecessarily specific (like ratio's of chemicals and fully explaining the fermentation process)
For a 5-8 markers, I would definitely be unnecessarily specific to ensure you get full marks. It takes four minutes to go through the whole fermentation process, and that's worth getting an extra mark for! Go through each step methodically, with every piece of information relevant in answering the question.
Post by: noonedoesnt on November 01, 2016, 10:12:25 am
Post by: RuiAce on November 01, 2016, 10:20:10 am
picked B, answer was ACareful, B is more suitable for the anode, not the cathode.
Because Pt is your cathode, you probably want something that involves a gas/aqueous half-cell.
Post by: JemexR on November 01, 2016, 01:23:40 pm
Post by: noonedoesnt on November 01, 2016, 01:29:17 pm
Post by: RuiAce on November 01, 2016, 01:34:36 pm
Could someone please do the working for HSC 2004 Q13 please? I have a ridiculously tiny number at the moment (not enough change in temperature). ThanksPlease provide the question or a link to the paper in the future
Do we need to know the structure of amino acids as a condensation polymer?Amino acids aren't in the HSC chemistry course...
Edit: Those notes ring a bell. Nothing wrong with it but I feel there's an overloaded amount of stuff there to memorise
Post by: wesadora on November 01, 2016, 02:22:57 pm
This makes sense calculation wise for tritrations to me (n=cv), but, I thought that for neutralisation, it's based on H+ + OH- --> H2O, and the concentration of H+ in weak acids is less than the concentration of the acid, wouldn't this affect the volume neeeded for neutralisation?
but it doesn't. ._.
Post by: noonedoesnt on November 01, 2016, 02:28:23 pm
why is it that the strength of an acid/base does not affect the volume required for titration (rather, the concentration of the acids affects the volumes)? E.g. if both acid/base solutions are 0.1M it does not matter whether one or the other is weak or strong - the same volumes for neutralisation will be required.
This makes sense calculation wise for tritrations to me (n=cv), but, I thought that for neutralisation, it's based on H+ + OH- --> H2O, and the concentration of H+ in weak acids is less than the concentration of the acid, wouldn't this affect the volume neeeded for neutralisation?
but it doesn't. ._.
I believe in a strong acid + strong base the equal moles of H+ and OH- neutralise, and the reaction goes to completion.
However in a weak acid + strong base, the equation is an equilibrium equation and the strong base forces the creation of more H+ as it reacts with OH-, thus the same volumes are required.
That's what I think anyways, Rui will be able to elaborate a bit more.
Post by: RuiAce on November 01, 2016, 02:31:16 pm
why is it that the strength of an acid/base does not affect the volume required for titration (rather, the concentration of the acids affects the volumes)? E.g. if both acid/base solutions are 0.1M it does not matter whether one or the other is weak or strong - the same volumes for neutralisation will be required.This is the exact same reaction that occurs in all titrations. Which is why we never use a weak base and weak acid because that will cause concentrations to matter too much.
This makes sense calculation wise for tritrations to me (n=cv), but, I thought that for neutralisation, it's based on H+ + OH- --> H2O, and the concentration of H+ in weak acids is less than the concentration of the acid, wouldn't this affect the volume neeeded for neutralisation?
but it doesn't. ._.
At the instant one of the species is strong, things happen. Note that the weak one, say, weak acid, does not fully ionise by itself.
But what happens when you add a strong base? The OH-'s there are fully ionised. Clearly the OH- will react with the H+.
Thus the weak acid lost H+. Hence by Le chatelier's principle, the system will want to replace the most H+, thus more of it gets ionised.
But this H+ also gets used by the OH-. Hence the weak acid is ultimately forced to keep ionising until all the OH- is gone. So ultimately the H+ gets forced into ionising completely because the base is strong and is, in a way, "hydroxide abundant"
Post by: wesadora on November 01, 2016, 02:43:19 pm
legends
Post by: kimmie on November 01, 2016, 03:08:51 pm
I uploaded what I did but the answer is 1.20 V
(http://uploads.tapatalk-cdn.com/20161031/771c311ebc62cba50e878f715827ab90.jpg)
(http://uploads.tapatalk-cdn.com/20161031/532d8069a0e45c8af3d4783044ff91b6.jpg)
Post by: wesadora on November 01, 2016, 03:34:19 pm
Hey for these types of Qs, how do you know that 0.16 turns to -0.16?
I uploaded what I did but the answer is 1.20 V
(http://uploads.tapatalk-cdn.com/20161031/771c311ebc62cba50e878f715827ab90.jpg)
(http://uploads.tapatalk-cdn.com/20161031/532d8069a0e45c8af3d4783044ff91b6.jpg)
remember, the table is of standard reduction potentials. Since SO2 is being oxidised, its -0.16V (reduction potential of SO42- is 0.16V, as you read off the table, but in this case the reaction is SO2 being oxidised (backwards reaction to what's shown on your reduction table- so SO2 + 2H2O --> SO42- + 4H+ + 2e-...and so you flip the sign and it's -0.16V). 1.36V + (-0.16V) = 1.2
Post by: JemexR on November 01, 2016, 04:20:34 pm
How do you convert mg/L -> mL?
It is the 2009 Multiple choice Q 15
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2009exams/pdf_doc/2009-hsc-exam-chemistry.pdf
Post by: ProfLayton2000 on November 01, 2016, 04:21:56 pm
Post by: wesadora on November 01, 2016, 04:42:28 pm
Just a quick converting question
How do you convert mg/L -> mL?
It is the 2009 Multiple choice Q 15
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2009exams/pdf_doc/2009-hsc-exam-chemistry.pdf
for that question, you gotta convert mg/L to mg/10L (as per the Q) and so times mg by 10. Convert mg to g (times by 0.001). use n=m/M to find number of moles (M = molar mass of oxygen, m = mass of oxygen in grams, which you worked out before).
Then, use V = n x 24.79, to find the volume in Litres. Convert Literes to mL. It's tricky, and not a straight conversion - but that's the process and it just requires you to know how your equations all link together REAAAALLY well. :P
Post by: wesadora on November 01, 2016, 04:50:04 pm
Is it true that nuclear reactors are used to make radioisotopes with atomic number <= 95 and accelerators (cyclotrons and linear accelerators) make radioisotopes with atomic number > 95?Nuclear reactors are used for neutron bombardment, which can be used for elements up to atomic number 98 (californium). Accelerators can be used for ANYTHING - you can make commercial radioisotopes less than atomic number 95 in linear accelerators, e.g. Nitrogen-14 + Helium-4 --> Fluorine-18. But, you can also use them to collide larger nuclei to make super large nucleuses (and pretty much all transuranic elements) beyond atomic number 98 that can't be produced in nuclear reactors (by neutron bombardment).
Post by: RuiAce on November 01, 2016, 04:50:29 pm
Is it true that nuclear reactors are used to make radioisotopes with atomic number <= 95 and accelerators (cyclotrons and linear accelerators) make radioisotopes with atomic number > 95?You can use whatever you want to make <=95 but for >95 you're bombarding too many protons and you're kinda forced to use accelerators. Because how are you going to get some element with atomic number 113 to stay there without decaying whilst you bombard with neutrons.
Though nuclear reactors would be more common than accelerators for <=95 I suppose
Post by: JemexR on November 01, 2016, 05:34:25 pm
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2010exams/pdf_doc/2010-hsc-exam-chemistry.pdf
For D : When the Ammonium and Chloride dissociate, wouldn't the positively charged ammonium ion ionise with the hydroxide in the water, creating water and ammonia?
And for HSC 2006 Q8, given that they have the same concentration, wouldn't that mean that they are equally dilute/concentrated?
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2006exams/pdf_doc/chemistry_06.pdf
Post by: proficles on November 01, 2016, 06:22:57 pm
Post by: kimmie on November 01, 2016, 06:45:15 pm
(http://uploads.tapatalk-cdn.com/20161101/5b0c111036fe3ad2d0728c29fbfad81a.jpg)
Post by: lha on November 01, 2016, 06:53:19 pm
Post by: RuiAce on November 01, 2016, 07:20:57 pm
Just wondering, what raw mark do you have to get for it to scale to 90?Use the raw marks database to see historic data.
Post by: RuiAce on November 01, 2016, 07:28:56 pm
For Q 18 of the 2010 paper, why is the answer C?So I'll assume that you figured out why A and B are wrong (neutral salts).
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2010exams/pdf_doc/2010-hsc-exam-chemistry.pdf
For D : When the Ammonium and Chloride dissociate, wouldn't the positively charged ammonium ion ionise with the hydroxide in the water, creating water and ammonia?
And for HSC 2006 Q8, given that they have the same concentration, wouldn't that mean that they are equally dilute/concentrated?
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2006exams/pdf_doc/chemistry_06.pdf
Be careful as to which is the conjugate acid and which is the conjugate base here. H+ is the conjugate acid of H2O
Yes, soon enough you'll have NH3 and what not forming but what happens at the start? What you're adding isn't NH3, it's NH4Cl. That's the first thing that comes into contact with the mixture
When the ammonium chloride dissociates, the NH4+ will rush to react with the H2O already there. This causes the equilibrium to shift to the RIGHT (as per LCP), which is what we DON'T want here.
Instead, by adding sodium acetate, the CH3COO- will rush to react with the H+ instead. This forces the equilibrium to shift to the left to produce the chromate ions that we do want.
Post by: RuiAce on November 01, 2016, 07:30:55 pm
For Q 18 of the 2010 paper, why is the answer C?
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2010exams/pdf_doc/2010-hsc-exam-chemistry.pdf
For D : When the Ammonium and Chloride dissociate, wouldn't the positively charged ammonium ion ionise with the hydroxide in the water, creating water and ammonia?
And for HSC 2006 Q8, given that they have the same concentration, wouldn't that mean that they are equally dilute/concentrated?
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2006exams/pdf_doc/chemistry_06.pdf
In question 8 they don't have equal concentrations. Acid X is 0.1mol L-1 and acid 2 is 1 mol L-1. So the second is more concentrated than the first because 1 > 0.1
Post by: RuiAce on November 01, 2016, 07:35:46 pm
Hi, this 2005 question has the answer as B, which i agree with. However i don't understand why option D is wrong, do electrons not flow through the voltmeter? Thanks in advance :)Because we can't "observe" it per se. Observe means that we see it happen.
A bit silly lol
Post by: lha on November 01, 2016, 07:36:07 pm
Use the raw marks database to see historic data.
What is reported exam mark?
Post by: RuiAce on November 01, 2016, 07:37:17 pm
What is reported exam mark?The first two columns are pretty much the same thing. It's just that because some options are harder they get a tiny bit scaled (usually at most +1 mark).
Treat the first column as accurate in my opinion, and compare it to the last column.
Post by: lha on November 01, 2016, 07:38:07 pm
The first two columns are pretty much the same thing. It's just that because some options are harder they get a tiny bit scaled (usually at most +1 mark).
Treat the first column as accurate in my opinion, and compare it to the last column.
So is the last column the scaled hsc mark?
Post by: RuiAce on November 01, 2016, 07:42:06 pm
Hey for this, I understand it's an exothermic reaction so the answer can be B or D only but how is the answer D? Do we look at the moles or something?Pressure is going to play a role here.
(http://uploads.tapatalk-cdn.com/20161101/5b0c111036fe3ad2d0728c29fbfad81a.jpg)
Observe that in reaction B, there are EQUAL moles of gas on the reactants and the products. This means that, if we should vary the pressure, it should NOT affect the position of the equilibrium, and thus the yield of product.
Whereas in equation D, there are MORE moles of gas in the reactants than on the products. If there are more moles of gas in the reactants, then higher pressures should FAVOUR the FORWARD reaction, and thus the yield is increased.
Which is what is demonstrated, so the answer is D.
So is the last column the scaled hsc mark?Yeah, basically
Post by: kimmie on November 01, 2016, 07:44:17 pm
Post by: RuiAce on November 01, 2016, 07:46:00 pm
Do we need to know the formula for cyclohexane and bromine water? Or just cyclohexene and bromine water?The reaction between cyclohexane and bromine water is similar.
It's just that:
a) You have to write UV on top of the arrow, cause otherwise the reaction doesn't happen
b) H2 gas is also produced
So it doesn't hurt to know that extra bit extra
Post by: kimmie on November 01, 2016, 07:48:22 pm
Post by: RuiAce on November 01, 2016, 07:49:21 pm
So like this? (http://uploads.tapatalk-cdn.com/20161101/3625d40158e3fd2730ee6be18c044f78.jpg)Yep excellent
Post by: bethjomay on November 01, 2016, 08:06:41 pm
For this dot point, where it says explain do we just need a chemical equation demonstrating it behaving as an acid/base?
Post by: RuiAce on November 01, 2016, 08:08:47 pm
"identify a range of salts which form acidic, basic or neutral solutions and explain their acidic, neutral or basic nature."Well you need to be able to relate to your equation properly.
For this dot point, where it says explain do we just need a chemical equation demonstrating it behaving as an acid/base?
Take something like sodium carbonate. It's basic because of the behaviour of the carbonate ion in water:
CO32- + H2O(l) <-> HCO3- + OH-
(Alternatively show how it reacts with an acid.)
But so long as you know how to refer to your equation then yeah you're all good.
Post by: bethjomay on November 01, 2016, 08:11:16 pm
Well you need to be able to relate to your equation properly.
Take something like sodium carbonate. It's basic because of the behaviour of the carbonate ion in water:
CO32- + H2O(l) <-> HCO3- + OH-
(Alternatively show how it reacts with an acid.)
But so long as you know how to refer to your equation then yeah you're all good.
Yep, that's what I have! Thank you!
Post by: MysteryMarker on November 01, 2016, 08:46:16 pm
Cheers.
Post by: RuiAce on November 01, 2016, 08:49:13 pm
Just a generic question, but for a question on 'Assess the effectiveness of AAS in pollution control', what would be some main points to talk about?Just compare with how poor historic methods were and how they really wouldn't find many things.
Cheers.
This would mostly be a history based question ending with realising oh wait AAS works miracles.
May be worth mentioning trace elements as well and how AAS helped reveal the presence and impacts of bad stuff like Pb2+
Post by: Sssssrr on November 01, 2016, 08:57:19 pm
thanks
Post by: RuiAce on November 01, 2016, 08:58:47 pm
for combustion in chemical monitoring, do we need to know about catalytic converters?Well I didn't. Maybe you should, but I didn't bother. I just remembered what catalytic cracking was and was like yep.
thanks
Post by: kimmie on November 01, 2016, 09:04:18 pm
(http://uploads.tapatalk-cdn.com/20161101/8cc0b1126ea60093005539a455d89d6e.jpg)
Post by: JemexR on November 01, 2016, 09:31:45 pm
Hey for this, I understand it's an exothermic reaction so the answer can be B or D only but how is the answer D? Do we look at the moles or something?
(http://uploads.tapatalk-cdn.com/20161101/5b0c111036fe3ad2d0728c29fbfad81a.jpg)
It's because of the pressure. As the pressure increases, the system will favour the side with less moles on it (Le Chatelier's Principle). B has equal moles of gas on both sides, therefore indicating that if B was the answer, pressure would not influence the yield of product. D is therefore the answer since there is less moles of gas on the side of the products when compared to the reactants, hence the system will favour the forward reaction if the pressure of the system increases.
Post by: JemexR on November 01, 2016, 09:33:10 pm
If they ask us to draw cellulose, would this be fine?
(http://uploads.tapatalk-cdn.com/20161101/8cc0b1126ea60093005539a455d89d6e.jpg)
I would play it safe and add the carbons -> you have around a minute a mark anyway, so how much extra time will adding a few carbons be? :) :P
Post by: jakesilove on November 01, 2016, 09:36:59 pm
I would play it safe and add the carbons -> you have around a minute a mark anyway, so how much extra time will adding a few carbons be? :) :P
I definitely agree, I would be drawing the entire molecule :)
Post by: RuiAce on November 01, 2016, 09:41:06 pm
If they ask us to draw cellulose, would this be fine?Draw the whole thing out with the carbons. It's boring but not hard and things are clearer that way
(http://uploads.tapatalk-cdn.com/20161101/8cc0b1126ea60093005539a455d89d6e.jpg)
Post by: onepunchboy on November 01, 2016, 09:55:22 pm
Post by: RuiAce on November 01, 2016, 09:55:56 pm
hello sorry if this is a stupid question, but what are the solubility rules used for? LOLa) To make sure you don't write down wrong states in your equation when things precipitate
b) cation/anion tests
Post by: massive on November 01, 2016, 09:59:20 pm
Also, what affect does increasing the concentration have on the equilibrium if some of the reactant/products are solids or liquids? does it matter??
Post by: RuiAce on November 01, 2016, 10:07:02 pm
guys for equilibrium reactions, do we say: "the change in conditions causes system to adjust and counteract the changes and therefore favour the forward reaction" OR do we say "the change in conditions causes the system to adjust and counteract the changes by shifting the equlibrium to the right"The things you said in bold are equivalent; they're both ok. But in either case you must explicitly state the words "Le Chatelier's principle"
Also, what affect does increasing the concentration have on the equilibrium if some of the reactant/products are solids or liquids? does it matter??
Increasing the concentration of solids and liquids do nothing. Because the increase in volume for a solid/liquid counteracts the increase in moles.
Post by: massive on November 01, 2016, 10:17:38 pm
Because the increase in volume for a solid/liquid counteracts the increase in moles.Thanks Rui, I get everything except this part, what do you mean??
Post by: RuiAce on November 01, 2016, 10:22:25 pm
Thanks Rui, I get everything except this part, what do you mean??In reality too technical and not required for the HSC course. Just take it for granted.
The way I see it, you can't increase the "concentration" of a solid or liquid.
Post by: Cindy2k16 on November 01, 2016, 10:36:18 pm
TIA
Post by: RuiAce on November 01, 2016, 10:39:36 pm
Hi am i correct in thinking that we only need to know either Zeiglar Natta polymerisation or free radical polymerisation in depth (and just have a simple understanding of the other process? Not both?Actually I think the process is the exact same. The only difference is that the former uses the Ziegler-Natta catalyst, whereas the latter uses a peroxide initiator.
TIA
It's activation -> propagation -> termination in all cases.
Post by: onepunchboy on November 01, 2016, 10:52:16 pm
How do i do q7 ? P.s the value of k is 1.7 x 10^-10
Thanksss
Post by: lha on November 01, 2016, 11:00:16 pm
Post by: massive on November 02, 2016, 01:28:37 am
Post by: massive on November 02, 2016, 03:06:15 am
Post by: onepunchboy on November 02, 2016, 04:09:52 am
Can someone explain question 8 and 9? Thank you , much appreciated!
Post by: RuiAce on November 02, 2016, 07:46:27 am
DO we write esterification reactions with an equilibrium or forward arrow?Esterification is reversible so equilibrium. In fact the equilibrium lies heavily to the left - hardly any products are formed.
do we always write aqueous for the state for salts??If they precipitate no, otherwise yes
guys how do you do neutralisation reactions with oxides?? :SYou lost me on this one. Example please
Post by: RuiAce on November 02, 2016, 07:53:40 am
(http://uploads.tapatalk-cdn.com/20161101/4e7cbf2f6223e7c0a155d76c7495e597.jpg)Interesting question
How do i do q7 ? P.s the value of k is 1.7 x 10^-10
Thanksss
Post by: RuiAce on November 02, 2016, 07:56:16 am
(http://uploads.tapatalk-cdn.com/20161101/5dd4c7591ebd49da64a74ebe716a45f9.jpg)For Q9 A and C are obviously wrong because one is a weak base and the other is strong
Can someone explain question 8 and 9? Thank you , much appreciated!
B is also wrong because they have different molar masses, so if n is the same, then because n=m/MM we have different masses
Similar discussions on why D is the correct answer can be found in posts #1170 and #1171
Post by: lha on November 02, 2016, 08:37:40 am
Post by: RuiAce on November 02, 2016, 08:39:34 am
WHen we talk about ethanol from renewable sources, do we talk about sugar cane or cellulose?Wouldn't you briefly mention both, because cellulose is derived from sugar cane?
Post by: lha on November 02, 2016, 08:40:53 am
Wouldn't you briefly mention both, because cellulose is derived from sugar cane?
So how would i say it?
Post by: RuiAce on November 02, 2016, 08:43:15 am
So how would i say it?Give a brief mention to what sugar cane is and why it's renewable, and then just explain that you can get cellulose from it to which you can get ethanol from cellulose.
Think about it logically. How would you structure it so that it makes sense to someone.
Everyone writes differently; I can't tell you "how" to write an extended response, or bits and pieces of it.
Post by: lha on November 02, 2016, 08:45:33 am
Give a brief mention to what sugar cane is and why it's renewable, and then just explain that you can get cellulose from it to which you can get ethanol from cellulose.
Think about it logically. How would you structure it so that it makes sense to someone.
Everyone writes differently; I can't tell you "how" to write an extended response, or bits and pieces of it.
I say that sucrose from sugar cane is hydrolysed to form glucose, then glucose ia fermented to get ethanol. Where does the cellulose come in?
Post by: wesadora on November 02, 2016, 08:52:31 am
I say that sucrose from sugar cane is hydrolysed to form glucose, then glucose ia fermented to get ethanol. Where does the cellulose come in?Cellulose is a constituent of sugar cane. Sugar cane = biomass. ALL biomass contains cellulose! Just say sugar cane is a source of cellulose.
Post by: RuiAce on November 02, 2016, 08:53:27 am
I say that sucrose from sugar cane is hydrolysed to form glucose, then glucose ia fermented to get ethanol. Where does the cellulose come in?Now that's going beyond the syllabus.
To briefly summarise it though (for sake of completeness), sucrose is the DIMER formed in condensation polymerisation. Two beta-glucose monomers form this sucrose molecules, and it's them which 'polymerise' into cellulose. So what actually happens is that you have your cellulose, which you break down into sucrose first, which then gets further broken down into glucose.
Post by: wesadora on November 02, 2016, 08:59:04 am
"A solution was made by mixing sodium acetate solid and a solution of acetic acid. Use an equation to predict and explain the impact on the pH of the solution of adding a very small volume of hydrochloric acid to this mixture".
I said: The impact of the mixture is negligible due to the buffer solution of CH3COOH and CH3COONa, as the CH3COO- ion is a weak conjugate base of the weak acid CH3COONa (as per required for a buffer solution). When HCl is added: HCl(aq) + CH3COONa(aq) --> NaCl (aq) + CH3COOH(aq), it is neutralised. pH change will thus be resisted, creating a weaker acid and neutral salt.
I got 1/2...is this wrong/how could I have gotten full marks? There was a cross on top of my equation - was this where I went wrong?
Thanks! 😊
Post by: RuiAce on November 02, 2016, 09:15:37 am
So I was looking through my trial paper:1. It is not negligible, because something small happens and it gets recovered but something still happens that's not minuscule.
"A solution was made by mixing sodium acetate solid and a solution of acetic acid. Use an equation to predict and explain the impact on the pH of the solution of adding a very small volume of hydrochloric acid to this mixture".
I said: The impact of the mixture is negligible due to the buffer solution of CH3COOH and CH3COONa, as the CH3COO- ion is a weak conjugate base of the weak acid CH3COONa (as per required for a buffer solution). When HCl is added: HCl(aq) + CH3COONa(aq) --> NaCl (aq) + CH3COOH(aq), it is neutralised. pH change will thus be resisted, creating a weaker acid and neutral salt.
I got 1/2...is this wrong/how could I have gotten full marks? There was a cross on top of my equation - was this where I went wrong?
Thanks! 😊
2. You did not properly relate to Le Chatelier's principle
3. You did not write the equation of the buffer system (identification that there was a buffer was probably the mark)
When the buffer system was formed, the following equilibrium was established (noting that the Na+ is a neutral, spectator cation.
CH3COO- + H3O+ <-> CH3COOH(aq) + H2O(l)
Because the solution was made by combining the substances, both of which are a weak base/acid respectively, the above buffer system is formed. The buffer will resist small changes in pH, which you mentioned but not explained in the correct way.
When a small quantity of HCl is added, we introduce some H3O+ into the system. So by LCP, the equilibrium will shift to the right, producing more acetic acid and water.
Now the purpose of a buffer to resist pH changes is like so. There are two things to keep in mind.
1. By shifting the equilibrium to the right, we have eliminated some of the strong HCl and introduced more weak CH3COOH.
2. The fact that there is both CH3COO- AND CH3COOH imply that the pH of the mixture is quite stable. There's both acidic and basic substances in there, and hence the concentrations of acidic and basic substances are pretty stable, implying the pH doesn't change so easily.
In adding the strong acid, note that by LCP it just gets converted to a weak acid, which is a part of the buffer. Hence the pH will increase, however by a smaller amount than if the buffer was not there.
You can determine where you lost the marks and what you would've needed to get them. Obviously since it was 2 marks not AS much depth is required here, but you can determine where the mark lost was.
Post by: WLalex on November 02, 2016, 09:18:20 am
Post by: wesadora on November 02, 2016, 09:20:24 am
Post by: RuiAce on November 02, 2016, 09:21:51 am
Im not getting any of these answers...If citric acid is triprotic then 3 moles of NaOH will react with 1 mol of citric acid.
So nC3H4OH(COOH)3 = 1/3 * nNaOH = 1/3 *CV = 3 * 0.550 * 0.02950 = 5.408...*10-3 mol
Converting the moles to mass using m=nMM we have m = 5.408...*10-3 *192.12 = 1.039049 g
Then just divide by 0.025
Edit: Cheers for picking the mistake up below me
Post by: wesadora on November 02, 2016, 09:29:44 am
Im not getting any of these answers...make sense?
Post by: wesadora on November 02, 2016, 09:32:27 am
If citric acid is triprotic then 3 moles of NaOH will react with 1 mol of citric acid.
So nC3H4OH(COOH)3 = 3 * nNaOH = 3 *CV = 3 * 0.550 * 0.02950 = 0.048675 mol
Converting the moles to mass using m=nMM we have m = 0.048675*192.12 = 9.351441 g
Then just divide by 0.025
sorry Rui...aren't you meant to divide by 3, not times by 3? 3 moles of NaOH will react with Citric. Therefore number of moles of citric will be 1/3 of NaOH.
edit: lol that's ok :)
Post by: massive on November 02, 2016, 10:09:00 am
You lost me on this one. Example pleaseLike you know how one of the condition for an acidic oxide is that it neutralises bases to form salt and water. How can you show that using an equation i.e. what's the general equation?
Post by: RuiAce on November 02, 2016, 10:14:16 am
Like you know how one of the condition for an acidic oxide is that it neutralises bases to form salt and water. How can you show that using an equation i.e. what's the general equation?There isn't a "general equation" per se. Because take aluminium oxide, which is amphoteric. If you look at how it reacts with an acid that's pretty straight forward but it reacts in the most peculiar ways possible with an acid. (Or other way around, I don't remember. This is mostly independent research)
Pretty sure Jake would have some. He literally made a whole list of chemical equations for you to use
Otherwise I'm certain that the Jacaranda textbook has a few. (But yeah, they're all examples. There is no "general" rule of thumb here)
Post by: RuiAce on November 02, 2016, 10:15:32 am
hey, would anyone happen to know when PLA was first introduced as a plastic and statistics of how commonly it is used?Not required.as per the syllabus.
use available evidence to gather and
present data from secondary sources
and analyse progress in the recent
development and use of a named
biopolymer. This analysis should
name the specific enzyme(s) used
or organism used to synthesise the
material and an evaluation of the
use or potential use of the polymer
produced related to its properties
Post by: Furrrball on November 02, 2016, 10:32:40 am
Post by: RuiAce on November 02, 2016, 10:38:44 am
I don't understand why the answer is d..(http://imgur.com/a/t9uko)A is just wrong because those aren't even -OH groups, they're just hydrogens. That's not going to be a site for polymerisation
B is wrong because that double bond shouldn't be there. Looking at the polymer clearly we only have single bonds there and two hydrogens leaning off each carbon.
C has the same reason as A
So in a way D is correct by process of elimination. You can also check D is correct by trying to join the two monomers to form the dimer. The fact that -OH groups are hanging off each side guarantee that we have a site for condensation polymerisation (recall: a small molecule such as water falls off during the polymerisation process). The monomers are right - the carbons off the alkane have no double bond and the carbons hanging off the benzene ring are indeed bonded with the oxygens
Post by: sherlockian on November 02, 2016, 10:46:12 am
Post by: RuiAce on November 02, 2016, 10:50:06 am
Does anyone know the vanadium redox cell cathode reaction? I just realised that I had 2e- on the LHS (which is what it says in my textbook) but all other sources are stating there is only one electron on the LHS. Thanks!Everything says 1.
Check your charges. Do they balance out? If not, then you know where the mistake is.
Post by: WLalex on November 02, 2016, 11:15:46 am
make sense?
Yes thank you to both you and Rui :)
Post by: Albertenouttaten on November 02, 2016, 11:18:22 am
I got a few questions so bare with me;
1) When you are asked to write the redox reactions and say one equasion gives 2 electrons and other equation accept 1 electron. When writing the two redox reaction, do you write them balanced or do you just balance them when you are writing your final net ionic equation?
2) Would Ammonia be considered a raw material in the Solvay process? (Industrial Chemistry Option topic)
3)Is a weak acid weak base neutralization an exothermic reaction?
4) When trying to determine the ions present in a solution, typically you would add in Nitric acid first to bubble off the CO3- ions (carbonate ions), what would be the equation for this reaction?
5) Do we have to remember every step in the AAS process (Atomic Absorption Spectrometry)?
Thanks to everyone who answers and very sorry for the large amount of questions!
Post by: Krijan on November 02, 2016, 11:22:45 am
I was reading over my notes and it seems like i forgot how to name isomers and stuff, so like for example C2H2CL2F2, and there is a structure there, and ur job is to name it, in this case its 1,1-dichloro-2,2-difluroethane. The issue with me is that i cant remember how to name that, was that some sort of rule?
please help hahah
Thanks
Post by: nimasha.w on November 02, 2016, 11:27:45 am
Post by: RuiAce on November 02, 2016, 11:47:54 am
Hi everyone,1. Given your half equations the way you put it, to combine them into the redox reaction equation you would have to multiply 2 to both the reactants and products, of the one with 1 electron. I don't fully understand what you're saying even though it looks like you probably know what you're doing
I got a few questions so bare with me;
1) When you are asked to write the redox reactions and say one equasion gives 2 electrons and other equation accept 1 electron. When writing the two redox reaction, do you write them balanced or do you just balance them when you are writing your final net ionic equation?
2) Would Ammonia be considered a raw material in the Solvay process? (Industrial Chemistry Option topic)
3)Is a weak acid weak base neutralization an exothermic reaction?
4) When trying to determine the ions present in a solution, typically you would add in Nitric acid first to bubble off the CO3- ions (carbonate ions), what would be the equation for this reaction?
5) Do we have to remember every step in the AAS process (Atomic Absorption Spectrometry)?
Thanks to everyone who answers and very sorry for the large amount of questions!
2. Yes but not sure if the term "raw materials" is necessary. The good thing is that it gets recycled.
3. All neutralisations are exothermic, including weak acid + weak base. It's always H+ + OH- -> H2O which is an exothermic reaction.
4. CO32- + 2 HNO3(aq) -> CO2(g) + H2O(l) + 2 NO3-
5. I did but according to Jake you don't have to. Might want to double check with him.
Post by: ProfLayton2000 on November 02, 2016, 11:49:12 am
Hi everyone,
I got a few questions so bare with me;
1) When you are asked to write the redox reactions and say one equasion gives 2 electrons and other equation accept 1 electron. When writing the two redox reaction, do you write them balanced or do you just balance them when you are writing your final net ionic equation?
2) Would Ammonia be considered a raw material in the Solvay process? (Industrial Chemistry Option topic)
3)Is a weak acid weak base neutralization an exothermic reaction?
4) When trying to determine the ions present in a solution, typically you would add in Nitric acid first to bubble off the CO3- ions (carbonate ions), what would be the equation for this reaction?
5) Do we have to remember every step in the AAS process (Atomic Absorption Spectrometry)?
Thanks to everyone who answers and very sorry for the large amount of questions!
1. When you write your half equations, you can keep them as you see on the data sheet (eg Zn->Zn2+ + 2e-, Ag+ + e- -> Ag), but when it comes to writing full redox reaction, you must balance it such that the number of electrons released by the oxidation reaction match the ones gained by the reduction reaction (eg. 2Ag+ + Zn -> Ag + Zn2+)
2. I do industrial Chem so Idk
3. Any acid base neutralisation is exothermic, but less so that strong acid/strong base neutralisations.
4. 2HNO3 + CO3 2- --> CO2 + H2O + 2NO3- (CO2 is gas, water is liquid and rest is aq)
5. Fairly sure! There is a monochromatic light source, a flame with the sample vapourised, a prism and detector which measures % absorbance. (While you're at it make sure you know it's advantages and practical uses)
*Can someone check this for correctness plz*
Post by: RuiAce on November 02, 2016, 11:51:09 am
Hii Im Krijan,There's heaps of "rules", but depending on what you get given you only apply the rules that are relevant.
I was reading over my notes and it seems like i forgot how to name isomers and stuff, so like for example C2H2CL2F2, and there is a structure there, and ur job is to name it, in this case its 1,1-dichloro-2,2-difluroethane. The issue with me is that i cant remember how to name that, was that some sort of rule?
please help hahah
Thanks
Because you just typed the chemical formula we aren't really sure as to what you mean. Naming questions involve a structural formula
So you're better off providing an example and ask us to guide you through the rules you need. Feel free to provide 2 at a time though (unless they're like identical)
Post by: RuiAce on November 02, 2016, 11:51:52 am
hey i don't know if this makes sense but when you have to calculate the hydrogen ion concentration of a diprotic or triprotic acid, do you have to find the hydrogen ion concentration and then divide by 2/3?For the moles of the actual acid itself? Yes
Post by: Krijan on November 02, 2016, 11:58:44 am
There's heaps of "rules", but depending on what you get given you only apply the rules that are relevant.
Because you just typed the chemical formula we aren't really sure as to what you mean. Naming questions involve a structural formula
So you're better off providing an example and ask us to guide you through the rules you need. Feel free to provide 2 at a time though (unless they're like identical)
I have attached, the structure image, But in simple words, just say in exam i was to get this structure and was asked to name it, can you please tell me how would i possibly name it?
Thank You :)
Post by: RuiAce on November 02, 2016, 12:02:28 pm
I have attached, the structure image, But in simple words, just say in exam i was to get this structure and was asked to name it, can you please tell me how would i possibly name it?So there are 3 fluorine atoms and 2 chlorine atoms.
Thank You :)
First thing I think about when I see a CFC - alphabetical order.
Second thing I think about when I see a CFC - minimise the sum of the 'locants' (i.e. the numbers I give).
That's how I decide which carbon I'm supposed to start on.
So notice how, preserving alphabetical order, the two options are:
1,1-dichloro-2,2,2-trifluoroethane (counting left to right)
2,2-dichloro-1,1,1-trifluoroethane (counting right to left)
Sum up the locants in each case:
1+1+2+2+2=8
2+2+1+1+1=7
Clearly the second one has a smaller sum, so the answer is
2,2-dichloro-1,1,1-trifluoroethane
Post by: jakesilove on November 02, 2016, 12:04:24 pm
I have attached, the structure image, But in simple words, just say in exam i was to get this structure and was asked to name it, can you please tell me how would i possibly name it?
Thank You :)
First, note that the groups (chlorine, flourine etc.) need to be named in alphabetical order. So, Chlorine then Flourine.
Note that there are two carbons. So, ethane group.
If there are two, we put the prefix 'di'. If there are three, we put the prefix 'tri'.
We number EACH of the molecules. So, in this case, we get something like
2,2-dichloro-1,1,1-trifluroethane.
Note that the numbers should be the minimum possible total value. We could have named the compound
1,1-dichloro-2,2,2-trifluroethane
But that would add up to a total number that is more than our original name!
Post by: :3 on November 02, 2016, 12:07:31 pm
For question B), why is the answer calculating the percentage of CO2 in the dry ice? Since it's asking for the purity, don't we find the mass of ice then find the percentage of that?
(https://i.gyazo.com/0ef04889f57fc4d5767e90c90698ca40.png)
Thanks.
Post by: JemexR on November 02, 2016, 12:13:04 pm
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2010exams/pdf_doc/2010-hsc-exam-chemistry.pdf
For D : When the Ammonium and Chloride dissociate, wouldn't the positively charged ammonium ion ionise with the hydroxide in the water, creating water and ammonia?
And for HSC 2006 Q8, given that they have the same concentration, wouldn't that mean that they are equally dilute/concentrated?
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2006exams/pdf_doc/chemistry_06.pdf
Post by: Krijan on November 02, 2016, 12:15:36 pm
Post by: Furrrball on November 02, 2016, 12:20:33 pm
Hiya,
For question B), why is the answer calculating the percentage of CO2 in the dry ice? Since it's asking for the purity, don't we find the mass of ice then find the percentage of that?
(https://i.gyazo.com/0ef04889f57fc4d5767e90c90698ca40.png)
Thanks.
It's talking about dry ice-that's solid carbon dioxide :)
Post by: jakesilove on November 02, 2016, 12:23:26 pm
Hiya,
For question B), why is the answer calculating the percentage of CO2 in the dry ice? Since it's asking for the purity, don't we find the mass of ice then find the percentage of that?
(https://i.gyazo.com/0ef04889f57fc4d5767e90c90698ca40.png)
Thanks.
Dry ice IS Carbon dioxide. So, finding the amount of CO2, and dividing that by the total mass, will get you the purity! They should specify that more explicitly in modern exams.
Post by: :3 on November 02, 2016, 12:25:47 pm
It's talking about dry ice-that's solid carbon dioxide :)
I'm so silly, of course it is.
Thank you. :)
Post by: jakesilove on November 02, 2016, 12:28:42 pm
For Q 18 of the 2010 paper, why is the answer C?
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2010exams/pdf_doc/2010-hsc-exam-chemistry.pdf
For D : When the Ammonium and Chloride dissociate, wouldn't the positively charged ammonium ion ionise with the hydroxide in the water, creating water and ammonia?
And for HSC 2006 Q8, given that they have the same concentration, wouldn't that mean that they are equally dilute/concentrated?
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2006exams/pdf_doc/chemistry_06.pdf
For question 18, all I can really say is that sodium acetate is a basic salt, and ammonium chloride is an acidic salt. We want to add a base, so that the equilibrium shifts to the left, and so we add sodium acetate! I was always bad with this sort of thing, so can't help out more than that.
For question 8, they don't actually have the same concentration? One is 0.1mol/L, the other is 1mol/L. Does that fix your issue?
Post by: RuiAce on November 02, 2016, 12:31:21 pm
For Q 18 of the 2010 paper, why is the answer C?Is there a particular reason why you asked the same questions again?
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2010exams/pdf_doc/2010-hsc-exam-chemistry.pdf
For D : When the Ammonium and Chloride dissociate, wouldn't the positively charged ammonium ion ionise with the hydroxide in the water, creating water and ammonia?
And for HSC 2006 Q8, given that they have the same concentration, wouldn't that mean that they are equally dilute/concentrated?
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2006exams/pdf_doc/chemistry_06.pdf
For Q 18 of the 2010 paper, why is the answer C?
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2010exams/pdf_doc/2010-hsc-exam-chemistry.pdf
For D : When the Ammonium and Chloride dissociate, wouldn't the positively charged ammonium ion ionise with the hydroxide in the water, creating water and ammonia?
And for HSC 2006 Q8, given that they have the same concentration, wouldn't that mean that they are equally dilute/concentrated?
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2006exams/pdf_doc/chemistry_06.pdf
So I'll assume that you figured out why A and B are wrong (neutral salts).
Be careful as to which is the conjugate acid and which is the conjugate base here. H+ is the conjugate acid of H2O
Yes, soon enough you'll have NH3 and what not forming but what happens at the start? What you're adding isn't NH3, it's NH4Cl. That's the first thing that comes into contact with the mixture
When the ammonium chloride dissociates, the NH4+ will rush to react with the H2O already there. This causes the equilibrium to shift to the RIGHT (as per LCP), which is what we DON'T want here.
Instead, by adding sodium acetate, the CH3COO- will rush to react with the H+ instead. This forces the equilibrium to shift to the left to produce the chromate ions that we do want.
Post by: :3 on November 02, 2016, 12:39:34 pm
Questions:
- 13: how do you know whether the curve continues up (B), instead of down (A) (thought equilbrium will move down).
- 15: what is the quickest way of doing these types of questions (all I can think of is doing them separately)?
- 17
http://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-hsc-chemistry.pdf
Thanks again, really appreciate that you're answering some of my questions (even though some are idiotic/simply there).
Post by: jakesilove on November 02, 2016, 12:42:31 pm
Sorry...multiple choice questions.
Questions:
- 13: how do you know whether the curve continues up (B), instead of down (A) (thought equilbrium will move down).
- 15: what is the quickest way of doing these types of questions (all I can think of is doing them separately)?
- 17
http://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-hsc-chemistry.pdf
Thanks again, really appreciate that you're answering some of my questions (even though some are idiotic/simply there).
For 13, I'm almost certain the answer is A, not B. Basically, I'm sure you're right, and the answer are wrong.
Unfortunately, there isn't a quick way for Q 15. You just need to do all of them, and see which is your answer?
For 17, you should be able to go straight to your table of standard potentials, find dichromate and and sulfer dioxide, invert the sign of the one highest on the list, and add up the two potentials!
Post by: Cindy2k16 on November 02, 2016, 12:45:34 pm
TIA
Post by: RuiAce on November 02, 2016, 12:45:54 pm
For 13, I'm almost certain the answer is A, not B. Basically, I'm sure you're right, and the answer are wrong.But if the volume is halved, then the pressure is increased right? Which favours the side with the fewer moles of gas?
I just glanced at it quickly, not too sure. Can only tune in for so long at a time
Post by: jakesilove on November 02, 2016, 12:48:00 pm
But if the volume is halved, then the pressure is increased right? Which favours the side with the fewer moles of gas?
I just glanced at it quickly, not too sure. Can only tune in for so long at a time
But after it's gone up, it should go back down a little to compensate for the change in concentration (ie. massive jump to favour the side with fewer moles of gas, then small decline to compensate)
Post by: jazzaa36 on November 02, 2016, 12:48:22 pm
Post by: jakesilove on November 02, 2016, 12:48:52 pm
Hi for this question Evaluate the impact on society of the environmental issues associated with 7 THREE of the industrial processes that you have studied in this option. (industrial chem) is the extraction of sulfur counted as an industrial process on it's own or if we wanted to discuss it would we have to also talk about the production of sulfuric acid from sulfur?
TIA
Hey! I didn't do this option, but it sounds totally fine to have an answer including that
Post by: RuiAce on November 02, 2016, 12:49:51 pm
But after it's gone up, it should go back down a little to compensate for the change in concentration (ie. massive jump to favour the side with fewer moles of gas, then small decline to compensate)You lost me there. I was just using LCP on the equation given.
The change in
Post by: jakesilove on November 02, 2016, 12:50:32 pm
Yo does anyone have a simple way of remebering what effect water has on the concentration when left on apparatus of volumetric analysis ?
Do you mean in terms of titration? You just sort of need to logic it out. If there is water left in the burrette, the base will be more diluted. That means you'll require MORE of it to neutralise the same amount of acid. So, you'll think that it is weaker than it actually is. Just think about it for a bit, there isn't really a quick way!
Post by: RuiAce on November 02, 2016, 12:51:11 pm
Hi for this question Evaluate the impact on society of the environmental issues associated with 7 THREE of the industrial processes that you have studied in this option. (industrial chem) is the extraction of sulfur counted as an industrial process on it's own or if we wanted to discuss it would we have to also talk about the production of sulfuric acid from sulfur?Oh I'll quickly address this one.
TIA
Technically, yes it IS involved with the production of sulfuric acid. We need to have a source of sulfur before we can start producing our sulfuric acid.
I'd say talking about it would be more beneficial here, because it's easy to link environmental concerns to the Frasch process
Post by: Furrrball on November 02, 2016, 12:57:31 pm
http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2005exams/pdf_doc/chemistry_05.pdf
Q.25
2005 HSC
Post by: jakesilove on November 02, 2016, 12:59:38 pm
How do I calculate the percentage of total dissolved solids? Do I have to subtract the mass of suspended solids from the total volume of water?
http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2005exams/pdf_doc/chemistry_05.pdf
Q.25
2005 HSC
No, you shouldn't have to, and again I think those HSC responses are wrong. If I have 400mL of water, which contains 5g of suspended solids, do I really have 395mL of water? No, obviously not, because 1) mL and g are totally different units and 2) we care about the volume of the solvent as a whole, which is 400mL. Later BOSTES exams don't subtract that amount, either.
Post by: wesadora on November 02, 2016, 01:12:18 pm
I am aware esters have a lower bp/mp than alkanoic acids - alkanoic acids have two sites for hydrogen bonding, dipole-dipole bonding as well as dispersion forces, while esters only have dipole and dispersion forces?
Why do esters not have hydrogen bonding - I mean...they have an OH and double bonded O group as well.
Post by: jakesilove on November 02, 2016, 01:35:14 pm
Esters vs alkanoic acid boiling points/MP.
I am aware esters have a lower bp/mp than alkanoic acids - alkanoic acids have two sites for hydrogen bonding, dipole-dipole bonding as well as dispersion forces, while esters only have dipole and dispersion forces?
Why do esters not have hydrogen bonding - I mean...they have an OH and double bonded O group as well.
I'm not actually sure why Esters don't form hydrogen bonds! The important thing to know is just that they don't; you don't need to understand why. However, the fact that they don't have hydrogen bonds explains the lower mp/bp!
Post by: Rd2487 on November 02, 2016, 01:50:13 pm
Ozone has a high molecular weight, is that why it has a high BP/MP than molecular oxygen? Or is it something to do with bond energy
Explanation please?
Post by: massive on November 02, 2016, 01:52:20 pm
Post by: jakesilove on November 02, 2016, 01:57:52 pm
Just a bit confused.
Ozone has a high molecular weight, is that why it has a high BP/MP than molecular oxygen? Or is it something to do with bond energy
Explanation please?
Yep, molecular weight is the important factor here. Additionally, it's polar, so has strong dipole-dipole intermolecular forces :)
Post by: jakesilove on November 02, 2016, 02:00:45 pm
how do you do this guys?? the answer is: 1x10-7 mol/L
Hey! Remember that, to find the pH of something, we use -log(H+). So, if we plugged in -log(10^-7), we would get 7! What does that mean? As we keep diluting the water, the pH will increase and increase and increase until it gets to neutral (7). But, it wouldn't make sense for the pH to keep rising. Like, we can't just keep adding water, decrease the concentration of H+, and make the solution basic, can we? That wouldn't make any sense. So, the minimum concentration is neutral, as the question states.
Post by: massive on November 02, 2016, 02:02:56 pm
Hey! Remember that, to find the pH of something, we use -log(H+). So, if we plugged in -log(10^-7), we would get 7! What does that mean? As we keep diluting the water, the pH will increase and increase and increase until it gets to neutral (7). But, it wouldn't make sense for the pH to keep rising. Like, we can't just keep adding water, decrease the concentration of H+, and make the solution basic, can we? That wouldn't make any sense. So, the minimum concentration is neutral, as the question states.
Ohh thanks, so whenever we dilute the pH ALWAYS increases? I thought that was only for weak acids :S
Post by: Cindy2k16 on November 02, 2016, 02:04:58 pm
TIA :)
Post by: jakesilove on November 02, 2016, 02:06:09 pm
Ohh thanks, so whenever we dilute the pH ALWAYS increases? I thought that was only for weak acids :S
Yeah, because remember that pH is the -log(concentration of H+). So, if we add more water, the CONCENTRATION will decrease (even if the NUMBER of H+ ions stay the same). Does that make sense?
Post by: lha on November 02, 2016, 02:08:33 pm
Post by: jakesilove on November 02, 2016, 02:13:37 pm
Hi how come the answer is A? I thought that when a weak acid/base is used, since it sets up an equilibrium, more of a strong acid/base is required to neutralise it?
TIA :)
Hey! The simple answer is that it just DOES require the same amount of to neutralise it. Check out Rui's summary here, because I was never very good at this, but really you can just learn that this is a thing that is true and move on with your life.
Post by: jakesilove on November 02, 2016, 02:14:33 pm
Can you use silver nitrate as the salt bridge for a galvanic cell that has a lead anode and platinum cathode with fe3+/fe2+ solution?
Yep! I would generally recommend using a potassium nitrate salt bridge, but either way works fine
Post by: Cindy2k16 on November 02, 2016, 02:16:00 pm
Hey! The simple answer is that it just DOES require the same amount of to neutralise it. Check out Rui's summary here, because I was never very good at this, but really you can just learn that this is a thing that is true and move on with your life.
ok thanks you!
Post by: JemexR on November 02, 2016, 02:18:36 pm
Is there a particular reason why you asked the same questions again?
Sorry Rui... I totally missed that
Post by: massive on November 02, 2016, 02:20:57 pm
Post by: noonedoesnt on November 02, 2016, 02:23:44 pm
Post by: jakesilove on November 02, 2016, 02:32:26 pm
Guys I don't get the sample answer, I always thought citric acid is more of a stronger acid than acetic because its triprotic :S
Nup, turns out that acetic acid is stronger than citric acid. That's just a fact, memorise it!
Post by: jakesilove on November 02, 2016, 02:32:57 pm
Can they actually ask this in hsc!
I didn't do Industrial, but if that is an HSC question, then yes they certainly can!
Post by: lha on November 02, 2016, 02:35:10 pm
During your practical work you performed a first-hand investigation to observe the reactions of sulfuric acid acting as an oxidising agent.
Outline a conclusion drawn from the results of this investigation and assess the validity of this conclusion.
Post by: :3 on November 02, 2016, 02:35:55 pm
Can they actually ask this in hsc!
Pretty sure they can ask this and the process itself (i.e. draw the structural equations of soap + glycerol being made from fat and base).
Post by: :3 on November 02, 2016, 02:37:51 pm
How do you do this question? I dont recall doing this experiment...
During your practical work you performed a first-hand investigation to observe the reactions of sulfuric acid acting as an oxidising agent.
Outline a conclusion drawn from the results of this investigation and assess the validity of this conclusion.
I believe one of them was sulfuric acid reacting with zinc metal to produce ZnSO4 and Hydrogen Gas.
Post by: lha on November 02, 2016, 02:39:19 pm
I believe one of them was sulfuric acid reacting with zinc metal to produce ZnSO4 and Hydrogen Gas.
How would you answer the question then? I have no idea what to write...
Post by: Cindy2k16 on November 02, 2016, 02:40:19 pm
TIA
Post by: jakesilove on November 02, 2016, 02:45:49 pm
Hi for this question the BOSTES sample answer has the depletion of ozone by CFCs. Is it still correct if i wrote equations for the natural depletion of ozone?
TIA
Technically, you'd definitely get the marks (the question doesn't specify CFCs), however I would recommend sticking to the dotpoints where applicable
Post by: massive on November 02, 2016, 02:55:50 pm
Post by: Cindy2k16 on November 02, 2016, 02:56:39 pm
Post by: noonedoesnt on November 02, 2016, 03:03:08 pm
Hi how come in this reaction Cl gas isn't being produced at the anode? TIA
Hi cindy :)
The question states that it is an 0.05 mol/L solution of NaCl
Thus it is a dilute solution. Electrolysis of a dilute solution results in the oxidation of water at the anode and the reduction of water at the cathode.
Note: In a concentrated NaCl solution, Cl is oxidised at anode, and water is reduced at cathode. Whilst in a liquid/molten solution, Na is reduced and Cl is oxidised.
Post by: noonedoesnt on November 02, 2016, 03:04:29 pm
Post by: Furrrball on November 02, 2016, 03:10:08 pm
Hey guys, are you meant to use c1v1=c2v2 for this question?? because i tried that and got 13.301... but the answer is 13.25
Hope this makes sense :D
(http://imgur.com/a/oLfDs)
http://imgur.com/Fc4ehZQ
Post by: massive on November 02, 2016, 03:12:17 pm
Post by: jakesilove on November 02, 2016, 03:14:27 pm
Anyone have an idea?
Hey! Figure out the moles of Lead chloride using the final mass. The moles of Lead chloride will equal the moles of Lead, so we can just plug this value for 'n' into C=n/v (using a volume of 50mL, the initial volume) to get a concentration out!
Post by: kimmie on November 02, 2016, 03:15:09 pm
Post by: massive on November 02, 2016, 03:16:38 pm
Hope this makes sense :DDamn thanks so much! can you have a look at the other q i posted as well please :D
(http://imgur.com/a/oLfDs)
http://imgur.com/Fc4ehZQ
Post by: noonedoesnt on November 02, 2016, 03:18:14 pm
Hey! Figure out the moles of Lead chloride using the final mass. The moles of Lead chloride will equal the moles of Lead, so we can just plug this value for 'n' into C=n/v (using a volume of 50mL, the initial volume) to get a concentration out!
the only part i don't understand is why we use the 50 not the 25mL
Post by: jakesilove on November 02, 2016, 03:22:28 pm
Can someone please do this :( the answer is 12.56
We know that the molar ratio is 1:2 (one mole of Base requires two moles of Acid).
If we use 0.01 moles of acid, that will take up 0.005 moles of base, leaving us with 0.00355 moles of base left. The concentration of Hydroxide is therefore
Now, pH is 14+pOH, so 1-log(0.0182)=12.26. I think I probably messed up some calculation error somewhere, but that's the general method, so you can go back over and check each step so you can understand how to answer a question like this!
Post by: jakesilove on November 02, 2016, 03:23:51 pm
the only part i don't understand is why we use the 50 not the 25mL
Because the lead collected was as a result of 50mL of the pond sample, and we're interested in the lead concentration in the pond :)
Post by: jakesilove on November 02, 2016, 03:26:00 pm
How do we measure biological oxygen demand?
(http://i.imgur.com/Wgkdi95.png)
Above is a super condensed version of what you need to know. You probably don't have time to go over the section in depth, and understand it properly, so just throw in some of the buzzwords I've outlined and you can still get all the marks :)
Post by: WLalex on November 02, 2016, 03:28:00 pm
Nup, turns out that acetic acid is stronger than citric acid. That's just a fact, memorise it!
buuuut...does not acetic acid only ionise around 1% whereas citric is around 8% (would that not mean citric is a stronger acid)??
Post by: jakesilove on November 02, 2016, 03:32:29 pm
buuuut...does not acetic acid only ionise around 1% whereas citric is around 8% (would that not mean citric is a stronger acid)??
Yep, you're definitely right about that actually. Sorry, I don't remember what the original question was or why I wrote what I did; presumably, the thing to get out of this is that in terms of ionisation, HCl ionises the most, then citric, then acetic acid.
Post by: massive on November 02, 2016, 03:48:48 pm
We know that the molar ratio is 1:2 (one mole of Base requires two moles of Acid).
If we use 0.01 moles of acid, that will take up 0.005 moles of base, leaving us with 0.00355 moles of base left. The concentration of Hydroxide is therefore
Now, pH is 14+pOH, so 1-log(0.0182)=12.26. I think I probably messed up some calculation error somewhere, but that's the general method, so you can go back over and check each step so you can understand how to answer a question like this!
Thanks for this, does the fact that calcium hydroxide releases 2 OH make a difference?? :S
Post by: jakesilove on November 02, 2016, 03:50:13 pm
Thanks for this, does the fact that calcium hydroxide releases 2 OH make a difference?? :S
Yeah, see my initial line (knowing that it is a 1:2 reactions allows me to find the limiting reagent)
Post by: kimmie on November 02, 2016, 04:33:54 pm
Post by: jakesilove on November 02, 2016, 04:49:25 pm
How does electroplating work?
Hey!
Electroplating works by setting up an electrolytic cell, with an anode and a cathode. Say we wanted to coat Iron is Silver. We would make Iron the Cathode, and Silver at the Anode. We would immerse both in a solution of Silver nitrate. The anode oxidises, thus producing more Silver ions. The cathode reduces, converting silver ions into silver metal, which plates the cathode. So, just set up the cathode with whatever you want to coat, set up the anode and the electrolyte with whatever you want the cathode to be coated in, and pump through some current!
Post by: ProfLayton2000 on November 02, 2016, 04:59:07 pm
(Specifically HSC 2004 q20)
Post by: jakesilove on November 02, 2016, 05:01:28 pm
How does including and outlier affect the validity, reliability and accuracy of results?
(Specifically HSC 2004 q20)
Hey! Including an outlier will effect your reliability, as the average you took will be incorrect (and averages speak to reliability), but it will also effect your validity, as your experimental design didn't include discounting outliers (so I guess it isn't so much the outlier itself that effects validity, but rather your decision not to exclude it, if that makes any sense what-so-ever). It probably doesn't effect the accuracy of your individual measurements, however it will obviously effect how close your solution is to the real value.
Post by: anotherworld2b on November 02, 2016, 05:13:40 pm
For electrons shells what is the pattern they follow to fill up the shells? I know that the maximum number of electrons is calculated using the 2n^2 but that's not how they fill up each shell?
I also wanted to ask for intermolecular forces how do you know what forces are present in a compound?
I also wanted to ask how do you know the effect of each force on properties like vapour pressure, boiling point and ect?
Post by: jakesilove on November 02, 2016, 05:19:00 pm
Hi I have some questions
For electrons shells what is the pattern they follow to fill up the shells? I know that the maximum number of electrons is calculated using the 2n^2 but that's not how they fill up each shell?
I also wanted to ask for intermolecular forces how do you know what forces are present in a compound?
I also wanted to ask how do you know the effect of each force on properties like vapour pressure, boiling point and ect?
I can't imagine you need to know how electrons fill shells, but if you do, check out the image here. Just draw consecutive arrows, and the first unit you hit is the next one you fill.
In terms of intermolecular forces; there are always dispersion forces, there are dipole-dipole forces if the molecule is polar, and there are hydrogen bonds if there are Hydrogen groups and F-O-N groups present in the compound.
No idea how to answer that last part, sorry!
THIS IS LARGELY NOT HSC MATERIAL!
Post by: RuiAce on November 02, 2016, 05:19:39 pm
Hi I have some questionsQ1 There actually IS no defined pattern in general. Every row of the periodic table follows a different trend.
For electrons shells what is the pattern they follow to fill up the shells? I know that the maximum number of electrons is calculated using the 2n^2 but that's not how they fill up each shell?
I also wanted to ask for intermolecular forces how do you know what forces are present in a compound?
I also wanted to ask how do you know the effect of each force on properties like vapour pressure, boiling point and ect?
Q2 Well, you just look at what you have, and see what substances are polar. Feel free to provide three questions though.
Q3 It's... the same for each forces I believe...
THIS IS LARGELY NOT HSC MATERIAL!Emphasis on this^
Post by: jakesilove on November 02, 2016, 05:21:10 pm
Q1 There actually IS no defined pattern in general. Every row of the periodic table follows a different trend.
Q2 Well, you just look at what you have, and see what substances are polar. Feel free to provide three questions though.
Q3 It's... the same for each forces I believe...Emphasis on this^
There is actually a defined pattern, which only messes up on elements who half fill their d-shells (I think? Something like that). Anyway, I did chem so long ago that all I remember is that image aha
Post by: g98 on November 02, 2016, 05:23:44 pm
1. what is the difference between nuclear reactors and cyclotrons? And what particles are used in each? And how do you know which one radioisotope is made in?
2. How do you know what type of radiation a radioisotope gives off when decaying?
3. Other than acid rain is there any other impact of SO2 and oxides of nitrogen in the atmosphere?
4. What's an example of a natural buffer? is CO2 + H20 --> H2CO3 (in equilibrium) one?
5. Whenever you use a sintered glass funnel do you use a side arm flask and vacuum and do you have to mention that in experiments?
Thanks!!
Post by: anotherworld2b on November 02, 2016, 05:25:42 pm
I was also wondering if I could get help understanding markonikoff's rule. I am not sure how it works :-\
Post by: RuiAce on November 02, 2016, 05:27:28 pm
There is actually a defined pattern, which only messes up on elements who half fill their d-shells (I think? Something like that). Anyway, I did chem so long ago that all I remember is that image ahaI wouldn't know of this lol
thank you for the help ;DThis isn't in the HSC course at all (unless Jake can handle it)
I was also wondering if I could get help understanding markonikoff's rule. I am not sure how it works :-\
Post by: jakesilove on November 02, 2016, 05:28:09 pm
Hi, I have a heap of questions so here I go :):
1. what is the difference between nuclear reactors and cyclotrons? And what particles are used in each? And how do you know which one radioisotope is made in?
2. How do you know what type of radiation a radioisotope gives off when decaying?
3. Other than acid rain is there any other impact of SO2 and oxides of nitrogen in the atmosphere?
4. What's an example of a natural buffer? is CO2 + H20 --> H2CO3 (in equilibrium) one?
5. Whenever you use a sintered glass funnel do you use a side arm flask and vacuum and do you have to mention that in experiments?
Thanks!!
1. Nuclear reactors bombard elements with neutrons. Cyclotrons accelerate charged particles (eg. electrons) and smash those into elements. You don't really need to know which is which in terms of your radioisotope, but if you ever have an equation where neutrons are added, then that's a nuclear reactor!
2. You just have to know. If it emits an electron, beta negative decay. If it emits a positron, beta positive decay. If it emits an alpha particle (He), alpha decay and if it emits a fast gamma ray, it's gamma radiation. Just know what type of emission your radioisotope undergoes, and why that is useful!
3. Nup, we really only care about acid rain. Potentially, it's a pollutant, as it's toxic to breathe in?
4. Yep, you can use that, or the standard HPO3/P2PO3 one
5. I have no idea what any of those words mean. Sorry aha
Post by: jakesilove on November 02, 2016, 05:28:37 pm
I wouldn't know of this lolThis isn't in the HSC course at all (unless Jake can handle it)
I don't think AnotherWorld is from NSW :)
Post by: RuiAce on November 02, 2016, 05:29:02 pm
3. Nup, we really only care about acid rain. Potentially, it's a pollutant, as it's toxic to breathe in?Yeah respiratory problems with sulfur dioxide matters. But yeah at the same time only just those two
I don't think AnotherWorld is from NSW :)I know aha
Post by: jakesilove on November 02, 2016, 05:29:13 pm
thank you for the help ;D
I was also wondering if I could get help understanding markonikoff's rule. I am not sure how it works :-\
But no, I have no clue
Post by: RuiAce on November 02, 2016, 05:30:23 pm
5. Whenever you use a sintered glass funnel do you use a side arm flask and vacuum and do you have to mention that in experiments?Umm if you used the sintered glass funnel yes I reckon both are necessary.
Post by: g98 on November 02, 2016, 05:34:32 pm
1. if the titrate is in the burette, what do you call the solution in the flask...is it always just standard solution?
2. do soaps form miscelles only when they are around particles are are the always in miscelles even when not surrounding grease...if that makes sense?
3. What is chain stiffening and what is it's effect?
4. Do you need to know the name and structure of a fat or oil like glycerol tristreate for saponification?
Thanks!
Post by: Ayyddaan on November 02, 2016, 05:35:23 pm
and the other in deep water (4000 m). Explain how the rusting processes differ
in these two ships. Include equations in your answer. (7m)
Need some help with this, could somebody write a proper response to it?
Post by: jakesilove on November 02, 2016, 05:36:40 pm
Thanks for your help! I do have a couple more questions though:
1. if the titrate is in the burette, what do you call the solution in the flask...is it always just standard solution?
2. do soaps form miscelles only when they are around particles are are the always in miscelles even when not surrounding grease...if that makes sense?
3. What is chain stiffening and what is it's effect?
4. Do you need to know the name and structure of a fat or oil like glycerol tristreate for saponification?
Thanks!
1. We call it the titrant, and the titre
2. No idea, hopefully someone who does industrial can help
3. Nope.
4. Nope
RUI!
Post by: anotherworld2b on November 02, 2016, 05:37:52 pm
I also wondering for hydrocarbons how do make sure you draw a isomer and not a completely different compound?
I sometimes accidently draw a entirely different compound instead of a isomer
Post by: jakesilove on November 02, 2016, 05:40:44 pm
Two identical ships are sunk in seawater. One is sunk in shallow water (60 m)
and the other in deep water (4000 m). Explain how the rusting processes differ
in these two ships. Include equations in your answer. (7m)
Need some help with this, could somebody write a proper response to it?
Not going to be able to give you a full response, but here are some things to discuss
Shallow waters
Increased oxygen, increased temperature, but decreased pressure. Rusting will occur as usual, by the usual equations, which you write out etc. etc. Nothing fancy here
Deep waters
Decreased oxygen, decreased temperature, but increased pressure. Rusting would be accelerated if not for the lack of Oxygen and cold temperatures, which should decrease the rate of corrosion. However, several factors increase the rate
Sulfate-reducing bacteria forces iron to oxidise (as something has to, for sulfate to reduce). Sulfate is leeched from volcanic rock etc. This leads to increased rusting
Acidic microenvironments due to areas of the ship etc. result in an 'easier' pathway for rust; check the voltage required for an H+ and Iron cell, and it will be less than an H2O/O2 and Iron cell, making it an easier process.
Hope that outline helps! Include loads of equations as well
Post by: jakesilove on November 02, 2016, 05:43:47 pm
Thank you for your help anyway ;D
I also wondering for hydrocarbons how do make sure you draw a isomer and not a completely different compound?
I sometimes accidently draw a entirely different compound instead of a isomer
Not sure how to help you here; just make sure you have the same number of elements. Potentially draw yourself a hint section, where you write how many Carbons, Hydrogens etc. you have. When you draw out an isomer, check that you still have the same number of each element!
Post by: kimmie on November 02, 2016, 05:45:32 pm
Post by: Ayyddaan on November 02, 2016, 05:50:27 pm
Not going to be able to give you a full response, but here are some things to discuss
Shallow waters
Increased oxygen, increased temperature, but decreased pressure. Rusting will occur as usual, by the usual equations, which you write out etc. etc. Nothing fancy here
Deep waters
Decreased oxygen, decreased temperature, but increased pressure. Rusting would be accelerated if not for the lack of Oxygen and cold temperatures, which should decrease the rate of corrosion. However, several factors increase the rate
Sulfate-reducing bacteria forces iron to oxidise (as something has to, for sulfate to reduce). Sulfate is leeched from volcanic rock etc. This leads to increased rusting
Acidic microenvironments due to areas of the ship etc. result in an 'easier' pathway for rust; check the voltage required for an H+ and Iron cell, and it will be less than an H2O/O2 and Iron cell, making it an easier process.
Hope that outline helps! Include loads of equations as well
That's perfect! Thanks heaps! Are there any specific equations that you think should be listed?
Post by: jakesilove on November 02, 2016, 05:50:43 pm
How does the concentration of electrolyte and nature of electrolyte affect electrolysis? Like aren't they both the same thing?
Not QUITE the same thing; nature of electrolyte could be like what the electrolyte is actually made up of (Silver Nitrate, Potassium Nitrate etc.) whereas concentration is... concentration. More concentrated = better electrolyte, nature of electrolyte must be soluble etc.
Post by: jakesilove on November 02, 2016, 05:52:54 pm
That's perfect! Thanks heaps! Are there any specific equations that you think should be listed?
Yep! You should have them in your notes somewhere, just be able to go from Water + Oxygen + Iron all the way to rust (I remember there being a couple steps), and then personally I would just write out the equations on the table of standard potentials regarding sulfate and Hydrogen ions
Post by: nimasha.w on November 02, 2016, 06:05:09 pm
Post by: RuiAce on November 02, 2016, 06:06:06 pm
3. Nope.YES SIR!
4. Nope
RUI!
Thanks for your help! I do have a couple more questions though:3. I haven't seen this term before. What context are we talking about?
1. if the titrate is in the burette, what do you call the solution in the flask...is it always just standard solution?
2. do soaps form miscelles only when they are around particles are are the always in miscelles even when not surrounding grease...if that makes sense?
3. What is chain stiffening and what is it's effect?
4. Do you need to know the name and structure of a fat or oil like glycerol tristreate for saponification?
Thanks!
4. You do need to know the name of one but I believe you're allowed off with the structure.
Post by: jakesilove on November 02, 2016, 06:07:26 pm
hey! how would you work this one out
Hey! If you look to the table of standard potentials, you'll realise that for the best cell, we want something very high on the list, and something very low on the list. What does that means? Something very reactive, and something very unreactive! Clearly, Q is the most reactive, and R is the least reactive, so we'll use those. Thus, the answer is C
Post by: kimmie on November 02, 2016, 06:13:58 pm
Post by: Sssssrr on November 02, 2016, 06:19:06 pm
Post by: Furrrball on November 02, 2016, 06:22:55 pm
Thanks for your help! I do have a couple more questions though:
1. if the titrate is in the burette, what do you call the solution in the flask...is it always just standard solution?
2. do soaps form miscelles only when they are around particles are are the always in miscelles even when not surrounding grease...if that makes sense?
3. What is chain stiffening and what is it's effect?
4. Do you need to know the name and structure of a fat or oil like glycerol tristreate for saponification?
Thanks!
For Q3. The soap molecules form a micelle when they carry out their job as an emulsifier for the water & grease. So only when the grease is present.
The hydrocarbon-tail of the soap forms dispersion forces with the grease and the anionic head forms ion-dipole interactions with the water molecule.
Hope that answers your question. :)
Post by: jakesilove on November 02, 2016, 06:23:26 pm
How do impressed current systems provide cathodic protection?
This one is pretty straight forward; the problem is that Iron has oxidised (lost electrons) right? So, we can pass a current along it (current = moving electrons) to reduce it again (gain electrons)!
Simple as that
Post by: jakesilove on November 02, 2016, 06:24:26 pm
can someone please explain how to use significant figures in chemistry? thanks
Hey! Just look for the least number of sig figs in the question. Eg.
13mol/L
41 degrees
1g
Whilst the first two measurements have 2 sig figs, the third only has one, so your answer needs to have one sig fig!
Post by: RuiAce on November 02, 2016, 06:24:51 pm
For Q3. The soap molecules form a micelle when they carry out their job as an emulsifier for the water & grease. So only when the grease is present.Actually I'm still a bit confused (because I don't have resources with me right now). Is this what they mean by 'chain stiffening' because I genuinely don't recall this term being coined
The hydrocarbon-tail of the soap forms dispersion forces with the grease and the anionic head forms ion-dipole interactions with the water molecule.
Hope that answers your question. :)
Plus it doesn't seem as though some 'stiffening' is going on
Post by: jakesilove on November 02, 2016, 06:28:23 pm
Actually I'm still a bit confused (because I don't have resources with me right now). Is this what they mean by 'chain stiffening' because I genuinely don't recall this term being coined
Plus it doesn't seem as though some 'stiffening' is going on
Industrial is a weird option
Post by: RuiAce on November 02, 2016, 06:29:30 pm
Industrial is a weird optionLol get any unnecessary thoughts out of your head Jake haha
Post by: Ayyddaan on November 02, 2016, 06:34:44 pm
(i) Write an equation to represent the process of rusting under neutral
conditions.
The sample answer was really weird and I don't think it was right. Please help!
Post by: jakesilove on November 02, 2016, 06:36:39 pm
Iron corrodes differently under acidic and neutral conditions.
(i) Write an equation to represent the process of rusting under neutral
conditions.
The sample answer was really weird and I don't think it was right. Please help!
(http://i.imgur.com/Zh1zN02.png)
Something that looks like this would be ideal
Post by: g98 on November 02, 2016, 06:45:42 pm
3. I haven't seen this term before. What context are we talking about?
Chain stiffening is used to describe properties of polymers like polyethylene and polystyrene - I think it has something to do with side groups maybe...
Post by: massive on November 02, 2016, 06:49:14 pm
Post by: Ayyddaan on November 02, 2016, 06:50:26 pm
(http://i.imgur.com/Zh1zN02.png)
Something that looks like this would be ideal
What is the 'x' infront of H2O
Post by: RuiAce on November 02, 2016, 06:50:32 pm
I DEFINITELY did not see that get mentioned in PoM.3. I haven't seen this term before. What context are we talking about?
Chain stiffening is used to describe properties of polymers like polyethylene and polystyrene - I think it has something to do with side groups maybe...
guys when do you put equilibrium arrows for all that conjugate acid and base stuff? that still confuses me :(Well analyse it.
If both substances are weak, then they're not fully ionised, so you have an equilibrium arrow.
If at least one thing is strong, that's going to drive the equation to completion, so you have a one-way arrow
Post by: jakesilove on November 02, 2016, 06:51:41 pm
What is the 'x' infront of H2O
It's just kind of a balancing factor; there could be any number of H2O molecules contained within the chemical structure of rust, such that the left hand side balances out with the right hand side.
Post by: anotherworld2b on November 02, 2016, 07:04:12 pm
Post by: Rd2487 on November 02, 2016, 07:06:15 pm
If we are asked to write an equation for incomplete combustion. Which one doe we write the one where CO and CO2 are formed or the one in which C and CO are formed. Or does it not matter?
Post by: tennis1 on November 02, 2016, 07:07:57 pm
or do you react it with H3O+?
Post by: jakesilove on November 02, 2016, 07:10:35 pm
If we are asked to write an equation for incomplete combustion. Which one doe we write the one where CO and CO2 are formed or the one in which C and CO are formed. Or does it not matter?
Doesn't matter! As long as some C or CO is formed, and the equation is balanced :)
Post by: jakesilove on November 02, 2016, 07:11:05 pm
When writing buffer equations, do you always make the acid react with H2O to form H3O+ ions and conj base
or do you react it with H3O+?
Always react an acid with water to show it's acidic properties, and a base with water to show its... basidic properties?
Post by: RuiAce on November 02, 2016, 07:12:45 pm
Always react an acid with water to show it's acidic properties, and a base with water to show its... basidic properties?Jake, you've lost the most basic word from your vocabulary
Post by: tennis1 on November 02, 2016, 07:19:44 pm
Always react an acid with water to show it's acidic properties, and a base with water to show its... basidic properties?
ohhh right yeah woops
so if it's a conj base I have to add the H3O+ but just generally write buffer equations using acid + water
Post by: daatarz on November 02, 2016, 07:30:07 pm
Is the equilibrium constant calculated using halved values because the volume has been doubled?
Also, is their anywhere I can find written solutions because for some HSC past papers honestly the solutions are really bad/way too short and simple imo
Post by: MysteryMarker on November 02, 2016, 07:37:03 pm
Cheers.
Post by: anotherworld2b on November 02, 2016, 07:43:09 pm
Also for addition reactions such as for pentane + Cl2 where do you put the cl in a structural formula? (is there a rule?)
Post by: bethjomay on November 02, 2016, 07:45:19 pm
Jake, you've lost the most basic word from your vocabulary
I lolled. Also: you could say the number of pages on this forum is about to get transuranic. 8)
Post by: jakesilove on November 02, 2016, 07:47:32 pm
For Industrial chemistry, 31. a ii) in 2015 http://www.boardofstudies.nsw.edu.au/hsc_exams/2015/exams/2015-hsc-chemistry.pdf
Is the equilibrium constant calculated using halved values because the volume has been doubled?
Also, is their anywhere I can find written solutions because for some HSC past papers honestly the solutions are really bad/way too short and simple imo
Sorry, gonna have to wait for someone that did Industrial!
Post by: jakesilove on November 02, 2016, 07:48:26 pm
For the formula H= -mCAT, should the mass be in g or Kg, and the H in J or KJ?
Cheers.
Check the constant that you use! Look at the value of C on your formula sheet; it will either be in g or kg, J or kJ. Whatever the units of the constant are, are the units you need to be using!
Post by: jakesilove on November 02, 2016, 07:49:15 pm
I lolled. Also: you could say the number of pages on this forum is about to get transuranic. 8)
I laughed for a long, long time
Post by: jakesilove on November 02, 2016, 07:49:23 pm
I was wondering is benzene a aromatic? What is the definition for a aromatic?
Also for addition reactions such as for pentane + Cl2 where do you put the cl in a structural formula? (is there a rule?)
No idea. Google tells me that Benzene is Aromatic.
As far as I'm aware, there isn't a rule, put them wherever you want
Post by: RuiAce on November 02, 2016, 07:52:23 pm
For Industrial chemistry, 31. a ii) in 2015 http://www.boardofstudies.nsw.edu.au/hsc_exams/2015/exams/2015-hsc-chemistry.pdfYeah that's why I used Excel Success One.
Is the equilibrium constant calculated using halved values because the volume has been doubled?
Also, is their anywhere I can find written solutions because for some HSC past papers honestly the solutions are really bad/way too short and simple imo
And regarding your question, yep exactly. Recall that the formula for the equilibrium constant uses concentrations, not moles. So if V = 2, then C = n/V = n/2
For the formula H= -mCAT, should the mass be in g or Kg, and the H in J or KJ?Yeah as Jake said you really need to watch out for the constants in what C you use.
Cheers.
This is why I always write my units in my equations. I force things to cancel out properly.
Post by: anotherworld2b on November 02, 2016, 07:53:18 pm
I am not sure how to do the complete hydration of but-1-yne?
Post by: RuiAce on November 02, 2016, 07:56:19 pm
oh okay thank you :DAlkynes are in the HSC either I'm afraid.
I am not sure how to do the complete hydration of but-1-yne?
Post by: WLalex on November 02, 2016, 07:56:55 pm
Post by: bethjomay on November 02, 2016, 07:58:02 pm
Post by: jakesilove on November 02, 2016, 07:59:57 pm
If a question doesn't ask specifically for a diagram, but you have to explain something, can we do that via a diagram and still get the marks?
For sure, although my recommendation is to draw the diagram AND give a brief explanation, to be 100% sure you get all the marks. Better to be safe than sorry!
Post by: jakesilove on November 02, 2016, 08:00:26 pm
Do we have to draw all diagrams in pencil? I am worried that it will not scan well
Pencil scans fine, and if they tell you to draw diagrams in pencil, then you probably should. All I can say is that I didn't even bring pencils into my Science exams
Post by: RuiAce on November 02, 2016, 08:00:47 pm
Do we have to draw all diagrams in pencil? I am worried that it will not scan wellIn science, this is what you're meant to do, as per front page of the exam. And tbh I reckon the scanning for the science papers was 10x better than that for math papers. (This is coming from someone who BOUGHT his exam responses.)
If a question doesn't ask specifically for a diagram, but you have to explain something, can we do that via a diagram and still get the marks?I for one do. I drew out my Gratzell cell and the structure of a soap last year in my exam.
Also, haha so many chemistree puns
Post by: nibblez16 on November 02, 2016, 08:34:54 pm
Post by: sl.sarlak on November 02, 2016, 08:37:54 pm
thanks :)
Post by: jakesilove on November 02, 2016, 08:42:11 pm
Hello, I need help with determining the concentration of hcl... Can you please explain with displaying the formulas ur using too. Thank You :)
We start off by figuring out how many moles of the Sodium Carbonate solution we have in the 25mL
Okay, now we need the chemical reaction between HCl and Sodium carbonate
Clearly, there is a 2:1 ratio. So, for every 1 mole of Sodium carbonate we have, we need 2 moles of HCl to neutralise. Therefore, if we have 0.0025 moles of Sodium carbonate, the amount of acid we must have added is 0.05 moles.
Now, we can figure out concentration by
Post by: jakesilove on November 02, 2016, 08:43:22 pm
Hi I was wondering how do you know a chemistry question is a limiting reagent question? (Sorry if it's a stupid question)
thanks :)
Basically, if you ever have two chemicals being added together, and have no idea what's going on, it's going to be a limiting reagent question
Post by: bethjomay on November 02, 2016, 08:50:13 pm
Post by: wesadora on November 02, 2016, 08:50:53 pm
Basically, if you ever have two chemicals being added together, and have no idea what's going on, it's going to be a limiting reagent questionHAHAHAHAHAHAHA THIS.
Post by: RuiAce on November 02, 2016, 08:51:11 pm
Are there any specific buffer systems we need to know or can we just choose one and memorise it, bc I only have one at the moment?Pick one and memorise it, cause otherwise they give you the buffer and its equation
Post by: jakesilove on November 02, 2016, 08:51:48 pm
Are there any specific buffer systems we need to know or can we just choose one and memorise it, bc I only have one at the moment?
Yep, just choose one and run with it!
Post by: bethjomay on November 02, 2016, 08:52:05 pm
Pick one and memorise it, cause otherwise they give you the buffer and its equation
Awesome, thank you! :)
Post by: nibblez16 on November 02, 2016, 08:54:52 pm
We start off by figuring out how many moles of the Sodium Carbonate solution we have in the 25mL
Okay, now we need the chemical reaction between HCl and Sodium carbonate
Clearly, there is a 2:1 ratio. So, for every 1 mole of Sodium carbonate we have, we need 2 moles of HCl to neutralise. Therefore, if we have 0.0025 moles of Sodium carbonate, the amount of acid we must have added is 0.05 moles.
Now, we can figure out concentration by
Thank You so much! Makes much more sense :)
Post by: wesadora on November 02, 2016, 08:56:49 pm
Post by: nibblez16 on November 02, 2016, 08:58:58 pm
Post by: MysteryMarker on November 02, 2016, 09:01:11 pm
Cheers. :D
Post by: RuiAce on November 02, 2016, 09:02:55 pm
How do we calculate TDS?You just have to use common sense.
The dissolved solids are the ones actually dissolved. Not the ones picked up by the filtration.
So whatever was left from evaporation.
Then you just find your concentration as a %(w/v)
Post by: RuiAce on November 02, 2016, 09:04:07 pm
What is the range in which the titres of a titration can range from? Like what is the maximum difference in volume between titres before it has to be omitted?What do you mean? Do you mean to find the average titre volume?
Cheers. :D
Because if you mean that then by common sense you shouldn't even be touching data off by more than 0.20mL
No "defined number". Just use common sense
Post by: bethjomay on November 02, 2016, 09:06:13 pm
How does this look as a plan for an ethanol 7 marker? (I took your advice Jake
) Sorry if it's hard to read. I've put (eq.) where I would put a relevant equation!
Post by: jakesilove on November 02, 2016, 09:06:42 pm
What is the range in which the titres of a titration can range from? Like what is the maximum difference in volume between titres before it has to be omitted?
Cheers. :D
Three values need to be within 0.1mL of each other
Post by: jakesilove on November 02, 2016, 09:08:31 pm
(http://uploads.tapatalk-cdn.com/20161102/01d3acbeeb205c2fdc1aa2a36de00399.jpg)
How does this look as a plan for an ethanol 7 marker? (I took your advice Jake
Looks fucking brilliant. Just make sure to have a final assessment, and you're sweet :)
Post by: bethjomay on November 02, 2016, 09:11:45 pm
Looks fucking brilliant. Just make sure to have a final assessment, and you're sweet :)
Yay thank you! Ah yes, I always forget that.
Post by: wesadora on November 02, 2016, 09:16:32 pm
Post by: jakesilove on November 02, 2016, 09:17:21 pm
why does 1 butanol have higher mp/bp than 2 butanol? Is it because there are hydrogen bonding (OH) is on the first carbon, or has more polarity?
No idea, and that definitely isn't something you need to know
Post by: wesadora on November 02, 2016, 09:19:40 pm
No idea, and that definitely isn't something you need to knowLMAO GOOD :'D
Post by: MysteryMarker on November 02, 2016, 09:43:36 pm
Is this just because NaHCO3 amphiprotic? So when it dissolves in water it can either produce H+ or OH-, and thus he can't define it as an acid/base. Or is there more to it?
Cheers.
Post by: jakesilove on November 02, 2016, 09:46:25 pm
Explain why Arrhenius acid/base definition is unable to account for the acid/base properties of NaHCO3, whereas the Bronsted - Lowry theory can.
Is this just because NaHCO3 amphiprotic? So when it dissolves in water it can either produce H+ or OH-, and thus he can't define it as an acid/base. Or is there more to it?
Cheers.
That sounds right to me!
Post by: tennis1 on November 02, 2016, 10:12:39 pm
* rinsing all glassware with water: dilutes--> Decreases conc in burette. so does that mean that more ___is added to burette? and then overall conc. will increase? percentage mass decrease?
*not drying Na2CO3= More water-->Dilutes conc.--> more __ added into the burette-->? more concentrated overall (final calc)??
* wrong indicator: will mean colour change doesn't match equivalence point and so more___ added in burette? Increasing concentration?
Thanks!
Post by: nickglyn on November 02, 2016, 10:20:46 pm
Post by: Furrrball on November 02, 2016, 10:21:23 pm
Actually I'm still a bit confused (because I don't have resources with me right now). Is this what they mean by 'chain stiffening' because I genuinely don't recall this term being coined
Plus it doesn't seem as though some 'stiffening' is going on
My bad, that was for Q2
Post by: Albertenouttaten on November 02, 2016, 10:27:02 pm
1) Is ammonia explosive
2) is NaHCO3 acid or base?
3) Do we have to know the method of how to create our biopolymer e.g. the polymersiation process or do we just have to know bacteria regarding polymerisation process?
4) Do we need to know the equations on how to make our chosen industrial and medical radioisotope e.g. equation on how to make Tc-99m and Co-60?
Good luck to everyone tommorow.
Post by: Furrrball on November 02, 2016, 10:27:51 pm
Hey guys, is the answer to this C? I can't find the answers to this paper online it's too old. Thanks
10. C
Molar mass of P2O5 = 2 x 31.0 + 5x 16.0 = 142.0 g/mol
Number of moles of P2O5 used = 1.42/142 = 0.010
Moles of H3PO4 formed = 0.020
Moles of NaOH needed = 3 x 0.020 = 0.060 mol
Volume of NaOH needed = 0.060/0.30 = 0.20 L
So yes, you're right! ;D
http://www.cci.net.au/conqchem/ExamComments06Frameset.htm
Post by: RuiAce on November 02, 2016, 10:32:09 pm
Hello smartest people in Sydney, few questions to ask:1. Ammonia itself isn't explosive. It's just used in the manufacturing of OTHER explosives.
1) Is ammonia explosive
2) is NaHCO3 acid or base?
3) Do we have to know the method of how to create our biopolymer e.g. the polymersiation process or do we just have to know bacteria regarding polymerisation process?
4) Do we need to know the equations on how to make our chosen industrial and medical radioisotope e.g. equation on how to make Tc-99m and Co-60?
Good luck to everyone tommorow.
2. It's both. It's amphiprotic. You should well know what this means.
(However, it might be worth mentioning that in water it is basic)
3. Don't remember, ask Jake
4. You don't but I memorised them anyway just in case, because it demonstrates skill. It can also be used as your equations for the production of a commercial radioisotope dot point.
Post by: RuiAce on November 02, 2016, 10:35:26 pm
Ok so I'm just a little bit confused about the effect of errors on overall concentration or percentage in a tabletIf you rinse all glassware with water and then don't do the required procedures afterwards, then you've decreased the concentration in both the burette AND the pipette. Which means you no longer wtf your calculated values even are anymore; they're not guaranteed to be too high or too low, we just don't know
* rinsing all glassware with water: dilutes--> Decreases conc in burette. so does that mean that more ___is added to burette? and then overall conc. will increase? percentage mass decrease?
*not drying Na2CO3= More water-->Dilutes conc.--> more __ added into the burette-->? more concentrated overall (final calc)??
* wrong indicator: will mean colour change doesn't match equivalence point and so more___ added in burette? Increasing concentration?
Thanks!
Using hydrous Na2CO3 and not anhydrous means that your calculations will be wrong at the start. You can't find the concentration of your primary solution because you don't even know what the mass is anymore.
Wrong endpoint is all you need for wrong indicator. You can work out using logic what might come afterwards.
Note: Endpoint. Not equivalence point. (You could potentially that the endpoint and equivalence points don't match up at all.)
Post by: massive on November 02, 2016, 10:36:06 pm
Post by: nibblez16 on November 02, 2016, 10:36:58 pm
Post by: RuiAce on November 02, 2016, 10:39:34 pm
Can I have a solution with formulas used plz. One thing I sometimes get confused with is where to start the calculation... any tip on tht? ThanksQuestion was addressed in post #993 including the starting point.
The wording should be enough for you to figure out what formulas got used.
Post by: onepunchboy on November 02, 2016, 10:45:06 pm
THANKS
Post by: Furrrball on November 02, 2016, 10:46:13 pm
Whats the difference between titre and titrant??
Titrant (solution of known conc.) and titrate (solution of unknown conc.)
Post by: g98 on November 02, 2016, 10:46:46 pm
Thanks :)
Post by: tennis1 on November 02, 2016, 10:49:01 pm
If you rinse all glassware with water and then don't do the required procedures afterwards, then you've decreased the concentration in both the burette AND the pipette. Which means you no longer wtf your calculated values even are anymore; they're not guaranteed to be too high or too low, we just don't know
Using hydrous Na2CO3 and not anhydrous means that your calculations will be wrong at the start. You can't find the concentration of your primary solution because you don't even know what the mass is anymore.
Wrong endpoint is all you need for wrong indicator. You can work out using logic what might come afterwards.
Note: Endpoint. Not equivalence point. (You could potentially that the endpoint and equivalence points don't match up at all.)
oh ok thanks. But if we dilute the burette with water, instead of washing with solution, does that mean that as the concentration decreases so lower pH, means that more of the [ ] in the burette is added. Would this then increase the overall concentration and increase the percentage mass in tablet?
And why won't you know what mass is if it's hydrous? because there's extra water?
Post by: RuiAce on November 02, 2016, 10:53:32 pm
oh ok thanks. But if we dilute the burette with water, instead of washing with solution, does that mean that as the concentration decreases so lower pH, means that more of the [ ] in the burette is added. Would this then increase the overall concentration and increase the percentage mass in tablet?When the water molecules dissolve into the water you end up messing up the volume of the water as well. Plus you don't know the chemical formula for hydrous sodium carbonate to properly calculate the moles
And why won't you know what mass is if it's hydrous? because there's extra water?
With the burette it honestly depends on if you put acid or base in the burette, so you need to figure it out for your own scenario.
Post by: RuiAce on November 02, 2016, 10:57:00 pm
Hey does anyone know an equation to describe the reaction of saponifcation that is easy to memorise? all the ones ive come across are really big and look really hard to memorise...All you need to remember is
THANKS
Fat/Oil + 3NaOH -> 3Soap + Glycerol
Post by: RuiAce on November 02, 2016, 11:02:36 pm
What units is the turbidity measured in? and how to TOMS work for measuring ozone?"Nephelometric Turbidity Units", but you don't even need to know that.
Thanks :)
Post by: g98 on November 02, 2016, 11:10:17 pm
"Nephelometric Turbidity Units", but you don't even need to know that.
Ok Thanks, and do we need to know the Winkler method?
Post by: RuiAce on November 02, 2016, 11:11:24 pm
Ok Thanks, and do we need to know the Winkler method?Yeah nope
Unless Jake did..
Post by: tennis1 on November 02, 2016, 11:13:18 pm
When the water molecules dissolve into the water you end up messing up the volume of the water as well. Plus you don't know the chemical formula for hydrous sodium carbonate to properly calculate the moles
With the burette it honestly depends on if you put acid or base in the burette, so you need to figure it out for your own scenario.
right that makes sense!
just one more thing sorry-
is there a 'general rule' that I can just memorise, because the burette thing confuses me, and I have a feeling it will be in the exam (as it's part of the new course.)
Like if there was acid in burette VS. if there was a base in burette (do we add more or less of the acid/base in burette if it's diluted, and what would be the overall effect on the concentration) thanks :)
Post by: gabe.ralph on November 03, 2016, 03:28:38 pm
(http://McChemdsmall.jpg)
Post by: FallonXay on November 06, 2016, 10:30:33 pm
So now that HSC is over, I was thinking of self-teaching HSC Chemistry. So in regards to this, I wanted to ask how relevant the preliminary course is (Because textbooks and resources are quite pricey - so if the prelim course isn't too big, I won't overly bother with getting resources for it ;) )
Thanks :)
Post by: RuiAce on November 06, 2016, 10:31:00 pm
Hiyaa~Actually, let me be honest with you.
So now that HSC is over, I was thinking of self-teaching HSC Chemistry. So in regards to this, I wanted to ask how relevant the preliminary course is (Because textbooks and resources are quite pricey - so if the prelim course isn't too big, I won't overly bother with getting resources for it ;) )
Thanks :)
Prelim chemistry is more useful for science than HSC chemistry. It's more worthwhile to learn that one instead.
P.S. Congrats on 100th post
Post by: Yasminpotts1105 on November 07, 2016, 08:27:33 pm
Post by: Yasminpotts1105 on November 07, 2016, 08:32:15 pm
Post by: RuiAce on November 07, 2016, 08:34:36 pm
Future applications of knowing how to ferment sucrose to ethanol?
Uses in society of knowing about the fermentation of sucrose to ethanol, apart from production of ethanol?Both of these are not in the scope of the HSC course.
The process of fermentation only serves as a means of going from cellulose to ethanol. Everything else is centered around ethanol itself, not the fermentation process.
Post by: mbdtHSC on November 09, 2016, 11:53:54 pm
Post by: Dolphax on November 10, 2016, 11:00:42 am
When evaluating the impact on society for silver-oxide button cell, is it enough to mention its use in wristwatches and calculators due to small size and high constant voltage? I know it is used in pacemakers to force the ventricle? I think to contract with greater force, but is that kind of detail even needed and how long would a button cell last in a pacemaker anyway?
No, it's definitely not needed to talk about "forcing ventricles." You really only need to list some properties of the button cell relating to its cost/practicality and relates it to impacts on society.
Some could include:
- very small and light
- relatively long life
- expensive metal
- constant output voltage
- non rechargeable
- non toxic metal
and then you can relate these to watches, cameras, hearing aids etc (e.g. small size allows for miniature electric appliances, non-toxic nature allows use inside body etc)
Post by: mjorfian on November 12, 2016, 12:30:06 pm
20a) Calculate the mass of ethanol that must be burnt to increase the temperature of 3 210 g of water by 65°C, if exactly half of the heat released by this combustion is lost to the surroundings.
The heat of combustion of ethanol is 1367 kJ mol −1.
I got through the first step which was calculating heat of combustion, but i don't understand why they they divided the answer by 1367 in the second step to find the moles? It's probs super basic but I just can't wrap my head around WHY. (the solutions used the formula n = q/n) Would you mind explaining it to me please?
link to solutions: http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2009exams/pdf_doc/chemistry-sample-answers-09.pdf
Post by: RuiAce on November 12, 2016, 12:46:29 pm
This is from the 2009 HSC paper:
20a) Calculate the mass of ethanol that must be burnt to increase the temperature of 3 210 g of water by 65°C, if exactly half of the heat released by this combustion is lost to the surroundings.
The heat of combustion of ethanol is 1367 kJ mol −1.
I got through the first step which was calculating heat of combustion, but i don't understand why they they divided the answer by 1367 in the second step to find the moles? It's probs super basic but I just can't wrap my head around WHY. (the solutions used the formula n = q/n) Would you mind explaining it to me please?
link to solutions: http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2009exams/pdf_doc/chemistry-sample-answers-09.pdf
Post by: Yasminpotts1105 on November 15, 2016, 07:01:53 am
Post by: RuiAce on November 15, 2016, 07:14:02 am
'The enthalpy of combustion of ethanol is 1364kJ mol. If you use ethanol to heat 250L of water, calculate the mass of ethanol you must use to heat the water from 20 degrees to 80 degrees. Show your working. (Data: density (H2O) = 1 g L , specific heat of water = 4.18 J g C.)Did you mean the density of water is 1 kg/L
Post by: Yasminpotts1105 on November 15, 2016, 07:58:42 am
Post by: RuiAce on November 15, 2016, 11:41:50 am
Subject to some minor inaccuracies but 75% of the question is done. Last bit I included the relevant formula
Post by: kiwiberry on November 15, 2016, 06:41:40 pm
Post by: jakesilove on November 15, 2016, 09:10:36 pm
why is polystyrene insulative to heat? and why is pvc weather resistant?
Thankfully, not part of the curriculum! We don't really care WHY, we just care that it is :)
Post by: kiwiberry on November 17, 2016, 02:37:51 pm
Thankfully, not part of the curriculum! We don't really care WHY, we just care that it is :)
Oh really? :o so for a question like "Describe the uses of three named addition polymers in terms of their structure and properties", would you have to relate structure to properties?
Post by: jakesilove on November 17, 2016, 05:09:50 pm
Oh really? :o so for a question like "Describe the uses of three named addition polymers in terms of their structure and properties", would you have to relate structure to properties?
Sorry, I sort of understand where you're coming from now; properties are different from 'WHY' the properties are apparent. So, the PROPERTY is that it is a good insulator, and the use is thermal cups etc. Why is it a good insulator? You don't need to know that. You just need to be able to list the properties themselves (eg. insulator, hard, brittle, strong) and then relate that to a use. Does that make sense?
Post by: Happy Physics Land on November 18, 2016, 11:06:03 am
Oh really? :o so for a question like "Describe the uses of three named addition polymers in terms of their structure and properties", would you have to relate structure to properties?
These are really all you need to know:
1. linear, crystalline structure = high tensile strength, high hardness, high impact resistance
2. non-linear, amorphous structure = low tensile strength, low hardness, low impact resistance
3. Bulky side groups/side chains = high stiffness because carbon chains cannot bend easily
4. Having a N, O or F in the structure (e.g. cellulose) = high tensile strength because hydrogen bonding between chains
5. Having double or triple bonds in the structure = higher boiling point, because more energy input required to overcome double or triple bonds
Post by: RuiAce on November 18, 2016, 11:11:34 am
These are really all you need to know:Fortunately even then those aren't that big of a deal, except for maybe thy double bond one. It doesn't hurt to know these though I reckon
1. linear, crystalline structure = high tensile strength, high hardness, high impact resistance
2. non-linear, amorphous structure = low tensile strength, low hardness, low impact resistance
3. Bulky side groups/side chains = high stiffness because carbon chains cannot bend easily
4. Having a N, O or F in the structure (e.g. cellulose) = high tensile strength because hydrogen bonding between chains
5. Having double or triple bonds in the structure = higher boiling point, because more energy input required to overcome double or triple bonds
Post by: kiwiberry on November 18, 2016, 02:32:16 pm
Fortunately even then those aren't that big of a deal, except for maybe thy double bond one. It doesn't hurt to know these though I reckon
These are really all you need to know:
1. linear, crystalline structure = high tensile strength, high hardness, high impact resistance
2. non-linear, amorphous structure = low tensile strength, low hardness, low impact resistance
3. Bulky side groups/side chains = high stiffness because carbon chains cannot bend easily
4. Having a N, O or F in the structure (e.g. cellulose) = high tensile strength because hydrogen bonding between chains
5. Having double or triple bonds in the structure = higher boiling point, because more energy input required to overcome double or triple bonds
Sorry, I sort of understand where you're coming from now; properties are different from 'WHY' the properties are apparent. So, the PROPERTY is that it is a good insulator, and the use is thermal cups etc. Why is it a good insulator? You don't need to know that. You just need to be able to list the properties themselves (eg. insulator, hard, brittle, strong) and then relate that to a use. Does that make sense?
yes, that makes sense!! thanks so much guys :)
Post by: anotherworld2b on November 23, 2016, 01:46:56 am
I am not sure how to write the equation needed for q5. For q7 iam not sure what to do ???
Post by: jakesilove on November 23, 2016, 12:52:46 pm
Hi i was wondering if i could please get help with these 2 questions.
I am not sure how to write the equation needed for q5. For q7 iam not sure what to do ???
For question 7, find the number of moles you have of each gas (using the weight), multiply that by the molar volume, and see which one equals 3.85L
For question 5, find the number of moles of the acid that have been added. If you multiply this by 6 (as there are 6 moles of flourine for every one mole of acid) you will get the moles of flourine released. Then, find the concentration in the solution, and you should be done.
Post by: anotherworld2b on November 24, 2016, 06:07:29 pm
I tried q5. Would this be correct? I was wondering how would you write the chemical reaction?
For question 7, find the number of moles you have of each gas (using the weight), multiply that by the molar volume, and see which one equals 3.85L
For question 5, find the number of moles of the acid that have been added. If you multiply this by 6 (as there are 6 moles of flourine for every one mole of acid) you will get the moles of flourine released. Then, find the concentration in the solution, and you should be done.
Post by: f_tan on November 24, 2016, 08:39:54 pm
Post by: jakesilove on November 24, 2016, 09:30:27 pm
I'm still a bit confused about q7. Could you please give me an example?
I tried q5. Would this be correct? I was wondering how would you write the chemical reaction?
Looks correct. If the molar mass of a gas was 3g, then you find the moles of the gas by 5.78/3. Then, multiply this by 24.71L to get the molar volume. Clearly, that isn't equal to the required moles, so that isn't the correct gas.
Post by: jakesilove on November 24, 2016, 09:31:17 pm
I've got a chem research task and one of the things I have to do is research the history of batteries. I'm a little confused - can anyone clarify the difference between the voltaic, galvani, and daniell cell, and who invented them? Thank you!
My understanding was that the cells were actually the same, just different names! You need to know the development of battery technologies, but I believe that the names of the batteries aren't very important.
Post by: RuiAce on November 24, 2016, 09:34:59 pm
My understanding was that the cells were actually the same, just different names! You need to know the development of battery technologies, but I believe that the names of the batteries aren't very important.Though gonna add on, pretty sure that the Daniell cell is a specific type of galvanic cell because it has to be between copper and zinc. (Going off memory here; my teacher did give us a worksheet.)
Post by: f_tan on November 26, 2016, 01:22:38 pm
Though gonna add on, pretty sure that the Daniell cell is a specific type of galvanic cell because it has to be between copper and zinc. (Going off memory here; my teacher did give us a worksheet.)
Hi, sorry I'm back with this question again :P
I'm still confused with which one the Daniell cell is, because different websites have different descriptions and diagrams?
Which one is the Daniell cell?
(http://i.imgur.com/q0jKY78.png) OR (http://i.imgur.com/TPMZ0wr.png)
Thanks!
Post by: RuiAce on November 26, 2016, 01:39:56 pm
Hi, sorry I'm back with this question again :PThey are the same cell. The only difference in that diagram is that one of them has the electrolytes in two seperate compartments, whereas the other has them in the same.
I'm still confused with which one the Daniell cell is, because different websites have different descriptions and diagrams?
Which one is the Daniell cell?
(http://i.imgur.com/q0jKY78.png) OR (http://i.imgur.com/TPMZ0wr.png)
Thanks!
That doesn't change the cell. The cell is the same because all the chemicals (copper etc.) are the same
Post by: J.B on November 30, 2016, 08:51:11 pm
Post by: RuiAce on November 30, 2016, 08:52:43 pm
I was just wondering in calculation questions, how do you know how many decimal places your answer should be to?Always round to the LEAST number of SIGNIFICANT FIGURES given in the question. Never rely solely on decimal points without good reason.
Post by: J.B on November 30, 2016, 09:12:12 pm
Always round to the LEAST number of SIGNIFICANT FIGURES given in the question. Never rely solely on decimal points without good reason.
Thank you
Post by: FallonXay on December 08, 2016, 08:07:51 am
[Prelim Question] I'm looking at examples of triatomic molecules in the textbook and one example is water (H2O). Why is water a molecule, I thought it was just a compound? Also what does 'Molecules are particles that can move independently of one another' mean?
Thanks.
Post by: RuiAce on December 08, 2016, 10:23:45 am
Hello~Compounds are a subclass of molecules.
[Prelim Question] I'm looking at examples of triatomic molecules in the textbook and one example is water (H2O). Why is water a molecule, I thought it was just a compound? Also what does 'Molecules are particles that can move independently of one another' mean?
Thanks.
Molecules just mean there's more than one atom in your substance. Note that a molecule can be a compound (H2O) or an element (O2)
All compounds are molecules. However elements are either molecules (O2) or single atoms (Al)
Not too sure what they really mean by move independently. I thought that was just a given; one molecule moving doesn't have to affect the other molecule moving.
Post by: FallonXay on December 08, 2016, 10:57:17 am
All compounds are molecules. However elements are either molecules (O2) or single atoms (Al)
And can all single atoms also be called monatomic molecules?
(It says helium atoms He and argon atoms Ar are examples monatomic molecules)
Post by: RuiAce on December 08, 2016, 10:59:38 am
And can all single atoms also be called monatomic molecules?I don't think 'monatomic molecules' is a part of the chemistry glossary. It's a contradiction.
(It says helium atoms He and argon atoms Ar are examples monatomic molecules)
Molecule implies two or more atoms. Monatomic implies just one atom.
Post by: Yasminpotts1105 on December 13, 2016, 06:49:27 pm
Post by: Yasminpotts1105 on December 13, 2016, 07:29:22 pm
Post by: RuiAce on December 13, 2016, 07:31:57 pm
Why do we specifically use cold water in the prac to determine and compare the heat of combustion of alcohols?I think this is to do with how heating works, but considering heat losses as well.
In a way, it is 'easier' to heat cold water, as opposed to hot water. This mainly has to do with how we have heat losses everywhere; heat gets absorbed by the calorimeter, lost to surroundings and all sorts of problems that damage the accuracy of the practical.
If we could heat the water, so that literally all of the heat went into the water and not any surroundings, I believe using cold water or hot water will not matter. But because there's all these other things to consider, cold water can potentially make our measurements more realistic and accurate.
That and it may also have to do with the thermometer as well. I feel like I rambled in helping you though.
Jake!!!
How do we produce large amounts of butanol?This is not in the syllabus. You may be able to just fugde it though and compare it to how ethanol is produced.
Post by: jakesilove on December 13, 2016, 07:39:48 pm
Why do we specifically use cold water in the prac to determine and compare the heat of combustion of alcohols?
The short answer is that we don't. The formula only cares about the change in temperature, so based on that the same amount of energy is required to raise water's temperature from 25 to 26 degrees as it takes to raise water's temperature from 89 to 90 degrees. So, we generally start with cold water and heat it up because that's just the easiest thing to do. What's the alternative? Heat some water up using a hot plate, then do the experiment? Then, you'll encounter problems with determining the initial temperature accurately (as it continues to drop before more heat is applied), consistent heating of the liquid, heating of glassware and metal etc.
However, Rui also has a point. A heated system (in comparison to the environment around it) is more likely to cool, because the 'aim' of a system is to reach a single temperature. So, if we start with room temperature water, it will be very hard for it to cool down. However, water at 80 degrees can cool down much more easily, as the environment REALLY wants to suck up some of that heat. To be honest, I think this affect has very, very little impact on the experiment as a whole. I think my first answer is more pertinent to your question.
Post by: jakesilove on December 15, 2016, 01:29:20 pm
Why do we actually need to know the molar heat of combustion of alcohols?
Do you mean, like, why is it important for chemists to understand the molar heat of combustion? Essentially, it allows us to understand and predict how much energy we can release, by burning a set quantity of a liquid. From that, we can determine which fuels are most efficient, and thus which we should use to power cars etc. For instance, the molar heat of combustion of Ethanol is 2/3 that of Octane, making it a worse fuel. Does that answer your question?
Post by: bluecookie on December 27, 2016, 11:48:52 am
Post by: RuiAce on December 27, 2016, 12:04:39 pm
Hello, I'm just a bit confused about the ionisation energy of alkali and alkaline metals. My handout sheets say alkaline metals are higher in ionisation energy, yet all elements to the left of the 4th row all have positive oxidation states, meaning they lose electrons to complete the octet rule. When I googled it, it said ionisation energy was the ability of a particle to strip electrons from another atom. None of them strip electrons from other atoms to form compounds but alkaline metals loses more electrons in the process than alkali metals to form compounds and satisfy the octet rule, so how do alkaline metals have a higher ionisation energy than alkali metals?I'm actually getting confused at what your point is.
The ionisation energy is the energy required, for another atom, to come take a specified electron FROM the atom in question. The first ionisation energy of potassium is the amount of energy required to take that single valence electron FROM sodium. In other words, it is the amount of energy required to make sodium go from K, to K+
Similarly, the first ionisation energy of calcium is the energy required, for another atom, to come take one of the two valence electrons FROM magnesium. It is the amount of energy required to make Mg become Mg+
As is the second ionisation energy, the amount of energy required to make Mg+ become Mg2+
So of course alkaline earth metals such as calcium have a higher ionisation energy than alkali earth metals such as potassium. Alkali metals are more reactive than alkaline earth metals because firstly, they only have one valence electron, not two. Secondly, remember that the electrons are held in orbit of the nucleus, because of the protons. Because something like potassium has less protons (19) in the nucleus than calcium (20), the electrons are held more tightly in alkaline earth metals. So it should be HARDER for that electron to come off an alkaline earth metal, than an alkali metal. Hence why the first ionisation energy would be higher.
Also, I'm not sure how the oxidation states matter here. The oxidation state of any PURE element is ALWAYS 0. You're probably thinking about the more common oxidation states of the ions formed.
That does not matter here though. I don't see how oxidation states relate to ionisation energies.
Post by: bluecookie on December 27, 2016, 12:17:01 pm
I'm actually getting confused at what your point is.
The ionisation energy is the energy required, for another atom, to come take a specified electron FROM the atom in question. The first ionisation energy of potassium is the amount of energy required to take that single valence electron FROM sodium. In other words, it is the amount of energy required to make sodium go from K, to K+
Similarly, the first ionisation energy of calcium is the energy required, for another atom, to come take one of the two valence electrons FROM magnesium. It is the amount of energy required to make Mg become Mg+
As is the second ionisation energy, the amount of energy required to make Mg+ become Mg2+
So of course alkaline earth metals such as calcium have a higher ionisation energy than alkali earth metals such as potassium. Alkali metals are more reactive than alkaline earth metals because firstly, they only have one valence electron, not two. Secondly, remember that the electrons are held in orbit of the nucleus, because of the protons. Because something like potassium has less protons (19) in the nucleus than calcium (20), the electrons are held more tightly in alkaline earth metals. So it should be HARDER for that electron to come off an alkaline earth metal, than an alkali metal. Hence why the first ionisation energy would be higher.
Also, I'm not sure how the oxidation states matter here. The oxidation state of any PURE element is ALWAYS 0. You're probably thinking about the more common oxidation states of the ions formed.
That does not matter here though. I don't see how oxidation states relate to ionisation energies.
Ohh, thank you :)
I thought the ionisation energy was the likelihood that the atom in question could take an electron from another atom. I think I got it the opposite way around! :P It all makes sense now, thanks for clarifying :)
Post by: bsdfjnlkasn on December 27, 2016, 07:19:49 pm
I'm getting a tad confused with galvanic cells and the list of standard potentials given on the back of the periodic table.
So I understand all the values are given as though the element is undergoing reduction and that you just reverse them when figuring out the cathode's half equation and the associated electrical potential/voltage.
But, when I was writing my study notes I came across something which really confused me and so I was wondering if someone could verify/explain the following dot points to me.
• The electrode oxidising will always have the lowest SRP
o Where a non-metal and a metal are present, the metal oxidises
o Where two non-metals are present, the least reactive oxidises
Thank you!!
Post by: RuiAce on December 27, 2016, 08:03:26 pm
Hey there,The second dot point can be verified by using the table on the data sheet, or checked by some general chemistry knowledge. The table approach is that all the gases are towards the bottom, below all the metals. You won't find a gas somewhere above a metal on there, so you can deduce that the metal is always oxidized and the gas is always reduced.
I'm getting a tad confused with galvanic cells and the list of standard potentials given on the back of the periodic table.
So I understand all the values are given as though the element is undergoing reduction and that you just reverse them when figuring out the cathode's half equation and the associated electrical potential/voltage.
But, when I was writing my study notes I came across something which really confused me and so I was wondering if someone could verify/explain the following dot points to me.
• The electrode oxidising will always have the lowest SRP
o Where a non-metal and a metal are present, the metal oxidises
o Where two non-metals are present, the least reactive oxidises
Thank you!!
The general knowledge approach is that, gases would rather gain electrons to have a full outer shell than lose electrons. Remember that oxidation is loss, so the metal would have a greater tendency to lose than the gas.
The third dot point isn't necessary at all. However, you can check the table and note that chlorine is below bromine, so if you form a cell between chlorine and bromine (+ required ions obviously), clearly the bromine will be oxidised. Indeed, bromine is also more reactive than chlorine.
This is also true in general for metals. Zinc is more reactive than copper, and you can check that between these two zinc will be the one getting oxidised.
As for the first one, well this goes completely back to what standard reduction potentials are. The data sheet displays each half equation for reductions, and the corresponding reduction potentials (Eo values). As a rule, we KNOW that the top one would always be the one undergoing oxidation. E.g. between sodium and magnesium, the sodium (found further up) will be undergoing oxidation.
Which makes sense. A more reactive metal will lose its electron more readily. The voltage produced by the cell should be higher if that's the one losing the electron, and giving it to the less reactive metal. From your studies in school, you should be well aware that the metal found closer to the top of the metal undergoes oxidation (and its half equation needs to be flipped).
Post by: bsdfjnlkasn on December 27, 2016, 10:06:44 pm
I just had a query about the first syllabus dot point for the Production of Materials module which follows: "Construct word and balanced formulae equations of chemical reactions as they are encountered"
Would it really be necessary to include word equations in year 12? Like how often does that come up in the HSC? You would hope the markers don't need the provided reactions spelled out for them and neither do you want us students wasting time writing out chlorine instead of Cl for example. I also don't really want to waste room on my notes including stuff that won't/is unlikely to be assessed so I would really appreciate having that minor issue clarified.
Thank you!!
Post by: RuiAce on December 27, 2016, 10:14:30 pm
Hey there,Quite rarely. An example of where it appears is actually in the industrial chemistry option, and the reason is because remembering the chemical formulae is a bit too difficult.
I just had a query about the first syllabus dot point for the Production of Materials module which follows: "Construct word and balanced formulae equations of chemical reactions as they are encountered"
Would it really be necessary to include word equations in year 12? Like how often does that come up in the HSC? You would hope the markers don't need the provided reactions spelled out for them and neither do you want us students wasting time writing out chlorine instead of Cl for example. I also don't really want to waste room on my notes including stuff that won't/is unlikely to be assessed so I would really appreciate having that minor issue clarified.
Thank you!!
But there is no reason for it to be unexaminable. Just because it's rare and usually dominated by the apparently superior balanced formulae equations, doesn't mean it's an impracticality. Plus it is extremely useful to describe general cases such as alkene + water -> alkanol (although you wouldn't if you don't have to).
Spelling, however, they are usually lenient in the HSC. They're not marking you on spelling (that is ridiculous); they are marking you on understanding of the content.
To be honest, IF you know the balanced chemical formulae equations, you should be able to derive the word equations on the spot. There's no good reason to make notes for word equations.
Post by: bsdfjnlkasn on December 28, 2016, 10:21:57 pm
What basic carbon-chain structures does cellulose have which makes it able to be used in the production of petrochemicals? And how are they helpful?
Post by: RuiAce on December 28, 2016, 11:18:15 pm
Hey there,(http://antoine.frostburg.edu/chem/senese/101/consumer/images/cellulose.gif)
What basic carbon-chain structures does cellulose have which makes it able to be used in the production of petrochemicals? And how are they helpful?
I see carbon atoms everywhere in cellulose. That is it. A basic carbon-chain structure is just a chain of carbons being there. That is not meant to be overanalysed.
Petrochemicals. Things such as ethylene and ethanol, and say octane and polyethylene. They're full of carbon chains. So if we break down cellulose and gather all the carbon chains, we should be able to make other petrochemicals.
(Easiest way to demonstrate how it works - just talk about how you can convert cellulose all the way down to ethanol. Which can be converted to ethylene, and then into a bunch of other stuff.)
How are they helpful? Well right now we produce our petrochemicals from crude oil. What about it? It's a non-renewable resource (will run out).
Cellulose on the other hand? Abundant, and renewable. Meaning we won't easily run out.
Post by: bluecookie on December 29, 2016, 03:29:43 pm
What is the mass of the potassium that reacted with the water in the scenario described above?
What volume of hydrogen gas would be produced during the reaction at 25 degrees c and 100 kpa?
Post by: RuiAce on December 29, 2016, 03:39:17 pm
Potassium reacts with water to form potassium hydroxide and hydrogen gas. When a piece of potassium reacted with 200ml of water, the resulting solution was found to have a concentration of 0.0046 mol L.Translating the words into a balanced chemical equation we have:
What is the mass of the potassium that reacted with the water in the scenario described above?
What volume of hydrogen gas would be produced during the reaction at 25 degrees c and 100 kpa?
2 K(s) + H2O(l) → 2 KOH(aq) + H2(g)
The resulting solution was dissolved in 200 mL of water. Its concentration is given as 0.0046 mol L-1.
So by using n = CV, we have nKOH = 0.0046 * 0.2 = 0.00092 mol
The reaction shows that 1 mol of KOH is produced alongside 0.5 mol of H2
Hence nH2 = 0.00046 mol
Since we are at 25oC and 100 kPa, the data sheet tells us that the molar volume is VM = 24.79 L mol-1
So by using n = V/VM we have V = 0.0046 * 24.79 = 0.114034 L
Post by: bsdfjnlkasn on December 29, 2016, 07:43:48 pm
How often does the HSC ask for literal methods of experiments conducted during the year? Should I spend a long time memorising them (as in the specific measurements and equipment)?
Thank you :)
Post by: RuiAce on December 29, 2016, 07:48:47 pm
Hey there,If the results are quantitative, then they probably will not matter.
How often does the HSC ask for literal methods of experiments conducted during the year? Should I spend a long time memorising them (as in the specific measurements and equipment)?
Thank you :)
However, you must know the experimental method to fine details if you want mark maximisation. As well as some results if they are qualitative.
It is possible to get away with writing a method using a numbered format, I believe (might want to double check with Jake). However, you absolutely need to know your method as it is always possible (sometimes likely) that you will be asked on any random method of the course. They will explicitly say something along the lines of "outline an experiment where you ..."
An experimental method without numeric quantities and not stating what materials are used (e.g. conical flask) can easily show signs of lack of understanding. There's situations where you would prefer a conical flask over a beaker because you're interested in things not spilling out, and measurements may not be required. And you never want to add too much of a substance either (e.g. you need to balance out the amount of alkanol v.s. alkanoic acid for esterification, as per LCP and other factors)
You also need to be able to assess the validity, reliability and accuracy of the experiment you conducted.
All of this is typically a pain to most students. However, understanding the experiments you performed and being able to describe each and every one of them is definitely examinable, and takes considerable time to know fully. Experiments are examined when you least expect it to be.
Post by: bsdfjnlkasn on December 29, 2016, 08:18:58 pm
When we are calculating the enthalpy of a certain fuel (q=mcAt), why do we use all the water measurements? i.e. it's mass, 4.18 and change in temp?
Thank you :)
Post by: RuiAce on December 29, 2016, 08:22:08 pm
Hey there,q is the energy required to heat the water so that the temperature changes by ΔH. So everything on the RHS is related to the water.
When we are calculating the enthalpy of a certain fuel (q=mcAt), why do we use all the water measurements? i.e. it's mass, 4.18 and change in temp?
Thank you :)
Post by: bsdfjnlkasn on December 31, 2016, 09:36:43 pm
Post by: RuiAce on December 31, 2016, 09:53:57 pm
What would the substitution reaction equation between an alkane (like hexane) and bromine water be when exposed to UV light ?The product is the same product as in the addition reaction involving alkenes. But hydrogen gas is also produced (you should be able to see why)
Post by: bsdfjnlkasn on January 01, 2017, 11:28:24 am
C6H12 + Br2 –UV light -> C6H11Br + HBr?
I'm not sure what you mean there being hydrogen produced because by drawing out the molecules this is what I got. Is there a better/quicker method to use?
Post by: RuiAce on January 01, 2017, 11:35:02 am
Saying we used cyclohexane, would this be correct:
C6H12 + Br2 –UV light -> C6H11Br + HBr?
I'm not sure what you mean there being hydrogen produced because by drawing out the molecules this is what I got. Is there a better/quicker method to use?
Cyclohexene addition reaction:
C6H10 + Br2 -> C6H10Br2
Cyclohexane substitution reaction:
C6H12 + Br2 -> C6H10Br2 + H2
Edit: Strikethrough - Or maybe it's correct. It makes sense, but was definitely not what I was taught for addition/substitution reactions. Time to call in Jake
Post by: Yasminpotts1105 on January 01, 2017, 02:28:30 pm
Because for a dry cell, I have the anode and cathode half equations but I don't understand how I condense it into one?
Post by: RuiAce on January 01, 2017, 02:41:12 pm
What is an overall reduction-oxidation equation?Give an example of two half equations and I'll remake the complete equation
Because for a dry cell, I have the anode and cathode half equations but I don't understand how I condense it into one?
Post by: jakesilove on January 01, 2017, 02:57:13 pm
That reaction is wrong.
Cyclohexene addition reaction:
C6H10 + Br2 -> C6H10Br2
Cyclohexane substitution reaction:
C6H12 + Br2 -> C6H10Br2 + H2
Edit: Strikethrough - Or maybe it's correct. It makes sense, but was definitely not what I was taught for addition/substitution reactions. Time to call in Jake
Definitely stick with Rui's reactions above. Whilst yours are possible, his are the 'likely' reaction products, and the most useful ones for your purpose
Post by: Yasminpotts1105 on January 01, 2017, 03:21:03 pm
2MnO2 + 2NH4 = Mn2O3 + H2O + 2NH3
Post by: RuiAce on January 01, 2017, 03:33:29 pm
Zn (s) = Zn^2+ + 2e^-That second one is incorrect. There should be two more electrons on the left.
2MnO2 + 2NH4 = Mn2O3 + H2O + 2NH3
2MnO2 + 2NH4 + 2 e^- = Mn2O3 + H2O + 2NH3
Now it should be clearer as to how they can be combined.
Post by: Aaron12038488 on January 03, 2017, 01:09:34 pm
Post by: jakesilove on January 03, 2017, 08:40:04 pm
for prelim chem, the first topic, it looks really boring. Is this the case for the rest of the topics. Also for chem and other sciences, should i memorise everything
Whether a topic is 'boring' will be entirely dependent on what you're interested in! Try to find something that you care about, and focus on that in your studies.
I think that most of Science is about understanding the content, and then there is some memorisation involved. You'll learn to strike the right balance over time
Post by: bluecookie on January 04, 2017, 12:23:56 pm
Post by: RuiAce on January 04, 2017, 12:28:48 pm
For the HSC, do we need to memorise the colour range of common indicators, and also the pH that they change? Or will those values be provided for us in the question?You will need to remember what colour they are in generic ranges (e.g. strongly acidic, weakly basic). And you do need to know where abouts it changes colour (e.g. phenolphthalein changes colour over the basic range). That's for the four common indicators in the course; there are tons of indicators out there but only the four common ones matter.
However, the specific values are usually stated. A band 6 student will probably memorise them anyway because writing it down impresses the examiner, but I've found that usually they tell you them if you need to refer to it.
Post by: bluecookie on January 04, 2017, 12:47:43 pm
Post by: RuiAce on January 04, 2017, 01:03:47 pm
Thanks :) On a similar note, is it expected we memorise the carbon dioxide water equilibrium equations?Yes that's crucial.
As a rule of thumb, memorise as many equations as you can. However Jake made a list a while back.
Post by: bluecookie on January 04, 2017, 01:39:31 pm
Yes that's crucial.
As a rule of thumb, memorise as many equations as you can. However Jake made a list a while back.
Thank you ^^
Post by: shreya_ajoshi on January 09, 2017, 11:22:01 am
The way I do it is to write Octane's combustion reaction and balance it.
C8H18 + O2 --> CO2 +H2O
When you balance it, the ratio of octane to carbon dioxide is 1:8. Wouldn't that mean that the amount of energy carbon dioxide releases is 5460 x 8 = 43680. But in the answers, they do 5460/8 = 682.5.
Why do they divide instead of multiply
Post by: jakesilove on January 09, 2017, 11:42:23 am
THe amount o energy released in kJ per mole during cobustion of octance is 5460. How would you find the amount of energy release in kJ/mol of Carbon Dioxide.
The way I do it is to write Octane's combustion reaction and balance it.
C8H18 + O2 --> CO2 +H2O
When you balance it, the ratio of octane to carbon dioxide is 1:8. Wouldn't that mean that the amount of energy carbon dioxide releases is 5460 x 8 = 43680. But in the answers, they do 5460/8 = 682.5.
Why do they divide instead of multiply
Very interesting question! Never seen anything like it before. So, looking at the reaction, we have
As you've stated, the relationship between Octane and Carbon dioxide is clearly
Well, if the forward reaction RELEASES 5460 kJ of energy, the backwards reaction (ie. Carbon dioxide and Water turning into Octane and Oxygen) must ABSORB 5460 kJ of energy (switch the arrow the other way around, and switch the sign of the heat of reaction). Therefore, we can say that the '8' moles of Carbon dioxide is ALSO associated with 5460 kJ of energy. Thus, to get ONE mole of Carbon dioxide, we divide the total energy (5460 kJ) by the number of moles (8) to get our answer (682.5 kJ). I have the say, this is a really strange question, and I've never seen anything like it in the HSC. I also don't think I've explained in brilliantly, but that's my answer to the best of my knowledge. Does it sort of make sense? Don't worry too much if it doesn't; it's not very important in the HSC.
Great question though! Very hard, but you were on the right track!
(Mod edit: Fixing the LaTex)
Post by: shreya_ajoshi on January 09, 2017, 12:06:45 pm
Very interesting question! Never seen anything like it before. So, looking at the reaction, we have
As you've stated, the relationship between Octane and Carbon dioxide is clearly. We know that 5460kJ of energy is released in the combustion of one mole of Octane. How can we think about this question?
Well, if the forward reaction RELEASES 5460 kJ of energy, the backwards reaction (ie. Carbon dioxide and Water turning into Octane and Oxygen) must ABSORB 5460 kJ of energy (switch the arrow the other way around, and switch the sign of the heat of reaction). Therefore, we can say that the '8' moles of Carbon dioxide is ALSO associated with 5460 kJ of energy. Thus, to get ONE mole of Carbon dioxide, we divide the total energy (5460 kJ) by the number of moles (8) to get our answer (682.5 kJ). I have the say, this is a really strange question, and I've never seen anything like it in the HSC. I also don't think I've explained in brilliantly, but that's my answer to the best of my knowledge. Does it sort of make sense? Don't worry too much if it doesn't; it's not very important in the HSC.
Great question though! Very hard, but you were on the right track!
(Mod edit: Fixing the LaTex)
Ahhh, i get it now! Thank you very much :)
Post by: shreya_ajoshi on January 09, 2017, 01:03:04 pm
I2O5 + 3CO --> I2 + 3CO2
Post by: RuiAce on January 09, 2017, 01:06:10 pm
Identify the species that is oxidised, and the species that is reducedWrite out the relevant oxidation states.
I2O5 + 3CO --> I2 + 3CO2
Reactants:
O: -2 each
I: +5 each
O: -2 each
C: +2
________
Products:
I: 0
C: +4
O: -2 each
Hence, iodine is reduced whilst carbon is oxidised.
The important thing is if you understand where those numbers came from
Post by: shreya_ajoshi on January 09, 2017, 01:40:33 pm
Write out the relevant oxidation states.
Reactants:
O: -2 each
I: +5 each
O: -2 each
C: +2
________
Products:
I: 0
C: +4
O: -2 each
Hence, iodine is reduced whilst carbon is oxidised.
The important thing is if you understand where those numbers came from
I understand the oxidation states for the reactants and the iodine product. I don't understand how you got the oxidation states for the carbon dioxide product.
Post by: RuiAce on January 09, 2017, 01:59:32 pm
I understand the oxidation states for the reactants and the iodine product. I don't understand how you got the oxidation states for the carbon dioxide product.Each oxygen always has an oxidation state of -2 unless it's the oxygen element or a member of the peroxide ion.
Carbon dioxide consists of 2 oxygens. Hence, in total, the two oxygens amount to an oxidation state of -4.
But carbon dioxide has no charge. Hence, the sum of oxidation states of all the atoms in carbon dioxide should be 0.
To ensure that happens, the oxidation state of carbon must be +4 to cancel it out
Post by: Iminschool on January 09, 2017, 05:19:20 pm
Post by: jakesilove on January 09, 2017, 06:00:06 pm
For this question, what does 'destructive and non-destructive testing' mean?
A destructive test is one in which the sample is destroyed (eg. if I were to test water by vapourising it, then the water is destroyed!). A non-destructive test is one in which the sample is NOT destroyed (eg. if I tested the reflectivity of a mirror by shining a light onto it, the mirror remains intact!).
Does that make sense?
Specifically, the pH probe is non destructive, as it doesn't do anything to the substance. However, adding an indicator will change the composition of the liquid (ie. raise or lower the pH, react, etc.) and thus is considered destructive.
Post by: RuiAce on January 09, 2017, 06:01:35 pm
A destructive test is one in which the sample is destroyed (eg. if I were to test water by vapourising it, then the water is destroyed!). A non-destructive test is one in which the sample is NOT destroyed (eg. if I tested the reflectivity of a mirror by shining a light onto it, the mirror remains intact!).Never even heard of these before Jake... are they actually in the course or is this your own knowledge?
Does that make sense?
Post by: jakesilove on January 09, 2017, 06:02:47 pm
Never even heard of these before Jake... are they actually in the course or is this your own knowledge?
Definitely in the course; eg. AAS is a destructive test, which is a disadvantage of it :)
Post by: RuiAce on January 09, 2017, 06:05:25 pm
Definitely in the course; eg. AAS is a destructive test, which is a disadvantage of it :)Man. Unless my memory is actually that bad, I feel like I just learnt something way too late...
Post by: Iminschool on January 09, 2017, 06:10:31 pm
A destructive test is one in which the sample is destroyed (eg. if I were to test water by vapourising it, then the water is destroyed!). A non-destructive test is one in which the sample is NOT destroyed (eg. if I tested the reflectivity of a mirror by shining a light onto it, the mirror remains intact!).
Does that make sense?
Specifically, the pH probe is non destructive, as it doesn't do anything to the substance. However, adding an indicator will change the composition of the liquid (ie. raise or lower the pH, react, etc.) and thus is considered destructive.
Yea it does make sense, great use of analogies. Thanks
Post by: Iminschool on January 09, 2017, 08:41:34 pm
Post by: RuiAce on January 09, 2017, 09:26:06 pm
I get C as my answer, requesting confirmation pleaseYep.
Post by: bsdfjnlkasn on January 09, 2017, 09:41:00 pm
I get C as my answer, requesting confirmation please
How would you go about solving this question? I'm not sure where to begin because we're not given any masses
Thank you!
Post by: RuiAce on January 09, 2017, 09:47:47 pm
How would you go about solving this question? I'm not sure where to begin because we're not given any massesIt says 1 kilogram of hydrazine. Which is 1000 g
Thank you!
Post by: bsdfjnlkasn on January 09, 2017, 09:58:19 pm
It says 1 kilogram of hydrazine. Which is 1000 g
Would we find the number of moles for N2H4 (given 1000g) and then x by 24.47?
This is the method I tried but it's obviously wrong, could you possibly start me off and I'll try again?
Post by: RuiAce on January 09, 2017, 10:05:58 pm
Would we find the number of moles for N2H4 (given 1000g) and then x by 24.47?You forgot to consider the equation.
This is the method I tried but it's obviously wrong, could you possibly start me off and I'll try again?
After finding the number of moles of N2H4, we note that 2 moles of N2H4 produces 7 moles of 'gas'. So to get the moles of GAS produced, we must multiply by 7.2 before we consider the 24.47
Post by: Mathew587 on January 13, 2017, 02:28:16 pm
How the hell do we know what the products are in chemcial reaction? :/
Like in most cases, I have no idea what it would be except if its something simple like combustion. Anyone have like a basic equation sheet that would be useful for chem cos I'd love that.
Thanks peeps :)
Post by: Iminschool on January 13, 2017, 02:37:40 pm
Hey guys,
How the hell do we know what the products are in chemcial reaction? :/
Like in most cases, I have no idea what it would be except if its something simple like combustion. Anyone have like a basic equation sheet that would be useful for chem cos I'd love that.
Thanks peeps :)
I think you'll need a sheet on the rules like this one:
Post by: Mathew587 on January 13, 2017, 02:43:42 pm
Do you happen to have one from prelim with all the equations we should know from then? I have a terrible memory btw :_
Post by: RuiAce on January 13, 2017, 02:45:26 pm
Hey guys,There's usually categories.
How the hell do we know what the products are in chemcial reaction? :/
Like in most cases, I have no idea what it would be except if its something simple like combustion. Anyone have like a basic equation sheet that would be useful for chem cos I'd love that.
Thanks peeps :)
If it's oxygen with something, it's probably combustion
If it's acid with base, it's probably neutralisation
etc. You will have to provide further examples if you are confused.
Post by: RuiAce on January 13, 2017, 02:48:09 pm
Yup exactly like that.I don't believe there are many you need to know "off by heart" in prelim.
Do you happen to have one from prelim with all the equations we should know from then? I have a terrible memory btw :_
Some things like acid + metal = salt + hydrogen might pop up.
Most of the time you need to figure it out yourself. If it's too hard they give you the equation, otherwise it should fall under a common category.
Post by: Mathew587 on January 13, 2017, 02:50:34 pm
I should probably just study more hehe
Post by: anotherworld2b on January 15, 2017, 01:02:09 am
I was also wondering what would be the best way to answer this question 'explain relative atomic mass?'
Could you explain it by saying the relative atomic mass of an elememt is the ration of the weighted average mass per atom of the naturally occuring form and reflexts the isotopic composition of the element?
Post by: jakesilove on January 15, 2017, 03:32:18 pm
For this question. Would you explain electrostatic repulsion and attraction as well as strong nuclear force? I am not sure how much depth should be included in this question.
I was also wondering what would be the best way to answer this question 'explain relative atomic mass?'
Could you explain it by saying the relative atomic mass of an elememt is the ration of the weighted average mass per atom of the naturally occuring form and reflexts the isotopic composition of the element?
Yep, to both of your questions. How in depth you answer the question will depend on the marks allocated; for now, just make sure that YOU understand the answer.
Post by: bsdfjnlkasn on January 16, 2017, 04:25:55 pm
Identify oxides of non-metals which act as acids and describe the conditions under which they act as acids
Post by: RuiAce on January 16, 2017, 06:38:57 pm
For the following outcome, do we ever need to address oxides of metals in the Acidic Environment syllabus?Yeah. Oxides such as sulfur dioxide and carbon dioxide are acidic, for example.
Identify oxides of non-metals which act as acids and describe the conditions under which they act as acids
Conditions? Just write out equations showing their acidic nature.
Post by: jakesilove on January 17, 2017, 08:54:04 am
For the following outcome, do we ever need to address oxides of metals in the Acidic Environment syllabus?
Identify oxides of non-metals which act as acids and describe the conditions under which they act as acids
The 'condition' they're talking about here is the reaction between the acidic oxide and water. If you walked into a room of sulfur dioxide, you wouldn't fizzle up from all the acid. The acid needs to become an aqueous solution (ie. sulfuric acid etc.) before it displays acidic properties.
Post by: Iminschool on January 17, 2017, 04:17:58 pm
HF(aq) + H2O(l) <---> F-(aq) + H3O+(aq)
When added to this equilibrium, which one of the following soluble chemicals will least affect its position?
(A) Copper(II)fluoride
(B) Hydrogen chloride
(C) Sodium hydroxide
(D) Copper(II)nitrate
I am having trouble applying Le Chatlier's principle for some of the possible answers listed, and no i don't have the solution to this question.
Post by: sweetcheeks on January 17, 2017, 05:32:47 pm
Question here:A system will attempt to correct/oppose any change that is made, this includes adding more reactants or products as increasing the concentration of a substance will take the system out of equilibrium. If we add CuF2 to the solution it dissociates into Cu2+ and F-, resulting in an increase in the F- concentration which will disrupt equilibrium, causing the system to favour the backwards reaction. Hydrogen chloride is more easily identified as HCl, which when added to water will form H3O+ ions, again this will disrupt equilibrium. Sodium hydroxide, NaOH will dissociate into Na+ and OH-, the OH- ions will react with the H3O+, reducing its concentration again disrupting equilibrium. Finally adding Cu(NO3)3 will result in Cu2+ and NO3- ions entering the solution, the reaction doesn't involve either ions (and neither will result in precipitation) therefore it won't interfere with the position of equilibrium.
HF(aq) + H2O(l) <---> F-(aq) + H3O+(aq)
When added to this equilibrium, which one of the following soluble chemicals will least affect its position?
(A) Copper(II)fluoride
(B) Hydrogen chloride
(C) Sodium hydroxide
(D) Copper(II)nitrate
I am having trouble applying Le Chatlier's principle for some of the possible answers listed, and no i don't have the solution to this question.
Post by: Iminschool on January 17, 2017, 06:08:41 pm
A system will attempt to correct/oppose any change that is made, this includes adding more reactants or products as increasing the concentration of a substance will take the system out of equilibrium. If we add CuF2 to the solution it dissociates into Cu2+ and F-, resulting in an increase in the F- concentration which will disrupt equilibrium, causing the system to favour the backwards reaction. Hydrogen chloride is more easily identified as HCl, which when added to water will form H3O+ ions, again this will disrupt equilibrium. Sodium hydroxide, NaOH will dissociate into Na+ and OH-, the OH- ions will react with the H3O+, reducing its concentration again disrupting equilibrium. Finally adding Cu(NO3)3 will result in Cu2+ and NO3- ions entering the solution, the reaction doesn't involve either ions (and neither will result in precipitation) therefore it won't interfere with the position of equilibrium.
Thank you :)
Post by: Iminschool on January 17, 2017, 09:09:56 pm
Post by: RuiAce on January 17, 2017, 09:15:23 pm
I know how to do this question and i get my answer as (D) as i get a degree of ionisation of 100% which is the absolute highest. So the problem i'm having is that the answers say the answer is (C), plus this question is from a CSSA paper. Is this an error in the answers? Or is there a trick that i am not aware of?Very inclined to say D is correct.
Post by: Iminschool on January 17, 2017, 09:35:05 pm
Very inclined to say D is correct.
Awesome, must be an error then
Post by: bsdfjnlkasn on January 20, 2017, 04:56:09 pm
Question 1
Why don't neutral oxides react with either acidic or basic oxides?
Question 2
• Metal oxides on the left side of Periodic Table are generally basic
Why is this the case? What makes them basic? Could someone please explain this with an equation or something? I just don't understand what property of metal oxides causes them to form bases. Same goes for non-metal oxides generally being acidic.
Why is this the case? What makes them basic? Could someone please explain this with an equation or something? I just don't understand what property of metal oxides causes them to form bases. Same goes for non-metal oxides generally being acidic.
Question 3
Hey there,
I just wanted to clarify some things just quickly:
If an equilibrium is said to be "moving away", does this mean it is favouring the backwards reaction?
Also, If the concentration of ions increases in the products, does the system (going by Le Chatelier's principle) want to oppose that change by going to a state with less ions (which inevitably decreases the concentration of ions to a more stable level)? Does it do this by favouring the backwards reaction?
Thank you!!
I just wanted to clarify some things just quickly:
If an equilibrium is said to be "moving away", does this mean it is favouring the backwards reaction?
Also, If the concentration of ions increases in the products, does the system (going by Le Chatelier's principle) want to oppose that change by going to a state with less ions (which inevitably decreases the concentration of ions to a more stable level)? Does it do this by favouring the backwards reaction?
Thank you!!
Question 4
How are oxidation states useful in determining whether an element (or specifically, a metalloid) will be amphoteric?
Question 5
I was just reading through a study guide online and it says:
"The left side of the periodic table generally consists of acidic oxides, while the right side is generally made of basic oxides."
Is this correct? Because I thought that metals form basic oxides. Metals are found on the LHS of the Periodic Table. Also, doesn't acidity follow the same trends as electronegativity, i.e. increases towards F? Further disproving the first half of the statement?
Or am I missing a crucial point somewhere in my above questions?
Thank you! ;D
"The left side of the periodic table generally consists of acidic oxides, while the right side is generally made of basic oxides."
Is this correct? Because I thought that metals form basic oxides. Metals are found on the LHS of the Periodic Table. Also, doesn't acidity follow the same trends as electronegativity, i.e. increases towards F? Further disproving the first half of the statement?
Or am I missing a crucial point somewhere in my above questions?
Thank you! ;D
Question 6
For the outcome "Describe the solubility of carbon dioxide in water under various conditions as an equilibrium process and explain in terms of Le Chatelier’s principle"
Why is Le Chaterlier's principle applied differently for an increase in concentration and volume of CO2?
"An increase in the concentration of CO2 (g) will shift the equilibrium to the right, converting carbon dioxide and water into carbonic acid in order to reduce the concentration of carbon dioxide."
I understand how this works for concentration, but for:
"An increase in the volume of CO2 (g) will shift the equilibrium to the right, converting carbon dioxide and water into carbonic acid in order to reduce the volume of carbon dioxide. Thus the system will attempt to counteract this change by favouring the backwards reaction."
it almost seems to me that LC's principle has been applied twice in the instance of increased volume (inferred in the first sentence then applied in the second). Why is this the case?
Thank you!
Why is Le Chaterlier's principle applied differently for an increase in concentration and volume of CO2?
"An increase in the concentration of CO2 (g) will shift the equilibrium to the right, converting carbon dioxide and water into carbonic acid in order to reduce the concentration of carbon dioxide."
I understand how this works for concentration, but for:
"An increase in the volume of CO2 (g) will shift the equilibrium to the right, converting carbon dioxide and water into carbonic acid in order to reduce the volume of carbon dioxide. Thus the system will attempt to counteract this change by favouring the backwards reaction."
it almost seems to me that LC's principle has been applied twice in the instance of increased volume (inferred in the first sentence then applied in the second). Why is this the case?
Thank you!
Mod Edit: Posts merged. Use the Modify button to add questions to your initial post to avoid chain posting.
Post by: bholenath125 on January 20, 2017, 04:57:04 pm
Im having a lot of trouble understanding how to label Hydrocarbons like 3,4 Dibromoethene
Is there a site I could use?
Post by: RuiAce on January 20, 2017, 11:41:15 pm
1. A substance is acidic if it reacts an actual acid/base in aqueous solution. Not with other oxides.Question 1Why don't neutral oxides react with either acidic or basic oxides?Question 2• Metal oxides on the left side of Periodic Table are generally basic
Why is this the case? What makes them basic? Could someone please explain this with an equation or something? I just don't understand what property of metal oxides causes them to form bases. Same goes for non-metal oxides generally being acidic.Question 3Hey there,
I just wanted to clarify some things just quickly:
If an equilibrium is said to be "moving away", does this mean it is favouring the backwards reaction?
Also, If the concentration of ions increases in the products, does the system (going by Le Chatelier's principle) want to oppose that change by going to a state with less ions (which inevitably decreases the concentration of ions to a more stable level)? Does it do this by favouring the backwards reaction?
Thank you!!Question 4How are oxidation states useful in determining whether an element (or specifically, a metalloid) will be amphoteric?Question 5I was just reading through a study guide online and it says:
"The left side of the periodic table generally consists of acidic oxides, while the right side is generally made of basic oxides."
Is this correct? Because I thought that metals form basic oxides. Metals are found on the LHS of the Periodic Table. Also, doesn't acidity follow the same trends as electronegativity, i.e. increases towards F? Further disproving the first half of the statement?
Or am I missing a crucial point somewhere in my above questions?
Thank you! ;DQuestion 6For the outcome "Describe the solubility of carbon dioxide in water under various conditions as an equilibrium process and explain in terms of Le Chatelier’s principle"
Why is Le Chaterlier's principle applied differently for an increase in concentration and volume of CO2?
"An increase in the concentration of CO2 (g) will shift the equilibrium to the right, converting carbon dioxide and water into carbonic acid in order to reduce the concentration of carbon dioxide."
I understand how this works for concentration, but for:
"An increase in the volume of CO2 (g) will shift the equilibrium to the right, converting carbon dioxide and water into carbonic acid in order to reduce the volume of carbon dioxide. Thus the system will attempt to counteract this change by favouring the backwards reaction."
it almost seems to me that LC's principle has been applied twice in the instance of increased volume (inferred in the first sentence then applied in the second). Why is this the case?
Thank you!
Mod Edit: Posts merged. Use the Modify button to add questions to your initial post to avoid chain posting.
Acidic oxides like SO2 don't react with basic oxides like MgO
2. The fact that an oxide is basic if it reacts with an acid. As it so stands, in general they ALWAYS react with acids. So just know it.
3. "moving away" needs more context. Could just mean the equilibrium being disturbed. Because that's NOT common chemistry terminology
And simply put yes
4. Provide a question. I can see how they might be useful but I cannot give a concrete answer
5. That quote looks like it is in the complete wrong order
I want to get back to 6 later (unless Jake beats me). That one needs a more comprehensive answer and right now I am really tired
For Production of Materials,Try providing an example for things like these. It's easier to illustrate thought processes.
Im having a lot of trouble understanding how to label Hydrocarbons like 3,4 Dibromoethene
Is there a site I could use?
(However, I think the Jacaranda textbook addresses it decently well.)
Post by: bsdfjnlkasn on January 23, 2017, 12:53:49 pm
How important is it that we know how to name acids?
Do we need to know the following indicators: Alizarine yellow and bromoscresol green?
Post by: ellipse on January 23, 2017, 01:59:16 pm
How important is it that we know how to name acids in The Acidic Environment topic?
Must we know the following indicators: Alizarine yellow and bromoscresol green?
It may be important, but just to be safe, make sure you know how to name the alkanoic acids, and there's a dot point in acidic environment that requires you to know the systematic and the common name for, iirc, HCl, H2SO4, ethanoic and citric acid, so try your best and memorise that as well. Learn the other acid names as you go through the course too, it may be useful.
As for the indicators, I'm pretty sure the syllabus doesn't require you to know them
Post by: bsdfjnlkasn on January 23, 2017, 02:10:49 pm
It may be important, but just to be safe, make sure you know how to name the alkanoic acids, and there's a dot point in acidic environment that requires you to know the systematic and the common name for, iirc, HCl, H2SO4, ethanoic and citric acid, so try your best and memorise that as well. Learn the other acid names as you go through the course too, it may be useful.
As for the indicators, I'm pretty sure the syllabus doesn't require you to know them
Perfect! I guess it's better to know than not to know ;)
Post by: bsdfjnlkasn on January 23, 2017, 02:12:59 pm
Question 1.
Spoiler
Is interpreting equilibrium graphs an important/necessary skill for the course?
Question 2.
Spoiler
For the outcome "Describe the solubility of carbon dioxide in water under various conditions as an equilibrium process and explain in terms of Le Chatelier’s principle"
Why is Le Chaterlier's principle applied differently for an increase in concentration and volume of CO2?
"An increase in the concentration of CO2 (g) will shift the equilibrium to the right, converting carbon dioxide and water into carbonic acid in order to reduce the concentration of carbon dioxide."
I understand how this works for concentration, but for:
"An increase in the volume of CO2 (g) will shift the equilibrium to the right, converting carbon dioxide and water into carbonic acid in order to reduce the volume of carbon dioxide. Thus the system will attempt to counteract this change by favouring the backwards reaction."
It almost seems to me that LC's principle has been applied twice in the instance of increased volume (inferred in the first sentence then applied in the second). Why is this the case?
Why is Le Chaterlier's principle applied differently for an increase in concentration and volume of CO2?
"An increase in the concentration of CO2 (g) will shift the equilibrium to the right, converting carbon dioxide and water into carbonic acid in order to reduce the concentration of carbon dioxide."
I understand how this works for concentration, but for:
"An increase in the volume of CO2 (g) will shift the equilibrium to the right, converting carbon dioxide and water into carbonic acid in order to reduce the volume of carbon dioxide. Thus the system will attempt to counteract this change by favouring the backwards reaction."
It almost seems to me that LC's principle has been applied twice in the instance of increased volume (inferred in the first sentence then applied in the second). Why is this the case?
Question 2a. (Same outcome)
Spoiler
For the dissolution of CO2 in water, given by:
CO2 (g) +H2O (l) ⇌ H2CO3 (aq)
Will the rate of the forward reaction increase if the concentration of either reactant is increased?
CO2 (g) +H2O (l) ⇌ H2CO3 (aq)
Will the rate of the forward reaction increase if the concentration of either reactant is increased?
Question 3
Spoiler
How is C6H8O7 (citric acid) triprotic if there are 8 Hydrogen's that will ionise not 3? (Taking that H+ ions are essentially protons) I could easily be misunderstanding what this means though.
Post by: ellipse on January 23, 2017, 02:47:36 pm
More questions on The Acidic Environment :D :D:
Question 1.SpoilerIs interpreting equilibrium graphs an important/necessary skill for the course?
Question 2.SpoilerFor the outcome "Describe the solubility of carbon dioxide in water under various conditions as an equilibrium process and explain in terms of Le Chatelier’s principle"
Why is Le Chaterlier's principle applied differently for an increase in concentration and volume of CO2?
"An increase in the concentration of CO2 (g) will shift the equilibrium to the right, converting carbon dioxide and water into carbonic acid in order to reduce the concentration of carbon dioxide."
I understand how this works for concentration, but for:
"An increase in the volume of CO2 (g) will shift the equilibrium to the right, converting carbon dioxide and water into carbonic acid in order to reduce the volume of carbon dioxide. Thus the system will attempt to counteract this change by favouring the backwards reaction."
It almost seems to me that LC's principle has been applied twice in the instance of increased volume (inferred in the first sentence then applied in the second). Why is this the case?
Question 2a. (Same outcome)SpoilerFor the dissolution of CO2 in water, given by:
CO2 (g) +H2O (l) ⇌ H2CO3 (aq)
Will the rate of the forward reaction increase if the concentration of either reactant is increased?
1. I think equilibrium graphs are very important. Take for instance q12 from 2012 HSC, which is a multiple choice with equilibrium graphs. I also remember there was another long response on equilibrium graphs, which requires you to explain all 3 factors (temperature, pressure and concentration). So yes, I think it is very important and try your best to master it
2. Im not really sure if quote thing is right (increasing volume doesn't really 'shift' it to the right, I may be wrong though). concentration is given by the formula c=n/v. So by increasing the volume, you're basically decreasing the concentration of the gases. So by LCP, the system will try to increase the concentration, and this is the backwards reaction. Another way to think about this is that volume is inversely proportional to the pressure. And pressure is directly proportional the amount of gaseous moles. Thus, by increasing the volume, the pressure is decreased. So to increase the pressure, the equilibrium shifts to the side with the most GAS moles, and this is the backwards reaction. This explanation might be really confusing, so I hope someone else is able to clearly explain this
2a. Recall that LCP is not applied to pure solids and liquids, ie they do not affect the equilibrium at all. So increasing/decreasing the concentration of water has no effect on the equilibrium. So the rate of forward reaction will only increase if the concentration of CO2 is increased.
3. Basically, its not necessary for all the hydrogen in a molecule to be 'removable'. eg in ethnaoic, the H's bonded to the carbon won't ionise, only the one that is bonded with the oxygen. Similarly, If you look at the structure of citric acid, only the 3 hydrogens sticking out of the carboxylic acid groups are able to ionise, hence its tri-protic.
Post by: bsdfjnlkasn on January 23, 2017, 02:56:49 pm
Question 1
Spoiler
Why are more dilute acids stronger?
Question 2
Spoiler
Do we need to know how to interpret pH graphs of titration neutralisations?
Question 3
Spoiler
Do the product lines in an equilibrium graph always travel inverse to the reactants?
Post by: bsdfjnlkasn on January 23, 2017, 03:30:18 pm
Another way to think about this is that volume is inversely proportional to the pressure. And pressure is directly proportional the amount of gaseous moles. Thus, by increasing the volume, the pressure is decreased. So to increase the pressure, the equilibrium shifts to the side with the most GAS moles, and this is the backwards reaction. This explanation might be really confusing, so I hope someone else is able to clearly explain this
2a. Recall that LCP is not applied to pure solids and liquids, ie they do not affect the equilibrium at all. So increasing/decreasing the concentration of water has no effect on the equilibrium. So the rate of forward reaction will only increase if the concentration of CO2 is increased.
3. Basically, its not necessary for all the hydrogen in a molecule to be 'removable'. eg in ethnaoic, the H's bonded to the carbon won't ionise, only the one that is bonded with the oxygen. Similarly, If you look at the structure of citric acid, only the 3 hydrogens sticking out of the carboxylic acid groups are able to ionise, hence its tri-protic.
Hey ellipse,
First of all, thank you so much for taking the time to send me such detailed answers :) !!
I just have a few things to clarify with your answers as I'm still a bit unclear on the theory behind some points.
1. What formula are you quoting for the second half of your answer to my 2nd question?
(I just can't remember if/when I learnt that volume is inversely proportional to the pressure etc.) But the rest of it makes perfect sense!
2. Why won't the H's bonds to the C ionise? Further, what allows the H in the O-H ionise?
Thanks again!! :)
Post by: Shadowxo on January 23, 2017, 04:20:34 pm
Hey ellipse,
First of all, thank you so much for taking the time to send me such detailed answers :) !!
I just have a few things to clarify with your answers as I'm still a bit unclear on the theory behind some points.
1. What formula are you quoting for the second half of your answer to my 2nd question?
(I just can't remember if/when I learnt that volume is inversely proportional to the pressure etc.) But the rest of it makes perfect sense!
2. Why won't the H's bonds to the C ionise? Further, what allows the H in the O-H ionise?
Thanks again!! :)
For the formula, P1V1 = P2V2. As you increase the volume of a container, the molecules hit the sides of it less and therefore the pressure is less. Also pressure is proportional to concentration, so an increase in volume means c=n/v and concentration and therefore pressure decreases.
Post by: jakesilove on January 23, 2017, 04:34:26 pm
Still going on The Acidic Environment 8)
Question 1SpoilerWhy are more dilute acids stronger?
Question 2SpoilerDo we need to know how to interpret pH graphs of titration neutralisations?
Question 3SpoilerDo the product lines in an equilibrium graph always travel inverse to the reactants?
Hey!
The question 'Why are more dilute acids stronger' doesn't make sense. Potentially, what you mean to ask is why more dilute acids ionise more? This is because the reaction will take place, a certain percentage of the time, between the acid and water. More water = more reactions. The more you dilute an acid, the more ionisation that occurs, but the higher the pH.
Yes, you need to know how to interpret pH graphs. You need to know whether the titration used a strong/weak acid and a strong/weak base, where the neutralisation point is, and what indicator to use.
I really don't understand your last question. Product lines? As reactants are used up (ie. the graph shows reactants decreasing), the products will increase.
Post by: bsdfjnlkasn on January 23, 2017, 04:59:08 pm
The question 'Why are more dilute acids stronger' doesn't make sense. Potentially, what you mean to ask is why more dilute acids ionise more? This is because the reaction will take place, a certain percentage of the time, between the acid and water. More water = more reactions. The more you dilute an acid, the more ionisation that occurs, but the higher the pH.
Hey thanks for clarifying my questions,
I have a few more if that's ok :)
1. Why do only strong acids fully ionise?
2. For "why more dilute acids ionise more":
When the reaction is concerning the acid and water, does the rate of reaction increase?
- Would this be to increase concentration which decreases with increased volume?
Since dilute acids have higher pH readings, does that mean dilute acids are more acidic?
Or is it sort of by definition that a high concentration of H+ ions means high pH - not really related to the acidity of the dilute acid? (because how can a dilute acid be really acidic? Isn't this inaccurate?)
3. Do you have any general tips for interpreting equilibrium graphs? Are there many tricks to it? Why are they useful?
Also, sorry for being unclear - I'm still new to this topic and am still trying to understand what I don't know if that makes sense :P
Thanks again :D
Post by: Shadowxo on January 23, 2017, 05:51:40 pm
Hey thanks for clarifying my questions,
I have a few more if that's ok :)
1. Why do only strong acids fully ionise?
2. For "why more dilute acids ionise more":
When the reaction is concerning the acid and water, does the rate of reaction increase?
- Would this be to increase concentration which decreases with increased volume?
Since dilute acids have higher pH readings, does that mean dilute acids are more acidic?
Or is it sort of by definition that a high concentration of H+ ions means high pH - not really related to the acidity of the dilute acid? (because how can a dilute acid be really acidic? Isn't this inaccurate?)
3. Do you have any general tips for interpreting equilibrium graphs? Are there many tricks to it? Why are they useful?
Also, sorry for being unclear - I'm still new to this topic and am still trying to understand what I don't know if that makes sense :P
Thanks again :D
Hi,
1. Strong acids want to give their H+ ions more, which is why they're strong acids, and that means they ionise more (create more H30+ ions)
2. A lower pH (eg 1 or 2) means more acidic, a higher pH means less acidic / more basic. So if a dilute acid has a higher pH reading it is less acidic, as the concentration of H30+ ions is lower.
3. Practice helps. See how many substances are going up/down, the ones going the same direction are on the same side of the equation, and see how fast it's changing, that should show you which one it is if it's eg 2a+b, the one being consumed faster is a.
About your earlier question about products/reactant lines, yes, as the reactants are consumed products are produced, so concentration of reactants decreases (goes down) and concentration of products increases (goes up) and vice versa for backwards reactions.
Post by: jakesilove on January 23, 2017, 05:52:46 pm
Hi,
1. Strong acids want to give their H+ ions more, which is why they're strong acids, and that means they ionise more (create more H30+ ions)
2. A lower pH (eg 1 or 2) means more acidic, a higher pH means less acidic / more basic. So if a dilute acid has a higher pH reading it is less acidic, as the concentration of H30+ ions is lower as it ionises less
3. Practice helps. See how many substances are going up/down, the ones going the same direction are on the same side of the equation, and see how fast it's changing, that should show you which one it is if it's eg 2a+b, the one being consumed faster is a.
About your earlier question about products/reactant lines, yes, as the reactants are consumed products are produced, so concentration of reactants decreases (goes down) and concentration of products increases (goes up) and vice versa for backwards reactions.
Shadow, you're a legend!
Post by: Shadowxo on January 23, 2017, 06:00:00 pm
Shadow, you're a legend!
:P
Post by: ellipse on January 23, 2017, 06:06:26 pm
Hey thanks for clarifying my questions,
I have a few more if that's ok :)
1. Why do only strong acids fully ionise?
2. For "why more dilute acids ionise more":
When the reaction is concerning the acid and water, does the rate of reaction increase?
- Would this be to increase concentration which decreases with increased volume?
Since dilute acids have higher pH readings, does that mean dilute acids are more acidic?
Or is it sort of by definition that a high concentration of H+ ions means high pH - not really related to the acidity of the dilute acid? (because how can a dilute acid be really acidic? Isn't this inaccurate?)
3. Do you have any general tips for interpreting equilibrium graphs? Are there many tricks to it? Why are they useful?
Also, sorry for being unclear - I'm still new to this topic and am still trying to understand what I don't know if that makes sensA e :P
Thanks again :D
A great answer by Shadow.
I would like to add some extra tips for q3 if you don't know already (to easily identity concentration/pressure/temperature changes);
a sharp verticle increase/decrease in one substance implies its change in concentration
If every substance sharply increases, then volume is decreased / pressure is increased
if every substance sharply decreases, then volume is increased / pressure is decreased
A gradual change of all substance imples a change in temperature
Post by: ellipse on January 23, 2017, 06:08:02 pm
Hey ellipse,
First of all, thank you so much for taking the time to send me such detailed answers :) !!
I just have a few things to clarify with your answers as I'm still a bit unclear on the theory behind some points.
1. What formula are you quoting for the second half of your answer to my 2nd question?
(I just can't remember if/when I learnt that volume is inversely proportional to the pressure etc.) But the rest of it makes perfect sense!
2. Why won't the H's bonds to the C ionise? Further, what allows the H in the O-H ionise?
Thanks again!! :)
Sorry, but Im not really sure for question 2
Post by: bsdfjnlkasn on January 23, 2017, 08:11:06 pm
So I have managed to write a few notes on my question "why do more dilute acids ionise more?"with a few questions remaining. Can someone please check over to see if they're correct and possible answer some of the questions?
More dilute acids have higher pH readings because:
-The reaction, when occurring between the acid and water will increase the rate of reaction (idk if this is relevant)
-The more you dilute an acid, the more ionisation that occurs (what ions are created here?) resulting in a higher pH reading
-> Additional H2O causes ionisation equilibrium to favour the forward reaction, in order to counteract the decrease in concentration (a result of the dilution)
-> More ionisation occurs and concentration of [H+] increases making the solution more basic
Also, I have some more questions regarding the most recent replies :)
1. What is it about strong acids that makes them "want to give their H+ ions more"?
(has it got something to do with concentration?)
2. Why won't the H's bonds to the C ionise in ethnoic acid? Further, what allows the H in the O-H ionise?
3. Are there any tricks to pH graphs of titration and neutralisations?
4. Do more reactions occur with more water because the concentration of the product ions decrease?
Thanks again ::)
Post by: Shadowxo on January 23, 2017, 09:15:24 pm
Hey thanks for the team effort everyone!
So I have managed to write a few notes on my question "why do more dilute acids ionise more?"with a few questions remaining. Can someone please check over to see if they're correct and possible answer some of the questions?
More dilute acids have higher pH readings because:
-The reaction, when occurring between the acid and water will increase the rate of reaction (idk if this is relevant)
-The more you dilute an acid, the more ionisation that occurs (what ions are created here?) resulting in a higher pH reading
-> Additional H2O causes ionisation equilibrium to favour the forward reaction, in order to counteract the decrease in concentration (a result of the dilution)
-> More ionisation occurs and concentration of [H+] increases making the solution more basic
Also, I have some more questions regarding the most recent replies :)
1. What is it about strong acids that makes them "want to give their H+ ions more"?
(has it got something to do with concentration?)
2. Why won't the H's bonds to the C ionise in ethnoic acid? Further, what allows the H in the O-H ionise?
3. Are there any tricks to pH graphs of titration and neutralisations?
4. Do more reactions occur with more water because the concentration of the product ions decrease?
Thanks again ::)
- don't think rate of reaction is relevant here
- the more you dilute an acid, the more ionisation that occurs creating more H3O+ ions (only a greater number, not concentration), meaning it has a higher % ionisation
but the more you dilute it, the less the concentration of H3O+ ions resulting in the higher pH (less acidic, concentration of H3O+ ions is less)
-Kind of like Le Chatlier's principle, if you decrease the concentration then the system partially opposes it by partially increasing (but still less than original value)
since the concentration of everything but the water (fairly constant) decreases and Ka = [H3O+]2/[CH3COOH], if all the concentrations decrease then the value of this expression is less than Ka so the concentration of H3O+ increases partially and concentration of CH3COOH decreases.
- More ionisation occurs but concentration of [H+] decreases making the solution more basic
1. The definition of a strong acid is one that "completely ionizes (dissociates) in a solution" (according to google) so basically strong acids want to give their H+ ions. You don't really need to know details, but the definition of a strong acid is basically one that wants to donate their H+ ions strongly. Concentration is irrelevant, different acidic molecules are either strong or weak, irrespective of their concentrations
2. Don't really need to know this, I think it's just about the structure. Just make sure you know the main acids/bases and which elements they donate/accept.
3. There are different graphs for different titrations. There should be some examples in your textbook, you should just know what each looks like. If it starts above 7 you begin with a base and add an acid. If it starts below 7 you begin with an acid and add a base. Closer to 7 the weaker acid/base it is. Steep long incline at equivalence point usually means strong acid and base, short and not steep usually means weak either one or both, I'm sure there are more rules but don't remember them all atm
4. Yes, as I explained above, as all concentrations decrease, with 2 of the product concentrations decreasing and 1 reactant concentration decreasing (water is pretty much constant) more H3O+ is created and CH3COOH consumed.
Hope I helped, if you need any clarification or if I made any mistakes let me know :)
Post by: bsdfjnlkasn on January 23, 2017, 11:50:53 pm
- don't think rate of reaction is relevant here
- the more you dilute an acid, the more ionisation that occurs creating more H3O+ ions (only a greater number, not concentration), meaning it has a higher % ionisation
but the more you dilute it, the less the concentration of H3O+ ions resulting in the higher pH (less acidic, concentration of H3O+ ions is less)
-Kind of like Le Chatlier's principle, if you decrease the concentration then the system partially opposes it by partially increasing (but still less than original value)
since the concentration of everything but the water (fairly constant) decreases and Ka = [H3O+]2/[CH3COOH], if all the concentrations decrease then the value of this expression is less than Ka so the concentration of H3O+ increases partially and concentration of CH3COOH decreases.
- More ionisation occurs but concentration of [H+] decreases making the solution more basic
1. The definition of a strong acid is one that "completely ionizes (dissociates) in a solution" (according to google) so basically strong acids want to give their H+ ions. You don't really need to know details, but the definition of a strong acid is basically one that wants to donate their H+ ions strongly. Concentration is irrelevant, different acidic molecules are either strong or weak, irrespective of their concentrations
2. Don't really need to know this, I think it's just about the structure. Just make sure you know the main acids/bases and which elements they donate/accept.
3. There are different graphs for different titrations. There should be some examples in your textbook, you should just know what each looks like. If it starts above 7 you begin with a base and add an acid. If it starts below 7 you begin with an acid and add a base. Closer to 7 the weaker acid/base it is. Steep long incline at equivalence point usually means strong acid and base, short and not steep usually means weak either one or both, I'm sure there are more rules but don't remember them all atm
4. Yes, as I explained above, as all concentrations decrease, with 2 of the product concentrations decreasing and 1 reactant concentration decreasing (water is pretty much constant) more H3O+ is created and CH3COOH consumed.
Hope I helped, if you need any clarification or if I made any mistakes let me know :)
Hey thanks shadowxo!
I'm not sure if this is taught in the HSC syllabus, but if you could please explain what you meant with
"Ka = [H3O+]2/[CH3COOH], if all the concentrations decrease then the value of this expression is less than Ka so the concentration of H3O+ increases partially and concentration of CH3COOH decreases."
it would probably help teach me another way of looking at the concept (as you were very clear with everything else :) )
Also, when you highlighted that "[H+] decreases making the solution more basic" does the H+ reference the hydrogens in H3O+? I'm just a bit confused because I thought high H+ concentrations meant a substance was very basic and high H3O+ concentrations meant a substance was highly acidic.
Thanks for everything :)
Post by: Shadowxo on January 24, 2017, 12:10:39 am
Hey thanks shadowxo!
I'm not sure if this is taught in the HSC syllabus, but if you could please explain what you meant with
"Ka = [H3O+]2/[CH3COOH], if all the concentrations decrease then the value of this expression is less than Ka so the concentration of H3O+ increases partially and concentration of CH3COOH decreases."
it would probably help teach me another way of looking at the concept (as you were very clear with everything else :) )
Also, when you highlighted that "[H+] decreases making the solution more basic" does the H+ reference the hydrogens in H3O+? I'm just a bit confused because I thought high H+ concentrations meant a substance was very basic and high H3O+ concentrations meant a substance was highly acidic.
Thanks for everything :)
Hi, I'm actually from Victoria so might be slightly different content :P
First of all [H3O+] = [H+] ( the H+ ions react with water to form [H3O+], they're used interchangeably)
So high [H3O+] means highly acidic, and high [H+] also means highly acidic, as they're basically the same thing
If something's basic, it has a low [H+] (or high [OH-])
And with the Ka, not sure if it's in HSC course but likely is. Do you know K? If so, Ka is just the acidity constant as [H2O] is constant, so it's like K*[H2O]
Basically for each weak acid, there's an equilibrium. We use the term Ka to represent the acidity constant, there is a defined Ka for each weak acid. If the acid is HA and it reacts with H2O to form H3O+ and A- ions, the Ka = [H3O+][A-]/[HA], and the [H3O+] = [A-] as they react 1:1, so Ka = [H3O+]2/[HA]
So the Ka for ethanoic acid is Ka = [H3O+]2/[CH3COOH]. This Ka is always constant at equilibrium, regardless of the concentrations so if all the concentrations decrease then this expression (sometimes called Q) isn't equal to the Ka, so in order for it to be in equilibrium again, the concentrations need to change so it's equal to the Ka. If all concentrations decrease, since it's [H3O+]2/[CH3COOH] then [H3O+] must increase and [CH3COOH] must decrease to return to equilibrium
Hope this helps :)
Post by: RuiAce on January 24, 2017, 12:39:39 pm
All the following questions relate to The Acidic Environment topic:Regarding this one you only need to know the 4 common off by heart:
How important is it that we know how to name acids?
Do we need to know the following indicators: Alizarine yellow and bromoscresol green?
Methyl orange
Bromothymol blue
Litmus
Phenolphthalein
Rest, nope you'll be given it on the exam
Post by: RuiAce on January 24, 2017, 12:40:50 pm
Hi, I'm actually from Victoria so might be slightly different content :PThe equilibrium constant stuff is only taught in the industrial chemistry option.
First of all [H3O+] = [H+] ( the H+ ions react with water to form [H3O+], they're used interchangeably)
So high [H3O+] means highly acidic, and high [H+] also means highly acidic, as they're basically the same thing
If something's basic, it has a low [H+] (or high [OH-])
And with the Ka, not sure if it's in HSC course but likely is. Do you know K? If so, Ka is just the acidity constant as [H2O] is constant, so it's like K*[H2O]
Basically for each weak acid, there's an equilibrium. We use the term Ka to represent the acidity constant, there is a defined Ka for each weak acid. If the acid is HA and it reacts with H2O to form H3O+ and A- ions, the Ka = [H3O+][A-]/[HA], and the [H3O+] = [A-] as they react 1:1, so Ka = [H3O+]2/[HA]
So the Ka for ethanoic acid is Ka = [H3O+]2/[CH3COOH]. This Ka is always constant at equilibrium, regardless of the concentrations so if all the concentrations decrease then this expression (sometimes called Q) isn't equal to the Ka, so in order for it to be in equilibrium again, the concentrations need to change so it's equal to the Ka. If all concentrations decrease, since it's [H3O+]2/[CH3COOH] then [H3O+] must increase and [CH3COOH] must decrease to return to equilibrium
Hope this helps :)
In the core topic "the acidic environment" they only deal with equilibrium principles from a qualitative perspective.
Post by: Shadowxo on January 24, 2017, 01:07:29 pm
The equilibrium constant stuff is only taught in the industrial chemistry option.
In the core topic "the acidic environment" they only deal with equilibrium principles from a qualitative perspective.
Ah, here it's a core topic and reasonably important
Post by: bsdfjnlkasn on January 24, 2017, 04:25:34 pm
Either way i'll need to learn it as we study industrial chemistry for our elective :)
Post by: anotherworld2b on January 25, 2017, 01:37:00 am
Post by: shreya_ajoshi on January 25, 2017, 01:11:04 pm
Hi i was wondering if anyone knew any tricks/tips on tackling hydrocarbon structural diagrams. I have difficulty when it comes to naming it correctly especially when it is a cyclo structure
This is a pretty helpful link: https://www.youtube.com/watch?v=Oz6z2HyNU-k
https://www.youtube.com/watch?v=LUOpbV__zU8 --> for the cyclo structures
Post by: anotherworld2b on January 25, 2017, 01:45:12 pm
This is a pretty helpful link: https://www.youtube.com/watch?v=Oz6z2HyNU-k
https://www.youtube.com/watch?v=LUOpbV__zU8 --> for the cyclo structures
Post by: hanaacdr on January 27, 2017, 03:12:16 pm
could i please get some help on this question,
explain in terms of bond type and strength
(i)- why addition polymers are generally hydrophobic?
(ii) why PVC loses structural integrity when placed in sunlight for long periods of time?
thank you
Post by: RuiAce on January 27, 2017, 03:19:17 pm
HiI think for ii you just need to know that it does, not the why part. Don't remember learning about the why, but it should have something to do with the chemical structure.
could i please get some help on this question,
explain in terms of bond type and strength
(i)- why addition polymers are generally hydrophobic?
(ii) why PVC loses structural integrity when placed in sunlight for long periods of time?
thank you
For i though, those strong carbon chains (the first part of the question) are what give it the hydrophobic nature. These strong carbon chains may form intermolecular forces with other carbon chains in the form of dispersion forces, however they don't really interact with water. As a rule of thumb, polar substances interact with other polar substances (such as water), and non polar with non polar (eg octane).
Yet again, forgot a lot of the intricacies but that's the basic idea. Someone else can input further.
Post by: hanaacdr on January 27, 2017, 03:36:21 pm
I think for ii you just need to know that it does, not the why part. Don't remember learning about the why, but it should have something to do with the chemical structure.
For i though, those strong carbon chains (the first part of the question) are what give it the hydrophobic nature. These strong carbon chains may form intermolecular forces with other carbon chains in the form of dispersion forces, however they don't really interact with water. As a rule of thumb, polar substances interact with other polar substances (such as water), and non polar with non polar (eg octane).
Yet again, forgot a lot of the intricacies but that's the basic idea. Someone else can input further.
thank you!
Post by: Noorijaz on January 28, 2017, 03:17:14 pm
i havent made any notes for chemistry for the beginning of year 12.POM
Can i get away with not making notes and just use someone elses notes or should i go through the pain to make notes.
Furthermore is it better to tpye or hand write notes.
thanks heaps
Post by: RuiAce on January 28, 2017, 03:29:51 pm
hiWell, personally, I never made notes for chem. I just downloaded another set of notes and combined that with my textbook.
i havent made any notes for chemistry for the beginning of year 12.POM
Can i get away with not making notes and just use someone elses notes or should i go through the pain to make notes.
Furthermore is it better to tpye or hand write notes.
thanks heaps
So "can you" get away with it? Yeah, I'd say so. Is it recommended? Probably not.
As I always say, typing v.s. handwriting is UP TO YOU. Typing usually makes it neater, but if you hand write them you're really activating your brain and have a better chance at storing it in your head. Everyone is different on what they choose to do.
Post by: anotherworld2b on January 28, 2017, 08:11:57 pm
The periodic table is arranged horizontally in order of increasing atomic number in such a way the trends can be seen. Would atomic number be the same as atomic weight?
I also wanted to ask for relative atomic mass is it usually written as a percentage or amu? I saw two questions that are nearly identical in wording but one is in amu and the other in percentage :-\
Post by: bsdfjnlkasn on January 28, 2017, 08:29:35 pm
Hi I just wanted to confirm whether that I wrote in my notes is correct
The periodic table is arranged horizontally in order of increasing atomic number in such a way the trends can be seen. Would atomic number be the same as atomic weight?
I also wanted to ask for relative atomic mass is it usually written as a percentage or amu? I saw two questions that are nearly identical in wording but one is in amu and the other in percentage :-\
The horizontal arrangement note is definitely correct. Just look at a periodic table, what do you see? You have hydrogen in the top left corner which has an atomic number of 1, then you move along to Helium, atomic number 2. Because He is at the end of the first horizontal group (called a period) we move down to the next period and look to the far left where Lithium (3) is sitting etc etc. The main trends that you can observe across periods are electronegativity and first ionisation energy, so that's right also.
Atomic number and atomic weight are not the same thing. Atomic number is the big round number we were discussing earlier that identifies an element's position. Atomic weight on the other hand, is the (usually) smaller number that tells us what the element's weight is. It is measured in atomic mass units (amu) and is usually corrected to 2 decimal places. Relative atomic mass and atomic weight are the same thing. I haven't ever seen either given as a percentage. Maybe post the questions you were referring so that we can take a look and see if the question was actually talking about something else :)
Finally for an example, let's look at Lithium - It's atomic number is 3 whereas it's atomic weight is 6.94 amu
Hopefully that helped with your queries and let me know if you need anything else cleared up ;D
Post by: Shadowxo on January 28, 2017, 08:46:29 pm
Hi I just wanted to confirm whether that I wrote in my notes is correct
The periodic table is arranged horizontally in order of increasing atomic number in such a way the trends can be seen. Would atomic number be the same as atomic weight?
I also wanted to ask for relative atomic mass is it usually written as a percentage or amu? I saw two questions that are nearly identical in wording but one is in amu and the other in percentage :-\
Just adding some extra information :P
Atomic number is the number of protons in the nucleus of that element. It is always the same for a specific element, and a whole number. Periodic table is arranged left to right with increasing atomic number (/ protons) and usually, as the number of protons increases, the number of neutrons increases too, to hold the element together. More protons -> more weight so usually left to right is also increasing weight.
Protons and neutrons are around 2000x heavier than electrons, so atomic weight is primarily made up of the mass of protons and neutrons
Atomic weight is the weight of the entire element, protons and neutrons (the average, as some elements have different isotopes due to different neutron numbers). Average number of neutrons can also be found by atomic weight - atomic number.
For relative atomic mass, I've usually seen it as amu, with the percentage abundance of different isotopes being given as information to work out the relative atomic mass
Post by: QC on January 29, 2017, 04:11:58 am
HiOk, depending on how many marks the Q is worth add more detail
could i please get some help on this question,
explain in terms of bond type and strength
(i)- why addition polymers are generally hydrophobic?
(ii) why PVC loses structural integrity when placed in sunlight for long periods of time?
thank you
(i) Addition polymers are generally hydrophobic as they are saturated (all electron bonds are used/single) which means they do not react with water. As well as this, they are non-polar and if you remember the water topic last year, that means that they do not form a solution as water doesn't create strong enough inter molecular bonds to break the existing bonds between the polymer molecule.
(ii) As for the PVC losing structural integrity, as Rui said you probably don't need to know this but if your interested/want to beef up your answer, in sunlight the PVC molecules are hit by UV radiation which emits a photon that is able to weaken the plastic (photodegredation). Weathering and other things can also cause PVC to lose its structural integrity. Side note: A lot of the time companies coat these plastics to enable them to withstand more conditions, you can talk about this in the properties and uses dot point.
Post by: anotherworld2b on January 29, 2017, 01:11:48 pm
I was also wondering if someone could explain the factor of shielding by inner electrons in periodic trends.
I also wanted to confirm whether a atom's nuclear charge is the same as core charge as a factor of periodic trends? Could it be defined as the atteaction between the nucleus and the outer electron shell?
I was also wondering in questions where you have explain elemental spectra, emission spectroscopy, atomic absorption spectroscopy and mass spectroscopy are there key points that should be explained? In my answers i generally write too little and lose marks :-\
Post by: RuiAce on January 29, 2017, 01:48:10 pm
Oh ok i was confused because i was looking at how the periodic table was arranged by increasing atomic weight( which is the atomic number?)These concepts are not in the HSC course.
I was also wondering if someone could explain the factor of shielding by inner electrons in periodic trends.
I also wanted to confirm whether a atom's nuclear charge is the same as core charge as a factor of periodic trends? Could it be defined as the atteaction between the nucleus and the outer electron shell?
I was also wondering in questions where you have explain elemental spectra, emission spectroscopy, atomic absorption spectroscopy and mass spectroscopy are there key points that should be explained? In my answers i generally write too little and lose marks :-\
Also no, atomic weight (aka atomic mass) is the decimal number associated with it. The atomic number is just the number of protons there are.
Post by: QC on January 29, 2017, 02:20:58 pm
Oh ok i was confused because i was looking at how the periodic table was arranged by increasing atomic weight( which is the atomic number?)Hey, just to add to what Rui said, I'm pretty sure that before Henry Mosley, the table was arranged by mass number (the average number of protons and neutrons that are known in all isotopes of that element. E.g hydrogen is 1.008, almost all isotopes of hydrogen exist as just 1 proton and one electron however, some isotopes such as deuterium and tritium which are 1 proton and 1 neutron and 1 proton and 2 neutrons respectively exist but in much lower quantities than the 1 proton no neutrons hydrogen isotope. Also, the atomic number is the number of protons which is unique to each element e.g hydrogen 1, helium 2, lithium 3 etc.
I was also wondering if someone could explain the factor of shielding by inner electrons in periodic trends.
I also wanted to confirm whether a atom's nuclear charge is the same as core charge as a factor of periodic trends? Could it be defined as the atteaction between the nucleus and the outer electron shell?
I was also wondering in questions where you have explain elemental spectra, emission spectroscopy, atomic absorption spectroscopy and mass spectroscopy are there key points that should be explained? In my answers i generally write too little and lose marks :-\
Also I think that mass spectroscopy is a way of measuring ionisation/radiation so that could be useful for that dotpoint.
Finally, you don't need to know about core charge (the nuclear charge experienced by an outershell electron) but it is the same as the atom's nuclear charge I think.
Post by: Shadowxo on January 29, 2017, 02:25:32 pm
Oh ok i was confused because i was looking at how the periodic table was arranged by increasing atomic weight( which is the atomic number?)
I was also wondering if someone could explain the factor of shielding by inner electrons in periodic trends.
I also wanted to confirm whether a atom's nuclear charge is the same as core charge as a factor of periodic trends? Could it be defined as the atteaction between the nucleus and the outer electron shell?
I was also wondering in questions where you have explain elemental spectra, emission spectroscopy, atomic absorption spectroscopy and mass spectroscopy are there key points that should be explained? In my answers i generally write too little and lose marks :-\
Atomic number = number of protons in that elements (number of protons is what defines the element) eg Oxygen, 8
Atomic weight = average weight of that element (protons and neutrons), eg Oxygen, 16.0
Bit rusty on this as I don't think it's assessed in VCE but
Shielding: electrons in their shells are attracted to the nucleus as they are negative and the nucleus is positive due to the protons. For the valence shells, they experience the attraction of the nucleus AND the repulsion from the other negative electrons in the non-valence shells closer to the nucleus, so the more electron shells -> more shielding. So as you go down the periodic table, more shells therefore more shielding, and number of inner shells stays the same from left to right so same shielding across.
If I remember correctly, nuclear charge = charge from protons (number of protons), core charge = charge from protons - charge from inner electrons (protons - inner shell electrons). So nuclear charge is the same as atomic number (increases both down and across periodic table), core charge increases across the periodic table and remains the same down the periodic table. Core charge takes into account the shielding effect from the inner shell electrons and is the attraction between the nucleus and outer electron shell, and nuclear charge is just the charge of the protons.
You should include what elements/molecules they can be used on, how they differentiate them if applicable (eg retention time) and what they measure (eg retention time, concentration). I would ask your teacher what you should include for them but these should be the main ones.
Post by: RuiAce on January 29, 2017, 02:25:42 pm
But any input for him is always appreciated.
Post by: beau77bro on January 29, 2017, 09:17:17 pm
thankyouuuuu
Post by: RuiAce on January 29, 2017, 09:24:06 pm
what is the best way to study for chemistry, generally but also, what is the best textbook or workbook to be using for questions. currently im using conquering as info source and also doing question but im also trying to do surfing. opinions? better way to study? i dont really have a teacher or non-snake classmates, so what should i be doing to maximise study both in actual study and in doing questions?Textbook discussion. Surfing feels more like a summary-based textbook; more of an aid. Spending a bit of time on it isn't too bad, but I wouldn't go for a lot of them. At the end of the day, past papers all the way.
thankyouuuuu
"Best way to study" - non-existent, because the most optimal strategy varies from person to person. As opposed to better ways. Note that study is a matter of you choosing to put your time into a subject, but in general has an objective - the EXAM. So a better approach would be to study in an appropriate manner - make all your notes ASAP, know them well, and then when you can do as many past papers as possible.
Post by: beau77bro on January 29, 2017, 09:26:55 pm
Textbook discussion. Surfing feels more like a summary-based textbook; more of an aid. Spending a bit of time on it isn't too bad, but I wouldn't go for a lot of them. At the end of the day, past papers all the way.
"Best way to study" - non-existent, because the most optimal strategy varies from person to person. As opposed to better ways. Note that study is a matter of you choosing to put your time into a subject, but in general has an objective - the EXAM. So a better approach would be to study in an appropriate manner - make all your notes ASAP, know them well, and then when you can do as many past papers as possible.
thankyou rui your my saviour hahaha im gonna start printing out and pumping out half yearly past papers asap, appreciate the advice and thankyou math god
Post by: RuiAce on January 29, 2017, 09:30:57 pm
thankyou rui your my saviour hahaha im gonna start printing out and pumping out half yearly past papers asap, appreciate the advice and thankyou math godGood luck 8)
Post by: bsdfjnlkasn on February 01, 2017, 10:27:13 pm
Here are some notes I took down in class today, and I was wondering if someone could help me clarify the details of acid rain as I'm still a bit iffy about it all :) - Questions are in bold
1. Acidic oxides are released into the atmosphere
• When the concentrations of these gases in the air reach high levels, acid rain can occur
2. Acidic oxides dissolve in condensing water and as a result are transformed into an acid
• Acids are then sometimes oxidised by ozone or O2 molecules
- Become more acidic, having stronger effects
When acids are oxidised are hydrogen ions formed, is this what makes the falling rain more acidic (as in what is referred to 'stronger effects')?
• Sulfur dioxide and nitrogen dioxide are soluble and so reduce the pH of rain water considerably
- What has solubility got to do with reducing pH? Is it because the dioxides don't ionise and hence increase H+ concentrations?
Thank you! Any help would be greatly appreciated ;D
Post by: QC on February 01, 2017, 11:11:18 pm
Hey there,When acids are oxidised are hydrogen ions formed, is this what makes the falling rain more acidic (as in what is referred to 'stronger effects')?
Here are some notes I took down in class today, and I was wondering if someone could help me clarify the details of acid rain as I'm still a bit iffy about it all :) - Questions are in bold
1. Acidic oxides are released into the atmosphere
• When the concentrations of these gases in the air reach high levels, acid rain can occur
2. Acidic oxides dissolve in condensing water and as a result are transformed into an acid
• Acids are then sometimes oxidised by ozone or O2 molecules
- Become more acidic, having stronger effects
When acids are oxidised are hydrogen ions formed, is this what makes the falling rain more acidic (as in what is referred to 'stronger effects')?
• Sulfur dioxide and nitrogen dioxide are soluble and so reduce the pH of rain water considerably
- What has solubility got to do with reducing pH? Is it because the dioxides don't ionise and hence increase H+ concentrations?
Thank you! Any help would be greatly appreciated ;D
So acid rain primarily is formed via the formation of sulfur and nitrogen oxides (N2O, NO2, SO2,SO3) In the presence of water, sulfur trioxide (SO3) is converted rapidly to sulfuric acid:
SO3 (g) + H2O (l) → H2SO4 (aq)
Nitrogen dioxide reacts with OH to form nitric acid:
NO2 + OH· → HNO3
So these are acids right so they therefore reduce the pH of the the water, (its like a dilute acid which has a low pH) And yes, as you said, hydrogen ions are formed as the HNO3 and H2SO4 dissociate to form H+ and NO3- ions and H+ and SO42- ions. And if you refer to the definition of pH (pH=-log[H+]). This therefore means that as the concentration of H+ ions increases, the strength of the acid increases. You then have to talk about the impacts of strong acid on the environment and human health etc.
What has solubility got to do with reducing pH? Is it because the dioxides don't ionise and hence increase H+ concentrations?
Think about it, if the oxide cannot dissolve, it cannot interact with the H2O and thus H+/H3O+ ions cannot be formed when they form their respective acids. To add to this, they are non-metal oxides so they are acidic and therefore also increase [H+]. I'm not sure what you mean by dioxides not ionising hence increase [H+]. The dioxides react with the H2O and then ionise as acids.
Post by: RuiAce on February 01, 2017, 11:17:50 pm
NO2 + OH· → HNO3I like the response, but I want to pinpoint that this equation is wrong.
What actually happens is
2 NO2 + H2O -> HNO2 + HNO3
The equation provided should, I hope, be possible. But it's not what actually happens.
Post by: QC on February 01, 2017, 11:34:43 pm
I like the response, but I want to pinpoint that this equation is wrong.Hey Rui, is there a marathon type thread on Atar Notes where rather than just answering questions, people ask exam style questions giving the amount of marks each question is worth and then others mark using a marking rubric. I think this is much more useful then even doing past papers since you actually get to know what your answer would be worth rather than the p bad sample answers/rubrics that are available.
What actually happens is
2 NO2 + H2O -> HNO2 + HNO3
The equation provided should, I hope, be possible. But it's not what actually happens.
Post by: RuiAce on February 02, 2017, 12:37:20 am
Hey Rui, is there a marathon type thread on Atar Notes where rather than just answering questions, people ask exam style questions giving the amount of marks each question is worth and then others mark using a marking rubric. I think this is much more useful then even doing past papers since you actually get to know what your answer would be worth rather than the p bad sample answers/rubrics that are available.Do you want one of these? I can try discussing it with some higher authorities.
Post by: Aussie1Italia2 on February 02, 2017, 04:23:48 pm
I have a titration calculation and I'm not entirely sure what I'm doing.
If it takes 50mL of 0.5M KOH to completely neutralise 125 mL of sulfuric acid solution (H2SO4), what is the concentration of the H2SO4 solution.
Post by: RuiAce on February 02, 2017, 04:36:53 pm
Hello! Please and thank you for the help!
I have a titration calculation and I'm not entirely sure what I'm doing.
If it takes 50mL of 0.5M KOH to completely neutralise 125 mL of sulfuric acid solution (H2SO4), what is the concentration of the H2SO4 solution.
Post by: QC on February 02, 2017, 05:00:05 pm
Do you want one of these? I can try discussing it with some higher authorities.Yeah, as I said, I think if active, it is possibly better than past papers and I think it is useful especially for long response/harder questions that may be hard for one person to objectively mark.
Post by: bsdfjnlkasn on February 04, 2017, 09:59:40 am
Do you want one of these? I can try discussing it with some higher authorities.
This sounds sick!!!
Post by: RuiAce on February 04, 2017, 10:31:35 am
This sounds sick!!!It's up. And it needs more participants :P
Post by: bsdfjnlkasn on February 04, 2017, 10:48:46 am
It's up. And it needs more participants :P
Just posted ;)
Post by: morning Glory on February 04, 2017, 11:04:01 am
Post by: beau77bro on February 04, 2017, 11:28:38 am
a link or something would be fine, if not still thankyou
Post by: jakesilove on February 04, 2017, 12:02:16 pm
Hi! I'm currently doing Chemical Monitoring and Management and I was wondering what's the difference between Total Dissolved Solids and Concentration of common ions. I know that the Concentration of common ions is for nitrates and phosphates whereas TDS is mainly measuring salts and other metal ions. However, I believe that some nitrates are soluble in water, so would these still be classified in Conc of common ions or in TDS? Also, couldn't the ions in TDS be considered 'common' ions?
Hey! The main difference is that the concentration of common ions test refers to a specific ion. So, you might try to figure out the specific concentration of nitrates in a waterway, using appropriate tests. However, total dissolved solids refers to ANY solid, and is expressed as a percentage by weight. This is used to determine turbidity, and general health of the waterway. Another thing to note is that ALL nitrates are soluble in water. If you were to test specifically for nitrates, then you would classify it as a common ion, but if you were testing for ALL impurities then it would fall within TDS. Does that make sense? Common ions often fall WITHIN TDS.
Post by: jakesilove on February 04, 2017, 12:03:30 pm
hello, i was wondering if jake still has those chemistry worksheets he talked about at the lecture? im making my own now however i dont quite remember what i dont quite remember so his stuff would be super helpful as a template
a link or something would be fine, if not still thankyou
Hey! I don't actually have Chemistry worksheets available, however I have written an article explaining the technique which you can find HERE. Whilst the article refers to Modern History, just do the same thing for Chemistry :) Feel free to post your worksheets here, would be happy to take a look!
Post by: beau77bro on February 04, 2017, 12:04:46 pm
Post by: jakesilove on February 04, 2017, 12:05:34 pm
ok sweet thankyou for the help and great advice. you guys rock im gonna go read that now thankyou lots.
No problem! Always happy to help :)
Post by: morning Glory on February 04, 2017, 12:49:15 pm
Hey! The main difference is that the concentration of common ions test refers to a specific ion. So, you might try to figure out the specific concentration of nitrates in a waterway, using appropriate tests. However, total dissolved solids refers to ANY solid, and is expressed as a percentage by weight. This is used to determine turbidity, and general health of the waterway. Another thing to note is that ALL nitrates are soluble in water. If you were to test specifically for nitrates, then you would classify it as a common ion, but if you were testing for ALL impurities then it would fall within TDS. Does that make sense? Common ions often fall WITHIN TDS.
Right, that makes a lot more sense, thank you! Have a nice day :)
Post by: RangaTurtle on February 04, 2017, 04:37:15 pm
Post by: jakesilove on February 04, 2017, 06:11:46 pm
Hello. I need to compare dry cell to lithium cell batteries for homework but having trouble finding what is the chemical reaction of a lithium-Ion cell as this is the one that the teachers has given us to compare.
A quick google search yielded this link here. Check it out!
Post by: bsdfjnlkasn on February 04, 2017, 07:31:22 pm
Is it because of something that happens in solution? Why is it's degree of ionisation so low if it's looking to give away it's electrons so desperately?
Post by: RuiAce on February 04, 2017, 07:35:09 pm
What is it about HF (and F's high electronegativity) that makes it such a weak acid?Fluorine's high electronegativity is mostly prelim content. And only barely; most of the reasoning is not explained until university.
Is it because of something that happens in solution? Why is it's degree of ionisation so low if it's looking to give away it's electrons so desperately?
The most electronegative elements are fluorine, oxygen and nitrogen. Incidentally, they're both non-metals, take electrons in to fill their outer shell, AND have only two electron shells.
Incidentally, it is this electronegativity that MAKES hydrofluoric acid a weak acid. Whilst in general Cl, Br and I follow similar rules, the electronegativity of F distinguishes it a bit from the other halogens.
That's all you might want to know, and it still isn't a part of the HSC course anyway. They just care that you know that HCl and etc. are strong acids.
Post by: lzxnl on February 04, 2017, 09:27:21 pm
So we have a tiny proton-seeking ion floating around in solution. How stable is that going to be? What's it going to do if it runs into a proton? It's going to be much more likely than the other halide ions to abstract a proton. The other halides are far larger, so this isn't nearly as much of a problem.
Post by: RangaTurtle on February 04, 2017, 10:07:06 pm
A quick google search yielded this link here. Check it out!
Thanks a lot for the help
Post by: Paul.I on February 05, 2017, 01:30:53 pm
"Sodium metal is heated in chlorine gas."
a. Use the standard electrode potentials to predict a reaction the could take place. (NaCl is what l have put down)
b. Write an equation to represent the overall oxidation/reduction reaction. (Here l have put the two theoretical half equations but the problem is both substances are oxidizing and therefore l cant work out what is reducing.)
c. Write the name and formula of the oxidant and the reductant in this reaction. (Again same problem with b. l just figgured being that an ionic compound is forming one isn't forcing the other to reduce or oxidize.)
Post by: QC on February 05, 2017, 01:57:36 pm
oxidant is chlorine gas (Cl2) and reductant is Na metal. I might just be overlooking something?
Post by: kiwiberry on February 05, 2017, 02:00:37 pm
Hi l was wondering if anyone could help me with this question?
"Sodium metal is heated in chlorine gas."
a. Use the standard electrode potentials to predict a reaction the could take place. (NaCl is what l have put down)
b. Write an equation to represent the overall oxidation/reduction reaction. (Here l have put the two theoretical half equations but the problem is both substances are oxidizing and therefore l cant work out what is reducing.)
c. Write the name and formula of the oxidant and the reductant in this reaction. (Again same problem with b. l just figgured being that an ionic compound is forming one isn't forcing the other to reduce or oxidize.)
So the reaction is 2Na + Cl2 --> 2NaCl
Na and Cl2 both have oxidation states of 0. In NaCl, Na+ has an oxidation state of +1, and Cl- has an oxidation state of -1
We can see that the state of sodium increases from 0 to +1, so it has oxidised. Chlorine's oxidation state decreases from 0 to -1, so it has reduced.
Post by: QC on February 05, 2017, 02:03:37 pm
So the reaction is 2Na + Cl2 --> 2NaClTrue, defs a better explanation then mine haha
Na and Cl2 both have oxidation states of 0. In NaCl, Na+ has an oxidation state of +1, and Cl- has an oxidation state of -1
We can see that the state of sodium increases from 0 to +1, so it has oxidised. Chlorine's oxidation state decreases from 0 to -1, so it has reduced.
Post by: Paul.I on February 05, 2017, 02:39:21 pm
So the reaction is 2Na + Cl2 --> 2NaCl
Na and Cl2 both have oxidation states of 0. In NaCl, Na+ has an oxidation state of +1, and Cl- has an oxidation state of -1
We can see that the state of sodium increases from 0 to +1, so it has oxidised. Chlorine's oxidation state decreases from 0 to -1, so it has reduced.
Thank you very much
Post by: Paul.I on February 05, 2017, 02:58:34 pm
For C l believe it is the Chlorine but l am not sure.
Post by: shreya_ajoshi on February 05, 2017, 03:23:03 pm
How do you do this?
The heat of combustion of 1-propanol is 2016 kJ mol-1.
What is the numerical value of the heat of combustion in kJ g–1?
The options are:
(A) 33.60
(B) 2016
(C) 3.360 × 104
(D) 1.210 × 105
Thank you :)
Post by: Shadowxo on February 05, 2017, 03:23:30 pm
Can someone please help me with B and C. I understand A and D fine, l am just confused in which part of the solution is reducing.
For C l believe it is the Chlorine but l am not sure.
b) Well you have Zinc nitrate, Zinc, and MnO4 with HCl, and K+ and NO3
To start with, K+ (aq) is fairly unreactive as it's down the bottom of the table - it would only react with Li(s) so it will not be appearing in this reaction, it's just there to balance out the charges. Platinum is unreactive so is ignored. NO3 is unreactive so is ignored.
As the equation \(\ce{MnO4^-(aq) + 8H+(aq) + 5e- -> Mn^2+(aq) + 4H2O(l)}\) is above \(\ce{Zn^2+(aq) + 2e- -> Zn(s)}\), we know that Zn(s) is oxidised to Zn2+, and the MnO4- along with the H+ from HCl is reduced to Mn2+ and H2O.
From you have your two equations, their standard reduction potentials, and which is oxidising and reducing
Does this help? :)
Post by: Shadowxo on February 05, 2017, 03:28:57 pm
Hi
How do you do this?
The heat of combustion of 1-propanol is 2016 kJ mol-1.
What is the numerical value of the heat of combustion in kJ g–1?
The options are:
(A) 33.60
(B) 2016
(C) 3.360 × 104
(D) 1.210 × 105
Thank you :)
Well first you need to find out how many grams in a mol of propanol
Propanol = C3H8O
M(Propanol) = 3*12.0 + 8*1.0 + 16.0 = 60.0gmol-1
So there are 60.0g per mol of propanol
So heat of combustion is 2016kJ per mol or 2016 kJ per 60.0g = 33.6kJ per gram
Post by: jakesilove on February 05, 2017, 03:29:29 pm
Hi
How do you do this?
The heat of combustion of 1-propanol is 2016 kJ mol-1.
What is the numerical value of the heat of combustion in kJ g–1?
The options are:
(A) 33.60
(B) 2016
(C) 3.360 × 104
(D) 1.210 × 105
Thank you :)
Hey! So, we have the quantity
If we want to turn this into a kJ/g, we need to multiply by moles, and divide by grams. I can tell by looking at the units! What do we know that has both grams and moles? Well, the molar mass of Propanol is
Remember, we need to MULTIPLY by moles, so let's divide our original number by the molar mass
So, our answer is A.
To be honest, that's the ONLY answer it could have been. The value for kJ/g should be LESS than the value for kJ/mol, as there are 60g in a mole. Therefore, the answer had to be less than 2016.
Post by: Paul.I on February 05, 2017, 03:52:52 pm
b) Well you have Zinc nitrate, Zinc, and MnO4 with HCl, and K+ and NO3
To start with, K+ (aq) is fairly unreactive as it's down the bottom of the table - it would only react with Li(s) so it will not be appearing in this reaction, it's just there to balance out the charges. Platinum is unreactive so is ignored. NO3 is unreactive so is ignored.
As the equation \(\ce{MnO4^-(aq) + 8H+(aq) + 5e- -> Mn^2+(aq) + 4H2O(l)}\) is above \(\ce{Zn^2+(aq) + 2e- -> Zn(s)}\), we know that Zn(s) is oxidised to Zn2+, and the MnO4- along with the H+ from HCl is reduced to Mn2+ and H2O.
From you have your two equations, their standard reduction potentials, and which is oxidising and reducing
Does this help? :)
This is much appreciated thank you
Post by: shreya_ajoshi on February 05, 2017, 06:33:40 pm
Hey! So, we have the quantity
If we want to turn this into a kJ/g, we need to multiply by moles, and divide by grams. I can tell by looking at the units! What do we know that has both grams and moles? Well, the molar mass of Propanol is
Remember, we need to MULTIPLY by moles, so let's divide our original number by the molar mass
So, our answer is A.
To be honest, that's the ONLY answer it could have been. The value for kJ/g should be LESS than the value for kJ/mol, as there are 60g in a mole. Therefore, the answer had to be less than 2016.
THANK YOUUUUU!!! Makes much more sense now :)
Post by: J.B on February 06, 2017, 07:07:17 pm
I have just learn't about Le Chatelier's Principle and discussed this in relation to the equation H2O(l) + CO2(g) ↔ H2CO2(aq).
I understand that this reaction is exothermic so heat is produced.
I also understand therefore when increasing the temperature this will shift the equilibrium to the left to 'counteract' the extra heat. And I understand that when increasing the pressure this will shift the equilibrium to the right as it will result in less stress due to there only being one molecule colliding with the wall of the container.
But I don't understand that as increasing temperature is a way of increasing pressure, how come when looked at separately they favour different directions of the reaction?
Thanks.
Post by: jakesilove on February 06, 2017, 08:02:01 pm
Hi,
I have just learn't about Le Chatelier's Principle and discussed this in relation to the equation H2O(l) + CO2(g) ↔ H2CO2(aq).
I understand that this reaction is exothermic so heat is produced.
I also understand therefore when increasing the temperature this will shift the equilibrium to the left to 'counteract' the extra heat. And I understand that when increasing the pressure this will shift the equilibrium to the right as it will result in less stress due to there only being one molecule colliding with the wall of the container.
But I don't understand that as increasing temperature is a way of increasing pressure, how come when looked at separately they favour different directions of the reaction?
Thanks.
Hey! You've picked up a solid flaw in the way that Le Chatelier's principle is taught; there is a lot of back-and-forward going on that the syllabus glosses over. So, my main answer is that you're sort of right. Let's take it step by step.
First, we increase the temperature. The reaction is exothermic, and so releases energy upon moving forward. The equilibrium will want to DECREASE it's temperature, to minimise the change, and thus move to the left (towards the reactants).
Okay, so we've moved the equilibrium to the left. There are now more reactants than there used to be; what does that mean? Well, the reactants contain a gas, so there is MORE gas than there used to be. As such, the pressure will have increased.
The HSC stops there. However, you've asked a very deep, very important question; then what?
Remember, the equilibrium shifts to MINIMISE the change, but not completely switch the system back to it's original state. You can imagine it like a percentage; there is a temperature change of x%, but the equilibrium will only shift y%, where y is much smaller than x.
Back to the question at hand. Well, the pressure has increased; so what? Well, if pressure increases, the equilibrium will shift to the side with fewer moles of gas. In this case, that's the products; thus the equilibrium will shift to the right (towards the products). However, it will only do so z% (where z is smaller than y, and much smaller than x. If this x,y,z thing is confusing, ignore it; think of it like a dampening effect with each successive equilibrium shift). So, yes, pressure increased, but the equilibrium shifted to minimise that increase!
Why stop there? By shifting to the right this second time, we've increased the concentration of products. The equilibrium doesn't like that; it wants to minimise the change in concentration, and shift back towards the left.
And then back towards the right.
And then back towards the left.
And the back again.
And again.
What you've picked up on is the endless nature of this interaction. We call it equilibrium because it doesn't stop, it merely stabilises. It's a deep, complicated solution to the seemingly obvious flaw that you pointed out. I hope I made sense of it, and that you can understand it.
This is beyond the syllabus, but I think it helps you to internalise what is physically going on when an equilibrium is introduced to a change. Thank you for your brilliant question!
Post by: J.B on February 07, 2017, 07:45:40 am
Post by: 1tankengine on February 07, 2017, 04:55:08 pm
Post by: RuiAce on February 07, 2017, 05:06:56 pm
Hey, I just had a practice exam covering Mod 1 and I am struggling with cell notation and cell potentials, I always mix up the direction of the potentials and I was wondering if there was an easy way to remember how to work these problems. Thanks :)The direction of the potentials depends on how high up in the standard reductions potential they are. Note that you're given the reduction potentials on your data sheet.
Given any cell, you should be able to identify the species (e.g. Mg|MgCl2). Once you've identified the species, it is always the one further up that needs to be flipped. The one getting flipped is always undergoing oxidation, whereas the one not flipped is always undergoing reduction.
And the mnemonic RED-CAT AN-OX is all you need to determine which electrode is going on. Reduction at Cathode, Anode is Oxidation.
Post by: Aaron12038488 on February 08, 2017, 06:32:31 pm
Also how do I write a scientific report. My teacher said it was different from junior science. Thx
Moderator action: Posts merged. At times like this, please resort to the 'modify' function to avoid repeat-posting.
Post by: RuiAce on February 08, 2017, 06:56:49 pm
I'm doing preliminary Chem, and I find the techniques really confusing for some reason. Should I basically memorise all of these techniques, which techniques do I have to know, and how much info for each technique. Any help would be appreciated.What do you mean by "technique"
Technique could mean anything in prelim. E.g separation of mixtures techniques, techniques in calculation questions
Also how do I write a scientific report. My teacher said it was different from junior science. ThxI question your teacher. What?
Because in Yr 11/12 science I still stuck with:
Aim
(Hypothesis - Maybe.)
Apparatus
Safety
Method
Results
Discussion
Conclusion <- Although, I've rarely needed one of these before.
Post by: Aaron12038488 on February 08, 2017, 06:59:38 pm
Post by: RuiAce on February 08, 2017, 07:02:04 pm
Solid + Solid = sieving, centrifugation etc.
Liquid + Undissolved solid - filtration
Liquid + Dissolved solid - evaporation, distillation etc.
Liquid + Liquid - distillation etc.
And then you have other things like using magnets which are for special cases where there's a magnetic metal involved.
Although, at the end of the day you could still argue it is most certainly memorisation.
Post by: bsdfjnlkasn on February 08, 2017, 10:29:39 pm
Any help would be greatly appreciated!!
Post by: RuiAce on February 08, 2017, 10:46:13 pm
Why is it that an increase in pressure results in a decrease of concentration in that same substance? Is there a physical way (idk maybe using an analogue) that we can imagine this, and then a mathematical expression that verifies the relationship of the two? I've just kind of learnt as a rule of thumb when applying LCP but would prefer a more concrete understanding if that's ok.Better give an example. Because depending on the equation, by LCP an increase in pressure just shifts the equilibrium to the side with fewer moles of gas and aqueous substances. Which means whilst it's decreased on one end, it's increased on the other end.
Any help would be greatly appreciated!!
The mathematical expression is taught in the industrial chemistry option - the equilibrium constant.
Post by: chexmixrocks on February 08, 2017, 11:00:28 pm
Post by: RuiAce on February 08, 2017, 11:07:41 pm
So isotopes become radioactive usually when the proton:neutron ratio is more than 1:1.5 right? So why is Cobalt-60 and Technetium-99 radioactive?Explanation of radioactive decay not related to the n-p ratio is beyond the HSC course.
Technetium is a special case. It's the lightest element that has no stable radioisotopes. Took them centuries to just discover that element. Link for further reading at your own leisure.
Cobalt-59 is the stable one in the cobalt bunch. It may also be worth mentioning that Nickel-60 is stable, but Nickel-59 is not.
The n:p ratio gives an indicator as to which are generally stable. The rule is not strictly binding to the stable ones. It's more binding towards which are unstable, if anything.
Post by: bsdfjnlkasn on February 09, 2017, 06:34:02 am
Better give an example. Because depending on the equation, by LCP an increase in pressure just shifts the equilibrium to the side with fewer moles of gas and aqueous substances. Which means whilst it's decreased on one end, it's increased on the other end.
The mathematical expression is taught in the industrial chemistry option - the equilibrium constant.
Hey Rui,
I understand what happens due to an increase in pressure according to LCP but the confusion arises when I try to connect how an increase in pressure leads to a decrease in concentration. Maybe for the example, H2O + CO2 --> H2CO3
could you run through with me what happens if the pressure of CO2 is increased? Like other than applying the blanket rule: the equilibrium shifts to the side of fewer gas moles. Would this increase in pressure, increase the concentration of CO2 which will then force the reaction forward? So is it safe to assume the opposite of what I think i've been taught in the first line? That being, an increase in pressure results in a decrease in concentration?
Sorry for inundating you with questions, I really appreciate the help
Post by: RuiAce on February 09, 2017, 09:29:21 am
When we talk about pressure, we talk about pressure on the entire system. If we're just increasing the quantity of one thing, by consequence we just increase the concentration of that one substance. Always make sure that we do not mix up the terminology here.
Hey Rui,
I understand what happens due to an increase in pressure according to LCP but the confusion arises when I try to connect how an increase in pressure leads to a decrease in concentration. Maybe for the example, H2O + CO2 --> H2CO3
could you run through with me what happens if the pressure of CO2 is increased? Like other than applying the blanket rule: the equilibrium shifts to the side of fewer gas moles. Would this increase in pressure, increase the concentration of CO2 which will then force the reaction forward? So is it safe to assume the opposite of what I think i've been taught in the first line? That being, an increase in pressure results in a decrease in concentration?
Sorry for inundating you with questions, I really appreciate the help
If we are adding more CO2 in, then we are making the concentration of CO2 greater. (Does this increase the 'pressure'? Yes, because we increased the concentration of a gas. But that's not what we mean by 'increasing the pressure'.)
Because there's too much CO2 on side, by LCP the system wants to minimise this disturbance by using up some of the CO2. Hence the equilibrium shifts to the right.
Also, remember that your equilibrium arrows are REVERSIBLE, not one-sided arrows.
When we talk about increasing the "pressure" in a broad context, we're talking about increasing the concentration of EVERYTHING. We increase the concentration of CO2, but ALSO the concentration of H2O and H2CO3.
Of course, in this example, the ONLY gas is CO2. If we increase the pressure, the only thing that we're going to be concerned in is CO2.
But in general, because we put too much gases in there, the equilibrium shifts to the side with the LEAST moles of gas. By doing this, the system eliminates moles of gas so that essentially, it's as if the system is a bit freed up (like making a room less stuffy). So the equilibrium also shifts to the right.
(Btw, forget that I said aqueous in my earlier post. I have no clue what I was thinking - probably juggled too many things at once.)
___________________________________
For this example, because CO2 was the only gas, the ultimate effect was that the concentration of CO2 DID go down due to an increase in pressure.
But if you take 2 NO2 <--> N2O4, where both substances are gases and on products and reactants, you can't just make these assumptions. Concentration and pressure should not be tied with each other for the sake of LCP analysis.
P.S. Do point out if something doesn't make sense. I literally woke up and just started typing an answer so I was a bit tired at the time, and I haven't checked what i wrote.
Post by: bsdfjnlkasn on February 09, 2017, 04:26:07 pm
Thanks for distinguishing between CO2 acting to increase pressure on it's own (in the provided example) and how any other gases present would be of similar effect in a different scenario.
I still don't understand the connection between pressure and concentration in your line:
I'm just not sure how you related the two for this example (especially since you said not to do this at the very end)
When we talk about increasing the "pressure" in a broad context, we're talking about increasing the concentration of EVERYTHING. We increase the concentration of CO2, but ALSO the concentration of H2O and H2CO3.
Finally, when you say the "ultimate effect" do you mean what happened after LCP was applied? Because the increase in pressure which comes with more CO2 will mean that the side with fewer gas moles would be favoured. By having this excess CO2 being made into products we can then say that CO2 decreased. Is that right? What I think you're saying is that I can't immediately anticipate that higher pressures will result in lower concentrations with because more substances could be involved. But in this case, I can say that the higher pressure results in lowered concentrations.
For this example, because CO2 was the only gas, the ultimate effect was that the concentration of CO2 DID go down due to an increase in pressure.
Sorry if there wasn't a clear question, I just wanted to make sure that I had consolidated my understanding. If I have said something wrong, could you please let me know? Thank you!
Post by: RuiAce on February 09, 2017, 05:58:32 pm
Hey Rui,My bad if I muddled some terms up.
Thanks for distinguishing between CO2 acting to increase pressure on it's own (in the provided example) and how any other gases present would be of similar effect in a different scenario.
I still don't understand the connection between pressure and concentration in your line:
I'm just not sure how you related the two for this example (especially since you said not to do this at the very end)
Finally, when you say the "ultimate effect" do you mean what happened after LCP was applied? Because the increase in pressure which comes with more CO2 will mean that the side with fewer gas moles would be favoured. By having this excess CO2 being made into products we can then say that CO2 decreased. Is that right? What I think you're saying is that I can't immediately anticipate that higher pressures will result in lower concentrations with because more substances could be involved. But in this case, I can say that the higher pressure results in lowered concentrations.
Sorry if there wasn't a clear question, I just wanted to make sure that I had consolidated my understanding. If I have said something wrong, could you please let me know? Thank you!
Pressure in an arbitrary context is related to the concentrations of every gas possible. Whilst yes, if we only increase the concentration we are increasing "pressure", simply because we're putting more gas in, it's not what we're talking about.
- Increasing the pressure by ONLY adding CO2 means only adding CO2
- Increasing the pressure and NO more info means adding all gases in.
_______________________________
Think of that as a flow chart
[ CO2 is only gas ] ====> *Increase pressure* ====> Only increase the concentration of CO2 ====> [Concentration of CO2 increased] ====> LCP acts to reduce concentration of CO2.
You don't go straight from "increase pressure" to the last line. You go through it step by step so that the final result is that the concentration of CO2 goes down (so yes after LCP applied)
Post by: bsdfjnlkasn on February 10, 2017, 06:57:41 pm
What is it about strong acids that makes them have a higher degree of ionisation?
Post by: RuiAce on February 10, 2017, 07:01:11 pm
Is Acid strength determined by the concentration of H+ ions and the degree of ionisation?Yes
What is it about strong acids that makes them have a higher degree of ionisation?
___________________________
That's just how they're defined. Strong acids are acids that ionise completely (degree of ionisation = 100%). Weak acids do not ionise completely. (70% ionisation = weak.)
As for WHY HCl and etc. ionise completely, that is out of the course.
Post by: bsdfjnlkasn on February 10, 2017, 07:45:12 pm
Analyse the relationship between the position of elements in the Periodic Table, and the acid-base behaviour of their oxides
Non-metals can be found on the right hand side of the periodic table. As their oxides are acidic in solution and donate protons. We can hence deduce that non-metal oxides are formed on the right hand side of the periodic table. Acidity follows the same trends as electronegativity as it increases across periods and up groups (i.e. towards F). Likewise, metals can be found on the right hand side of the periodic table. As their oxides are basic and accept protons when in solution, we can deduce that metal oxides are formed on the left hand side of the periodic table. Alkalinity/basicity is the reverse of trends in electronegativity as it increases across periods from right to left and increases down groups. Oxides which can act as a base or acid in solution are known as amphoteric oxides and we usually find them in the middle of the periodic table, some notable ones include Zn, Pb and Al.
Post by: Paul.I on February 10, 2017, 08:41:35 pm
Calculate the pH of the solution produced by:
Mixing 50mL of 0.1 mol/L HCl with 20mL of 0.05 mol/L NaOH
Post by: Shadowxo on February 10, 2017, 09:08:05 pm
Hi can someone please help answer this question:
Calculate the pH of the solution produced by:
Mixing 50mL of 0.1 mol/L HCl with 20mL of 0.05 mol/L NaOH
pH is -log10[H3O+] or 14 - pOH
First you find the number of H3O+ and the number of OH-
n (HCl) = cV = 0.1*0.050 = 0.0050 mol
n (NaOH) = cV = 0.05*0.020 = 0.001 mol
n (H+) = n (HCl) = 0.0050 mol
n (OH-) = n (NaOH) = 0.001 mol
As they react in a 1:1 ratio, H+ is in excess by 0.0050 - 0.001 = 0.004 mol
[H+] = n/V = 0.004/0.070 = 0.057 = 0.06 molL-1 rounded
pH = -log10[H+] = 1.24 = 1.2 (I rounded to 2 sig figs but with the information it should be 1, but with pH questions pH can be a bit weird)
Hope this helps a bit :)
Post by: Butterflygirl on February 10, 2017, 09:41:07 pm
just a quick question for the compound FeSiO3, why is the oxidation number of Si +4 and not -4? because doesn't it need 4 electrons to have a full outershell? or is it because it could lose the 4 electrons too?
Thanks!
Post by: RuiAce on February 10, 2017, 09:46:06 pm
Hi guyss :)Basically that.
just a quick question for the compound FeSiO3, why is the oxidation number of Si +4 and not -4? because doesn't it need 4 electrons to have a full outershell? or is it because it could lose the 4 electrons too?
Thanks!
Technically carbon, silicon, germanium etc. can lose or gain 4, and if they choose to lose 4 then they completely lose their outer shell, which is fine. It just so happens that they have a tendency to choose one over the other, in general.
(However, don't think of it like ionic bonding though. Take CO2. C has an oxidation state of +4 but it's covalently bonded to the two oxygens though. It's quite complicated and the entire deal isn't explained in the HSC.)
Post by: Paul.I on February 10, 2017, 09:57:00 pm
pH is -log10[H3O+] or 14 - pOH
First you find the number of H3O+ and the number of OH-
n (HCl) = cV = 0.1*0.050 = 0.0050 mol
n (NaOH) = cV = 0.05*0.020 = 0.001 mol
n (H+) = n (HCl) = 0.0050 mol
n (OH-) = n (NaOH) = 0.001 mol
As they react in a 1:1 ratio, H+ is in excess by 0.0050 - 0.001 = 0.004 mol
[H+] = n/V = 0.004/0.070 = 0.057 = 0.06 molL-1 rounded
pH = -log10[H+] = 1.24 = 1.2 (I rounded to 2 sig figs but with the information it should be 1, but with pH questions pH can be a bit weird)
Hope this helps a bit :)
Thank You Very Much
Post by: anotherworld2b on February 11, 2017, 02:01:52 am
1. Are atmospheric gases soluble? How does the equilibrium between the atmospheric concentration and ocean concentration of these gases help decrease greenhouse gases in the atmosphere?
I believe atmospheric gases are soluble?
2. Give the sequence of equations that represent the CO2 gas in the atmosphere eventually forming hydrogen ions in the ocean.
Post by: bsdfjnlkasn on February 11, 2017, 09:23:46 am
Hi, I posted this a while back on the Chem Megathread (everyone go check it out :D) but the time has come for me to answer it myself. So, could someone please check that I have addressed this 4-marker correctly? Thank you!
Analyse the relationship between the position of elements in the Periodic Table, and the acid-base behaviour of their oxides
Non-metals can be found on the right hand side of the periodic table. As their oxides are acidic in solution and donate protons. We can hence deduce that non-metal oxides are formed on the right hand side of the periodic table. Acidity follows the same trends as electronegativity as it increases across periods and up groups (i.e. towards F). Likewise, metals can be found on the right hand side of the periodic table. As their oxides are basic and accept protons when in solution, we can deduce that metal oxides are formed on the left hand side of the periodic table. Alkalinity/basicity is the reverse of trends in electronegativity as it increases across periods from right to left and increases down groups. Oxides which can act as a base or acid in solution are known as amphoteric oxides and we usually find them in the middle of the periodic table, some notable ones include Zn, Pb and Al.
Hey sorry to be annoying but could I please get a little bit of feedback on this question? Thank you so much!!
Post by: shreya_ajoshi on February 12, 2017, 11:05:33 am
In class we did the 'decarbonating soft drink' experiment. But we used a control along with a can of soft drink to help us. The control was a can of water. I don't quite understand the point of the control. Could someone please explain it to me?
Thanks :)
Post by: sweetcheeks on February 12, 2017, 11:30:38 am
Hi!Could you further elaborate on the experiment? Like how was it performed, what were you measuring? Perhaps the water was used to show CO2 concentration of water in equilibrium with the atmosphere, showing that the CO2 inside the soft drink was being lost due it not being in equilibrium with the environment.
In class we did the 'decarbonating soft drink' experiment. But we used a control along with a can of soft drink to help us. The control was a can of water. I don't quite understand the point of the control. Could someone please explain it to me?
Thanks :)
Post by: Vaike on February 12, 2017, 12:06:51 pm
I was hoping someone could help me with this question. I understand that A is endothermic and thus has +ΔH, and I also figure that C must have +ΔH as it is the reverse of a combustion reaction. However, I am unsure of B and D and how to identify their ΔH value as positive or negative.
Thanks :)
Post by: RuiAce on February 12, 2017, 12:19:39 pm
Hi all,This is completely stretching the boundaries the HSC course, but I see your Victorian tag so you might've posted in the wrong section of the forum.
I was hoping someone could help me with this question. I understand that A is endothermic and thus has +ΔH, and I also figure that C must have +ΔH as it is the reverse of a combustion reaction. However, I am unsure of B and D and how to identify their ΔH value as positive or negative.
Thanks :)
Intuitive explanation: The reactions that happen spontaneously are the exothermic ones; you need to provide energy for an endothermic one. Sodium metal, being as reactive as it is, would more likely spontaneously lose off that electron to become a sodium ion. Reaction B (the reverse) is unlikely to be spontaneous and is thus also the endothermic one.
Post by: kylesara on February 12, 2017, 01:48:57 pm
Post by: RuiAce on February 12, 2017, 02:00:23 pm
Hi, if basic oxides contain metals, and acidic oxides contain non-metals, why is NH3 basic? ThanksNH3 is not an oxide.
You don't have to be an oxide to be basic/acidic. They're defined by the Bronsted-Lowry definition of proton acceptors/donors, not the presence of an oxygen.
Post by: legorgo18 on February 14, 2017, 07:22:33 pm
1)Do you need to know about nuclear fusion
2)Are there any cons for tc 99m and na24 (i cant find any)
3) What is synchrotron and do you need to know it
ty
Post by: jakesilove on February 14, 2017, 07:31:13 pm
Hi guys, just a few questions on nuclear chemistry from pom 9.2.5
1)Do you need to know about nuclear fusion
2)Are there any cons for tc 99m and na24 (i cant find any)
3) What is synchrotron and do you need to know it
ty
Hey!
1) For the Nuclear chem dotpoints, no you don't. However, if you're doing Industrial chemistry as an option, you learn about it there!
2) Just use standard disadvantages. Eg. radioactive waste, cost to produce, ability to potentially harm humans, over exposure etc.
3) A synchrotron is just a particle accelerator! That's definitely something you need to know about; it's basically a massive circle that accelerates charged particles to near light speed, then smashes it into another particle.
Let me know if you have any more questions!
Post by: legorgo18 on February 14, 2017, 07:38:16 pm
And i just need some clarification on the transuranic and commercial production dot points,
so basically they have the same methods but commercial can only be produced by cyclotron and nuclear reactor? Sorry i just want to make sure i get this correct in my test if it pops up :)
Post by: jakesilove on February 14, 2017, 07:44:06 pm
Hi jake! Thanks for the response, for the synchrotron if i use copernicium as an example, what would be the nuclear eqn?
And i just need some clarification on the transuranic and commercial production dot points,
so basically they have the same methods but commercial can only produced by cyclotron and nuclear reactor? Sorry i just want to make sure i get this correct in my test if it pops up :)
Don't choose an element and then see if it works for something like a particle accelerator; spend some time googling particle accelerators, and find a reaction that occurs.
I don't quite understand your question; cyclotrons and nuclear reactors are two completely different things, and have different methods of producing transuranic elements. You need to understand how both of them work, and the differences between the two. They are both used for commercially produced isotopes.
Post by: RuiAce on February 14, 2017, 07:46:30 pm
Hey!I don't think so
1) For the Nuclear chem dotpoints, no you don't. However, if you're doing Industrial chemistry as an option, you learn about it there!
Hi jake! Thanks for the response, for the synchrotron if i use copernicium as an example, what would be the nuclear eqn?Your nuclear equation does not have to be related to the synchrotron. There's no point in memorising an equation just for a specific type of particle accelerator.
And i just need some clarification on the transuranic and commercial production dot points,
so basically they have the same methods but commercial can only be produced by cyclotron and nuclear reactor? Sorry i just want to make sure i get this correct in my test if it pops up :)
You only need an equation to demonstrate production of a transuranic element (which Copernicium suffices for), and one for transuranic element. Not for each type of particle accelerator.
Nuclear equations are a thing to which Wikipedia can be deemed reliable.
Post by: legorgo18 on February 14, 2017, 07:48:45 pm
transuranic can be produced by nuclear reactor, linear accelerator and cyclotron and commercial can only be produced by nuclear reactor and cyclotron
Post by: kiwiberry on February 15, 2017, 02:16:37 pm
SO2(g) + H2O(l) --> H2SO3(aq) (also is this supposed to be in equilibrium?)
SO3(g) + H2O(l) --> H2SO4(aq)
but my textbook has this one instead of the second one:
2H2SO3(aq) +O2(g) --> 2H2SO4(aq)
Or are they both correct?
Post by: RuiAce on February 15, 2017, 02:30:32 pm
Hey, I'm a bit confused about which equations to use for the formation of acid rain. My teacher gave me these:There's actually TWO reaction pathways to make sulfuric acid.
SO2(g) + H2O(l) --> H2SO3(aq) (also is this supposed to be in equilibrium?)
SO3(g) + H2O(l) --> H2SO4(aq)
but my textbook has this one instead of the second one:
2H2SO3(aq) +O2(g) --> 2H2SO4(aq)
Or are they both correct?
The O2 can bump into the SO2 first, or the H2O does.
Post by: kiwiberry on February 15, 2017, 03:06:41 pm
There's actually TWO reaction pathways to make sulfuric acid.
The O2 can bump into the SO2 first, or the H2O does.
Ah I see, thank you :)
Post by: Butterflygirl on February 16, 2017, 07:13:01 pm
For combustion, we did a prac with a burner that contained ethanol in liquid form but the teacher said that in the equation for combustion of ethanol, it should be in gas state.
Can someone tell me how to figure out the state of different hydrocarbons?
Thankyou!! :)
Post by: RuiAce on February 16, 2017, 07:27:19 pm
Guys, I'm so confused about the states of hydrocarbons...I think the reason why it's gaseous is simply because what does the burner do? Heat it. Ethanol has a BP of somewhere around 78 degrees and the flame usually brings the temperature up enough so that the reaction doesn't take place until ethanol has entered the gaseous state.
For combustion, we did a prac with a burner that contained ethanol in liquid form but the teacher said that in the equation for combustion of ethanol, it should be in gas state.
Can someone tell me how to figure out the state of different hydrocarbons?
Thankyou!! :)
Now, with the following information - don't take it as law; best ask Jake to confirm. As far as I know of, you should be treating propanol and below as a gas. But then once you reach butanol (with a BP of about 110 degrees according to Google) it won't matter too much. Yet, you really don't encounter the combustion of butanol and above that often. (The only time you do is where you have to compare the heat of combustion of alkanols in the experiment.)
Post by: Kle123 on February 16, 2017, 07:50:49 pm
Could you please explain to me what a control experiment is and its purpose. Thank you
Post by: RuiAce on February 16, 2017, 07:56:06 pm
I have a question about a 'control experiment'. In trying to find an answer on the web, all that came up were things about controlled variables. To elaborate, the control experiment in which I'm not completely certain of its purpose is always seen in the discussion of a first hand experiment, addressing improvements in validity. E.g. in the acidic topic the experiment of decarbonating a carbonated drink through heating on a hotplate/boiling water bath is not completely valid. To improve validity a control of water is used.Having a control is basically having a basis. The same experiment with a carbonated drink should not yield the same results as water.
Could you please explain to me what a control experiment is and its purpose. Thank you
In the experiment, you could say that the variable is what your drink is. If it's (deionized) water, which is "probably" liquid in its most purest form, you should not expect results. I think of it like a "ground state" - we do nothing to it whatsoever.
Whereas a carbonated drink is full of all sorts of things to give it flavour. These "things" should be what's producing the CO2
(Also, note that if the WATER yielded positive results, then supposedly water would be a carbonated drink. If that's the case, in your coke can what if it was the water producing the CO2 and not something else?)
Sorry - Much like the above answer, all over the place
Post by: Butterflygirl on February 16, 2017, 10:01:26 pm
2H2 + O2 --> 2H20 (g), enthalpy change= -484kJ/mol
2H2 + O2 --> 2H20 (l)
Would you expect the heat change for the second reaction to be greater than, less than or the same as the heat change in the first reaction? Explain your answer.
Thanks!! :)
Post by: RuiAce on February 16, 2017, 10:21:21 pm
Also another question...Less.
2H2 + O2 --> 2H20 (g), enthalpy change= -484kJ/mol
2H2 + O2 --> 2H20 (l)
Would you expect the heat change for the second reaction to be greater than, less than or the same as the heat change in the first reaction? Explain your answer.
Thanks!! :)
We are told that the reaction between hydrogen and oxygen to produce gaseous water is an exothermic reaction. But then we switch out the product with water in its liquid state. So the only thing to analyse is the product.
So consider this: How does water go from a liquid state into a gaseous state? The answer is that it simply gets heated. The water absorbs the heat energy around it to go from the liquid state upwards.
Essentially, if gaseous water is immediately produced, then this has already happened. The water has already absorbed the heat in its surroundings (which INCLUDES the heat released from the reaction) and changed states. But if liquid water is produced, then this did not happen. It may or may not have absorbed some of the heat that was already in the surroundings, but it did NOT absorb the heat produced from the reaction.
If the system (i.e. the water product) does not absorb this heat, then since the reactants are the same, we can only assume that the heat was simply released into the surroundings. This "unused" heat released to the surroundings makes the reaction more exothermic, thus lowering the enthalpy change value.
Post by: legorgo18 on February 17, 2017, 11:20:54 pm
A studemt was told to dissolve 2g of NaOH in 2L of water. Calculate the concentration and pH of this solution
I searched on net and the eqn seems to be: NaOH + H2O > Na + OH + H2O
If this eqn is legit, then what is the solution? The na? or what is the actual equation?
Post by: Kle123 on February 17, 2017, 11:52:22 pm
Hey um got a question
A studemt was told to dissolve 2g of NaOH in 2L of water. Calculate the concentration and pH of this solution
I searched on net and the eqn seems to be: NaOH + H2O > Na + OH + H2O
If this eqn is legit, then what is the solution? The na? or what is the actual equation?
I had to reduce the resolution to post. The working out doesn't show all steps. Hope you still understand.
Post by: Butterflygirl on February 18, 2017, 02:32:34 pm
For the question:
A cold pack contains 32.0g of ammonium nitrate that dissolves in 120g of water at 20 degrees celsius. The energy absorbed when the salt dissolves in the water is 10.28kJ. Calculate the minimum temperature that could be reached by this solution (the specific heat capacity of the solution is 4.18 J/g/degrees celsius).
I can calculate up to finding delta T but I'm not sure how to find the minimum temperature. I thought the minimum temperature was 20 because the solution absorbs energy so it should increase when the ammonium nitrate is dissolved in the water?
Help please, I'm finding hard to visualise what's actually going on
Thankyou so much :)
Post by: sweetcheeks on February 18, 2017, 02:58:12 pm
Thanks for the help guys :) I have another question also...
For the question:
A cold pack contains 32.0g of ammonium nitrate that dissolves in 120g of water at 20 degrees celsius. The energy absorbed when the salt dissolves in the water is 10.28kJ. Calculate the minimum temperature that could be reached by this solution (the specific heat capacity of the solution is 4.18 J/g/degrees celsius).
I can calculate up to finding delta T but I'm not sure how to find the minimum temperature. I thought the minimum temperature was 20 because the solution absorbs energy so it should increase when the ammonium nitrate is dissolved in the water?
Help please, I'm finding hard to visualise what's actually going on
Thankyou so much :)
When ammonium nitrate dissolves it is an endothermic reaction, resulting in the solution cooling down. The minimum temperature will be 20-deltaT. In general, if there is an absorption of energy there will be a decrease in temperature. If there is energy released the temperature will increase.
What is happening in an endothermic reaction is that the bonds of the products has a higher energy then the bonds of the reactants. This energy needs to come from somewhere, it is taken from the environment.
Post by: Butterflygirl on February 18, 2017, 03:05:37 pm
When ammonium nitrate dissolves it is an endothermic reaction, resulting in the solution cooling down. The minimum temperature will be 20-deltaT. In general, if there is an absorption of energy there will be a decrease in temperature. If there is energy released the temperature will increase.
What is happening in an endothermic reaction is that the bonds of the products has a higher energy then the bonds of the reactants. This energy needs to come from somewhere, it is taken from the environment.
Makes so much sense now, thankyou!! :)
Post by: parthie on February 18, 2017, 05:41:01 pm
Bromine Water Decolourisation
Heat of Combustion of Alcohols
Galvanic Cells - measure voltage
Using flower petals as an indicator
Thankyou
Also if anyone has any tips about how to properly write up experiments because I lose all my marks there and practicals are worth 60% of my school mark
Post by: anotherworld2b on February 18, 2017, 09:09:23 pm
Post by: RuiAce on February 18, 2017, 09:24:44 pm
I was wondering would anyone know a good site/notes for learning the haber process and sulfuric acid the contact process?When I was learning the contact process I just used my textbook with "The Student's Guide to HSC Chemistry"
You could try "easychem" for the Haber process.
Post by: kiwiberry on February 18, 2017, 09:27:05 pm
If anyone has any of these experiments written up and would like to lend to me that would be a huge help I have a practical exam coming up and its worth 25% and I am really bad at writing up experiments
Bromine Water Decolourisation
Heat of Combustion of Alcohols
Galvanic Cells - measure voltage
Using flower petals as an indicator
Thankyou
Also if anyone has any tips about how to properly write up experiments because I lose all my marks there and practicals are worth 60% of my school mark
Hey! So the general things you should be including in your prac reports are:
- Aim
- Hypothesis
- Diagram of set-up - labelled!
- Method - not sure if it's just my school but this should be written in past tense
Eg. Change "Measure 100mL of water and place it into an aluminium can" to "100mL of water was measured and placed into an aluminium can"
- Risk assessment - state the risk, and ways to minimise this risk
- Results - could include observations, tables/graphs, calculations
For graphs, always remember to include a title, label your axes with units, and make sure your scale is appropriate (increments of 1, 2, 5, 10 etc.)
For tables, also remember to write a title, and make sure you draw in the borders - I know this might sound stupid but the teachers at my school are really picky and they took marks off people who didn't draw the outside borders on their tables :/
- Discussion - here you have to talk about accuracy, reliability and validity, as well as any improvements you could make to your experiment.
Accuracy refers to your measuring instruments - How sensitive were the instruments that you used? Could you have used something more accurate (scales with more decimal places or volumetric flask vs. measuring cylinder)? For things like the heat of combustion prac, how close were your results to the actual value?
Reliability refers to consistency of results - How many times did you repeat it? Did you obtain similar results everytime? Important to note that your results aren't reliable if you repeat 100 times but obtain completely different results
Validity refers to whether your method actually tests your aim - Did you control all your variables? Did you pick the most appropriate equipment?
- and finally, Conclusion - just a sentence or two about your results with reference to your aim/hypothesis
Hope this helps :)
Post by: RuiAce on February 18, 2017, 09:29:18 pm
- Method - not sure if it's just my school but this should be written in past tenseI think at my school, we used third person language and avoided the use of tense. Our steps were more like instructions.
Post by: anotherworld2b on February 18, 2017, 10:29:25 pm
thank you
When I was learning the contact process I just used my textbook with "The Student's Guide to HSC Chemistry"
You could try "easychem" for the Haber process.
Post by: anotherworld2b on February 19, 2017, 01:41:53 am
Post by: Shadowxo on February 19, 2017, 01:46:19 am
Would anyone have advice on how to do these questions?
Exothermic means heat is produced (feels hot) and the products have less energy than the reactants. Endothermic means heat is absorbed (feels cold) and the products have more energy than the reactants.
Burning kerosene -> produces heat -> exothermic
Melting ice -> heat absorbed & water has more energy than ice -> endothermic
See if you can do the rest and post any you can't do :)
Post by: bsdfjnlkasn on February 19, 2017, 10:07:46 am
I'm pretty sure my last line is incorrect as doesn't the presence of H3O+ determine a substance to be acidic not basic as I have concluded? I just tried to get to the fact that dilute acids have higher pH readings so can be seen as more basic than before but I don't get the link ... Hopefully that made some sense. Any help would be greatly appreciated!!
o The more you dilute an acid, the more ionisation that occurs
• LCP aims to maintain same ion concentration, which with an increase in volume naturally decreases
• Maintains same ion concentration by increasing % ionisation
• But comparatively, there are more H3O+ ions than before which results in a higher pH reading, so essentially it becomes more basic
Thank you!!
Post by: RuiAce on February 19, 2017, 10:43:49 am
Could someone please check if the following notes I have written are correct?More H3O+ ions results in a lower pH reading.
I'm pretty sure my last line is incorrect as doesn't the presence of H3O+ determine a substance to be acidic not basic as I have concluded? I just tried to get to the fact that dilute acids have higher pH readings so can be seen as more basic than before but I don't get the link ... Hopefully that made some sense. Any help would be greatly appreciated!!
o The more you dilute an acid, the more ionisation that occurs
• LCP aims to maintain same ion concentration, which with an increase in volume naturally decreases
• Maintains same ion concentration by increasing % ionisation
• But comparatively, there are more H3O+ ions than before which results in a higher pH reading, so essentially it becomes more basic
Thank you!!
The scale is a negative logarithmic scale
Post by: Shadowxo on February 19, 2017, 11:03:38 am
Could someone please check if the following notes I have written are correct?
I'm pretty sure my last line is incorrect as doesn't the presence of H3O+ determine a substance to be acidic not basic as I have concluded? I just tried to get to the fact that dilute acids have higher pH readings so can be seen as more basic than before but I don't get the link ... Hopefully that made some sense. Any help would be greatly appreciated!!
o The more you dilute an acid, the more ionisation that occurs
• LCP aims to maintain same ion concentration, which with an increase in volume naturally decreases
• Maintains same ion concentration by increasing % ionisation
• But comparatively, there are more H3O+ ions than before which results in a higher pH reading, so essentially it becomes more basic
Thank you!!
I'm not familiar with LCP, but I'll see if I can help a little.
First of all, just some info about H3O+ and OH- ions in water, and whether something is an acid or a base:
In pure water, which is neutral, the pH is 7. Some water molecules react with each other to form H3O+ and OH- ions but these concentrations and amounts are equal, so water is neutral. So water, being completely neutral with a pH of 7 (at 25şC) has both H3O+ and OH- ions.
Formula is H2O + H2O ⇌ H3O+ + OH−
And as [H3O+] decreases, the pH increases due to the negative logarithmic scale. 1 - very acidic, 7- neutral, 14 - very basic. If the [H3O+] decreases, the pH increases as it becomes more basic, getting closer to the basic end.
pH measures acidity by using the concentration of H3O+ ions in the formula pH = -log10[H3O+]. The number of H3O+ ions increases during dilution, but the [H3O+] decreases, as c=n/V and while n is larger, V is much larger, resulting in a lower concentration. pH can also be found by using 14- pOH, as pH + pOH = 14 at 25şC. I don't know about LCP, but if the same ion concentration is maintained, that is, if [H3O+] remains the same, the pH and therefore acidity will remain the same also. It only becomes more basic if [H3O+] decreases.
If [H3O+] decreases, such as through dilution, it becomes more basic / less acidic and the pH rises. A substance can be acidic while being "more basic" than another acid. More basic = less acidic.
Also, something can be basic even if there are H3O+ ions. To be basic and have a higher pH than 7, the [H3O+] must be very low, but this cannot happen by adding water to an acid as there will always be more H3O+ than OH- ions and water will react with itself to create more H3O+, therefore keeping the pH below 7. In any acidic or basic solution that contains water at 25şC, there will be H3O+ and OH- ions.
Hope this helps a bit :)
Post by: anotherworld2b on February 19, 2017, 01:35:42 pm
I was also wondering would solid -> liquid -> gas be endothermic or exothermic?
Exothermic means heat is produced (feels hot) and the products have less energy than the reactants. Endothermic means heat is absorbed (feels cold) and the products have more energy than the reactants.
Burning kerosene -> produces heat -> exothermic
Melting ice -> heat absorbed & water has more energy than ice -> endothermic
See if you can do the rest and post any you can't do :)
Post by: exit on February 19, 2017, 01:38:03 pm
Oh I see :)
I was also wondering would solid -> liquid -> gas be endothermic or exothermic?
endothermic.
Post by: Shadowxo on February 19, 2017, 01:47:00 pm
Oh I see :)
I was also wondering would solid -> liquid -> gas be endothermic or exothermic?
Gases have lots of kinetic energy, liquids have a bit and solids have the least. so solid -> liquid -> gas are all increasing energy, therefore as the product has more energy than the reactant, the reaction is endothermic.
Post by: RuiAce on February 19, 2017, 01:47:00 pm
Oh I see :)A brief (maybe not clear - sorry) explanation as to why it's endothermic:
I was also wondering would solid -> liquid -> gas be endothermic or exothermic?
For something to change states in that order, the temperatures need to be HIGHER. This is because the chemicals capture the heat energy around them to go into the next state. If the temperatures were lower, the energy the chemical (say, water) contains will be released back out as it reverts down a state.
Post by: beau77bro on February 19, 2017, 04:29:33 pm
15.a. Hahaha could someone explain this?
(http://uploads.tapatalk-cdn.com/20170218/d3fe10eb124ff2e53a1378aa64b61c0a.jpg)
And then 23. I have 0 clue, a worked solution would be amazing and massively appreciated
(http://uploads.tapatalk-cdn.com/20170218/afb39ca0b8bd031671c717cb53b8e282.jpg)
Post by: thenerdygangster on February 19, 2017, 04:43:01 pm
Post by: kiwiberry on February 19, 2017, 04:44:26 pm
Hey struggling on a couple of conquering chem questions. I'll post them together but thankyou.
15.a. Hahaha could someone explain this?
(http://uploads.tapatalk-cdn.com/20170218/d3fe10eb124ff2e53a1378aa64b61c0a.jpg)
And then 23. I have 0 clue, a worked solution would be amazing and massively appreciated
(http://uploads.tapatalk-cdn.com/20170218/afb39ca0b8bd031671c717cb53b8e282.jpg)
For 15a), in solution, NaHCO3 --> Na+ + HCO3-. Therefore, the concentration of HCO3- will increase, and by Le Chatelier, the equilibrium will shift left to decrease the concentration of HCO3-. Thus the concentration of H+ will decrease as well.
Post by: beau77bro on February 19, 2017, 06:17:09 pm
Post by: Mathew587 on February 19, 2017, 06:51:57 pm
Post by: thenerdygangster on February 19, 2017, 06:57:23 pm
Quote
why does H+ ions decrease if HCO3 decreases? I get why that decreases but not why H+ ions do.
You're producing H2CO3, in this question. In order to form H2CO3,, H+ ions need to be used up. This means that the amount of H+ ions decrease. Hope that helps :)
Post by: Mathew587 on February 19, 2017, 07:43:35 pm
Post by: J.B on February 20, 2017, 02:56:15 pm
Today in class, my teacher showed us how that if you have 0.1 molar concentration of Hydrochloric acid solution, the pH is 1. Due to the pH=-log10[H+].
Then we learn't that 0.1 molar concentration of Sulfuric acid solution which produces 2 hydronium ions thus, times the 0.1 by two and the pH is 0.7.
Then he said that 0.1 molar concentration of Acetic acid has pH 3. But I don't understand how you would work this out?
I was just wondering if you would be able to explain this?
Thank you.
Post by: RuiAce on February 20, 2017, 02:57:59 pm
Hello,Between HCl and H2SO4, which are both strong acids, it is quite clear to work out as you can look at the formula and count the number of H's you have there.
Today in class, my teacher showed us how that if you have 0.1 molar concentration of Hydrochloric acid solution, the pH is 1. Due to the pH=-log10[H+].
Then we learn't that 0.1 molar concentration of Sulfuric acid solution which produces 2 hydronium ions thus, times the 0.1 by two and the pH is 0.7.
Then he said that 0.1 molar concentration of Acetic acid has pH 3. But I don't understand how you would work this out?
I was just wondering if you would be able to explain this?
Thank you.
Acetic acid is a weak acid. It does not ionise to completion. This means that its DEGREE of ionisation is not 100%.
You are not expected to memorise the degree of ionisation of CH3COOH
Post by: kylesara on February 20, 2017, 03:21:26 pm
Post by: jakesilove on February 20, 2017, 03:41:01 pm
Hi I was just wondering if ill have to remember how to draw a Vanadium redox battery during any test this year. whenever i draw it its really bad. thanks.
Hey! If one of your chosen batteries is the Vanadium redox battery, then it's probably a good idea to know how to draw it! This is really just an 'icing on the cake' type situation, so if you're not great at it now, don't worry! You'll get used to it over time, so that when the HSC comes you can smash any battery question out of the park.
Again, you don't HAVE to know how to draw it, but I personally think it's helpful!
Post by: J.B on February 20, 2017, 05:35:19 pm
Between HCl and H2SO4, which are both strong acids, it is quite clear to work out as you can look at the formula and count the number of H's you have there.
Acetic acid is a weak acid. It does not ionise to completion. This means that its DEGREE of ionisation is not 100%.
You are not expected to memorise the degree of ionisation of CH3COOH
Oh right, thank you so much!!!
Post by: Aaron12038488 on February 20, 2017, 05:41:28 pm
Thanks.
Post by: jakesilove on February 20, 2017, 10:37:54 pm
I was wondering in future if I could post like my prac report, will I need 15 posts similarly to the Essay Policy?
Thanks.
Hey!
Don't worry about the post count, but I don't think I'll be able to mark an assessment in super-detail. I will be able to give you pointers, though; where to focus your attention, where you can improve, that sort of thing!
Jake
Post by: parthie on February 21, 2017, 08:19:18 pm
Hey! So the general things you should be including in your prac reports are:
- Aim
- Hypothesis
- Diagram of set-up - labelled!
- Method - not sure if it's just my school but this should be written in past tense
Eg. Change "Measure 100mL of water and place it into an aluminium can" to "100mL of water was measured and placed into an aluminium can"
- Risk assessment - state the risk, and ways to minimise this risk
- Results - could include observations, tables/graphs, calculations
For graphs, always remember to include a title, label your axes with units, and make sure your scale is appropriate (increments of 1, 2, 5, 10 etc.)
For tables, also remember to write a title, and make sure you draw in the borders - I know this might sound stupid but the teachers at my school are really picky and they took marks off people who didn't draw the outside borders on their tables :/
- Discussion - here you have to talk about accuracy, reliability and validity, as well as any improvements you could make to your experiment.
Accuracy refers to your measuring instruments - How sensitive were the instruments that you used? Could you have used something more accurate (scales with more decimal places or volumetric flask vs. measuring cylinder)? For things like the heat of combustion prac, how close were your results to the actual value?
Reliability refers to consistency of results - How many times did you repeat it? Did you obtain similar results everytime? Important to note that your results aren't reliable if you repeat 100 times but obtain completely different results
Validity refers to whether your method actually tests your aim - Did you control all your variables? Did you pick the most appropriate equipment?
- and finally, Conclusion - just a sentence or two about your results with reference to your aim/hypothesis
Hope this helps :)
Thankyou so much this really helps!!
Post by: anotherworld2b on February 21, 2017, 08:26:15 pm
Melting ice -> heat absorbed & water has more energy than ice -> endothermic
I was also wondering if i could have some help with these questions. In particular for q3 how do you know what is being consumed? Is it always the first reactant in the equation? If not, how do you tell?
Post by: bananna on February 21, 2017, 08:50:10 pm
can someone pls help with this q the answer's (C)
i sorta get the calculations, but don't know WHY you'd use mol weight
its from the 2003 HSC q 6
thank u!!
Post by: Mathew587 on February 21, 2017, 09:20:30 pm
I am bit confused about ice melting. Why does water have more energy than ice? Does that mean that if you freeze ice the opposite will happen?Water has more energy than ice simply because water flows while ice doesn't. This means water must have kinetic energy which MAKES it flow. Also steam has more energy than water and ice simply as to create steam, you need to ADD heat to water just as how you need to ADD heat to ice to make it water. :D
Melting ice -> heat absorbed & water has more energy than ice -> endothermic
I was also wondering if i could have some help with these questions. In particular for q3 how do you know what is being consumed? Is it always the first reactant in the equation? If not, how do you tell?
Idek how you can freeze ice?!?
Yes as far as what you know now, the reactant is whats being consumed (obv...). When solid water i.e. ice is transformed into liquid water i.e. water, the ice i.e. the reactant in this case is the one being consumed :D
Post by: parthie on February 21, 2017, 09:50:02 pm
and
For heat of combustion of alkaniols experiment - if this was done at school would this automatically be considered invalid if so why??
Any help would be great thanks
Post by: bsdfjnlkasn on February 21, 2017, 09:54:03 pm
hi!
can someone pls help with this q the answer's (C)
i sorta get the calculations, but don't know WHY you'd use mol weight
its from the 2003 HSC q 6
thank u!!
Hey, I'd also like some help approaching the calculations part of the question if that's ok?
Post by: bsdfjnlkasn on February 21, 2017, 10:05:56 pm
Can someone please explain validity in experiments to me?? I am so confused
and
For heat of combustion of alkaniols experiment - if this was done at school would this automatically be considered invalid if so why??
Any help would be great thanks
Validity is mostly concerned with maintaining all the variables the same to ensure the experiment, as implied by the name, valid. If you changed what you were measuring in the middle of an experiment then your experiment wouldn't have valid results as your results are not specific to the dependent variable anymore as you've introduced another one.
The heat of combustion experiment wouldn't be considered invalid if you kept your controlled variables controlled, measured the change in mass and changed the fuels accordingly. If you've kept these all as they should be, then you have yourself a valid experiment. But, what I think you're hinting at in your question is actually accuracy rather than validity. Accuracy is concerned with how close your values are to the accepted ones and the way in which you recorded your results. Because the latter is far less significant (only really concerned with parallax error in observing thermometer), we don't really need to focus on it as much. This is because our results do not drastically change when small readings on the thermometer are taken incorrectly.
Now on to what really shakes up our results:
Heat loss is by far, the most significant contributor to inaccurate results; i.e. results which deviate from the accepted values for the heat of combustion of your independent fuels.
Your heat of combustion experiment depends on the mass changes which is a result of the fuel having evaporated. This is what we are measuring. We obtain these mass changes by heating the fuel. If all the heat from the flame was conserved and directed straight into the alkanol, (i.e. through use of a bomb calorimeter) then we wouldn't see large discrepancies. But, because the experiment wasn't performed in a closed system, heat inevitably escaped - dissipated into the surroundings. By not having the most efficient heat transfer, our results are no longer reliable as such a big portion of what our results depended on was unreliable.
I hope that makes sense. Essentially you need to know that the heat losses to the surroundings significantly decreases the reliability of the experiment. Let me know if you want me to clarify the above further.
Post by: kiwiberry on February 21, 2017, 10:06:55 pm
hi!
can someone pls help with this q the answer's (C)
i sorta get the calculations, but don't know WHY you'd use mol weight
its from the 2003 HSC q 6
thank u!!
So for this question, we're trying to convert heat of combustion from kJ/g to kJ/mol. Molar mass is in g/mol. Looking at the units, if we times heat of combustion in kJ/g by molar mass in mol/g, we'll get kJ/mol, which is what we want!
Does that make sense? So to find heat of combustion in kJ/mol, you would multiply the values in the table by their respective molar masses :)
Post by: RuiAce on February 21, 2017, 10:09:52 pm
Can someone please explain validity in experiments to me?? I am so confusedValidity - Does your method address the aim?
and
For heat of combustion of alkaniols experiment - if this was done at school would this automatically be considered invalid if so why??
Any help would be great thanks
Your experiment in the laboratory is very poor, as the method doesn't properly test what we want to at all. It's both invalid and inaccurate (although can be very reliable), because you have heat losses occurring everywhere - heat absorbed into the calorimeter, heat released in the environment, the specific heat capacity of the calorimeter not being concerned in calculations etc. all affecting the final values.
By not having the most efficient heat transfer, our results are no longer reliable as such a big portion of what our results depended on was unreliable.Everything is good up to here. Reliability is specifically concerned with consistency.
I hope that makes sense. Essentially you need to know that the heat losses to the surroundings significantly decreases the reliability of the experiment. Let me know if you want me to clarify the above further.
You can have an experiment that's quite trashy, but still reliable because you consistently got the same wrong results every time you did it.
Post by: bsdfjnlkasn on February 21, 2017, 10:16:54 pm
So for this question, we're trying to convert heat of combustion from kJ/g to kJ/mol. Molar mass is in g/mol. Looking at the units, if we times heat of combustion in kJ/g by molar mass in mol/g, we'll get kJ/mol, which is what we want!
Does that make sense? So to find heat of combustion in kJ/mol, you would multiply the values in the table by their respective molar masses :)
Hey thanks for giving that conversion, the units make more sense. Would it be ok to assumed that m = 1g? How would we then proceed with the rest of the calculations?
Post by: kiwiberry on February 21, 2017, 10:28:37 pm
Hey thanks for giving that conversion, the units make more sense. Would it be ok to assumed that m = 1g? How would we then proceed with the rest of the calculations?
We're using the molar mass, so the mass of one mole of each substance. So for propanol (C3H8O):
Post by: RuiAce on February 21, 2017, 10:30:55 pm
The heat change per mole is in general, ALWAYS greater than that for the mass. And I know that n=m/MM so the molar mass is what distinguishes between the masses and moles.
Because the molar masses are always numbers greater than 1, and the heat change per mole is the larger one:
Given heat change per mole: Divide by MM to get heat change per gram
Given heat change per gram: Multiply by MM to get heat change per mole
(However, thanks for the proper explanation kiwiberry. This is just my quick way out.)
Post by: anotherworld2b on February 21, 2017, 10:51:51 pm
Water has more energy than ice simply because water flows while ice doesn't. This means water must have kinetic energy which MAKES it flow. Also steam has more energy than water and ice simply as to create steam, you need to ADD heat to water just as how you need to ADD heat to ice to make it water. :D
Idek how you can freeze ice?!?
Yes as far as what you know now, the reactant is whats being consumed (obv...). When solid water i.e. ice is transformed into liquid water i.e. water, the ice i.e. the reactant in this case is the one being consumed :D
Post by: anotherworld2b on February 21, 2017, 10:52:52 pm
Post by: Mathew587 on February 21, 2017, 11:15:59 pm
I was also wondering how to do this question
tip- kinetic energy. revise that bit and try again :)
Post by: anotherworld2b on February 21, 2017, 11:23:18 pm
tip- kinetic energy. revise that bit and try again :)
Post by: J.B on February 22, 2017, 07:59:12 am
This might be a silly question. I understand that Sulfuric acid is diprotic, and Hydrochloric acid is monoprotic as you can see this in their equations. But I was doing a practise HSC question, and in the answers it stated that Citric Acid is triprotic. I was just wondering, if I am supposed to have learnt this?, and how do you know that it is triprotic, as to me it is not obvious through its equation?
Thank you
Post by: jakesilove on February 22, 2017, 10:27:15 am
Hi,
This might be a silly question. I understand that Sulfuric acid is diprotic, and Hydrochloric acid is monoprotic as you can see this in their equations. But I was doing a practise HSC question, and in the answers it stated that Citric Acid is triprotic. I was just wondering, if I am supposed to have learnt this?, and how do you know that it is triprotic, as to me it is not obvious through its equation?
Thank you
Hey! This is just a stupid Chemistry thing that you'll have to memorise; Citric acid is triprotic. If you look at the structural formula, it sort of makes sense (there are three 'loose' Hydrogen). But, that doesn't matter; just memorise the point!
Post by: strawberriesarekewl on February 22, 2017, 10:27:59 am
Post by: anotherworld2b on February 22, 2017, 11:23:13 am
Post by: kiwiberry on February 22, 2017, 01:26:54 pm
What are some good tips on answering short answer and extended responses? I know we need to address the verb (i suck at that) and make a plan of what we are going to write but what else is required for full marks-Equations and diagrams!!! Always always always include relevant equations where you can, and for prac questions especially, it is much easier to draw a quick diagram of the set-up than to describe it in words
-If you haven't already, take a look at the BOSTES verbs list! Remember that assess/evaluate questions always have an easy mark allocated to a judgement
-For long responses, using underlined headings to separate parts of your answer instead of having a page full of writing will help ensure that the marker doesn't miss any part of your answer, eg in assess/evaluate questions, I always write Judgement: before I write my judgement.
-And yes always plan long answers before you start writing them! It is very easy to start 'brain dumping' and go off on tangents unrelated to the question, teachers get really pissed off at this
Other than that, what helps for me is going through many past papers and looking at the marking criteria for different questions. Eventually, you'll have a better idea at how marks are allocated, and in exams this helps with knowing what to write to gain full marks :)
Post by: kiwiberry on February 22, 2017, 02:03:39 pm
I was wondering if my answers are correct. I am a bit confused about how to know what colour the reaction will turn into
Hey! From your answers, I think you've got the right idea with the colours. Basically, if the equilibrium favours the forward reaction, the concentration of the products on the right will increase and thus their colour will become more prominent, vice versa
Q2 first photo- for the concentration of Cl- to increase, it will shift left
Q3 first photo- this is an exothermic reaction, meaning that heat is released. So when temperature is decreased, equilibrium will favour the exothermic reaction to produce more heat, which is the forward reaction in this case. Does that make sense?
Same goes for Q4 on the same photo, except this time it will favour the backward endothermic reaction since temperature is increased
Q5 first photo- you're right that OH- will react with H+ to form water! This reduces the concentration of H+. As a result, equilibrium will shift left to oppose the change and produce more H+
I'll have a look at the second photo and edit this post in a bit! Unless someone beats me to it haha.
Edit: ok I'm back!
Q3 second photo- there is actually no change to equilibrium here. Remember that the three factors affecting equilibrium are concentration, temperature and pressure. By removing solid iodine, its concentration/density isn't affected! Therefore, there will be no shift. Same goes for adding a solid. Good to note that Le Chatelier's Principle does not apply to solids
Q4 second photo- pressure will only affect gases in equilibrium. So because iodine is in solid form on the right, the mole ratio of gases is actually 1:0. Therefore, when volume is increased, pressure decreases, so equilibrium will shift to the reaction which produces more moles of gas, ie the backward reaction
Q5 second photo- same with 3&4 of the first photo, the equilibrium will shift to favour the backward endothermic reaction to reduce the temperature
Hope this helps :) Let me know if anything doesn't make sense!
Post by: strawberriesarekewl on February 22, 2017, 03:05:13 pm
-Equations and diagrams!!! Always always always include relevant equations where you can, and for prac questions especially, it is much easier to draw a quick diagram of the set-up than to describe it in words
-If you haven't already, take a look at the BOSTES verbs list! Remember that assess/evaluate questions always have an easy mark allocated to a judgement
-For long responses, using underlined headings to separate parts of your answer instead of having a page full of writing will help ensure that the marker doesn't miss any part of your answer, eg in assess/evaluate questions, I always write Judgement: before I write my judgement.
-And yes always plan long answers before you start writing them! It is very easy to start 'brain dumping' and go off on tangents unrelated to the question, teachers get really pissed off at this
Other than that, what helps for me is going through many past papers and looking at the marking criteria for different questions. Eventually, you'll have a better idea at how marks are allocated, and in exams this helps with knowing what to write to gain full marks :)
Thanks Kiwiberry
Post by: bananna on February 22, 2017, 03:30:06 pm
So for this question, we're trying to convert heat of combustion from kJ/g to kJ/mol. Molar mass is in g/mol. Looking at the units, if we times heat of combustion in kJ/g by molar mass in mol/g, we'll get kJ/mol, which is what we want!
Does that make sense? So to find heat of combustion in kJ/mol, you would multiply the values in the table by their respective molar masses :)
thank you! :)
Post by: bananna on February 22, 2017, 04:03:27 pm
can someone help me solve this q from hsc 2012
thank you!
Post by: kiwiberry on February 22, 2017, 04:40:12 pm
hi!13) First of all, remember that the oxidising agent does the oxidising, so is being reduced. This means that the oxidising agent has to be on the reactants side, so this gets rid of B. Also note that a substance is being reduced when its oxidation state reduces.
can someone help me solve this q from hsc 2012
thank you!
Let's look at the oxidation states of the others
A) H+: +1 at first, and stays at +1 in water
C) Fe2+: +2 at first, +3 after. Iron is being oxidised, so the answer isn't c)
D) Cr2O72-: Looking at Cr, +6 at first (let me know if you don't understand how I got this), +3 after! So it has been reduced, therefore it is the oxidising agent and the answer is D :)
14) So the oxidation and reduction eqns are:
Oxidation: 6Fe2+ <--> 6Fe3++6e-
Eoxidation= -0.77 V (remember to change the sign for oxidation when using the values on the data sheet)
Reduction: Cr2O72-+14H++6e- <--> 2Cr3+ + 7H2O
Ereduction= 1.36V
Therefore Etotal= 1.36 - 0.77 = 0.59 V! So the answer is A :)
Post by: J.B on February 22, 2017, 05:35:54 pm
Hey! This is just a stupid Chemistry thing that you'll have to memorise; Citric acid is triprotic. If you look at the structural formula, it sort of makes sense (there are three 'loose' Hydrogen). But, that doesn't matter; just memorise the point!
Ok thank you so much! and apart from
sulfuric acid - diprotic
Hydrochloric acid, acetic acid - monoprotic
Citric, Phosphoric acid - triprotic.
Are there any other ones that I will need to know?
Thank you.
Post by: legorgo18 on February 22, 2017, 05:47:17 pm
Calculate the volume of a 0.025 mol/L solution of KOH required to neutralise a 250 Ml of the 0.100 mol/L solution of HCl (ph of 2)
I just wrote the equation and was thinking, moles? c = n/v? degree of ionisation?
Please dont solve it for me, want to have some playtime :))
Post by: RuiAce on February 22, 2017, 05:53:14 pm
Hey how would you approach this questionThe equation is HCl + KOH -> KCl + H2O
Calculate the volume of a 0.025 mol/L solution of KOH required to neutralise a 250 Ml of the 0.100 mol/L solution of HCl (ph of 2)
I just wrote the equation and was thinking, moles? c = n/v? degree of ionisation?
Please dont solve it for me, want to have some playtime :))
1 mole of HCl reacts with 1 mole of KOH, So the more ratio of HCl and KOH is 1:1, so for perfect neutralisation you need the same amount of moles of each. You need to adapt to scenarios where you compare the moles of two different substances.
(Remark: The degree of ionisation of both substances is 100% here.)
Post by: J.B on February 22, 2017, 08:13:24 pm
I was just wondering, as 1M of HCL has a ph of 0, what does say 10M of HCL have as a pH?
Post by: kiwiberry on February 22, 2017, 08:22:45 pm
Hi,It would have a pH of -1 in theory! The pH scale is not restricted to 0-14, it can go negative and above 14 as well :)
I was just wondering, as 1M of HCL has a ph of 0, what does say 10M of HCL have as a pH?
Post by: Zainbow on February 22, 2017, 08:26:50 pm
this answer the question below is A.
Can you please explain why it's that answer? And what do they mean by 'acidified' when the potassium dichromate doesn't have H?
Post by: RuiAce on February 22, 2017, 08:37:18 pm
Hello :)Something being 'acidified' just means that it got mixed with an acid. Hence, H+ ions are involved in the entire mixture, not the compound being referred to.
this answer the question below is A.
Can you please explain why it's that answer? And what do they mean by 'acidified' when the potassium dichromate doesn't have H?
The relevant half-equations from your table of standard reduction potentials are thus:
SO24- + 4 H+ + 2 e- ⇌ SO2(aq) + 2 H2O(l)
1/2 Cr2O72- + 7 H+ + 3 e- ⇌ Cr3+ + 7/2 H2O(l)
I will leave the rest as your exercise. Come back if you are still stuck - the process should be standard from here.
Post by: Zainbow on February 22, 2017, 08:51:16 pm
Something being 'acidified' just means that it got mixed with an acid. Hence, H+ ions are involved in the entire mixture, not the compound being referred to.
The relevant half-equations from your table of standard reduction potentials are thus:
SO24- + 4 H+ + 2 e- ⇌ SO2(aq) + 2 H2O(l)
1/2 Cr2O72- + 7 H+ + 3 e- ⇌ Cr3+ + 7/2 H2O(l)
I will leave the rest as your exercise. Come back if you are still stuck - the process should be standard from here.
thank you, I think I've got it now
Post by: anotherworld2b on February 22, 2017, 09:43:05 pm
I was also wondering what would happen if you add water to a reaction?
Would it always decrease the concentration of reactants so it'll favour the side that produces more molecules/particles? Or will there be situations where nothing happens to the equilibrium position? I also wanted confirm that adding a inert gas (noble gas) to a reaction will not change the equilibrium position
For a rate vs time equilibrium graph if you add heat would the graph dramatically increase(vertical line?)
Post by: strawberriesarekewl on February 22, 2017, 09:56:35 pm
In HSC chemistry what are the hardest parts in production of materials, Acidic environment, Chemical management and monitoring and Industrial chemistry (option topic)
Post by: RuiAce on February 22, 2017, 10:13:36 pm
Hey guysHardest is biased towards each person.
In HSC chemistry what are the hardest parts in production of materials, Acidic environment, Chemical management and monitoring and Industrial chemistry (option topic)
What I felt:
PoM: (roting = rote learning)
- Roting all the stuff on nuclear chemistry involving commercial radioisotopes and blah
- Roting all the elctrochemistry
- Ethanol bullshitting (although this got better)
AE:
- Whole ton of experiments to memorise
CMM:
- The entire water section. I seriously hated it.
- Ways of measuring ozone concentrations in the atmosphere
- AAS
Industrial chem:
Tbh, the whole thing was hard. But the calculations in this section are intense (equilibrium constant stuff) and the Solvay process is really hard to know off by heart. So are the industrially used electrolytic cells for NaOH
Post by: strawberriesarekewl on February 22, 2017, 10:43:36 pm
How did you do about approaching these parts? Did you watch videos on them or do specific practice questions? (Sorry I'm on mi phone at the moment using the atarnotes app an I can't quote you)
Post by: RuiAce on February 22, 2017, 10:44:48 pm
Cheers ruiA few - Yes I did. Easychem has some good videos.
How did you do about approaching these parts? Did you watch videos on them or do specific practice questions? (Sorry I'm on mi phone at the moment using the atarnotes app an I can't quote you)
Most of them I just drilled though.
Post by: kiwiberry on February 22, 2017, 10:46:50 pm
Thank you for your help kiwiberryI'm not sure about the water and inert gas, but my guess is that water will dilute any aqueous substances, and the equilibrium will shift to favour the side which increases the concentrations of these substances again. However in theory, other liquids shouldn't affect position of equilibrium because, like solids, their concentration is fixed. I don't think inert gases would affect equilibrium, unless they affect the pressure in some way? Don't take my word for this!!
I was also wondering what would happen if you add water to a reaction?
Would it always decrease the concentration of reactants so it'll favour the side that produces more molecules/particles? Or will there be situations where nothing happens to the equilibrium position? I also wanted confirm that adding a inert gas (noble gas) to a reaction will not change the equilibrium position
For a rate vs time equilibrium graph if you add heat would the graph dramatically increase(vertical line?)
If you're talking about the graphs like these, heat added wouldn't result in a sudden change. It would be a gradual increase like at t3:
(http://www.smartlearner.mobi/science/VideoPastPapers/Equilibrium/Images/equilibrium_papers_10.jpg)
Post by: strawberriesarekewl on February 22, 2017, 10:49:18 pm
What do you mean by you "drilled through" them? Do you mean you just "did it"
Post by: anotherworld2b on February 22, 2017, 10:56:22 pm
would the graph be different for rate vs time?
I'm not sure about the water and inert gas, but my guess is that water will dilute any aqueous substances, and the equilibrium will shift to favour the side which increases the concentrations of these substances again. However in theory, other liquids shouldn't affect position of equilibrium because, like solids, their concentration is fixed. I don't think inert gases would affect equilibrium, unless they affect the pressure in some way? Don't take my word for this!!
If you're talking about the graphs like these, heat added wouldn't result in a sudden change. It would be a gradual increase like at t3:
(http://www.smartlearner.mobi/science/VideoPastPapers/Equilibrium/Images/equilibrium_papers_10.jpg)
Post by: legorgo18 on February 22, 2017, 11:10:20 pm
When their electrical conductivities are measured, 1M HCl is a good conductor but 1M NH3 isn't.
Explain why this occurs using equations.
Conjugates? Acid/Base? Clues pref :)
Post by: RuiAce on February 22, 2017, 11:13:12 pm
Hey got another questionHint: Electrical conductivity increases when things are ionised.
When their electrical conductivities are measured, 1M HCl is a good conductor but 1M NH3 isn't.
Explain why this occurs using equations.
Conjugates? Acid/Base? Clues pref :)
Post by: legorgo18 on February 22, 2017, 11:20:03 pm
Post by: RuiAce on February 22, 2017, 11:21:20 pm
hcl is a strong acid so it completely ionises hence high conductivity and conversely ammonia is a weak base?What I was thinking of
Post by: kiwiberry on February 22, 2017, 11:24:58 pm
I see
would the graph be different for rate vs time?
Sorry, I'm unfamiliar with this but this site (scroll down a bit) seems to have a good explanation of it! I think you're right with the vertical line when temperature increases
Post by: anotherworld2b on February 22, 2017, 11:41:23 pm
Sorry, I'm unfamiliar with this but this site (scroll down a bit) seems to have a good explanation of it! I think you're right with the vertical line when temperature increases
Post by: Shadowxo on February 23, 2017, 12:03:45 am
Thank you for your help kiwiberry
I was also wondering what would happen if you add water to a reaction?
Would it always decrease the concentration of reactants so it'll favour the side that produces more molecules/particles? Or will there be situations where nothing happens to the equilibrium position? I also wanted confirm that adding a inert gas (noble gas) to a reaction will not change the equilibrium position
For a rate vs time equilibrium graph if you add heat would the graph dramatically increase(vertical line?)
I'm not sure about the water and inert gas, but my guess is that water will dilute any aqueous substances, and the equilibrium will shift to favour the side which increases the concentrations of these substances again. However in theory, other liquids shouldn't affect position of equilibrium because, like solids, their concentration is fixed. I don't think inert gases would affect equilibrium, unless they affect the pressure in some way? Don't take my word for this!!
If you're talking about the graphs like these, heat added wouldn't result in a sudden change. It would be a gradual increase like at t3:
(http://www.smartlearner.mobi/science/VideoPastPapers/Equilibrium/Images/equilibrium_papers_10.jpg)
Just clearing things up a bit :)
If you add water, it dilutes the solution so the concentration of all the products and reactants decreases. Kiwi, while adding water doesn't change the number of each molecule, it does increase the volume and therefore decreases concentration, this is what causes equilibrium to shift, as it relies on concentration. So if a solution is diluted, equilibrium will shift to favour the side with more molecules (you can figure this out using K - not sure if you're familiar with it - but in order for K to remain constant, if the concentration of everything decreases, the side with more molecules is going to partially increase in concentration, and the side with less is going to decrease, to keep K constant. If you haven't been taught this, ignore it.) Nothing happens to the equilibrium position if the number of molecules on each side is the same.
Adding a noble/inert gas will not change the equilibrium position. Equilibrium depends on concentration. So while adding an inert gas will increase the overall pressure (/concentration) of the molecules in the vessel, the concentration (/pressure) of each substance will remain the same, therefore nothing changes.
For a rate vs time equilibrium graph, yes the graph would go vertically up as the rate is suddenly increasing
Hope this explains things a bit :)
Post by: anotherworld2b on February 23, 2017, 01:31:31 am
How can you tell if its endo or exo without the change of enthalpy being indicated?
Post by: anotherworld2b on February 23, 2017, 01:36:57 am
Post by: strawberriesarekewl on February 23, 2017, 02:05:05 pm
I have a prac coming up at the end of term. For this prac I have considered revising my scientific skills thoroughly, making notes on the prac that will come up and watching youtube videos to actually look at how the actual prac is performed
What other ways should I study for this upcoming prac? There arent really any prac exams online that I could do for practice so...
Post by: kiwiberry on February 23, 2017, 02:29:33 pm
Hey guys
I have a prac coming up at the end of term. For this prac I have considered revising my scientific skills thoroughly, making notes on the prac that will come up and watching youtube videos to actually look at how the actual prac is performed
What other ways should I study for this upcoming prac? There arent really any prac exams online that I could do for practice so...
Tbh I think that's all you can really do to prepare for a prac exam. For your notes, include the method, set-up, risk assessment and the validity, accuracy and reliability of each experiment because you'll probably get questions based on these. Make sure you know how to draw proper tables and graphs (line of best fit, title, units etc). If anyone has any other advice I would like to know as well, I have a prac exam this term too :(
Post by: jakesilove on February 23, 2017, 03:15:19 pm
Could i have help with q11 a?
How can you tell if its endo or exo without the change of enthalpy being indicated?
If the answer is a), I really don't know how you were supposed to get that without a delta H value!
I was also wondering how would you do e for this question?
Again, I really don't know. Look for whichever bond is 'strongest'; which that is, I'm not sure! Falls outside the HSC curriculum. However, I reckon it's the second one: Oxygen and Hydrogen bonds are very strong, as one is very electro negative and one is very electro positive
Post by: Fahim486 on February 23, 2017, 03:17:39 pm
I was just wondering if anyone had notes on interpreting equilibrium graphs.
Thanks!
Post by: f_tan on February 24, 2017, 11:23:13 pm
1. A household cleaning agent contains a weak base of general formula NaX. 1.00 g of this compound was dissolved in 100.0 mL of water. A 20.0 mL sample of the solution was titrated with 0.1000 mol L–1 hydrochloric acid and required 24.4 mL of the acid for neutralisation. What is the molar mass of this base?
2. The following equation represents a chemical system in equilibrium:
OCl−(aq) + H2O(l) <--> HOCl(aq) + OH−(aq)
Which of the following is an acid/base conjugate pair?
(A) H2O / HOCl
(B) HOCl / OH−
(C) HOCl / OCl−
(D) OCl− / H2O
Thank you! :)
Post by: kiwiberry on February 24, 2017, 11:54:23 pm
Hi! I need help with these past HSC questions, I'm not sure why/how to get the answer:
1. A household cleaning agent contains a weak base of general formula NaX. 1.00 g of this compound was dissolved in 100.0 mL of water. A 20.0 mL sample of the solution was titrated with 0.1000 mol L–1 hydrochloric acid and required 24.4 mL of the acid for neutralisation. What is the molar mass of this base?
2. The following equation represents a chemical system in equilibrium:
OCl−(aq) + H2O(l) <--> HOCl(aq) + OH−(aq)
Which of the following is an acid/base conjugate pair?
(A) H2O / HOCl
(B) HOCl / OH−
(C) HOCl / OCl−
(D) OCl− / H2O
Thank you! :)
1) Finding the moles of HCl used first:
n(HCl) = 0.1*0.0244 = 0.00244 mol
Because the reaction is NaX +HCl --> NaCl + HX
When neutralisation occurs, number of moles of NaX = number of moles of HCl
Therefore n(NaX) = 0.00244 mol
Because only 20mL out of the 100mL solution of NaX was titrated, m(NaX) = 1*0.2 = 0.2g
So there are 0.00244 moles in 0.2g of NaX!
0.00244 mol = 0.2 g
1 mol = 0.2/0.00244 = 81.97 g
So the molar mass is 81.97 g/mol :)
2) When an acid donates a proton, it forms its conjugate base, and when a base accepts a proton, it forms its conjugate acid
Looking at OCl-, it accepts a proton from water to form HOCl. Thus, the conjugate acid-base pair is OCl-/HOCl
Looking at H2O, it donates a proton to OCl- to form OH-. So the pair is H2O/OH-
Therefore the answer is C!
Post by: RuiAce on February 26, 2017, 09:16:11 am
Hi everyone,As rules of thumb:
I was just wondering if anyone had notes on interpreting equilibrium graphs.
Thanks!
Sudden spike in ALL substances - change in pressure
Sudden spike in ONE substance - change in concentration of just one thing
No spike but the graph suddenly changes - change in temperature
Reasoning:
The graph is concentration v.s. time and we know that C=n/V. So a spike, i.e. a sudden change in concentration, only occurs as a result of a change in the moles or the volume.
If the volume gets changed, then the concentration of EVERYTHING is changed. Hence this relates to a change in pressure.
If the moles gets changed, well then the moles of only one thing changes.
If no spike happens, but the concentrations start gradually changing afterwards, well the last thing left on the list is temperature. Which makes sense; temperature still causes the equilibrium to shift, but we haven't abruptly affected the concentration.
You must then use LCP as appropriate to determine which side the equilibrium shifted to and etc.
Post by: Fahim486 on February 26, 2017, 06:08:07 pm
As rules of thumb:
Sudden spike in ALL substances - change in pressure
Sudden spike in ONE substance - change in concentration of just one thing
No spike but the graph suddenly changes - change in temperature
Reasoning:
The graph is concentration v.s. time and we know that C=n/V. So a spike, i.e. a sudden change in concentration, only occurs as a result of a change in the moles or the volume.
If the volume gets changed, then the concentration of EVERYTHING is changed. Hence this relates to a change in pressure.
If the moles gets changed, well then the moles of only one thing changes.
If no spike happens, but the concentrations start gradually changing afterwards, well the last thing left on the list is temperature. Which makes sense; temperature still causes the equilibrium to shift, but we haven't abruptly affected the concentration.
You must then use LCP as appropriate to determine which side the equilibrium shifted to and etc.
Thank you so much for your help RuiAce :)
Post by: kylesara on February 28, 2017, 03:40:45 pm
Post by: jakesilove on February 28, 2017, 03:45:53 pm
Hi, i was wondering if you have to number your steps in the method of a report or if dot points are alright. Thanks.
Hey! Either works fine; I much prefer to number though.
Post by: J.B on February 28, 2017, 05:56:06 pm
Write Two chemical equations to show that the dihydrogen phosphate ion is amphiprotic. I can't seem to find any worked solutions for this so I was wondering if my answer was correct:
As an acid: H2PO4- (aq) + OH- (aq) > HPO4 2- (aq) + H20 (l)
As a base: H2PO4- (aq) + H3O+ (aq) > H3PO4 (aq) + H2O (l)
Thank you.
Post by: kiwiberry on February 28, 2017, 06:00:45 pm
Hi I was just doing Q22 b) from the 2004 chemistry paper. And the question is:
Write Two chemical equations to show that the dihydrogen phosphate ion is amphiprotic. I can't seem to find any worked solutions for this so I was wondering if my answer was correct:
As an acid: H2PO4- (aq) + OH- (aq) > HPO4 2- (aq) + H20 (l)
As a base: H2PO4- (aq) + H3O+ (aq) > H3PO4 (aq) + H2O (l)
Thank you.
Looks good!! :)
Post by: RuiAce on February 28, 2017, 06:21:45 pm
Post by: bananna on March 01, 2017, 07:14:41 am
I have a q from the 2012 chem paper
All the lead ions present in a 50.0 mL solution were precipitated by reaction with excess chloride ions. The mass of the dried precipitate was 0.595 g.
What was the concentration of lead in the original solution?
thanks!
Post by: bananna on March 01, 2017, 07:16:10 am
I also have this question
The heat of combustion of propan-1-ol is 2021kJmol–1. Combustion takes place according to the equation:
2C3H7OH(l) + 9O2(g) → 6CO2(g) + 8H2O(l)
What mass of water is formed when 1530 kJ of energy is released?
thanks again :)
Post by: bananna on March 01, 2017, 07:35:49 am
can you please check my response to this question its out of 4 marks
An indicator is placed in water. The resulting solution contains the green ion, Ind −, and 4
the red molecule, HInd.
Explain why this solution can be used as an indicator. In your response, include a
suitable chemical equation that uses Ind − and Hind.
HSC 2013
Hind<--> H^+ + Ind^-
red green
This solution is an indicator because it has a differentiating power (colour) between pHs. It will recognise the lower pH substance as red, and will change to green when a substance of higher pH is added. Thus, it is a valid indicator.
I wasn't really sure how to answer this q
thank you :)
Post by: kiwiberry on March 01, 2017, 08:12:15 am
HiPb2++ 2Cl- --> PbCl2
I have a q from the 2012 chem paper
All the lead ions present in a 50.0 mL solution were precipitated by reaction with excess chloride ions. The mass of the dried precipitate was 0.595 g.
What was the concentration of lead in the original solution?
thanks!
We know that v(Pb2+) = 0.05 L and m(PbCl2) = 0.595 g. Our goal is to find c(Pb2+), so somehow we need to find n(Pb2+)!
We can find n(PbCl2) since we are given its mass
n(PbCl2) = 0.595/(207.2+2*35.45) = 0.0021395… mol
Since n(PbCl2) = n(Pb2+),
n(Pb2+) = 0.0021395 mol
m(Pb2+) = 0.0021395*207.2 = 0.44308… g
Now we can find concentration!
c(Pb2+) = 0.44308.../0.05 = 8.866.. g/L = 8.87 g/L :)
Post by: kiwiberry on March 01, 2017, 08:31:30 am
Hi again (sorry)Here we can find the number of moles of propanol that were combusted since we are given heat of combustion per mol
I also have this question
The heat of combustion of propan-1-ol is 2021kJmol–1. Combustion takes place according to the equation:
2C3H7OH(l) + 9O2(g) → 6CO2(g) + 8H2O(l)
What mass of water is formed when 1530 kJ of energy is released?
thanks again :)
n(propanol) = 1530/2021 = 0.75707... mol
Now you can find n(H2O) using the mole ratio in the equation, and find the mass of water using the moles!
Post by: kiwiberry on March 01, 2017, 08:37:19 am
Again, I am sooooo sorryYou've got the right idea! Since the question states that the indicator has to be placed in water, it might be better to use HInd + H2O <--> Ind- + H3O+ as your equation. Because it's 4 marks, I would expand a bit and talk about Le Chatelier's Principle, so explain how if an acid is added, [H3O+] will increase and thus equilibrium will shift left, resulting in a red colour. Vice versa for bases. :)
can you please check my response to this question its out of 4 marks
An indicator is placed in water. The resulting solution contains the green ion, Ind −, and 4
the red molecule, HInd.
Explain why this solution can be used as an indicator. In your response, include a
suitable chemical equation that uses Ind − and Hind.
HSC 2013
Hind<--> H^+ + Ind^-
red green
This solution is an indicator because it has a differentiating power (colour) between pHs. It will recognise the lower pH substance as red, and will change to green when a substance of higher pH is added. Thus, it is a valid indicator.
I wasn't really sure how to answer this q
thank you :)
Post by: bananna on March 01, 2017, 11:45:26 am
You've got the right idea! Since the question states that the indicator has to be placed in water, it might be better to use HInd + H2O <--> Ind- + H3O+ as your equation. Because it's 4 marks, I would expand a bit and talk about Le Chatelier's Principle, so explain how if an acid is added, [H3O+] will increase and thus equilibrium will shift left, resulting in a red colour. Vice versa for bases. :)
i get it thank you :)
Post by: roygbivmagic on March 03, 2017, 09:12:33 pm
I understand that in solution KHSO4 --> K+ and HSO4-
K+ cannot accept or donate protons, and HSO4- can donate its H+. But can't HSO4- also accept a H+?
Post by: RuiAce on March 03, 2017, 09:16:29 pm
Hi, could you please explain why KHSO4 is an acidic salt?It can, but it tends not to. Whilst HSO4- is technically amphiprotic, in water it has a tendency to be acidic.
I understand that in solution KHSO4 --> K+ and HSO4-
K+ cannot accept or donate protons, and HSO4- can donate its H+. But can't HSO4- also accept a H+?
i.e. It has a tendency to go from HSO4- to SO42- in water instead. Not backwards into becoming H2SO4 again.
Post by: Kle123 on March 03, 2017, 10:06:02 pm
Post by: kiwiberry on March 03, 2017, 10:54:16 pm
Could someone explain to me how this is an example of a Bronsted and Lowry acid-base reaction.The Bronsted-Lowry definition states that acids are proton donors, and bases are proton acceptors.
Here, HCl is the acid, and donates a proton to form Cl-. NH3, the base, accepts this proton to form NH4+. These two ions combine together to form NH4Cl
Post by: bsdfjnlkasn on March 04, 2017, 09:24:59 am
Hi, could you please explain why KHSO4 is an acidic salt?
I understand that in solution KHSO4 --> K+ and HSO4-
K+ cannot accept or donate protons, and HSO4- can donate its H+. But can't HSO4- also accept a H+?
Hey I just wanted to extend this question to bicarbonate HCO3
Is it also technically amphiprotic as it is can either become carbonate or carbonic acid - are we expected to know which will be favoured more generally? Also on the quoted question, what is it about K+, Na+, these non-metal ions that prevents them from accepting hydrogens? I understand Na is inert but if someone could please offer a more in-depth explanation for why not just it being so I would really appreciate it (but it's totally ok if I have to accept it for the HSC)
Thank you! :)
Post by: bsdfjnlkasn on March 04, 2017, 09:54:10 am
I was just wondering if there was anything particularly notable about neutral oxides which make them unable to react with acidic or basic oxides? Or are we just expected to learn some in which case are there any I'm missing from the following list? CO,NO, N2O
More questions:
I was also wondering if I could get some clarification with how increasing the volume of a substance decreases pressure if the equilibrium can only be in a closed system?
Because if we increase volume of say water in a bottle, aren't we increasing the pressure of the air inside as the same volume is now forced to fit into a smaller area?
But for now taking that fact for granted, does an increase in pressure cause the system to favour side with fewer gas moles because it's trying to maintain the same mole ratio?
For the following where all reactants/products are gases, would increasing the volume be the same as increasing the pressure of that substance? What does pressure specifically refer to? That of the gas or the entire system?
CH3OH (g) <--> CO (g) + 2H2 (g)
2. Is it true that if the pressure of a system is increased the concentration of all the substances increases? Because I don't understand how pressure (only concerned with gases) influences concentration (only concerned with aqueous solutions)
Sorry I know - a lot of questions, any guidance/answers would be greatly appreciated :)
Post by: Shadowxo on March 04, 2017, 05:17:16 pm
Hey there,
I was just wondering if there was anything particularly notable about neutral oxides which make them unable to react with acidic or basic oxides? Or are we just expected to learn some in which case are there any I'm missing from the following list? CO,NO, N2O
More questions:
I was also wondering if I could get some clarification with how increasing the volume of a substance decreases pressure if the equilibrium can only be in a closed system?
Because if we increase volume of say water in a bottle, aren't we increasing the pressure of the air inside as the same volume is now forced to fit into a smaller area?
But for now taking that fact for granted, does an increase in pressure cause the system to favour side with fewer gas moles because it's trying to maintain the same mole ratio?
For the following where all reactants/products are gases, would increasing the volume be the same as increasing the pressure of that substance? What does pressure specifically refer to? That of the gas or the entire system?
CH3OH (g) <--> CO (g) + 2H2 (g)
2. Is it true that if the pressure of a system is increased the concentration of all the substances increases? Because I don't understand how pressure (only concerned with gases) influences concentration (only concerned with aqueous solutions)
Sorry I know - a lot of questions, any guidance/answers would be greatly appreciated :)
Hi :)
1. Volume and pressure:
First of all, the pressure of a substance is proportional to the concentration of a substance, and so the two are often interchangeable when referring to a specific substance.
With gases, increasing volume is increasing the volume of the container holding the gas. This means the same number of moles of gas occupy more space, therefore as c=n/V, and V increases while n stays the same, the concentration decreases, and therefore the pressure decreases (molecules don't hit the sides of the container as much - more space in the container).
Yes, an increase in pressure causes the system to favour the side with fewer gas molecules. This is due to wanting to maintain equilibrium - using the K value (not sure if you know this) you can see it'll favour the side with less gas molecules. This can also be explained by Le Chatlier's principle, it wants to partially oppose the change by decreasing the overall pressure of the system.
Pressure is proportional to concentration for gases, so a decrease in pressure is the same as saying a decrease in concentration. Pressure can refer to either the overall system or the individual substances, they'll usually specify. Often it'll refer to both. If it says "pressure of [substance]" it'll refer to that substance, if it says "the pressure was decreased" it'll usually refer to the whole system.
Increasing the volume would be the same as decreasing the pressure/concentration of each individual substance, and decreasing the overall pressure/concentration.
2. Concentration can be used both for aqueous solutions and gases. c=n/v, both liquids and gases have a number of moles per volume. eg a gas having 2 moles of it in a 2L container, c=n/V = 1molL-1. Pressure is only used for gases though.
The pressure of a system can be increased in two ways:
- Adding an inert gas. This increases the pressure of the system but doesn't affect the concentration/pressure of each individual substance, so has no effect on equilibrium.
- Decreasing volume / adding more of each substance (usually the former). This increases the pressure/concentration of both the overall system and each substance. So if the volume is decreased, the concentration of each substance increases.
Hope this helps, if anything need further clarification just ask :)
Post by: bsdfjnlkasn on March 04, 2017, 09:16:17 pm
Could I get some help with the following questions? I feel like there's a trick somewhere because my usual calculations aren't getting the correct answers - any help would be greatly appreciated :)
1. To determine the concentration of ammonia in ‘cloudy ammonia’ solution, a chemist first accurately diluted 25mL (by pipette) to 500mL (volumetric flask), then titrated 25mL (by pipette) of the diluted solution with 0.151 mol/L nitric acid solution; it required 27.2mL. Calculate the molarity of the original ammonia solution.
2. 25mL of H2SO4 (aq) from a car battery was accurately diluted to 500mL. 25mL of the diluted solution was titrated with 0.206 mol/L NaOH. It required 38.8mL. Calculate the molarity of the original battery acid and its concentration in grams per litre. If 50mL of this acid were split on the bench, how much anhydrous sodium carbonate would be needed to neutralise it?
Post by: kiwiberry on March 04, 2017, 09:54:08 pm
Hey there,
Could I get some help with the following questions? I feel like there's a trick somewhere because my usual calculations aren't getting the correct answers - any help would be greatly appreciated :)
1. To determine the concentration of ammonia in ‘cloudy ammonia’ solution, a chemist first accurately diluted 25mL (by pipette) to 500mL (volumetric flask), then titrated 25mL (by pipette) of the diluted solution with 0.151 mol/L nitric acid solution; it required 27.2mL. Calculate the molarity of the original ammonia solution.
2. 25mL of H2SO4 (aq) from a car battery was accurately diluted to 500mL. 25mL of the diluted solution was titrated with 0.206 mol/L NaOH. It required 38.8mL. Calculate the molarity of the original battery acid and its concentration in grams per litre. If 50mL of this acid were split on the bench, how much anhydrous sodium carbonate would be needed to neutralise it?
1. NH3 + HNO3 --> NH4NO3
We know that v(diluted NH3)=0.025 L, c(HNO3)=0.151 and v(HNO3)=0.0272 L, so we can find n(HNO3)
n(HNO3) = 0.151 x 0.0272 = 0.0041072 mol = n(NH3)
so c(diluted NH3) = 0.0041072/0.025 = 0.164288 mol/L
Now, because it was a twenty-fold dilution (500/25=20),
concentration of the original ammonia solution = 20 x 0.164288 = 3.28576 = 3.3 mol/L (2sf)
2. This is basically the same method as the first question, except you have to convert concentration in mol/L to g/L by mulitplying by molar mass. For the second part of the question, you use the concentration of H2SO4 you calculated to calculate the moles of Na2CO3 :)
Post by: kiwiberry on March 04, 2017, 10:13:25 pm
Hey I just wanted to extend this question to bicarbonate HCO3
Is it also technically amphiprotic as it is can either become carbonate or carbonic acid - are we expected to know which will be favoured more generally? Also on the quoted question, what is it about K+, Na+, these non-metal ions that prevents them from accepting hydrogens? I understand Na is inert but if someone could please offer a more in-depth explanation for why not just it being so I would really appreciate it (but it's totally ok if I have to accept it for the HSC)
Thank you! :)
I would like some clarification on this as well! Someone told me that Na+ is extremely weak because it's the conjugate acid of NaOH, a strong base, is this right?
Post by: bsdfjnlkasn on March 04, 2017, 10:42:55 pm
I would like some clarification on this as well! Someone told me that Na+ is extremely weak because it's the conjugate acid of NaOH, a strong base, is this right?
Yeah, I see how that explanation makes sense when the equation is relevant to NaOH but not every example will have Na+'s being the result of it previously reacting with the really strong base if that makes sense? Also thank you for the calculation replies, will reattempt now ;D
Post by: jakesilove on March 05, 2017, 10:51:17 am
I would like some clarification on this as well! Someone told me that Na+ is extremely weak because it's the conjugate acid of NaOH, a strong base, is this right?
To be honest, I think that at this point you just need to accept it for the purpose of the HSC and move on. I could make up any number of explanations (like the one you stated above) but I don't know which would be actually correct. Suffice to say that this is a tiny hole in the curriculum, and you won't be asked to explain something like this. Great questions though!
Post by: HighTide on March 05, 2017, 11:43:36 am
I would like some clarification on this as well! Someone told me that Na+ is extremely weak because it's the conjugate acid of NaOH, a strong base, is this right?Sup,
Not sure if this has been answered before, but thinking about it like this should help:
I'll just take a strong acid HA as the example. So HA + water --> A- and hydronium. For HA to be a strong acid, it must ionize to completion in water. Hence if you set up an acidity constant Ka will be very high.
Now consider, the reverse of the equation, so: A- + hydronium --> HA + water. In this case, if A- was a strong conjugate base, then, it would accept the hydrogen and form HA, and hence its basicity constant would be high. So the problem with this is that you can't have a high Ka for one reaction, and a high Kb for its backwards reaction, because then you're not having complete ionization. Hence it should be: strong acid --> weak conjugate base, strong base--> weak conjugate acid, weak base --> strong conjugate acid, weak acid --> strong conjugate base.
So in the case of NaOH, which is a very strong base, we will have a very weak conjugate acid Na+. It's basically an inverse relationship. The higher the acidity, the lower the basicity. eg. HCl is a strong acid, Cl- is a weak conjugate base.
Hope this helps
Post by: ch3ntastic on March 05, 2017, 08:32:12 pm
Post by: armtistic on March 05, 2017, 09:14:22 pm
I've noticed when it comes to the equations for the batteries i.e. dry cell or silver-button cell in POM the equations vary widely between sources.
I was wondering someone could point out which exactly would be most desirable to use in the HSC and also how do you even remember such complex equations?
Also how important are the solubility rules?
Post by: kiwiberry on March 05, 2017, 09:28:08 pm
For the "Standard Potentials" sheet supplied in the exams. Potassium is at the top and fluorine is at the bottom. Which one is the stronger oxidant/reductant? I get really confused between the whole OIL-RIG and its agents. Thanks ;)
As you go up the standard potentials sheet, ease of oxidation increases (as does reactivity), so K(s) is the strongest reductant. This means that K+ is the weakest oxidant. All the species on the right are reducing agents, and all the ones on the left are oxidising agents :)
Post by: jakesilove on March 06, 2017, 09:15:31 am
Hey Guys.
I've noticed when it comes to the equations for the batteries i.e. dry cell or silver-button cell in POM the equations vary widely between sources.
I was wondering someone could point out which exactly would be most desirable to use in the HSC and also how do you even remember such complex equations?
Also how important are the solubility rules?
Hey! It's important to have SOME kind of equation, but as long as it balances, it won't really matter what it is. As you've said, there are plenty of variations on the internet. Pick the easiest one, and learn it off by heart!
Solubility rules are vital. You need to know them, as you could have up to 8 marks in an HSC exam relying on these principles. They're tough to remember, but with enough practice, you'll do it!
Post by: J.B on March 06, 2017, 02:55:43 pm
I was just wondering why is sodium ethanoate written as CH3COONa, if the Na is the cation and the ethanoate the anion? As I would have automaticallly swapped them around?
Thanks.
Post by: jakesilove on March 06, 2017, 03:02:11 pm
Hi,
I was just wondering why is sodium ethanoate written as CH3COONa, if the Na is the cation and the ethanoate the anion? As I would have automaticallly swapped them around?
Thanks.
Hey! It's really only written like that to indicate how the bonding has occurred. The Sodium atom is bonded to one of the Oxygen atoms (the one that isn't double bonded), as suggested by the empirical formula. Make sense?
Post by: J.B on March 06, 2017, 03:10:27 pm
Hey! It's really only written like that to indicate how the bonding has occurred. The Sodium atom is bonded to one of the Oxygen atoms (the one that isn't double bonded), as suggested by the empirical formula. Make sense?
Ok thankyou, so in an exam is it best to write it like this?
Post by: jakesilove on March 06, 2017, 04:51:48 pm
Ok thankyou, so in an exam is it best to write it like this?
I reckon so! It isn't a big deal, but I think that form is just easier to work with :) Also, it makes it easier for you to remember the ionisation (ie. the Na just popping off).
Post by: Kle123 on March 07, 2017, 05:14:36 pm
Possible neutralising substances available: 300mL of 2M NaOH(aq) solution, 200g packet of solid sodium hydrogen carbonate (NaHCO3) or 100 g of NaOH pellets
Question: which substance would be most appropriate clean up neutralising agent and why?
Post by: Newbalance101 on March 07, 2017, 05:20:03 pm
Post by: kiwiberry on March 07, 2017, 05:52:04 pm
200mL of a same concentrated sulfuric acid (18M) was dropped by accident.
Possible neutralising substances available: 300mL of 2M NaOH(aq) solution, 200g packet of solid sodium hydrogen carbonate (NaHCO3) or 100 g of NaOH pellets
Question: which substance would be most appropriate clean up neutralising agent and why?
NaOH isn't a good neutralising agent in general because it's a strong base, so if excess is used by accident, the spill would become dangerous again. This leaves NaHCO3, which is good neutralising agent because it is weak, bubbles during neutralisation so you're able to see when the reaction has finished, and because it's solid, it helps prevent the spill from spreading. Not relevant here, but NaHCO3 is also amphiprotic which is beneficial if the spilt substance is unknown because it can neutralise both acids and bases
Post by: kiwiberry on March 07, 2017, 05:59:22 pm
How do you add half equations together? I'm really bad at it :/
Balance the half equations first so there's the same number of electrons in both. After that, all you have to do is add the two equations together (combine everything on the reactants side and products side) and cancel the electrons on both sides :)
Post by: beau77bro on March 09, 2017, 08:44:21 am
thankyou
Post by: jakesilove on March 09, 2017, 09:10:07 am
Hey how does one write net ionic equations? I'm confused with the whole different sets of equations used, so there's half, complete, nuclear and net, are there any others I need to know?
thankyou
Hey! Yep, those are the equations you'll need to be able to figure out. Net ionic equations essentially leave out spectator ions; other than that, it's just the complete equation. Half equations break the reaction up into reduction and oxidation halves (so one side should be adding electrons, one side should be losing electrons). Not sure whether you're looking for clarification or actual help, so if there's a specific equation that you're struggling with post it up and we can help you out!
Post by: beau77bro on March 09, 2017, 11:17:44 am
Post by: beau77bro on March 09, 2017, 11:18:33 am
Post by: jakesilove on March 09, 2017, 12:50:52 pm
Omg sorry that doesn't even make sense, thanks Jake I'll do better next time hopefully
Aha nah I understand what you mean! There's just so much going on when it comes to Chemistry, it's hard to stay on top of all the terminology :)
Post by: Aaron12038488 on March 10, 2017, 06:56:18 pm
Post by: jakesilove on March 10, 2017, 07:07:42 pm
So I'm in prelim and my first assessment is a Prac. My class has not done the separating techniques only gravimetric analysis. So is it possible that one of the separating techniques is in the Prac. The other class has done all the techniques, so I'm confused why my class hasn't.
Hey!
As a general rule, your teacher will not assess you on anything they haven't taught you in class. It would be very easy for you to go to the head of Science, or the school, if they unfairly assessed you (particularly if one class has done the work, and another class has not). So, I think it's worth talking to some classmates about whether you REALLY haven't done the learning required for gravimetric analysis. If not, I wouldn't worry about it. Also, the best course of action is always to just ask your teacher :)
Post by: mylinh-nguyen on March 10, 2017, 11:11:40 pm
also does it matter how much indicator you use in the solution with unknown concentration?
Post by: shreya_ajoshi on March 10, 2017, 11:30:04 pm
Can someone explain how reverse titration works and the difference between normal titration? I'm concerned my assessment task it will be that.By reverse titration, do you mean back titration?
also does it matter how much indicator you use in the solution with unknown concentration?
No, the amount of indicator used doesn't really matter. I normally add 5 drops
Post by: jakesilove on March 11, 2017, 11:31:41 am
Can someone explain how reverse titration works and the difference between normal titration? I'm concerned my assessment task it will be that.
also does it matter how much indicator you use in the solution with unknown concentration?
The amount of Indicator matters a little bit; the indicator itself is acidic or basic, so by adding it you're changing to pH. This affects the validity of the experiment. So, we try to use as few drops as possible!
Post by: legorgo18 on March 12, 2017, 12:22:10 pm
So im revising for half yearly and each of my sources have different colours for the cabbage indicator
Can someone please give me the legit ones for the red cabbage indicator
All ive got is pink for acid, blue for neutral and green for base
Thank you
Post by: kiwiberry on March 12, 2017, 01:16:28 pm
Hey have a question about the cabbage indicator
So im revising for half yearly and each of my sources have different colours for the cabbage indicator
Can someone please give me the legit ones for the red cabbage indicator
All ive got is pink for acid, blue for neutral and green for base
Thank you
This is what I have:
- Pink/red for acidic (1-4)
- Purple for slightly acidic to neutral (4-7)
- Blue for neutral to slightly basic (7-10 but slightly green at 10)
- Green for basic (10-12)
Not sure if legit though!
Post by: jakesilove on March 12, 2017, 01:30:47 pm
This is what I have:
- Pink/red for acidic (1-4)
- Purple for slightly acidic to neutral (4-7)
- Blue for neutral to slightly basic (7-10 but slightly green at 10)
- Green for basic (10-12)
Not sure if legit though!
Aha at the end of the day, it genuinely varies for cabbage to cabbage. So, this isn't super important, and different sources will say different things. That being said, the above looks legit.
Post by: Mathew587 on March 12, 2017, 04:57:02 pm
Write overall Redox equations for the following reaction in an acidic environment and identify the oxidising and reducing agent in each case
A) MNO4^-1 (aq) + H2S (aq) -> Mn^2+ (aq) + S(s)
B) ClO^-1 (aq) + SO2 (g) -> Cl^-1(aq) + SO4^2- (aq)
Post by: Ellie__ on March 12, 2017, 07:38:30 pm
You've probably answered this like 10000000 times, but I was wondering if there is any 'cheat sheet' thing or what the best way to remember likely acids, bases, formulas and valencies for tests. I'm doing the acidic environment currently and have completed the production of materials and there is A LOT of formulas and stuff to remember for 1/2 yearlies so I really want to know how it's best to remember
Thanks Guys!!!!!!!!
Post by: jakesilove on March 12, 2017, 10:21:42 pm
Hey guys,
You've probably answered this like 10000000 times, but I was wondering if there is any 'cheat sheet' thing or what the best way to remember likely acids, bases, formulas and valencies for tests. I'm doing the acidic environment currently and have completed the production of materials and there is A LOT of formulas and stuff to remember for 1/2 yearlies so I really want to know how it's best to remember
Thanks Guys!!!!!!!!
Hey! Check out this formula sheet that I made a while ago. In my opinion, it has everything you need to memorise! Let me know if you have any questions :)
Post by: Bubbly_bluey on March 13, 2017, 06:27:23 pm
A) Mg + H2SO4 --> MgSO4 + H2 (for thisone i said yes b/c Mg is oxidising ang H2 is reducing???)
B) 2MnO4- + 16H+ + 10I- --> 2Mn2++ 5I2 + 8H2O
Post by: Mathew587 on March 13, 2017, 06:43:50 pm
Hi can someone help me with this question.Grt help btw :/
Write overall Redox equations for the following reaction in an acidic environment and identify the oxidising and reducing agent in each case
A) MNO4^-1 (aq) + H2S (aq) -> Mn^2+ (aq) + S(s)
B) ClO^-1 (aq) + SO2 (g) -> Cl^-1(aq) + SO4^2- (aq)
Post by: jakesilove on March 13, 2017, 06:45:35 pm
Hey guys i'm having trouble in determining whether these equations are redox reactions. How can i tell?
A) Mg + H2SO4 --> MgSO4 + H2 (for thisone i said yes b/c Mg is oxidising ang H2 is reducing???)
B) 2MnO4- + 16H+ + 10I- --> 2Mn2++ 5I2 + 8H2O
Hey! For the first one you're absolutely correct! Great job :)
Let's take a closer look at the second one. Hydrogen looks like it stays exactly the same, as does Oxygen, so we can attack the equation by looking at the Magnesium and the Iodine.
What is the initial charge of Magnesium? Well,
So, the initial charge of magnesium must have been 7+. Clearly, the final charge on Magnesium is now 2+, therefore it has REDUCED (gained electrons, or decreased in oxidation number). If one species reduces, another MUST oxidise. In this case, Iodine oxidises (in this case, goes from -1 to 0).
Does that make sense? It's definitely worth being able to write out the half equations for these reactions, leaving out spectator ions.
Post by: jakesilove on March 13, 2017, 06:46:15 pm
Grt help btw :/
Sorry Mathew, must have missed that question! Will get on it right now :)
Post by: jakesilove on March 13, 2017, 06:55:40 pm
Hi can someone help me with this question.
Write overall Redox equations for the following reaction in an acidic environment and identify the oxidising and reducing agent in each case
A) MNO4^-1 (aq) + H2S (aq) -> Mn^2+ (aq) + S(s)
B) ClO^-1 (aq) + SO2 (g) -> Cl^-1(aq) + SO4^2- (aq)
Now, I'm not entirely sure what the 'overall redox equation... in an acidic environment' means. Perhaps it means that there is an excess of Hydrogen ions? Regardless, the equation is hopelessly wrong, so let's address that first.
Let's keep the reactants the same, and see what products really are
Okay, I'm almost certain that the 'acidic environment' part of the question means we should ADD Hydrogen ions to the system
That's looking a lot nicer! It's balanced, everything is happy, as expected.
Now, let's look at what is oxidising, and what is reducing. The Hydrogen ions are staying the same, as is the Oxygen atoms. Generally, it's going to be the metals/solids that oxidise/reduce anyway, so it's a fair guess that we should look to the Manganese and the Sulfer.
What is the initial charge on the Manganese? Well,
Great! So, the initial charge was +7, and it goes to +2. The oxidation number has decreased, therefore the substance has reduced! Accordingly, Sulfur has oxidised (gone from -2 to 0).
Working on the second question now :)
Okay, we have
Whilst I suppose we could balance the equation like this, I'm more comfortable making everything as stable as possible, and introducing an 'acidic environment'. As such, the equation will likely look more like this
Beautiful, balanced, awesome. Again, Hydrogen and Oxygen are unlikely to have oxidised or reduced. So, let's look at the Chlorine and the Sulfur.
Sulfur clearly starts as +4, and becomes +6. Therefore, it oxidises (increases in Oxidation number). Chlorine starts as +1, and ends as -1. Therefore, it reduces!
Great! Difficult question, and I'm sure the chemical formulas could have been completed in a number of ways, but hey this worked. Did all that make sense? Sorry for the delayed response!
Post by: Bubbly_bluey on March 13, 2017, 07:26:42 pm
Two quick questions: in the chemistry data sheet, there is a list of standard potentials, some with negative and positive voltages.
1) what does the sign mean (like chemically what is happening to determine the sign of voltage)?
2) when you work out the redox reaction (like choosing two half equations but flipping one of them) we can calculate the voltage when you add them. So with a voltaic cell, assuming we have a perfect experiment, is the reading on the volt meter suppose to match the value we worked out from the equations? Hope this makes sense.
Thank you ;D
Post by: jakesilove on March 13, 2017, 07:52:10 pm
Hello again! :)
Two quick questions: in the chemistry data sheet, there is a list of standard potentials, some with negative and positive voltages.
1) what does the sign mean (like chemically what is happening to determine the sign of voltage)?
2) when you work out the redox reaction (like choosing two half equations but flipping one of them) we can calculate the voltage when you add them. So with a voltaic cell, assuming we have a perfect experiment, is the reading on the volt meter suppose to match the value we worked out from the equations? Hope this makes sense.
Thank you ;D
Hey! Essentially, if the sign is positive, the species will reduce very readily. If the sign is negative, the species will be be difficult to reduce (and therefore oxidise very readily). If the overall reaction has a positive sign, the cell will move forward (ie. react, causing a current to flow). If the overall reaction as a negative sign, the cell will not move forward (ie. no reaction, no current).
Yep, you're absolutely right about the second bit! Assuming one mol/L concentration of solution, perfect transmission of current, no energy loss etc. that it was you would expect a voltmeter to read :)
Post by: Mathew587 on March 13, 2017, 08:36:06 pm
Now, I'm not entirely sure what the 'overall redox equation... in an acidic environment' means. Perhaps it means that there is an excess of Hydrogen ions? Regardless, the equation is hopelessly wrong, so let's address that first.
Let's keep the reactants the same, and see what products really are
Okay, I'm almost certain that the 'acidic environment' part of the question means we should ADD Hydrogen ions to the system
That's looking a lot nicer! It's balanced, everything is happy, as expected.
Now, let's look at what is oxidising, and what is reducing. The Hydrogen ions are staying the same, as is the Oxygen atoms. Generally, it's going to be the metals/solids that oxidise/reduce anyway, so it's a fair guess that we should look to the Manganese and the Sulfer.
What is the initial charge on the Manganese? Well,
Great! So, the initial charge was +7, and it goes to +2. The oxidation number has decreased, therefore the substance has reduced! Accordingly, Sulfur has oxidised (gone from -2 to 0).
Working on the second question now :)
Okay, we have
Whilst I suppose we could balance the equation like this, I'm more comfortable making everything as stable as possible, and introducing an 'acidic environment'. As such, the equation will likely look more like this
Beautiful, balanced, awesome. Again, Hydrogen and Oxygen are unlikely to have oxidised or reduced. So, let's look at the Chlorine and the Sulfur.
Sulfur clearly starts as +4, and becomes +6. Therefore, it oxidises (increases in Oxidation number). Chlorine starts as +1, and ends as -1. Therefore, it reduces!
Great! Difficult question, and I'm sure the chemical formulas could have been completed in a number of ways, but hey this worked. Did all that make sense? Sorry for the delayed response!
yup that helped. ty :)
Post by: Bubbly_bluey on March 14, 2017, 01:41:37 pm
Citiric acid, the predominant acid in lemon juice, is a triprotic acid. A student titrated 25mL samples of lemon juice with 0.55mol/L NaOH.The mean titration volum was 29.50mL. The molar mass of citric acid is 192.12g/mol. What was the concentration of the citric acid in the lemon?
I went to calulate the moles of NaOH but i dont know how to use the fact that citric acid is a triprotic acid. The answer says i have to divide by 3 but why do you have to do that?
Thanks :)
Post by: kiwiberry on March 14, 2017, 01:59:46 pm
Hi! i need a little help with this question.
Citiric acid, the predominant acid in lemon juice, is a triprotic acid. A student titrated 25mL samples of lemon juice with 0.55mol/L NaOH.The mean titration volum was 29.50mL. The molar mass of citric acid is 192.12g/mol. What was the concentration of the citric acid in the lemon?
I went to calulate the moles of NaOH but i dont know how to use the fact that citric acid is a triprotic acid. The answer says i have to divide by 3 but why do you have to do that?
Thanks :)
Because citric acid is triprotic, it is able to donate 3 H+ in solution. So for every mole of citric acid, 3 moles of OH- are needed to neutralise the H+. Therefore 3 moles of NaOH are needed to completely neutralise 1 mole of citric acid, meaning that you have to divide n(NaOH) by 3 to get n(citric acid) :)
Post by: J.B on March 14, 2017, 02:22:30 pm
Thanks
(http://uploads.tapatalk-cdn.com/20170313/f1bccd9632d0606a87d0cc12d3272fd0.jpg)
Post by: 1tankengine on March 14, 2017, 05:11:18 pm
A student was investigating the heat of combustion of ethanol. She used an ethanol
burner that had an initial mass of 68.0 g. She then lit the burner and placed it under a
beaker containing 500 g of water. After a few minutes, she noticed that the water
temperature had risen from 24°C to 38°C and the burner now weighed 66.5 g.
She made the assumption that only the water was heated.
What would be the student’s value for the heat of combustion for ethanol?
It is from a past half yearly paper at my school and it says the answer is 859 kJ mol–1 but I'm getting a different answer.
Post by: J.B on March 14, 2017, 05:23:01 pm
I know it sounds stupid but I can't get this question right-
A student was investigating the heat of combustion of ethanol. She used an ethanol
burner that had an initial mass of 68.0 g. She then lit the burner and placed it under a
beaker containing 500 g of water. After a few minutes, she noticed that the water
temperature had risen from 24°C to 38°C and the burner now weighed 66.5 g.
She made the assumption that only the water was heated.
What would be the student’s value for the heat of combustion for ethanol?
It is from a past half yearly paper at my school and it says the answer is 859 kJ mol–1 but I'm getting a different answer.
I received 897KJ mol-1
But I could be wrong.
Post by: kiwiberry on March 14, 2017, 05:23:30 pm
I was wondering if someone would be able to explain this question (12) to me?
Thanks
(http://uploads.tapatalk-cdn.com/20170313/f1bccd9632d0606a87d0cc12d3272fd0.jpg)
I think it's A! All Bronsted-Lowry acids must contain a H+ to donate. When this proton is donated, the acid's conjugate base is formed, which will have an overall negative charge since a positive charge (H+) was removed from it. eg. HCl/Cl-, HNO3/NO3- etc.
Post by: 1tankengine on March 14, 2017, 05:30:32 pm
I received 897KJ mol-1
But I could be wrong.
That's closer than I got.. Would you be able to tell me how you worked it out?
Post by: Ellie__ on March 14, 2017, 05:32:05 pm
Hey! Check out this formula sheet that I made a while ago. In my opinion, it has everything you need to memorise! Let me know if you have any questions :)
THANKYOUUU!!
Post by: kiwiberry on March 14, 2017, 05:40:38 pm
I know it sounds stupid but I can't get this question right-
A student was investigating the heat of combustion of ethanol. She used an ethanol
burner that had an initial mass of 68.0 g. She then lit the burner and placed it under a
beaker containing 500 g of water. After a few minutes, she noticed that the water
temperature had risen from 24°C to 38°C and the burner now weighed 66.5 g.
She made the assumption that only the water was heated.
What would be the student’s value for the heat of combustion for ethanol?
It is from a past half yearly paper at my school and it says the answer is 859 kJ mol–1 but I'm getting a different answer.
I got 899 kJ/mol haha this is weird
ΔH = mcΔt = (500)(4.18)(38-24)= 29260 J
m(C2H5OH) = 68 - 66.5 = 1.5g
n(C2H5OH) = 1.5/[2(12.01) + 6(1.008) + 16]= 0.0325... mol
ΔH per mol = 29260/0.0325... = 898633.12 = 899 kJ/mol
Post by: 1tankengine on March 14, 2017, 05:55:13 pm
I got 899 kJ/mol haha this is weird
Oh I see where I went wrong, I wrote 68-66.5=2.5 ...
But I got 897 as well. It was a multiple choice question so maybe the working out was done wrong.
Anyway thanks for your help!
Post by: dux99.95 on March 14, 2017, 06:01:29 pm
My question is, if you had to quickly study for Production of Materials and The Acidic Environment, how would you do it?
Like, whats most important in each e.g focus on pracs more in POM, focus on calculations more in acidic environment (these are just examples, idk if you should actually do that).
Obviously I should study EVERYTHING but half yearlies are soon - so what should I do?
Post by: Aaron12038488 on March 14, 2017, 06:58:23 pm
Post by: jakesilove on March 15, 2017, 09:17:07 am
Hey
My question is, if you had to quickly study for Production of Materials and The Acidic Environment, how would you do it?
Like, whats most important in each e.g focus on pracs more in POM, focus on calculations more in acidic environment (these are just examples, idk if you should actually do that).
Obviously I should study EVERYTHING but half yearlies are soon - so what should I do?
Hey! Unfortunately, you're going to have to focus on EVERYTHING in both modules. However, you can get a good idea of what the HSC assesses more regularly by doing HSC past papers! So, the best method of study from here on in is doing past papers, and this will get you prepared for the actual exam. Do any past papers you can get your hands on (HSCs, Trials, whatever) and do the paper in timed conditions. Refer to your notes if absolutely necessary, but try to avoid this. Make sure to mark your work, and write out any mistakes that you make in a separate book. Then, revise those mistakes to try and avoid making them again.
Jake
Post by: jakesilove on March 15, 2017, 09:19:07 am
So i have a chemistry practical tommorow, any tips. The week after I have to hand in my prac report. Also what parts do I need for a prac report.
Hey! Clearly a bit late to help you in the prac today; let's talk about the prac report.
In the report, I would recommend you have
- Aim
- Hypothesis
- Materials/Equipment
- Method
- Results
- Discussion
- Conclusion
By providing all of the above information, you'll have gone into sufficient depth (and sufficiently shown your scientific prowess) to get a really good mark! Whilst we can't do an assignment for you, let us know if you're really struggling with anything or not sure how to interpret some of your data, and we'd be happy to help you out :)
Jake
Post by: J.B on March 15, 2017, 12:59:07 pm
I think it's A! All Bronsted-Lowry acids must contain a H+ to donate. When this proton is donated, the acid's conjugate base is formed, which will have an overall negative charge since a positive charge (H+) was removed from it. eg. HCl/Cl-, HNO3/NO3- etc.
Yeah that's what I thought as well! , but the answer said it was D?
Post by: RuiAce on March 15, 2017, 01:05:34 pm
I think it's A! All Bronsted-Lowry acids must contain a H+ to donate. When this proton is donated, the acid's conjugate base is formed, which will have an overall negative charge since a positive charge (H+) was removed from it. eg. HCl/Cl-, HNO3/NO3- etc.
Yeah that's what I thought as well! , but the answer said it was D?It's not A.
NH4+ is a B-L acid but clearly its conjugate base NH3 is neutral, not negatively charged. Donating a proton does NOT necessarily imply that it will become negatively charged. Hence A is wrong.
B is obviously wrong (consider NO3-) and the above example also contradicts C. I haven't figured out why D is correct yet but I may come back to it later.
Post by: Aaron12038488 on March 15, 2017, 04:59:49 pm
Post by: bholenath125 on March 16, 2017, 12:34:45 am
Post by: jakesilove on March 16, 2017, 09:17:27 am
what does it mean by priniciple in a scientific report? Also for the method, can I say like what the compound looked like before heating? Or should I put this in my results section?
Not entirely sure what a 'principle' of a scientific report is; maybe formal manner? Succinct? Scientific language? Including the right subsections and analysis?
Observations should be put in the results section, not the method. The method should just be numbered, past tense, exactly what you did to carry out the experiment!
Post by: jakesilove on March 16, 2017, 09:21:59 am
I have a lot of trouble understanding the structural formation of condensation polymers. Anyone know a site which explains this concept well?
Which part of condensation polymers are you confused about? I've pasted an example below
(https://www.materialsworldmodules.org/resources/polimarization/amidelinkeqn.gif)
You can think of condensation polymers as like two molecules, being smashed together, but leaving out some small parts from each molecule. In the above example, the OH is 'removed' from the acid, and H is 'removed' from the Amine (whatever that is!). The final molecule is just the connection of the two molecules, with the OH+H=H2O. Condensation polymers are just polymers that, when formed, have a small molecule removed :)
Post by: parthie on March 16, 2017, 02:35:44 pm
Thanks
Post by: jakesilove on March 16, 2017, 03:08:54 pm
Hey can someone please try and explain what buffers are to me...I am really confused?
Thanks
Hey! A buffer is any solution that resists small changes in pH. That's the definition, and always the first thing to say in any answer relating to buffers. However, the chemical properties of a buffer are a bit more complicated!
Buffers operate using Le Chatelier's principle. Le Chatelier's principle states that when an equilibrium is introduced to a change, the equilibrium shifts to minimise that change. We've just said that a buffer resists small changes in pH; the only way it could do this is by shifting AWAY from an acidic solution when an acid is added, and TOWARDS an acidic solution when a base is added (to neutralise the added acid).
Generally, a buffer is made up of a weak acid and it's conjugate base. You don't need to understand why; it just helps if you get a multiple choice question asking you to identify a buffer. Look for a conjugate acid/base pair, and then discard any strong Acids.
What does a buffer look like? Well, the most important buffer is the one in our blood.
So, what does this look like? Well, when the blood becomes too acidic, the equilibrium will shift to the LEFT (ie. away from the Hydrogen ion). This will decrease the acidity of the solution (ie. resists small changes in pH!). WHen the buffer becomes too basic, the equilibrium will shift to the RIGHT (ie. towards the Hydrogen ion). This will increase acidity of the solution (ie. again, resisting small changes in pH!). This buffer stops our blood from becoming too acidic, killing us from the inside.
Hope that all makes sense!
Post by: bananna on March 17, 2017, 11:51:20 am
how do you work out the concentration of an acid given the pH of a substance?
for example,
A solution of HNO3 has a pH of 2.05. Calculate the concentration of the acid.
ALSO, (sorry)
how do calculate the pH of 0.055M Ba(OH)2?
do you assume the 2xOH molecules are included in the molar concentration? (if that makes sense)
thank you!
Post by: jakesilove on March 17, 2017, 11:55:49 am
hi!
how do you work out the concentration of an acid given the pH of a substance?
for example,
A solution of HNO3 has a pH of 2.05. Calculate the concentration of the acid.
thank you!
Hey! We have a nice little formula for this, on your Chemistry formula sheet. It says,
Remember that, if
then
So, we can rearrange the above given equation to read
ie. 10 to the power of the negative pH will give you the concentration! In this case,
Post by: bananna on March 17, 2017, 12:02:53 pm
Hey! We have a nice little formula for this, on your Chemistry formula sheet. It says,
Remember that, if
then
So, we can rearrange the above given equation to read
ie. 10 to the power of the negative pH will give you the concentration! In this case,
thank you!
forgot about that formula
also,
how do calculate the pH of 0.055M Ba(OH)2?
do you assume the 2xOH molecules are included in the molar concentration? (if that makes sense)
thank you!
Post by: jakesilove on March 17, 2017, 03:24:38 pm
thank you!
forgot about that formula
also,
how do calculate the pH of 0.055M Ba(OH)2?
do you assume the 2xOH molecules are included in the molar concentration? (if that makes sense)
thank you!
That's exactly what you do! So,
As the concentration of OH will be twice the concentration of the base.
Post by: Kekemato_BAP on March 17, 2017, 03:56:21 pm
Post by: ellipse on March 17, 2017, 05:11:05 pm
Students measured the mass change in a flask containing glucose solution and yeast. The initial mass of the
flask and contents was 428.1 g and the mass of the flask and contents after a week was 390.1 g.
What is the mass of alkanol produced?
Post by: jakesilove on March 17, 2017, 05:16:12 pm
Hi, are there any previous guides on how to properly draw Lewis dot diagrams? I always lose a mark for non-proper scientific drawing of it, thanks :)
Hey! This site here has heaps of examples! Let me know if there is a specific question that you're struggling with.
Post by: jakesilove on March 17, 2017, 05:21:09 pm
Need help on this questions please
Students measured the mass change in a flask containing glucose solution and yeast. The initial mass of the
flask and contents was 428.1 g and the mass of the flask and contents after a week was 390.1 g.
What is the mass of alkanol produced?
Hey! First, let's look at the chemical formula
So, where would mass be lost? In the Carbon dioxide gas! We can assume that all mass change (ie. 38g) was a result of lost Carbon dioxide. So,
Well, if there are 0.863 moles of Carbon dioxide, there will also be 0.863 moles of the Alkanol (Ethanol). Therefore
Post by: ellipse on March 17, 2017, 05:25:01 pm
Hey! First, let's look at the chemical formula
So, where would mass be lost? In the Carbon dioxide gas! We can assume that all mass change (ie. 38g) was a result of lost Carbon dioxide. So,
Well, if there are 0.863 moles of Carbon dioxide, there will also be 0.863 moles of the Alkanol (Ethanol). Therefore
Hey Jake! I got the same mass as well, but the answer says that it is 19.9g. So is 19.9g wrong?
Post by: Kekemato_BAP on March 17, 2017, 05:46:10 pm
Hey Jake! I got the same mass as well, but the answer says that it is 19.9g. So is 19.9g wrong?
Yeah, it should be the 39.7g. The 19.9 value is half of the 39.7 value so the textbook may have had the wrong stoichiometry ratio
Post by: Kekemato_BAP on March 17, 2017, 06:09:54 pm
Eg. 20mL of 1M citric acid + 25mL of 0.5M sodium hydroxide? How do you calculate H+ concentration of a weak acid when it doesn't ionise fully? Thanks
Post by: Kle123 on March 17, 2017, 07:17:03 pm
These are thoughts that pop up when i learned titration. Teacher never explained it. Could someone please help me? THANKS
Post by: kiwiberry on March 17, 2017, 08:12:30 pm
Post by: bsdfjnlkasn on March 17, 2017, 08:23:41 pm
After you distill an ester, how would you confirm its purity?
I'm guessing you would weigh out the ester and continue to fractionally distill your sample until the weight plateaus - this would indicate that all impurities have filtered out. Mind you, I am just applying general theory about obtaining pure samples but since esters are liquids it maybe a bit different when discussing impurities and how to get rid of them.
Post by: Kekemato_BAP on March 17, 2017, 08:39:49 pm
Post by: Bubbly_bluey on March 17, 2017, 09:15:36 pm
A student predicts 100mL of 10mol/L NaOH mixed with 100mL H2SO4 of 10mol/L with release about 114kJ. Is this prediction true?
Thank you :D
Post by: Kekemato_BAP on March 17, 2017, 09:22:41 pm
Hey! I have a question:
A student predicts 100mL of 10mol/L NaOH mixed with 100mL H2SO4 of 10mol/L with release about 114kJ. Is this prediction true?
Thank you :D
Does it give you the enthalpy heat value?
Post by: Bubbly_bluey on March 17, 2017, 09:28:45 pm
Does it give you the enthalpy heat value?Is enthalpy value same as heat of neutralisation? because the previous question has a reaction between a strong acid and base to have 57kJ/mol of heat neutralisation.
Post by: Bubbly_bluey on March 17, 2017, 09:49:58 pm
Can someone explain the uses of Ka, Kb, Kw, etc? What are they used for? Thanks
Hey!
I think Ka is used to show the strength and weakness of acids. So stronger acids have a higher value of Ka because they completely dissociate.
The formula: Ka= [H+][A-] / [HA] shows the dissociation. When the [H+] is bigger = strong acid
Kb is for bases and Kw is for water = 1x10-14.
The formula: ka x kb = kw shows that if we know the value Ka of an acid we can work out its conjugate base's Kb.
hope this makes sense :) i could be wrong but i haven't seen a lot of questions in HSC relating to these :P
Post by: Shadowxo on March 17, 2017, 11:06:56 pm
Hey! I have a question:
A student predicts 100mL of 10mol/L NaOH mixed with 100mL H2SO4 of 10mol/L with release about 114kJ. Is this prediction true?
Thank you :D
The heat of neutralisation shows how much heat/energy is released during this reaction, so is therefore equal to the enthalpy / ∆H.
First find the number of each (n=cV), write out the equation, then figure out how many mols of the equation occur (should be equal to the number of mols of NaOH/H2SO4). E=∆H*n = 57*n kJ.
Would you need to know how to calculate pH of a non-neutral salt?
Eg. 20mL of 1M citric acid + 25mL of 0.5M sodium hydroxide? How do you calculate H+ concentration of a weak acid when it doesn't ionise fully? Thanks
A weak acid and a weak base (and weak acid and strong base) react completely with each other, even though they don't with water. So figure out the number of each , figure out which is in excess then figure out the concentration of the excess reactant, and therefore the concentration of H3O+ / OH-, taking into account how much it ionises.
Post by: parthie on March 18, 2017, 03:57:23 pm
Im just confused what i am meant to write like how to phrase it
Thankyou
Post by: f_tan on March 18, 2017, 05:00:21 pm
How do I work out the answer for this question?
Thanks!
Post by: jakesilove on March 18, 2017, 06:59:18 pm
Hey If anyone can help with the explanation in part b) of the question attached that would be great
Im just confused what i am meant to write like how to phrase it
Thankyou
Hey! Definitely a confusing statement; I would just figure out some equations for the reaction between carbonate and water, and see what we get!
Easy! Turns out, when the salt is added to a solution it produces Hydroxide ions. Hydroxide ions are what makes a solution basic, and thus the pH will be greater than 7. In this particular case, the pH is 8, but that isn't something we can really comment on except to say that it is a very weak base :)
Post by: jakesilove on March 18, 2017, 07:07:45 pm
(http://i.imgur.com/eBWEALt.png)
How do I work out the answer for this question?
Thanks!
I've always really liked this question; at first, it seems really difficult, but once you figure out the approach it isn't too bad!
Let's start off by calculating the number of moles of Charcoal.
Okay, great. We know that equal amounts of charcoal are used in each reaction. So, 16.65 moles of Charcoal must be consumed in each reaction! The first reaction 1:1 for C:CO. Thus, 16.65 moles of CO are produced as well. We can quickly find the mass of gas.
Okay. The second reaction is 1:1 C:CO2. Thus, 16.65 moles of CO2 are produced as well. We can quickly find the mass of the gas.
In total, this gives us a mass of 1.2kg, so the answer is B :)
Post by: f_tan on March 19, 2017, 01:09:32 am
In total, this gives us a mass of 1.2kg, so the answer is B :)
Thank you!
Also, could you explain why the graph should have two lines of best fit?
(http://i.imgur.com/VVWSSdC.png)
(http://i.imgur.com/UjlnkVf.png)
Post by: Butterflygirl on March 19, 2017, 07:50:17 am
Why is it better to use a highly conductive aqueous gel containing KNO3 for a salt bridge compared to a strip of filter paper dipped in KNO3 solution?
What effect would it have on measurements taken from a galvanic cell?
I don't understand how conductivity works and how it relates to the salt bridge.
Also,
If copper wire is used as a salt bridge, the voltage generated is higher but how does this happen? because doesn't the copper only contain cations (Cu2+)? How does it balance the charges in the half-cells and increase the voltage compared to using KNO3 solution or something?
Thankyou so much :)
Post by: jakesilove on March 19, 2017, 11:30:19 am
Thank you!
Also, could you explain why the graph should have two lines of best fit?
(http://i.imgur.com/VVWSSdC.png)
(http://i.imgur.com/UjlnkVf.png)
Honestly, I have absolutely no idea! I'm sure you can get full marks using one line of best fit. Potentially, they used two different scales on their graph (to take into account the quite large range over which Pressure was calculated). Still, there is no way I would have done that, and you would definitely get full marks by drawing one line of best fit.
Post by: jakesilove on March 19, 2017, 11:33:08 am
I really need help with these questions because my SAC is tomorrow :(
Why is it better to use a highly conductive aqueous gel containing KNO3 for a salt bridge compared to a strip of filter paper dipped in KNO3 solution?
What effect would it have on measurements taken from a galvanic cell?
I don't understand how conductivity works and how it relates to the salt bridge.
Also,
If copper wire is used as a salt bridge, the voltage generated is higher but how does this happen? because doesn't the copper only contain cations (Cu2+)? How does it balance the charges in the half-cells and increase the voltage compared to using KNO3 solution or something?
Thankyou so much :)
Hey! I'm in NSW, so I didn't do the same curriculum, and can only really guess at answers. Using a 'highly conductive' strip will be better for simply that reason; it is highly conductive. When ions are transferred along the salt bridge, energy will not be lost as easily because the ions are free to flow. Thus, current will not be impeded as much (reducing total resistance), increasing the efficiency of the circuit. The measurement will likely read a higher voltage.
Think of a galvanic cell as a regular circuit. The only important thing is the conduction of ELECTRONS; in a salt bridge, that comes in the form of ions, however the electrons can be transmitted in any way. Thus, using a copper wire works well because it is very, very conductive! I can't imagine you need to be able to answer these questions in any more depth than that :)
Jake
Post by: Aaron12038488 on March 19, 2017, 02:23:31 pm
Post by: jakesilove on March 19, 2017, 03:01:56 pm
So I'm having trouble with the validity of thermal decomposition of Copper Carbonate. I wrote that as the experiment was qualitatively instead of quantitatively, that validity was needed for the exp.
Hey! Is this for one of the options? Not sure if I've heard of the decomposition of Copper carbonate as part of the course?
Post by: Kekemato_BAP on March 19, 2017, 03:58:17 pm
Post by: jakesilove on March 19, 2017, 04:58:04 pm
Hi, how do i calculate the percentage by mass of a mixture? And also finding the empirical formulas? I'm having trouble understanding this concept
Could you please provide a specific question? That way, we can figure out what you're struggling with.
Post by: Kekemato_BAP on March 19, 2017, 05:05:20 pm
Could you please provide a specific question? That way, we can figure out what you're struggling with.
What is the percentage by mass of oxygen (O) in sodium hydroxide (NaOH)?
Suppose 3.2g of sulfur reacts with oxygen to produce 6.4g of sulfur oxide. What is the formula of the oxide?
Thank you so much :)
Post by: jakesilove on March 19, 2017, 05:24:36 pm
What is the percentage by mass of oxygen (O) in sodium hydroxide (NaOH)?
Suppose 3.2g of sulfur reacts with oxygen to produce 6.4g of sulfur oxide. What is the formula of the oxide?
Thank you so much :)
Hey! To find the percentage mass of Oxygen in Sodium hydroxide, we just find the mass of Oxygen, and divide by the total mass of Sodium hydroxide. So,
Now, the next one is a bit tougher. We have some reaction,
Where A and x are some constants. Now, the molar ratio is 1:1, so there are
of the Sulfur oxide. If 0.1 moles of sulfur oxide weighs 6.4 grams (in the question), then the molar mass of the sulfur oxide must be 64g!
So, the molar mass of sulfur, plus the molar mass of how ever many oxygen atoms we have, must be 64.
Clearly, x=2, and so the oxide is SO2(g)
Post by: Kekemato_BAP on March 19, 2017, 06:18:31 pm
Hey! To find the percentage mass of Oxygen in Sodium hydroxide, we just find the mass of Oxygen, and divide by the total mass of Sodium hydroxide. So,
Now, the next one is a bit tougher. We have some reaction,
Where A and x are some constants. Now, the molar ratio is 1:1, so there are
of the Sulfur oxide. If 0.1 moles of sulfur oxide weighs 6.4 grams (in the question), then the molar mass of the sulfur oxide must be 64g!
So, the molar mass of sulfur, plus the molar mass of how ever many oxygen atoms we have, must be 64.
Clearly, x=2, and so the oxide is SO2(g)
Thank you :) that helps a lot
Post by: Kle123 on March 20, 2017, 03:51:24 pm
Why do we need a secondary solution? Is its concentration supposed to be more accurate than the primary solution? How can measure something to be more accurate than the solution it was used to create it?
These are thoughts that pop up when i learned titration. Teacher never explained it. Could someone please help me? THANKS
My question kinda got lost in the vast ocean of posts. I'm just going to re submit it. If anyone can help it'd be much appreciated thank you!
Post by: bsdfjnlkasn on March 20, 2017, 04:05:01 pm
I was wondering if anyone could help me out with clarifying some things about Le Chatellier's principe - I wrote these notes but am unsure of how true they are
o Pressure refers to the pressure acting on the entire system
• Adding more of one thing just increases its concentration (if aqueous)
- This then increases the systems volume and actually decreases pressure
• If pressure is increased, the concentration of all the products and reactants increases also
EDIT:
Why do dilute acids have higher pH readings (I know it's kind of intuitive but could I get an explanation through Le Chatelier's principle)
THANK YOU!
Post by: jakesilove on March 21, 2017, 09:14:18 am
My question kinda got lost in the vast ocean of posts. I'm just going to re submit it. If anyone can help it'd be much appreciated thank you!
Hey! I have to say, I never actually learned about the 'secondary' solution. I've got questions about it before, but the fact that I don't know anything about it means that you don't need to know it for the HSC. However, if your teacher would expect you to use it in a method, then you probably should. Maybe just clarify with the rest of the class (or maybe someone on the forum can help!). Unfortunately, I'm not your man here :(
Post by: jakesilove on March 21, 2017, 09:20:22 am
Hey there,
I was wondering if anyone could help me out with clarifying some things about Le Chatellier's principe - I wrote these notes but am unsure of how true they are
o Pressure refers to the pressure acting on the entire system
• Adding more of one thing just increases its concentration (if aqueous)
- This then increases the systems volume and actually decreases pressure
• If pressure is increased, the concentration of all the products and reactants increases also
EDIT:
Why do dilute acids have higher pH readings (I know it's kind of intuitive but could I get an explanation through Le Chatelier's principle)
THANK YOU!
Hey! Just a quick summary of Le Chatelier's Principle, which states that a system in equilibrium, when introduced to a change, will shift in equilibrium to minimise that change.
Concentration
If an equation is in equilibrium, and you add more of the reactants, then the system will want to 'use up' the reactants. Thus, the system will move towards the products.
If an equation is in equilibrium, and you add more products, then the system will want to 'use up' the products. Thus, the system will move towards the products.
Temperature
If the process is exothermic, then it RELEASES heat when it moves forward. Thus, if you increase the temperature, moving forward will only increase the temperature more. So, the system will move BACKWARDS (ie towards the reactants), to 'use up' the heat.
If the process is endothermic, then it ABSORBS heat when it moves forward. Thus, if you increase the temperature, moving forward will 'use up' the excess heat. So, the system will move FORWARD (ie towards the products), to 'use up' the heat.
Pressure
If a system of GASES are at equilibrium, and you increase the pressure, then the system will move towards the side with fewer moles of gas. If a system of gases are at equilibrium, and you decrease the pressure, then the system will move towards the side with greater moles of gas.
With regards to your Acid question; when you dilute an acid, then by Le Chatelier's principles the equilibrium will shift towards the side with Hydrogen ions (ie. more of the acid will ionise, to increase the concentration of ions). However, despite the increase in ion numbers, the concentration will actually DECREASE overall as you have increased the volume of the solution. ie. you may have gone from 100 ions to 1000 ions, but the solution has gone from 10mL to 1000mL, and so the concentration of ions has actually decreased. Thus, the pH will increase, as it will be LESS acidic.
Post by: kiwiberry on March 21, 2017, 09:50:01 am
My question kinda got lost in the vast ocean of posts. I'm just going to re submit it. If anyone can help it'd be much appreciated thank you!
Is this about how you have to standardise NaOH for example by titrating it against oxalic acid first before you can use it in a titration as a secondary standard? I believe it's because NaOH reacts with CO2 in the air, which neutralises part of the solution and so affects its concentration. NaOH is also hydroscopic (it absorbs water from the atmosphere) so it's very hard to measure out solid NaOH accurately. Basically it's an unsuitable primary standard solution because it's concentration is always changing, so you have to titrate it first with a suitable primary standard (like oxalic acid) to find its actual concentration at the time :)
Post by: bsdfjnlkasn on March 23, 2017, 04:46:30 pm
I have a few more questions :)
Why do alkenes have lower boiling points than alkanes?
Wouldn't a double bond be more stable as it's stronger than any of the others?
Or should we be looking at the bonding they exhibit? There are more dispersion forces associated with alkanes because they have 2 extra hydrogens than the corresponding alkenes - right? Realistically, the actual single bonds themselves are weaker but because there are so many of them this factor isn't really considered as it does little in determining the BP. When talking about boiling points we're discussing the energy required to break intermolecular bonds right? Because that's the only way I can understand us ignoring the intramolecular double bond...
What is it about the double bond that makes it so reactive and susceptible to addition reactions? One more general question - do we need to know all of the reactions associated with alkanes and alkenes?
Could someone please clarify if i've justified the differences in boiling points appropriately/address the associated queries? I know it's a lot so any help would be appreciated, thank you again!!
EDIT: Is an alkyl what replaces the hydrogen in an alkane or is it the name for the entire new molecule formed?
Is Ethylbenzene and phenylethene the same thing (i.e. styrene)?
Post by: Kle123 on March 23, 2017, 06:26:21 pm
How is it shown that NaOH + H20 is a B-L base?
Thank YOU.
Post by: jakesilove on March 23, 2017, 06:53:02 pm
Hey Jake - THANK YOU SO MUCH for that awesome response.
I have a few more questions :)
Why do alkenes have lower boiling points than alkanes?
Wouldn't a double bond be more stable as it's stronger than any of the others?
Or should we be looking at the bonding they exhibit? There are more dispersion forces associated with alkanes because they have 2 extra hydrogens than the corresponding alkenes - right? Realistically, the actual single bonds themselves are weaker but because there are so many of them this factor isn't really considered as it does little in determining the BP. When talking about boiling points we're discussing the energy required to break intermolecular bonds right? Because that's the only way I can understand us ignoring the intramolecular double bond...
What is it about the double bond that makes it so reactive and susceptible to addition reactions? One more general question - do we need to know all of the reactions associated with alkanes and alkenes?
Could someone please clarify if i've justified the differences in boiling points appropriately/address the associated queries? I know it's a lot so any help would be appreciated, thank you again!!
EDIT: Is an alkyl what replaces the hydrogen in an alkane or is it the name for the entire new molecule formed?
Is Ethylbenzene and phenylethene the same thing (i.e. styrene)?
Alkenes and Alkanes actually have very similar boiling points. Remember, boiling point is to do with INTERmolecular bonds, not INTRAmolecular bonds. So, the double bond etc plays no role; you have to look at the molecule as a whole. You're absolutely right; this tends to come down to dispersion forces, which increase for bigger molecules.
A double bond ISN'T more stable; it would much prefer to be two single bonds. You don't need to know specific reactions, but you do need to understand what addition/condensation reactions are, and be able to show the 'splitting' of a double bonded alkene.
You've definitely justified it! Make your point a bit clearer (ie. 'The difference in boiling point is associated with the different in intermolecular dispersion forces' etc) and you'll be golden :)
Not sure about the Alkyl; I would just ignore that if I were you. Yes, they are the same
Post by: jakesilove on March 23, 2017, 06:56:18 pm
It can be shown that HCl is a B-L acid: HCl+H2O -> Cl^- +H3O^+
How is it shown that NaOH + H20 is a B-L base?
Thank YOU.
Hey! We just show it causing water to donate a Hydrogen ion; the easiest way is like this
The first Hydroxide ion has PULLED a hydrogen ion off the water, forming water itself. The important thing is that a component of the base acted as a proton acceptor; NaOH is a hard example, though, and one which I tend to avoid.
Post by: bsdfjnlkasn on March 23, 2017, 07:56:25 pm
I was just wondering if I could get an indication of how common the following topics are in exams:
1. The different types of cracking and their purposes (catalytic and thermal)
2. The LDPE process
3. Explanations of what the actual properties of polymers (i.e. chain branching and stiffening)
Also just a general question:
What additional information apart from crude oil/petroleum can we include in the following outcome: "1.1 Identify the industrial source of ethylene from the cracking of some of the fractions from the refining of petroleum:"
Post by: RuiAce on March 23, 2017, 08:11:09 pm
Thanks again Jake for your comprehensive response!Not really that often for the first one. It's nice to know both exist but it suffices to know just one of them, and you can include a description if you wish. Haven't seen them get examined explicitly because I think the description is not necessary for the course.
I was just wondering if I could get an indication of how common the following topics are in exams:
1. The different types of cracking and their purposes (catalytic and thermal)
2. The LDPE process
3. Explanations of what the actual properties of polymers (i.e. chain branching and stiffening)
Also just a general question:
What additional information apart from crude oil/petroleum can we include in the following outcome: "1.1 Identify the industrial source of ethylene from the cracking of some of the fractions from the refining of petroleum:"
The second one was actually in the HSC exam of my year. That's worth considering. That being said, don't recall if it appeared before that.
With chain-branching vs stiffening the important thing is you know roughly what they look like first. LDPE vs HDPE is a pretty good example. That being said, you should know a few properties (e.g. flexible, transparent v.s. stiff, transluscent).
This has a somewhat high tendency of appearing in half yearly exams, but only an average tendency in HSC exams.
I'll leave Jake for that general question. I didn't really study that dot point in depth; I just knew that it was true and went with it.
Post by: jakesilove on March 23, 2017, 08:14:22 pm
I'll leave Jake for that general question. I didn't really study that dot point in depth; I just knew that it was true and went with it.
You're absolutely right here; there's nothing really else you can know. Maybe identify a specific fraction of petroleum? It's really not that important.
Post by: Annie657 on March 23, 2017, 10:01:00 pm
Is anyone out there doing chem of art? I'm doing it next term and everyone says it's pretty hard, my teacher says it's not popular, but is it really that bad?
Post by: jakesilove on March 24, 2017, 09:12:07 am
Hi,
Is anyone out there doing chem of art? I'm doing it next term and everyone says it's pretty hard, my teacher says it's not popular, but is it really that bad?
Hey! It's definitely not a popular course, however I think I've done a lot of the content at University! So, hopefully, I can help you out as you move through the option :)
Post by: Annie657 on March 24, 2017, 03:46:46 pm
Post by: Annie657 on March 24, 2017, 07:06:28 pm
In the alkenes and alkanes experiment with bromine water, does it matter if you start with bromine water first in your test tube or should you start with the hydrocarbon and add it? Thankyou :)
Post by: lzxnl on March 25, 2017, 12:28:47 am
Ok so I have another question,
In the alkenes and alkanes experiment with bromine water, does it matter if you start with bromine water first in your test tube or should you start with the hydrocarbon and add it? Thankyou :)
You should start off with your coloured bromine, as the discolouring will be much easier to see if you start with that.
Post by: Ellie__ on March 25, 2017, 08:44:21 am
I'm really unsure on picking out what a titration calculation question is actually asking for, and what I have to do to get there, do you have any suggestions on like a possible general formula type thing that I can use in all titration calculations
Thankyou!
Sorry if this doesn't make sense!!
Post by: Wren on March 25, 2017, 11:57:43 am
Hey guys,
I'm really unsure on picking out what a titration calculation question is actually asking for, and what I have to do to get there, do you have any suggestions on like a possible general formula type thing that I can use in all titration calculations
Thankyou!
Sorry if this doesn't make sense!!
Hey,
I was having the same problem initially and I don't know if you already do this but what helped me was clearly listing out all the information I was already given example. n(C2H6O)= 0.01mol etc.... and then list the formulas that I might need in titration like c=n/V. It's a bit time consuming at first but once you get used to answering different types of questions you won't need it as much. Hope it helps.
Post by: Ellie__ on March 25, 2017, 07:17:13 pm
Hey,
I was having the same problem initially and I don't know if you already do this but what helped me was clearly listing out all the information I was already given example. n(C2H6O)= 0.01mol etc.... and then list the formulas that I might need in titration like c=n/V. It's a bit time consuming at first but once you get used to answering different types of questions you won't need it as much. Hope it helps.
Ah ok that makes sense!! Thankyou
Post by: bsdfjnlkasn on March 26, 2017, 09:26:48 am
A student measured out 100 mL of 0.01 HCl and labels it A
25mL of A was poured into a beaker. 75mL of distilled water was added until 100mL was made up. They called this solution B
Compare the pH of the 2 solutions.
I understand we have to calculate the moles of HCl but I don't know what to do with the next step where the dilution occurs. The concentration changes and the moles do too so I don't know which formula to apply or if there is a handy shortcut that works with 75mL being 3/4 of the total 100mL - does that make sense? I want to kind of get to know the shortcuts so I don't waste time applying formulas when I don't need to.
Thank you so much for your help!
EDIT: another question :)
The heat of combustion of ethanol is 1346 kJ/mol. What mass of ethanol would be required to produce 250 kJ of energy upon its complete combustion?
Post by: ag5946 on March 26, 2017, 11:03:53 am
Could someone please help me out with the following:
A student measured out 100 mL of 0.01 HCl and labels it A
25mL of A was poured into a beaker. 75mL of distilled water was added until 100mL was made up. They called this solution B
Compare the pH of the 2 solutions.
I understand we have to calculate the moles of HCl but I don't know what to do with the next step where the dilution occurs. The concentration changes and the moles do too so I don't know which formula to apply or if there is a handy shortcut that works with 75mL being 3/4 of the total 100mL - does that make sense? I want to kind of get to know the shortcuts so I don't waste time applying formulas when I don't need to.
Thank you so much for your help!
EDIT: another question :)
The heat of combustion of ethanol is 1346 kJ/mol. What mass of ethanol would be required to produce 250 kJ of energy upon its complete combustion?
Dont you just use C1V1=C2V2, calculate the C2 and then calculate pH of the resulting solution and compare it to the original pH of 2
Post by: jakesilove on March 26, 2017, 11:20:13 am
Could someone please help me out with the following:
A student measured out 100 mL of 0.01 HCl and labels it A
25mL of A was poured into a beaker. 75mL of distilled water was added until 100mL was made up. They called this solution B
Compare the pH of the 2 solutions.
I understand we have to calculate the moles of HCl but I don't know what to do with the next step where the dilution occurs. The concentration changes and the moles do too so I don't know which formula to apply or if there is a handy shortcut that works with 75mL being 3/4 of the total 100mL - does that make sense? I want to kind of get to know the shortcuts so I don't waste time applying formulas when I don't need to.
Thank you so much for your help!
EDIT: another question :)
The heat of combustion of ethanol is 1346 kJ/mol. What mass of ethanol would be required to produce 250 kJ of energy upon its complete combustion?
Yep, ag is exactly right! Could can quickly calculate the initial pH (ie. -log(0.01). Then, we use C1V2=C2V2 to calculate the final concentration, and sub that straight into our pH formula again!
Now, the heat of combustion question is always a tough one. We know that
Now, we want to release 250kJ of energy. Just as a matter of logic, this means we must be burning LESS than one mole of Ethanol (as one mole produces MORE energy). So, we divide the smaller number by the bigger number, which will be equal to the number of moles. This is probably just a technique to memorise.
Then, you can multiply by the molar mass to get the total mass
Post by: bsdfjnlkasn on March 26, 2017, 11:24:42 am
Yep, ag is exactly right! Could can quickly calculate the initial pH (ie. -log(0.01). Then, we use C1V2=C2V2 to calculate the final concentration, and sub that straight into our pH formula again!
Now, the heat of combustion question is always a tough one. We know that
Now, we want to release 250kJ of energy. Just as a matter of logic, this means we must be burning LESS than one mole of Ethanol (as one mole produces MORE energy). So, we divide the smaller number by the bigger number, which will be equal to the number of moles. This is probably just a technique to memorise.
Then, you can multiply by the molar mass to get the total mass
Thank you so much Jake!!
Could I get help with approaching this 5 mark question? I don't know how you can involve equations with polymerisation? I know it might be annoying to ask you to draw it but I was never taught this properly and it just came up in a practice paper and it's obviously worth a lot of marks so if you could maybe refer me to a website that shows it well that would be really awesome!
EDIT: I'm not sure what the difference between an oxidising agent is and something which oxidises. They're not the same but I don't know how to remember a good definition to not get confused. Any advice would be greatly appreciated :)
Post by: bsdfjnlkasn on March 26, 2017, 02:37:08 pm
I have a few miscellaneous questions that I was hoping to get some help with. I've attached them as screenshots and because I've got mostly pH related questions wrong, it would be really nice to have someone explain the theory behind it (if there is any) to help me clarify.
Thank you!!
For Q2: I answered A because I thought molar heat of combustion was independent of the mole ratio. Sorry if that's really stupid because they both have the word mole in them.... I'm just confused. Ans - D
For Q10: I'm not sure what the best approach would be to answering this because there are 5 different elements and their spread through multiple reactants/products. Ans - B
Q11: I was almost certain the answer was C but it's A
Q12: I was also pretty sure the answer was D not C
Post by: jakesilove on March 26, 2017, 03:27:55 pm
Thank you so much Jake!!
Could I get help with approaching this 5 mark question? I don't know how you can involve equations with polymerisation? I know it might be annoying to ask you to draw it but I was never taught this properly and it just came up in a practice paper and it's obviously worth a lot of marks so if you could maybe refer me to a website that shows it well that would be really awesome!
EDIT: I'm not sure what the difference between an oxidising agent is and something which oxidises. They're not the same but I don't know how to remember a good definition to not get confused. Any advice would be greatly appreciated :)
Hey,
The polymerisation reactions of these two reactions are something you need to know. There is a dotpoint about PVC and Polystyrene. I would recommend going through the HSC dot points, and figuring out if there's anything you've missed out on.
You need to be able to distinguish between addition polymerisation and condensation polymerisation. Addition polymerisation occurs when the monomers 'link' up without any elimination of a small molecule. Condensation polymerisation occurs when the monomers 'link' up and elimination a small molecule. The first one will look like this
(http://www.tangram.co.uk/images/TI-Plastics-Pinking2.jpg)
You can tell that, since the monomer has a double bond, the bond will just 'split' open and react with other monomers.
The second one doesn't have a double bond, so a small molecule will be eliminated. One of the -OH groups will be eliminated from one side, and one -H from the -OH will be eliminated from the other side. This means that a product of the reaction will be water (H2O).
Additionally, you can talk about the standard initiation, propagation, termination steps required of polymerisation.
An oxidising agent is something that causes something else to oxidise. Thus, it reduces.
Molar heat of combustion has to do with how many bonds are broken. Thus, the more a molecule 'breaks down', the higher the molar heat of combustion. The bigger the molecule, the more it can combust into CO2 and H2O. So, the answer is D
Oxygen rarely changes oxidation state, so let's ignore it for now.
The initial charge of KMnO4 is zero. Thus,
The oxidation state of Manganese, initially, is +7. Looking at the right hand side
The final oxidation state of Manganese is +3. Therefore, it has decreased in oxidation number (gained electrons). The answer is B
Each step on the pH scale is a factor of 10. So, the Hydrogen ions in Milk 10 times more concentrated than in Distilled Water. The Hydrogen ions in Rain water is 10*10=100 times more concentrated than in Distilled water. the concentration in Wine is 10*10*10=1000 times more concentrated that in Distilled water. The answer is A.
The initial pH is going to be
We want the final pH to be five, so we need
Thus, initial concentration is 0.001 and final concentration is 0.00001.
So, we need to ADD 1980mL, as there is already 20mL and the total volume needs to be 2000mL
Post by: bsdfjnlkasn on March 26, 2017, 04:33:07 pm
I have one more question (attached). Do you have any suggestions for tackling pH questions or are they really not that hard :P
Post by: jakesilove on March 26, 2017, 05:05:26 pm
Thank you SO much Jake!! I understood everything
I have one more question (attached). Do you have any suggestions for tackling pH questions or are they really not that hard :P
Most of the time, pH questions are just simple application of the log formula. However, sometimes they get trickier, like the question you attached. In those cases, just slowly work your way through, finding out any information you can. Then, see if you can string it together to form an answer.
First, we need a chemical equation
Clearly, the reaction is 1:1. Now, the moles of each substance is
Clearly, HCl is the limiting reagent. All of the acid will react, leaving
moles of the base left. We need to find the concentration of the Hydroxide ions, which will be
Post by: bluecookie on March 26, 2017, 06:32:36 pm
b) 0.2M potassium hydroxide and 0.15M acetic acid.
c) 0.25M butanoic acid and 0.20 sodium carbonate
d) 0.2M hydrochloric acid and 0.2M sodium carbonate
Post by: Annie657 on March 26, 2017, 08:33:10 pm
Thankyou
Post by: Bubbly_bluey on March 26, 2017, 10:50:36 pm
Hi, just wondering what saturated and unsaturated means in regards to alkenes and alkanes, because I keep using them as words but dont actually know what they mean! ;D ;DHey! I have a pretty "dumb" way (if you like to call it) of remembering it. So alkanes have more hydrogens bonding to the carbons because of single bonds. So they are kind of like "saturated with hydrogens" . Chemically I believe it means that since that all carbons are filled with single bonds, they dont have the potential to react with other compound (ie. all the bonds are full)
Thankyou
Alkenes have less hydrogens because of double bonding. They can react with other compounds by breaking up that bonding.
Not sure if this is the best way to explain it, but I hope it helps :D
Post by: jakesilove on March 27, 2017, 09:31:13 am
What are the indicators suitable for these titrations:
b) 0.2M potassium hydroxide and 0.15M acetic acid.
c) 0.25M butanoic acid and 0.20 sodium carbonate
d) 0.2M hydrochloric acid and 0.2M sodium carbonate
Hey! You generally ignore the concentration of the solution, and just look at the acid/base used.
If a strong acid and a strong base is used, then the appropriate indicator is Bromothymol Blue, which has an end point around a pH of 7
If a strong acid and a weak base is used, then the appropriate indicator is Methyl Orange, which has an end point around a pH of 4-6
If a weak acid and a strong base is used, then the appropriate indicator is Phenophthalein, which has an end point around a pH of 8-10
Post by: shreya_ajoshi on March 27, 2017, 10:11:15 am
how would you do this?
If 1kg of 10% solution of glucose in water in apparatus weighing 350g was completely fermented, and any gas formed allowed to escape, by what amount would the mass of the apparatus and its contents decrease? What percentage decrease does this correspond to?
Post by: shreya_ajoshi on March 27, 2017, 12:58:23 pm
The answers don't have sample answers.
thank you :)
Post by: jakesilove on March 27, 2017, 01:07:20 pm
Hi,
how would you do this?
If 1kg of 10% solution of glucose in water in apparatus weighing 350g was completely fermented, and any gas formed allowed to escape, by what amount would the mass of the apparatus and its contents decrease? What percentage decrease does this correspond to?
Hey! First, let's look at the chemical reaction for fermentation.
Assuming that the reaction goes to completion (which, in reality, it won't. Remember that the yeast dies at 15% concentration of Ethanol), all Glucose will be converted into Ethanol and Carbon dioxide.
We know we have 100g of Glucose. In moles, this is
So, this is how many moles of Glucose reacts. Thus, as the reaction is 1:2:2, 0.55*2=1.10 moles of Carbon dioxide is produced. This gas is LOST! So, we can find the mass of the Carbon dioxide, and subtract that from the total mass to get our answer.
Post by: Fahim486 on March 27, 2017, 01:31:22 pm
Thanks!
Post by: Fahim486 on March 27, 2017, 01:32:07 pm
Cheers!
Post by: jakesilove on March 27, 2017, 02:23:38 pm
Hi how would you answer these?
The answers don't have sample answers.
thank you :)
Hey! I'll just do one as an example, and you can use my response to work out the answers to the others. Let's look at petrol.
We have kJ/mol, and we want kJ/g. This is pretty easy, if you think of the question in terms of units. Remember that
Therefore,
I usually think about this by asking whether I expect the number to get smaller or larger. Clearly, the energy released per gram will be less than the energy released per mole. Thus, we divide.
Now, the find the kJ/L, we use the answer we just found. Remember,
So, if one gram of Petrol releases 48.2kJ of energy, and there are 690g of petrol in a litre, then
Easy! Use the same working out for the rest.
In discussing the potential use of a fuel source as an alternative to petrol, first and foremost is renewability. Due to the limited nature of petrochemicals, being able to produce substances such as Ethanol is very important.
Then, look to the table. Does the fuel source provide as much energy per gram as Petrol? How about energy per litre? Will cars need to fill up with more fuel if they use a non-petrol substance? Is the substance even a liquid? How can it be stored? Will it corrode the engine? Those are the sorts of discussion points that will get you full marks.
Post by: jakesilove on March 27, 2017, 02:28:12 pm
Hi everyone, I'm having trouble understanding these questions so could someone pls explain them.
Thanks!
Hey! The first question is sort of just a by-definition one; radioactivity is when elements shed electrons/helium atoms due to an instability in the nucleus. This can occur because of three reasons; One, the ratio of neutrons to protons is too high. Two, the ratio of neutrons to protons is too low. Three, there are just too many neutrons and protons. Thus, the answer is definitely D.
Question 9 is the same; the salt bridge is literally used just to complete the circuit. Remember from like year 8 science that, if you want a circuit to work, it needs to be a complete loop. Thus, the electrons might flow one way (through the external circuit), but the 'left over' electrons/ions need some way to continue to flow. Thus, we use a salt bridge soaked in a conductive substance.
For your last question, looks like you need to do some research on radioactive isotopes. The questions you need to ask yourself are;
- Is a half life of 12 days long or short? If it is long, then you don't want it in the human body (as it will continue to damage tissue for ages!). If it is short, it is likely suitable.
- What is beta decay? Is it harmful to the human body?
- What is low energy gamma radiation? Is it harmful to the human body?
As this is only a two marker, a very brief over view of the above points is likely enough. Generally, however, isotopes used for medicine have a much shorter half-life (ie. 6 hours), and will only emit very low energies of one type of radiation. So, this substance is unlikely to be suitable for medical purposes.
Post by: shreya_ajoshi on March 27, 2017, 04:26:44 pm
Hey! First, let's look at the chemical reaction for fermentation.
Assuming that the reaction goes to completion (which, in reality, it won't. Remember that the yeast dies at 15% concentration of Ethanol), all Glucose will be converted into Ethanol and Carbon dioxide.
We know we have 100g of Glucose. In moles, this is
So, this is how many moles of Glucose reacts. Thus, as the reaction is 1:2:2, 0.55*2=1.10 moles of Carbon dioxide is produced. This gas is LOST! So, we can find the mass of the Carbon dioxide, and subtract that from the total mass to get our answer.
Thanks!!!
Post by: shreya_ajoshi on March 27, 2017, 04:40:48 pm
[/quote] Then, look to the table. Does the fuel source provide as much energy per gram as Petrol? How about energy per litre? Will cars need to fill up with more fuel if they use a non-petrol substance? Is the substance even a liquid? How can it be stored? Will it corrode the engine? Those are the sorts of discussion points that will get you full marks.
[/quote]
Thanks so much :)
So could I say that Ethanol and Kerosene do not provide as much energy per gram but hydrogen does. Only kerosene releases as much and more energy per Litre. If cars are to use these fuels, they would need to use them such as Ethanol as a petrol extender. Hydrogen is not even liquid hence will be very hard to contain and use as a fuel. When the ethanol is subjected to fractional distillation after fermentation, it will still have water in it and this will corrode the engines. Hence, Kerosene and ethanol have the most potential as alternative fuels based on their physical and chemical properties. However, Kerosene is obtained from crude oil which is a non-renewable resource, hence its supply is slowly decreasing. Hence the only alternative left is ethanol which is derived from a renewable resource (biomass) but it does have disadvantages as the engines will need modification.
I know it's a messy response but I'm just making sure my ideas are correct and then I would structure it.
Post by: Fahim486 on March 27, 2017, 07:39:19 pm
Hey! The first question is sort of just a by-definition one; radioactivity is when elements shed electrons/helium atoms due to an instability in the nucleus. This can occur because of three reasons; One, the ratio of neutrons to protons is too high. Two, the ratio of neutrons to protons is too low. Three, there are just too many neutrons and protons. Thus, the answer is definitely D.
Question 9 is the same; the salt bridge is literally used just to complete the circuit. Remember from like year 8 science that, if you want a circuit to work, it needs to be a complete loop. Thus, the electrons might flow one way (through the external circuit), but the 'left over' electrons/ions need some way to continue to flow. Thus, we use a salt bridge soaked in a conductive substance.
For your last question, looks like you need to do some research on radioactive isotopes. The questions you need to ask yourself are;
- Is a half life of 12 days long or short? If it is long, then you don't want it in the human body (as it will continue to damage tissue for ages!). If it is short, it is likely suitable.
- What is beta decay? Is it harmful to the human body?
- What is low energy gamma radiation? Is it harmful to the human body?
As this is only a two marker, a very brief over view of the above points is likely enough. Generally, however, isotopes used for medicine have a much shorter half-life (ie. 6 hours), and will only emit very low energies of one type of radiation. So, this substance is unlikely to be suitable for medical purposes.
Thanks so much for your help!!!!
Post by: jakesilove on March 27, 2017, 09:57:23 pm
Then, look to the table. Does the fuel source provide as much energy per gram as Petrol? How about energy per litre? Will cars need to fill up with more fuel if they use a non-petrol substance? Is the substance even a liquid? How can it be stored? Will it corrode the engine? Those are the sorts of discussion points that will get you full marks.
Thanks so much :)
So could I say that Ethanol and Kerosene do not provide as much energy per gram but hydrogen does. Only kerosene releases as much and more energy per Litre. If cars are to use these fuels, they would need to use them such as Ethanol as a petrol extender. Hydrogen is not even liquid hence will be very hard to contain and use as a fuel. When the ethanol is subjected to fractional distillation after fermentation, it will still have water in it and this will corrode the engines. Hence, Kerosene and ethanol have the most potential as alternative fuels based on their physical and chemical properties. However, Kerosene is obtained from crude oil which is a non-renewable resource, hence its supply is slowly decreasing. Hence the only alternative left is ethanol which is derived from a renewable resource (biomass) but it does have disadvantages as the engines will need modification.
I know it's a messy response but I'm just making sure my ideas are correct and then I would structure it.
Your answer is perfect! Great job!!
Post by: bluecookie on March 27, 2017, 11:06:51 pm
Hey! You generally ignore the concentration of the solution, and just look at the acid/base used.
If a strong acid and a strong base is used, then the appropriate indicator is Bromothymol Blue, which has an end point around a pH of 7
If a strong acid and a weak base is used, then the appropriate indicator is Methyl Orange, which has an end point around a pH of 4-6
If a weak acid and a strong base is used, then the appropriate indicator is Phenophthalein, which has an end point around a pH of 8-10
Thank you ^-^
Post by: shreya_ajoshi on March 28, 2017, 09:54:50 pm
How would you do part c) of the question. Im talking about PHB as my biopolymer.
Thanks :)
Post by: jakesilove on March 29, 2017, 11:16:51 am
Hey there,
How would you do part c) of the question. Im talking about PHB as my biopolymer.
Thanks :)
Hey! This is just something you need to have in your notes somewhere, and will depend on the research that you have conducted. Figure out what the deficiencies in PHB are (ie. is it expensive? Time consuming to produce?). Then, all you have to do is suggest that research must be done in those areas to improve the potential of the biopolymer as a resource more widely used!
Post by: hanaacdr on March 29, 2017, 09:59:05 pm
could you please go through everyday uses of soil testing on pH please
thank you
Post by: hanaacdr on March 29, 2017, 10:09:21 pm
is the titrant the unknown solution or the known solution?
thanks
Post by: cookiedream on March 29, 2017, 10:21:21 pm
also, in titration
is the titrant the unknown solution or the known solution?
thanks
The titrant is the solution of the known concentration.
Post by: jakesilove on March 30, 2017, 08:19:25 am
hi
could you please go through everyday uses of soil testing on pH please
thank you
Hey! We test the pH of soil to ensure it isn't too acidic/basic. This is important, as crops and trees will only grow in soil of a certain (approximately neutral) pH. So, if we don't quickly identify acidic soil, crops will die. Additionally, plants growing in acidic soil may be acidic themselves. This will damage any animals who rely on those plants for food. Finally, soil will leach it's pH into waterways adjacent. So, we want to make sure soil is neutral else waterways become acidified, killing marine life and anything relying on the waterway
Post by: bananna on March 30, 2017, 11:00:36 am
Just wondering what's the difference between amphoteric and amphiprotic elements? Also, examples would be greatly appreciated !
Thank you :)
Post by: jakesilove on March 30, 2017, 12:24:11 pm
Hi!
Just wondering what's the difference between amphoteric and amphiprotic elements? Also, examples would be greatly appreciated !
Thank you :)
Hey! This is really beyond the curriculum, and confuses a lot of students. Suffice it to say that if you treat them as exactly the same, you will ALWAYS get the marks.
An amphiprotic substance is anything that can both accept and donate protons (thus, it acts as a B-L acid/base). And amphoteric substance, however, ACTS as an acid and a base without necessarily donating/accepting a proton. For instance, Aluminium trioxide will act as an acid, despite not containing any hydrogen atoms!
Again, you don't need to know this. For the purpose of the HSC, just know that these substances act as an acid and as a base.
Post by: Gabby Mangion on March 30, 2017, 01:32:51 pm
Post by: jakesilove on March 30, 2017, 01:48:16 pm
Hi. I was just going over my notes for my chem half yearly and came across the oxidation states and their rules. I was wondering if someone could please explain it to me and give an example of how to solve it in a question??
Hey Gabby! Welcome to the forums!
Oxidations states is a useful tool we can use to decide whether a substance is oxidising or reducing. There are some pretty simple rules that we need to know before we can attack any question.
1. Oxygen has an oxidation state of -2
Ever Oxygen we see will have an oxidation state of -2. So, if there are two oxygen atoms, we will say that the total state is -4!
Elements have an oxidation state of 0
This is because it is 'neutral'
The oxidation state of each element will add up to the oxidation state of the molecule
This is just sort of intuitive. Let's first look at an easy example.
Now, the initial oxidation state of Copper is 0 (as it is in elemental form). The final compound containing Copper has an overall charge of zero. However, we know that CO3 has an oxidation state of -1. Therefore, Copper must have a change of +1. So, as it has increased in oxidation state, it has oxidised.
The initial oxidation state of Silver is +1, as NO3 is -1. The final oxidation state of Silver is 0. Thus, the oxidation state has decrease, so Silver has reduced!
Let me know if you have a specific question you want us to work through, to help you better understand the theory!
Post by: Gabby Mangion on March 30, 2017, 02:08:02 pm
Hey Gabby! Welcome to the forums!
Oxidations states is a useful tool we can use to decide whether a substance is oxidising or reducing. There are some pretty simple rules that we need to know before we can attack any question.
1. Oxygen has an oxidation state of -2
Ever Oxygen we see will have an oxidation state of -2. So, if there are two oxygen atoms, we will say that the total state is -4!
Elements have an oxidation state of 0
This is because it is 'neutral'
The oxidation state of each element will add up to the oxidation state of the molecule
This is just sort of intuitive. Let's first look at an easy example.
Now, the initial oxidation state of Copper is 0 (as it is in elemental form). The final compound containing Copper has an overall charge of zero. However, we know that CO3 has an oxidation state of -1. Therefore, Copper must have a change of +1. So, as it has increased in oxidation state, it has oxidised.
The initial oxidation state of Silver is +1, as NO3 is -1. The final oxidation state of Silver is 0. Thus, the oxidation state has decrease, so Silver has reduced!
Let me know if you have a specific question you want us to work through, to help you better understand the theory!
Thank you!! I know this is probably a really simple question so please bear with me.
There is a question in a past paper I'm doing and it says (It's a multiple choice):
Zn + 2MnO2 -> ZnO + MnO3
In this state the change in oxidation state of Manganese is:
A) From O to +2
B) From +4 to +3
C) From +4 to +6
D) no change
I understand it with Zinc but sadly not Manganese.
Thanks
Post by: jakesilove on March 30, 2017, 02:15:47 pm
Thank you!! I know this is probably a really simple question so please bear with me.
There is a question in a past paper I'm doing and it says (It's a multiple choice):
Zn + 2MnO2 -> ZnO + MnO3
In this state the change in oxidation state of Manganese is:
A) From O to +2
B) From +4 to +3
C) From +4 to +6
D) no change
I understand it with Zinc but sadly not Manganese.
Thanks
No worries! So, the initial state is
Which has a net charge of zero (or they would have specified a net charge). As oxygen has a charge of -2, and the charges add up to form the net charge, we can say that
Where x is the charge on Manganese. So, the initial oxidation state of Manganese is going to be 4 (solving for x)
Now, the final state is
Again, we create a formula, noting that the net charge is zero
So, the oxidation number has INCREASED to 6. Thus, the answer is C.
If you figured out the correct answer for Zinc, you can use logic to figure out what's going on with Manganese. You would have found that zinc REDUCES (ie oxidation number decreases). Therefore, the Manganese must OXIDISE). This limits it to A and C. Finally, note that the initial condition of Manganese is not in an elemental form. Thus, it cannot have an initial oxidation state of 0. So, the answer can only be C :)
Post by: bananna on March 30, 2017, 02:16:56 pm
Hey! This is really beyond the curriculum, and confuses a lot of students. Suffice it to say that if you treat them as exactly the same, you will ALWAYS get the marks.
An amphiprotic substance is anything that can both accept and donate protons (thus, it acts as a B-L acid/base). And amphoteric substance, however, ACTS as an acid and a base without necessarily donating/accepting a proton. For instance, Aluminium trioxide will act as an acid, despite not containing any hydrogen atoms!
Again, you don't need to know this. For the purpose of the HSC, just know that these substances act as an acid and as a base.
What kind of questions could they ask regarding amphiprotic substances? (I haven't come across very many)
thank you!
Post by: Gabby Mangion on March 30, 2017, 02:28:44 pm
No worries! So, the initial state is
Which has a net charge of zero (or they would have specified a net charge). As oxygen has a charge of -2, and the charges add up to form the net charge, we can say that
Where x is the charge on Manganese. So, the initial oxidation state of Manganese is going to be 4 (solving for x)
Now, the final state is
Again, we create a formula, noting that the net charge is zero
So, the oxidation number has INCREASED to 6. Thus, the answer is C.
If you figured out the correct answer for Zinc, you can use logic to figure out what's going on with Manganese. You would have found that zinc REDUCES (ie oxidation number decreases). Therefore, the Manganese must OXIDISE). This limits it to A and C. Finally, note that the initial condition of Manganese is not in an elemental form. Thus, it cannot have an initial oxidation state of 0. So, the answer can only be C :)
Thank you so much!!! It's making a lot more sense now. :)
Post by: bananna on March 30, 2017, 03:04:58 pm
why is acetic acid a weaker acid than citric?
And how would I answer a question comparing the degree of ionisation of citric, acetic and HCl acids?
thank you :) :) :)
Post by: jakesilove on March 30, 2017, 03:15:10 pm
Hi,
why is acetic acid a weaker acid than citric?
And how would I answer a question comparing the degree of ionisation of citric, acetic and HCl acids?
thank you :) :) :)
Hey! Acetic acid is only monoprotic; thus, one molecule of acetic acid will release only one Hydrogen ion. However, citric acid is triprotic; thus, one molecule of citric acid will release three Hydrogen ions. So, citric acid will produce a more acidic solution (assuming equal concentrations) than acetic acid. However, only a small percentage of acetic acid/citric acid molecules actually dissociate in water. Most of them stay as a compound, rather than ionising. This is the definition of a weak acid. On the other hand, HCl is a strong acid, and so ionises completed in solution. This is the definition of a strong acid, and is why a solution of HCl (assuming equal concentration) will be more acidic than a solution of citric acid, which is more acidic that a solution of acetic acid.
Post by: Alalamc on March 30, 2017, 04:59:52 pm
1. A compound has a percent composition of 38.1% C, 6.3% H and 55.6% Cl and a molar mass of approximately 125 g/mol. What is the empirical and molecular formula?
2. What is the concentration of a sodium carbonate solution prepared by diluting 10.0mL of a 5.0 M Na2CO3 solution to 250mL?
Thanks!
Post by: bsdfjnlkasn on March 30, 2017, 07:20:14 pm
Hi I was having a bit of trouble with the following 2 questions, I was hoping you could explain them to me :)\
1. A compound has a percent composition of 38.1% C, 6.3% H and 55.6% Cl and a molar mass of approximately 125 g/mol. What is the empirical and molecular formula?
2. What is the concentration of a sodium carbonate solution prepared by diluting 10.0mL of a 5.0 M Na2CO3 solution to 250mL?
Thanks!
Hiya there,
I managed to solve your second question and hopefully it's right.
2. We will be using the: c1v1=c2v2 formula to help us solve this question :)
First let's write out all the information we've been given:
c1 = 5M
c2 = ?
v1 = 0.01 L (always covert mL to L)
v2 = 0.25 L
Next we sub in our values to the above formula and get the answer
i.e. 5 x 0.01 = c2 x 0.25
and you should get 0.2M :)
Also, I did some rough working for your first question but was wondering if you could send the solution through so that I can double check that what I got was correct (I won't be surprised if I got it wrong)
Hopefully this was helpful :)
Post by: chelseam on March 30, 2017, 07:56:26 pm
Post by: Alalamc on March 30, 2017, 08:08:53 pm
Hopefully this was helpful :)Legend! I haven't got the solutions/answers :( but i did get oddly big numbers for Q1 :/
Post by: bsdfjnlkasn on March 30, 2017, 08:25:40 pm
Hi! Can someone please explain the method to solve these questions? I'm especially confused by b) since it's just NaOH! Thank you :D
Hey don't be thrown off by NaOH, all that needs to be done differently is we need to calculate pOH before finding the pH
(http://uploads.tapatalk-cdn.com/20170330/1af2d40be6cb0a4a9f88ed9a1b4649de.jpg)
Post by: Kira Leigh on March 30, 2017, 08:35:40 pm
I was doing a past mid course my teacher gave me today and came across the term 'amphoteric' I know that in class we covered amphiprotic but not amphoteric. Is there a difference?
Post by: Wren on March 30, 2017, 08:47:44 pm
Hey there!There's a similar question on the previous page thats been answered so you can just check that. But if you can't be bothered it says its basically the same thing for the purpose of HSC.
I was doing a past mid course my teacher gave me today and came across the term 'amphoteric' I know that in class we covered amphiprotic but not amphoteric. Is there a difference?
Post by: katnisschung on March 30, 2017, 08:49:12 pm
what would happen if you heated hydrogen peroxide to like 100°C?
Post by: chelseam on March 30, 2017, 08:59:23 pm
Hey don't be thrown off by NaOH, all that needs to be done differently is we need to calculate pOH before finding the pH
Post by: jakesilove on March 30, 2017, 09:59:42 pm
Hey there!
I was doing a past mid course my teacher gave me today and came across the term 'amphoteric' I know that in class we covered amphiprotic but not amphoteric. Is there a difference?
Yep! Check out my post here
Post by: jakesilove on March 30, 2017, 10:02:01 pm
just out of curiosity ( i don't take chem but this is somewhat related to bio test and i don't have a chem background)
what would happen if you heated hydrogen peroxide to like 100°C?
Look, when it comes to complex Chemistry, I'm a bit out of my depth aha. However, it has a boiling point around 150 degrees, so my guess is going to be... Nothing?
Also, I just want to say a massive thank you to all the users answering questions. Most of you are current Year 12s yourself, so the fact that you're taking the time to help others is just phenomenal. Plus, it's an insanely good way to study; if you can teach, then you understand. You keep ATAR Notes running :)
Post by: Kekemato_BAP on March 31, 2017, 12:17:17 am
just out of curiosity ( i don't take chem but this is somewhat related to bio test and i don't have a chem background)
what would happen if you heated hydrogen peroxide to like 100°C?
Cool question! Hydrogen peroxide is an unstable compound of H2O2 which slowly decomposes into H2 gas and O2 gas at room temperature. The drastic increase in heat rapidly speeds up the decomposition rate. Even though the boiling point is 150'C, it would likely have already decomposed into the explosive mixture of hydrogen and oxygen gas.
Post by: Kekemato_BAP on March 31, 2017, 12:28:58 am
Hi I was having a bit of trouble with the following 2 questions, I was hoping you could explain them to me :)\
1. A compound has a percent composition of 38.1% C, 6.3% H and 55.6% Cl and a molar mass of approximately 125 g/mol. What is the empirical and molecular formula?
2. What is the concentration of a sodium carbonate solution prepared by diluting 10.0mL of a 5.0 M Na2CO3 solution to 250mL?
Thanks!
Hey! The second question has already been answered, but not the first one. Here's my answer:
First put the percentages as grams.
38.1g C, 6.3g H, 55.6g Cl
Now you can convert these into moles.
38.1/12.01= ~3.17
6.3/1.008= ~5.95
55.6/35.45= ~1.57
It can be seen that the moles of C, H, and Cl are about a 3:6:1.5 ratio if you round them up. But you can't have 1.5 of a Cl, so it would be doubled to 6:12:3 which simplifies into 2:4:1.
Therefore, the empirical formula is C2H4Cl.
The molar mass of C2H4Cl is ~63.5g/mol and the molar mass is supposed to be 125g/mol. Thus, the molecular formula is C4H8Cl2 because 63.5 is about half of 125.
Hope this makes sense!! :)
Post by: bananna on March 31, 2017, 07:48:04 am
Please help!!
If during combustion an alcohol lost 30% of heat to the environment, would I multiply the MOLES by 70/100 or is it totally different
Thank youuuuuu
Much appreciated
Post by: Kekemato_BAP on March 31, 2017, 08:00:30 am
Hi
Please help!!
If during combustion an alcohol lost 30% of heat to the environment, would I multiply the MOLES by 70/100 or is it totally different
Thank youuuuuu
Much appreciated
Hi! If the combustion loses 30% of its heat, this means that the recorded value of heat is 70% of the actual value of heat. Divide the recorded value by 7 then multiply by 10. That should work in giving you the actual heat of combustion. Divide this value by the number of moles of fuel consumed in the combustion to get your kJ/mol.
Hope this helps :)
Post by: bananna on March 31, 2017, 08:22:51 am
Hi! If the combustion loses 30% of its heat, this means that the recorded value of heat is 70% of the actual value of heat. Divide the recorded value by 7 then multiply by 10. That should work in giving you the actual heat of combustion. Divide this value by the number of moles of fuel consumed in the combustion to get your kJ/mol.
Hope this helps :)
is there any way you could put it in terms of multiplying ?
like value multiplied by 30/100 or 70/100 ?
my teacher explained it once like that and I got it
but now i forgot
thanks
Post by: Kekemato_BAP on March 31, 2017, 04:28:47 pm
is there any way you could put it in terms of multiplying ?
like value multiplied by 30/100 or 70/100 ?
my teacher explained it once like that and I got it
but now i forgot
thanks
The "theoretical" heat of combustion is the heat released without any loss. It is measured in kJ/mol or J/mol such as ethanol 1360kJ/mol.
I don't think there is a formula for it, but I made this little formula for calculating heat of combustion:
Finding actual heat (heat with loss):
Theoretical x [(100-%)/100]
%=how much was lost
Finding theoretical heat (heat without loss):
(Actual) / [(100-%)/100]
%=how much was lost
For example, you combust 1 mole of ethanol with 30% of heat lost to the environment. Ethanol=1360kJ/mol.
To calculate the actual heat, this is 1360kJ x [(100-30%)/100]
=1360kJ x 70/100
=952kJ
Therefore, the heat absorbed by the thing being heated is 952kJ.
Like you said, you multiply it by 70/100. (100-%/100)
Hope this makes sense!! :)
Post by: Bubbly_bluey on March 31, 2017, 09:07:23 pm
(I'm having trouble uploading the picture vv)
13)The gases sulfur dioxide and hydrogen sulfide reacted when mixed, to form sulfur and water. SO2(g) + 2H2S (g) --> 3S (s) + 2H2O (l)
What is the role of hydrogen sulfide in the above reaction?
a) Bronsted Lowry acid
b) Arrhenius acid
c)oxidant
d) reactant
14) What mass of sulfur is formed when 2L of sulfur dioxide is mixed with 2L of hydrogen sulfide, under 273K and100kPa?
a) 1.5g
b)2g
c)4g
d) 8g
Thanks :)
Post by: thenerdygangster on March 31, 2017, 09:47:43 pm
Hey guys I need help with these questions especially Q14.
Not too sure about this question but I'll give it a crack. For 13, you'll need to keep the basic definitions of both Arrhenius acid and Bronsted Lowry acid in your head. An Arrhenius acid is any species that can increase the concentration of H+ in an aqueous solution. A Bronsted Lowry acid can be used even without an aqueous solution present. Oxidants are electron acceptors and oxygen donators so you can cross (c) out . I would say since the two reactants are gases, H2S would be a Bronsted Lowry acid.
For 14, you'll have to figure out which reactant is the limiting reagent. In this case, you have 1 mole of SO2 reacting with 2 moles of H2S. This means that for every one mole of SO2 used, 2 moles of H2S would be used as well. So finding the number of moles of each reactant (in this particular reaction), you can figure out the limiting reactant (think Ideal Gas Law; PV = nRT). From there, use stoichiometry to figure out the mole of S formed and consequently, the mass.
Hope that helps :)
Post by: Kekemato_BAP on March 31, 2017, 09:56:10 pm
Hey guys I need help with these questions especially Q14.
(I'm having trouble uploading the picture vv)
13)The gases sulfur dioxide and hydrogen sulfide reacted when mixed, to form sulfur and water. SO2(g) + 2H2S (g) --> 3S (s) + 2H2O (l)
What is the role of hydrogen sulfide in the above reaction?
a) Bronsted Lowry acid
b) Arrhenius acid
c)oxidant
d) reactant
14) What mass of sulfur is formed when 2L of sulfur dioxide is mixed with 2L of hydrogen sulfide, under 273K and100kPa?
a) 1.5g
b)2g
c)4g
d) 8g
Thanks :)
Great question!
13) I'm not that great at the topic, but the hydrogen sulfide acts as a base because sulfur dioxide is an acidic oxide, and the solid sulfur is the salt equivalent in the neutralisation (?)
14) This one I like :)
SO2 + 2H2S -> 3S + 2H2O
(1:2:3:2 stoichiometric mole ratio)
(n=moles)
v(SO2)= 2L
n(SO2)= 2L / 22.71L/mol (273K=0'C)
=0.0881mol
1:3 mole ratio, n(S)= 0.2642mol
m(S)= n*MM = 0.2642mol*32.07g/mol
=8.4g (round to 8g)
Therefore, the answer is D!!
Hope this helps :) sorry I couldn't answer the first one properly!!
Post by: Bubbly_bluey on March 31, 2017, 10:17:12 pm
Not too sure about this question but I'll give it a crack. For 13, you'll need to keep the basic definitions of both Arrhenius acid and Bronsted Lowry acid in your head. An Arrhenius acid is any species that can increase the concentration of H+ in an aqueous solution. A Bronsted Lowry acid can be used even without an aqueous solution present. Oxidants are electron acceptors and oxygen donators so you can cross (c) out . I would say since the two reactants are gases, H2S would be a Bronsted Lowry acid.Yea i agree both Arrhenius acid and oxidants are incorrect. I was stuck between reductant and bronsted lowry. Maybe I missed apoint but could you explain why its bronsted lowrys? Because I was thinking since I dont know which reactant is acting as an acid, so I looked on to how sulfur dioxide has been reduce??
Hope that helps :)
Thanks for the help tho! much appreciated ;D
Great question!YES! thats what i did at first(taking the stoichiometric ratios of SO2: 3S) but when you take the ratios of 2H2S: 3S its a different answer (which is apparently the correct one C). But I don't understand why you have the only consider a certain ratio ???
14) This one I like :)
SO2 + 2H2S -> 3S + 2H2O
(1:2:3:2 stoichiometric mole ratio)
(n=moles)
v(SO2)= 2L
n(SO2)= 2L / 22.71L/mol (273K=0'C)
=0.0881mol
1:3 mole ratio, n(S)= 0.2642mol
m(S)= n*MM = 0.2642mol*32.07g/mol
=8.4g (round to 8g)
Therefore, the answer is D!!
Hope this helps :) sorry I couldn't answer the first one properly!!
Post by: kiwiberry on March 31, 2017, 10:30:03 pm
YES! thats what i did at first(taking the stoichiometric ratios of SO2: 3S) but when you take the ratios of 2H2S: 3S its a different answer (which is apparently the correct one C). But I don't understand why you have the only consider a certain ratio ???
We're given the volumes of both reactants, so we have to find the limiting reagent first. In this case, n(H2S) = n(SO2) = 0.0881.. mol. However, we have to consider the 1:2 stoichiometric ratio- 2 moles of H2S are required to react with 1 mole of SO2. Hence SO2 will be in excess and H2S will be the limiting reagent, meaning we must use the 2H2S:3S ratio when calculating :)
Post by: Kekemato_BAP on March 31, 2017, 10:33:38 pm
We're given the volumes of both reactants, so we have to find the limiting reagent first. In this case, n(H2S) = n(SO2) = 0.0881.. mol. However, we have to consider the 1:2 stoichiometric ratio- 2 moles of H2S are required to react with 1 mole of SO2. Hence SO2 will be in excess and H2S will be the limiting reagent, meaning we must use the 2H2S:3S ratio when calculating :)
Yea i agree both Arrhenius acid and oxidants are incorrect. I was stuck between reductant and bronsted lowry. Maybe I missed apoint but could you explain why its bronsted lowrys? Because I was thinking since I dont know which reactant is acting as an acid, so I looked on to how sulfur dioxide has been reduce??
Thanks for the help tho! much appreciated ;D
YES! thats what i did at first(taking the stoichiometric ratios of SO2: 3S) but when you take the ratios of 2H2S: 3S its a different answer (which is apparently the correct one C). But I don't understand why you have the only consider a certain ratio ???
Yes!!! It actually is C!! I didn't read the full question sorry!! I didn't see that there was 2L of H2S which would have been a limiting reagent. My calculations were correct, but completely ignored the 2L of H2S sorry!!
Post by: Alalamc on April 01, 2017, 11:22:25 am
Thanks so I am actually doing a chemistry online course because I haven't done it in yr 11/12, so do you have to round up the moles to the nearest .5 before you start to take them into consideration for the subscripts? :)
Now you can convert these into moles.
38.1/12.01= ~3.17
6.3/1.008= ~5.95
55.6/35.45= ~1.57
Post by: Kekemato_BAP on April 01, 2017, 03:53:34 pm
Thanks so I am actually doing a chemistry online course because I haven't done it in yr 11/12, so do you have to round up the moles to the nearest .5 before you start to take them into consideration for the subscripts? :)
It depends.. sometimes the values are exact whole numbers like 2, 4, 3. Sometimes, they're like 1.98, 4.2, 2.99; and you just have to round them to their nearest half-number like 1, 2, 1.34. You have round to nearest .5 and then make them all whole numbers, eg. 1, 2, 1.5 then change to 2, 4, 3 because you can't have half an atom.
Hope this makes sense :)
Post by: ellipse on April 01, 2017, 09:54:12 pm
Post by: Bubbly_bluey on April 01, 2017, 10:07:53 pm
Just to confirm, is the water and ethanol gas or liquid during hydration and dehydration process?
im pretty sure liquid for both
Post by: Bubbly_bluey on April 01, 2017, 10:09:35 pm
thanks :)
Post by: cxmplete on April 02, 2017, 04:57:41 pm
Would it be okay if someone explained titration calculations? Our teacher just rushed over it and I haven't really grasped it.
Post by: Bubbly_bluey on April 02, 2017, 05:16:03 pm
Hi,Hey! I usually go by this method when tackling titration questions. Of course it pretty general but if you have a specific question i may be able to help you out :)
Would it be okay if someone explained titration calculations? Our teacher just rushed over it and I haven't really grasped it.
1) I always start with a diagram and label everything im given in the question like how much volume used, concentration.
So for example in burette: what solution is it? concentration? volume? can i work out the number of moles?
2) write out a balanced equation of the reaction
3) calculate any missing information and add it to your diagram.
4) Depending on the question, this should be enough info find concentration of unknown. From balanced equation deduce the stiochimetric ratios. Use the number of moles you calculated in step 1 to find the number of moles for unknown. Now for the unknown you have both volume and number of moles so just use C=n/v to find concentration.
Again this may or may not answer ur question (sorry)
Post by: Alalamc on April 02, 2017, 06:05:49 pm
I was hoping that someone could actually see if Q2 of PH titrations answers are correct because I keep getting the PH's in decimals and also can someone help with Q1. would be great help!
Q1
Calculate the pH in the titration of 10.00mL of 1.0M HCl with 1.0M NaOH after the addition of 10mL NaOH?
Balanced equation shows reaction ratio of 1:1
n(OH-)= 1.0mol/L x 0.010= 0.010mol
Excess n[H+]= 0.01-0.010= 0
Total Volume: 0.020 L
M= 0/0.020= 0
pH= -log[0]
= ERROR
Q2
Calculate the pH in the titration of 10.00mL of 1.0M HCl with 1.0M NaOH after the addition of 2.0mL NaOH?
Balanced equation shows reaction ratio of 1:1
n(OH-)= 1.0mol/L x 0.002= 0.002mol
Excess n[H+]= 0.01-0.002= 0.008
Total Volume: 0.012 L
M= 0.008/0.012= 0.67
pH= -log[0.67]
= 0.18
Post by: kiwiberry on April 02, 2017, 06:25:18 pm
Hey Guys
I was hoping that someone could actually see if Q2 of PH titrations answers are correct because I keep getting the PH's in decimals and also can someone help with Q1. would be great help!
Q1
Calculate the pH in the titration of 10.00mL of 1.0M HCl with 1.0M NaOH after the addition of 10mL NaOH?
Balanced equation shows reaction ratio of 1:1
n(OH-)= 1.0mol/L x 0.010= 0.010mol
Excess n[H+]= 0.01-0.010= 0
Total Volume: 0.020 L
M= 0/0.020= 0
pH= -log[0]
= ERROR
Q2
Calculate the pH in the titration of 10.00mL of 1.0M HCl with 1.0M NaOH after the addition of 2.0mL NaOH?
Balanced equation shows reaction ratio of 1:1
n(OH-)= 1.0mol/L x 0.002= 0.002mol
Excess n[H+]= 0.01-0.002= 0.008
Total Volume: 0.012 L
M= 0.008/0.012= 0.67
pH= -log[0.67]
= 0.18
1) If there is no excess H+, the solution will be neutral! Hence it will have a pH of 7
2) You've got it right! The pH scale isn't restricted to whole numbers :)
Post by: Kekemato_BAP on April 03, 2017, 05:42:39 am
Just to confirm, is the water and ethanol gas or liquid during hydration and dehydration process?
The water in hydration is steam and ethene is naturally a gas. Both hydration and dehydration are conducted at temperatures of 170-300 degrees Celsius so I assume they're all going to be gas since it exceeds their boiling points.
Post by: Bubbly_bluey on April 03, 2017, 10:37:13 am
The water in hydration is steam and ethene is naturally a gas. Both hydration and dehydration are conducted at temperatures of 170-300 degrees Celsius so I assume they're all going to be gas since it exceeds their boiling points.
yes sorry you're right! a terrible mistake on my part. I did a quick search and found that:
In dehydration
ethanol (g) --> ethylene (g) + H2O (g) because it is occuring at high temperatures as you said.
In Hydration: the process in occuring at 300 degrees
ethylene(g) + H2O (l)--> ethanol (g)
water in this case could also be a gas; it can start off as a liquid and turn into steam.
Post by: Bubbly_bluey on April 03, 2017, 11:35:08 am
Thanks :)
Post by: jakesilove on April 03, 2017, 12:09:36 pm
Hey guys! So I did a fermentation prac a while ago, we added sugar, yeast and orange juice into our test tubes and left it to ferment for a few days. Does anyone know a possible reason why orange juice was added?seems odd lol.
Thanks :)
Aha that is pretty bloody odd. Basically, what you need is some sort of substance to ferment (eg. sugar), and a fermentor (ie. Yeast). Potentially the Orange Juice was just like extra 'food' to be digested and fermented by the Yeast? Definitely an odd experiment, I would recommend just learning the standard experiment (which involves putting glucose and yeast into a beaker, along with standard conditions re heat, anaerobic etc).
Seriously weird prac aha
Post by: Alalamc on April 04, 2017, 09:41:04 am
1) 20.0 g of N2 and 20.0 g of O2 are placed in a metal container with a volume of 12.0 L at a temperature of 200°C. What is the pressure in the container?
Pressure of N2:
n: 20g/ 28.0= 0.71
V: 12L
T: 200 degrees or 473K
P= 0.71x8.31x473/12
P= 232.6kPa
Pressure of O2:
n= 20g/32.0= 0.625
P= 0.63x8.31x473/12
P= 206.4kPa
Total Pressure: 206.4+ 232.6= 439kPa
2)
A spark causes the gases in question 14 to react to form NO2. Assuming the reaction goes to completion and that there is no charge in temperature or volume, what is the pressure in the container after reaction?
Post by: jakesilove on April 04, 2017, 09:48:59 am
So sorry to be here again! its just that I don't have access to teachers and I don't even go to school anymore, if someone can explain this to me it'll be real good! :) I worked out Q1 (hope its right)
1) 20.0 g of N2 and 20.0 g of O2 are placed in a metal container with a volume of 12.0 L at a temperature of 200°C. What is the pressure in the container?
Pressure of N2:
n: 20g/ 28.0= 0.71
V: 12L
T: 200 degrees or 473K
P= 0.71x8.31x473/12
P= 232.6kPa
Pressure of O2:
n= 20g/32.0= 0.625
P= 0.63x8.31x473/12
P= 206.4kPa
Total Pressure: 206.4+ 232.6= 439kPa
2)
A spark causes the gases in question 14 to react to form NO2. Assuming the reaction goes to completion and that there is no charge in temperature or volume, what is the pressure in the container after reaction?
Hey! This out pretty far outside the HSC curriculum, but I've done PV=nRT in uni and it looks like the first question was done correctly! For the second one, we start by writing out a chemical equation.
Clearly, the molar ratio is 1:2. Now, let's work out the moles of each of the substance. Actually, you've already done that!
For everyone one mole of Nitrogen, two moles of Oxygen is required. So, Oxygen will be the limiting reagent (ie. it will be completely reacted before the Nitrogen is completely reacted). So, if 0.625 moles of Oxygen is reacted, nitrogen will react in HALF of the molar quantity (0.3125 moles).
So, we know that there are 0 moles of Oxygen, 0.71-0.3125=0.3975 moles of Nitrogen, and 0.625 moles of NO2 in the final system. Use the partial pressure technique above to find the total pressure!
Post by: Bubbly_bluey on April 04, 2017, 03:06:38 pm
Two beakers X and Y; either contains 0.1mol/L of HCL or 0.1mol/ acetic acid. A student performs two tests:
test 1: stuck a probe metre in to determine the pH
test2: add a volume of o.1mol/L of NaOH needed to neutralise each acid and using phenolpthalein indicator.
Assess the effectiveness of determining which acid is in which beaker when performing the two tests.
I was able to do test 1 but I need help for test 2. If HCL + NaOH--> NaCl + H2O then wouldn't the solution turn colourless and if CH3COOH + NaOH --> CH3COONa + H2O be purple (slightly basic salt?)
Thanks :)
Post by: J.B on April 04, 2017, 05:17:46 pm
(http://uploads.tapatalk-cdn.com/20170404/37175ed49a5ebf1ef6ba39afe9806726.jpg)
Post by: Bubbly_bluey on April 04, 2017, 06:04:41 pm
Hi I was just wondering in Q6 of 2005 HSC exam, how do you know that the answer is A and not D? And do we have to know certain esters that are used for flavouring or solvents?Hey! the answer is not D because you would have done esters I believe? So ethly propanoate is an ester that is produced from an alkanol and alkanoic acids.Usually esters produce nice fruity smells that are commonly used for perfumes, cosemetics and flavourings. They are not usually used for dissolving anything (I'm not sure) just mainly to produce smells.
(http://uploads.tapatalk-cdn.com/20170404/37175ed49a5ebf1ef6ba39afe9806726.jpg)
As for the second question, it wouldnt hurt to know one or two just in case. Use as an example for long responses? Generally they wouldn't ask for specific flavouring come from which reactants.
Hope this helps :)
Post by: jakesilove on April 04, 2017, 06:52:06 pm
Hey guys!
Two beakers X and Y; either contains 0.1mol/L of HCL or 0.1mol/ acetic acid. A student performs two tests:
test 1: stuck a probe metre in to determine the pH
test2: add a volume of o.1mol/L of NaOH needed to neutralise each acid and using phenolpthalein indicator.
Assess the effectiveness of determining which acid is in which beaker when performing the two tests.
I was able to do test 1 but I need help for test 2. If HCL + NaOH--> NaCl + H2O then wouldn't the solution turn colourless and if CH3COOH + NaOH --> CH3COONa + H2O be purple (slightly basic salt?)
Thanks :)
Hey! Definitely a seriously weird question, with loads of ways you could interpret it. In one, we're titrating a strong acid with a weak base. In the other, we're titrating a weak acid with a strong base. Phenophthalein will only be appropriate for the first titration, so maybe the question is getting you to say that this will limit the effectiveness of the second titration? Alternatively, the weaker acid will require less base to neutralise, and so we can use the amount of NaOH used as a determination of the substance? Basically, you're totally on the right track, and the question is very stupid.
Post by: Bubbly_bluey on April 04, 2017, 07:22:43 pm
The answer marked is...
Spoiler
C
Post by: kiwiberry on April 04, 2017, 07:31:57 pm
Lets hope this isn't another weird question ::)
The answer marked is...SpoilerC
I got B! You just times it by its molar mass, right?
Post by: J.B on April 04, 2017, 08:14:14 pm
Hey! the answer is not D because you would have done esters I believe? So ethly propanoate is an ester that is produced from an alkanol and alkanoic acids.Usually esters produce nice fruity smells that are commonly used for perfumes, cosemetics and flavourings. They are not usually used for dissolving anything (I'm not sure) just mainly to produce smells.
As for the second question, it wouldnt hurt to know one or two just in case. Use as an example for long responses? Generally they wouldn't ask for specific flavouring come from which reactants.
Hope this helps :)
Ok thank you,
It was just that my teacher told us they are also used as solvents.
Post by: Bubbly_bluey on April 04, 2017, 09:06:22 pm
I got B! You just times it by its molar mass, right?I got B as well! Not sure why C is the answer. Could be an error or something :/
Post by: Kekemato_BAP on April 05, 2017, 01:41:46 pm
Post by: jakesilove on April 05, 2017, 03:39:51 pm
I need clarification on this question.. I'm not sure if it is A or D because all acids have hydrogen, but metal salts are also formed
It was my understanding that a metal + acid = salt + hydrogen! So you're totally correct to be confused, A and D are both correct :)
Stupid question,
Post by: legorgo18 on April 06, 2017, 12:33:39 pm
A chemist mixes 270mL of 4M zinc nitrate solution with 185 mL of 5M magnesium sulfate solution to form a precipitate. She then ........
so part a is to write a balanced chemical equation to represent the reaction.
My question is, what is the precipitate???? All nitrates are soluble, sulfate has no restriction with zinc.... is this an error in my homework? Thanks for answering :)
Post by: Kekemato_BAP on April 06, 2017, 02:49:54 pm
Hey so this question is in my homework:
A chemist mixes 270mL of 4M zinc nitrate solution with 185 mL of 5M magnesium sulfate solution to form a precipitate. She then ........
so part a is to write a balanced chemical equation to represent the reaction.
My question is, what is the precipitate???? All nitrates are soluble, sulfate has no restriction with zinc.... is this an error in my homework? Thanks for answering :)
This question is designed to test knowledge on solubility rules. Good job on identifying that both products are soluble! The answer would be "none" since no insoluble precipitate is formed.
Post by: legorgo18 on April 06, 2017, 03:07:05 pm
This question is designed to test knowledge on solubility rules. Good job on identifying that both products are soluble! The answer would be "none" since no insoluble precipitate is formed.
Bruh the funniest thing is the next part says calculate the theoretical mass of precipitate formed...
Post by: Kekemato_BAP on April 07, 2017, 11:24:53 pm
I know the main definition but need help on the disadvantages of each theory. Thanks :)
Post by: RuiAce on April 08, 2017, 09:48:05 am
I need help understanding the different acid/base theories like Lavoisier, Davy, Arrhenius, B/LFor the acid-base theories, the disadvantages are essentially what is not addressed by each theory.
I know the main definition but need help on the disadvantages of each theory. Thanks :)
Lavoisier stated that all acids contain oxygen. Counterexample: HCl.
Davy stated that all acids contain hydrogen. Counterexample: NaHCO3.
Arrhenius stated that all acids ionise in water to produce H+. This was quite an improvement from theories in the past and is quite accurate. He also defined degree of ionisation, as well as taking the concept over to bases as well, claiming they must contain OH-. This also lead to the concept of neutralisation.
The counterexample to this one is mostly with bases. Again, NaHCO3 is a counterexample, but for a different reason. It's a base that doesn't contain the hydroxide ion.
As far as things go, modern day acids and bases are always defined based off B/L theory. At least, that's the convention, and how HSC goes. In reality acids and bases are categorised into two types: B/L and Lewis acids/bases. But you do not need to go any further - take it for granted that B/L is the most accurate of the lot. It also has the ability to address the amphiprotic nature of substances as well.
Post by: jakesilove on April 08, 2017, 09:49:14 am
I need help understanding the different acid/base theories like Lavoisier, Davy, Arrhenius, B/L
I know the main definition but need help on the disadvantages of each theory. Thanks :)
Hey! Let's take a look at the scientists in the curriculum.
Lavoisier claimed that acids were substances that contained Oxygen. This is because he noticed that non-metal oxides were acidic in solution. However, this theory was disproven, as many basic substances contain Oxygen (eg. NaOH) and some acids did not contain Oxygen (eg. HCl).
Davy updated the theory, claiming that Acids were substances that contained Hydrogen atoms that could be replaced by metal ions. He claimed this because he notices that, when acids react with metals, the metal seemed to replace Hydrogen, creating Hydrogen gas. However, thus notion only allows acids to be defined in conjunction with metals. Clearly, acids can be acidic without being introduced to a metal species
Arrhenius suggested that acids were substances that ionised in solution to produce Hydrogen ions. This was a really solid theory, as it defined acids independently of any other species. He also defined strong and weak acids! Now, the reason that Arrhenius' theory is wrong gets a bit technical (you can find more information on that here) which is why for most cases his theory is actually fine to use! However, it has been updated.
The B-L theory of acids and bases simplifies everything. Acids are proton donors, and bases are proton acceptors. Easy!
Post by: hanaacdr on April 10, 2017, 01:07:55 pm
just 2 questions
in the Haber process why is monitoring the reaction vessel for level of oxygen critical?
also why does the concentration of hydrogen and nitrogen have to be in a ratio of 1:3 in the Haber process?
thank you
Post by: kiwiberry on April 10, 2017, 01:23:33 pm
Hi,
just 2 questions
in the Haber process why is monitoring the reaction vessel for level of oxygen critical?
also why does the concentration of hydrogen and nitrogen have to be in a ratio of 1:3 in the Haber process?
thank you
Oxygen reacts with hydrogen explosively under high temperatures, so the ingoing gas stream must be monitored so all oxygen is removed before it enters the reaction vessel! The 1:3 ratio for N:H maximises efficiency of conversion into ammonia, because a buildup of excess gas will take up space in the vessel and also pose a risk of explosion.
Post by: hanaacdr on April 10, 2017, 01:36:26 pm
Oxygen reacts with hydrogen explosively under high temperatures, so the ingoing gas stream must be monitored so all oxygen is removed before it enters the reaction vessel! The 1:3 ratio for N:H maximises efficiency of conversion into ammonia, because a buildup of excess gas will take up space in the vessel and also pose a risk of explosion.
THANK YOU!
Post by: Kekemato_BAP on April 10, 2017, 08:59:34 pm
Post by: arunasva on April 10, 2017, 09:14:55 pm
Post by: Kekemato_BAP on April 11, 2017, 01:35:47 am
Someone help me. Pleeze.
Hi, do you have the full question?
This question would be done by first calculating the moles of SO2 from the data of the question, then multiplying by 24.79mol/L to get volume of gas.
It depends on the rest of the question.
Post by: jakesilove on April 11, 2017, 08:40:59 am
Are metal hydroxides salts? I get mixed answers whenever I google it..
Hey! This is where definitions fail us; in Chemistry, it's not always helpful to lump something in under a particular definition. However, we can come to some sort of answer if we try hard enough!
A salt is just an ionic compound, containing an anion and a cation. Strictly, therefore, a metal hydroxide is NOT a salt. There is no 'anion', but rather a compound which we could call the 'anion'. So, by stretching the definition a bit, I would be more than happy to call a metal hydroxide a salt, claiming that the -OH is the anion.
The real question to ask yourself is this; is determining that a metal hydroxide is a salt helpful in any way? Definitions help us to easily describe a substance, and describing NaOH as a metal hydroxide is plenty (although a strong base would be better!).
Overall, I would say that the answer is no, but you could probably convince some chemists to the contrary.
Post by: Wren on April 11, 2017, 10:19:45 am
Post by: jakesilove on April 11, 2017, 03:18:21 pm
Hey I was just wondering does it matter which reagent is added in excess in esterification or can it just be any?
I can't imagine it really matters, as the reaction won't go to completion anyway! I would just add plenty of both :)
Post by: RuiAce on April 11, 2017, 04:49:19 pm
Hey I was just wondering does it matter which reagent is added in excess in esterification or can it just be any?
I can't imagine it really matters, as the reaction won't go to completion anyway! I would just add plenty of both :)Usually the alkanoic acid is added in excess.
Two reasons:
1. The excess is easy to neutralise. It's hard trying to eliminate excess alkanol
2. Only one is added in excess to force the equilibrium further than otherwise. If the concentrations of both are increased then the effect of LCP isn't as significant.
Post by: Wren on April 11, 2017, 08:33:45 pm
Usually the alkanoic acid is added in excess.Thanks guys
Two reasons:
1. The excess is easy to neutralise. It's hard trying to eliminate excess alkanol
2. Only one is added in excess to force the equilibrium further than otherwise. If the concentrations of both are increased then the effect of LCP isn't as significant.
Post by: bsdfjnlkasn on April 12, 2017, 05:29:52 pm
I've just started chemical monitoring and management and was wondering if we were required to know the 3 steps of the Haber process?
1. Preparing the reactants
2. The reaction vessel
3. Condensing the NH3 (g) to liquid ammonia, ready for distribution
We haven't covered them at school but since there is such a big focus on it, it just seemed strange that we just identified simple things to do with the reaction and the monitoring aspects of how ammonia is produced. I mean great if we don't have to because that's less to memorise but if we do, then it'd be good to know because we completely skipped over it in class.
Thank you!! :)
Post by: jakesilove on April 13, 2017, 09:59:02 am
Hey there,
I've just started chemical monitoring and management and was wondering if we were required to know the 3 steps of the Haber process?
1. Preparing the reactants
2. The reaction vessel
3. Condensing the NH3 (g) to liquid ammonia, ready for distribution
We haven't covered them at school but since there is such a big focus on it, it just seemed strange that we just identified simple things to do with the reaction and the monitoring aspects of how ammonia is produced. I mean great if we don't have to because that's less to memorise but if we do, then it'd be good to know because we completely skipped over it in class.
Thank you!! :)
You definitely DON'T need to know about any of those things! The curriculum focuses very specifically on certain aspects of the Haber process, but leaves out the vast majority. That's because the interesting Chemistry occurs during the actual process, rather than during the practical elements required to distribute the product. So, it's good that you have the broader picture in mind, but you don't need to know it in any great detail.
Post by: RuiAce on April 13, 2017, 10:26:05 am
However there's a bit more to know about the reaction vessel
- Importance of 3:1 H:N ratio
- Effect of impurities
- Inert impurities such as argon
- Reactive impurities such as oxygen
There's also the delicate glassware but that's under LCP and the pressure stuff.
Post by: beau77bro on April 16, 2017, 11:57:42 am
Post by: RuiAce on April 16, 2017, 12:04:51 pm
theres seems to be a very heavy amount of content on the actual steps of the haber process, is it essential to know all of it? conquering talks about how they remove certain products and things that might impede the reaction. but where do all these steps fit in the actual syllabus? sorry if im a bit vague but just abit unsure. thanksImpurities definitely matters.
The monitoring of the Haber process is certainly a part of the course. If you increase the pressure by adding impurities then all you're doing is slowing the reaction down, and you definitely need to know about this. Increasing the pressure by adding something like Argon does not shift the equilibrium any way because you're actually hindering both the forward and reverse reactions.
You also need to know that you definitely do not want any reactive impurities either like oxygen. Reactions involving oxygen are quite explosive and would potentially break the delicate glassware (which leads to a whole chain of complications you should be able to figure out yourself).
Relevant syllabus dot point as addressed on easychem
Post by: Nicko912 on April 16, 2017, 12:57:24 pm
If I were to mix Cu(NO3)2 solution in a single beaker with ZnSO4 solution with a copper and zinc electrode in the single beaker, what are two predictions why the cell won't work? ( wire and load is still present )
Post by: bluecookie on April 17, 2017, 02:30:15 pm
Post by: jakesilove on April 17, 2017, 02:39:05 pm
Hi quick question ( Daniell cell ),
If I were to mix Cu(NO3)2 solution in a single beaker with ZnSO4 solution with a copper and zinc electrode in the single beaker, what are two predictions why the cell won't work? ( wire and load is still present )
I didn't do this cell, so to be honest I'm genuinely unsure! Potentially the total voltage is negative (check the table of standard reduction potentials), or the mixture will create some sort of chemical reaction that results in a solid being precipitated, before any current can be produced.
Post by: jakesilove on April 17, 2017, 02:43:32 pm
Any tips for remembering solubility rules and colour charts? I'm currently trying to remember the solubility and colours of certain substances and nothing is sticking help >.<
Hey! I can definitely help you out a little bit :)
The below device is what I used to learn my solubility rules.
(http://i.imgur.com/tGzfOge.png)
It essentially says that EVERYTHING is soluble, except for certain exceptions (which you can find on the right). Let me know if you're struggling to understand what it means.
For the flame test colours, I found it helpful to come up with my own way of remembering them. Here are mine:
(http://i.imgur.com/lMveOhB.png)
Jake
Post by: Nicko912 on April 17, 2017, 03:08:23 pm
I didn't do this cell, so to be honest I'm genuinely unsure! Potentially the total voltage is negative (check the table of standard reduction potentials), or the mixture will create some sort of chemical reaction that results in a solid being precipitated, before any current can be produced.
Thanks for the response Jake, appreciate it.
Post by: jakesilove on April 17, 2017, 03:17:13 pm
Thanks for the response Jake, appreciate it.Sorry I couldn't be of more help :( Maybe shoot your teacher a message?
Post by: Nicko912 on April 17, 2017, 04:04:22 pm
Sorry I couldn't be of more help :( Maybe shoot your teacher a message?
No problem. I'll find out soon enough from my teacher :)
Post by: bluecookie on April 18, 2017, 09:02:05 am
Thank you! (:
Post by: bananna on April 18, 2017, 08:32:48 pm
why is ammonia liquidated during the haber process?
Thanks!!! 😀
Post by: RuiAce on April 18, 2017, 08:35:51 pm
Hi I have a q in chemical monitoring ...It's not liquidated until we need to extract it.
why is ammonia liquidated during the haber process?
Thanks!!! 😀
And well, the reason why we liquidate it to collect it is kinda obvious. If we didn't liquidate it and left it as a gas, it's too easy for us to accidentally capture some N2 and H2 as well. We are exploiting the fact that NH3 has a higher boiling/condensation point than N2 and H2.
Post by: bsdfjnlkasn on April 18, 2017, 09:44:00 pm
Just a few questions that I can't seem to find an answer to in my textbook, any guidance would be appreciated!
a) In the context of AAS, explain what is meant by the phrase: “a fingerprint of a cation.”
b) Explain what is meant by 9.5 ppm and then convert it to a concentration of mol/L.
(I don't really understand ppm, all I know is what it stands for)
Thanks again :)
Post by: RuiAce on April 18, 2017, 09:54:58 pm
Hiiiiii,Personally never heard of that first one before.
Just a few questions that I can't seem to find an answer to in my textbook, any guidance would be appreciated!
a) In the context of AAS, explain what is meant by the phrase: “a fingerprint of a cation.”
b) Explain what is meant by 9.5 ppm and then convert it to a concentration of mol/L.
(I don't really understand ppm, all I know is what it stands for)
Thanks again :)
Ppm is similar to %. 1% is basically 1 in 100, so 1 ppm is 1 in 1,000,000.
Note that this implies that we need to know the substance. If we don't know the substance, we're stuck at 9.5 ppm, because we don't have a molar mass.
Post by: bsdfjnlkasn on April 18, 2017, 10:32:08 pm
Personally never heard of that first one before.
Ppm is similar to %. 1% is basically 1 in 100, so 1 ppm is 1 in 1,000,000.
Note that this implies that we need to know the substance. If we don't know the substance, we're stuck at 9.5 ppm, because we don't have a molar mass.
Hey thanks for the reply!
Sorry, the ppm question was being specific to lead, so would we just divide by the formula mass?
Post by: RuiAce on April 18, 2017, 10:38:45 pm
Hey thanks for the reply!Yep
Sorry, the ppm question was being specific to lead, so would we just divide by the formula mass?
Post by: anotherworld2b on April 19, 2017, 12:10:56 pm
mass weighed = 2.65g
Spoiler
Q2. What does it mean when a substance is anhydrous?
Anhydrous literally means 'no water'. Substances without water are anhydrous. The term is most often applied to crystalline substances when the water of crystallization is removed.
Q3. Why was the anhydrous sodium carbonate placed in the oven? In brief:
The primary standard, Na2CO3 (s) is made anhydrous by heating it in an oven at 260-270 degrees celcius. To prevent the anhydrous Na2CO3 (s) from reabsorbing water vapour from the air, it is kept in a desiccator. Silica gel crystals inside the dessicator keep the air dry by absorbing water vapour.
Q4. Calculate the precise concentration in mol.L-1
N = c x v
N = 0.05 x 0.5 = 0.025 mol
m = n x M
m = 0.025 x { (23 x 2) + 12 + (16 x 3) }
m = 0.025 x 106 = 2.65g
Q5. List any factors (systematic or random errors) that may have contributed to minor inaccuracies.
Instrument Error: error caused by an inaccurate instrument
Post by: jakesilove on April 20, 2017, 03:02:56 pm
Hi I was wondering if someone could check my answers to these questions please. I am having a bit of trouble in particular or Q4 and Q5
mass weighed = 2.65gSpoiler
Q2. What does it mean when a substance is anhydrous?
Anhydrous literally means 'no water'. Substances without water are anhydrous. The term is most often applied to crystalline substances when the water of crystallization is removed.
Q3. Why was the anhydrous sodium carbonate placed in the oven? In brief:
The primary standard, Na2CO3 (s) is made anhydrous by heating it in an oven at 260-270 degrees celcius. To prevent the anhydrous Na2CO3 (s) from reabsorbing water vapour from the air, it is kept in a desiccator. Silica gel crystals inside the dessicator keep the air dry by absorbing water vapour.
Q4. Calculate the precise concentration in mol.L-1
N = c x v
N = 0.05 x 0.5 = 0.025 mol
m = n x M
m = 0.025 x { (23 x 2) + 12 + (16 x 3) }
m = 0.025 x 106 = 2.65g
Q5. List any factors (systematic or random errors) that may have contributed to minor inaccuracies.
Instrument Error: error caused by an inaccurate instrument
All looks great! Other errors include human error (not quite reading to the calibration point when filling up the 500mL of distilled water), purity error (maybe the 2.65g was not purely the substance you were looking for), splashing of the water (potentially losing some of the substance) etc.
Post by: itssona on April 20, 2017, 04:17:47 pm
is it that is has low melting point so that means you can easily melt it and join pipes together?
or that its unreactive or smth
I can't find an answer anywhere :/
Post by: jakesilove on April 20, 2017, 04:32:44 pm
WHeey, which of the following best explains the use of solder to join metal pipes and wires?
is it that is has low melting point so that means you can easily melt it and join pipes together?
or that its unreactive or smth
I can't find an answer anywhere :/
Could you provide us with the possible answers? I would actually say that it has a HIGH melting point; whilst it makes it more difficult to join the pipes together, once you've done that they will be very well attached, and you don't have to worry about them disassembling if temperatures are raised :)
Post by: itssona on April 20, 2017, 04:39:02 pm
Could you provide us with the possible answers? I would actually say that it has a HIGH melting point; whilst it makes it more difficult to join the pipes together, once you've done that they will be very well attached, and you don't have to worry about them disassembling if temperatures are raised :)the possible answers are:
high melting point
low melting point
reactive
not reactive
But your answer actually makes so much sense- thank you Jake!! :D :D
Post by: itssona on April 20, 2017, 05:14:12 pm
if i have a question: write the equation for the reaction of calcium and water (in a test)
so I would know its Ca + H2O --> OH
but how am I meant to know that hydrogen is also a product? is that like a rule?
Post by: jakesilove on April 20, 2017, 05:38:56 pm
another question sorry:
if i have a question: write the equation for the reaction of calcium and water (in a test)
so I would know its Ca + H2O --> OH
but how am I meant to know that hydrogen is also a product? is that like a rule?
Hey! There is a rule that a metal + water will result in a metal oxide + hydrogen gas. So, in this case
Post by: itssona on April 20, 2017, 06:17:50 pm
Hey! There is a rule that a metal + water will result in a metal oxide + hydrogen gas. So, in this case
omg thank you for explaining Jake:)
Post by: anotherworld2b on April 20, 2017, 06:19:16 pm
I was also wondering for this reaction:
Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)
i'm not sure how Q2 is different from Q1
Q1. Using the equation for the reaction, calculate the number of moles of sodium carbonate used in each titration
Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)
n(HCl) = 0.05
n( Na2CO3 ) = ˝ n(HCl)
n(Na2CO3) = ˝ x 0.05 = 0.025 moles
Q2. From the equation determine the number of moles of HCL that react with each mole of sodium carbonate. Use this to determine the number of moles of HCL used in the titration.
Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)
I was also a bit confused about these questions. I am not sure what it is asking
Q5. Is the equivalence point of a reaction acidic, basic or neutral? Explain.
It should be a bit acidic?
Q7. Phenolphthalein colour change is in the vicinity of pH 9. Suppose phenolphthalein had been used in this titration:
a.Would the volume of acid required for the titration be more or less than that obtained?
less?
b.Would the calculated concentration of the acid be lower of higher than the result obtained?
All looks great! Other errors include human error (not quite reading to the calibration point when filling up the 500mL of distilled water), purity error (maybe the 2.65g was not purely the substance you were looking for), splashing of the water (potentially losing some of the substance) etc.
Post by: jakesilove on April 20, 2017, 06:27:30 pm
thank you for your help :D
I was also wondering for this reaction:
Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)
i'm not sure how Q2 is different from Q1
Q1. Using the equation for the reaction, calculate the number of moles of sodium carbonate used in each titration
Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)
n(HCl) = 0.05
n( Na2CO3 ) = ˝ n(HCl)
n(Na2CO3) = ˝ x 0.05 = 0.025 moles
Q2. From the equation determine the number of moles of HCL that react with each mole of sodium carbonate. Use this to determine the number of moles of HCL used in the titration.
Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)
I was also a bit confused about these questions. I am not sure what it is asking
Q5. Is the equivalence point of a reaction acidic, basic or neutral? Explain.
It should be a bit acidic?
Q7. Phenolphthalein colour change is in the vicinity of pH 9. Suppose phenolphthalein had been used in this titration:
a.Would the volume of acid required for the titration be more or less than that obtained?
less?
b.Would the calculated concentration of the acid be lower of higher than the result obtained?
Hey, sorry without looking at the full original question, Question 2 just seems like nonsense (since it looks like they gave you that n(HCl)=0.05 moles?)
The equivalence point depends on the titration itself. If we are titrating a strong acid and a weak base (as here), the equivalence point will be in the acidic zone (ie. pH around 4-5).
We were titrating a strong acid and a weak base, so the equivalence point should be a lower pH. Instead, using Phenolphathein will require the pH to be raised a lot more than necessary. So, LESS acid would be used (as you've noted).
If we use LESS acid to neutralise the same amount of base, but THINK that there are more moles in that small quantity of acid, then we will calculate that the acid is stronger (less volume, but same number of moles, equals higher concentration).
Post by: itssona on April 20, 2017, 06:58:28 pm
and in say, OH- it's simple since O is -2 and H is +1 so then adding it gives -1 which is the charge
but what about the other ones?
sorry for asking whats probably such a dumb question
Post by: RuiAce on April 20, 2017, 06:59:43 pm
Hey so I have a chem test coming up and we need to know all polyatomic ions and their charges - thing is I remember them but not the charges,Being blunt, you're just going to have to know them. E.g. sulfate is -3
and in say, OH- it's simple since O is -2 and H is +1 so then adding it gives -1 which is the charge
but what about the other ones?
sorry for asking whats probably such a dumb question
Post by: itssona on April 20, 2017, 07:01:34 pm
Being blunt, you're just going to have to know them. E.g. sulfate is -3great -_-
lol thanks for telling me tho, guess there's no easy way out argh XD
Post by: RuiAce on April 20, 2017, 07:02:37 pm
great -_-Well, as dorky as it sounds, sporcle was what I used to memorise them.
lol thanks for telling me tho, guess there's no easy way out argh XD
Post by: itssona on April 20, 2017, 09:24:02 pm
Well, as dorky as it sounds, sporcle was what I used to memorise them.OMG I haven't used sporcle in a long time- perhaps I should use it again!
Dorky but works I suppose ;)
Post by: itssona on April 21, 2017, 10:05:53 am
and would i get marked down by doing so?
Post by: RuiAce on April 21, 2017, 10:09:37 am
hey another dumb question but when naming covalent compounds, if its like N2O, then do we not write dinitrogen? since its diatomic can I just say nitrogen oxide?That causes confusion with NO.
and would i get marked down by doing so?
NO is nitrogen monoxide
N2O is dinitrogen monoxide.
Post by: itssona on April 21, 2017, 10:14:55 am
That causes confusion with NO.ohhhhhhhhhhhhh okay, thank you :)
NO is nitrogen monoxide
N2O is dinitrogen monoxide.
Post by: Kekemato_BAP on April 21, 2017, 02:08:40 pm
Hey! There is a rule that a metal + water will result in a metal oxide + hydrogen gas. So, in this case
Doesn't a metal + water reaction produce a metal hydroxide + H2?
Calcium should react with water to produce calcium hydroxide and hydrogen.
Ca+ 2H2O -> Ca(OH)2 + H2
Post by: jakesilove on April 21, 2017, 05:01:39 pm
Doesn't a metal + water reaction produce a metal hydroxide + H2?
Calcium should react with water to produce calcium hydroxide and hydrogen.
Ca+ 2H2O -> Ca(OH)2 + H2
That is absolutely correct, no idea why I went with a Metal oxide rather than a Metal hydroxide.
Post by: itssona on April 21, 2017, 05:03:40 pm
but can someone explain the half equations for aluminium + dilute nitric acid?
I understand the aluminium part but then we have this? 2H+ + 2e– → H2
also please tell me any rules like for acids and others, thank you!
Post by: anotherworld2b on April 22, 2017, 12:47:00 pm
I was wondering how could I further explain why the equivalence point is in the acidic region?
I have been trying to write a procedure to find the acidity of white wine. We are required to write a step by step method, which allows someone to determine the total acidity of wine. Expressing the final determination of total acidity of the wine in grams per litre (g.L-1) Include the proposed calculations in this plain, using x and multiples of x to represent unknown value/s. Assume the main and only contribution of the acidic properties of wine is tartaric acid.
This is what I have so far but I'm not sure if I'm doing it correctly or validly.
Materials:
•White wine
•200ml o a standardised hydroxide solution
•Calibrated pH meter
•50mL burette
•Retort stand and burette clamp
•150mL beaker
•250mL conical flask or beaker
•1 small label to label the 150mL beaker
•Wash bottle with distilled water
Procedure:
1.Rinse the 150mL Beaker with a small amount of sodium hydroxide solution, empty it, label it and fill it with about 100mL of the sodium hydroxide solution
2.Prepare the burette, then ill it with the sodium hydroxide solution
3.Rinse the reaction vessel (conical flask or 250 mL beaker) with water
4.Weigh the reaction vessel
5.Pour about X mL of white wine into the reaction vessel and reweigh it
6.Add about YmL of water to the reaction vessel so the depth is sufficient to cover the glass electrode o the pH meter
7.Place the pH meter in the reaction vessel and record the pH
8.Place the reaction vessel under the burette and begin the titration, stopping after each milliliter of sodium hydroxide has been added, to record the pH.
9.Continue adding sodium hydroxide until the PH is fairly constant in the basic region
10.Repeat the titration a couple more times with more frequent sampling of pH near the large change in ph
Hey, sorry without looking at the full original question, Question 2 just seems like nonsense (since it looks like they gave you that n(HCl)=0.05 moles?)
The equivalence point depends on the titration itself. If we are titrating a strong acid and a weak base (as here), the equivalence point will be in the acidic zone (ie. pH around 4-5).
We were titrating a strong acid and a weak base, so the equivalence point should be a lower pH. Instead, using Phenolphathein will require the pH to be raised a lot more than necessary. So, LESS acid would be used (as you've noted).
If we use LESS acid to neutralise the same amount of base, but THINK that there are more moles in that small quantity of acid, then we will calculate that the acid is stronger (less volume, but same number of moles, equals higher concentration).
Post by: jakesilove on April 22, 2017, 02:22:52 pm
hey so I think I missed on learning half equations and I understand the basics like Sulfur + 2e- --> S2- which is common logic
but can someone explain the half equations for aluminium + dilute nitric acid?
I understand the aluminium part but then we have this? 2H+ + 2e– → H2
also please tell me any rules like for acids and others, thank you!
Hey! Basically, all you have to think about is which species is oxidising, which species is reducing, and which species is a spectator ion. So, let's look at aluminium and nitric acid
Just as a side note; Aluminium will barely react with Nitric acid. But, that's beside the point.
Now, we look at the equation as a whole. What changes? What stays the same?
Well, we can see that the NO3 ions stays EXACTLY the same. It is in aqueous form, it has a change of -1. So, we can REMOVE it, as it is a 'spectator ion'
Now that we have the net ionic equation, we can easily find the half equations. Clearly, Aluminium oxidises (as the oxidation number increases, it loses electrons).
So, Hydrogen must reduce
And there you have it! Not too difficult, hey?
Post by: jakesilove on April 22, 2017, 02:29:40 pm
thank you for your help :)
I was wondering how could I further explain why the equivalence point is in the acidic region?
I have been trying to write a procedure to find the acidity of white wine. We are required to write a step by step method, which allows someone to determine the total acidity of wine. Expressing the final determination of total acidity of the wine in grams per litre (g.L-1) Include the proposed calculations in this plain, using x and multiples of x to represent unknown value/s. Assume the main and only contribution of the acidic properties of wine is tartaric acid.
This is what I have so far but I'm not sure if I'm doing it correctly or validly.
Materials:
•White wine
•200ml o a standardised hydroxide solution
•Calibrated pH meter
•50mL burette
•Retort stand and burette clamp
•150mL beaker
•250mL conical flask or beaker
•1 small label to label the 150mL beaker
•Wash bottle with distilled water
Procedure:
1.Rinse the 150mL Beaker with a small amount of sodium hydroxide solution, empty it, label it and fill it with about 100mL of the sodium hydroxide solution
2.Prepare the burette, then ill it with the sodium hydroxide solution
3.Rinse the reaction vessel (conical flask or 250 mL beaker) with water
4.Weigh the reaction vessel
5.Pour about X mL of white wine into the reaction vessel and reweigh it
6.Add about YmL of water to the reaction vessel so the depth is sufficient to cover the glass electrode o the pH meter
7.Place the pH meter in the reaction vessel and record the pH
8.Place the reaction vessel under the burette and begin the titration, stopping after each milliliter of sodium hydroxide has been added, to record the pH.
9.Continue adding sodium hydroxide until the PH is fairly constant in the basic region
10.Repeat the titration a couple more times with more frequent sampling of pH near the large change in ph
Your method looks good! Perhaps make it a bit more formal; instead of saying 'a couple more times', say 'Repeat the procedure outlined above 3 times' or something like that. However, you look like you've got the right idea.
Re equivalence point; this is actually something I didn't know myself, so had to look up! With a strong acid and strong base, the equivalence point will be neutral; that just makes sense. But, with a strong acid and a weak base, the reaction forms an ACIDIC SALT. This acidic salt brings down the pH, even as the substances themselves are being neutralised. So, the overall pH will be acidic!
Post by: itssona on April 22, 2017, 03:19:46 pm
Hey! Basically, all you have to think about is which species is oxidising, which species is reducing, and which species is a spectator ion. So, let's look at aluminium and nitric acidOMG I FINALLY GET IT AHHHHHHHHHHHHHHHH
Just as a side note; Aluminium will barely react with Nitric acid. But, that's beside the point.
Now, we look at the equation as a whole. What changes? What stays the same?
Well, we can see that the NO3 ions stays EXACTLY the same. It is in aqueous form, it has a change of -1. So, we can REMOVE it, as it is a 'spectator ion'
Now that we have the net ionic equation, we can easily find the half equations. Clearly, Aluminium oxidises (as the oxidation number increases, it loses electrons).
So, Hydrogen must reduce
And there you have it! Not too difficult, hey?
THANK YOU JAKE
Post by: anotherworld2b on April 22, 2017, 05:27:18 pm
Would my method be able to allow someone to find the acidity of wine?
What kind of method could you use if you wanted to use a titration to find the acidity of wine? Would it be possible?
Your method looks good! Perhaps make it a bit more formal; instead of saying 'a couple more times', say 'Repeat the procedure outlined above 3 times' or something like that. However, you look like you've got the right idea.
Re equivalence point; this is actually something I didn't know myself, so had to look up! With a strong acid and strong base, the equivalence point will be neutral; that just makes sense. But, with a strong acid and a weak base, the reaction forms an ACIDIC SALT. This acidic salt brings down the pH, even as the substances themselves are being neutralised. So, the overall pH will be acidic!
Post by: anotherworld2b on April 22, 2017, 07:28:54 pm
Q3. Why was the anhydrous sodium carbonate placed in the oven?In brief:
The primary standard, Na2CO3 (s) is made anhydrous by heating it in an oven at 260-270 degrees celsius. To prevent the anhydrous Na2CO3 (s) from reabsorbing water vapour from the air, it is kept in a desiccator. Silica gel crystals inside the dessicator keep the air dry by absorbing water vapour.
Post by: Aaron12038488 on April 22, 2017, 08:48:17 pm
Post by: jakesilove on April 23, 2017, 11:18:59 am
thank you for your help :D
Would my method be able to allow someone to find the acidity of wine?
What kind of method could you use if you wanted to use a titration to find the acidity of wine? Would it be possible?
Yep, your method will do the trick! To find the acidity of a substance, you need to titrate it. You also need to know the chemical making the wine acidic, so that you can write out the chemical formula. From there, you carry out the usual mathematical steps to find the concentration (and therefore pH) of the wine.
As for your second question, you haven't really explained WHY it is placed in an oven; merely how it is heated up/kept dry etc. Obviously, you know the answer; it is to ensure the substance doesn't absorb water, affecting the purity and the way in which you measure to mass/moles used to create a standard solution. However, you haven't directly answered the question.
Post by: jakesilove on April 23, 2017, 11:19:31 am
is it possible if I post up a past preliminary paper with my answers, and receive marking?
Hey! Probably not an entire paper, but feel free to chuck up a few questions you're not sure about, and I'll take a look at your answers!
Post by: Alalamc on April 23, 2017, 03:47:55 pm
Metal A reacts with B2+ but does not react with C2+.
Metal D reacts with A2+.
Only B does not react with H+ to form H2 gas.
hey guys I know this may seem reasonable simply but I havent come across this type of question before
Post by: RuiAce on April 23, 2017, 04:10:40 pm
Draw up an electrochemical series consisting of metals A, B, C and D and their ions and H+ and H2 given the following information:A reacts with B2+ so A is more reactive than B. (Because A will turn into A2+ and B2+ will go back to B.)
Metal A reacts with B2+ but does not react with C2+.
Metal D reacts with A2+.
Only B does not react with H+ to form H2 gas.
hey guys I know this may seem reasonable simply but I havent come across this type of question before
A does not react with C2+ so C is more reactive with A. (Conversely)
C > A > B
D reacts with A2+ so D is also more reactive than A. (D gets oxidised to become D2+ whereas A2+ is reduced.)
B does not react with H+ to become B2+, but A will react with H+ to become A2+
So the reactivity of H is between A and B
C > A > H > B
D > A > H > B
D is also more reactive than A but we aren't told any information to compare between D and C to properly rank them.
(I just hope I did not do this in reverse.)
Post by: mcheema on April 23, 2017, 07:28:55 pm
Ive noticed that majority of the exams and quizzes I do I mainly get half equations wrong.
Can you please provide some tips and explain how to do half equations
Thanks
Post by: Kekemato_BAP on April 23, 2017, 11:40:52 pm
Hi,
Ive noticed that majority of the exams and quizzes I do I mainly get half equations wrong.
Can you please provide some tips and explain how to do half equations
Thanks
Hi, this is how I do all my half equations and net ionics. We'll use calcium with hydrochloric acid as an example:
1: Write the equation first. Ca + 2HCl -> CaCl2 + H2
2: Write the oxidation equation for the calcium metal. Ca -> Ca2+ + 2e-
3: Write the reduction equation for the hydrogen ions from the HCl. 2H+ + 2e- -> H2
4: The net ionic equation is basically the two half equations combined but minus the electrons. Ca + 2H+ -> Ca2+ + H2
That's how I do it.
Post by: anotherworld2b on April 24, 2017, 12:04:01 am
Yep, your method will do the trick! To find the acidity of a substance, you need to titrate it. You also need to know the chemical making the wine acidic, so that you can write out the chemical formula. From there, you carry out the usual mathematical steps to find the concentration (and therefore pH) of the wine.
As for your second question, you haven't really explained WHY it is placed in an oven; merely how it is heated up/kept dry etc. Obviously, you know the answer; it is to ensure the substance doesn't absorb water, affecting the purity and the way in which you measure to mass/moles used to create a standard solution. However, you haven't directly answered the question.
Post by: mcheema on April 24, 2017, 02:54:24 am
Hi, this is how I do all my half equations and net ionics. We'll use calcium with hydrochloric acid as an example:
1: Write the equation first. Ca + 2HCl -> CaCl2 + H2
2: Write the oxidation equation for the calcium metal. Ca -> Ca2+ + 2e-
3: Write the reduction equation for the hydrogen ions from the HCl. 2H+ + 2e- -> H2
4: The net ionic equation is basically the two half equations combined but minus the electrons. Ca + 2H+ -> Ca2+ + H2
That's how I do it.
Thanks for the help. Yes that makes more sense now
Post by: jakesilove on April 24, 2017, 12:10:04 pm
I just had another read of my assignment. I'm not sure what calculations could be used to determine tartaric acid concentration in wine using this method to be honest. I'm sorry for asking so many questions. I don't really understand titrations very well.
Hey!
So, we're titrating Tartaric acid and Sodium hydroxide. We have the concentration (and thus moles) of Sodium hydroxide; and we can find the volume (and thus concentration) of Tartaric acid using the titration process. First, we need a chemical equation.
I had to google this. From here, we can do straight forward titration calculations to find the concentration of acid. Find the number of moles of NaOH used, and halve it to get the number of moles of Tartaric acid. Finally, divide the moles of acid by the volume used to get the concentration.
Post by: anotherworld2b on April 24, 2017, 03:41:35 pm
I was also wondering how do you choose the correct indicator for a tit ration involving sodium hydroxide and hydrochloric acid?
methyl orange or bromophenol blue?
Hey!
So, we're titrating Tartaric acid and Sodium hydroxide. We have the concentration (and thus moles) of Sodium hydroxide; and we can find the volume (and thus concentration) of Tartaric acid using the titration process. First, we need a chemical equation.
I had to google this. From here, we can do straight forward titration calculations to find the concentration of acid. Find the number of moles of NaOH used, and halve it to get the number of moles of Tartaric acid. Finally, divide the moles of acid by the volume used to get the concentration.
Post by: jakesilove on April 24, 2017, 05:43:30 pm
thank you for your help :D
I was also wondering how do you choose the correct indicator for a tit ration involving sodium hydroxide and hydrochloric acid?
methyl orange or bromophenol blue?
Strong acid + strong base will have an end point that's about neutral. So, we use Bromothymol Blue
Post by: anotherworld2b on April 24, 2017, 06:38:53 pm
Post by: armtistic on April 24, 2017, 06:48:00 pm
I was wondering how do you do this question?
You already found the moles of CaO and now you're finding them again in your last step when you should be using m = n x mm to find mass
Post by: anotherworld2b on April 24, 2017, 07:07:29 pm
Thank you very much for your help :D
You already found the moles of CaO and now you're finding them again in your last step when you should be using m = n x mm to find mass
Post by: anotherworld2b on April 24, 2017, 07:28:43 pm
Post by: bsdfjnlkasn on April 24, 2017, 10:49:54 pm
I was wondering if someone could help me with the calculations in the following question. Specifically the "alternate calibration method" where you don't use the calibration curve. I haven't been formally taught this yet and the textbook explanation is a bit confusing so any help would be greatly appreciated :) :)
EDIT: I also have another question - Why does afternoon peak hour traffic have much less effect on city ozone concentrations than the morning peak?
From my textbook, I seem to be under the impression that the ozone concentration is higher in the afternoon because over the day, the NOx which are initially emitted by car exhausts in the morning peak, are oxidised and then photodissociated into free radical which then react with O2. Am I missing a step where the ozone just randomly disappears because that seems to be the only missing piece that would allow me to properly explain what the question implies.
Post by: jakesilove on April 25, 2017, 10:27:17 am
I was trying to solve this question but I'm not sure what to do. Can i have some help please?
This is just a standard titration question. Find the chemical equation. Find the number of moles of the known substance. Use the chemical equation to divide/multiply this value to get the moles of the unknown. Then, divide my the volume to get the concentration
Post by: jakesilove on April 25, 2017, 10:32:12 am
Hey there,
I was wondering if someone could help me with the calculations in the following question. Specifically the "alternate calibration method" where you don't use the calibration curve. I haven't been formally taught this yet and the textbook explanation is a bit confusing so any help would be greatly appreciated :) :)
EDIT: I also have another question - Why does afternoon peak hour traffic have much less effect on city ozone concentrations than the morning peak?
From my textbook, I seem to be under the impression that the ozone concentration is higher in the afternoon because over the day, the NOx which are initially emitted by car exhausts in the morning peak, are oxidised and then photodissociated into free radical which then react with O2. Am I missing a step where the ozone just randomly disappears because that seems to be the only missing piece that would allow me to properly explain what the question implies.
Hey! I'm not sure what you mean by the 'alternate calibration' method; as far as I can tell, you need to sketch a calibration curve using the standard solutions (ie Absorbance vs. ppm) and then find on that graph where the absorbance is 0.32, 0.46, 0.21 and 0.055 to find the ppm value of each of the solutions A,B,C and D. Does that kind of make sense? First, you'll need to convert each standard value into ppm. Then, plot it against absorbance and join a line of best fit.
As for your second question, that's way, way outside of the curriculum and is definitely something that you DO NOT need to know. I had no idea that ozone concentrations were higher in the afternoon. It is important to note that Ozone can be formed by combustion in car exhausts, but beyond that you seriously will never need to know any of the information you've described above.
Post by: anotherworld2b on April 25, 2017, 12:40:34 pm
This is just a standard titration question. Find the chemical equation. Find the number of moles of the known substance. Use the chemical equation to divide/multiply this value to get the moles of the unknown. Then, divide my the volume to get the concentration
Post by: beau77bro on April 25, 2017, 03:51:49 pm
Post by: itssona on April 25, 2017, 05:49:35 pm
Metals high up in the reactivity series (such as sodium, calcium and magnesium) have carbonates that need a lot of energy to decompose them.
?
If metals that are more reactive need more energy to decompose, then that means they have stronger bonds..but how?
Thank you :)
Post by: jakesilove on April 25, 2017, 05:55:32 pm
Hey would anyone be able to explain this:
Metals high up in the reactivity series (such as sodium, calcium and magnesium) have carbonates that need a lot of energy to decompose them.
?
If metals that are more reactive need more energy to decompose, then that means they have stronger bonds..but how?
Thank you :)
Hey! You've confused the ELEMENT, and the associated COMPOUND.
Sodium is very, very reactive. However, NaCl (table salt) is very unreactive! This makes sense; since Sodium is so react, it will try really, really hard to stay as an unreactive compound, rather than decompose into its constituent elements. So, a lot of energy is required to force the stable Sodium to become unstable (ie. to go from a compound back to an element).
Does that kind of make sense? A lot of energy is required to break intermolecular forces in a compound containing reactive elements, BECAUSE the element is reactive and wants to stay bonded!
Post by: itssona on April 25, 2017, 05:58:55 pm
Hey! You've confused the ELEMENT, and the associated COMPOUND.OMFG THANK YOU SO MUCH DJFSISK
Sodium is very, very reactive. However, NaCl (table salt) is very unreactive! This makes sense; since Sodium is so react, it will try really, really hard to stay as an unreactive compound, rather than decompose into its constituent elements. So, a lot of energy is required to force the stable Sodium to become unstable (ie. to go from a compound back to an element).
Does that kind of make sense? A lot of energy is required to break intermolecular forces in a compound containing reactive elements, BECAUSE the element is reactive and wants to stay bonded!
Post by: jakesilove on April 25, 2017, 09:49:10 pm
OMFG THANK YOU SO MUCH DJFSISK
No worries; totally understand why you thought what you though!
Post by: anotherworld2b on April 25, 2017, 10:48:19 pm
Post by: MisterNeo on April 26, 2017, 02:28:57 pm
Hi i attempted to do these two questions but I'm having some trouble
Hi!! I see what you did wrong in Q15. The equation you wrote was correct but your calculations didn't take into account that NaOH was 2 moles for each H2SO4. Thus, your calculations needed to divide moles of NaOH by 2.
I have attached my calculations for Q16. Hope this helps!! Feel free to ask for any clarification.. :D
Post by: ellipse on April 27, 2017, 06:18:46 pm
So in most of the titration methods i read, they wash the conical flask with distill water, but they don't dry it or something. Isn't this inaccurate? Because when we transfer 25 ml of acid (or base) into it, the volume will increase (due to the left over distill water) but the moles of the acid (or base) will be the same, thus leading to a lower concentration of the acid (or base) than what was intended. So when we calculate the concentration, we will get a different answer than what was intended.
Could someone clarify this please
Post by: jakesilove on April 27, 2017, 07:00:16 pm
Would someone please help me with a titration doubt please
So in most of the titration methods i read, they wash the conical flask with distill water, but they don't dry it or something. Isn't this inaccurate? Because when we transfer 25 ml of acid (or base) into it, the volume will increase (due to the left over distill water) but the moles of the acid (or base) will be the same, thus leading to a lower concentration of the acid (or base) than what was intended. So when we calculate the concentration, we will get a different answer than what was intended.
Could someone clarify this please
Hey!
Once we've transferred the solution to the conical flask, the concentration no longer matters. The important thing is the number of moles in the flask for the Base to neutralise. Again, the concentration is totally irrelevant at this stage. We need to figure out the number of moles required of base to neutralise the number of moles of acid present in the flask. For this, we use the chemical formula etc. and carry out the usual steps.
I could pour as much water (if it's pH was exactly 7) into the conical flask as I want! The amount of base required to neutralise it would be the same.
Post by: jakesilove on April 27, 2017, 07:04:07 pm
Hey!
Once we've transferred the solution to the conical flask, the concentration no longer matters. The important thing is the number of moles in the flask for the Base to neutralise. Again, the concentration is totally irrelevant at this stage. We need to figure out the number of moles required of base to neutralise the number of moles of acid present in the flask. For this, we use the chemical formula etc. and carry out the usual steps.
I could pour as much water (if it's pH was exactly 7) into the conical flask as I want! The amount of base required to neutralise it would be the same.
I don't think I explained myself very well
Let me know if you need me to clarify!
Post by: anotherworld2b on April 27, 2017, 07:16:35 pm
I was just wondering
We are required to write a step by step method, which allows someone to determine the total acidity of wine. Expressing the final determination of total acidity of the wine in grams per litre (g.L-1) Include the proposed calculations in this plain, using x and multiples of x to represent unknown value/s. Assume the main and only contribution of the acidic properties of wine is tartaric acid.
I rewrote the method. I was wondering if someone could have a look at it please?
I also tried to the calculations which will determine tartaric acid concentration in wine but I'm not sure how to find the total acidity of the wine in grams per litre.
Hey!
So, we're titrating Tartaric acid and Sodium hydroxide. We have the concentration (and thus moles) of Sodium hydroxide; and we can find the volume (and thus concentration) of Tartaric acid using the titration process. First, we need a chemical equation.
I had to google this. From here, we can do straight forward titration calculations to find the concentration of acid. Find the number of moles of NaOH used, and halve it to get the number of moles of Tartaric acid. Finally, divide the moles of acid by the volume used to get the concentration.
Post by: bsdfjnlkasn on April 30, 2017, 03:38:32 pm
(http://uploads.tapatalk-cdn.com/20170430/95e924f48a4370a5428c1e520934be78.jpg)
Post by: RuiAce on April 30, 2017, 03:49:17 pm
Hey there! I sort of guessed my way through this question but can't find any solid answers online, all there is are marking guidelines but I'm not finding them too helpful because I'm stuck. If someone could check my part a) and explain why it's those two solutions as well as an approach for q17 I'd really appreciate it. Thank you!!I actually disagree with the answers. I think the reaction between solutions 1 and 2 is this one
(http://uploads.tapatalk-cdn.com/20170430/95e924f48a4370a5428c1e520934be78.jpg)
Post by: anotherworld2b on April 30, 2017, 04:54:05 pm
Post by: anotherworld2b on April 30, 2017, 05:32:49 pm
I was trying to do this question. But i havent dealt with density before so I'm not sure how to start.
This is how I did it. Find moles of HCl used, then minus the excess by finding moles of NaOH used to neutralise. Use that final HCl used to find the moles of CaCO3 reacted. This ends up being 0.00975mol. Multiply that by the MM of CaCO3 to get 97.5%, which is divided by 1g of calcite which is also 97.5%, and it rounds up to 98% like the answer says.
Hope this makes sense. Feel free to ask for clarification :)
Post by: MisterNeo on April 30, 2017, 05:35:36 pm
Hey there! I sort of guessed my way through this question but can't find any solid answers online, all there is are marking guidelines but I'm not finding them too helpful because I'm stuck. If someone could check my part a) and explain why it's those two solutions as well as an approach for q17 I'd really appreciate it. Thank you!!
Hey there! The answers are in the pic below. The first reaction is between a metal carbonate and an acid because it produces bubbles of CO2.
Solution 3-4 requires you to know your solubility rules as they form precipitates. PbCl2 and BaCO3 are insoluble.
Hope this helps :)
Post by: Shadowxo on May 03, 2017, 07:51:55 pm
I see now thank you for your help :)
I was trying to do this question. But i havent dealt with density before so I'm not sure how to start.
Hi :)
So in this question, it gives you the density (grams per mL) and volume (mL) so you multiply them together to get the mass (grams)
mass = 19.6*1.67 = 32.732g
The thing is, it's not pure sulphuric acid (it has an unknown concentration) so you can't continue like Neo and use n=m/M as some of that mass is water.
With the NaOH:
n(NaOH)=cV = 0.390*0.0126 = 0.004914 mol
Using the equation H2SO4 + 2NaOH => Na2SO4 + 2H2O
n(H2SO4) in 25mL = 1/2 * n(NaOH) = 2 * 0.004914 = 0.002457
n(H2SO4) in 1L = n(H2SO4) in original solution = 0.002457 * 40 = 0.09828
Now convert that into mass using m=n * M = 0.09828 * 98.08 = 9.639g
% mass = m(pure sulphuric acid) / m(original solution) = 9.639 / 32.732 * 100% = 29.45% = 29.4%
I haven't taken into account sig figs and my wording may not be the best but this is the solution :)
Post by: Shadowxo on May 03, 2017, 11:02:08 pm
Thanks for pointing that out! Didn't know why I did that.. :-[
It's an easy mistake to make as sulphuric acid can be liquid (so all sulphuric acid) or aqueous (diluted). Just have to look at the question and what information they do/don't give.
Post by: RuiAce on May 04, 2017, 08:48:20 am
Post by: RuiAce on May 04, 2017, 08:54:23 am
It's an easy mistake to make as sulphuric acid can be liquid (so all sulphuric acid) or aqueous (diluted). Just have to look at the question and what information they do/don't give.Well, whilst we do take it as a liquid it isn't actually "all" acid. It's just concentrated to the point it may as well be (don't remember exactly how concentrated but it's at least 98%, which basically becomes 18M). It's better to say "almost all" acid.
Post by: bsdfjnlkasn on May 06, 2017, 09:29:21 pm
I was wondering if I could get some solid feedback for this 4 marker, I typed it up as it's neater and the HSC success one book I have, doesn't include papers between 2004 and 2008. I can't seem to find any concrete answers online and so would appreciate a mark estimate and areas for improvement as I'm sure I haven't included everything that I should have. Were we required to write two points for each detail asked? So two details of the method and two more for the trends?
Any advice would be greatly appreciated, thank you!!
Post by: anotherworld2b on May 07, 2017, 03:46:40 pm
Post by: jakesilove on May 08, 2017, 09:18:59 am
Hey there,
I was wondering if I could get some solid feedback for this 4 marker, I typed it up as it's neater and the HSC success one book I have, doesn't include papers between 2004 and 2008. I can't seem to find any concrete answers online and so would appreciate a mark estimate and areas for improvement as I'm sure I haven't included everything that I should have. Were we required to write two points for each detail asked? So two details of the method and two more for the trends?
Any advice would be greatly appreciated, thank you!!
Hey! Thanks for posting up your solution, always happy to mark things like this :)
I would break up the marks into two sections; firstly, 'explaining' how the information was obtained (one mark) and secondly, outlining changes that have occurred (three marks).
You've clearly and easily received the first mark. No comment there except to say that it's potentially a little long (if you hand write those first three sentences, that's potentially a whole paragraph!).
Now, moving onto the more interesting part. I would have used subheadings to separate the two sections; I would recommend you get used to doing things like that!
Whilst outside of the timeframe, the natural first sentence would have been something like 'Ozone had been severely depleted above Antarctica due to the use of CFCs'. This sets you up for everything else you're about to say, and just makes logical sense to begin with.
Including the date of the Montreal Protocol is brilliant! I like how you've worked in specific detail, that's really important with questions like this. You've clearly discussed natural Ozone depletion (not how I would have done it, but totally legit), but explained that CFCs were more significant. The rest of your paragraph is great, and clearly explains changes over time.
I think there are a few areas of improvement;
See above re first sentence and logical flow
You could easily have improved the specific detail (when were CFCs banned? What were they replaced with? A throw away line on all this is plenty)
You could easily have improved the inclusion of chemical detail (Why was Ozone depleted by CFCs? Relevant equations?)
Basically, I think this could easily be a three out of four, or a four out of four. However, by including the above suggestions, it will definitely be a four out of four!
Post by: jakesilove on May 08, 2017, 09:20:58 am
Can I have some help with these 2 questions please? I'm not sure if I'm doing them correctly
The first one is absolutely right! No redox there.
Second one is also absolutely right! No redox either.
Post by: bsdfjnlkasn on May 08, 2017, 06:52:36 pm
Hey! Thanks for posting up your solution, always happy to mark things like this :)
I would break up the marks into two sections; firstly, 'explaining' how the information was obtained (one mark) and secondly, outlining changes that have occurred (three marks).
You've clearly and easily received the first mark. No comment there except to say that it's potentially a little long (if you hand write those first three sentences, that's potentially a whole paragraph!).
Now, moving onto the more interesting part. I would have used subheadings to separate the two sections; I would recommend you get used to doing things like that!
Whilst outside of the timeframe, the natural first sentence would have been something like 'Ozone had been severely depleted above Antarctica due to the use of CFCs'. This sets you up for everything else you're about to say, and just makes logical sense to begin with.
Including the date of the Montreal Protocol is brilliant! I like how you've worked in specific detail, that's really important with questions like this. You've clearly discussed natural Ozone depletion (not how I would have done it, but totally legit), but explained that CFCs were more significant. The rest of your paragraph is great, and clearly explains changes over time.
I think there are a few areas of improvement;
See above re first sentence and logical flow
You could easily have improved the specific detail (when were CFCs banned? What were they replaced with? A throw away line on all this is plenty)
You could easily have improved the inclusion of chemical detail (Why was Ozone depleted by CFCs? Relevant equations?)
Basically, I think this could easily be a three out of four, or a four out of four. However, by including the above suggestions, it will definitely be a four out of four!
Awesome Jake! Thanks for all this incredible detail, it's really helped guide me through questions of a similar mark allocation :) How would you have described ozone depletion? Would the whole process of chlorine free radicals decomposing ozone into oxygen be necessary to write equations for? Also, more of a general question, when providing equations in answers, how much should we explain what is going on because often it seems that the equations speak for themselves - and obviously you don't want to be wasting time repeating yourself :P
Thanks again for your help, I really appreciate it!!
Post by: jakesilove on May 09, 2017, 10:22:58 am
Awesome Jake! Thanks for all this incredible detail, it's really helped guide me through questions of a similar mark allocation :) How would you have described ozone depletion? Would the whole process of chlorine free radicals decomposing ozone into oxygen be necessary to write equations for? Also, more of a general question, when providing equations in answers, how much should we explain what is going on because often it seems that the equations speak for themselves - and obviously you don't want to be wasting time repeating yourself :P
Thanks again for your help, I really appreciate it!!
Most of the time, the equation will speak for itself! In a question like this, I don't think you need to run through the whole CFC process, maybe just include the overall reaction? ie. Ozone + Oxygen free radical --> 2 Oxygen etc.
Post by: Alalamc on May 09, 2017, 07:51:45 pm
1)
The molar enthalpy of combustion of camphor, C10H16O, is -5898 kJ/mol. How much energy is released on combustion of 1 gram of camphor?
2) Determine the enthalpy of reaction when 1 mole of ethene reacts with oxygen to form carbon dioxide and water. Using the bond energies (attached)
Thankyou guys !!!
Post by: jakesilove on May 10, 2017, 12:25:27 pm
Hey Guys these 2 questions are really bugging me can someone EXPLAIN them to me :)
1)
The molar enthalpy of combustion of camphor, C10H16O, is -5898 kJ/mol. How much energy is released on combustion of 1 gram of camphor?
2) Determine the enthalpy of reaction when 1 mole of ethene reacts with oxygen to form carbon dioxide and water. Using the bond energies (attached)
Thankyou guys !!!
Hey! Here's how we work out the first one. We know that the heat of combustion is
But, we want the answer to be in kJ/g, right? To get from kJ/mol to kJ/g, we would need to do something like this
Cool! So, what do we know that has units of mol/g (or, rather, g/mol and then we can just divide it)?. The molar mass! The molar mass of Camphor is
So,
A more intuitive way of doing this is thinking about what kJ per mol actually means. It means kJ per 152.23 grams! So, just divide by the grams to get kJ per gram :)
Now, onto your second, question, which I really don't know about WAY OUTSIDE OF HSC
It looks like we do it like this. Our equation is
Okay. Now it gets tough. In Ethene, there is one double-bonded Carbon-Carbon bond, and four single-bonded Carbon-Hydrogen bond. From the formula sheet, the total bond energy will be
As there is only one mole, the bond energy of Ethene is 2276 kJ. In Oxygen, there is one double-bonded Oxygen-Oxygen bond.
As there are three moles, the bond energy will be 1497kJ.
Now, we need to look at the products. Carbon dioxide has two double-bonded Carbon-Oxygen bonds. From the formula sheet
Since there are two moles, the bond energy will be 3196 kJ. In Water, there are two single-bonded Hydrogen-Oxygen bonds. Thus,
As there are two moles of water, the bond energy will be 1840 kJ.
Adding up all the reactant energies, we get 3773 kJ. Adding up all the product energies, we get 5036 kJ. To find the enthalpy change, we subtract the products from the reactants, giving us
Note that I've never done this before, so I may be very wrong.
Post by: anotherworld2b on May 10, 2017, 10:48:24 pm
Post by: jakesilove on May 11, 2017, 09:54:56 am
Hi i was wondering if I could have some help finding the oxidation numbers for this question please.
Hey! Not sure which question in particular you're looking for help with; could you provide some working out, so I can point you in the right direction?
Post by: Fahim486 on May 12, 2017, 11:18:38 pm
Cheers!
Post by: MisterNeo on May 13, 2017, 09:01:06 am
Hey so I was just wondering is the precipitation test only for cations or can it also be or anions? Because recently I did a prac in class to test for anions and so after adding 5 drops of HNO3 followed by 5 drops of Ag+ to my phosphate anion, I got a yellow precipitate but i'm not sure whether that identified my silver cation or phosphate anion.
Cheers!
Hi!
The precipitation test can be used for both cations and anions.
Cations can be identified using:
-Flame test
-Atomic Absorption Spectroscopy
-Precipitation
Anions can be identified using:
-Precipitation
Precipitating Anions
For example, you have two unknown solutions of sulfate ions and chloride ions but you forgot which is which.
How do you know which anion is present?
Add a barium nitrate solution to the both solutions and the precipitating solution is sulfate. Barium sulfate is INSOLUBLE and will precipitate as a white solid.
Precipitating Cations
Another example, you have two unknown solutions of sodium ions and calcium ions, how do you identify them?
Add a sulfate solution to both and the precipitating solution is calcium. All Group 1 are soluble thus sodium will remain in solution. Calcium sulfate is insoluble.
Your school prac
The solution started as a phosphate solution only I assume.
Adding nitric acid would introduce nitrate ions to the solution, no biggie, all nitrates are soluble.
Adding silver ions would precipitate the phosphate ions because it forms a yellow precipitate. You identified the anion present as phosphate. The silver ion is a known solution that you added, so it was used to identify phosphate not the other way around.
I hope this clears thing up :)
Post by: Fahim486 on May 13, 2017, 02:56:52 pm
Hi!
The precipitation test can be used for both cations and anions.
Cations can be identified using:
-Flame test
-Atomic Absorption Spectroscopy
-Precipitation
Anions can be identified using:
-Precipitation
Precipitating Anions
For example, you have two unknown solutions of sulfate ions and chloride ions but you forgot which is which.
How do you know which anion is present?
Add a barium nitrate solution to the both solutions and the precipitating solution is sulfate. Barium sulfate is INSOLUBLE and will precipitate as a white solid.
Precipitating Cations
Another example, you have two unknown solutions of sodium ions and calcium ions, how do you identify them?
Add a sulfate solution to both and the precipitating solution is calcium. All Group 1 are soluble thus sodium will remain in solution. Calcium sulfate is insoluble.
Your school prac
The solution started as a phosphate solution only I assume.
Adding nitric acid would introduce nitrate ions to the solution, no biggie, all nitrates are soluble.
Adding silver ions would precipitate the phosphate ions because it forms a yellow precipitate. You identified the anion present as phosphate. The silver ion is a known solution that you added, so it was used to identify phosphate not the other way around.
I hope this clears thing up :)
That was a great explanation. Thanks so much!!!!
Post by: marioedd on May 14, 2017, 04:12:33 pm
Need help with part d.
Thanks in advance
Post by: MisterNeo on May 14, 2017, 07:21:05 pm
(http://uploads.tapatalk-cdn.com/20170514/8c1782086e971f40f3762e444d820f12.jpg)
Need help with part d.
Thanks in advance
Hi!
If the concentration of the sodium chloride was too low, the chlorine gas will not form at the anode but the hydroxide ions will become discharged to give water and oxygen gas. This is because hydroxide is favoured over chlorine as chlorine ions drops in concentration due to reduced sodium chloride.
Also, for Part C, you need 2 chlorine ions as you only wrote one.
Hope this helps!! Sorry for the late reply, I spent like 30mins learning how to type equations :)
Post by: arunasva on May 14, 2017, 08:50:01 pm
Post by: RuiAce on May 14, 2017, 08:53:23 pm
Ok I am so stuck. How are there 4 sample when there were 3 divisions made ? Overall just stuck man. Please help, Ill be grateful ;D ;D ;D ;D ;D ;DThe first three are standardized, i.e. those concentrations were known from the start. The other four samples are random samples you've obtained and know nothing about their concentration. There's a difference between known concentrations and unknown concentrations; you don't have to have the same number of known and unknown concentrations.
To draw the calibration curve, you first plot the standard concentration against absorbance (may be other way around; I've forgotten the right order), and draw the line of best fit through only those three points and (0,0). Then, you use your graph to infer the values of A, B, C and D based off the given absorbances.
Post by: legorgo18 on May 15, 2017, 07:43:10 pm
Why is the water flow in microscopic membrane filters parallel to the surface rather than perpendicular?
a) The holes in the polymer membrane are produced at an angle to the surface.
b) In fact, the water flow is perpendicular to increase pressure.
c) It increases the rate of flow.
d) The water pressure would be too high, clogging the pores up.
Tyvm :)
Post by: Kekemato_BAP on May 16, 2017, 09:45:51 am
(http://i.imgur.com/NEXfXBT.jpg)
Post by: RuiAce on May 16, 2017, 09:56:57 am
Could someone explain how to do calibration curves for AAS? This question is new content for me and is confusing. :-[This same question was posted only a few posts ago. Please check out that post first and then come back for further help
(http://i.imgur.com/NEXfXBT.jpg)
Post by: marioedd on May 16, 2017, 05:13:26 pm
Hi!Thanks so much
If the concentration of the sodium chloride was too low, the chlorine gas will not form at the anode but the hydroxide ions will become discharged to give water and oxygen gas. This is because hydroxide is favoured over chlorine as chlorine ions drops in concentration due to reduced sodium chloride.
Also, for Part C, you need 2 chlorine ions as you only wrote one.
Hope this helps!! Sorry for the late reply, I spent like 30mins learning how to type equations :)
Post by: beau77bro on May 17, 2017, 10:58:51 pm
Post by: beau77bro on May 17, 2017, 11:00:00 pm
im really struggling with monitoring and management because im not sure how much of it to know or not. it seems ridiculous memorise heavy. i have atarnotes, would i be correct in saying i would need to memorise a majority of that info for the course? i need some direction to continue as i am lacking much direction. like im reading all of this info and its going in one ear and out the other because i dont know what is actually important.
Post by: MisterNeo on May 17, 2017, 11:49:37 pm
im really struggling with monitoring and management because im not sure how much of it to know or not. it seems ridiculous memorise heavy. i have atarnotes, would i be correct in saying i would need to memorise a majority of that info for the course? i need some direction to continue as i am lacking much direction.
Hey! Glad you asked!
Monitoring and management can be quite content heavy and requires some degree of memorising but also being able to apply the knowledge.
Things you should memorise/be familiar with
-Solubility rules are a must for identifying cations and anions.
-Flame test colours of the syllabus elements (Ca, Ba, Pb, Fe, Cu, Li, etc)
-Equilibrium conditions of the Haber process. How to maximise yield of ammonia. How to get the reactants. Why does the process have to be carefully monitored. History and Germany uses. Haber process is a big chunk of memorising really
-CFC's, we haven't done this yet, but you need to memorise the layers of the atmospheres and the conditions of each layer. Also equations of ozone depletion by free radical.
-Also, water quality. Turbidity, hardness, etc.
So yeah, the topic does have a lot of memorising involved. Howeve, Band 4-5 students know how to memorise the course whereas Band 6 know how to apply the information effectively. You catch my drift? ;)
Hope this helps :)
Post by: seventeenboi on May 18, 2017, 09:56:57 am
What is the purpose of using standard solutions in atomic absorption spectroscopy???
Thanks :)
Post by: jakesilove on May 18, 2017, 10:35:14 am
Hi!
What is the purpose of using standard solutions in atomic absorption spectroscopy???
Thanks :)
Hey! In essence, you need something to compare your finding to!
Say that you spray a solution containing 1ppm of some metal into the AAS. It tells you the absorbance is some value, let's call it X. So what? How can you tell from that absorbance what the ppm value of the metal is?
You have to compare it to a standard curve. So, before doing our experiment, we spray a liquid with a known concentration of the metal. We spray 0ppm, 0.5ppm, 1ppm and 1.5ppm (for instance). By sketching a standard curve function, we can compare our unknown solution to the known solution!
Post by: seventeenboi on May 18, 2017, 02:41:28 pm
Hey! In essence, you need something to compare your finding to!
Say that you spray a solution containing 1ppm of some metal into the AAS. It tells you the absorbance is some value, let's call it X. So what? How can you tell from that absorbance what the ppm value of the metal is?
You have to compare it to a standard curve. So, before doing our experiment, we spray a liquid with a known concentration of the metal. We spray 0ppm, 0.5ppm, 1ppm and 1.5ppm (for instance). By sketching a standard curve function, we can compare our unknown solution to the known solution!
oooohhhh thanks so much! :)
Post by: scienceislife on May 18, 2017, 05:34:27 pm
b) This acid solution was used to standardise an approximately 0.02mol/L NaOH solution. 20.00mL of NaOH required 20.61mL of the oxalic acid solution to neutralise it. Calculate the exact concentration of the NaOH solution.
c) The standard NaOH solution was then used to standardise an approximately 0.02mol/L HCl solution. 20.00mL of the NaOH solution was placed in a conical flask. The HCl solution was placed in the burette and the initial and final readings were 1.83mL and 20.26mL respectively. Calculate the concentration of the HCl solution.
Post by: jakesilove on May 18, 2017, 05:47:30 pm
a) 0.1362 of oxalic acid was dissolved in water and the solution made up to 250.0mL. Calculate the concentration of the solution.
b) This acid solution was used to standardise an approximately 0.02mol/L NaOH solution. 20.00mL of NaOH required 20.61mL of the oxalic acid solution to neutralise it. Calculate the exact concentration of the NaOH solution.
c) The standard NaOH solution was then used to standardise an approximately 0.02mol/L HCl solution. 20.00mL of the NaOH solution was placed in a conical flask. The HCl solution was placed in the burette and the initial and final readings were 1.83mL and 20.26mL respectively. Calculate the concentration of the HCl solution.
Hey! I assume you left the 'grams' off the first number :)
To answer part a), we just need the formula
where 'n' is moles and 'V' is volume in Litres. First, we calculate moles by
Oxalic acid is
and has a molar mass of 90.03g. Thus,
Now, we convert from mL to litres. 250mL is 0.25L. So
Now, onto part b). We know that the neutralisation reaction is
Thus, two moles of Sodium hydroxide requires one mole of Oxalic acid to react! We use 20.61mL Oxalic acid solution. We can quickly calculate how many moles are present here.
There will be twice as many moles of NaOH, so we have 0.00025 moles of NaOH. Putting this into our concentration equation;
You can use a similar methodology to attack part c)! Let me know if you have any trouble :)
Post by: itssona on May 19, 2017, 08:53:43 pm
So basically besides the HSC syllabus, we need to know the prelim stuff well???
like do we need to know the history and society impact stuff that's in prelim chem?
Post by: MisterNeo on May 19, 2017, 11:27:51 pm
Heey, so it says for chemistry: The Preliminary course contains content that is considered assumed knowledge for the HSC course.
So basically besides the HSC syllabus, we need to know the prelim stuff well???
like do we need to know the history and society impact stuff that's in prelim chem?
Hi, preliminary chem in Year 11 builds onto HSC chem because they do assume you know the previous content. This means that you should remember the important things from the preliminary syllabus.
This would include:
-Calculating things with moles. (Maybe the history of moles with Avogardo's number+Gay Lussac and its impact?)
-Memorising valences and ion charges
-Water has 4.18J/g specific heat capacity, what specific heat capacity is, AND why water is so important in relation to its high resistance to temperature change.
-Reactivity series and first ionisation energy trends.
And much more. I have seen HSC/Trial papers where there was a bunch of questions on first ionisation energy, which is Year 11 content. Hence, it would be ideal to know the preliminary course.
As for the history and impacts in the preliminary course, you most likely will have to know the history of the periodic table and how Mendeleev arranged the tables, etc. Maybe Gay Lussac+Avogadro's contributions to chemistry. Social impacts will likely focus on the environmental impacts of certain properties of water like its high heat capacity AND its less-dense solid state. You would need to explain how these are essential in sustaining aquatic and human life, etc.
Ultimately, yes I would know both syllabi inside and out for the HSC.
Hope this helps :)
Post by: RuiAce on May 20, 2017, 12:26:06 am
As for the history and impacts in the preliminary course, you most likely will have to know the history of the periodic table and how Mendeleev arranged the tables, etc. Maybe Gay Lussac+Avogadro's contributions to chemistry. Social impacts will likely focus on the environmental impacts of certain properties of water like its high heat capacity AND its less-dense solid state. You would need to explain how these are essential in sustaining aquatic and human life, etc.That being said, you aren't expected to give a full, comprehensive description in how they work anymore.
In my responses, I always coined relevant prelim content but usually stopped after just a mention, or a brief description.
Also, when I was completing the HSC course I never had to use Gay-Lussac's law or Avodagro's law.
Post by: itssona on May 20, 2017, 10:50:52 am
Hi, preliminary chem in Year 11 builds onto HSC chem because they do assume you know the previous content. This means that you should remember the important things from the preliminary syllabus.Omg this helped so much, thank youuuuu!! :)
This would include:
-Calculating things with moles. (Maybe the history of moles with Avogardo's number+Gay Lussac and its impact?)
-Memorising valences and ion charges
-Water has 4.18J/g specific heat capacity, what specific heat capacity is, AND why water is so important in relation to its high resistance to temperature change.
-Reactivity series and first ionisation energy trends.
And much more. I have seen HSC/Trial papers where there was a bunch of questions on first ionisation energy, which is Year 11 content. Hence, it would be ideal to know the preliminary course.
As for the history and impacts in the preliminary course, you most likely will have to know the history of the periodic table and how Mendeleev arranged the tables, etc. Maybe Gay Lussac+Avogadro's contributions to chemistry. Social impacts will likely focus on the environmental impacts of certain properties of water like its high heat capacity AND its less-dense solid state. You would need to explain how these are essential in sustaining aquatic and human life, etc.
Ultimately, yes I would know both syllabi inside and out for the HSC.
Hope this helps :)
That being said, you aren't expected to give a full, comprehensive description in how they work anymore.Thanks Rui! I'm glad that briefly describing it will be sufficient enough *phew*
In my responses, I always coined relevant prelim content but usually stopped after just a mention, or a brief description.
Also, when I was completing the HSC course I never had to use Gay-Lussac's law or Avodagro's law.
Post by: Jyrgal on May 20, 2017, 07:58:17 pm
Post by: RuiAce on May 20, 2017, 08:17:00 pm
hello! just wondering about partial/total pressure in a gaseous environment. lets say for a simple gas equation of N2+3H2=2NH3, if i was to add an inert gas into the container for example, my understanding is that the partial pressures of the reactants & products doesnt get affected as the inert gas only affects total pressure. my problem is since your adding another gas into the container, wouldnt the container available to the products & reactants decrease in volume, so itd increase the concentrations of them? so why would the equilibrium stay even after adding an inert gas?A common misconception among students is what "increasing the pressure means".
Without further description, when the pressure is increased it's assumed that you added only equilibrium mixture. In your example, it would mean you added a bit of N2, H2 and NH3. When a bit of all of that is added in, your analysis is valid.
Suppose instead we add argon. Let's consider what happens.
The presence of argon hinders the forward reaction because there is a greater chance that the reactants end up colliding with the inert argon instead. Hence the forward reaction is slowed down.
Its presence also hinders the products bumping into each other, so the reverse reaction is also slowed down.
Therefore, the overall effect is that both the forward and reverse reactions are affected. As a consequence, no change will occur to the equilibrium position. This change is more like a counter-catalyst; the rate that the equilibrium is achieved is slowed down.
Post by: chelseam on May 22, 2017, 09:36:07 pm
Post by: MisterNeo on May 22, 2017, 10:56:00 pm
Hi! I'm really confused about ionic equations in general, including how to get to the net ionic equation and how to determine if there are spectator ions and what they would be. If someone could explain this to me I would appreciate this so so much! Thank you :)
Hi! These redox reactions have 2 half equations and a net ionic equation. Each half equation is usually an oxidation and a reduction species.
The spectator ion is a species that is not involved in the redox reaction and remains unchanged.
For example, magnesium metal is placed into a solution of copper nitrate.
Normal Equation
Half Equations: Oxidation
Half Equations: Reduction
Net Ionic Equation
With spectator ions
(The spectator ion is NO3- because it is not involved in the redox reaction and remains aqueous throughout the reaction. Thus, the NO3-, on both sides, would be crossed out when writing the equation.)
Hope this helps :)
Post by: chelseam on May 22, 2017, 11:03:38 pm
Hope this helps :)Thank you so so much! Wow, I wish I could upvote this more than once :D This is so helpful!
Post by: anotherworld2b on May 23, 2017, 10:19:28 pm
I am not sure how you can tell what the product of the reaction would be. All i now is that there will be Mn 2+ on the right hand side
Post by: jakesilove on May 24, 2017, 09:49:24 am
Hi could i have some help with this question please?
I am not sure how you can tell what the product of the reaction would be. All i now is that there will be Mn 2+ on the right hand side
Hey! So we have
How did I know that? I googled it. There's no way you need to be able to 'guess' this reaction!
Post by: anotherworld2b on May 24, 2017, 08:07:21 pm
I also had a question about the effect of water on equilibrium.
I am quite confused about it because sometimes it has a effect and sometimes it doesn't. How do you know whether or not water has an effect on equilibrium?
Hey! So we have
How did I know that? I googled it. There's no way you need to be able to 'guess' this reaction!
Post by: scienceislife on May 25, 2017, 07:16:50 am
Post by: Piccolo on May 25, 2017, 08:48:16 pm
I've recently been revising my enthalpy of combustion work, and I had a question regarding why kerosene's enthalpy is so high. The answer is that it forms low energy bonds in the final products, and the lower the energy of the bond the more heat released. Why is that? Are low energy bonds strong or weak bonds, and why is more heat released in the formation of a low energy bond?
Thanks!
Post by: jakesilove on May 26, 2017, 04:35:14 pm
Any advice for making the most of chemistry lessons when you're barely learning anything from the teacher?
Hey! I had a not-so-incredible teacher as well; check out an article I wrote here: https://atarnotes.com/how-to-top-chemistry/. I reckon it's got all the info you need :)
Post by: jakesilove on May 26, 2017, 04:38:24 pm
Hey fellas,
I've recently been revising my enthalpy of combustion work, and I had a question regarding why kerosene's enthalpy is so high. The answer is that it forms low energy bonds in the final products, and the lower the energy of the bond the more heat released. Why is that? Are low energy bonds strong or weak bonds, and why is more heat released in the formation of a low energy bond?
Thanks!
Hey! This is definitely way beyond the syllabus, so I'll just briefly give you a conceptual understanding of the Chemistry behind this.
Essentially, Kerosene goes from a reactant containing high-energy bonds, to a state of low-energy products. So, you're question is; why is so much heat released in combustion? How does bond energy relate to enthalpy?
Essentially, the answer is simple; there is a lot of energy in the bonds of the reactants, and not much energy is the bonds of the products. Where did that energy go? Heat. Because we know that the bond energy is low, we also know that energy must have been released! Thus, the heat of enthalpy is high.
Does that kind of make sense? Again, this is definitely not useful in the HSC syllabus
Post by: bsdfjnlkasn on May 27, 2017, 11:24:49 am
I was just wondering if someone could help me with the second part of this question - is it because CO is toxic? Causing respiratory problems?
Sulfur acid can dehydrate formic acid (methanoic acid, HCOOH) to carbon monoxide. Write an equation for the reaction. Why is this reaction too dangerous to perform in a school laboratory?
Post by: jakesilove on May 27, 2017, 11:27:34 am
Hey there,
I was just wondering if someone could help me with the second part of this question - is it because CO is toxic? Causing respiratory problems?
Sulfur acid can dehydrate formic acid (methanoic acid, HCOOH) to carbon monoxide. Write an equation for the reaction. Why is this reaction too dangerous to perform in a school laboratory?
Yep, you're absolutely right; just spend some time discussing the dangers of CO, and you'll get the marks :)
Post by: seventeenboi on May 27, 2017, 03:33:06 pm
the answer to this is apparently A, but I don't understand why :( wouldn't the pH drop sharply as indicated by C in the introduction of a strong acid ??
Thanks!
Post by: RuiAce on May 27, 2017, 03:37:54 pm
HI :) (2003 HSC q15)Because we're adding it to a buffer.
the answer to this is apparently A, but I don't understand why :( wouldn't the pH drop sharply as indicated by C in the introduction of a strong acid ??
Thanks!
A buffer is made by mixing certain concentrations of an acid with its conjugate base. However the purpose of doing so is that a system is created whereby small changes in amounts of acids and bases added do NOT produce a strong effect on the system's pH. This is essentially how the pH in our blood is regulated should extra acidic/basic contents be added; the buffer will withstand the acid/base for a fair while before it starts breaking down.
If, on the other hand, we added it to something like water, then C would be valid.
Post by: J.B on May 27, 2017, 03:47:34 pm
I was just wondering with the definition of a free radical: species with one or more lone or unpaired electrons.
I'm a bit confused as to what "lone or unpaired electrons" actually refers too, does this refer to elements that don't have a full outer shell? Like oxygen etc.
Thank you
Post by: jakesilove on May 27, 2017, 04:06:15 pm
Hi,
I was just wondering with the definition of a free radical: species with one or more lone or unpaired electrons.
I'm a bit confused as to what "lone or unpaired electrons" actually refers too, does this refer to elements that don't have a full outer shell? Like oxygen etc.
Thank you
Hey! This goes to a point in Chemistry that you have not actually been taught. Basically, even ignoring the idea of a 'full' outer shell, electrons like to exist in pairs. So, an empty shell with two electrons is more stable than an empty shell with only one electron. You don't really need to know/understand much about this!
Post by: J.B on May 27, 2017, 04:26:44 pm
Hey! This goes to a point in Chemistry that you have not actually been taught. Basically, even ignoring the idea of a 'full' outer shell, electrons like to exist in pairs. So, an empty shell with two electrons is more stable than an empty shell with only one electron. You don't really need to know/understand much about this!
Ok so for the HSC is it something along the lines of this that I need to understand:
Like are some single oxygen elements not free-radicals if all 6 electrons are in pairs, and then some single oxygen elements are free-radicals if it has two pairs of electrons, and then two single unpaired electrons?
Post by: jakesilove on May 27, 2017, 04:44:46 pm
Ok so for the HSC is it something along the lines of this that I need to understand:
Like are some single oxygen elements not free-radicals if all 6 electrons are in pairs, and then some single oxygen elements are free-radicals if it has two pairs of electrons, and then two single unpaired electrons?
Yep, that's right! Alternatively, Oxygen could lose an electron, leaving it with 5 valence electrons. Obviously, one of them would be unpaired! But, most commonly, one of the electrons jumps into a higher shell (for some complicated reason), leaving two sets of paired electrons, and two non-paired electrons
Post by: J.B on May 27, 2017, 04:46:17 pm
Yep, that's right! Alternatively, Oxygen could lose an electron, leaving it with 5 valence electrons. Obviously, one of them would be unpaired! But, most commonly, one of the electrons jumps into a higher shell (for some complicated reason), leaving two sets of paired electrons, and two non-paired electrons
Ok thank you :)
Post by: J.B on May 27, 2017, 04:51:16 pm
In this haloalkane when naming it. Is it 2,3-dibromo-4-chloropentane or 3,4-dibromo-2-chloropentane?
As the first one has a lower sum for the bromine, but my teacher gave us a rule to give the most electronegative halogen (Cl>Br) the lowest numbers which would be the second one?
Post by: jakesilove on May 27, 2017, 06:19:45 pm
(http://uploads.tapatalk-cdn.com/20170527/67e3d44e9035c66425a87c32ad0f080d.jpg)
In this haloalkane when naming it. Is it 2,3-dibromo-4-chloropentane or 3,4-dibromo-2-chloropentane?
As the first one has a lower sum for the bromine, but my teacher gave us a rule to give the most electronegative halogen (Cl>Br) the lowest numbers which would be the second one?
Both would be accepted in the HSC; the rules are complicated, and not taught well. A multiple choice question will always stick with the lowest sum rule, but if you write out the answer and use the electronegativity rule (and justify it) you'll get the marks :)
Post by: J.B on May 27, 2017, 06:30:38 pm
Both would be accepted in the HSC; the rules are complicated, and not taught well. A multiple choice question will always stick with the lowest sum rule, but if you write out the answer and use the electronegativity rule (and justify it) you'll get the marks :)
Ok, Thank you :)
Post by: seventeenboi on May 27, 2017, 11:18:50 pm
Because we're adding it to a buffer.
A buffer is made by mixing certain concentrations of an acid with its conjugate base. However the purpose of doing so is that a system is created whereby small changes in amounts of acids and bases added do NOT produce a strong effect on the system's pH. This is essentially how the pH in our blood is regulated should extra acidic/basic contents be added; the buffer will withstand the acid/base for a fair while before it starts breaking down.
If, on the other hand, we added it to something like water, then C would be valid.
ohhhh makes sense :O
thanks so much!
Post by: Bubbly_bluey on May 29, 2017, 07:27:13 pm
But the answer was1,2,2-tribromo-1,1,2-trifluroethane. Is it something to do with adding something up? I'm not sure what to add up though.
Thanks :)
Post by: bsdfjnlkasn on May 29, 2017, 08:25:21 pm
Hi! I got this question about naming this halogen incorrect. I got 1,1,2-tribromo-1,2,2-trifluroethane
But the answer was1,2,2-tribromo-1,1,2-trifluroethane. Is it something to do with adding something up? I'm not sure what to add up though.
Thanks :)
Hey there,
So what the answers have done is given the lowest numbers to the more electronegative element (fluorine) whilst naming the whole compound in alphabetical order (so bromine first then fluorine).
Hopefully this helped!
Post by: seventeenboi on May 31, 2017, 03:56:06 pm
Just 2 questions about galvanic cells:
'If a galvanic cell was allowed to run for 6 hours, describe what changes you would expect to see.'
'Over time cell voltages will fall. Identify 2 possible reasons for this.'
Thank you!
Post by: sirecg02 on May 31, 2017, 09:27:58 pm
Post by: f_tan on May 31, 2017, 10:16:18 pm
(http://i.imgur.com/aVI7XRH.png)
Post by: kiwiberry on May 31, 2017, 10:37:37 pm
I'm having trouble answering the second part of the question - what is the impact of decreasing the temperature on the equilibrium constant K?
(http://i.imgur.com/aVI7XRH.png)
When temperature is decreased, by Le Chatelier's Principle, equilibrium will shift to favour the reaction which produces more heat, ie the reverse exothermic reaction (as the forward reaction is endothermic). Thus, the concentration of the products will decrease, and the concentration of the reactants will increase. As K=[products]/[reactants], the equilibrium constant will therefore be lower.
Note that temperature is the only factor that affects the equilibrium constant - the decrease in volume will only have an effect on the position of equilibrium, but K will remain the same. :)
Post by: anotherworld2b on June 01, 2017, 05:05:51 pm
Post by: MisterNeo on June 01, 2017, 06:09:39 pm
Hi i was just wondering if someone could have a look at my answers please.
(http://i.imgur.com/lebIfPj.jpg)
This is my attempt at the question for part B.
I'm a bit skeptical of your final value though...135 tons of SO3 from 1 ton of Sulfur.
Part c/d:
(http://i.imgur.com/2lYkWAU.jpg)
This is what I got for part c/d. I may be wrong though, so answers would be nice.
Hope this helps :)
Post by: ellipse on June 01, 2017, 06:56:21 pm
So back to the question, if 1 tonnes of sulfur reacts than that is equal to 31181 moles of S. This produces 31181 moles of SO2 (as its 1:1).
This 31181 moles of SO2 reacts with 31181/2 moles of O2 (cause its SO2:O2 is 2:1) to produce 31181 moles of SO3 (cause SO2:SO3 is 1:1)
Taking the 95% eficiency, this equates to 2.37*10^6 grams.
I think the rest of the question works similarly
Hope someone could check over this, I might be completely wrong
Post by: anotherworld2b on June 01, 2017, 09:08:14 pm
I don't have answers to this question
Post by: chloe9756 on June 01, 2017, 11:02:42 pm
Post by: MisterNeo on June 01, 2017, 11:36:27 pm
What are some points about accuracy, validity and reliability that can be discussed in the anion and cation precipitation tests?
Hi!! ;)
Validity: Is the experiment actually suitable for your aim?
Eg. I want to determine if sulfate is in this solution. What I would do is add a barium nitrate solution. What I would not do is conduct a flame test because that is only for cation testing. Am I adding two nitrate solutions together? Lol that would be pointless.
Reliability: Do you get identical results upon repetition?
Eg. Some salts are slightly soluble such as calcium hydroxide. Sometimes it will produce a notable precipitate, sometimes it will not. So you would need to repeat this experiment multiple times to ensure that the precipitate either stays or goes away fully.
Accuracy: How close does it come to the true result or other people's results?
Eg. If I put two soluble solutions together (barium niitrate+sodium chloride) and somehow get a precipitate, this will mean either two things: Someone labelled the chemical wrong, or someone contaminated the solution. This will need to be taken into account if something doesn't appear right compared to other people's results and the actual result.
Hope this helps :D
Post by: Piza on June 02, 2017, 02:36:01 pm
I have found the concentration of the diluted vinegar, but I need to find the concentration of the undiluted vinegar.
Would I multiply the concentration of the diluted vinegar by 6 or by 7 (from adding the ratio numbers)?
Post by: Piza on June 02, 2017, 06:48:19 pm
You would multiply the concentration of the diluted vinegar by 6 because it is 6/1. It is the opposite of diluting which multiplies by 1/6.
Hope this helps :)
I understand now, thank you for your answer!! :D
Post by: Kle123 on June 04, 2017, 01:30:46 pm
Post by: Aaron12038488 on June 04, 2017, 02:09:42 pm
identify a geological, physical, biological phenomenons.
Post by: bananna on June 05, 2017, 05:32:26 am
What would happen if acid was added to this equation in terms of equilibrium:
CO2(g) + H2O(l) ⇌ H2CO3(aq)
And what would happen if a base is added?
What does increase/decrease the solubility of gas mean? Which way does it mean the equilibrium is shifted?
THANK YOU!!!
Post by: MisterNeo on June 05, 2017, 09:27:39 am
Hi!
What would happen if acid was added to this equation in terms of equilibrium:
CO2(g) + H2O(l) ⇌ H2CO3(aq)
And what would happen if a base is added?
What does increase/decrease the solubility of gas mean? Which way does it mean the equilibrium is shifted?
THANK YOU!!!
Adding acid: The equilibrium shifts to the left.
This is because there is an increased concentration of hydrogen ions and the system will shift to the left to counter it according to LCP.
Adding base: The equilibrium shifts to the right.
This is because the base neutralises the carbonic acid, thus pushing the system to the right to replace the missing acid.
The solubility of gases refers to the ability of gases to dissolve in water. In this equilibrium, a shift to the right increases the solubility of CO2, whereas a shift to the left makes more CO2 become gaseous out of aqueous state.
Hope this helps :D
Post by: Annie657 on June 05, 2017, 10:05:18 am
Post by: Jyrgal on June 05, 2017, 10:18:00 am
Is BOD5 calculated by DO5 - DO0 or the other way round for water testing? Ahh I have my assesment today :-[
You should be subtracting the level of dissolved oxygen from day (1) (im assuming ur DO0 is your original measurement) to the level of dissolved oxygen at the end of 5 days (DO5), not the other way around, cause dissolved oxygen on day 1 should be higher than day 5. It's also measured in ppm with a dissolved oxygen sensor. Any BOD of 1-5ppm is considered somewhat clean, any numbers above is polluted/has bacteria. Hope this helps, goodluck for your test :)
Post by: sarah96037885 on June 05, 2017, 12:15:04 pm
1. Complete the following equation showing the main organic products. You do not have to balance the equation but all organic compounds should be shown in structural form. If no reaction occurs, write NR.
- propan-2-ol+KMnO4 →
- pent-2-ene+HOH and H+ →
- HO(CH2)3OH+HOOC(CH2)4COOH →
2. Show by means of equations how you could convert
- propan-1-ol to propanoic acid
- but-2-yne to 2,2,3-trichlorobutane
3. How could you distinguish experimentally between an alkane, an alkene and an alkyne ?
4. How could you distinguish experimentally between primary, secondary and tertiary alcohols?
5. Arrange the following in order of increasing boiling point and use bonding theory to explain your reasoning:
CH3(CH2)4CH3
C5H11OH
C4H9COOH
CH3C(CH3)2CH2CH3
C4H10
6. Calculate the percent composition of the elements in 3-chloro-2-methylbutanoic acid.
7. 5.00 g of methanol are mixed with 20.0 g of octanoic acid and allowed to react to form an ester. If the reaction has an average yield of 75.0%, what mass of product should be formed?
Post by: jakesilove on June 05, 2017, 12:30:44 pm
Why is it always best for First Hand Investigations to be undertaken individually?
Genuinely not sure about this one; perhaps it minimises the error that different people bring? Don't think this is an important thing to understand
Post by: jakesilove on June 05, 2017, 12:31:56 pm
i have a question regarding my prelim assessment. (Metals and Water
identify a geological, physical, biological phenomenons.
Hey! This is probably too broad a question; is it a dot point? You should easily be able to identify general phenomena; Geological includes techtonic plate shifts, physical includes gravity (?) and biological includes photosynthesis.
Post by: Jyrgal on June 05, 2017, 02:08:43 pm
If so, how would you answer a question that asks "Using bronsted-lowry acid/base theory, describe what happens when ammonia is placed into water"
Post by: MisterNeo on June 05, 2017, 04:11:55 pm
hello! i was just flicking over textbooks, and according to bronsted-lowry acid/base theory, a strong/weak acid has a conjugate weak/strong base. Is ammonia and ammonium an exception to this? since both of them are considered 'weak' (medium?)
If so, how would you answer a question that asks "Using bronsted-lowry acid/base theory, describe what happens when ammonia is placed into water"
Hi!
A conjugate base/acid of an strong acid/base is very weak.
A conjugate base/acid of a weak acid/base is weak.
The conjugate acid of ammonia (a weak base) is the ammonium ion, which is mildly strong.
The conjugate acid of a strong base like sodium hydroxide is the sodium ion, which is very weak.
Ammonia in Water
Water is amphiprotic, meaning that it can be both an acid and a base, which is why water has donated a proton to ammonia to form the ammonium ion.
Post by: Annie657 on June 05, 2017, 05:21:43 pm
You should be subtracting the level of dissolved oxygen from day (1) (im assuming ur DO0 is your original measurement) to the level of dissolved oxygen at the end of 5 days (DO5), not the other way around, cause dissolved oxygen on day 1 should be higher than day 5. It's also measured in ppm with a dissolved oxygen sensor. Any BOD of 1-5ppm is considered somewhat clean, any numbers above is polluted/has bacteria. Hope this helps, goodluck for your test :)
Thankyou so much! Hopefully I did ok, we had to do a winkler titration 😫
Post by: Aaron12038488 on June 05, 2017, 05:33:04 pm
Post by: jakesilove on June 06, 2017, 10:33:01 am
how would i go about this question. Describe the cause and effect relationship between the appropriate properties of water to earthquakes?
I'm honestly unsure! I imagine that the answer goes something like this. Earthquakes are caused by seismic shifts in tectonic plates. However, usually, these shifts occur in the middle of the ocean. The energy released by earthquakes are able to travel to land by propagating through water, as water is a medium that easily allows the transfer of energy. The ocean can also cause Tsunamis to form, increasing the effect of the earthquake.
Post by: Jyrgal on June 06, 2017, 04:35:56 pm
Post by: jakesilove on June 06, 2017, 05:12:46 pm
hey! i've been practicing in titrating using phenolphthalein with sodium carbonate as standard and nitric acid. I've noticed that whenever i get some colour change, after some few seconds of swirling the colour disappears, and I know that this happens because CO2 dissolves to make the pH slightly lower so colour goes away. I was just wondering how do you exactly determine which colour change to stop since 'permanent' colour change doesnt rly work for calculating concentration if co2 reacts with the sodium carbonate as well
Hey! The general rule I always used was 'if the colour stays the same for 10 seconds, you've reached the end point'. If you then let it rest, and noticed a few minutes later that the colour had reverted back, you don't need to go and change your results! However, I don't know if this is necessarily a 'cover-all' rule, it's just what I use! As long as you have a solid 'method', so that you get reliable results (by using the same criteria each time), you'll be fine :)
Post by: Jyrgal on June 06, 2017, 06:35:34 pm
For esterification, is there an explanation to why the -OH group comes from the alkanoic acid rather than the alkanol? (in other words: why is the -OH group in alkanoic acid weaker than the -OH group in alkanol?)
Cheers!
Post by: jakesilove on June 06, 2017, 06:57:03 pm
hello again another Q cause my exams r soon ;D ;D
For esterification, is there an explanation to why the -OH group comes from the alkanoic acid rather than the alkanol? (in other words: why is the -OH group in alkanoic acid weaker than the -OH group in alkanol?)
Cheers!
Nope, there absolutely isn't. It's a nice thing to know, but also not entirely necessary. Basically, we figured that out by carefully looking at the molecules when physically doing the experiment.
Post by: Piza on June 06, 2017, 08:28:01 pm
Post by: jakesilove on June 06, 2017, 08:38:22 pm
I need some guidance in answering these types of questions, can someone do their working out for A so that I can do B and C, thank you!
Definitely a weird question! We can use the hint to realise that ppm=mg/L. So,
We know that absorbance is proportional to concentration. We need to 'convert' our known standard of 0.85 to 0.8, and then perform the same operation on the concentration.
Great! Hopefully you followed the above; we know have the concentration of Iron in the 250mL volumetric flask. To find the total mass of Iron we divide by four (as there are 4.71mg per L, therefore 0.471/4 mg per 0.25L).
This results in a total mass of 1.178mg of Iron in the original tablet. You can divide this by 100mg to find the w/w concentration of Iron in the tablet.
Hope that all made sense!
Post by: Piza on June 06, 2017, 08:44:30 pm
Definitely a weird question! We can use the hint to realise that ppm=mg/L. So,
We know that absorbance is proportional to concentration. We need to 'convert' our known standard of 0.85 to 0.8, and then perform the same operation on the concentration.
Great! Hopefully you followed the above; we know have the concentration of Iron in the 250mL volumetric flask. To find the total mass of Iron we divide by four (as there are 4.71mg per L, therefore 0.471/4 mg per 0.25L).
This results in a total mass of 1.178mg of Iron in the original tablet. You can divide this by 100mg to find the w/w concentration of Iron in the tablet.
Hope that all made sense!
Thank you so much, this helped me a lot!!
Post by: beau77bro on June 08, 2017, 05:14:44 pm
Post by: jakesilove on June 08, 2017, 05:25:21 pm
Hey guys so I have a prac assessment followed shortly by a processing skills assessment. But I need to worry more about the practical because they have gotten me in strife a lot before. Are there any (and all) prac tips you guys could give me? Even basic ones to make sure I do as much right as possible
Hey! Bit of a tough question to answer, but I'll try my best.
First and foremost, make sure you totally understand the theory behind whatever practical task you could be asked to do. Presumably, you don't know what the prac is? Perhaps go through the curriculum, picking out pracs and making sure you understand them. However, don't limit yourself there; make sure there aren't areas of the syllabus that you're particularly weak with, in case the particular talk assesses those points.
Now, the practical task itself will usually require a couple of things.
Firstly, you will probably be asked to write a method/procedure/experimental plan. Get used to the structure; something along the lines of aim, hypothesis, equipment/material, method, results, discussion, conclusion. You probably won't need to do this entire process, however having the general shape of an experimental design in mind is very helpful when completing a practical task. Make sure you understand each of the above mentioned sections; if not, let me know, and I'll elaborate!
When it comes to methods particularly, make sure to write them in past tense, and number them. Some people say you must being with a verb; you are more than welcome to do this, although I didn't really focus on that.
With results etc. make you sure you are comfortable with drawing tables to summarise data. Make sure you put units in the heading of each row/column, and you box in your table.
With your discussion, be very familiar with the terms accuracy, reliability and validity. Know how to assess each of these, and how they inter relate. If you have question on any of this, feel free to ask, I just don't want to go over anything you already know really well!
Be knowledgeable of the names of various glass ware, and the way by which you can write a risk assessment (ie risk, precaution, response table).
Uhhhh I'm trying to think of anything else...
Will keep thinking. I'm sure others will contribute. Please let me know if I can elaborate on any of the above!!!
Post by: beau77bro on June 08, 2017, 05:36:59 pm
Hey! Bit of a tough question to answer, but I'll try my best.
First and foremost, make sure you totally understand the theory behind whatever practical task you could be asked to do. Presumably, you don't know what the prac is? Perhaps go through the curriculum, picking out pracs and making sure you understand them. However, don't limit yourself there; make sure there aren't areas of the syllabus that you're particularly weak with, in case the particular talk assesses those points.
Now, the practical task itself will usually require a couple of things.
Firstly, you will probably be asked to write a method/procedure/experimental plan. Get used to the structure; something along the lines of aim, hypothesis, equipment/material, method, results, discussion, conclusion. You probably won't need to do this entire process, however having the general shape of an experimental design in mind is very helpful when completing a practical task. Make sure you understand each of the above mentioned sections; if not, let me know, and I'll elaborate!
When it comes to methods particularly, make sure to write them in past tense, and number them. Some people say you must being with a verb; you are more than welcome to do this, although I didn't really focus on that.
With results etc. make you sure you are comfortable with drawing tables to summarise data. Make sure you put units in the heading of each row/column, and you box in your table.
With your discussion, be very familiar with the terms accuracy, reliability and validity. Know how to assess each of these, and how they inter relate. If you have question on any of this, feel free to ask, I just don't want to go over anything you already know really well!
Be knowledgeable of the names of various glass ware, and the way by which you can write a risk assessment (ie risk, precaution, response table).
Uhhhh I'm trying to think of anything else...
Will keep thinking. I'm sure others will contribute. Please let me know if I can elaborate on any of the above!!!
hmmm yea im going through the pracs and picking out the ones to revise and go through, its only the ones we did as a class. but the problem is, there isnt actually a conclusion or discussion. there is a plan, results and the majority will consist on carrying out the practical well, and unfortunately that's where im worst at ahahahha. so like i need to know all the basic practical procedures and the tricks to them, like whenever you make a standardised solution you rinse the flask with water first, then put the substance to be dissolved in the flask and then fill. because filling and then adding the solid doesnt take into account volume of the solid. like that kinda stuff, i know im asking a weird and ambiguous/broad question.
to narrow it down the pracs are only the first two mods, minus esters.
thankyou and sorry ahahha
Post by: jakesilove on June 08, 2017, 06:18:35 pm
hmmm yea im going through the pracs and picking out the ones to revise and go through, its only the ones we did as a class. but the problem is, there isnt actually a conclusion or discussion. there is a plan, results and the majority will consist on carrying out the practical well, and unfortunately that's where im worst at ahahahha. so like i need to know all the basic practical procedures and the tricks to them, like whenever you make a standardised solution you rinse the flask with water first, then put the substance to be dissolved in the flask and then fill. because filling and then adding the solid doesnt take into account volume of the solid. like that kinda stuff, i know im asking a weird and ambiguous/broad question.
to narrow it down the pracs are only the first two mods, minus esters.
thankyou and sorry ahahha
Ah, okay, I see! Could you do me a favour and go through, figure out which pracs you might be assessed on, and throw up a list? Then, I can talk through each, giving you points/methods etc. Most of them are pretty straight forward; you'll probably be doing a Titration aha. Still, pop up a list, and I'll give you whatever I think you need for each :)
Post by: beau77bro on June 08, 2017, 06:51:39 pm
Molar heat of combustion:
Galvanic Cells:
Potential difference of metals in an electrode:
Natural Indicators:
Decarbonation of a soft drink:
Titrations:
thanks so much jake your a life saver
Post by: jakesilove on June 08, 2017, 08:12:04 pm
this looks like most of the ones we could actually be assessed on (theres also a fermentation prac but it would take to long and i doubt very much sir would do it)Let's do this
Molar heat of combustion:
Galvanic Cells:
Potential difference of metals in an electrode:
Natural Indicators:
Decarbonation of a soft drink:
Titrations:
thanks so much jake your a life saver
Molar heat of combustion:
Here, we are trying to extract the heat released by combusting one mole of a substance. For arguments sake, let's assume your prac is Ethanol, however it could foreseeably be any other alcohol.
We look at the formula, which is
This will tell us how much energy has been released, depending on the increased temperature of the water.
We set up the below prac
(http://www.ausetute.com.au/images/aphrspbu.gif)
First, we weight the mass of the substance (eg. Ethanol). Then, we light it, and let it heat up the water. Finally, we extinguish the Ethanol, measuring A) the change in mass of the Ethanol and b) the change in temperature of the water.
Let's say the water heated up by 20 degrees, and one gram of Ethanol was combusted. First, we find the energy that must have been 'used' to heat up the water by this amount, using the formula above
Where m is the mass of the water, and C is the specific heat capacity. This will give us some value of H, which was released when one gram of Ethanol was burned. Then, we convert this kJ/g measurement into kJ/mol (my multiplying by the molar mass), and we have a total value!
Some notes on the 'reliability' etc part of this experiment. Firstly, we would perform it multiple times, and average the result. Second, validity is quite limited in this experiment for two reasons: 1) A lot of heat energy is lost to the environment and 2) incomplete combustion can occur (if there is a black smudge underneath the tripod. These are all things you would talk about in the written portion of the prac.
One down. Like 100 to go.
Galvanic Cells:
For a Galvanic Cell, you need to be really confident re how to draw one out/set one up/label it. The below is a bit of a blurry example, but you get the idea.
(http://i.imgur.com/2dcLq6T.png)
So, set it up by placing the liquids in two beakers, and the appropriate electrode in each beaker, connected to a voltmeter in series. Also, make sure to connect the two half cells with a salt bridge, to allow ions to travel between (essentially completing the circuit!).
For an experiment like this, there isn't much to it. You might be asked to change the concentration of the solution, and measure the affect of this on the voltage etc. You don't need to know anything about any possible variances in advance; you'll have to actually do it on the day.
Be comfortable with half-cell notation, redox equations, etc.
I've never heard of a practical task that was a Galvanic cell, so I think this is pretty unlikely :)
Potential difference of metals in an electrode:
This is the same as a Galvanic cell. Potentially (lol) you'll need to switch metals, compare the output voltage etc. This can all be predicted using the table of standard potentials on your formula sheet; you find the was oxidising, and switch the signs of the potential on the sheet. Then, you add it to the one reducing, and presto! You've got a voltage out. So, the highest voltage would be a cell between a substance very high on the list, and a substance very low on the list. Be comfortable with this table, with balancing equations, which creating net ionic equations and half equations. Happy to give some examples if you think that will help.
Natural Indicators:
Also a pretty straight forward practical task. Usually, you'll use something like red cabbage. Here's how I did the prac.
Cut up the cabbage into smallish pieces, and place it in a beaker full of water. Smoosh it around, so it releases its delicious delicious juices. Then, boil it for about 5 minutes, also to ensure that you get it's sick ass ability to detect changes in pH. Drain the remaining liquid, so you retain only the solution, and throw away the soggy Cabbage.
Now, you've got your indicator! We need to test how useful it is, and what range it will be able to recognise. So, prepare a range of test tubes (ideally, multiple at each pH scale for reliability) such that each test tube has a different pH. I would try to get a test tube with pH 1, a test tube with pH 2.... a test tube with pH 12 and a test tube with pH 13. If not possible, just ensure you have the full range. Drip a few drops of the indicator into each test tube, and record any colour change. It will be obvious what 'sort' of indicator yours is once you've done this! ie. if it's red for all substances up until a pH of 9, and then blue for pHs higher than 10, it would be useful in determining if something is very basic.
Ooft. Getting there.
Decarbonation of a soft drink:
This one is pretty chilled. However, there is a bit of nuance to it. First, you'll get a can of soft drink. Weigh it. Then, place it on a scale, and open the can. MAKE SURE NOT TO RIP THE TAGGY BIT OFF, OR YOU'LL BE CHANGING THE WEIGHT! Let the soft drink sit, and record changes in mass over whatever time period you have (if it's an assessment, probably not long). Plot this change in weight against time, and you'll have a nice curvey/straight lineish graph.
What's happening is that Carbon dioxide is being released. Based on the change in mass, you can calculate exactly how much Carbon dioxide has been released, based on its molar mass. I assume you can do this calculation.
Now, if you want to be really smart, consider the main issue with validity. So, you're watching Carbon dioxide escape, but what else? Water is evaporating. So, if you want to be clever, get a different can, empty it, and fill it with water. Simultaneously measure any change in mass in this can, which would be entirely due to water loss. Approximate that the evaporation in the second can is the same as the first, and 'add' the additional weight to your data. eg. if you notice that the water can loses 3g, but the soft drink can loses 5g, you can only actually attribute 2g to Carbon dioxide escaping.
Remember that this is all done in terms of Le Chatelier's principle, although the experiment itself doesn't seem to involve it. Just be comfortable with the principle and it's applications in order to answer any theoretical questions.
Titrations:
AAAAAAAND now we get to the tough one. Let me see if I have any helpful images...
I DO INDEED!
(http://i.imgur.com/drxMCbb.png)
Hell yeah. This genuinely has everything you need. The LHS is about creating a standard, the RHS is about performing the titration.
I think it's actually fairly comprehensive; let me know if I can elaborate on this at all.
Phew.
Post by: jakesilove on June 08, 2017, 08:15:31 pm
this looks like most of the ones we could actually be assessed on (theres also a fermentation prac but it would take to long and i doubt very much sir would do it)
I love that you call him sir aha I do not miss High School
Post by: haony98 on June 08, 2017, 08:34:08 pm
Can someone explain molar heat of neutralisation
It's not an actual dot point in the syllabus but we will get tested on it and it's kinda a mix of of the concept of molar heat + neutralization reactions
Post by: jakesilove on June 08, 2017, 08:39:20 pm
Hi
Can someone explain molar heat of neutralisation
It's not an actual dot point in the syllabus but we will get tested on it and it's kinda a mix of of the concept of molar heat + neutralization reactions
Hey! I'm not sure I quite understand your question, but I think you're asking about the heat absorbed/released when an acid and a base react? This is sort of a dot point, and I have definitely seen marks allocated to this topic area. It's a really simple concept, and you don't need to know much about it.
At it's heart, every neutralisation reaction is exactly the same;
Now, it just so happens that this reaction is exothermic; ie. as the reaction moves forward, heat is released. In fact, for a strong acid and a strong, base, about 56kJ of energy per mole is released! Thus
Now, this is only every 'assessable' when it comes to the practicality of cleaning up large chemical spills etc. If you mix a large amount of base (say, NaOH) with a huge spill of acid (say, HCl) then a huge amount of energy can be released. This is dangerous for people nearby, for the region on which the spill has occurred, that sort of thing. Again, tiny tiny point in the syllabus, but sometimes assessed!
Key take aways:
1. Acid/Base neutralisation is always essentially the same reaction
2. That reaction is exothermic
Hope that all made sense! Great question :)
Post by: beau77bro on June 08, 2017, 08:39:50 pm
I love that you call him sir aha I do not miss High Schoolhshahahahah i full call all my teachers and everyone sir even the female teachers ahahahah, bad habit sorry. I AM GETTING A BOOK AND TAKING NOTES RIGHT NOW, THANKYOU JAKE, always so helpful thankyou.
when i finish typing up the pracs from this and my prac work ill post and ask? in probably a billion years. but thankyou this will help so much, esp those titration notes. mint btw. which im worst at and will probably be the prac hhahahahah but thankyou again i actually cant thankyou enough.
Post by: jakesilove on June 08, 2017, 08:40:54 pm
hshahahahah i full call all my teachers and everyone sir even the female teachers ahahahah, bad habit sorry. I AM GETTING A BOOK AND TAKING NOTES RIGHT NOW, THANKYOU JAKE, always so helpful thankyou.
when i finish typing up the pracs from this and my prac work ill post and ask? in probably a billion years. but thankyou this will help so much, esp those titration notes. mint btw. which im worst at and will probably be the prac hhahahahah but thankyou again i actually cant thankyou enough.
No problem, seriously glad the above helped. Let me know if you have ANY questions about anything I've posted, and of course I would be more than happy to take a look at notes you put together :)
Post by: Wren on June 08, 2017, 09:21:26 pm
Post by: jakesilove on June 08, 2017, 09:31:43 pm
hey hoping someone can clarify for me in the testing for anion prac my school says for the last step add Fe2+ in an alkaline solution to check if there is phosphate. I was just wondering why am I adding Fe2+
Hey! If we add Iron ions (say that five times fast), we end up producing Iron phosphate, which is insoluble! So, to test for phosphate, if we add Iron ions and see the precipitation of a solid, we can be confident that Phosphate was present :) If we add excess iron, we can get all the Phosphate out!
Post by: Yagami Light on June 08, 2017, 11:08:17 pm
Post by: jakesilove on June 09, 2017, 10:34:24 am
thanks guys
Glad you're finding the forum useful! Feel free to pop up a question anytime, and welcome!
Post by: imda.beast on June 09, 2017, 03:07:53 pm
Post by: jakesilove on June 09, 2017, 03:39:45 pm
hi jake, can you also run through the identifying different ions in solution practice and the main point I have to remember. it is for a prac exam, thank you
Hey! Basically, you just need to remember your solubility rules/flame tests. Then, you'll likely be given a solution containing a number of ions. Use the solubility rules/flame test colours to figure out a way of determining each ion. There isn't much to this one in terms of prep, except you really do need to memorise the rules (Assuming they don't give them to you!). I've put the way I remember each rule below.
(http://i.imgur.com/0t4KRgD.png)
(http://i.imgur.com/RFS1RIl.png)
Let me know if I can add anything else!
Post by: MisterNeo on June 09, 2017, 06:10:27 pm
Hey! Basically, you just need to remember your solubility rules/flame tests. Then, you'll likely be given a solution containing a number of ions. Use the solubility rules/flame test colours to figure out a way of determining each ion. There isn't much to this one in terms of prep, except you really do need to memorise the rules (Assuming they don't give them to you!). I've put the way I remember each rule below.
(http://i.imgur.com/0t4KRgD.png)
(http://i.imgur.com/RFS1RIl.png)
Let me know if I can add anything else!
(http://i.imgur.com/RFS1RIl.png)
Isn't it spelled "mnemonic"? Pneumonic is a lung thing... :-\
Post by: jakesilove on June 09, 2017, 11:18:20 pm
(http://i.imgur.com/RFS1RIl.png)
Isn't it spelled "mnemonic"? Pneumonic is a lung thing... :-\
This may very well be true...
Post by: chelseam on June 09, 2017, 11:34:07 pm
Let me know if I can add anything else!The flame test one is the most brilliant thing I have ever seen
Post by: RuiAce on June 09, 2017, 11:44:28 pm
This may very well be true...Rip.
The flame test one is the most brilliant thing I have ever seenIt's the whole basis of fireworks
Post by: Wren on June 10, 2017, 04:24:13 pm
Hey! If we add Iron ions (say that five times fast), we end up producing Iron phosphate, which is insoluble! So, to test for phosphate, if we add Iron ions and see the precipitation of a solid, we can be confident that Phosphate was present :) If we add excess iron, we can get all the Phosphate out!
Thanks Jake. And also I've seen another chart where barium is added. Is there any difference?
Post by: jakesilove on June 10, 2017, 04:38:12 pm
Thanks Jake. And also I've seen another chart where barium is added. Is there any difference?
Barium is way more common; most students/tests/assessments use Barium. Either will work!
Post by: Wren on June 11, 2017, 10:35:32 am
Barium is way more common; most students/tests/assessments use Barium. Either will work!Thank you :)
Post by: Kekemato_BAP on June 13, 2017, 11:49:24 am
What prac would this be? What should I know beforehand?
I have this prac exam this Thursday
(http://i.imgur.com/uPeM5Tl.jpg)
Thanks so much 8)
Post by: jakesilove on June 13, 2017, 11:55:23 am
Hey guys..
What prac would this be? What should I know beforehand?
I have this prac exam this Thursday
(http://i.imgur.com/uPeM5Tl.jpg)
Thanks so much 8)
Hey! I suspect this would be about finding the concentration of common ions in a solution? Check the top post on this page for ways to memorise solubility rules! Honestly, though, it's impossible to predict; I would just be brushing up on the whole of Chemical Monitoring and Management (maybe a bit of the Acidic Environment) as well as ways to write practical tasks up, use reliability/validity/accuracy etc.
Post by: Kekemato_BAP on June 13, 2017, 03:03:27 pm
Hey! I suspect this would be about finding the concentration of common ions in a solution? Check the top post on this page for ways to memorise solubility rules! Honestly, though, it's impossible to predict; I would just be brushing up on the whole of Chemical Monitoring and Management (maybe a bit of the Acidic Environment) as well as ways to write practical tasks up, use reliability/validity/accuracy etc.
If it were a finding ion experiment, what could I write about validity/reliability/accuracy? I usually struggle with those areas.
Post by: parthie on June 13, 2017, 04:11:57 pm
Thanks
Post by: jakesilove on June 13, 2017, 04:16:46 pm
If it were a finding ion experiment, what could I write about validity/reliability/accuracy? I usually struggle with those areas.
So validity will go to whether you are testing what you've been trying to test; did you use the right materials? Did you perform the right steps? etc. Reliability asks whether you repeated the experiment, and accuracy would be whether you a 'right'. ie. How close to the real answer were you?
Post by: jakesilove on June 13, 2017, 04:19:16 pm
Does anyone know the validity/accuracy of titration experiment, esters, and identifying anion and cation experiments?
Thanks
Hey! See above re anion and cation.
For titration, Validity will also be about whether you washed every piece of glassware with the right substance. If you didn't you're not 'testing' what you're trying to test, as the concentrations of the liquids will change. Validity would also be called into question if you used the wrong indicator. Accuracy just asks how 'close' you are to the real result. You use equipment that is fairly accurate, as Burettes can read to 0.05 of a mL etc. Reliability is increased by repeating the titration multiple times.
For esters, you're not really testing anything quantitative (so no accuracy), you're not repeating the experiment (no reliability), and you don't really check if there is in fact an ester in the final solution (no validity). It's more of a demonstrative experiment than a 'proper' experiment
Post by: Aussie1Italia2 on June 13, 2017, 11:05:20 pm
Post by: Aussie1Italia2 on June 13, 2017, 11:09:57 pm
Drawing a chemical model in this case of an ester formation, can I please have an example? How to do it has slipped my mind.
Thank you for all the help!
Post by: parthie on June 14, 2017, 04:05:37 pm
Hey! See above re anion and cation.
For titration, Validity will also be about whether you washed every piece of glassware with the right substance. If you didn't you're not 'testing' what you're trying to test, as the concentrations of the liquids will change. Validity would also be called into question if you used the wrong indicator. Accuracy just asks how 'close' you are to the real result. You use equipment that is fairly accurate, as Burettes can read to 0.05 of a mL etc. Reliability is increased by repeating the titration multiple times.
For esters, you're not really testing anything quantitative (so no accuracy), you're not repeating the experiment (no reliability), and you don't really check if there is in fact an ester in the final solution (no validity). It's more of a demonstrative experiment than a 'proper' experiment
Thanks so much! but how exactly would you word the validity/accuracy of anions/cations bc I'm still confused about that??
sorry one last thing...what are improvements that can be made for all those experiments?
Post by: jakesilove on June 14, 2017, 04:14:29 pm
Thanks so much! but how exactly would you word the validity/accuracy of anions/cations bc I'm still confused about that??
sorry one last thing...what are improvements that can be made for all those experiments?
Hey! With regards to anion/cation, I would literally just say that if the steps carried out were legit, then the experiment is valid (ie. testing what you are trying to test). If the results you gain are close to the 'true' results, and the equipment you use is accurate (ie. high number of decimal places etc) then the experiment is accurate. The important part is identifying issues in validity, accuracy and reliability, which is entirely dependant on the experiment you perform (therefore cannot be predicted).
Improvements, again, come down to the experiment. You need to have a good idea of the theory behind each prac, and be ready to improve the experiment yourself. I can't tell you how to improve an experiment that you haven't planned yet! Again, know the theory, know your experimental design, and you'll be fine. In terms of identifying improvements, I would 1) Find issues in accuracy/reliability/validity and 2) Solve them by improving experimental design.
Jake
Post by: hansolo9 on June 14, 2017, 09:54:43 pm
How do I draw a line of best fit?
I always have trouble knowing how to draw them and where to stop the line.
Especially for calibration curves in AAS.
Thank you
Post by: RuiAce on June 14, 2017, 10:19:21 pm
Hello!Whilst in the real world it's possible to determine the equation of the actual line of best fit, as far as HSC science goes your toolbox is quite limited. Given a set of (plotted) data, to draw your line of best fit simply draw in the line that best represents the slope of the curve itself. The following are some tips you may choose to employ
How do I draw a line of best fit?
I always have trouble knowing how to draw them and where to stop the line.
Especially for calibration curves in AAS.
Thank you
- If almost all of your data can be represented on a smooth line, and by almost all I mean only a few weird ones (maybe 2), you should ignore the weird ones and treat them as outliers or mistakes in the experiment and just draw the line through the other points.
- If your data is very scattered (unlikely to appear in the HSC) and you insist on drawing a line of best fit, focus on the slope of the line of best fit.
- Stop at least the largest value on the x-axis given. We draw lines of best fit for the sake of an 'interpolation' - determining a trend within the extreme values given. Alternatively, stop until you go over the grid that you're given (because in the HSC they usually give grids), because you may need it later for an 'extrapolation'.
- If your data doesn't look linear go for a curve of best fit. Never happens with AAS in particular though.
Post by: beau77bro on June 15, 2017, 10:10:56 pm
" First test small quantities of each solution to find their pH. This tells you which indicator to use; if they are a strong acid and a strong base, you want an indicator with an equivalence point around 7(bromothymol blue )"
or close too, and i don't really get. Testing their pH how? As well as how will you know if it’s a strong acid or strong base? What if it’s just a really concentrated weak acid, and how does this affect the equivalence point, because it will be an acidic salt formed? Or basic salt, what if both are weak? Then use 7? But how do we know? Confused ahahah.
like what do you treat as the equivalence point, can't you just make it 7, and then just add acid to force the equilibrium of the weak base across enough so that its completely neutralised - i am very confused. sorry if it doesnt make sense, im not great at this part.
Post by: jakesilove on June 16, 2017, 09:35:37 am
hey so in the atarnotes by you guys it says this about preparing for titration:
" First test small quantities of each solution to find their pH. This tells you which indicator to use; if they are a strong acid and a strong base, you want an indicator with an equivalence point around 7(bromothymol blue )"
or close too, and i don't really get. Testing their pH how? As well as how will you know if it’s a strong acid or strong base? What if it’s just a really concentrated weak acid, and how does this affect the equivalence point, because it will be an acidic salt formed? Or basic salt, what if both are weak? Then use 7? But how do we know? Confused ahahah.
like what do you treat as the equivalence point, can't you just make it 7, and then just add acid to force the equilibrium of the weak base across enough so that its completely neutralised - i am very confused. sorry if it doesnt make sense, im not great at this part.
Hey! To test the pH of a substance (roughly) you would just use some sort of indicator eg. Universal indicator, Litmus paper etc. However, the general rule is: Strong acid + Strong base = end point around neutral. Strong acid + Weak base = end point towards the acidic end. Weak acid + Strong base = end point towards the basic end. You just need to learn that. Also, you need to be able to identify whether an acid/base is strong or weak. In the HSC, Hydrochloric acid, Nitric acid and Sulfuric acid are the strong acids, and NaOH is the strong base. Everything else is weak.
So yeah, this is sort of stuff you just need to memorise, rather than really internalise and understand. I certainly didn't really think when performing titrations; just did what I knew was true.
Post by: beau77bro on June 16, 2017, 06:05:31 pm
Post by: beau77bro on June 17, 2017, 04:30:57 pm
i hope this helps anyone else doing pracs right now - i went pretty in depth (i think), but i didnt do discussion as my assessment wont have any of that end stuff.
Post by: jakesilove on June 17, 2017, 04:36:26 pm
hey so this is the molar heat of combustion prac? i was wondering if i covered everything or if there was any issue with my procedure. Any and all advice appreciated ;D
i hope this helps anyone else doing pracs right now - i went pretty in depth (i think), but i didnt do discussion as my assessment wont have any of that end stuff.
Bloody brilliant. Literally the only thing I would change is the wording of your assessment of validity etc. Instead of saying 'not valid', say something along the lines of 'validity is severely limited due to...'. It is still, to a certain extent, valid; however, you're just indicating that there is much to be improved.
Again, brilliant, in-depth summary. Great job!
Post by: beau77bro on June 17, 2017, 04:38:41 pm
Bloody brilliant. Literally the only thing I would change is the wording of your assessment of validity etc. Instead of saying 'not valid', say something along the lines of 'validity is severely limited due to...'. It is still, to a certain extent, valid; however, you're just indicating that there is much to be improved.ok yay, duly noted i will change that now.
Again, brilliant, in-depth summary. Great job!
im annoyed though because the next prac, galvanic cells - i already did this research task on it and i cant find it :'( :'( so now gotta start from scratch. will post if i find it
Post by: bsdfjnlkasn on June 17, 2017, 05:29:51 pm
Does anyone have any resource they're willing to share for processing second hand data relating to AAS? I can't seem to find any practice questions and I have an assessment with this in it- any help would be really appreciated! :) :)
Post by: jakesilove on June 17, 2017, 05:39:16 pm
Hey there!
Does anyone have any resource they're willing to share for processing second hand data relating to AAS? I can't seem to find any practice questions and I have an assessment with this in it- any help would be really appreciated! :) :)
What do you mean re processing second hand data relating to AAS? Can you like 'make up' the sort of question you mean, or just give me a better indication of what information you'd like? Happy to help!
Post by: beau77bro on June 17, 2017, 07:07:40 pm
theres also some questions she posted (she said not to share them because copyright ahahha), so here yah go.
again im not really sure what your asking for, and i didnt do this, but hopefully this helps even a little? even in helping jake or the moderators (aka legends) find more questions for you.
Post by: bsdfjnlkasn on June 17, 2017, 07:45:45 pm
What do you mean re processing second hand data relating to AAS? Can you like 'make up' the sort of question you mean, or just give me a better indication of what information you'd like? Happy to help!
Hey sorry about the lack of clarity!
On my notification one of the task outcomes is: process second hand data relating to AAS and sulfate content in fertiliser. Some examples would probably be the questions given in conquering chemistry as I find them quite difficult but any general questions about sketching or interpreting AAS graphs would be really helpful. I'm sort of getting the hang of them now but would like to expose myself to all the question types/harder versions of them. Also any ideas what they could ask in regards to the sulfate content in fertiliser apart from calculating the percentage in a sample?
Thanks again :D
Post by: beau77bro on June 17, 2017, 07:50:02 pm
Hey bsdfjnlkasn did the things I posted help?
Hey sorry about the lack of clarity!
On my notification one of the task outcomes is: process second hand data relating to AAS and sulfate content in fertiliser. Some examples would probably be the questions given in conquering chemistry as I find them quite difficult but any general questions about sketching or interpreting AAS graphs would be really helpful. I'm sort of getting the hang of them now but would like to expose myself to all the question types/harder versions of them. Also any ideas what they could ask in regards to the sulfate content in fertiliser apart from calculating the percentage in a sample?
Thanks again :D
Post by: bsdfjnlkasn on June 17, 2017, 07:50:43 pm
this is a video from my teacher, its abit slow, but i think it has what you want. https://www.youtube.com/watch?v=b4H0QPwWeDI
theres also some questions she posted (she said not to share them because copyright ahahha), so here yah go.
again im not really sure what your asking for, and i didnt do this, but hopefully this helps even a little? even in helping jake or the moderators (aka legends) find more questions for you.
Hey thank you so much, these are the sorts of questions I was looking for :D! I'll definitely give these a crack, sorry to be really annoying but do you think you could possibly send the answers to these questions? I'm still not confident enough to be sure of my working at this point, but if you don't have any then I can just post them on here and get some confirmation. Thanks again, I really appreciate it :)
Post by: beau77bro on June 17, 2017, 07:53:45 pm
Post by: beau77bro on June 17, 2017, 08:42:32 pm
also how do u deal with molar concentration in more complicated overall reactions? like for a reaction where it takes 2 moles of one metal to displace 1 mole of the other, does that mean you use 2 molar solution of the first metal ions, or what? - how does that affect charge flow?
thankyou, sorry for squeezing soo many questions in was kind of a train of thought thing
Post by: bsdfjnlkasn on June 17, 2017, 08:56:30 pm
hey so i was wondering why are galvanic cell voltages so often different to the ones worked out from the data sheet? like a list of reasons would be nice - is it just that the standard conditions were used or it wasn't in 1 molar solutions. we found in class that increasing the size of the salt bridge would increase the voltage dramatically, because of course the metal wires wont be the limiting factor in the movement of charge, it would be the salt bridge right?
also how do u deal with molar concentration in more complicated overall reactions? like for a reaction where it takes 2 moles of one metal to displace 1 mole of the other, does that mean you use 2 molar solution of the first metal ions, or what? - how does that affect charge flow?
thankyou, sorry for squeezing soo many questions in was kind of a train of thought thing
Hey there!
Here are some reasons for the discrepancies in read voltage and the theoretical value obtained through the standard potentials:
1. Not maintaining SLC is a major reason as we can't exactly ensure that the concentrations are all 1M (except always annotate this in your diagrams)
- Higher concentrations will increase the rate of reaction/current flow
- Lab conditions and concentrations are linked as if we were to increase the temperature of the room, then we make the solutions more concentrated (as water molecules would have evaporated)
2. Current flow is inhibited by resistance in wires (from external circuit)
3. There may be impurities on the electrodes so cleaning them will make it easier for the respective redox reactions to occur
4. Difficult to regulate the dispersion of ions in the salt bridge which also adds resistance
- If the ions haven't moved into their respective beakers, then we don't have a complete circuit preventing charge from flowing. But this doesn't stop the redox reactions from occurring, so the reactions go without powering anything because of the slow dispersion in the salt bridge. By the time there is a full circuit, the voltage would have decreased as there is less metal than what we started off with (as some have already reacted when we couldn't get an accurate reading)
NB: This is a really hazy explanation so would appreciate it if someone could step in and clarify things :)
Hopefully these helped :)
Post by: beau77bro on June 17, 2017, 08:59:30 pm
is it to do with the base reacting with the glass - wont that still happen in the flask? or maybe its to do with clogging the tap somehow? i really dont know
Post by: bsdfjnlkasn on June 17, 2017, 09:06:13 pm
also, for titrations my teacher says to always put the base in the flask - im not sure why though?
is it to do with the base reacting with the glass - wont that still happen in the flask? or maybe its to do with clogging the tap somehow? i really dont know
Hi hopefully this is of some help too :)
With titrations, you should always be putting the substance you're trying to find the concentration of in the burette as you will be able to obtain an exact volume and then deduce it's concentration (n = cv).
Perhaps you were always doing the same type of titration where you knew the base's concentration so put it in the volumetric flask to detect the colour change and so calculate the unknown acid concentration. But I think you should be approaching titrations a bit more open-mindedly and think about what you're trying to find out and how that affects the set up of the apparatus. It's got nothing to do with the base as a substance, don't worry it won't clog a burette up. Titration is about using a known concentration of either a base or acid (being the standard solution or analyte when sitting in the flask) to deduce an unknown concentration (titrant being delivered by burette).
Let me know if you have any more questions :)
Post by: beau77bro on June 17, 2017, 09:09:19 pm
Hey there!
Here are some reasons for the discrepancies in read voltage and the theoretical value obtained through the standard potentials:
1. Not maintaining SLC is a major reason as we can't exactly ensure that the concentrations are all 1M (except always annotate this in your diagrams)
- Higher concentrations will increase the rate of reaction/current flow
- Lab conditions and concentrations are linked as if we were to increase the temperature of the room, then we make the solutions more concentrated (as water molecules would have evaporated)
2. Current flow is inhibited by resistance in wires (from external circuit)
3. There may be impurities on the electrodes so cleaning them will make it easier for the respective redox reactions to occur
4. Difficult to regulate the dispersion of ions in the salt bridge which also adds resistance
- If the ions haven't moved into their respective beakers, then we don't have a complete circuit preventing charge from flowing. But this doesn't stop the redox reactions from occurring, so the reactions go without powering anything because of the slow dispersion in the salt bridge. By the time there is a full circuit, the voltage would have decreased as there is less metal than what we started off with (as some have already reacted when we couldn't get an accurate reading)
NB: This is a really hazy explanation so would appreciate it if someone could step in and clarify things :)
Hopefully these helped :)
ahahha sorry to be a noob but whats SLC? thankyou for all the extra points (esp the temperature one and evaporation-never thought of that). also i think we agree about the salt bridge idea, but i kind of imagine it as the fact that 'you're only as strong as your weakest link' kinda dealio. so if the current in the wire can move through very efficiently (though there will be resistance like you said) then it will, however we have to preserve electrical neutrality throughout the circuit and maintain a closed circuit. so if the ions are moving slowly through, or at a decreased rate (due to small cross sectional area), then current can only move through the wire at a similarly limited rate - it is possible we r saying the same idea and have misunderstood. it is equally likely i am completely wrong ahahahha but hopefully more students can clarify. thanks bsdfjnlkasn
Post by: beau77bro on June 17, 2017, 09:14:08 pm
Hi hopefully this is of some help too :)
With titrations, you should always be putting the substance you're trying to find the concentration of in the burette as you will be able to obtain an exact volume and then deduce it's concentration (n = cv).
Perhaps you were always doing the same type of titration where you knew the base's concentration so put it in the volumetric flask to detect the colour change and so calculate the unknown acid concentration. But I think you should be approaching titrations a bit more open-mindedly and think about what you're trying to find out and how that affects the set up of the apparatus. It's got nothing to do with the base as a substance, don't worry it won't clog a burette up. Titration is about using a known concentration of either a base or acid (being the standard solution or analyte when sitting in the flask) to deduce an unknown concentration (titrant being delivered by burette).
Let me know if you have any more questions :)
yea i totally agree, i thought the unknown 'titrant' was placed in the burrette always. but my teacher said there was an issue due to bases having some affect on the burrette - something i will have to check with him (im not sure i trust him ahahhhaha because hes not a great chem teacher). but conversely - it really shouldnt matter which substance you place in which - if you know the volume of each part used and the concentration of one you will be able to determine the concentration of the second independant of if it was in the beaker or not. i do recognise that it is the standard procedure so im more questioning my teacher -.-
thanks again bsdfjnlkasn
Post by: bsdfjnlkasn on June 17, 2017, 09:22:18 pm
ahahha sorry to be a noob but whats SLC? thankyou for all the extra points (esp the temperature one and evaporation-never thought of that). also i think we agree about the salt bridge idea, but i kind of imagine it as the fact that 'you're only as strong as your weakest link' kinda dealio. so if the current in the wire can move through very efficiently (though there will be resistance like you said) then it will, however we have to preserve electrical neutrality throughout the circuit and maintain a closed circuit. so if the ions are moving slowly through, or at a decreased rate (due to small cross sectional area), then current can only move through the wire at a similarly limited rate - it is possible we r saying the same idea and have misunderstood. it is equally likely i am completely wrong ahahahha but hopefully more students can clarify. thanks bsdfjnlkasn
No worries :) SLC is standard lab conditions. They're detailed at the back of the periodic table and according to which "condition" an experiment is conducted in, you can determine gas volumes. This is the only calculation application I can think of, but it's generally a good thing to mention when discussing the accuracy of particular experiments. Also, I think we are saying the same thing about the salt bridge which is a relief - your way is a much clearer way of explaining it too.
Post by: beau77bro on June 17, 2017, 09:24:08 pm
No worries :) SLC is standard lab conditions. They're detailed at the back of the periodic table and according to which "condition" an experiment is conducted in, you can determine gas volumes. This is the only calculation application I can think of, but it's generally a good thing to mention when discussing the accuracy of particular experiments. Also, I think we are saying the same thing about the salt bridge which is a relief - your way is a much clearer way of explaining it too.
well im glad me helping the great help u gave me helped u, which helped me ahahaha
Post by: bsdfjnlkasn on June 17, 2017, 09:27:46 pm
yea i totally agree, i thought the unknown 'titrant' was placed in the burrette always. but my teacher said there was an issue due to bases having some affect on the burrette - something i will have to check with him (im not sure i trust him ahahhhaha because hes not a great chem teacher). but conversely - it really shouldnt matter which substance you place in which - if you know the volume of each part used and the concentration of one you will be able to determine the concentration of the second independant of if it was in the beaker or not. i do recognise that it is the standard procedure so im more questioning my teacher -.-
thanks again bsdfjnlkasn
Yeah, you're definitely right about the titrant going in the burette. I personally haven't heard anything about bases affecting glassware, because like you said, we wouldn't be able to hold the base in the beaker/flask either if that were the case... But you seem to have a good understanding of titrations regardless, but just quickly, I do think the standard procedure is the best way to find exact concentrations of unknown substances and I don't think we should really be thinking about it in reverse :P Just keep it simple and work with the precision of the burette to give you the volume for the unknown you need 8)
Post by: beau77bro on June 17, 2017, 09:33:32 pm
Yeah, you're definitely right about the titrant going in the burette. I personally haven't heard anything about bases affecting glassware, because like you said, we wouldn't be able to hold the base in the beaker/flask either if that were the case... But you seem to have a good understanding of titrations regardless, but just quickly, I do think the standard procedure is the best way to find exact concentrations of unknown substances and I don't think we should really be thinking about it in reverse :P Just keep it simple and work with the precision of the burette to give you the volume for the unknown you need 8)
agreed. im sticking by that unless i find out otherwise from my teacher - he will be marking the practical after all. but it is interesting. thanks so much bsdfjnlkasn massive help and enjoyed the discussion honestly
Post by: bsdfjnlkasn on June 17, 2017, 09:38:28 pm
agreed. im sticking by that unless i find out otherwise from my teacher - he will be marking the practical after all. but it is interesting. thanks so much bsdfjnlkasn massive help and enjoyed the discussion honestly
No worries! It was good revision for me haha. Best of luck with everything, if you have any more questions post them up and hopefully someone will be free to reply :)
Post by: bsdfjnlkasn on June 17, 2017, 09:41:06 pm
I was just wondering if the line of best fit for an AAS curve always has to go through the origin? Usually we would base our line off the data given and only from the first data point to the last, but because the line is defined c = ka should we make passing the line through the origin a priority? The particular example I'm working on ends up having all the points above the line if I pass it through the origin. If anyone has any advice that would be super handy - thank you :D
Post by: beau77bro on June 17, 2017, 09:45:59 pm
Hey there,
I was just wondering if the line of best fit for an AAS curve always has to go through the origin? Usually we would base our line off the data given and only from the first data point to the last, but because the line is defined c = ka should we make passing the line through the origin a priority? The particular example I'm working on ends up having all the points above the line if I pass it through the origin. If anyone has any advice that would be super handy - thank you :D
what are the variables of c = ka, is C speed of light or what?
Post by: kiwiberry on June 17, 2017, 10:52:08 pm
what are the variables of c = ka, is C speed of light or what?C is concentration and A is absorbance :)
Post by: jakesilove on June 17, 2017, 11:54:50 pm
Hey there,
I was just wondering if the line of best fit for an AAS curve always has to go through the origin? Usually we would base our line off the data given and only from the first data point to the last, but because the line is defined c = ka should we make passing the line through the origin a priority? The particular example I'm working on ends up having all the points above the line if I pass it through the origin. If anyone has any advice that would be super handy - thank you :D
You definitely DO NOT want to create a point that goes through the origin, unless you actually measured that point. Otherwise, you've essentially fabricated results. I agree that the line SHOULD go through the origin, however just ASSUMING that it does is scientific fraud. In fact, a line that does not pass through the origin is almost a good thing. It means you can describe a systematic error; clearly, it should have gone through the origin, but it didn't. Why? What does that mean for the rest of your results? What does that mean for your experimental design?
In summary: do NOT assume anything, unless you've measured it. If you get weird results; GREAT! Explain it, using scientific methodology. Potentially, EXPLAIN what you've done to the marker, and why you've done it :)
Jake
Post by: chelseam on June 18, 2017, 12:09:26 am
Post by: chelseam on June 18, 2017, 01:07:31 am
Basically, the titration procedure requires a lot of rinsing to get the most accurate reading of concentration. Even the slightest mess-up can ruin the experiment.Thank you so much!! I really appreciate it ;D
I suggest this really good site!!
8)
Post by: bsdfjnlkasn on June 18, 2017, 09:20:52 am
You definitely DO NOT want to create a point that goes through the origin, unless you actually measured that point. Otherwise, you've essentially fabricated results. I agree that the line SHOULD go through the origin, however just ASSUMING that it does is scientific fraud. In fact, a line that does not pass through the origin is almost a good thing. It means you can describe a systematic error; clearly, it should have gone through the origin, but it didn't. Why? What does that mean for the rest of your results? What does that mean for your experimental design?
In summary: do NOT assume anything, unless you've measured it. If you get weird results; GREAT! Explain it, using scientific methodology. Potentially, EXPLAIN what you've done to the marker, and why you've done it :)
Jake
Hey Jake!
Thanks so much for the reply, I definitely agree with you here because the way graph an AAS calibration curve shouldn't differ from any other graph. BUT all the worked solutions I have seen (both in textbook and online) have considered the graph going through the origin which has me a bit worried. As a part of my assessment we do have to analyse secondary data relating to AAS so if you could possibly suggest a few errors that I would find in secondary experimental data, i'd really appreciate it! Then I could at least argue why I didn't graph the data going through the origin ... I just don't know why resources that HSC students rely on have always shown the graph going through the origin, it's really misleading.
Thanks again so much and hoping to hear back soon :D :D
Post by: jakesilove on June 18, 2017, 10:32:21 am
Hey Jake!
Thanks so much for the reply, I definitely agree with you here because the way graph an AAS calibration curve shouldn't differ from any other graph. BUT all the worked solutions I have seen (both in textbook and online) have considered the graph going through the origin which has me a bit worried. As a part of my assessment we do have to analyse secondary data relating to AAS so if you could possibly suggest a few errors that I would find in secondary experimental data, i'd really appreciate it! Then I could at least argue why I didn't graph the data going through the origin ... I just don't know why resources that HSC students rely on have always shown the graph going through the origin, it's really misleading.
Thanks again so much and hoping to hear back soon :D :D
As I understand it, you actually took measurements, but you didn't take a point where there was a concentration of zero in the apparatus? If that is so then, regardless of other factors, adding that 'zero point' would be fabricating data. This would be easy to justify in a write-up: 'I didn't take that measurement, so I didn't include it'. This is something you can say you would improve for next time, to increase the validity of the experiment. You would argue there is some systematic error in your apparatus to explain why it DIDN'T go through the origin (ie. maybe there was a tiny bit of matter floating around, even when you hadn't put anything in there, left over from past experiments. Maybe the air absorbs some of the light. etc. etc. etc.).
All in all, I would shoot your teacher an email to confirm what I'm saying. Whilst I'm definitely 'right', there is a slim chance that they would prefer what is 100% 'wrong'. That's a decision for you to make, but yeah the above is how I would handle this write up.
Post by: bsdfjnlkasn on June 18, 2017, 11:05:14 am
I was wondering if anyone could help me with deducing the reason we do some steps in the gravimetric analysis where we deduce the sulfate content in fertiliser. Why do we add acid? Does it matter which we use? HNO3 or HCl do they do the same thing? If someone could please provide some chemical equations that would be so helpful! Also, what's the point of adding BaCl2?
Thank you!!
Post by: jakesilove on June 18, 2017, 11:12:20 am
Hey there,
I was wondering if anyone could help me with deducing the reason we do some steps in the gravimetric analysis where we deduce the sulfate content in fertiliser. Why do we add acid? Does it matter which we use? HNO3 or HCl do they do the same thing? If someone could please provide some chemical equations that would be so helpful! Also, what's the point of adding BaCl2?
Thank you!!
Hey! First, we grind the fertiliser so it is easily dissolvable. Then, we weigh out a known quantity, and place it in a known volume of water. We add acid (generally, HCl, but really any acid is fine) to try and dissolve the soil as much as possible. Obviously, soil is not soluble in water, so we're just trying to break up all the little particles that it contains using that acid.
Then, we add excess barium chloride. This is to form a barium sulfate precipitate, which we can weigh. The relevant equation is
Then, we filter off the precipitate, weigh it, find the moles of sulfate originally present, and compare that to the mass of soil. Make sense?
Post by: bsdfjnlkasn on June 18, 2017, 11:45:29 am
Hey! First, we grind the fertiliser so it is easily dissolvable. Then, we weigh out a known quantity, and place it in a known volume of water. We add acid (generally, HCl, but really any acid is fine) to try and dissolve the soil as much as possible. Obviously, soil is not soluble in water, so we're just trying to break up all the little particles that it contains using that acid.
Then, we add excess barium chloride. This is to form a barium sulfate precipitate, which we can weigh. The relevant equation is
Then, we filter off the precipitate, weigh it, find the moles of sulfate originally present, and compare that to the mass of soil. Make sense?
Yes that was a helpful run down :)
If we were to add HCl instead of HNO3 what would the differences be? I've got that they neutralise any unwanted carbonates, phosphates and hydroxides which then could precipitate later on when we want to obtain BaSO4. This increases the overall solubility of the solution. I understand NO3 is more general as it is soluble - sorry if this is unnecessarily specific. The main point is that it neutralises and so removes any unneeded ions - right?
Thank you for clarifying!
Post by: jakesilove on June 18, 2017, 12:18:43 pm
Yes that was a helpful run down :)
If we were to add HCl instead of HNO3 what would the differences be? I've got that they neutralise any unwanted carbonates, phosphates and hydroxides which then could precipitate later on when we want to obtain BaSO4. This increases the overall solubility of the solution. I understand NO3 is more general as it is soluble - sorry if this is unnecessarily specific. The main point is that it neutralises and so removes any unneeded ions - right?
Thank you for clarifying!
Yep, add whatever acid you'd like. No need to go into details re why you're adding the particular acid
Post by: beau77bro on June 18, 2017, 02:02:10 pm
but it makes sense to have the burrette with the known, because if say you make a 1 molar solution of the standard - and you have a super weak acid or base as the unknown - it would take forever to do the experiment if you had the unknown in the burrette
but i guess the reverse is true. very unsure about this prac - but very sure it is gonna be the prac we do.
Post by: beau77bro on June 18, 2017, 02:16:32 pm
Post by: Alalamc on June 18, 2017, 02:27:18 pm
The ionisation constant of hypobromous acid, HBrO, is 2.06x10-9 at a temperature at which Kw is 1.2x10-14. Determine both the pH and the pOH of a 0.25 M solution of this acid at that temperature.
Also how can we tell if a reaction (at equilibrium) is endothermic or exothermic?
Also does my working out seem correct? (considering the fact that H+ & OH- are the same in water.
What is the value of Kw if the pH of a sample of pure water is 6.77? Is the temperature of this solution above or below 25oC?
pH = -log[H+]
6.77 = -log[H+]
[H+] = 1 times 10^ (-6.77)
[H+] = 1.6982 times 10^ (-7)
Since at the given pH for the given temperature water is neutral H+ = OH-, [OH-] =1.6982 times 10^ (-7).
Thus Kw= [H+] [OH-]
Kw= 1.6982 × 10^ (-7) times 1.6982 × 10^ (-7)
= 2.88388324 × 10^ (-14)
Post by: jakesilove on June 18, 2017, 02:31:13 pm
hey im now very confused. what is the standard procedure, is it the unknown in the burrette or the flask. conquering chem says in the flask. people and teacher say in burrette. IDK WHAT TO DO
but it makes sense to have the burrette with the known, because if say you make a 1 molar solution of the standard - and you have a super weak acid or base as the unknown - it would take forever to do the experiment if you had the unknown in the burrette
but i guess the reverse is true. very unsure about this prac - but very sure it is gonna be the prac we do.
Literally will not make any difference; you will find the same value, no matter which way you do it. Most people just always put the base in the burrette, and the acid in the conical flask, regardless of whether they are the standard or the unknown. However, it genuinely doesn't matter, as long as you know which is which, and perform your calculations accordingly.
Post by: jakesilove on June 18, 2017, 02:33:13 pm
for galvanic cell calculations, how do u deal with molar concentration in more complicated overall reactions? like for a reaction where it takes 2 moles of one metal to displace 1 mole of the other, does that mean you use 2 molar solution of the first metal ions, or what? - how does that affect charge flow? - and what does that have to do with the standard potentials
Is this part of the core course, or your option? If it's part of the core, steer clear of it. Basically, the higher the concentration of ions, the more current will flow. This only matter if they give you data, indicating this trend, and you need to analyse it. You definitely don't need to know it, or anything about it. There are really complicated formulas dealing with this sort of this; your table of standard reduction potentials assume a concentration of 1mol/L, and you should too.
Post by: jakesilove on June 18, 2017, 02:34:26 pm
Hi guys this question has me baffled? :) :)
The ionisation constant of hypobromous acid, HBrO, is 2.06x10-9 at a temperature at which Kw is 1.2x10-14. Determine both the pH and the pOH of a 0.25 M solution of this acid at that temperature.
Also how can we tell if a reaction (at equilibrium) is endothermic or exothermic?
Also does my working out seem correct? (considering the fact that H+ & OH- are the same in water.
What is the value of Kw if the pH of a sample of pure water is 6.77? Is the temperature of this solution above or below 25oC?
pH = -log[H+]
6.77 = -log[H+]
[H+] = 1 times 10^ (-6.77)
[H+] = 1.6982 times 10^ (-7)
Since at the given pH for the given temperature water is neutral H+ = OH-, [OH-] =1.6982 times 10^ (-7).
Thus Kw= [H+] [OH-]
Kw= 1.6982 × 10^ (-7) times 1.6982 × 10^ (-7)
= 2.88388324 × 10^ (-14)
Unfortunately, I really can't help you out here. I didn't do this option, and so I'm not even clear on the terminology. Hopefully someone else on the forum can help!
Post by: tashlyc on June 18, 2017, 02:37:31 pm
Post by: beau77bro on June 18, 2017, 02:49:56 pm
Literally will not make any difference; you will find the same value, no matter which way you do it. Most people just always put the base in the burrette, and the acid in the conical flask, regardless of whether they are the standard or the unknown. However, it genuinely doesn't matter, as long as you know which is which, and perform your calculations accordingly.
ok sweet i thought so - what would u suggest?
Is this part of the core course, or your option? If it's part of the core, steer clear of it. Basically, the higher the concentration of ions, the more current will flow. This only matter if they give you data, indicating this trend, and you need to analyse it. You definitely don't need to know it, or anything about it. There are really complicated formulas dealing with this sort of this; your table of standard reduction potentials assume a concentration of 1mol/L, and you should too.its core but it's a prac, i was just wondering if we make the solutions in ratio to the overall reaction equation - because will that effect the potential difference if it isnt? - or will one jsut run out faster(just realised this is probably the case)?
Post by: jakesilove on June 18, 2017, 03:12:12 pm
ok sweet i thought so - what would u suggest?
its core but it's a prac, i was just wondering if we make the solutions in ratio to the overall reaction equation - because will that effect the potential difference if it isnt? - or will one jsut run out faster(just realised this is probably the case)?
No suggestion; personally, I always put the base in the burrette.
Yep, it'll just run out faster!
Post by: jakesilove on June 18, 2017, 03:18:06 pm
Can someone please explain how to figure this equilibrium question, I'm getting different answers to the back of the book.
My understanding is that
Now, we know that the final moles of NO was 0.062 moles. Therefore, 0.038 moles of NO has been reacted. The same number of moles of Hydrogen gas must have been reacted (2:2) leaving 0.051-0.038=0.013 moles. Note that finding the moles is the same as finding the concentration, as there is one litre of substance. Now, the final concentration of water will be 0.038 moles/L (1:1 reaction with NO) and the final concentration of Nitrogen gas will be 0.019 moles/L (2:1 reaction with NO). Therefore
Post by: bsdfjnlkasn on June 18, 2017, 03:19:10 pm
Yep, add whatever acid you'd like. No need to go into details re why you're adding the particular acid
Awesome thank you very much! :D
I was just wondering if you could please clarify whether the following (final) step in titration is necessary:
8. To ensure permanent colour change, the nozzle was sprayed from a wash bottle with water and the analyte observed for consistent colour
Why is this allowed? I've seen this in a couple places online and on videos but am convinced that this dilutes the primary standard meaning more volume is required to hit the endpoint. Doesn't this final step then make the whole procedure inaccurate as it suggests more volume of the titrant is needed to neutralise the primary standard?
Would love some clarity on this issue, here's a link that detailed this final step (https://www.youtube.com/watch?v=sFpFCPTDv2w from 5:55).
Thank you!!
Post by: RuiAce on June 18, 2017, 03:19:46 pm
Can someone please explain how to figure this equilibrium question, I'm getting different answers to the back of the book.(http://uploads.tapatalk-cdn.com/20170618/d77145c3f42229e0f7bd36742f14989f.jpg)I've found that drawing arrows may help.
Post by: jakesilove on June 18, 2017, 03:22:26 pm
Awesome thank you very much! :D
I was just wondering if you could please clarify whether the following (final) step in titration is necessary:
8. To ensure permanent colour change, the nozzle was sprayed from a wash bottle with water and the analyte observed for consistent colour
Why is this allowed? I've seen this in a couple places online and on videos but am convinced that this dilutes the primary standard meaning more volume is required to hit the endpoint. Doesn't this final step then make the whole procedure inaccurate as it suggests more volume of the titrant is needed to neutralise the primary standard?
Would love some clarity on this issue, here's a link that detailed this final step (https://www.youtube.com/watch?v=sFpFCPTDv2w from 5:55).
Thank you!!
Diluting whatever is in the conical flask makes absolutely no difference to the experiment. Remember, once we've put stuff into the flask, we no longer care about concentration. Rather, we care about the number of moles/atoms in that conical flask. You can add as much water as you'd like, and it won't affect the overall calculation. Essentially, we only use concentration so that we can put a set number of moles into the flask. Once we've done this, concentration no longer becomes important!
Still, I don't rate the first half of that last step. No need to wash the nozzle with a wash bottle. However, you should wait 10 seconds to ensure the colour change is permanent (sometimes, the colour change reverts after a few seconds, and you have to add another drop!). As long as your method is consistent (eg. 10 seconds, 20, 30, 5) then you've got yourself a good experiment :)
Post by: jakesilove on June 18, 2017, 03:23:23 pm
(http://uploads.tapatalk-cdn.com/20170618/d77145c3f42229e0f7bd36742f14989f.jpg)I've found that drawing arrows may help.
Yeah, in my working out I totally ignored the original 0.1M of water! So, my answer is 'NEARLY' correct, you just need to add 0.1 to the concentration of water in the final solution :)
Post by: RuiAce on June 18, 2017, 03:36:58 pm
Yeah, in my working out I totally ignored the original 0.1M of water! So, my answer is 'NEARLY' correct, you just need to add 0.1 to the concentration of water in the final solution :)What's the bet that the answer changes drastically with that 0.1 forgotten? 8)
Post by: jakesilove on June 18, 2017, 03:37:45 pm
What's the bet that the answer changes drastically with that 0.1 forgotten? 8)
Oh, only by like a factor of 10
Post by: RuiAce on June 18, 2017, 03:40:32 pm
Oh, only by like a factor of 10Yeah lol. But that's still funny in an unfortunate way
Post by: kiwiberry on June 18, 2017, 03:40:49 pm
ok sweet i thought so - what would u suggest?
its core but it's a prac, i was just wondering if we make the solutions in ratio to the overall reaction equation - because will that effect the potential difference if it isnt? - or will one jsut run out faster(just realised this is probably the case)?
Just as a note, when you're using phenolphthalein, it's better to put the base in the burette and acid in the conical flask because it's easier to see the colour change from colourless --> light pink than dark pink --> light pink
Can someone please explain how to figure this equilibrium question, I'm getting different answers to the back of the book.
I like using an ice table for these type of questions :)
(http://i.imgur.com/sdLyWDT.png)
Post by: bsdfjnlkasn on June 18, 2017, 03:43:41 pm
Diluting whatever is in the conical flask makes absolutely no difference to the experiment. Remember, once we've put stuff into the flask, we no longer care about concentration. Rather, we care about the number of moles/atoms in that conical flask. You can add as much water as you'd like, and it won't affect the overall calculation. Essentially, we only use concentration so that we can put a set number of moles into the flask. Once we've done this, concentration no longer becomes important!
Still, I don't rate the first half of that last step. No need to wash the nozzle with a wash bottle. However, you should wait 10 seconds to ensure the colour change is permanent (sometimes, the colour change reverts after a few seconds, and you have to add another drop!). As long as your method is consistent (eg. 10 seconds, 20, 30, 5) then you've got yourself a good experiment :)
Hey thanks again Jake!
Why wouldn't diluting the solution in the conical flask change the number of moles? Doesn't n = cv?
As you can probably tell I've got an assessment tomorrow and want to clarify lots of last minute things.
One of the outcomes we're being assessed is on our ability to evaluate the validity of our conclusions and I was wondering if you could give a general explanation of what this means as well as some points on validity for a titration experiment.
Thanks so much :D
EDIT: This is a reply I received regarding including the origin in AAS calibration curves:
"The line of best fit should only go through the origin if it is appropriate to do so.
Hope that helps."
So should I take that as confirmation only to include (0,0) if it is a specific data point given?
Finally, how should I account for the rough titration in my method for the titration?
Post by: jakesilove on June 18, 2017, 03:54:09 pm
Hey thanks again Jake!
Why wouldn't diluting the solution in the conical flask change the number of moles? Doesn't n = cv?
As you can probably tell I've got an assessment tomorrow and want to clarify lots of last minute things.
One of the outcomes we're being assessed is on our ability to evaluate the validity of our conclusions and I was wondering if you could give a general explanation of what this means as well as some points on validity for a titration experiment.
Thanks so much :D
EDIT: This is a reply I received regarding including the origin in AAS calibration curves:
"The line of best fit should only go through the origin if it is appropriate to do so.
Hope that helps."
So should I take that as confirmation only to include (0,0) if it is a specific data point given?
Finally, how should I account for the rough titration in my method for the titration?
Yes, n=CV. However, if you increase the volume of water, you're decreasing the concentration. If I put 10 tennis balls in a beaker (equivalent to moles of a substance), and add loads of water, there will still only be 10 tennis balls in the beaker. So, diluting the conical flask with water does not change the moles, just the concentration.
Validity asks the question: have you tested what you're trying to test? Specific to titration, this comes down to using correct washing techniques, the correct indicator, and the correct standard solution.
Yep, don't use (0,0) unless that was a measurement actually taken.
In your method, just say something like 'Allow _____ to drip from the Burrette in to the Conical flask until a colour change is detected. Record the change in volume, and use this is a 'rough' run-through, from which future 'precise' experiments can be estimated'
Post by: bsdfjnlkasn on June 18, 2017, 05:28:56 pm
What would happen if we rinsed the burette with the acid (if the base was supposed to go in there). Would a neutralisation reaction begin to occur in the burette increasing it's pH? Does that then reduce the volume required to neutralise the standard solution if it were put in the conical flask?
Is the standard solution often the base?
What difference would it make if we were to put base through a pipette designate to transfer the acid?
Post by: jakesilove on June 18, 2017, 05:32:40 pm
Hey there,
What would happen if we rinsed the burette with the acid (if the base was supposed to go in there). Would a neutralisation reaction begin to occur in the burette increasing it's pH? Does that then reduce the volume required to neutralise the standard solution if it were put in the conical flask?
Is the standard solution often the base?
What difference would it make if we were to put base through a pipette designate to transfer the acid?
If you rinse the burrette with the acid, and then put the base into it, then some of the base will be neutralised. This means that the concentration of the base would be lower than expected, and would therefore require MORE base to neutralise the acid in the conical flask. Therefore, your final calculations would show a LOWER concentration than the original, 'real' concentration.
Yes, the standard solution is usually NaOH. But it could really be anything.
Again, if you had already washed the pipette in Acid, and then you use it to transfer the Base, the Base will decrease in concentration. Same as above.
Post by: beau77bro on June 18, 2017, 06:18:34 pm
like what's the difference between a volumetric flask and a conical flask? and how much standard solution should we make up for a titration. expect a posted prac before this night ends as well as possibly a galvanic cell one or a soft drink one
thanks guys
also where do we use pipettes in titrations even?
how do we accurately move the solution to be tested to conical flask under the burrette? do we just rinse the measuring cylinder endlessly with distilled water because it doesnt effect the number of moles
endless questions omg
Mod edit: Merged posts. If you post multiple times, and you haven't received an answer, you can always modify your original post to add questions
Post by: jakesilove on June 18, 2017, 08:29:46 pm
hey so what is the basic apparatus/ equipment used in titrations - i know generally, but im struggling with what they actually look like and what they are used for.
like what's the difference between a volumetric flask and a conical flask? and how much standard solution should we make up for a titration. expect a posted prac before this night ends as well as possibly a galvanic cell one or a soft drink one
thanks guys
also where do we use pipettes in titrations even?
how do we accurately move the solution to be tested to conical flask under the burrette? do we just rinse the measuring cylinder endlessly with distilled water because it doesnt effect the number of moles
endless questions omg
Mod edit: Merged posts. If you post multiple times, and you haven't received an answer, you can always modify your original post to add questions
Glassware in titration:
Volumetric flask: used to bring your standard solution up to a set volume. ie. 250mL, 500mL etc.
Conical flask: Used to store a set quantity of some solution, below the burrette
Burrette: Used to store a quantity of some solution, and be able to measure the change in solution levels after titration
Pipette: Used to transfer solution from the volumetric flask to the conical flask.
You can make as much standard as you want. You'll generally use around 25mL of standard per titration: generally, you'll make about 250mL of standard solution.
You shouldn't be using a measuring cylinder in a titration. The only thing you can wash with only water (ie. not the solution going into it) is the conical flask.
I think that answers everything!
Post by: beau77bro on June 18, 2017, 08:39:11 pm
also how do we deal with diprotic acids when testing?? do we assume they go to completion, how would we assume they didnt? and what indicator? like i really have no clue what to do with that. so an explanation/ steps would be great (with calculations)
Post by: bsdfjnlkasn on June 18, 2017, 09:08:21 pm
Post by: beau77bro on June 18, 2017, 09:10:26 pm
Hey could someone please explain to me why the answer for this question is D. I got B and feel really confused... Thank you!
hey bsdfjnlkasn you need to remove the 3rd measurement as it is an outlier, then take the average of 4 left over measurements.
Post by: bsdfjnlkasn on June 18, 2017, 09:10:40 pm
how should u move the solution being tested in the conical flask? like how do u fill it accurately? like will all conical flasks have measurements all down the side?
You use the pipette to fill the conical flask (same thing as the volumetric flask). These flasks don't have measurements down the side and don't actually need them since we are accurately drawing up solution with pipettes and transferring them this way.
Here's a useful video that will help you visualise the titration a bit better :D :
https://www.youtube.com/watch?v=sFpFCPTDv2w
Post by: beau77bro on June 18, 2017, 09:14:39 pm
You use the pipette to fill the conical flask (same thing as the volumetric flask). These flasks don't have measurements down the side and don't actually need them since we are accurately drawing up solution with pipettes and transferring them this way.our schools have very bad pipettes to put it lightly and im attrocious with them. idk how to deal with that fact, how do u even get better at that omg
Here's a useful video that will help you visualise the titration a bit better :D :
https://www.youtube.com/watch?v=sFpFCPTDv2w
theres a second question added above btw:
'also how do we deal with diprotic acids when testing?? do we assume they go to completion, how would we assume they didnt? and what indicator? like i really have no clue what to do with that. so an explanation/ steps would be great (with calculations)'
Post by: bsdfjnlkasn on June 18, 2017, 09:24:22 pm
our schools have very bad pipettes to put it lightly and im attrocious with them. idk how to deal with that fact, how do u even get better at that omg
See if you have the opportunity to practice with them before your exam - I find them tricky too!
Post by: bsdfjnlkasn on June 18, 2017, 09:28:37 pm
Could someone please give me a guide as to where the 4 marks in this question would be awarded. It's from the 2004 HSC paper and I can't seem to find proper solutions anywhere.
I've identified the outlier in trial 4 and stated that this led to invalid results as unreliable results were used. Further, the calculated result for [Hg2+] would end up being lower since the provided average is lower. I mentioned that regardless of the experiment being repeated, the results weren't consistent and so the overall experiment was made unreliable. Since the experiment wasn't reliable, we can't completely engage the aim which means that our validity is also compromised.
Is there anything else that I should add? How do I make the link between validity and reliability clearer? Have I missed any important information out?
Thank you, any help would be super appreciated :D
P.S: Here are the guidelines
The better candidates were able to clearly state the relevant points to support their judgement about both validity and reliability. There were a large number of responses that linked both terms together and showed no clear understanding of the difference between them. Most responses identified the outlier but stated incorrectly that including it in the average made the result more valid or more reliable or more accurate.
Post by: beau77bro on June 18, 2017, 09:35:25 pm
forgot the requirements for the standard - but im not sure it's in the prac so im not gonna focus too much yet
Post by: jakesilove on June 18, 2017, 09:42:50 pm
Post by: beau77bro on June 18, 2017, 11:12:21 pm
which leads to my question - what exactly are processing skills? and what would a processing skills assessment for chem consist of?
Post by: Alalamc on June 19, 2017, 12:09:02 pm
Question: Consider the reaction X2(g)+2Y(g) ↔ 2XY(g). 4.0 moles of X2 are mixed with 6.0 mole of Y in a 1.0-L container. When equilibrium is achieved at 30°C, it is found that 4.0 moles of XY are present. What is the equilibrium constant for this reaction at this temperature?
2 Mols XY / 2 mols of Y= 4 Mols XY/ x
x= 2 Mols of Y present (6-2=4 Mols of Y reacted)
2 Mols Y/ 1 Mols X2= 4 Mols/x
x= 2 Mols X2 reacted(4-2=2 Mols present at Eqm)
At eqm in 1 L
[X2]= 4-2= 2M
[Y]= 6-4= 2M
[XY]= 0+4= 4M
K= [XY] / [Y] [X2]
K= [4]^(2) / [2]^(2) times [2]
K = 2
Post by: bluecookie on June 19, 2017, 12:11:01 pm
Okay so, in a galvanic cell the chemical reactions between the anode and cathode electrodes with the solutions produce a current which causes oxidation or reduction to occur. But in electrolytic cells there's no anode and cathode, because in the diagram in my book they're just made of carbon, and there for the purposes of conducting electricity. So to my understanding in galvanic cells the negative/positive charges of the electrodes interaction with the negative/positive solutions generate the current and reaction they undergo at each electrode. But what happens in an electrolytic cell where there's no negative/positive electrodes?
Also, in a galvanic cell there is a solid form of both electrodes as well as a liquid form. So as the reduction/oxidation reactions occur, one of the element's electrolytes gains electrons and forms a solid on the solid electrode of itself? Whilst the other electrode loses electrons and dissolves into a liquid into it's own electrolyte solution? I'm confused because in an electrolytic cell there's no solid form of either of the elements used - they're just aqueous ions in a solution. So how does oxidation occur? When there's no solid electrode to give of an electron in order to become a liquid? I'm doing this question in my textbook (which doesn't have answers for this section, probs cause they thought it was too basic lmao) where its asking the reactions that occur at the electrodes and give equations (for an electrolytic cell of MgCl) and I'm confused cause since there's no solid electrode, wouldn't it just be reduction reactions occuring? Because the aqueous ions don't have another electron to give, so the most they can do is gain electrons from the battery at either electrode and become solids? So yeahhh, confused about that :/ And how/where the oxidation reaction occurs in an electrolytic cell.
Post by: johnk21 on June 19, 2017, 05:36:33 pm
Post by: beau77bro on June 19, 2017, 05:54:26 pm
ALL PRAC TIPS WANTED -will add them all to this post in edit for reference for other users
- the water goes into the BOTTOM of a condenser and out the TOP.
Post by: johnk21 on June 19, 2017, 05:59:19 pm
The addition of a catalyst as no effect on the yield as it only increases the rate of reaction and the time it takes to reach equilibrium.
An increase in pressure will shift to the right and increase the yield of N2O4. This is because an increase in pressure will shift towards the side with fewer moles (2:1).
The equilibrium forward reaction is exothermic because new bonds are formed from the two NO2 into one N2O4. I'm not familiar with the K-thing but I assume that's important.
8)
Thanks so much :)
Post by: beau77bro on June 19, 2017, 06:26:12 pm
Post by: ringring on June 19, 2017, 06:53:20 pm
Would appreciate any help/explanation for the following question..
An electrolytic cell contained platinum electrodes and 100.0 mL of an aqueous solution containing magnesium, nickel(II), silver, and sodium ions. The concentration of each of the ions in the solution was 0.200 M. A current of 0.800 A was passed through the cell for 2.50 hours. After this time the mass of the cathode would have increased by...
(answer is 3.33g btw)
Post by: kiwiberry on June 19, 2017, 07:00:10 pm
Can i please have help with this question??
You can also do part ii) by comparing the K value at different temperatures
At 25oC, 0.04 moles of N2O4 forms. Thus, 0.08 moles of NO2 were reacted (2:1 ratio) and 0.132-0.08=0.052 moles are left. Concentration of both are the same as the moles as v=1L. So K at 25oC is \(\frac{0.04}{(0.052)^2} = 14.79...\)
This tells us that when the temperature decreases from 100 --> 25oC, K increases. So to oppose the decrease in temperature, the concentration of products has increased, ie. equilibrium has shifted to the right to produce more heat (by Le Chatelier). Because the forward reaction is the one which produces more heat, it is exothermic. :)
Post by: jakesilove on June 19, 2017, 07:01:51 pm
what is a solid procedure for carrying out the decarbonation practical - im finding it hard to discern between just letting the can sit on the bench and coool - or using a warm bath - in which case how do i warm the water (school doesnt exactly have a good lab facility so a hot plate is very unlikely - pretty much why we havent done any computer based pracs or even physics cathode ray ones)
Already answered this here. As for prac techniques, I'm afraid there are just too many. Like, correct washing technique, meniscus, stuff like that is all specific to the prac. So, the best study you can do for yourself is write up complete practical reports for you to learn from
Post by: jakesilove on June 19, 2017, 07:03:25 pm
Hey!
Would appreciate any help/explanation for the following question..
An electrolytic cell contained platinum electrodes and 100.0 mL of an aqueous solution containing magnesium, nickel(II), silver, and sodium ions. The concentration of each of the ions in the solution was 0.200 M. A current of 0.800 A was passed through the cell for 2.50 hours. After this time the mass of the cathode would have increased by...
(answer is 3.33g btw)
I have literally never seen any question like this. Is it part of an HSC elective topic? I could maybe sort of think about it, if I had resistance or voltage, but with Current? Genuinely no idea.
Post by: ringring on June 19, 2017, 07:06:18 pm
I have literally never seen any question like this. Is it part of an HSC elective topic? I could maybe sort of think about it, if I had resistance or voltage, but with Current? Genuinely no idea.
sorry, I didn't realise this was the HSC thread, the question is part of the VCE study design.
Post by: jakesilove on June 19, 2017, 07:07:39 pm
sorry, I didn't realise this was the HSC thread, the question is part of the VCE study design.
Aha no worries! You can find the VCE Chemistry question thread here :)
Post by: beau77bro on June 19, 2017, 07:14:05 pm
Already answered this here. As for prac techniques, I'm afraid there are just too many. Like, correct washing technique, meniscus, stuff like that is all specific to the prac. So, the best study you can do for yourself is write up complete practical reports for you to learn from
hey jake, i already took notes on this :) the control idea was really helpful - essential to my prac. but im just confused on how to deal with condensation - and whether or not to use a water bath - is simplest always best or will the heated water provide a better source for removing carbon dioxide? i will post 2 example methods below - any input appreciated.
thankssss
Post by: chelseam on June 19, 2017, 07:17:11 pm
Post by: jakesilove on June 19, 2017, 07:18:12 pm
hey jake, i already took notes on this :) the control idea was really helpful - essential to my prac. but im just confused on how to deal with condensation - and whether or not to use a water bath - is simplest always best or will the heated water provide a better source for removing carbon dioxide? i will post 2 example methods below - any input appreciated.
thankssss
Either experiment is fine, totally up to you which you prefer. I did the first one, which is definitely easier.
Post by: jakesilove on June 19, 2017, 07:19:31 pm
Hi! Can someone please explain what the range of an indicator is? How you can tell what the range is if they give you a bunch of pHs and the colours they show when reacting with an indicator? Sorry if this is a silly question but I've just had a complete mind blank :( Thank you! :)
The 'range' of an indicator just asks where the indicator is useful. For instance, Bromothymol Blue changes colour around a pH of 7. So, it wouldn't be useful at all to tell the difference between two acidic substances. However, it would be very useful to determine the difference between an acidic and a basic substance! So, range just discusses in what region of the pH scale the indicator is useful to us as scientists :)
Post by: beau77bro on June 19, 2017, 07:26:01 pm
Post by: chelseam on June 19, 2017, 07:44:54 pm
So, range just discusses in what region of the pH scale the indicator is useful to us as scientists :)Thanks so much Jake!
Also, what would be the main points that we need to know about AAS? Is knowing how to draw a diagram of it important? My notes include stuff on hollow-cathode lamp selection and preparing the standard solution - are things like this necessary? Thank you :D
Post by: parthie on June 19, 2017, 08:00:17 pm
Post by: beau77bro on June 19, 2017, 08:09:10 pm
Post by: beau77bro on June 19, 2017, 08:22:07 pm
Hey does anyone know if titrations have dependent, independent and controlled variables if ur just titrating one thing
OK so to me - the independant is the colour of the solution/the neutralisation of the acid or base being tested
the dependant is what you measure - so it's the volume used to titrate it
the controlled will be the indicator and the primary standard used (pH constant, as well as how much is tested (if it is in the flask))
Post by: jakesilove on June 20, 2017, 09:56:07 am
Thanks so much Jake!
Also, what would be the main points that we need to know about AAS? Is knowing how to draw a diagram of it important? My notes include stuff on hollow-cathode lamp selection and preparing the standard solution - are things like this necessary? Thank you :D
That's already more information than you need! You need to understand the purpose of AAS, the method to perform AAS (ie. aspirate liquid in a flame, shine light through, compare with calibration curve) and the benefits of AAS, but that's basically it! Having those extra few details is nice, but definitely not necessary :)
Post by: jakesilove on June 20, 2017, 09:58:23 am
wait for glavanic cells, if it is the case that a the voltage is independent of the concentration in the cells - that is the molar ratio doesnt matter as it will simply use one faster than the other - then should we use 1 molar solutions? - may eat up prac time.
Always use 1 molar solutions. Voltage is not independent of molar concentration. There is just a complicated relationship between concentration and voltage, which you do not need to know.
Post by: chelseam on June 20, 2017, 07:24:35 pm
Post by: jakesilove on June 20, 2017, 07:36:59 pm
Hi! Could someone please explain how a susbtance can be polar? Thank you so much :D
Hey! A polar substance is just any molecule with a positive end and a negative end (we call these dipoles). Alternatively, a non-polar substance is uniform in change everything, and thus does not have a positive and negative end.
Below is an example;
(https://d2gne97vdumgn3.cloudfront.net/api/file/5ELWVak9Qcar4kOVpMmw)
Water is polar because it has a positive end (around the Hydrogen atoms) and a negative end (towards the oxygen atom). This arises because of the bent shape of Oxygen.
Hope this helps!
Post by: beau77bro on June 20, 2017, 08:13:23 pm
any comments or helpful tips are greatly appreciated. thankyou and wish me luck ahhaha
Post by: beau77bro on June 20, 2017, 08:15:54 pm
Post by: chelseam on June 20, 2017, 08:40:17 pm
Hope this helps!
When hydrogen bonding occurs in water, the positive hydrogen attracts to the negative oxygen and its lone pairs.Thank you! This is so helpful :D
Post by: chelseam on June 20, 2017, 08:55:02 pm
Post by: legorgo18 on June 20, 2017, 09:14:32 pm
Hi! How can you identify when a reaction is exothermic or endothermic? Thank you :)
If delta H <0, then exo >0 is endo
or do you mean like specific reactions like decomposition?
Post by: chelseam on June 20, 2017, 09:42:17 pm
If delta H <0, then exo >0 is endoI mean in specific reactions! Thanks :)
or do you mean like specific reactions like decomposition?
Post by: MisterNeo on June 20, 2017, 10:29:54 pm
I mean in specific reactions! Thanks :)
You can identify if a reaction is exothermic or endothermic, if no ΔH is given, by looking at the type of atom rearrangement that is taking place in the reaction.
Forming new bonds releases energy (exothermic), whereas braking bonds requires energy (endothermic).
Most reactions are a combination of both breaking and forming bonds BUT one will always outweigh the other, hence exothermic and endothermic.
Some examples:
COMBUSTION
You should be able to identify a combustion reaction as exothermic. This is because new bonds are being formed. Also because the reaction is spontaneous and doesn't require further energy input since it supplies its own energy.
NEUTRALISATION
Also an exothermic reaction because new bonds are being formed.
DECOMPOSITION
Most decomposition is endothermic because heat causes instability in molecules and breaks the bonds at certain temperatures.
SYNTHESIS
Usually the opposite of decomposition, hence it being exothermic as new bonds are formed.
Basically, bonds formed is exothermic; bonds broken is endothermic.
But I would recommend just memorising certain types of reactions being this and that, instead of trying to spot it through an equation.
Hope this helps ;)
Post by: Kekemato_BAP on June 20, 2017, 10:48:13 pm
I need help with this syllabus dot point for Industrial chem..
-discuss the issues associated with shrinking world resources with regard to one identified natural product that is not a fossil fuel, identifying the replacement materials used and/or current research in place to find a replacement for the named material
Other than fossil fuels, which are the best/easiest materials to do? Our teacher suggested rubber or soap, but don't know how to relate the advantages to its shrinking supply.
Thank you!! :D
Post by: chelseam on June 20, 2017, 11:15:48 pm
Basically, bonds formed is exothermic; bonds broken is endothermic.This clears up so much of my confusion! Thank you so much!! ;D
But I would recommend just memorising certain types of reactions being this and that, instead of trying to spot it through an equation.
Hipe this helps ;)
Post by: chelseam on June 21, 2017, 08:05:48 pm
Post by: MisterNeo on June 21, 2017, 08:58:14 pm
Hi! Could someone please explain Le Chatelier's principle in terms of pressure and what happens on an equilibrium graph? Thank you :D
According to LCP, an increase in pressure will shift the equilibrium system to the side with fewer moles of gas, whereas a decrease in pressure will shift to the side with more moles of gas.
Changes in volume are inverse to the above changes in pressure because increasing volume decreases pressure, and decreasing volume increases pressure.
For example, assume the following reaction:
In this hypothetical equilibrium reaction, the products have no moles of gas; but the reactants have 2 moles of gas.
This means that if you wanted to maximise the yield of the products, the pressure would be increased to push the system to the right; or the volume was decreased to increase pressure to push the system to the right.
On an equilibrium graph, it would look like this if pressure were increased:
(https://www.siyavula.com/science/grade-12/08-chemical-equilibrium/pspictures/3308628bd516fdecf2dcb6ef6bc5db81.png)
That sharp change in concentration represents an increase in pressure as the system acts to minimise the change.
Hope this helps!! ;)
Post by: chelseam on June 21, 2017, 09:06:25 pm
Hope this helps!! ;)Thank you so much! :D
Post by: beau77bro on June 21, 2017, 09:26:13 pm
Post by: bsdfjnlkasn on June 21, 2017, 09:36:56 pm
what are some risks related to using acids and how to prevent/deal with them like as a risk assessment - so like natural indicator pracs and titrations - help please ahhahaha prac tomorrow
Hey there,
For any acids or bases you use, you can be sure writing that either way, the substance will be corrosive. To minimise this risk, we wear gloves and safety glasses (as they are also skin/eye irritants).
If you have any questions, send them through and good luck!
Post by: kiwiberry on June 21, 2017, 09:44:29 pm
what are some risks related to using acids and how to prevent/deal with them like as a risk assessment - so like natural indicator pracs and titrations - help please ahhahaha prac tomorrow
Also have a neutralising agent eg. NaHCO3 available nearby to clean up any spills, and a source of running water to wash any spills on skin. Good luck! :)
Post by: MisterNeo on June 21, 2017, 10:13:31 pm
Also have a neutralising agent eg. NaHCO3 available nearby to clean up any spills, and a source of running water to wash any spills on skin. Good luck! :)
Also, make sure that the spill neutralising is not a strong acid/base because it is quite exothermic. Sodium bicarbonate is perfect since it is weak, and amphiprotic.
If you get a spill on your skin, do not neutralise on skin because it will burn. Wash with water quickly to remove acid or base. Base will form a slimy soap layer on skin, which is pretty gross.
Post by: Kekemato_BAP on June 23, 2017, 10:15:36 am
Post by: jakesilove on June 23, 2017, 10:39:31 am
Can someone explain the K equilibrium value and the reaction quotient? The K equilibrium is easier but I don't know how they relate to each other.
Hey! The reaction QUOTIENT a value used to describe a reaction at a particular point in time. However, the equilibrium constant is a value used to describe a reaction when it reaches equilibrium. They are calculated in exactly the same way (ie. same formula). However, you can use the reaction quotient (Q), when compared with the equilibrium constant (k) in order to determine which direction the reaction will shift.
If Q>k
Where the reaction quotient is greater than the given equilibrium value, the reaction will favour the reactants. Thus, it will move to the left until it reaches equilibrium.
If Q<k
Where the reaction quotient is less than the given equilibrium value, the reaction will favour the products. Thus, it will move to the right until it reaches equilibrium.
If Q=k
Where there reaction quotient is equal to the given equilibrium value, the reaction is at equilibrium!
In a question, you would likely be given an equilibrium value (eg. 2.4), and then a number of concentrations from which you can calculate the current reaction quotient. From that, you can decide in which direction the reaction will move!
Post by: Piza on June 24, 2017, 03:01:29 pm
After doing quite a few past papers, I have noticed the number of lines given for 6-7 mark questions. Did you guys adhere to this line limit or did you go on to ask for more paper? I am a bit worried because I am not sure if I can fit the required information into the lines provided (especially because I have large hand writing and I don't know if I could ask for paper during trials).
An example would be HSC 2013 Q30
Post by: jakesilove on June 24, 2017, 04:23:00 pm
Hey guys, just a question about the trials and HSC.
After doing quite a few past papers, I have noticed the number of lines given for 6-7 mark questions. Did you guys adhere to this line limit or did you go on to ask for more paper? I am a bit worried because I am not sure if I can fit the required information into the lines provided (especially because I have large hand writing and I don't know if I could ask for paper during trials).
An example would be HSC 2013 Q30
Hey hey!
You should definitely be using the number of lines given as a guide of how much to write. Try not to go over those lines, and definitely don't ask for more paper.
Answering long response questions in science becomes an exercise of identifying what a marker is looking for, and compacting as much detail into each sentence as possible. Make sure to answer the question, as succinctly as possible, but also try to 'throw in' extra information (literally by adding a few words, every here and there, with 'smart sounding' information). Practice makes perfect; once you've practised enough long responses, the technique needed becomes much easier to quickly perform.
Feel free to throw up an example answer, and I can take a look!
Post by: Piza on June 24, 2017, 05:27:42 pm
HSC (2016)
Question 30 (7 marks)
The use of CFCs has caused ozone depletion in the stratosphere.
Explain the steps that have been taken to reduce this problem. Include relevant chemical equations in your answer.
CFCs have been extensively used in many appliances, though in turn this has led to the destruction of ozone in the stratosphere.
In this region, CFCs photo dissociate in the presence of UV to form chlorine free radicals. As seen in the equations below, this free radical will go on to destroy an ozone molecule, and then create a chain reaction.
CCIF3(g) + UV → Cl. (g) + CF3(g)
Cl. (g) + O3(g) → ClO. (g) + O2(g)
ClO. (g) + O. (g) → Cl. (g) + O2(g)
Because of this, there have been steps taken to reduce this problem. In 1987, the Montreal Protocol was signed which called for cutting CFC use in half by 1998. Though, NASA’s Ozone Trends Panel indicated that the protocol’s limitations were not enough to save the ozone layer, and thus most or all of CFC use would need to be cut.
Thus, substances with a lower ozone depletion potential (ODP) were required to replace CFCs. Today, hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) are globally used as alternatives to CFCs. This is due to the presence of hydrogen in their structures which enable them to easily break down in the troposphere. This thereby lowers their ODP.
Therefore, the steps take to reduce the use of CFCs have in turn reduced the amount of ozone depletion.
Post by: jakesilove on June 25, 2017, 09:36:14 am
Thanks Jake! Here is an example answer, I tried to condense as much as I could.
HSC (2016)
Question 30 (7 marks)
The use of CFCs has caused ozone depletion in the stratosphere.
Explain the steps that have been taken to reduce this problem. Include relevant chemical equations in your answer.
CFCs have been extensively used in many appliances, though in turn this has led to the destruction of ozone in the stratosphere.
In this region, CFCs photo dissociate in the presence of UV to form chlorine free radicals. As seen in the equations below, this free radical will go on to destroy an ozone molecule, and then create a chain reaction.
CCIF3(g) + UV → Cl. (g) + CF3(g)
Cl. (g) + O3(g) → ClO. (g) + O2(g)
ClO. (g) + O. (g) → Cl. (g) + O2(g)
Because of this, there have been steps taken to reduce this problem. In 1987, the Montreal Protocol was signed which called for cutting CFC use in half by 1998. Though, NASA’s Ozone Trends Panel indicated that the protocol’s limitations were not enough to save the ozone layer, and thus most or all of CFC use would need to be cut.
Thus, substances with a lower ozone depletion potential (ODP) were required to replace CFCs. Today, hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) are globally used as alternatives to CFCs. This is due to the presence of hydrogen in their structures which enable them to easily break down in the troposphere. This thereby lowers their ODP.
Therefore, the steps take to reduce the use of CFCs have in turn reduced the amount of ozone depletion.
Hey! Good answer; here's how I would have structured it, to engage a bit more in the question, and the additional information I would add/stuff I would leave out.
CFCs were developed in the 1930s to be used as refrigerants. They were widely used, as they were cheap, and easy to produce. However, in 1974 it was demonstrated that CFCs caused Ozone Depletion (in fact, this won the Nobel prize).
Insert Chemical equations you used above
The 1985 Vienna Convention, and the 1987 Montreal Protocol, aimed to restrict our use of CFCs. By this time, over 50% of the Ozone concentration over Antarctica had been decimated.
Further, the 1997 Kyoto Protocol required Halon produced to cease by 2010, in line with recommendations made by NASA's Ozone Trends Panel.
From then, countries have agreed to entirely phase out CFCs, which were replaced by HCFCs. However, whilst these HCFCs have a lower ODP, they do not have an ODP of zero. Thus, these too will need to be phased out in favour of HFCs.
An that's it! I suspect my answer is shorter (note that I have ONLY included steps taken to reduce the use of CFCs, rather than sentences containing too much extraneous detail. However, I have included LOADS of information, which is sure to impress the maker).
Again, it is all about making every word of every sentence count. Just load your sentences with information, and make sure to really engage with the question.
Post by: beau77bro on June 25, 2017, 12:04:18 pm
What's the overall equation for 16? And if you guys could I would really appreciate a solution. THANKS
Post by: winstondarmawan on June 25, 2017, 01:17:48 pm
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/19441136_1251456198313234_1798191632_n.png?oh=f2983a459a9a0f6288e114cc1b331e56&oe=59516987
Post by: MisterNeo on June 25, 2017, 01:19:11 pm
(http://uploads.tapatalk-cdn.com/20170625/c811a4c5960aaaaa07c2b1d949cb6b50.jpg)
What's the overall equation for 16? And if you guys could I would really appreciate a solution. THANKS
There are two equations for this question:
The question requires you to first find the concentration of the hydrochloric acid solution. Your result is then used to find the concentration of the barium hydroxide. So if you get the first one wrong, the second one will also be wrong.
Part A: hydrochloric acid
Only 10mL of this was used to neutralise 21.3mL of acid
BUT it is a 1:2 mole ratio...
Part B: barium hydroxide
BUT it is a 2:1 mole ratio...
Only 25mL of Ba(OH)2 was needed to neutralise 27.1mL of acid
Find it in g/L.
I may be wrong, so answers would be nice. Hope this helps ;D
Post by: MisterNeo on June 25, 2017, 01:40:00 pm
Would appreciate help with the following Q, TIA.
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/19441136_1251456198313234_1798191632_n.png?oh=f2983a459a9a0f6288e114cc1b331e56&oe=59516987
I'd say the answer is A.
The carbon in methane has an oxidation state of -4 and hydrogen always has a state of +1 unless bonded as a metal hydride; and molecules like methane must have a total oxidation state of 0.
The carbon on the reactant side (methane) gets converted into carbon dioxide on the product side. Carbon dioxide has 2 oxygens of -2, thus carbon must be +4 to bring the charge down to 0.
Therefore, the carbon has changed oxidation states from -4 to +4, which is being oxidised by losing electrons to become a positive species.
The O2 oxygen also has a change in oxidation state by going from 0 (diatomic) to -2 (carbon dioxide). However, the options only had oxidised oxygen, which is incorrect.
Hope this helps :) Look out for changes in oxidation state!!
Post by: winstondarmawan on June 25, 2017, 02:17:23 pm
I'd say the answer is A.
The carbon in methane has an oxidation state of -4 and hydrogen always has a state of +1 unless bonded as a metal hydride; and molecules like methane must have a total oxidation state of 0.
The carbon on the reactant side (methane) gets converted into carbon dioxide on the product side. Carbon dioxide has 2 oxygens of -2, thus carbon must be +4 to bring the charge down to 0.
Therefore, the carbon has changed oxidation states from -4 to +4, which is being oxidised by losing electrons to become a positive species.
The O2 oxygen also has a change in oxidation state by going from 0 (diatomic) to -2 (carbon dioxide). However, the options only had oxidised oxygen, which is incorrect.
Hope this helps :) Look out for changes in oxidation state!!
Yes!! Thank you heaps.
Post by: Piza on June 25, 2017, 04:01:32 pm
Hey! Good answer; here's how I would have structured it, to engage a bit more in the question, and the additional information I would add/stuff I would leave out.
...
Again, it is all about making every word of every sentence count. Just load your sentences with information, and make sure to really engage with the question.
Thank you, this is super helpful!!
Post by: beau77bro on June 25, 2017, 04:46:27 pm
thanks
Post by: Kekemato_BAP on June 26, 2017, 12:58:05 pm
Hey! The reaction QUOTIENT a value used to describe a reaction at a particular point in time. However, the equilibrium constant is a value used to describe a reaction when it reaches equilibrium. They are calculated in exactly the same way (ie. same formula). However, you can use the reaction quotient (Q), when compared with the equilibrium constant (k) in order to determine which direction the reaction will shift.
If Q>k
Where the reaction quotient is greater than the given equilibrium value, the reaction will favour the reactants. Thus, it will move to the left until it reaches equilibrium.
If Q<k
Where the reaction quotient is less than the given equilibrium value, the reaction will favour the products. Thus, it will move to the right until it reaches equilibrium.
If Q=k
Where there reaction quotient is equal to the given equilibrium value, the reaction is at equilibrium!
In a question, you would likely be given an equilibrium value (eg. 2.4), and then a number of concentrations from which you can calculate the current reaction quotient. From that, you can decide in which direction the reaction will move!
Could you explain the things about excluding solids and liquids in the equilibrium? And also the thing about temperature being the only affecting variable.
Post by: beau77bro on June 26, 2017, 05:05:08 pm
Could you explain the things about excluding solids and liquids in the equilibrium? And also the thing about temperature being the only affecting variable.
ok so the temperature being the only effecting variable, is basically the fact that at a stable temperature - when you add more of a substance (concentration) or decrease the volume the equilibrium is taking place it will change concentrations on both sides accordingly to the reaction. so for the extra products it would balance out essentially. similarly in the change of volume the pressure is proportional to concentration, so it will move to the side with less gaseous moles increasing the concentration on the other side. but since the pressure has changed and there are now more gaseous molecules squeezed in a smaller space (and there are more moles on the side which the equilibrium has moved away) so it essentially balances out. the more moles has decreased in overall molecules - but there are more in a small space so increased concentration, the less moles side has gained more overall molecules and these take up a smaller space increasing the concentration but since there are less moles overall it's concentration increases more, but equals out in the equation.
now temperature is different because it doesn't balance out like the other changes do. it makes the reaction tend more to one side full stop. so for an endothermic if you increase the temperature, you push it to the right increasing the products. so in the equation the numerator increases drastically and the denominator shrinks - changing the K value. that's why they are for given temperatures.
now im not super sure on the solids and liquids part - i think its to do with the fact gas solubility increases as it cools. or not. but either way gaseous-solid or liquid reactions will favour the side with less gaseous moles, so if you increase the pressure they will just dissolve or make solids in a way that the equation doesnt allow for. i hope a moderator can shine some more light on this.
Post by: kiwiberry on June 26, 2017, 05:50:07 pm
Could you explain the things about excluding solids and liquids in the equilibrium? And also the thing about temperature being the only affecting variable.
Solids and liquids have a fixed concentration - we exclude them from the equilibrium constant because their concentrations do not change over time
hey so what would a processing skills test for chem look like? like what can i expect it to include?
thanks
In my processing skills test we got asked to consolidate information from a passage into a table and answer questions about it, draw graphs and do some basic calculations, so make sure you remember how to draw proper graphs/tables (line of best fit, title, etc.) :)
Post by: kylesara on June 28, 2017, 03:16:49 pm
Thanks.
Post by: kiwiberry on June 28, 2017, 03:45:24 pm
Hi, I was just going through the 2016 Chem past paper and don't understand why the answer for question 11 in Multiple choice is D and not A.
Thanks.
The numbers in D add up to 7 whereas the numbers in A add up to 8 - we pick the one with the lowest sum, so D is the answer :)
Post by: beau77bro on June 28, 2017, 11:25:54 pm
Please help Q26b - how do u do percentage (w/v)?
Thanks - beau
Post by: Shadowxo on June 30, 2017, 03:40:40 pm
(http://uploads.tapatalk-cdn.com/20170628/6a3c41cbbd2c501b456b637e6842438f.jpg)
Please help Q26b - how do u do percentage (w/v)?
Thanks - beau
So if you have the molarity of it, you have the number of mols per L = number of mols per 1000mL
1 mol of acetic acid = 60.0 g (rounding to one decimal place using the periodic table, your one may have more decimal places and you can use this)
So convert the number of mols of acetic acid to the weight of it (g) (multiply the number of mols by 60.0 to get the weight in g per 1000mL)
%w/v would be weight in grams / volume in mL * 100%
So divide the grams of acetic acid in each litre by 1000 (the mLs ie volume of vinegar) then multiply by 100 to get the percentage
Hope this helps :) bit rusty on my chem though
Post by: winstondarmawan on June 30, 2017, 07:49:52 pm
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/19679939_1257428117716042_1386574065_n.jpg?oh=a0a1bdfe654c145b7f2552a18f855b8d&oe=5957A740
Post by: MisterNeo on June 30, 2017, 09:27:59 pm
Hey can someone please help with Q1, all parts except (iii) and (v) and an explanation would help alot! TIA
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/19679939_1257428117716042_1386574065_n.jpg?oh=a0a1bdfe654c145b7f2552a18f855b8d&oe=5957A740
I
Neutralisation reactions are NOT redox reactions. There is no change in oxidation state or exchange of electrons as hydrogen in HCl is already in a +1 state and ends as +1.
II
Oxidation
Reduction
This combustion one is a bit complex because there are a crapload of electrons involved for longer alkanes. Anyhow, carbon can have oxidation states from -4 to +4, and ethane carbon exists as -3 because of the 3 +1 hydrogens bonded to each; and molecules must have a total state of 0. Therefore, carbon changes from a -3 to a +4 from ethane to carbon dioxide. And, oxygen goes from 0 in diatomic to -2 in carbon dioxide and water.
IV
Esterification is a redox reaction.
The alcohol is oxidised, and the alkanoic acid is reduced.
This guide explains it.
VI
This decomposition reaction is not a redox reaction and has no oxidation/reduction involved because the calcium, oxygen, and carbon all start and end with the same oxidation states of 2+, 2-, and 4+ respectively.
VII
This is the same with question I because it is also a neutralisation reaction. There is an exchange in proton to form ammonium, which then bonds with the already negative chlorine. Hence, there are no changes in oxidation states.
VIII
This one is a redox reaction because aluminium 3+ ions are reduced into solid aluminium by gaining 3 electrons each.
The oxygen is oxidised because it starts as a 2- ion and gets gains the electrons from aluminium ions to form oxygen gas.
This explains it nicely with equations.
IX
Oxidation
Reduction
This is a redox reaction because hydrogen goes from +1 in water to 0 in diatomic hydrogen, and oxygen goes from -2 in water to 0 in diatomic oxygen.
Basically, you have to watch out for changes in oxidation states where electrons are being exchanged.
Hope this helps ;)
Post by: winstondarmawan on July 01, 2017, 01:32:57 pm
I
Neutralisation reactions are NOT redox reactions. There is no change in oxidation state or exchange of electrons as hydrogen in HCl is already in a +1 state and ends as +1.
II
Oxidation
Reduction
This combustion one is a bit complex because there are a crapload of electrons involved for longer alkanes. Anyhow, carbon can have oxidation states from -4 to +4, and ethane carbon exists as -3 because of the 3 +1 hydrogens bonded to each; and molecules must have a total state of 0. Therefore, carbon changes from a -3 to a +4 from ethane to carbon dioxide. And, oxygen goes from 0 in diatomic to -2 in carbon dioxide and water.
IV
Esterification is a redox reaction.
The alcohol is oxidised, and the alkanoic acid is reduced.
This guide explains it.
VI
This decomposition reaction is not a redox reaction and has no oxidation/reduction involved because the calcium, oxygen, and carbon all start and end with the same oxidation states of 2+, 2-, and 4+ respectively.
VII
This is the same with question I because it is also a neutralisation reaction. There is an exchange in proton to form ammonium, which then bonds with the already negative chlorine. Hence, there are no changes in oxidation states.
VIII
This one is a redox reaction because aluminium 3+ ions are reduced into solid aluminium by gaining 3 electrons each.
The oxygen is oxidised because it starts as a 2- ion and gets gains the electrons from aluminium ions to form oxygen gas.
This explains it nicely with equations.
IX
Oxidation
Reduction
This is a redox reaction because hydrogen goes from +1 in water to 0 in diatomic hydrogen, and oxygen goes from -2 in water to 0 in diatomic oxygen.
Basically, you have to watch out for changes in oxidation states where electrons are being exchanged.
Hope this helps ;)
Thank you for clearing this up! My school doesn't teach oxidation states which is why I struggle with these questions.
Post by: jakesilove on July 02, 2017, 04:26:40 pm
Post by: Annie657 on July 03, 2017, 12:05:17 pm
So I'm just writing my notes for steps in titration, and was wondering is the end point when the solution first changes colour or when it sustains a colour for 10 seconds? If so when should you record your measurement?
Thanks! Annabelle
Post by: itssona on July 03, 2017, 04:13:41 pm
im comparing the densities of water and ice, but not too sure if this is valid, and our teacher often contradicts sites so idk
• Structure of Liquid Water
- Water molecules are randomly orientated
- Overall structure of water is random
- Molecules much closer together than in ice (so there is h bonding as h bonding is the strongest)
- The more h bonding present, the bigger the density
• Structure of Ice
- More structured than liquid water
- Adjacent water molecules hydrogen bonding to form hexagonal clusters
- This causes large vacant spaces within the hexagonal clusters
- As the volume of ice is greater, it is less dense
Post by: jakesilove on July 03, 2017, 04:16:15 pm
heeey would someone please be able to tell me if this sounds right:
im comparing the densities of water and ice, but not too sure if this is valid, and our teacher often contradicts sites so idk
• Structure of Liquid Water
- Water molecules are randomly orientated
- Overall structure of water is random
- Molecules much closer together than in ice (so there is h bonding as h bonding is the strongest)
- The more h bonding present, the bigger the density
• Structure of Ice
- More structured than liquid water
- Adjacent water molecules hydrogen bonding to form hexagonal clusters
- This causes large vacant spaces within the hexagonal clusters
- As the volume of ice is greater, it is less dense
Yep, that is 100% correct! Looks like you've got it! Might be worth looking at the actual, relative density of ice/water (so that you can give a sexy looking statistic to back up your theoretical response). However, great job!
Post by: itssona on July 03, 2017, 04:26:00 pm
Yep, that is 100% correct! Looks like you've got it! Might be worth looking at the actual, relative density of ice/water (so that you can give a sexy looking statistic to back up your theoretical response). However, great job!
Omg thank you Jake!! :)
Post by: hansolo9 on July 04, 2017, 03:10:43 pm
Do we assume that sulfuric acid fully ionises both its hydrogens?
Post by: jakesilove on July 04, 2017, 04:15:02 pm
Is the answer A or B?
Do we assume that sulfuric acid fully ionises both its hydrogens?
Sulfuric acid is a STRONG acid. So, yes it fully ionises in solution. As you say, it produces two hydrogen ions per mole of acid, therefore the concentration of hydrogen ions is 0.2 mol/L. Plug this into the pH formula, and you'll get the answer!
Post by: hansolo9 on July 04, 2017, 05:03:29 pm
Sulfuric acid is a STRONG acid. So, yes it fully ionises in solution. As you say, it produces two hydrogen ions per mole of acid, therefore the concentration of hydrogen ions is 0.2 mol/L. Plug this into the pH formula, and you'll get the answer!
But doesn't sulfuric acid ionise the first hydrogen to become a weak acid? Then is that second hydrogen fully or partially ionised?
Post by: kiwiberry on July 04, 2017, 05:15:39 pm
But doesn't sulfuric acid ionise the first hydrogen to become a weak acid? Then is that second hydrogen fully or partially ionised?
Yeah the second hydrogen only partially ionises. But either way, H+ concentration will be between 0.1 mol/L and 0.2 mol/L, so pH will be less than 1. :)
Post by: seventeenboi on July 04, 2017, 07:56:00 pm
I have two questions:
1. why is it that CFCs are so unreactive that they take up to 7 years to diffuse into the stratosphere?
2. net effect of ozone depletion by CFC photodissociation is given by this equation: O3 + O* → O2 + O2.. in conjunction with the other equations this suggests the continual regeneration of chlorine radicals which in turn catalyse more and more ozone depletion... then it would make sense that 1 molecule of CFC has the potential to destroy an infinite amount of ozone molecules, but why doesn’t it?
THANKS :)))
Post by: MisterNeo on July 04, 2017, 09:43:53 pm
HEYOO,
I have two questions:
1. why is it that CFCs are so unreactive that they take up to 7 years to diffuse into the stratosphere?
2. net effect of ozone depletion by CFC photodissociation is given by this equation: O3 + O* → O2 + O2.. in conjunction with the other equations this suggests the continual regeneration of chlorine radicals which in turn catalyse more and more ozone depletion... then it would make sense that 1 molecule of CFC has the potential to destroy an infinite amount of ozone molecules, but why doesn’t it?
THANKS :)))
CFC's unreactivity may have something to do with chlorine's electronegativity compared to hydrogen in methane. (probably don't need to know this, you just need to know that it is unreactive)
They are also water insoluble, which means that they don't get washed out by rain. This allows them to survive the journey into the stratosphere where they are decomposed by UV rays into free radicals.
You would assume that the chain reaction goes on forever, but there are millions of chlorine free radicals in affected areas. This means that they will bump into each other and form chlorine gas, which ends the chain reaction. Some free radicals may react with moisture or whatever to form hydrogen chloride, whereas some may dissipate into space. But by the time it does end, each radical would have destroyed countless ozone molecules.
Post by: seventeenboi on July 05, 2017, 04:58:19 pm
CFC's unreactivity may have something to do with chlorine's electronegativity compared to hydrogen in methane. (probably don't need to know this, you just need to know that it is unreactive)
They are also water insoluble, which means that they don't get washed out by rain. This allows them to survive the journey into the stratosphere where they are decomposed by UV rays into free radicals.
You would assume that the chain reaction goes on forever, but there are millions of chlorine free radicals in affected areas. This means that they will bump into each other and form chlorine gas, which ends the chain reaction. Some free radicals may react with moisture or whatever to form hydrogen chloride, whereas some may dissipate into space. But by the time it does end, each radical would have destroyed countless ozone molecules.
ohhh ok thanks :DD
Post by: bsdfjnlkasn on July 06, 2017, 10:42:02 am
Could someone please help out with the following question:
Write a general structural formula for the saponification of vegetable oil. Use symbols such as R1 and R2 to represent the carbon chains in the organic molecules.
Also could someone help me answer:
The equilibrium equation being referred to is attached :)
Draw a diagram to show the arrangement of (a) water molecules and (b) chloride ions around the cobalt ion. Why do the particles behave this way? Hint: Your answer should use the term “lone pairs of electrons” and coordinate covalent bonds may be involved
Thank you so much :D
Post by: MisterNeo on July 06, 2017, 11:44:54 am
Hey there!
Could someone please help out with the following question:
Write a general structural formula for the saponification of vegetable oil. Use symbols such as R1 and R2 to represent the carbon chains in the organic molecules.
Also could someone help me answer:
The equilibrium equation being referred to is attached :)
Draw a diagram to show the arrangement of (a) water molecules and (b) chloride ions around the cobalt ion. Why do the particles behave this way? Hint: Your answer should use the term “lone pairs of electrons” and coordinate covalent bonds may be involved
Thank you so much :D
Hey! :)
For saponification, the general reaction between an ester and a base to form an alcohol and a salt.
The most common ester used are a group called triglyceride found in fats and oils. Most common one in textbooks is tristearin.
The alcohol produced is usually glycerol, plus the salt is the sodium stearate (soap).
(http://cronodon.com/sitebuilder/images/Saponification2-600x198.jpg)
Idk much about the cobalt question, but hydrous compounds have water loosely bonded to the metal in the middle. This is partly due to the water's oxygen having two lone pairs of electrons that can form bonds with cobalt's electrons. Other hydrous compounds can have water hydrogen bond onto the anion of a salt such as sulfates.
But it looks something like this:
(http://www.chemspider.com/imageshandler.ashx?id=23045&w=200&logo=500)
Post by: limtou on July 07, 2017, 12:03:41 am
Thanks !
Post by: MisterNeo on July 07, 2017, 12:25:55 am
Could someone kindly explain the answer to this multiple choice?
Thanks !
Hi!! ;)
A pH increase is a decrease in acidity/hydrogen ion concentration.
So are you familiar with the whole buffer system with hydrogen carbonate?
Adding water
Remember the equation for pH?
The higher the concentration of H+, the lower the pH will be (acidic).
Adding more water will dilute the entire system, thus reducing the concentration. If you were to plug that dilute concentration into the formula, you would get a higher pH (less acidic) than the current concentration.
Increasing temperature
Carbon dioxide is a gas. Increasing the temperature of water will reduce its ability to dissolve gases, and most of the carbon dioxide will be evolved as a gas. This reduces the concentration of aqueous carbon dioxide.
According to LCP, the lost aqueous CO2 will cause the carbonic acid to reverse react to reform aqueous CO2 and water. This will also cause the hydrogen ions to shift to the left to replace the carbonic acid. Ultimately, the hydrogen ion concentration will fall, and become a more dilute solution. As stated in the water section, dilute solutions of H+ will be a higher pH because it is less of an acid and more towards neutral/basic.
Hope this helps ;)
Post by: limtou on July 07, 2017, 12:44:43 am
Hi!! ;)
A pH increase is a decrease in acidity/hydrogen ion concentration.
So are you familiar with the whole buffer system with hydrogen carbonate?
Adding water
Remember the equation for pH?
The higher the concentration of H+, the lower the pH will be (acidic).
Adding more water will dilute the entire system, thus reducing the concentration. If you were to plug that dilute concentration into the formula, you would get a higher pH (less acidic) than the current concentration.
Increasing temperature
Carbon dioxide is a gas. Increasing the temperature of water will reduce its ability to dissolve gases, and most of the carbon dioxide will be evolved as a gas. This reduces the concentration of aqueous carbon dioxide.
According to LCP, the lost aqueous CO2 will cause the carbonic acid to reverse react to reform aqueous CO2 and water. This will also cause the hydrogen ions to shift to the left to replace the carbonic acid. Ultimately, the hydrogen ion concentration will fall, and become a more dilute solution. As stated in the water section, dilute solutions of H+ will be a higher pH because it is less of an acid and more towards neutral/basic.
Hope this helps ;)
Ahh, thank you :)
Post by: seventeenboi on July 07, 2017, 06:39:02 pm
could someone help me with this question?? thanks :P the answer is B
Post by: kiwiberry on July 07, 2017, 07:24:02 pm
hellooo
could someone help me with this question?? thanks :P the answer is B
Hello!
If a 10 ppm solution has an absorbance of 0.4, then a solution with an absorbance of 0.5 will have a concentration of \(\frac{10}{0.4} \times 0.5=12.5 \text{ ppm} = \frac{0.0125 \text{ g/L}}{63.55 \text{ g/mol}} = 1.966...\times 10^{-4} \text{ mol/L} \) copper ions. Now considering the reaction between copper ions and sodium carbonate:
Here, copper carbonate is the precipitate. We're able to find the number of moles of Cu2+ since we have its concentration and volume, and so we're able to find the mass of CuCO3 :)
Post by: bsdfjnlkasn on July 07, 2017, 10:54:40 pm
What mass of 1,2 - dibromoethane is produced when 2 L of ethylene gas is reacted with 11.18g of bromine at SLC?
Any help would be greatly appreciated :D
Post by: kiwiberry on July 07, 2017, 11:11:56 pm
Can someone please help me out with the following question. I've tried looking at limiting reagents but I don't think it's relevant here.
What mass of 1,2 - dibromoethane is produced when 2 L of ethylene gas is reacted with 11.18g of bromine at SLC?
Any help would be greatly appreciated :D
Hey! :)
Whenever the amounts of both reactants are given, the limiting reagent must be found so the correct mole ratio is used. Remember that SLC = 24.79 L
Post by: bsdfjnlkasn on July 08, 2017, 09:28:50 am
Hey! :)
Whenever the amounts of both reactants are given, the limiting reagent must be found so the correct mole ratio is used. Remember that SLC = 24.79 LTherefore bromine is the limiting reagent, and we can use the mole ratio to find the mass of C2H4Br2
Thank you - legend! :)
I just have a few more questions, if that's ok (for anyone to answer)
I sort of understand why the answer here is C, it's the most obvious, but if someone could possibly illustrate the point with a chemical equation that would probably help clarify things further. I'm just not sure which one to write ???
The pH of a solution of sodium acetate was measured to be 9.05. Which of the following statements best explains the measured pH?
(A) sodium ions donate protons to water
(B) acetate ions donate protons to water
(C) sodium ions accept protons from water
(D) acetate ions accept protons from water
How are we supposed to know this one? It's probably got something to do with the BP's but i'm not sure what
Ethanol (boiling point 78.1°C) and butanoic acid (boiling point 163.5°C)will react under reflux to produce ethyl butanoate (boiling point 120.0°C). When the reflux mixture is distilled, four pure liquids are collected in separate beakers labelled 1, 2, 3 and 4 in order of their collection.
Which beaker contains the ester?
(A) 1
(B) 2
(C) 3
(D) 4
Are we supposed to know the entire flowchart for both anion and cation tests?
What would the method for this question be?
The concentration of argon in the atmosphere is 0.93% (v/v). What is the concentration expressed in ppm?
(A) 9.3 ppm
(B) 93 ppm
(C) 930 ppm
(D) 9300 ppm
Would it be valid to discuss Pb2+ as a product of industrial or agricultural processes and then justify we need to monitor it's levels? The sample answer discusses phosphate but for lead, what agricultural process does it come from?
For the attached question, does the following reasoning sound ok? So if X was able to precipitate with Cu2+ then it means that it is more reactive, so higher on the standard potentials? Does it mean that it's a better reducing agent because it's forcing the Cu to oxidise? I'm just not really sure what i'm looking for in this table and how I can use it to answer the question ...
Thanks so much again :D
Post by: kiwiberry on July 08, 2017, 11:15:48 am
I sort of understand why the answer here is C, it's the most obvious, but if someone could possibly illustrate the point with a chemical equation that would probably help clarify things further. I'm just not sure which one to write ???May be wrong but I think the answer is D? The sodium ion is not able to accept or donate protons because it's a cation. Because acetate is the conjugate base of a weak acid, it tends to accept protons from water, leaving behind hydroxide ions which make the solution basic
The pH of a solution of sodium acetate was measured to be 9.05. Which of the following statements best explains the measured pH?
(A) sodium ions donate protons to water
(B) acetate ions donate protons to water
(C) sodium ions accept protons from water
(D) acetate ions accept protons from water
Quote
How are we supposed to know this one? It's probably got something to do with the BP's but i'm not sure whatDistillation separates liquids with different bp's- the mixture is heated and the liquid with the lowest bp evaporates first and is collected first. So the liquids will be collected from lowest to highest bp. Remember that water (100oC) is also produced! Therefore, the order will be ethanol, water, ethanol butanoate, butanoic acid- the ester is 3
Ethanol (boiling point 78.1°C) and butanoic acid (boiling point 163.5°C)will react under reflux to produce ethyl butanoate (boiling point 120.0°C). When the reflux mixture is distilled, four pure liquids are collected in separate beakers labelled 1, 2, 3 and 4 in order of their collection.
Which beaker contains the ester?
(A) 1
(B) 2
(C) 3
(D) 4
Quote
Are we supposed to know the entire flowchart for both anion and cation tests?I think it should be ok just to know which ions are used to precipitate what, although it may be good to know things like why carbonate is tested first and why sulfate is tested before chloride etc. because I saw a hsc question on that somewhere. You should be fine with that if you know your solubility rules though! Sorry that was a vague answer, but personally I memorised the order the ions were tested in to be on the safe side.
Quote
What would the method for this question be?0.93% (v/v) refers to 0.93mL/100mL. Multiplying both sides by 10000, we get 9300mL/1000000mL which equals 9300 ppm
The concentration of argon in the atmosphere is 0.93% (v/v). What is the concentration expressed in ppm?
(A) 9.3 ppm
(B) 93 ppm
(C) 930 ppm
(D) 9300 ppm
Quote
Would it be valid to discuss Pb2+ as a product of industrial or agricultural processes and then justify we need to monitor it's levels? The sample answer discusses phosphate but for lead, what agricultural process does it come from?Yes you're able to discuss lead! Not sure about agricultural processes, but for industrial processes lead can be a product of the manufacturing of lead-acid batteries, as well as from old paints.
Quote
For the attached question, does the following reasoning sound ok? So if X was able to precipitate with Cu2+ then it means that it is more reactive, so higher on the standard potentials? Does it mean that it's a better reducing agent because it's forcing the Cu to oxidise? I'm just not really sure what i'm looking for in this table and how I can use it to answer the questionThat's right! So X will be higher on the standard potentials than all the other ions.
Hope this helps :)
Post by: bsdfjnlkasn on July 08, 2017, 11:47:11 am
For the question on how the pH of a solution of sodium acetate would be affected, the answer was actually D I must have slipped on the keyboard and hit C, but your explanation is really clear. Thanks again for everything :) :) , I'll probably be sending a lot more questions through now considering I only recently started revising ... whoops
Post by: bsdfjnlkasn on July 08, 2017, 01:56:48 pm
I was just wondering why "reduction of sulfur to hydrogen sulphide" is an environmental concern of the Frasch process? Is it because it then oxidises to form sulphurous acid (which then leads to acid rain)? What would the chemical equation for this reduction be?
Also does thermal pollution lead to eutrophication?
Thanks you :D
Post by: bsdfjnlkasn on July 08, 2017, 03:15:02 pm
I guess you can talk about hydrogen sulfide, however it gets oxidised into sulfur dioxide and water.
It's these products that then go on to react to form acidic rain. But sulfur dioxide can also be formed if the molten sulfur isn't cooled fast enough. So if you talk about sulfur dioxide, I wouldn't talk about hydrogen sulfide since they both end up as acid rain.
Hydrogen sulfide does smell like crap, and is a very poisonous gas to humans...so you can talk about that as an environmental issue.
With your second question, thermal pollution doesn't directly cause eutrophication (high nutrient levels) Heating water causes decreased gas solubility, which reduces dissolved oxygen levels and kills aquatic life and promotes algae blooms.
If you have time to go into more depth (ie. long response), you can argue that warm water dissolves minerals easier, hence increasing the nutrient levels in waterways. You can also say that the dead fish decompose into nutrients.
Basically, thermal pollution CAN cause eutrophication, but not directly. So I wouldn't make that link if you don't have enough writing space.
Hope this helps ;)
Thanks dude :)
Was just wondering why increasing temperature (for an exothermic reaction) decreases yield? Surely increasing the number of collusions will allow more to react and so more product to form?
I guess you can talk about hydrogen sulfide, however it gets oxidised into sulfur dioxide and water.
It's these products that then go on to react to form acidic rain. But sulfur dioxide can also be formed if the molten sulfur isn't cooled fast enough. So if you talk about sulfur dioxide, I wouldn't talk about hydrogen sulfide since they both end up as acid rain.
Hydrogen sulfide does smell like crap, and is a very poisonous gas to humans...so you can talk about that as an environmental issue.
With your second question, thermal pollution doesn't directly cause eutrophication (high nutrient levels) Heating water causes decreased gas solubility, which reduces dissolved oxygen levels and kills aquatic life and promotes algae blooms.
If you have time to go into more depth (ie. long response), you can argue that warm water dissolves minerals easier, hence increasing the nutrient levels in waterways. You can also say that the dead fish decompose into nutrients.
Basically, thermal pollution CAN cause eutrophication, but not directly. So I wouldn't make that link if you don't have enough writing space.
Hope this helps ;)
Hey thanks for that :)
Was just wondering what you would write then for the environmental issues of the Frasch process? Would you mostly be concerned with earth subsidence and the possibility for acid rain to occur?
Also which equation should I write to show that an increase in heat will decrease gas solubility?
Thanks again :)
Post by: kiwiberry on July 08, 2017, 03:58:05 pm
Thanks dude :)
Was just wondering why increasing temperature (for an exothermic reaction) decreases yield? Surely increasing the number of collusions will allow more to react and so more product to form?
Hey thanks for that :)
Was just wondering what you would write then for the environmental issues of the Frasch process? Would you mostly be concerned with earth subsidence and the possibility for acid rain to occur?
Also which equation should I write to show that an increase in heat will decrease gas solubility?
Thanks again :)
For an equilibrium reaction with an exothermic forward reaction, increasing temperature would increase the rate of both the forward and reverse reactions due to the increased number of collisions, but not the amount of yield. By Le Chatelier's Principle, equilibrium will shift to favour the reaction which decreases the temperature, ie. the reverse endothermic reaction, and as a result yield will decrease.
Yeah I would talk about the subsidence and the fact that hydrogen sulfide and sulfur dioxide are both serious air pollutants, as well as the fact that sulfur dioxide can cause acid rain.
Not sure about the gas solubility but someone else will probably be able to answer that :)
Post by: bsdfjnlkasn on July 09, 2017, 10:10:20 am
Was just wondering how the number of moles for OH- were calculated? The concentrations and volumes were all substituted correctly but you can see the question below. I'm just not sure why they multiplied it by two? Are they sort of considering the internals mole ratio in Ba(OH)2? Like how Ba:OH is 1:2? I've seen this a few times and if this is what is happening, I'd love an explanation because I don't really understand what's going on aha and why they do it.
Additionally, could someone explain why the attached method (my working) is wrong? I still got a pH of 13, but on the lower side...
(http://uploads.tapatalk-cdn.com/20170709/964356a844469f6a881f363e01fe3641.jpg)
Thanks again! :)
More questions: :D
1. How are we to determine which compounds are the most volatile? Can all those that we've encountered be ordered or is there a structural way of deducing this?
2. For the MC attached (Q7), why are A and C wrong? The answer is B. Sorry if this is annoying, but could someone please illustrate with oxidation numbers for the elements involved? I'm still really rusty on it all. Thank you!!
3. How do we figure out Q14?
4. How can we tell from an equilibrium graph whether it is exo/endothermic?
5. How would I do part 3 of the equilibrium constant question? I'm still not really familiar with the ICE method and would appreciate an explanation
THANK YOU :) :D
Post by: bsdfjnlkasn on July 09, 2017, 04:17:06 pm
Hope this helps ;) Let me know if something is unclear!
Hey thanks for that! I'm still a bit confused about why D in question 7 is incorrect. Maybe it's because I don't completely understand the meaning of "oxidising agent that undergoes reduction". Isn't oxygen just a spectator because it's oxidation number doesn't change through the whole reaction? I thought an oxidising agent was a species that made something else reduce ... I was never too confident on all the terminology :/ Hopefully you can make sense of my confusion and offer an explanation (don't worry if you can't lol)
Also with calculating the equilibrium constant, I've attached the solution but don't understand how they deduced the equilibrium values for the products - any ideas?
Thanks again for all your help! :) :)
Post by: jakesilove on July 09, 2017, 05:03:00 pm
Hey thanks for that! I'm still a bit confused about why D in question 7 is incorrect. Maybe it's because I don't completely understand the meaning of "oxidising agent that undergoes reduction". Isn't oxygen just a spectator because it's oxidation number doesn't change through the whole reaction? I thought an oxidising agent was a species that made something else reduce ... I was never too confident on all the terminology :/ Hopefully you can make sense of my confusion and offer an explanation (don't worry if you can't lol)
Also with calculating the equilibrium constant, I've attached the solution but don't understand how they deduced the equilibrium values for the products - any ideas?
Thanks again for all your help! :) :)
An oxidising agent is something that causes another substance to oxidise. Thus, it itself reduces. Does that make sense/answer your first question?
For the equilibrium constant, we just use the chemical equation that MisterNeo posted above. We know that 0.2 moles of CO reacted, therefore we multiple that by 2 to find how much Hydrogen reacted (0.4 moles) and know that 0.2 moles of the product was produced. Since the initial concentration of the product was zero, there is now a total of 0.2 moles of the product. Divide this by the volume to get a concentration, and you're done!
Post by: MisterNeo on July 09, 2017, 05:03:31 pm
Hey thanks for that! I'm still a bit confused about why D in question 7 is incorrect. Maybe it's because I don't completely understand the meaning of "oxidising agent that undergoes reduction". Isn't oxygen just a spectator because it's oxidation number doesn't change through the whole reaction? I thought an oxidising agent was a species that made something else reduce ... I was never too confident on all the terminology :/ Hopefully you can make sense of my confusion and offer an explanation (don't worry if you can't lol)
Also with calculating the equilibrium constant, I've attached the solution but don't understand how they deduced the equilibrium values for the products - any ideas?
Thanks again for all your help! :) :)
No problem ;)
So the definition of a reducing agent is a species that causes the reduction of another, and underoes oxidation itself.
Oxygen is always an oxidising agent because it only has oxidation states of 0 and -2 (reduction is gain).
Hence, the answer can't be D.
If oxygen was a reducing agent, this means that it could undergo oxidation to lose electrons, but it can't (oxygen can't be +1 or +2)
For example, rusting of iron is the oxidation of iron caused by oxygen being an oxidising agent.
It is quite confusing at first, but you'll get the hang of it after a few practice Redox questions. :)
For the equilibrium question, the K constant is the conditions once it has reached equilibrium.
-You need to write out the reaction equation (see my other answer).
-You first calculate the concentration of each reactant/product initially. (Divide moles by the 5L volume of the chamber- there was no product to begin with since it is initial.)
Same with hydrogen because they both start with 1mol.
-So after the equilibrium has reached equilibrium, 0.2mol of product was formed. You have to find out how many moles of each reactant was used according to the stoichiometry of the equation. Then determine the remaining reactants concentration.
The moles of CO used was 0.2mol (1:1 ratio with methanol).
The moles of H2 used was 0.4mol (2:1 ratio with methanol)
Now subtract these used values from the initial value.
So at equilibrium, the concentration is:
-CO: (1.0-0.2)/5=0.16mol/L
-H2: (1.0-0.4)/5=0.12mol/L
-CH3OH: 0.2/5=0.04mol/L
Now put these values into the K-constant formula (see my other answer), products go on the numerator/reactants on the denominator.
It's better if you express decimal constants in scientific notation.
Post by: jakesilove on July 09, 2017, 05:04:18 pm
No problem ;)
So the definition of a reducing agent is a species that causes the reduction of another, and underoes oxidation itself.
Oxygen is always an oxidising agent because it only has oxidation states of 0 and -2 (reduction is gain).
Hence, the answer can't be D.
It's better if you express decimal constants in scientific notation.
Why do I even try: MisterNeo, as always you are a legend, and your answer is impeccable
Post by: Bubbly_bluey on July 13, 2017, 10:32:05 am
Thanks :)
Post by: seventeenboi on July 13, 2017, 05:52:39 pm
Why is the first step of the ionisation of polyprotic acid the one that occurs to the greatest extent ??
Post by: MisterNeo on July 13, 2017, 06:13:59 pm
Hi :)
Why is the first step of the ionisation of polyprotic acid the one that occurs to the greatest extent ??
Hi! ;)
Polyprotic acids ionisation becomes weaker with after each proton.
Let's look at sulfuric acid:
Sulfuric acid is a strong acid that will fully ionise into its conjugate base, hydrogen sulfate, which is amphiprotic. So it becomes less of an acid because of it becoming a conjugate base.
Hence, hydrogen sulfate will only partially ionise in a equilibrium into its conjugate base, sulfate ion.
They basically (lol) get more basic as they donate hydrogen ions and become less of an acid, thus less extent of ionisation. :)
Also, it's worth noting that weak acids have stronger conjugate bases than strong acids. So, polyprotic weak acids like citric acid become more basic than sulfuric acid after each proton donation.
Post by: seventeenboi on July 13, 2017, 07:11:39 pm
Hi! ;)
Polyprotic acids ionisation becomes weaker with after each proton.
Let's look at sulfuric acid:
Sulfuric acid is a strong acid that will fully ionise into its conjugate base, hydrogen sulfate, which is amphiprotic. So it becomes less of an acid because of it becoming a conjugate base.
Hence, hydrogen sulfate will only partially ionise in a equilibrium into its conjugate base, sulfate ion.
They basically (lol) get more basic as they donate hydrogen ions and become less of an acid, thus less extent of ionisation. :)
Also, it's worth noting that weak acids have stronger conjugate bases than strong acids. So, polyprotic weak acids like citric acid become more basic than sulfuric acid after each proton donation.
OHHH mm that makes a lot of sense!! thank youu nice pun HAHA
Post by: RuiAce on July 13, 2017, 10:14:54 pm
Post by: winstondarmawan on July 13, 2017, 11:45:32 pm
This is a question from shipwrecks: Steel rusts more rapidly than pure iron. Give one reason for this in terms of the structure of steel.Hey! I do shipwrecks.
Thanks :)
Okay so basically, rusting is a special type of corrosion (that occurs with iron), which is a redox reaction.
So, we know that in pure iron, this occurs:
Anode: Fe(s) > Fe2+(aq) + 2e
Cathode: H2O (l) + 1/2 O2 (g) + 2e > 2 OH- (aq)
However, in steel (which is composed of iron and carbon), not only this reaction occurs, but another galvanic cell is set up with iron as the anodic site and carbon as the cathodic site. Pretty sure you do not need to know the half reactions for this, it should be in your theory that impurities (with less ease of oxidation, which carbon is) act as cathodic sites whilst iron acts as the anodic site. Hence, this accelerates the corrosion oxidation-reduction process and thus rusting occurs more rapidly in steel.
Hope this helps!
Post by: anotherworld2b on July 14, 2017, 12:14:47 am
Are there constant key differences that would help to differentiate between the two when given a diagram?
Post by: anotherworld2b on July 14, 2017, 01:20:12 am
thank you for your help :)
The only difference I see between them is that the anode/cathode are different metals in electroplating, whereas they are the same metal in electrorefining.
They both work off the same concept of electrolysis, so it's not that big of a deal if you can't distinguish between them. They usually tell you what's happening in the question like "Student A performed this electroplating experiment, and here are his results." ;D
Post by: Bubbly_bluey on July 14, 2017, 06:42:41 pm
Post by: Bubbly_bluey on July 14, 2017, 06:45:47 pm
Hey! I do shipwrecks.
Okay so basically, rusting is a special type of corrosion (that occurs with iron), which is a redox reaction.
So, we know that in pure iron, this occurs:
Anode: Fe(s) > Fe2+(aq) + 2e
Cathode: H2O (l) + 1/2 O2 (g) + 2e > 2 OH- (aq)
However, in steel (which is composed of iron and carbon), not only this reaction occurs, but another galvanic cell is set up with iron as the anodic site and carbon as the cathodic site. Pretty sure you do not need to know the half reactions for this, it should be in your theory that impurities (with less ease of oxidation, which carbon is) act as cathodic sites whilst iron acts as the anodic site. Hence, this accelerates the corrosion oxidation-reduction process and thus rusting occurs more rapidly in steel.
Hope this helps!
Yes it makes sense! Thanks so much! :)
Post by: seventeenboi on July 14, 2017, 07:37:24 pm
what are some problems associated with high levels of TDS in an aquatic body ???
Post by: MisterNeo on July 14, 2017, 08:08:10 pm
hello :))
what are some problems associated with high levels of TDS in an aquatic body ???
Total dissolved solids is everything except gases that is dissolved in water. This includes cations/anions (salts) and faeces.
-Metals like copper and zinc are very common dissolved metals, which cause health problem because they are a trace element, which is naturally present in very small concentrations in the human body and should not be high.
-Lead can also be present in the water, which is a neurotoxin that can be transferred to humans through seafood.
-pH can be affected.
-An increase in metal ions such as magnesium and calcium result in water hardness, which stops soap from lathering.
-Dissolved poo causes eutrophication, which leads to excessive plant.algae growth. It also causes the BOD to increase and kill the fish in the water.
-Polluted water usually has less oxygen dissolved in it for aquatic life.
-Water with high TDS also has high turbidity, which indicates contaminated water and is unsuitable for aquatic life.
These are the main concerns with high TDS that affect not only the fish, but also has impacts on humans.
Post by: Bubbly_bluey on July 14, 2017, 08:25:17 pm
Just a little confused between the Vienna Convention 1985 and the Montreal Protocol 1987. I know they were both associated with world leaders coming together to stop release of CFCs and introducing alternatives. But what is the difference between them.
Thanks ;D
Post by: winstondarmawan on July 14, 2017, 08:50:07 pm
TIA
Post by: MisterNeo on July 14, 2017, 09:15:12 pm
Hello! Just wanted to ask if anyone had any clever ways (e.g. mnemonics) to remember the anion/cation testing, as well as ion flame test colours. They always slip out of my mind.This is Jake's older post on mnemonics for the cation/anions and flame tests.
TIA
I use these for my solubility rules and flame memorising. ;D
(Just click the "image removed from quote".)
Hey! Basically, you just need to remember your solubility rules/flame tests. Then, you'll likely be given a solution containing a number of ions. Use the solubility rules/flame test colours to figure out a way of determining each ion. There isn't much to this one in terms of prep, except you really do need to memorise the rules (Assuming they don't give them to you!). I've put the way I remember each rule below.
(http://i.imgur.com/0t4KRgD.png)
(http://i.imgur.com/RFS1RIl.png)
Let me know if I can add anything else!
Post by: bsdfjnlkasn on July 14, 2017, 11:31:13 pm
I thought the answer for Q2 was D as chlorine is more electronegative than bromine so get's the lowest number (i.e. is the beginning of the carbon chain)
With 9, could I possibly get an explanation, I just chose D by a process of elimination since I knew the species had to be the same. Isn't PH3 acting as a base in the forward reaction? Or does conjugate acid/base pair imply the backwards reaction?
For Q16, I got 18.2mL...
Any help would be super appreciated! :D
Post by: andiyousif1 on July 15, 2017, 11:15:43 am
Post by: winstondarmawan on July 15, 2017, 12:05:04 pm
It was listed as one of the equipment but I don't see how it is necessary..
TIA
Post by: Fahim486 on July 15, 2017, 12:16:24 pm
Thanks!
Post by: MisterNeo on July 15, 2017, 12:56:49 pm
Hi so I am looking back at Production of Materials and kinda forgot the difference between Hexene and Hex-2-ene and like what the 2 indicates. Also another question, is bromine water used to distinguish between alkanes and alkenes e.g. propane and propene?
Thanks!
Hi! :)
The numbers on hexene represent which carbon the double bond is on.
Hex-1-ene is the same as 1-Hexene, and Hex-2-ene is the same as 2-Hexene.
Bromine water is used to distinguish between alkanes and alkenes. The practical experiments usually use cyclohexane and cyclohexane, but any alkane and alkene can be used because something like hexene will still open its double bond to react with bromine.
Post by: kdawgs on July 15, 2017, 02:11:49 pm
I only ask because I've been looking over chemistry study notes and for most of the notes under this dot point, people have included large sums of information relating to the physical and chemical properties of alkanes and alkenes.
Post by: MisterNeo on July 15, 2017, 02:41:35 pm
For the dot point "identify that ethylene, because of the high reactivity of it's double bond, is readily transformed into many useful products" if you were to answer a question on this in an HSC exam, do you have to include information regarding the properties of alkanes and alkenes?
I only ask because I've been looking over chemistry study notes and for most of the notes under this dot point, people have included large sums of information relating to the physical and chemical properties of alkanes and alkenes.
For this dotpoint, you only need to identify that ethylene is more reactive than its alkane counterpart - ethane, because of the high electron density in the double bond that allows ethylene to undergo addition reactions. You should include the fact that alkenes are unsaturated, and alkanes are saturated (maximum hydrogen bonded). These are the only two properties you would need to explain the reactivity comparison between alkanes and alkenes.
After that, they may ask you to identify properties of products like vinyl chloride and styrene that are formed by ethylene. ;)
Post by: kdawgs on July 15, 2017, 02:54:57 pm
Post by: anotherworld2b on July 16, 2017, 12:30:05 am
The student prepared 1.00L of a 0.0248mol.L-1 Na2CO3 solution. He titrated three 25mL aliquots of this solution against the HCL and found a average titre of 24.35mL.
a) Calc concentration of the standardized HCL solution
Na2CO3 + 2HCl -> 2NaCl + H20 + CO2
n(Na2CO3) in 1 litre = 1 x 0.0248 = 0.0248 moles
n(HCl) = 2 x 0.0248 moles = 0.0496 moles
c(HCl) = 0.0496 / 24.35 = 0.0020 mol -1
I was hoping to get help get some help with part b please. I am not how to start
The antacid suspensions were thoroughly shaken and 20mL of each transferred to separate 250mL volumetric flasks. Both were made up to the mark with distilled water and shaken vigorously. 10mL aliquots of the distilled suspension were transferred to conical flasks for titration and an appropriate indicator added.
The tire values obtained for the Al(OH)3 suspension.
Average Titre volume HCL mL
Trials
1 - 21.94
2 - 22.62
3 - 21.98
4 - 21.94
Average titre - 21.90
b) calculate the concentration, Mol.L-1 of Al(OH)3 in the original Al(OH)3 suspension
Post by: MisterNeo on July 16, 2017, 01:06:55 am
Hi I was wondering if someone could check my answer for part a in this question pleaseSpoilerThe student prepared 1.00L of a 0.0248mol.L-1 Na2CO3 solution. He titrated three 25mL aliquots of this solution against the HCL and found a average titre of 24.35mL.
a) Calc concentration of the standardized HCL solution
Na2CO3 + 2HCl -> 2NaCl + H20 + CO2
n(Na2CO3) in 1 litre = 1 x 0.0248 = 0.0248 moles
n(HCl) = 2 x 0.0248 moles = 0.0496 moles
c(HCl) = 0.0496 / 24.35 = 0.0020 mol -1
I was hoping to get help get some help with part b please. I am not how to start
The antacid suspensions were thoroughly shaken and 20mL of each transferred to separate 250mL volumetric flasks. Both were made up to the mark with distilled water and shaken vigorously. 10mL aliquots of the distilled suspension were transferred to conical flasks for titration and an appropriate indicator added.
The tire values obtained for the Al(OH)3 suspension.
Average Titre volume HCL mL
Trials
1 - 21.94
2 - 22.62
3 - 21.98
4 - 21.94
Average titre - 21.90
b) calculate the concentration, Mol.L-1 of Al(OH)3 in the original Al(OH)3 suspension
For Part A, the volume is supposed to be in litres (0.02435L) so your answer is 1000x too small.
For Part B, you exclude Titre-2 because it's an outlier and you calculate a new average using the 3 consistent results (21.95).
The vigorous shaking is due to aluminium hydroxide being only very slightly soluble.
So we work off the 21.95mL average for HCl used and the concentration obtained in Part A.
-Moles of HCl used is 2.037x0.02195=0.045mol.
-Moles of aluminium hydroxide=0.015mol.
The amount of each aluminium sample is 10mL so we divide 0.015 by 0.01L which is 1.5mol/L in the diluted sample.
The diluted sample is 250mL so we divide the 1.5 by 4 to get 0.375 moles.
These moles were from 20mL of the original sample, so divide 0.375 by 0.02L to get 18.75mol/L.
Annnd that's your original volume.
I think I did that right, do you have answers? ???
Hope this helps :)
Post by: anotherworld2b on July 16, 2017, 01:22:15 am
part a) would be
c(HCl) = 0.0496 / 0.02435 = 2.0369 mol L-1
I get it now thanks for your help :D
I was also hoping to get some help with these last parts for the question please because I don't have any answers available
f) Mg(OH)2 diluted suspension, found the mass of Mg(OH)2 in the 250mL diluted suspension to be 1.13g.Determine the concentration of Mg(OH)2 in the original undiluted suspension and express your answer in mol.L-1
g) Which of the preparations would be more effective (neutralise more HCl) for a given volume
For Part A, the volume is supposed to be in litres (0.02435L) so your answer is 1000x too small.
For Part B, you exclude Titre-2 because it's an outlier and you calculate a new average using the 3 consistent results (21.95).
The vigorous shaking is due to aluminium hydroxide being only very slightly soluble.
So we work off the 21.95mL average for HCl used and the concentration obtained in Part A.
-Moles of HCl used is 2.037x0.02195=0.045mol.
-Moles of aluminium hydroxide=0.015mol.
The amount of each aluminium sample is 10mL so we divide 0.015 by 0.01L which is 1.5mol/L in the diluted sample.
The diluted sample is 250mL so we divide the 1.5 by 4 to get 0.375 moles.
These moles were from 20mL of the original sample, so divide 0.375 by 0.02L to get 18.75mol/L.
Annnd that's your original volume.
I think I did that right, do you have answers? ???
Hope this helps :)
Post by: phoebegresham on July 16, 2017, 01:42:36 pm
Post by: jakesilove on July 16, 2017, 02:53:42 pm
I am going through the Jakes Chem notes that I purchased at one of the lectures. For Equilibria, it says that for endothermic reactions, an increase in temperature will cause the equilibrium to shift to the right, which I totally understand. However in the next syllabus point, it says that the CO2(g) -><- CO2(aq) is endothermic, however that an increase in temperature would cause a shift to the left. Isn't this contradicting the earlier statement? Thanks :)
Hey! You're 100% right, there is a typo there in some of the notes (thought we had sorted it out, sorry about that). The reaction, when shown in that order, is EXOTHERMIC. However, if you put the reaction the other way (ie. aqueous turning into gas) then the reaction is ENDOTHERMIC. Thus, in that case, when you increase the temperature the aqueous carbon dioxide turns into gaseous carbon dioxide, and escapes the solution. In that order, the reaction moves to the right. The issue here is the ambiguity of the whole 'moving right' vs 'moving left' thing, given that equilibrium reactions can technically be written either way. Anyway, I'm sorry for the mistake (totally my fault) but glad that you picked it up and understand it well enough to have come to your own conclusion.
The easiest way to think about this is that when you leave a fizzy coke in the sun, open, the soft drink will go flat quicker than if you had left the same coke in the fridge :)
Post by: phoebegresham on July 16, 2017, 03:07:39 pm
Post by: Fahim486 on July 16, 2017, 05:45:41 pm
Post by: Fahim486 on July 16, 2017, 06:04:37 pm
Post by: MisterNeo on July 16, 2017, 06:52:19 pm
Hi, I'm confused as to how you would approach this question on addition polymers. Thanks!Hi! :)
For this question you would explain how a longer polymer chain results in those properties based on the flowchart. You just need to structure your response according to what is given by the flowchart. A longer chain results in higher dispersion forces that increase the intermolecular forces between polymer chains, hence high melting/boiling points and increased hardness.
Hi again I've got another question, do LDPE have strong intermolecular forces or weak intermolecular forces? Thanks!LDPE have weaker intermolecular forces than HDPE because the frequent side branching prevents proper alignment of atoms, thus reducing the strength of the dispersion forces. ;D
Post by: seventeenboi on July 16, 2017, 08:54:08 pm
for the question 'state 3 ways that AAS has had a great impact on scientific understanding of tracelements in organisms' .. how would you answer it in a detailed way .. i'm also having trouble coming up with answers ??
so far I can only really think of
1. the essentiality of trace elements as they aid enzyme function - their requirement for the proper functioning of organisms in biological systems (i.e. iron in haemoglobin, magnesium, calcium, zinc in our systems)
2. criticality of trace element deficiencies in organisms - in humans, lack of micronutrients in plants, lack of cobalt on pasteur land, so animal health could not be maintained ..
3. ?????
could it possibly that the discovery of trace elements itself be considered an 'impact'??
in what other ways could you answer this question ?
thanks :)
Post by: Natasha.97 on July 16, 2017, 09:23:01 pm
hello
for the question 'state 3 ways that AAS has had a great impact on scientific understanding of tracelements in organisms' .. how would you answer it in a detailed way .. i'm also having trouble coming up with answers ??
so far I can only really think of
1. the essentiality of trace elements as they aid enzyme function - their requirement for the proper functioning of organisms in biological systems (i.e. iron in haemoglobin, magnesium, calcium, zinc in our systems)
2. criticality of trace element deficiencies in organisms - in humans, lack of micronutrients in plants, lack of cobalt on pasteur land, so animal health could not be maintained ..
3. ?????
could it possibly that the discovery of trace elements itself be considered an 'impact'??
in what other ways could you answer this question ?
thanks :)
Hi! :)
I think you're on the right track for the first and second points! The third way could possibly be measuring the concentration of toxic ions such as lead, as it is toxic to the body (brain, kidneys, neurotoxin). As for the level of detail, it is all dependent on the number of marks this is worth (e.g. if it is 6 marks, 3 would be the ways that AAS has an impact, and the other 3 would be an explanation of why it had a great impact). Hope this helps :D
Post by: kiwiberry on July 16, 2017, 09:27:05 pm
hello
for the question 'state 3 ways that AAS has had a great impact on scientific understanding of tracelements in organisms' .. how would you answer it in a detailed way .. i'm also having trouble coming up with answers ??
so far I can only really think of
1. the essentiality of trace elements as they aid enzyme function - their requirement for the proper functioning of organisms in biological systems (i.e. iron in haemoglobin, magnesium, calcium, zinc in our systems)
2. criticality of trace element deficiencies in organisms - in humans, lack of micronutrients in plants, lack of cobalt on pasteur land, so animal health could not be maintained ..
3. ?????
could it possibly that the discovery of trace elements itself be considered an 'impact'??
in what other ways could you answer this question ?
thanks :)
Hey! In addition to the above answer, you should definitely mention the fact that the development of AAS led to the discovery of trace elements in the first place - previous methods like gravimetric analysis were not sensitive enough to detect the presence of trace elements and so the causes of deficiency diseases in humans and plants were unknown. AAS was sensitive and accurate enough to detect their presence, and so a correlation between these elements and the health of organisms could be drawn. You can then talk about the points you mentioned about the importance of trace elements. :)
Post by: winstondarmawan on July 17, 2017, 12:44:46 am
So, my notes says increasing temperature increases rate of corrosion. However, doesn't increasing temperature also result in reducing of solubility of gases (e.g. O2), which wold reduce the rate of corrosion?
Also, I don't understand how there can be anaerobic corrosion, according to this solution:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20136777_1272903182835202_353827078_n.png?oh=dd044f1c45b808da2e920e39970ce5c3&oe=596E7E05
TIA!
Post by: MisterNeo on July 17, 2017, 01:13:58 am
Hello! Some questions for shipwrecks.
So, my notes says increasing temperature increases rate of corrosion. However, doesn't increasing temperature also result in reducing of solubility of gases (e.g. O2), which wold reduce the rate of corrosion?
Also, I don't understand how there can be anaerobic corrosion, according to this solution:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20136777_1272903182835202_353827078_n.png?oh=dd044f1c45b808da2e920e39970ce5c3&oe=596E7E05
TIA!
According to the EasyChem website (I don't do shipwreck sorry :P), the oxygen solubility does decrease but it's negligible because most corrosion is done by anaerobic bacteria at depths.
Anaerobic bacteria corrode by reducing sulfate to hydrogen sulfide ion using electrons from iron. (This explains it all.)
Post by: winstondarmawan on July 17, 2017, 08:51:51 am
According to the EasyChem website (I don't do shipwreck sorry :P), the oxygen solubility does decrease but it's negligible because most corrosion is done by anaerobic bacteria at depths.
Anaerobic bacteria corrode by reducing sulfate to hydrogen sulfide ion using electrons from iron. (This explains it all.)
Thank you! Also, does a non-Industrial student need to know equilibrium constant?
Post by: jakesilove on July 17, 2017, 11:48:19 am
Thank you! Also, does a non-Industrial student need to know equilibrium constant?
Absolutely not!
Post by: hansolo9 on July 18, 2017, 11:51:12 pm
"Explain why sulfuric acid is added into water instead of water being added into sulfuric acid?"
TIA
Post by: MisterNeo on July 18, 2017, 11:53:29 pm
Hi :) can some who does industrial chem help me with this question?
"Explain why sulfuric acid is added into water instead of water being added into sulfuric acid?"
TIA
Hey! So the reason why the pure acid is added to the water and not vice versa is solely because acid ionisation is exothermic.
If you were to add a beaker of acid to a beaker of water, the first drop of acid being ionised make the solution dilute and not pose any threat from the heat released.
However, if you were to add a beaker of water to a beaker of acid, the first drop of water will make the solution an extremely high concentration, and the exothermic reaction can boil the water and spray concentrated acid everywhere. Thus, fuck up your day - and your experiment.
So the general rule here is acid first so it doesn't burst. ;D
Post by: bigsweetpotato2000 on July 19, 2017, 01:17:35 am
Especially for chemistry since the multiple choice is quite hard to tackle :o
Thanks!
Bigsweetpotato Farm
Post by: MisterNeo on July 19, 2017, 01:58:12 am
To start the exams with the Multiple Choice or to not start the exams with the Multiple Choice?
Especially for chemistry since the multiple choice is quite hard to tackle :o
Thanks!
Bigsweetpotato Farm
I would have a look through MC during reading time to get an idea on how much time you should allocate for the section.
Personally, I like to do the first half of the MC at the start, then do the rest in between other sections to give my hand a break from the longer responses. ;)
Post by: Shadowxo on July 19, 2017, 10:39:53 am
To start the exams with the Multiple Choice or to not start the exams with the Multiple Choice?
Especially for chemistry since the multiple choice is quite hard to tackle :o
Thanks!
Bigsweetpotato Farm
What I tend to do is see how much time the MC questions are worth with respect to marks. Then, I use reading time to look over SA questions and get an idea of what they're talking about, and complete those first. I leave MC until the end unless I'm struggling with a SA question, if I am struggling I take a break and either go to the next question or MC to give myself time to think about it.
- You can guess MC questions but not SA
- SA you have to include all relevant information, but in MC you only need the right answer
- MC questions are only worth 1 mark and past questions don't affect current answers
- Towards the end of the exam you're likely to be more tired and don't want to do those long tough questions
Just what I tend to do :)
Post by: winstondarmawan on July 19, 2017, 05:05:39 pm
8. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20136376_1271772089614978_682044855_n.png?oh=ee5739c2bbd68d91e9daaa41e7fd0420&oe=5970AF0E
10. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20067722_1271778759614311_792814971_n.png?oh=c0f901e116ad084b5f9724edd3508723&oe=5971A02A
11. https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/20107688_1271783416280512_709372620_o.png?oh=b7b0ad8fcad5d15843f1ea220447f1be&oe=5971C7FF
TIA!
Post by: bsdfjnlkasn on July 19, 2017, 06:37:45 pm
Was just wondering which trial papers are the best to be doing now?
My exam is next week
Thank you! :)
Post by: MisterNeo on July 19, 2017, 07:12:14 pm
Hello! Would appreciate help for the following questions:
8. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20136376_1271772089614978_682044855_n.png?oh=ee5739c2bbd68d91e9daaa41e7fd0420&oe=5970AF0E
10. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20067722_1271778759614311_792814971_n.png?oh=c0f901e116ad084b5f9724edd3508723&oe=5971A02A
11. https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/20107688_1271783416280512_709372620_o.png?oh=b7b0ad8fcad5d15843f1ea220447f1be&oe=5971C7FF
TIA!
Hi! According to the syllabus, an acidic oxide reacts water to form an acidic solution and reacts with a base to form a salt. Calcium oxide is a base so I'd say the answer is both 2 and 3.
For the second one, 1 and 3 are immediately eliminated because conjugate pairs can only be one proton in difference. So the answer would be 2 and 4 since oxide ions can accept protons to form hydroxide. (This is why metal oxides are very basic in water.) The NH2- can accept a proton to become ammonia.
And for the last one, you need to find the excess reagent (after balancing the equation) and put those moles in a 100mL solution. Then plug them into the pH equation.
Moles of OH-=2(0.102x0.02)=0.00408mol
Moles of H+=(0.15x0.04)=0.06mol
Excess H+=0.00192mol
Concentration=0.0192mol/L
pH=1.71
Hope this helps ;D
Post by: bigsweetpotato2000 on July 19, 2017, 10:30:50 pm
Hey there,
Was just wondering which trial papers are the best to be doing now?
My exam is next week
Thank you! :)
For your trials? Definitely your school ones!
To maximise your score for trials, get familiar with your school's personal layout - even though they structure it like the HSC they tend to get... creative.
After trials though, head straight for HSC past papers!
Bigsweetpotato Farm
Post by: statues on July 19, 2017, 10:40:29 pm
In the hydration of Ethylene should I write the catalyst to be conc. phosphoric acid or dil. sulphuric acid?
I've got sources saying different things.
Thanks :)
Post by: kiwiberry on July 19, 2017, 11:02:51 pm
Quickie:
In the hydration of Ethylene should I write the catalyst to be conc. phosphoric acid or dil. sulphuric acid?
I've got sources saying different things.
Thanks :)
Both dilute phosphoric acid or sulfuric acid can be used! :)
Post by: bsdfjnlkasn on July 20, 2017, 07:36:20 pm
I'm not getting anything close to the answers given so would appreciate some guidance - thank you very much! :D
Post by: Shadowxo on July 20, 2017, 07:52:40 pm
Energy released from 1.635g:
E=mc∆T
m=200mL, c=4.18, ∆T = 47 - 22 = 25 şC
E = 200*4.18*25 = 20,900 J = 20.900 kJ
Energy released per gram = 20.900 /1.635 = 12.783 kJ
Energy released from 1.50g = 12.783 * 1.50 = 19.174 kJ
*remember sig figs, I ignored them but you shouldn't*
Hope that helps :)
Post by: seventeenboi on July 20, 2017, 08:07:44 pm
I know the answer to this question is a since all the other options contain substances which do not form a coordinate covalent bond. however, i was wondering why water forms a coordinate covalent bond???? is it perhaps referring to the hydronium ion ???
Post by: MisterNeo on July 20, 2017, 08:19:44 pm
hello :)
I know the answer to this question is a since all the other options contain substances which do not form a coordinate covalent bond. however, i was wondering why water forms a coordinate covalent bond???? is it perhaps referring to the hydronium ion ???
Yes, you are correct! The answer would be A because oxygen, water, and ammonia can all form coordinate covalent bonds (ozone, hydronium ion, ammonium ion). :)
Post by: bsdfjnlkasn on July 20, 2017, 09:00:23 pm
Could someone please explain the following solution to me?
I got that it was B but still (I know it's annoying) the terminology is confusing me
Any help would be super appreciated!
Thanks :D
Post by: MisterNeo on July 20, 2017, 09:19:10 pm
Hey there,
Could someone please explain the following solution to me?
I got that it was B but still (I know it's annoying) the terminology is confusing me
Any help would be super appreciated!
Thanks :D
Hey! So an oxidising agent (also known as oxidant) causes oxidation and undergoes reduction itself (gains electrons).
On the reduction potential list that you get in exams, the strongest oxidising agents are the the very top with the largest positive potential, whereas the strongest reducing agents (reductants) are at the very bottom with negative potentials.
(http://ch302.cm.utexas.edu/images302/std-pots-shortlist.png)
So with this question, the most powerful oxidant is the one with the most positive potential as it would appear at the very top of the potentials list, hence A+ is the answer. ;D
Post by: Mathew587 on July 20, 2017, 11:43:20 pm
Can someone explain to me how to calculate E* for electrolytic cells? I'm confused as to which undergoes oxidation, reduction and when water is involved.
Ty :)
Post by: beau77bro on July 21, 2017, 11:27:41 am
is it sufficient to say that there is very little water in the 98% concentrate, as well as the fact that most of the water forms hydrates resulting in very few ions being present. this then results in far greater ionisation when water is added and the corresponding heat produced from the strong ionisation, both of which contribute to the boiling and spititng of acid.
Post by: MisterNeo on July 21, 2017, 11:45:52 am
with the ionisation of concentrated sulfuric acid, is the reaction highly exothermic, or is it just that there is so much ionisation that the energy produced is enormous. I really can't tell from conquering, but I'm assuming it is a combination of both that produces a large amount of energy.
is it sufficient to say that there is very little water in the 98% concentrate, as well as the fact that most of the water forms hydrates resulting in very few ions being present. this then results in far greater ionisation when water is added and the corresponding heat produced from the strong ionisation, both of which contribute to the boiling and spititng of acid.
It's a combination of both the ionisation of acid is highly exothermic, and that the acid is in its pure liquid form since there are a lot of molecules being ionised.
The reaction is so exothermic that the acid is added into water, not water into acid or else it will boil/splash everywhere.
Post by: MisterNeo on July 21, 2017, 12:02:56 pm
Hey guys,
Can someone explain to me how to calculate E* for electrolytic cells? I'm confused as to which undergoes oxidation, reduction and when water is involved.
Ty :)
Hey! :)
So for electrolytic cells, it is different to galvanic cells because the anode is the positive terminal and the cathode is negative.
For the NaCl example of an electrolytic cell:
A battery power source is used to pull electrons from the anode to the cathode (inert platinum rods)), and the anions (Cl-) in the solution migrate towards the anode because it is positively charged from its electrons being taken (negative anode means that negative ions go towards it).
The anions (Cl-) then get oxidised (electrons lost) at the anode to form chlorine gas with the electrons being taken to the cathode by the battery.
The cations (Na+) get reduced (electrons gained) at the cathode to form sodium metal deposit on the cathode rod, and the electrons come from the chlorine.
Electrolytic cells are non-spontaneous, thus the E* will be negative.
To calculate it, you just get the value of chlorine's oxidation (-1.36), and add it to sodium's reduction (-2.71).
(-1.36) + (-2.71) = -4.07V
If the concentration of NaCl is too low (<2M), hydrogen ions will be reduced instead of sodium ions, and oxygen ions will be oxidised instead of chlorine ions. So hydrogen gas will evolve at the cathode and oxygen gas will evolve at the anode.
Hope this helps ;D
Post by: bluecookie on July 21, 2017, 12:07:48 pm
Post by: beau77bro on July 21, 2017, 07:46:09 pm
Post by: Bubbly_bluey on July 21, 2017, 07:52:18 pm
can someone please explain this magic?? honestly no clue how you get from parts per million to g/L in the solutionsHi! ppm is the same as saying mg/L. So you just convert the milligrams to grams by divide by 1000 and that would be how many grams you have in 1L.
Hope this makes sense :)
Post by: bsdfjnlkasn on July 21, 2017, 07:54:14 pm
Still struggling with the equilibrium constant questions...
I've provided an attachment of the question and honestly thought that I was on the right track, but got the moles of O2 after equilibrium had been established, wrong. How do I got about working this out properly?
Thank you so much!
Post by: kiwiberry on July 21, 2017, 08:27:38 pm
Hey there!
Still struggling with the equilibrium constant questions...
I've provided an attachment of the question and honestly thought that I was on the right track, but got the moles of O2 after equilibrium had been established, wrong. How do I got about working this out properly?
Thank you so much!
Hey! The amount of NO that reacted is 0.25 - 0.05 = 0.2 mol. Because n(NO):n(NO2) = 1:1, the amount of NO2 formed at equilibrium will be 0.2 mol. Because n(NO):n(O2) = 2:1, the amount of O2 that reacted will be half that of NO, so 0.1 mol. Therefore at equilibrium, there will be 0.12 - 0.1 = 0.02 mol of O2 left. Concentrations will be the same as moles because v=1. I find it helps to process the information by drawing an ice table :)
Post by: limtou on July 22, 2017, 01:29:13 am
Post by: bsdfjnlkasn on July 22, 2017, 01:53:41 am
Was wondering what info I needed for the following dot point:
Process information from secondary sources to summarise the use of ethanol as an alternative car fuel, evaluating the success of current usage
What current usages do we address? I have some info on Brazil but am getting conflicting advice about how successful it was - some sources say that it was abandoned and others, really successful. I would just like to know in case we aren't given secondary sources, or so that I can know what to say if we are given them.
Thank you!!
Post by: kiwiberry on July 22, 2017, 02:03:17 am
Hello! Please help with the following multiple choice~ Thanks!
Hello! The first question has been addressed here
For the second one, titrations can only be used to determine the concentration of an acid. The amount of base required to neutralise any monoprotic acid will be the same, regardless of their degree of ionisation, so we wouldn't be able to tell the difference in strength of HB and HA. Suppose HB is a weak acid:
Post by: kiwiberry on July 22, 2017, 02:12:44 am
Hey there,
Was wondering what info I needed for the following dot point:
Process information from secondary sources to summarise the use of ethanol as an alternative car fuel, evaluating the success of current usage
What current usages do we address? I have some info on Brazil but am getting conflicting advice about how successful it was - some sources say that it was abandoned and others, really successful. I would just like to know in case we aren't given secondary sources, or so that I can know what to say if we are given them.
Thank you!!
Hm, I've got that Brazil has had success with ethanol concentrations > 25% in fuel. Not sure about this though
Post by: bsdfjnlkasn on July 22, 2017, 08:53:36 am
I'm a bit confused about how soap acts as an emulsifying agent
When the non-polar tails attach themselves to the oil so that their negatively charged heads are facing out into the water, what allows the emulsion to occur? Is it dipole-dipole interactions?
Is it also dipole-dipole interactions for when the tails are facing out? OR just dispersion forces because the tail isn't positively charged?
Sorry if this doesn't make any sense... And let me know if we don't need to know these details! But I would still like to understand - thank you!
Post by: MisterNeo on July 22, 2017, 10:08:05 am
Hey there,
I'm a bit confused about how soap acts as an emulsifying agent
When the non-polar tails attach themselves to the oil so that their negatively charged heads are facing out into the water, what allows the emulsion to occur? Is it dipole-dipole interactions?
Is it also dipole-dipole interactions for when the tails are facing out? OR just dispersion forces because the tail isn't positively charged?
Sorry if this doesn't make any sense... And let me know if we don't need to know these details! But I would still like to understand - thank you!
Hey! I found this great guide on google that should be able to answer your question.
Basically, the non-polar tails attract to fats, and the negative head hydrogen bonds/dipoles with water to keep itself soluble in water. :)
Post by: beau77bro on July 22, 2017, 12:02:31 pm
Hi! ppm is the same as saying mg/L. So you just convert the milligrams to grams by divide by 1000 and that would be how many grams you have in 1L.
Hope this makes sense :)
I dont really understand the conversion?? like how does that work with everything having different molar weights and all?
Post by: Sine on July 22, 2017, 12:16:26 pm
can someone please explain this magic?? honestly no clue how you get from parts per million to g/L in the solutions
I dont really understand the conversion?? like how does that work with everything having different molar weights and all?So basically it is what it sounds like - how many parts of this species is there per total million parts
Say we have 100ppm this means 100/1000000 which is 0.01%
Molar weights have nothing to do with it since we aren't calculating the mole.
ppm is mg/L and it is also g/m^3 which is g/1000L
so here we have parts per million (ppm) = mg/L = g/1000L
Now say we have 125 ppm that means 125mg/L
Let's convert 125mg to g since 1000mg is in a gram we divide by 1000 which gets us to 0.125g
which means 125ppm = 0.125g/L
With conversions it's best to do more steps and be careful since any small mistake will give you an answer that is wrong.
Post by: jakesilove on July 22, 2017, 01:50:15 pm
So basically it is what it sounds like - how many parts of this species is there per total million parts
Say we have 100ppm this means 100/1000000 which is 0.01%
Molar weights have nothing to do with it since we aren't calculating the mole.
ppm is mg/L and it is also g/m^3 which is g/1000L
so here we have parts per million (ppm) = mg/L = g/1000L
Now say we have 125 ppm that means 125mg/L
Let's convert 125mg to g since 1000mg is in a gram we divide by 1000 which gets us to 0.125g
which means 125ppm = 0.125g/L
With conversions it's best to do more steps and be careful since any small mistake will give you an answer that is wrong.
Great explanation! I always struggled with this sort of thing
Post by: limtou on July 22, 2017, 02:27:38 pm
Hello! The first question has been addressed here
For the second one, titrations can only be used to determine the concentration of an acid. The amount of base required to neutralise any monoprotic acid will be the same, regardless of their degree of ionisation, so we wouldn't be able to tell the difference in strength of HB and HA. Suppose HB is a weak acid:As H+ reacts with the OH- in NaOH, its concentration decreases, so by LCP equilibrium will continue to shift right until all its H+ has reacted - hence we're unable to compare its degree of ionisation with HA. The pH meter will be able to measure the concentration of H+ ions of the acid that dissociate in solution and therefore its degree of ionisation. And B is definitely more suitable than D to keep the experiment fair! :)
Thanks! Just wondering, acid has to have H+ right? So are acidic oxide and acid different things?
Post by: jakesilove on July 22, 2017, 02:31:52 pm
Thanks! Just wondering, acid has to have H+ right? So are acidic oxide and acid different things?
An acid is a substance that donates protons (ie. H+)
An acidic oxide is a substance that, when combined with water, forms an acid.
So yeah, they are totally different things! Can get a bit confusing, but just keep those definitions in mind :)
Post by: limtou on July 22, 2017, 02:36:39 pm
An acid is a substance that donates protons (ie. H+)
An acidic oxide is a substance that, when combined with water, forms an acid.
So yeah, they are totally different things! Can get a bit confusing, but just keep those definitions in mind :)
Ah I see. Since the syllabus states that non-metal oxides act as acid, I was confused. Thanks for the clarification Jake :)
Post by: Jenny_trn on July 22, 2017, 05:02:42 pm
Post by: beau77bro on July 22, 2017, 05:24:10 pm
What is the monomer of polystyrene? I'm going through the HSC chemistry topic tests and the answer is ethenylbenzene, but i thought that was the systematic name for polystyrene, so styrene would be the monomer name? I'm honestly not sure.
YES OMG THIS GOT ME TOO, but yea generally the systematic name of the monomer is very similar to the polymer minus 'poly' so the systematic of the monomer name is ethenylbenzene, and for the polymer it is polyethenylbenzene. just like 1-chloroethane has a polymer of poly(1-chloroethane). i hope i smelt this all right ahhaha, and hope it helps
Post by: limtou on July 22, 2017, 05:35:52 pm
What is the monomer of polystyrene? I'm going through the HSC chemistry topic tests and the answer is ethenylbenzene, but i thought that was the systematic name for polystyrene, so styrene would be the monomer name? I'm honestly not sure.
ethenylbenzene is the systematic name for styrene, not polystyrene :)
I also have a question~
I read somewhere that: Anionic detergents are alkyl benzene sulfonates with a negatively charged end. The negative end on the ion allows it to strip oil from negative surfaces.
Can someone explain the negative surface bit? Does it matter that the surface is negative :O
Overall, I don't think I understand how the charge of the anionic/cationic heads have anything to do with their cleaning action.
Please explain! Thanks :)
Post by: winstondarmawan on July 23, 2017, 12:26:21 am
13. https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/20314573_1278708645587989_1998027790_o.png?oh=38e3a434d7181b6baec60d24e42d5f18&oe=5975528E
20. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20217105_1278709285587925_1449257879_n.png?oh=47b49a7b666c0b83837f22bedf9b96e1&oe=5975AB3B
27. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20289766_1278717925587061_773511986_n.png?oh=9985c24d14c7c4b1ace4f78626b04414&oe=597550CE
TIA
Post by: kiwiberry on July 23, 2017, 01:02:08 am
ethenylbenzene is the systematic name for styrene, not polystyrene :)
I also have a question~
I read somewhere that: Anionic detergents are alkyl benzene sulfonates with a negatively charged end. The negative end on the ion allows it to strip oil from negative surfaces.
Can someone explain the negative surface bit? Does it matter that the surface is negative :O
Overall, I don't think I understand how the charge of the anionic/cationic heads have anything to do with their cleaning action.
Please explain! Thanks :)
Not sure about anionic detergents, but cationic detergents are used in hair conditioners as the surface of hair is negatively charged :)
Post by: kiwiberry on July 23, 2017, 01:12:33 am
Hello! Would appreciate help for the following qs:
13. https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/20314573_1278708645587989_1998027790_o.png?oh=38e3a434d7181b6baec60d24e42d5f18&oe=5975528E
20. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20217105_1278709285587925_1449257879_n.png?oh=47b49a7b666c0b83837f22bedf9b96e1&oe=5975AB3B
27. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20289766_1278717925587061_773511986_n.png?oh=9985c24d14c7c4b1ace4f78626b04414&oe=597550CE
TIA
13) has been addressed here
20) Bromine has a higher ozone depleting potential than chlorine, so B is the answer. (However CFCs have contributed more to ozone depletion in the atmosphere because they are more abundant and have longer lifetimes)
27) Reading off the graph, concentration of a sample with absorbance 0.45 will have a concentration of about 0.45 micrograms/25mL
Post by: winstondarmawan on July 23, 2017, 07:58:38 am
13) has been addressed hereThank you! How about 27.b)?
20) Bromine has a higher ozone depleting potential than chlorine, so B is the answer. (However CFCs have contributed more to ozone depletion in the atmosphere because they are more abundant and have longer lifetimes)
27) Reading off the graph, concentration of a sample with absorbance 0.45 will have a concentration of about 0.45 micrograms/25mL
Post by: Shadowxo on July 23, 2017, 09:09:38 am
Thank you! How about 27.b)?Ppm is by weight, and like it says it's parts per million, eg mg per kg, micrograms per gram, etc. Easiest way to convert the answer to ppm is by making it micrograms / mL ie micrograms / g (as 1mL water = 1g).
Answering the question:
There are 0.45micrograms in the 25mL solution, so 0.45 micrograms of lead from the precipitate from the 100mL sample. This means there was 0.45 micrograms of lead in 100 mL of original water. Changing this to micrograms per mL (or ppm) just divide it by 100 for 0.0045 micrograms per mL or ppm
Hope this helps :)
Post by: phoebegresham on July 23, 2017, 11:25:38 am
In Example 2.1 in the Chemistry ATAR notes (Acidic Environment), the question 'is NaOH a good substance to use as a standard solution?' is asked, which i recently also saw in one of our topic tests, but I had no idea what the answer was! I don't think its answered in the book however, it just calculates the concentrations (also asked), but i don't think is says why or why not it is a good substance as a standard solution. I was just wondering what the answer is to that question?
Thanks heaps :)
Post by: kiwiberry on July 23, 2017, 11:54:53 am
Hey Jake!
In Example 2.1 in the Chemistry ATAR notes (Acidic Environment), the question 'is NaOH a good substance to use as a standard solution?' is asked, which i recently also saw in one of our topic tests, but I had no idea what the answer was! I don't think its answered in the book however, it just calculates the concentrations (also asked), but i don't think is says why or why not it is a good substance as a standard solution. I was just wondering what the answer is to that question?
Thanks heaps :)
NaOH is not a good standard solution - it is hygroscopic, which means that it absorbs water from the atmosphere, so it's very hard to accurate measure the mass of solid NaOH. It can also react with CO2 in the air, changing its concentration. Standard solutions need to have a known concentration, so NaOH is not appropriate as its concentration is always changing. :)
Post by: phoebegresham on July 23, 2017, 12:05:08 pm
NaOH is not a good standard solution - it is hygroscopic, which means that it absorbs water from the atmosphere, so it's very hard to accurate measure the mass of solid NaOH. It can also react with CO2 in the air, changing its concentration. Standard solutions need to have a known concentration, so NaOH is not appropriate as its concentration is always changing. :)thank you!
Post by: winstondarmawan on July 23, 2017, 02:21:49 pm
14. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20270053_1279294878862699_251626619_n.png?oh=0833834537ad91acfa67f1eec4e6a948&oe=59763A9F
The answer is D and I understand, but can someone explain why it is not C?
Shipwrecks Q:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20270053_1279294878862699_251626619_n.png?oh=0833834537ad91acfa67f1eec4e6a948&oe=59763A9F
The sample answer in the booklet was literally 2 pages and far too detailed for 6 marks. What would this Q require for the 6?
Post by: VanillaRice on July 23, 2017, 02:27:03 pm
Would appreciate help with the following Qs:AAS is typically used to measure concentrations of metal ions in solutions. The light source is a hollow cathode lamp made of the metal being analysed, and emits light of a certain frequency. Hence, C is incorrect, as white light is not used.
14. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20270053_1279294878862699_251626619_n.png?oh=0833834537ad91acfa67f1eec4e6a948&oe=59763A9F
The answer is D and I understand, but can someone explain why it is not C?
Post by: Natasha.97 on July 23, 2017, 02:32:22 pm
Would appreciate help with the following Qs:
14. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20270053_1279294878862699_251626619_n.png?oh=0833834537ad91acfa67f1eec4e6a948&oe=59763A9F
The answer is D and I understand, but can someone explain why it is not C?
Shipwrecks Q:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20270053_1279294878862699_251626619_n.png?oh=0833834537ad91acfa67f1eec4e6a948&oe=59763A9F
The sample answer in the booklet was literally 2 pages and far too detailed for 6 marks. What would this Q require for the 6?
Hi!
For AAS, the sample being shone through the sample is usually already known so we know which wavelengths are absorbed. AAS is used to determine the concentrations, so it should be measuring the intensity of light absorbed.
Hope this helps :)
Post by: bsdfjnlkasn on July 23, 2017, 05:23:34 pm
Was just wondering what were the most common covalent bond molecules that we are asked to draw in exams - sorry if that seems really random, just want to know what to expect
Thank you :D
Post by: MisterNeo on July 23, 2017, 05:36:10 pm
Hey there!
Was just wondering what were the most common covalent bond molecules that we are asked to draw in exams - sorry if that seems really random, just want to know what to expect
Thank you :D
I know that in Production of Materials, 99.99% of the time they will ask you to draw ethanol and glucose/cellulose polymerisation.
In Chemical Monitoring, they will definitely ask you to draw oxygen and ozone (you must draw coordinate covalents using an arrow).
In Acidic Enviro, they might ask you to draw ionisation of strong vs weak acid, or an ester.
Post by: kiwiberry on July 23, 2017, 06:05:51 pm
Hey there!
Was just wondering what were the most common covalent bond molecules that we are asked to draw in exams - sorry if that seems really random, just want to know what to expect
Thank you :D
Also citric acid! Make sure you remember its super long name and know how to draw it haha, that came up in my half yearly. If you do industrial, you need to know how to draw the saponification reactions as well as the structure of all the different types of detergents :)
Post by: Annie657 on July 23, 2017, 07:34:53 pm
Thankyou!
Post by: Natasha.97 on July 23, 2017, 09:26:14 pm
So at my school we do chem of art :/ And I was wondering how to explain what a hydrated ion in solution is? Like does it have to be an ionic molecule that disassociates in water or can they be covalent? My sheet says AlCl3 is covalent but I'm not sure why.
Thankyou!
Hi!
I don't do that option, but found some stuff online:
Definitions:
- Hydrated ions: Transition metals in solution that form hydrated ions as the polar water atoms surround the metal cations
- Ligands: Anions/polar molecules attached to a metal cation by coordinate bonding (one atom donates both electrons, also known as a dative covalent bond), using available empty orbitals in the metal ion
Explanation of hydrated ion:
- Water molecule is polar (2 lone pairs on O), resulting in the O side of the H2O molecule partially negative, and the H2 side partially positive
- Partially negative side of H2O molecule is attracted to the anhydrous (without water) cation in solution
Explanation of AlCl3:
From the above, I agree that it is covalent: Al is the transition metal in solution, and the Cl ions should be the ligands that attach to Al
Hope this helps! :D
Edited explanation of hydrated ion
Post by: Annie657 on July 23, 2017, 09:32:20 pm
Hi!
I don't do that option, but found some stuff online:
Definitions:
- Hydrated ions: Transition metals in solution that form hydrated ions as the polar water atoms surround the metal cations
- Ligands: Anions/polar molecules attached to a metal cation by coordinate bonding (one atom donates both electrons, also known as a dative covalent bond), using available empty orbitals in the metal ion
Explanation of hydrated ion:
- Water molecule is polar (2 lone pairs on O2), resulting in the O2 side of the H2O molecule partially negative, and the H2O side partially positive
- Partially negative side of H2O molecule is attracted to the anhydrous (without water) cation in solution
Explanation of AlCl3:
From the above, I agree that it is covalent: Al is the transition metal in solution, and the Cl ions should be the ligands that attach to Al
Hope this helps! :D
Thankyou so much for all that research! Most of it makes sense now, except isn't a covalent bond between a non metal and non metal atom? Sorry this is what I remember from junior science :/
Post by: Natasha.97 on July 23, 2017, 09:42:01 pm
Thankyou so much for all that research! Most of it makes sense now, except isn't a covalent bond between a non metal and non metal atom? Sorry this is what I remember from junior science :/
Hi Annie!
Yes, a covalent bond is usually between 2 non-metal atoms. Hopefully someone else can answer why AlCl3 is covalent, as I'm not completely sure :P
Post by: anotherworld2b on July 23, 2017, 10:23:24 pm
Post by: Daniyahasan on July 23, 2017, 10:28:40 pm
I was wondering when naming organics would you name bromo before ethy;?check this out; maybe itll help you
http://www.chemguide.co.uk/basicorg/conventions/names.html
Post by: bluecookie on July 23, 2017, 11:13:30 pm
Degree of ionisation = [H+]/[Acid] x (amount of mol)/(amount of h ions) x the whole thing by 100%. How do we find the hydrogen concentration and amount of h ions?
Post by: Bubbly_bluey on July 23, 2017, 11:45:28 pm
Hey guys, you somehow missed my question a few posts ago >.< XD (I think it got buried under the mountain of questions lol)Hey! I don't think I'll be much help but do you have the question so I can have a look. From what i understad degree of ionisation is the calulation of the hydronium ions conc/ acid conc as you expressed above. If you want to find hydronium conc and given degree ionisation and acid conc you can work that out by rearranging the formula:
Degree of ionisation = [H+]/[Acid] x (amount of mol)/(amount of h ions) x the whole thing by 100%. How do we find the hydrogen concentration and amount of h ions?
acid conc x degree ionisation = hydronium conc
By the amount of h ions do you mean the number of moles?
If so you can use concentration found above so you can multiply by given volume using : n=CxV
Of course I'm not sure if this is even what you are asking. Just let me know if i even answered your question. Sorry
Post by: MisterNeo on July 23, 2017, 11:48:50 pm
Hey guys, you somehow missed my question a few posts ago >.< XD (I think it got buried under the mountain of questions lol)
Degree of ionisation = [H+]/[Acid] x (amount of mol)/(amount of h ions) x the whole thing by 100%. How do we find the hydrogen concentration and amount of h ions?
Do you have a question for this?
I'm pretty sure this area of weak acids aren't covered in the Acidic Environment module as we stick to calculating H+ in strong acids using that 10-pH formula.
Post by: anotherworld2b on July 24, 2017, 12:28:08 am
check this out; maybe itll help you
http://www.chemguide.co.uk/basicorg/conventions/names.html
Post by: Bubbly_bluey on July 24, 2017, 01:08:41 pm
The second question can someone explain why the answer can't be B?
Thanks ;D
Post by: Bubbly_bluey on July 24, 2017, 05:53:14 pm
Hey! A half cell just refers to a beaker where either oxidation or reduction occurs. Remember that a galvanic cell is made up of 2 separate cells.Thanks I understand the second question (didn't read how there was a fresh sample was used for each). For the first question I got C as well but the answer said that it was A which is HCl. Not sure if I'm misunderstanding something or a typo.
So the electrolyte will be platinum iodide because the platinum electrode will undergo reduction at the cathode.
For the second question, B isn't correct because a new sample was used each time a reagent was added, so it doesn't really matter which order they added them. The correct answer would be that many precipitates are white. Lead iodide is a yellow precipitate and the particular test doesn't account for the presence of the iodide ion or any other anions that also make white precipitates.
Hope this helps ;D
Post by: Britany24 on July 24, 2017, 06:26:03 pm
Post by: kiwiberry on July 24, 2017, 06:27:04 pm
Thanks I understand the second question (didn't read how there was a fresh sample was used for each). For the first question I got C as well but the answer said that it was A which is HCl. Not sure if I'm misunderstanding something or a typo.
Platinum is inert - so it wouldn't be able to be reduced, it's used in galvanic cells when gases are involved. From the table of standard potentials, the oxidation potential of Mg is 2.36 V. Because the cell's potential is 3.8 V, the substance that gets reduced in the cathode half cell needs to have a reduction potential of around 1.44 V - the closest one on the data sheet is chlorine gas with 1.40 V! So HCl is the answer :)
Post by: jakesilove on July 24, 2017, 06:32:27 pm
Hey, can someone help identify the difference between anode and cathode in cells as I'm having trouble doing that, Thanks
Hey Britany! Welcome to the forums :) In any Chemical cell, there is an Anode and a Cathode. The Anode is where oxidation occurs (oxidation state goes up, ie. the substances LOSES an election). The Cathode is where reduction occurs (oxidation state goes down, ie the substances GAINS electrons). So, when you've got a chemical cell, all you have to do is identify which side oxidises, and which side reduces! Then, use REDCAT (Reduction @ Cathode) and ANOX (Anode is Oxidation) to identify which is which!
Hope that made sense :)
Edit: Also, as the Cathode gain electrons, and the anode loses electrons, current runs from Anode to Cathode :)
Post by: winstondarmawan on July 24, 2017, 07:17:28 pm
Shipwrecks Q:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20270053_1279294878862699_251626619_n.png?oh=0833834537ad91acfa67f1eec4e6a948&oe=59763A9F
The sample answer in the booklet was literally 2 pages and far too detailed for 6 marks. What would this Q require for the 6?
I think my Q got missed :(
EDIT: Sorry, wrong link! This is the right link.
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/20317181_1279305045528349_178776397_o.png?oh=762a343efa70030204d6bfa7bf0ccdbf&oe=59780E00
Post by: jakesilove on July 24, 2017, 07:27:40 pm
I think my Q got missed :(
EDIT: Sorry, wrong link! This is the right link.
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/20317181_1279305045528349_178776397_o.png?oh=762a343efa70030204d6bfa7bf0ccdbf&oe=59780E00
Hey! I don't remember too much from Shipwrecks, but basically I would be going through:
Initially, Wood was used to build ships. Look at positives (easy to find resource) and negatives (decays very quickly).
Then, ships started being covered in various metals. Honestly, I just don't remember this part of the curriculum very well, so you may have to look this up yourself. Basically, you want to discuss the importance of having some sort of sacrificial hull, that doesn't cause the nails on the ship to corrode (by setting up a galvanic cell). Um. Yeah. Hopefully someone else can help me out here!
Post by: Bubbly_bluey on July 24, 2017, 09:53:41 pm
Thanks :D
Post by: kiwiberry on July 24, 2017, 10:18:44 pm
Hey! I'm confused for part (e) in this question which is about electrolytic cells. So there is two possible choices for reduction: water or Zinc. I chose Zn because it requires less energy for it to reduce. However the answer chose water to reduce instead? Is it something to do with them being close together?
Thanks :D
In electrolytic cells, energy is inputted to drive a non-spontaneous redox reaction, so you would pick the reactions that would not occur by themselves. With Zn and water, Zn would be reduced spontaneously because it requires less energy than water, so in an electrolytic cell it would be oxidised - vice versa for water :)
Post by: Bubbly_bluey on July 24, 2017, 10:28:53 pm
In electrolytic cells, energy is inputted to drive a non-spontaneous redox reaction, so you would pick the reactions that would not occur by themselves. With Zn and water, Zn would be reduced spontaneously because it requires less energy than water, so in an electrolytic cell it would be oxidised - vice versa for water :)I'm so sorry but still confused. In a spontaneous reaction (with is a galvanic?) water is more reactive because its above Zn on the standard potentials so it will oxidise readily than Zn. But in a eletrolytic cell shouldn't Zinc be reducing because it has -0.76V compared to water -0.83V because it requires less energy?
Post by: anotherworld2b on July 25, 2017, 12:25:06 am
Post by: MisterNeo on July 25, 2017, 10:58:55 am
I'm so sorry but still confused. In a spontaneous reaction (with is a galvanic?) water is more reactive because its above Zn on the standard potentials so it will oxidise readily than Zn. But in a eletrolytic cell shouldn't Zinc be reducing because it has -0.76V compared to water -0.83V because it requires less energy?This is for electrolytic cells.
Galvanic cells preference those on the very top and bottom (most energy), whereas electrolytic cells prefer those closest to 0V because they require less energy.
(http://i.imgur.com/6vCj4pe.png)
I was wondering if anyone had any tips on how to remember the order in which to name organics?Are you referring to those fluoro-chloro naming rules, or prefix and suffix when naming things like aldehydes, ketones, etc?
I recommend this for naming different compounds.
;D
Post by: ellipse on July 25, 2017, 02:05:22 pm
Could someone please explain why the answer is C?
Post by: Bubbly_bluey on July 25, 2017, 08:45:18 pm
Thanks ;D
Post by: jakesilove on July 25, 2017, 09:40:08 pm
Hi again! When cobalt-60 is used for medicinal treatment of cancer, the gamma radiation emitted kills these cancer cells. But when cobalt-60 decays it also releases beta radiation, so is this useful in anyway for the treatment as well?
Thanks ;D
Hey! Honestly, I can't find anything about Beta radiation and cancer therapy. I would just stick to the Gamma radiation, and the point you've probably already got in your notes :)
Post by: vanessa mbogo on July 27, 2017, 10:04:20 pm
a) mercury cell
a) diaphragm cell
c) membrane cell
please help !
Mod Edit: Moved to Q&A :)
Post by: f_tan on July 27, 2017, 10:22:14 pm
1. 5.00 g of magnesium carbonate has reacted with 25.0 mL of 0.500 M HCl. The volume of CO2 produced at 0 degrees C and 100 kPa is closest to:
A) 0.142 L
B) 0.155 L
C) 0.284 L
D) 1.35 L
(http://i.imgur.com/JPfIPUN.png)
Post by: Natasha.97 on July 27, 2017, 10:37:16 pm
Hi! Could someone please explain how to answer these questions? Thanks!
1. 5.00 g of magnesium carbonate has reacted with 25.0 mL of 0.500 M HCl. The volume of CO2 produced at 0 degrees C and 100 kPa is closest to:
A) 0.142 L
B) 0.155 L
C) 0.284 L
D) 1.35 L
Hi!
1) Write out a BALANCED chemical equation (important)
2) Calculate the moles of HCl using c = n/v (make sure that volume is in litres!)
3) Calculate moles of magnesium carbonate, and determine the LIMITING reagent (i.e. is there sufficient A to react with B)
4) Calculate the moles of CO2 produced using limiting reagent
4) Refer to data sheet for molar volumes of gas (I think for this particular Q, it is 22.71L for 1 mole of gas)
5) Multiply the value calculated in step 4 with 22.71, and you should get the answer
Hope this helps! :)
Note: Not 100% sure if this is correct, can anyone else confirm?
Post by: MisterNeo on July 27, 2017, 10:55:05 pm
Hi! Could someone please explain how to answer these questions? Thanks!
1. 5.00 g of magnesium carbonate has reacted with 25.0 mL of 0.500 M HCl. The volume of CO2 produced at 0 degrees C and 100 kPa is closest to:
A) 0.142 L
B) 0.155 L
C) 0.284 L
D) 1.35 L
(http://i.imgur.com/JPfIPUN.png)
Hey! ;)
The first thing you should always do is write out the chemical equation.
This requires you to find the limiting reagent (the one that runs out first).
So you find the moles of magnesium carbonate used by dividing mass by molar mass.
5 / 84.31 = 0.0593mol
And you find the moles of the acid by multiplying the concentration by the volume.
0.5 x 0.025 = 0.0125mol
^This is the limiting reagent since it has less moles than the other reactant.
So there will be 0.00625mol of CO2 because of the 1:2 mole ratio.
To find volume of gas, multiply the moles of gas by 22.71L at 0'C
0.00625 x 22.71 = 0.142L
Thus the answer is A. ;D
For the galvanic cell, you'd refer to the reduction table for the nickel potential (-0.25 negative means it prefers oxidation over reduction).
Chlorine's potential is (+1.4 positive means it prefers reduction over oxidation).
Anode: Oxidation
Cathode: Reduction
Thus, the anode is nickel, and the cathode is chlorine gas at the inert platinum electrode.
It will flow from nickel to chlorine because it always flows from anode to cathode across the external wire.
The chlorine ions in solution do not produce a gas because it does not lose electrons, but nickel does.
The E* value is just nickel plus chlorine (remember to multiply nickel by -1 since it undergoes oxidation.
E*=0.25+1.4=1.65V (spontaneous reaction coz it's positive)
The colour change observed would be that the anode cell (nickel) becomes more green as more ions of nickel are produced.
Hope this helps :)
Post by: f_tan on July 28, 2017, 01:23:44 pm
Thank you!
(http://i.imgur.com/RjXRkNG.png)
(http://i.imgur.com/1PWZCvC.png)
Post by: Graceful on July 28, 2017, 02:11:51 pm
What is the monomer for PHB?
Post by: kiwiberry on July 28, 2017, 02:43:31 pm
Can anyone explain how you get the answer to these two questions?
Thank you!
(http://i.imgur.com/RjXRkNG.png)
(http://i.imgur.com/1PWZCvC.png)
Hello! :)
Remember that for condensation polymerisation, both monomers must have a functional group on either end. B is the only option which has monomers which fit this criteria - the first monomer has an OH group and the second one has a COOH group, so this is the answer. Ethene and chloroethene don't have a functional group and instead undergo addition polymerisation.
13) seems odd, I'm pretty sure they're all conjugate bases of weak acids, unless I'm missing something
Post by: kiwiberry on July 28, 2017, 02:47:35 pm
Hey Jake, thanks for all your help,
What is the monomer for PHB?
Hey! The monomer for PHB is 3-hydroxybutanoic acid :)
Post by: winstondarmawan on July 28, 2017, 05:10:20 pm
Do both anions and cations allow for the conduction of electricity?
Does the extent of conductivity depend entirely on the charge of the ions? Or do other factors such as molar mass play a role.
EDIT: Also any tips for practical exams (especially titration) are appreciated!
TIA
Post by: seventeenboi on July 28, 2017, 06:14:27 pm
Post by: MisterNeo on July 28, 2017, 06:22:59 pm
HELLO COULD SOMEONE PLS EXPLAIN THIS :)
The answer would likely be A.
It doesn't fully ionise that second hydrogen, so it would release less than the hydrochloric acid even though the concentrations are 1:2.
Hello! Just a few questions on conductivity of ions.
Do both anions and cations allow for the conduction of electricity?
Does the extent of conductivity depend entirely on the charge of the ions? Or do other factors such as molar mass play a role.
EDIT: Also any tips for practical exams (especially titration) are appreciated!
TIA
It's mainly the concentration of ions in solution that impact the electrical conductivity. The current is carried by both cations and anions (all charged particles) because you can't have a solution of only cations and no anions, vice versa.
For titration pracs, I'd recommend revising the roles of each equipment and the rinsing procedure.
This is a decent guide to titration methods.
Knowing how to calculate the concentration is vital because that's the whole point of the experiment. :)
Post by: J.B on July 28, 2017, 09:03:08 pm
Outline a procedure that could be used to positively identify this substance."
In this question does the fact that it's a crystalline substance make a difference to the tests you would use?
Post by: MisterNeo on July 28, 2017, 10:10:22 pm
"A white crystalline substance is found on a shelf in a chemical storeroom, however the label is unreadable. It is suspected to be either lead(II) carbonate, barium carbonate or barium chloride.
Outline a procedure that could be used to positively identify this substance."
In this question does the fact that it's a crystalline substance make a difference to the tests you would use?
Hey! :)
If it's a crystalline substance, it means that it is a solid salt not yet in contact with water.
This would be my method:
Barium chloride (the only soluble salt possibility) can be ruled out if the solid does not dissolve when placed in water. (Utilise the info given)
So you're left with either lead/barium carbonate...
Add HCl to decompose the carbonate to leave behind barium chloride or lead chloride.
Lead chloride is insoluble, barium chloride is not.
Write this into a flowchart with those (Yes?/No?) arrows and fancy boxes. :)
ps. I would avoid mentioning flame test because lead ion vapour is toxic.
Post by: elizag on July 28, 2017, 11:03:09 pm
Any guidance would be appreciated! Thank you
Post by: MisterNeo on July 28, 2017, 11:56:17 pm
Hey, I am currently preparing for my trial Chemistry exam, and was just wondering which questions are considered to be the common extended responses
Any guidance would be appreciated! Thank you
Based on the past papers I have done, the most common long responses are those on the syllabus with a high-order verb.
"Assess the potential of ethanol as an alternative fuel..."
Look for the dotpoints in the syllabus that begin with "analyse", "assess", "discuss".
I would write a good response for each of these to prepare for any long response they might throw at you. :)
Post by: phoebegresham on July 29, 2017, 09:59:26 am
Post by: J.B on July 29, 2017, 10:02:57 am
I was also wondering, if you receive a question like this
"A student added 1.71g of barium hydroxide to 50.0mL of distilled water. After stirring to dissolve all the solid, the student then added more distilled water to make the final volume 100.0mL of solution. Calculate the pH of the final solution"
And my answer is 13.3 pH. I am a bit confused with how many significant figures to answer to this, as is the 100.0 mL to 1s.f? Meaning that my answer should also be to 1s.f as this is the lowest s.f in the answer?
Post by: RuiAce on July 29, 2017, 10:14:45 am
Hey! When writing a molecular formula, the general formula for alkanes is C(n)H(n+2) and for alkenes C(n)H(2n), isn't it? When something is cyclo, does it change it to alkanes: C(n)H(2n) and alkenes: C(n)H(2n-2)? Very confused :-\ thank you!!Yeah. The reason why it goes down is (informally) due to the fact that you don't have a straight line molecule, but rather a looped one where the ends are joined.
Thanks MisterNeo,Assuming that your calculations are right, it would be to 3 s.f.
I was also wondering, if you receive a question like this
"A student added 1.71g of barium hydroxide to 50.0mL of distilled water. After stirring to dissolve all the solid, the student then added more distilled water to make the final volume 100.0mL of solution. Calculate the pH of the final solution"
And my answer is 13.3 pH. I am a bit confused with how many significant figures to answer to this, as is the 100.0 mL to 1s.f? Meaning that my answer should also be to 1s.f as this is the lowest s.f in the answer?
100.0mL is correct to 4 s.f.
This is because at the moment you see a 0 after the decimal point, you know it is significant. There's only ambiguity if all the 0's are before the decimal point.
Post by: smshs2017 on July 29, 2017, 01:01:33 pm
Thank you.
Post by: MisterNeo on July 29, 2017, 01:25:09 pm
Hello, i was past papers and i realised i need help with naming compounds with their proper IUPAC/ systematic name. i have attached an example of a quetsion. if someone could please explain to me how this works, it would be greatly appreciated!!
Thank you.
Hey! This area of chem is always very confusing, so glad you asked! ;D
Numbering priority goes to the most electronegative group
So fluorine is most electronegative, so you start counting from the left. (2)
Naming priority is alphabetical, ignoring prefixes like meth-, di-, etc.
So chloro comes before fluoro in the dictionary, thus chloro comes first.
The final answer would be 3-chloro-2-fluorobutane. ;)
Post by: phoebegresham on July 29, 2017, 01:59:30 pm
'A manufacturer claims his vinegar is 98% pure acetic acid with no additives. 25 mL of this vinegar was diluted to make 250 mL solution. 25.00mL aliquots were titres against a standardised 0.1086 M sodium hydroxide solution. The following tires were obtained during the investigation:
1. V(NaOH) (mL) = 26.25
2. V(NaOH) (mL) = 27.25
3. V(NaOH) (mL) = 27.30
4. V(NaOH) (mL) = 27.20
Calculate the concentration of the original acetic acid and evaluate the manufacturer's claims.'
Thank you!
Post by: J.B on July 29, 2017, 02:17:01 pm
"Compare TWO technologies used to detect and/or measure nuclear radiation."
Apart from describing the way the two technologies work, would the similarities and differences between say Geiger Counter and Scintillation counter just be:
Similarities: Both can measure the amounts of ionising radiation such as alpha and beta particles, and gamma radiation.
Differences: Their methods, and sensitivity are different.
And is there any way to expand on the above details?
Also I was wondering if someone could tell me there answer for this question:
"5.0g of anhydrous calcium carbonate was reacted with 150mL 0.1 mol.L -1 nitric acid. If the reaction was carried out at 25 o C and 100kPa, calculate the volume of gas that would be produced. And then calculate the pH of the final solution, once the reaction is complete."
As I don't have the answers for this question, but my answer was 0.19L and pH1.
Thank you.
Post by: J.B on July 29, 2017, 04:05:58 pm
For a polymer question when drawing polyvinylchloride and polystyrene, is it best to alternate the sides the chlorine and benzene are on due to their electronegativity forcing them to repel each other. Or just draw them on the same side (i.e bottom or top) to show that the monomer is a repeating unit?
Also can addition polymerisation be between different monomers like condensation polymerisation?
Thanks.
Post by: MisterNeo on July 29, 2017, 04:58:18 pm
I was just wondering in the question
Spoiler"Compare TWO technologies used to detect and/or measure nuclear radiation."
Apart from describing the way the two technologies work, would the similarities and differences between say Geiger Counter and Scintillation counter just be:
Similarities: Both can measure the amounts of ionising radiation such as alpha and beta particles, and gamma radiation.
Differences: Their methods, and sensitivity are different.
And is there any way to expand on the above details?
Also I was wondering if someone could tell me there answer for this question:Spoiler"5.0g of anhydrous calcium carbonate was reacted with 150mL 0.1 mol.L -1 nitric acid. If the reaction was carried out at 25 o C and 100kPa, calculate the volume of gas that would be produced. And then calculate the pH of the final solution, once the reaction is complete."
As I don't have the answers for this question, but my answer was 0.19L and pH1.
Thank you.
For the compare question, maybe (depending on the lines given) talk about how it is more sensitive or how it measures ionised particles in terms of the chemistry (argon becomes charged in Geiger counter and completes circuit).
For the calcium question, I got 0.19L of gas too, but I'm not too sure on the pH since all the acid is used up.
I'm guessing it might be pH 7?
Also, just another question,In our school, you will lose marks for not alternating the functional groups in polymers.
For a polymer question when drawing polyvinylchloride and polystyrene, is it best to alternate the sides the chlorine and benzene are on due to their electronegativity forcing them to repel each other. Or just draw them on the same side (i.e bottom or top) to show that the monomer is a repeating unit?
Also can addition polymerisation be between different monomers like condensation polymerisation?
Thanks.
Addition polymers can be more than one monomer type, such as SBR (styrene butadiene) where styrene addition polymerises with butadiene monomers.
Hope this helps ;)
Hello!! I was just wondering if I could please get some help with this question! Thanks :)For this question, you would find the average titre volumes by first eliminating any outliers and averaging the consistent results.
'A manufacturer claims his vinegar is 98% pure acetic acid with no additives. 25 mL of this vinegar was diluted to make 250 mL solution. 25.00mL aliquots were titres against a standardised 0.1086 M sodium hydroxide solution. The following tires were obtained during the investigation:
1. V(NaOH) (mL) = 26.25
2. V(NaOH) (mL) = 27.25
3. V(NaOH) (mL) = 27.30
4. V(NaOH) (mL) = 27.20
Calculate the concentration of the original acetic acid and evaluate the manufacturer's claims.'
Thank you!
So the average titre is 27.25mL.
-Write out the chemical equation.
-Find the moles of NaOH by multiplying conc. by volume.
-Use stoichiometry to find moles of acetic acid.
-Multiply the moles in 25mL of vinegar by the molar mass of acetic acid.
-Does it equal 98% pure acid? 98% pure acid is 24.5 grams of acetic acid for every 25mL of water (water is 1g/mL).
;D
Post by: winstondarmawan on July 29, 2017, 05:11:26 pm
TIA
Post by: Mounica on July 29, 2017, 07:08:41 pm
can someone pls tell me, how i would answer this question:
Polyvinyl alcohol can be reacted with acetic acid to form polyvinyl acetate.
(i) describe the reaction conditions you would use to produce polyvinyl acetate in this reaction
Post by: J.B on July 29, 2017, 07:19:18 pm
I was also just wondering in the alkaline dry cell that I studied the Cathode reaction is:
2MnO2 (s) + H2O (l) + 2e− →Mn2O3 (s) + 2OH− (aq)
But I'm just a bit confused as if the MnO2 is being reduced by gaining 2 electrons, shouldn't its oxidation state go from 4+ to 2+ rather than 4+ to 3+ as seen in this reaction?
Thanks
Post by: Shadowxo on July 29, 2017, 08:49:21 pm
Thanks MisterNeo,*apologies if I use incorrect terms or don't explain it well/right
I was also just wondering in the alkaline dry cell that I studied the Cathode reaction is:
2MnO2 (s) + H2O (l) + 2e− →Mn2O3 (s) + 2OH− (aq)
But I'm just a bit confused as if the MnO2 is being reduced by gaining 2 electrons, shouldn't its oxidation state go from 4+ to 2+ rather than 4+ to 3+ as seen in this reaction?
Thanks
If you're just trying to find the oxidation state, you know Mn2O3 is overall 0, and each of the O atoms is -2, therefore the oxidation state of the Mn is +3.
Understanding the logic of it: All the atoms are rearranged and molecules are created which have different overall charges.
An H+ atom splits from the H2O atom leaving it as an OH-, and an O2- atom splits from one of the MnO2 atoms to join it, creating another OH- atom. The Mn2O3 is left with an overall charge of +2, and the electrons join to make it +0 overall. Each of these reduces each Mn to change it's oxidation state from +4 to +3 as there's one electron for each.
Maybe you also didn't realise there are 2 MnO2 molecules?
Hope this helps :)
Post by: itssona on July 29, 2017, 08:57:09 pm
And why covalent network and covalent molecular are insoluble in water?
Thankyou :)
Post by: MisterNeo on July 29, 2017, 10:57:45 pm
Heey can anyone explain why ionic substances are soluble in water?
And why covalent network and covalent molecular are insoluble in water?
Thankyou :)
Ionic substances dissolve in water by dissociating into ions. Ionic molecules always have a positive ion and a negative ion. The hydrogens in water form dipoles with the anion, and the oxygen forms dipoles with the cation.
They essentially rip the ionic molecule apart.
However, you may realise that some ionic substances don't disassociate in water (insoluble precipitates) such as lead chloride, so be careful when explaining how ionic things dissolve.
Covalent molecules aren't all insoluble. They are only insoluble because they are either non-polar or polar.
Sugar is covalent, yet it dissolves in water. This is because sugar (glucose) has those -OH hydroxyl functional groups in its ring-like structure. Those are polar and is able to form hydrogen bonds with water because it is also polar.
Something like...methane...is a covalent molecule and is insoluble in water. This is because it has an even charge distribution and is non-polar, so it does not attract to water in any way but actually repels it.
Basically, it's polar/non-polar of covalent molecules that determine its solubility.
Covalent networks are insoluble because they are chemically bonded to each atom and require a shit-ton of energy to break them (intramolecular). So they remain solid.
Something like ionic lattices are bonded intermolecularly, and can easily be broken up by dipoles, unless an insoluble salt.
Hope this helps ;D
Post by: winstondarmawan on July 30, 2017, 06:51:26 am
Hello! Can someone please clarify buffer solutions for me, like how would you prepare one in a laboratory situation. I know the theory behind it.Just bumping my post
TIA
Post by: J.B on July 30, 2017, 09:22:48 am
Just bumping my post
Don't quote me, as I may not be right, but my understanding is that a buffer solution is formed when you mix a weak acid with the salt of its conjugate base, or a weak base with the salt of its conjugate acid.
This is a paper from James Ruse, and the question was:
"Outline how a buffer solution could be prepared from two or more of the above compounds.
Sample Answer:
2NH3 is a weak base and NH4Cl gives NH4+ which is the conjugate acid. Combining equimolar solutions would form a buffer solution that inhibits the change in pH."
Hope this helps :)
Post by: J.B on July 30, 2017, 09:25:16 am
I just have some more questions:
I was just wondering if Scintillation counters can detect both non-ionising radiation, and ionising radiation? As my school notebook, says only "non-ionising" radiation, but my textbook states ionising radiation.
And are all nuclear radiation ionising radiation?
I also just answered a question "Outline a valid procedure you would use to distinguish between acidic, basic and neutral chemicals using a pH meter or pH probe."
And in the answers it stated that all the solutions need to be dilute solutions? I was just wondering why this is?
Thank you :)
Post by: beau77bro on July 30, 2017, 10:39:45 am
Hi,
I just have some more questions:
I was just wondering if Scintillation counters can detect both non-ionising radiation, and ionising radiation? As my school notebook, says only "non-ionising" radiation, but my textbook states ionising radiation.
And are all nuclear radiation ionising radiation?
I also just answered a question "Outline a valid procedure you would use to distinguish between acidic, basic and neutral chemicals using a pH meter or pH probe."
And in the answers it stated that all the solutions need to be dilute solutions? I was just wondering why this is?
Thank you :)
the diluting is most likely because the acids and bases need to ionise and make Hydronium ions as well as hydroxide ions so that the electrodes can actually measure their concentrations - after all it measures the concentrations of OH- and H3O+
Post by: itssona on July 30, 2017, 11:01:12 am
Ionic substances dissolve in water by dissociating into ions. Ionic molecules always have a positive ion and a negative ion. The hydrogens in water form dipoles with the anion, and the oxygen forms dipoles with the cation.Ur amazing THANK YOU :)
They essentially rip the ionic molecule apart.
However, you may realise that some ionic substances don't disassociate in water (insoluble precipitates) such as lead chloride, so be careful when explaining how ionic things dissolve.
Covalent molecules aren't all insoluble. They are only insoluble because they are either non-polar or polar.
Sugar is covalent, yet it dissolves in water. This is because sugar (glucose) has those -OH hydroxyl functional groups in its ring-like structure. Those are polar and is able to form hydrogen bonds with water because it is also polar.
Something like...methane...is a covalent molecule and is insoluble in water. This is because it has an even charge distribution and is non-polar, so it does not attract to water in any way but actually repels it.
Basically, it's polar/non-polar of covalent molecules that determine its solubility.
Covalent networks are insoluble because they are chemically bonded to each atom and require a shit-ton of energy to break them (intramolecular). So they remain solid.
Something like ionic lattices are bonded intermolecularly, and can easily be broken up by dipoles, unless an insoluble salt.
Hope this helps ;D
Post by: seventeenboi on July 30, 2017, 11:14:18 am
why does the pH of identical concentrations of HCl and Acetic acid change by different amounts when diluted with the same volumes of water ??
thanks
Post by: MisterNeo on July 30, 2017, 11:55:25 am
Hi :)
why does the pH of identical concentrations of HCl and Acetic acid change by different amounts when diluted with the same volumes of water ??
thanks
It's got to do with strong/weak acids and LCP.
Strong Acid
Let's assume that we have a 1L solution of HCl with pH 1.
It's a strong acid and will ionise completely. When you add 9L of water, you decreased the concentration of H+ by tenfold, and the pH will be 2.
Weak Acid
Let's assume that we have 1L of this weak HF acid with a pH of 3.
If I were to dilute with water, the equilibrium will shift to the right to produce more H3O+ and decrease pH. However, the volume of water has also increased and has lowered the concentration. So after adding 9L of water to our pH 3 HF acid, our resulting pH will be around 3-4 because it it had produced more H3O+ during diluting.
Thus, strong/weak acids have different pH changes in dilution.
Make sense? ;D
Post by: J.B on July 30, 2017, 02:02:15 pm
Thanks.
Post by: MisterNeo on July 30, 2017, 02:25:17 pm
I was just wondering, are Total Ozone Mapping Spectrophotometers (TOMS) measuring the UV radiation from satellites like UV spectrophotometers or measuring the ozone levels directly?
Thanks.
I'm pretty sure TOMS is attached to a satellite that measures ozone according to the UV radiation emitted by Earth.
EasyChem has a pretty good guide on both UV and ozone spectrometers. ;)
Post by: J.B on July 30, 2017, 05:22:37 pm
I was just doing the 2004 HSC chemistry question "What is the purpose of the light source and the flame in an AAS?"
My understanding is that the light source contains the metal element that is the same as the metal being tested in the sample in the flame. So that the light source emits the specific light wavelength of the element to allow the element being tested in the flame to absorb this wavelength. This then allows the absorbance of this wavelength to be measured, as this is relative to the concentration in the sample.
Although the marking criteria stated that the flame excites the electrons to shift orbits.
I understand that this happens when in a flame, but I'm a bit unsure of how this relates to the sample absorbing the specific wavelength? As I thought that the flame excites the electrons to shift orbit, and a sample emits its wavelength when the electrons fall back to their orbit. But I don't understand where the absorbance of it's own wavelength in the flame occurs?
(sorry if this makes no sense)
Thanks.
Post by: jakesilove on July 30, 2017, 05:32:50 pm
HI
I was just doing the 2004 HSC chemistry question "What is the purpose of the light source and the flame in an AAS?"
My understanding is that the light source contains the metal element that is the same as the metal being tested in the sample in the flame. So that the light source emits the specific light wavelength of the element to allow the element being tested in the flame to absorb this wavelength. This then allows the absorbance of this wavelength to be measured, as this is relative to the concentration in the sample.
Although the marking criteria stated that the flame excites the electrons to shift orbits.
I understand that this happens when in a flame, but I'm a bit unsure of how this relates to the sample absorbing the specific wavelength? As I thought that the flame excites the electrons to shift orbit, and a sample emits its wavelength when the electrons fall back to their orbit. But I don't understand where the absorbance of it's own wavelength in the flame occurs?
(sorry if this makes no sense)
Thanks.
Can't say I quite understand that final paragraph, except to say that you seem to be completely correct in your understanding! The process is as follows:
A lamp using the element to be tested shines light of a specific wavelength (the wavelength that can be emitted/absorbed by the element to be tested) into a flame, which contains the aspirated sample. The light is absorbed by the relevant element, causing electrons to jump up in orbital/energy. When the electrons jump back down to their original orbital, they release light at the same wavelength as they absorb. This light is picked up by a receiver!
Basically, I think you are completely correct in your understand, and that the answer to the question that you looked at just confused you because they used different wording. Let me know if that makes sense :)
Post by: J.B on July 30, 2017, 05:47:12 pm
Can't say I quite understand that final paragraph, except to say that you seem to be completely correct in your understanding! The process is as follows:
A lamp using the element to be tested shines light of a specific wavelength (the wavelength that can be emitted/absorbed by the element to be tested) into a flame, which contains the aspirated sample. The light is absorbed by the relevant element, causing electrons to jump up in orbital/energy. When the electrons jump back down to their original orbital, they release light at the same wavelength as they absorb. This light is picked up by a receiver!
Basically, I think you are completely correct in your understand, and that the answer to the question that you looked at just confused you because they used different wording. Let me know if that makes sense :)
Thank you,
This makes sense, I just didn't understand that the flame atomises the element, while the light source is the cause for the electrons to jump shells.
Thanks.
Post by: ekhan_01 on July 30, 2017, 06:40:47 pm
Sorry can you answer this question, the answer is D
I am not too sure what to do as there are two values // not the ordinary pH question
Thanks :D
Post by: Shadowxo on July 30, 2017, 07:01:14 pm
Hi!For this, you'd calculate the number of mols of each (n=cV for each), use this to find the number of H+ ions for each (in this case it's 1:1 ie 1 mol of HCl has 1 mol of H+ ions). Then find out the concentration using c=n/V (n being number of H+ total - add together their individual number of H+ ions, and V being total volume). Then use this to find the pH.
Sorry can you answer this question, the answer is D
I am not too sure what to do as there are two values // not the ordinary pH question
Thanks :D
I can solve this step by step if you need but hopefully this pushes you in the right direction! :)
Note: In this case they both have the same concentration of 0.1M and each of the molecules results in one H+ ion, so the concentration would be 0.1M regardless of each of their volumes. However, this is usually not the case and you'd have to use the method outlined above
Post by: ekhan_01 on July 30, 2017, 07:22:07 pm
For this, you'd calculate the number of mols of each (n=cV for each), use this to find the number of H+ ions for each (in this case it's 1:1 ie 1 mol of HCl has 1 mol of H+ ions). Then find out the concentration using c=n/V (n being number of H+ total - add together their individual number of H+ ions, and V being total volume). Then use this to find the pH.
I can solve this step by step if you need but hopefully this pushes you in the right direction! :)
Note: In this case they both have the same concentration of 0.1M and each of the molecules results in one H+ ion, so the concentration would be 0.1M regardless of each of their volumes. However, this is usually not the case and you'd have to use the method outlined above
Ahh okay I get you but sorry can you please solve it step by step ")
Post by: Shadowxo on July 30, 2017, 07:45:22 pm
n(HNO3)= c*V=0.1*0.020 = 0.002 mol
n(H+ total) = 1*n(HCl) + 1*n(HNO3) = 0.003 mol (if it were eg H2SO4 then you'd make it 2*n(H2SO4))
c(H+)=n/V = 0.003/0.030 = 0.1
pH = -log[H+] = -log(0.1) = 1
They probably made a mistake in the answers, should be A
Post by: Mymy409 on July 30, 2017, 08:14:03 pm
Post by: ekhan_01 on July 30, 2017, 08:59:36 pm
n(HCl) = c*V = 0.1*0.010 = 0.001 mol
n(HNO3)= c*V=0.1*0.020 = 0.002 mol
n(H+ total) = 1*n(HCl) + 1*n(HNO3) = 0.003 mol (if it were eg H2SO4 then you'd make it 2*n(H2SO4))
c(H+)=n/V = 0.003/0.030 = 0.1
pH = -log[H+] = -log(0.1) = 1
They probably made a mistake in the answers, should be A
Thank you!
Is there another way to go about the question, something to do with the limiting reagent?? < (would this be the preferred method if the ratio is NOT 1:1)
Post by: Shadowxo on July 30, 2017, 09:29:15 pm
Thank you!In this case it's just two acids so they don't react. You can just calculate the number of H+ ions by adding their individual amounts.
Is there another way to go about the question, something to do with the limiting reagent?? < (would this be the preferred method if the ratio is NOT 1:1)
If it were an acid and a base (which it often is) you have to calculate how much reacts and what's left over. You then use what's left over to calculate the final pH.
Post by: vanessa mbogo on July 31, 2017, 12:27:54 am
Post by: MisterNeo on July 31, 2017, 12:43:26 am
What is The typical voltage and temperature of mercury cell, membrane cell and diaphgram cellHey! ;D I answered your question a while ago. I just moved your post to Q&A.
Hey! ;D
From my textbook it says that:
-Mercury cell: 3-4V
-Diaphragm cell: 3.5-5V
-Membrane cell: It doesn't say but it could be the same as diaphragm cell since "membrane cell is a diaphragm cell with an improved diaphragm". But I'd ask your teacher to clarify. :)
It doesn't seem to mention the temperatures so I would try and use some other properties.
Post by: Bubbly_bluey on July 31, 2017, 03:28:56 pm
Thanks ;D
Post by: seventeenboi on July 31, 2017, 04:21:33 pm
in atmospheric ozone concentration graphs, the general trend is a decrease over the decades, however, there are also fluctuations.. what is the reason for these fluctuations?? and i also read somewhere about seasonal fluctuation or something
could someone explain this???
thanks :D
Post by: MisterNeo on July 31, 2017, 11:01:30 pm
Hi! What are some of the sources of errors in a galvanic cell prac? Could I also use these errors with electrolytic cells as well?
Thanks ;D
In galvanic cell pracs, make sure that the electrodes are clean and not wet from a different prac. You would also make sure that both electrodes and the salt bridge are properly immersed in the solutions without them touching (salt bridge cannot touch electrodes coz it stops circuit).
You may be asked to calculate the E* value and account for any differences in actual and theoretical value (resistance in wires causes lower E*)
Electrolytic pracs would be similar but without the salt bridge. Concentration of the ions should be of the right amount so that the desired products are formed.
That's all I can think of ;D
hihi :(
in atmospheric ozone concentration graphs, the general trend is a decrease over the decades, however, there are also fluctuations.. what is the reason for these fluctuations?? and i also read somewhere about seasonal fluctuation or something
could someone explain this???
thanks :D
Yeah! So the ozone concentration does decrease over time, but there are those sudden jumps in the curve. These are likely to be from protocols enacted to phase out the use of certain chemicals like CFCs, which made industries switch to HCFCs as they seemed less damaging than CFCs. However, they were also bad for ozone, hence there is a fluctuation in ozone concentration. Then came along halons and stuff, which were also bad. So the fluctuations were probably from certain bans on CFCs. I think seasonal fluctuations may have something to do with the climate, like summer has more UV rays? :)
Post by: anotherworld2b on August 01, 2017, 12:53:54 am
Post by: MisterNeo on August 01, 2017, 01:57:58 am
I was wondering if anyone had any tips on how to identify which intermolecular forces are present in hydrocarbons
The general rule is that all carbon-hydrogen only molecules have only dispersion forces, whereas anything with -OH have hydrogen bonding and dipoles because hydroxyl groups are polar. :)
Post by: limtou on August 01, 2017, 05:45:33 pm
Post by: MisterNeo on August 01, 2017, 09:23:31 pm
Hello! Somehow I managed to get an answer not in the options given? Please help me out, thanks :)
Did you get 29.13mL?
I think the question is incorrect since they multiplied the mass by density instead of dividing by density.
I'm guessing that their "correct answer" would have been C but that's not the case.
Which paper is this from? ???
Post by: limtou on August 02, 2017, 12:03:04 am
Did you get 29.13mL?
I think the question is incorrect since they multiplied the mass by density instead of dividing by density.
I'm guessing that their "correct answer" would have been C but that's not the case.
Which paper is this from? ???
Oh yay that's what I got :) Was confused for a second.
It's from a recent Ruse paper
Post by: winstondarmawan on August 02, 2017, 08:40:41 am
EDIT: Also for experimental design to test the rate of corrosion of iron and steel, would it be more valid to leave the nails in water exposed to air throughout the experiment or initially aerate the water and cap with oil? TIA
Post by: anotherworld2b on August 02, 2017, 08:33:52 pm
The general rule is that all carbon-hydrogen only molecules have only dispersion forces, whereas anything with -OH have hydrogen bonding and dipoles because hydroxyl groups are polar. :)
Post by: Bubbly_bluey on August 03, 2017, 11:21:17 am
Thanks ;D
Post by: MisterNeo on August 03, 2017, 01:15:46 pm
hey guys! remember the experiment with the can soda opening to let thecarbon dioxide out and we have to measure how much is release? So why is it not possible for ALL the carbon dioxide to be released using this method? It is because of Le Chatelier's Principle?
Thanks ;D
It depends on the method you use.
If you just leave the soda open in room temperature, it will establish an "equilibrium with the air" until concentration of CO2 is equal in both the soda and the air.
If you were to heat the soda with a Bunsen, it will release more gas because dissolving gas is exothermic, and the rapid movement of water molecules kick the gas molecules out.
So you can't release all of the gas unless you boil it. :P
Post by: phoebegresham on August 03, 2017, 04:25:30 pm
Post by: MisterNeo on August 03, 2017, 04:39:27 pm
hey!! could you please explain the formation of cellulose from glucose?! is it condensation polymerisation? it seems to say that in the atar notes book however on the formula sheet on this forum it says that it does not give off a water molecule? also what is the deal with alpha and beta glucoses, do we need to know them? thank you so much !!!
Hey! :)
Cellulose is a condensation polymer as it gives off water when formed.
The difference between alpha and beta glucose is the polymer they form and which way the -OH points.
Alpha glucose polymerises to form starch, whereas beta glucose polymerises to form cellulose.
They are both isomers. ;D
2013 CSSA Trial paper
Question: A water sample was found to have a calcium chloride concentration of 1.5 x 10-3
What is the concentration of chloride ions in this sample expressed in ppm??
Not sure how to do the conversion to PPM
I assume that is 1.5 x 10-3 mol/L?
If so, you can deduce that there are 3 x 10-3 because of the 1:2 mole ratio of calcium and chloride.
Then multiply the moles of chloride by its molar mass to find the mass of chloride ions.
Convert the mass into milligrams.
And that is your parts per million because mg/L is equivalent to ppm.
Hope this helps ;D
Post by: Rainbowhippocampus on August 03, 2017, 05:28:27 pm
Just wondering if you have some tips on trying to learn, make notes and self teach the whole course in a week? I know it sounds stupid but we have only just started our option and have skimmed over the dot points. Any advice would be greatly appreciated for this. I also have no notes as the workbook we are provided with only has worksheets with little to no theory for revision. I have purchased your atar notes book and the excel book as well in an attempt to improve, but I am a visual learner more than anything. These are helping but I struggle with the content and remembering it. Please suggest anything that could help me out with this, other than my last resort of recording myself and listening to it. Thank you for any help.
Post by: Rainbowhippocampus on August 03, 2017, 05:42:07 pm
Post by: elizag on August 03, 2017, 06:01:12 pm
What is the final pH of a soluation obtained by mixing 25.0mL of 0.120mol/L HNO3 with 40mL of 0.056 mol/L KOH?
A) 2.2
B) 2.6
C) 3.0
D) 3.4
The correct answer is A.
I can't seem to recall the process of how to figure these questions out, any help would be appriciated! :)
Post by: supersmartkid on August 03, 2017, 06:09:28 pm
SO2 and HCl are both gases at room temp and pressure and both are soluble in water. Aqueous solutions of SO2 and HCl were prepared and the pH of each solution was measured.
Solution of SO2
Volume of Gas at 25 degrees and 100 kPa (L) = 2.48
Volume of solution= 1.00
pH= 1.43
Solution of HCl
Volume of Gas at 25 degrees and 100 kPa (L) = 0.49
Volume of solution= 2.00
pH= 2.0
a) Calculate the concentration of HCl
b) Write an ionic equation to represent the ionisation of the acid formed when SO2 gas is formed in water.
c) Contrast the concentration and the acid strength of the solution
Post by: winstondarmawan on August 03, 2017, 06:13:14 pm
TIA
EDIT: For a question like:
Describe the principle of atomic absorption spectroscopy and its application in
environmental monitoring. Include a diagram in your answer.
Would describe the principle be asking for the theory of absorption spectrum or the process itself? Or both? Thanks.
Post by: Rainbowhippocampus on August 03, 2017, 06:16:00 pm
Hey! ;)
Which option are you doing?
I do Industrial and I manage to memorise most things by writing lines every day.
Drawing diagrams of processes in the dotpoints can help remember info visually. :)
Hey the option I'm doing is forensic chemistry. I would do something like that but I don't have enough time. That's the main issue I'm having is I need to do a massive amount of study to learn it and it could impact on my other subjects because of it. Thanks
Post by: bsdfjnlkasn on August 03, 2017, 06:17:39 pm
Was just wondering how you would justify the different conditions in which esterification is carried out.
Post by: winstondarmawan on August 03, 2017, 06:31:05 pm
Hey there!
Was just wondering how you would justify the different conditions in which esterification is carried out.
Here you are looking at safety and maximising yield.
The condensing column (not sure if thats what its called lol) is used to prevent waste of gaseous reactants, by turning them back into liquids and hence they fall back into the reactant chamber.
Concentrated sulfuric acid is acts as a catalyst. (Also a dehydrating agent, which by LCP also increases production of esters.
Boiling chips to spread heat evenly, maximising collisions of particles.
Water bath/electronic heating plate to prevent combustion of volatile reactants.
Hope this helps. :)
Post by: blasonduo on August 03, 2017, 07:53:55 pm
Hey the option I'm doing is forensic chemistry. I would do something like that but I don't have enough time. That's the main issue I'm having is I need to do a massive amount of study to learn it and it could impact on my other subjects because of it. Thanks
Hey, I'm doing Forensics too!
My main question is how far are you into the topic?
If it isn't too far, the first 2 subtopics, are really just common sense when you look at it
"Why is contamination at a crime scene a bad thing" - Well it would skew the results and could wrongly convict someone!
it's not much off the core, as its then distinguishing alkanoic acids, hydrocarbons, alkanols. Or acidic basic or neutral salts.
It's not until subtopic 3, where it starts to get BIG!. So if you are past there, I would be happy to answer them for you!
Post by: supdood101 on August 03, 2017, 08:04:01 pm
A solution was made by mixing 75.00 mL of 0.120 mol/L hydrochloric acid with 25.00 mL of 0.200 mol/L sodium hydroxide. what is the pH of the solution?
thank you xx
Post by: Natasha.97 on August 03, 2017, 08:27:51 pm
What is the purpose of a monochromator in AAS?
TIA
EDIT: For a question like:
Describe the principle of atomic absorption spectroscopy and its application in
environmental monitoring. Include a diagram in your answer.
Would describe the principle be asking for the theory of absorption spectrum or the process itself? Or both? Thanks.
Hi!
1) The monochromator splits light to be able to differentiate different wavelengths
2) My interpretation is both the theory and the process
Hope this helps :)
Hi can you please help me with this question?
A solution was made by mixing 75.00 mL of 0.120 mol/L hydrochloric acid with 25.00 mL of 0.200 mol/L sodium hydroxide. what is the pH of the solution?
thank you xx
Here you go :)
(http://i.imgur.com/M8orcDy.png)
Post by: winstondarmawan on August 03, 2017, 08:49:39 pm
Hello! Does anyone have a Shipwrecks chemical equation cheat sheet they wish to share?
EDIT: Also for experimental design to test the rate of corrosion of iron and steel, would it be more valid to leave the nails in water exposed to air throughout the experiment or initially aerate the water and cap with oil? TIA
Bumping my old post, with a few more questions:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20614653_1289240094534844_726534848_n.png?oh=86a5489b0e6e7dcac465cc5aaa54c45d&oe=59853C31
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20614243_1289243467867840_2102123953_n.png?oh=ab87acd3f18aa0a1375791ff45a8d16f&oe=5984693D
Shipwrecks:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20614718_1289249957867191_1685056291_n.png?oh=868cc278b32576eae68aaa1f33fbb489&oe=59852592 Part ii
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20614288_1289250227867164_1611102850_n.png?oh=4176c79252bc8099ae3879c76d52219b&oe=59846FFE Part ii
TIA!
Post by: Natasha.97 on August 03, 2017, 09:39:02 pm
Bumping my old post, with a few more questions:Here you go :)
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20614243_1289243467867840_2102123953_n.png?oh=ab87acd3f18aa0a1375791ff45a8d16f&oe=5984693D
TIA!
(http://i.imgur.com/ABc2rjy.png)
Post by: chelseam on August 03, 2017, 09:48:27 pm
Post by: winstondarmawan on August 03, 2017, 10:12:24 pm
Hey! Could someone please explain the idea of isomers to me? Even when I try to draw them out if the question gives the formula, I have no idea how to tell if different structures are the same isomer or not! Thank you :)Count the number of each element in the molecule. If they are the same across 2 molecules, then they are isomers.
Anyways, another question:
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/20622970_1289303244528529_252745868_o.png?oh=2f0014b840d867573ddf2dc7367ce925&oe=59852752
I thought esterifaction resulted in only 2 liquids? I'm confused.
Post by: kiwiberry on August 03, 2017, 10:21:36 pm
Count the number of each element in the molecule. If they are the same across 2 molecules, then they are isomers.Because esterification is an equilibrium reaction, there will be some reactants left over. So ethanol and butanoic acid will be in the reflux mixture too :)
Anyways, another question:
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/20622970_1289303244528529_252745868_o.png?oh=2f0014b840d867573ddf2dc7367ce925&oe=59852752
I thought esterifaction resulted in only 2 liquids? I'm confused.
Post by: winstondarmawan on August 03, 2017, 10:31:35 pm
Because esterification is an equilibrium reaction, there will be some reactants left over. So ethanol and butanoic acid will be in the reflux mixture too :)Thank you! So what would the answer be?
Post by: chelseam on August 03, 2017, 10:33:39 pm
Count the number of each element in the molecule. If they are the same across 2 molecules, then they are isomers.Thank you :) How can you tell if two structures are actually the same isomer or not though?
Post by: kiwiberry on August 03, 2017, 10:40:37 pm
Thank you! So what would the answer be?With distillation, the mixture is heated and the component with the lowest bp will evaporate and be collected first, followed by the one with the next highest bp and so on. So the liquids will be collected from lowest to highest bp - ethanol, water, ethyl butanoate, butanoic acid - the ester is 3!
Post by: kiwiberry on August 03, 2017, 10:56:10 pm
Thank you :) How can you tell if two structures are actually the same isomer or not though?If you can give them the same systematic name then they're the same isomer. Like this:
(http://community.boredofstudies.org/attachments/17/chemistry/27986d1365941923-regarding-drawing-structural-isomers-1-metal-question-screen-shot-2013-04-14-10.15.22-pm.jpg) is the same molecule as this:(http://alevelchem.com/img/structures/1-chloropropane.gif) just flipped! They're both 1-chloropropane.
An isomer of this would be 2-chloropropane: (http://ibchem.com/IB/ibfiles/organic/org_img/structures/200/2-chloropropane.gif)
Does that make sense? ^-^
Post by: chelseam on August 03, 2017, 11:18:23 pm
Does that make sense? ^-^Yes!! Thank you so so much!! ;D
Post by: winstondarmawan on August 04, 2017, 01:53:09 pm
What is the conjugate acid of H2SO4?
Can someone please tell me that it is a misprint and meant to be conjugate base? I didn't know H2SO4 could have a conjugate acid...
Post by: Natasha.97 on August 04, 2017, 02:00:42 pm
I got an MC question in my trials that was:
What is the conjugate acid of H2SO4?
Can someone please tell me that it is a misprint and meant to be conjugate base? I didn't know H2SO4 could have a conjugate acid...
Hi! imo it probably is a misprint
Post by: MisterNeo on August 04, 2017, 03:39:27 pm
I got an MC question in my trials that was:
What is the conjugate acid of H2SO4?
Can someone please tell me that it is a misprint and meant to be conjugate base? I didn't know H2SO4 could have a conjugate acid...
Yeah, it's a misprint.
They either mean conjugate acid of HSO4-, or the conjugate base of H2SO4.
A strong acid can't act as a base. :P
Post by: winstondarmawan on August 04, 2017, 08:29:20 pm
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20624270_1290314487760738_1325675218_n.jpg?oh=1a078c3cce34e549affedbf93cc9e819&oe=5986AAAD
Post by: kiwiberry on August 04, 2017, 09:18:44 pm
What is the IUPAC name of the following CFC?Is that a carbon in the top right?
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20624270_1290314487760738_1325675218_n.jpg?oh=1a078c3cce34e549affedbf93cc9e819&oe=5986AAAD
Post by: Aaron12038488 on August 04, 2017, 09:33:44 pm
can i just plug in the numbers without the -?
Post by: winstondarmawan on August 04, 2017, 09:36:02 pm
Is that a carbon in the top right?No, sorry that's meant to be a Chlorine.
Post by: Natasha.97 on August 04, 2017, 09:41:20 pm
so for the molar heat equation:
can i just plug in the numbers without the -?
Hi!
Yes, you can:
Q is representative of the enthalpy change (chemical energy of products - chemical energy of reactants)
- Heat of combustion reactions involve the release of heat
- Products store less chemical energy than the reactants
- Enthalpy change is negative, therefore there is a minus sign in the front
Hope this helps :)
Post by: MisterNeo on August 04, 2017, 09:45:44 pm
No, sorry that's meant to be a Chlorine.
It would be 2-bromo-2-chloro-1,1,1-trifluoroethane. :D
You find the most electronegative element there and give it numbering priorities, then list alphabetically.
Post by: Aaron12038488 on August 04, 2017, 10:14:30 pm
Post by: Natasha.97 on August 05, 2017, 09:51:56 am
i have a prac test coming up. how should i remember all the colours of the preciptates.Hi!
I've attached my notes for reference :)
Anions
(http://i.imgur.com/mas8gYg.png)
Cations
(http://i.imgur.com/F9VpiC1.png)
Hope this helps ;D
EDIT: For Step 3 in cation testing, the ion that precipitates out is Ba2+, not SO42-
Post by: Aaron12038488 on August 05, 2017, 07:51:11 pm
delta h = mc delta t
OR
delta h = -mc delta t
so for my prac test coming up, i will be answering calculation questions. Which formula would i use? thx.
Does it have to do with endo and exothermic reactions?
so if the temperature increases does this mean its going to have the - sign.
Post by: MisterNeo on August 05, 2017, 08:01:58 pm
so in my textbook theres equations for calculating molar heat:
delta h = mc delta t
OR
delta h = -mc delta t
so for my prac test coming up, i will be answering calculation questions. Which formula would i use? thx.
Does it have to do with endo and exothermic reactions?
so if the temperature increases does this mean its going to have the - sign.
I would always use it with the negative since that's the actual formula.
Exothermic reactions have a negative value, whereas endothermic have a positive value. :)
Post by: Aaron12038488 on August 05, 2017, 08:24:36 pm
Post by: anotherworld2b on August 06, 2017, 12:36:37 am
Post by: MisterNeo on August 06, 2017, 02:10:20 am
I am not quite sure how to do this
This reaction is actually very similar to the reaction of sodium in water to produce NaOH.
2HOH + 2Na
2NaOH + H2
2C2H5OH + 2Na
2C2H5ONa + H2
You see, ethanol is just water with one of its hydrogens being an ethyl group, so it undergoes similar redox.
Oxidation
Reduction
Net Ionic
Hope this helps ;D
like for a calculation question, how do i do know if they are asking for an endothermic reaction or exothermic?Do you have an example question? The calculation questions with -mCΔT always involve exothermic reaction of a fuel, and you calculate variable of the equation.
Post by: anotherworld2b on August 06, 2017, 09:47:56 am
Could I get some help with this question please?
This reaction is actually very similar to the reaction of sodium in water to produce NaOH.
2HOH + 2Na
2NaOH + H2
2C2H5OH + 2Na
2C2H5ONa + H2
You see, ethanol is just water with one of its hydrogens being an ethyl group, so it undergoes similar redox.
Oxidation
Reduction
Net Ionic
Hope this helps ;DDo you have an example question? The calculation questions with -mCΔT always involve exothermic reaction of a fuel, and you calculate variable of the equation.
Post by: Aaron12038488 on August 06, 2017, 12:47:01 pm
how much energy will be required to raise the temperature of 1L of water in a kettle from 17 degrees ->100 degrees.
The answer is using the equation without the negative.
but later on the textbook.
When 1 g of NaOH is dissolved in 100 g in water, in an insulated cup, the temperature of the water rises from 20 degress --> 22.66 degrees. Calculate the heat of solution for sodium hydroxide.
For this question^^, its using the negative sign.
Post by: MisterNeo on August 06, 2017, 01:35:46 pm
heres an example question mister neo.
how much energy will be required to raise the temperature of 1L of water in a kettle from 17 degrees ->100 degrees.
The answer is using the equation without the negative.
but later on the textbook.
When 1 g of NaOH is dissolved in 100 g in water, in an insulated cup, the temperature of the water rises from 20 degress --> 22.66 degrees. Calculate the heat of solution for sodium hydroxide.
For this question^^, its using the negative sign.
When it asks for "how much energy", it asks for the amount of joules needed, which is a positive value because it requires an absolute value.
Kinda like me giving someone $5, instead of giving -$5.
Heat of solution refers to the change in enthalpy when something dissolves. This will either be exothermic or endothermic so you must use the negative sign for NaOH to represent the heat released. :)
Post by: anotherworld2b on August 06, 2017, 03:55:56 pm
1-butanol
2-butanol
2-methyl-propan-2ol
i'm not sure what the fourth isomer is
Thank you for your help
Could I get some help with this question please?
Post by: MisterNeo on August 06, 2017, 04:06:23 pm
I found three isomers so far
1-butanol
2-butanol
2-methyl-propan-2ol
i'm not sure what the fourth isomer is
2-methylpropan-1-ol
(https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcSFEhN2XkWlHOhStWEIKdWg18BOVtJRU8IX7tP9F8xD7AASp-9wq3bP6jZY)
Post by: Mathew587 on August 06, 2017, 11:16:59 pm
Anyone know how to this q- http://imgur.com/a/T2SuY
the answer is in bold.
ty :)
Post by: MisterNeo on August 06, 2017, 11:27:00 pm
Hey guys,
Anyone know how to this q- http://imgur.com/a/T2SuY
the answer is in bold.
ty :)
Hey! ;D
Firstly, write out the equation.
You need to find the moles of both HCl and NaOH (conc. times volume), and identify the limiting reagent.
n(HCl)= 0.0025mol
n(NaOH)= 0.00318mol
Since HCl is the limiting reagent (1:1 mole ratio), you subtract it from NaOH to find excess NaOH.
n(NaOH)= 0.00068mol
Now you divide these remaining moles by the total volume (0.055L) for the concentration.
Then use the pH formula with this conc. and subtract it from 14.
pH= 12.1
:)
Post by: Mathew587 on August 06, 2017, 11:36:36 pm
I didn't know we had to convert the moles into conc. before using the ph formula lolol. Saved me ^-^
Post by: Aaron12038488 on August 07, 2017, 08:23:44 pm
Post by: Natasha.97 on August 07, 2017, 08:25:01 pm
is a milky precipitate same as a cloudy ppt?
Hi! I think it is :)
Post by: Jenny_trn on August 07, 2017, 10:36:18 pm
Which of the following substances is best analysed by atomic absorption spectroscopy (AAS)?
(A) Calcium
(B) Iodine
(C) Nitrogen
(D) Silicon
Thanks!
Post by: MisterNeo on August 07, 2017, 10:47:44 pm
Hi, what would be the answer to the following question and why?
Which of the following substances is best analysed by atomic absorption spectroscopy (AAS)?
(A) Calcium
(B) Iodine
(C) Nitrogen
(D) Silicon
Thanks!
Hey! :)
AAS is used to detect concentrations of metal ions.
Iodine, nitrogen, and silicon are not metals.
Calcium is a metal, so the answer is A.
Post by: Jenny_trn on August 08, 2017, 02:00:02 pm
Solutions containing copper ions were analysed by AAS. A standard solution of 10ppm copper had an AAS absorbance of 0.400. A second solution of unknown concentration was found to have an absorbance of 0.500.
100mL of this second solution was reacted with excess sodium carbonate solution. The precipitate was then dried and weighed.
What mass of precipitate was formed?
(A) 1.25 x 10^-3] g
(B) 2.45 x 10^-3 g
(C) 1.54 g
(D) 2.43 g
Thanks!
Post by: kiwiberry on August 08, 2017, 02:38:20 pm
Hi ! I'm not sure how to do this;
Solutions containing copper ions were analysed by AAS. A standard solution of 10ppm copper had an AAS absorbance of 0.400. A second solution of unknown concentration was found to have an absorbance of 0.500.
100mL of this second solution was reacted with excess sodium carbonate solution. The precipitate was then dried and weighed.
What mass of precipitate was formed?
(A) 1.25 x 10^-3] g
(B) 2.45 x 10^-3 g
(C) 1.54 g
(D) 2.43 g
Thanks!
Hey! :)
If a solution with concentration 10 ppm has absorbance 0.4, a solution with absorbance 0.5 will have concentration \(\frac{10}{4} \times 5 = 12.5 ppm = 0.0125g/L\). Dividing by the molar mass of copper, this concentration equals 1.96.. x 10-4 mol/L. Now, writing out the reaction between copper ions and sodium carbonate:
Post by: Jenny_trn on August 08, 2017, 03:11:16 pm
ty :D
Post by: Natasha.97 on August 08, 2017, 03:44:17 pm
Hi! currently stuck on part (b) of this question
ty :D
Hi!
1) Addition of excess HNO3 ensures that all CO32- ions have been taken out of the solution
(CO32- + 2H+ → CO2 + H2O)
2) Barium is usually used to precipitate sulphate ions to become BaSO4, which is white. Barium chloride is soluble, so none of the precipitate formed would be barium chloride.
(Ba2+ + SO42- → BaSO4)
3) Silver chloride is used to precipitate chloride ions to become AgCl, which is also white. Silver sulphate (Ag2SO4) is insoluble, so if silver was added first, the white precipitate may contain samples of both substances (silver chloride and silver sulphate), rendering the sample invalid.
(Ag+ + Cl- → AgCl)
Hope this helps :)
Edit: Net ionic equation for first step (had an equation involving sodium nitrate for some reason!)
Post by: beau77bro on August 08, 2017, 10:46:32 pm
What is an Ozone measuring instrument? why did it replace TOMS
Post by: bsdfjnlkasn on August 09, 2017, 07:00:20 pm
I was wondering if someone could please help me find the concentration of chloride ions in the original undiluted seawater for the following method? The average titre was found to be 12mL
1. Dilute estuary water by pipetting a 20 mL sample into a 100 mL volumetric flask and making it up to the mark with distilled water.
2. Pipette a 10 mL aliquot of diluted estuary water into a conical flask and add about 50 mL distilled water and 1 mL of chromate indicator. The solution should be pale yellow, at this stage.
3. Titrate the sample with 0.1 mol L−1 silver nitrate solution. The silver chloride that forms will be a white precipitate. The endpoint of the titration is identified as the first appearance of a red-brown colour of silver chromate. It is a good idea to perform a rough titre, so you know what the end point should look like. Your teacher has put colour photos on your lab tray to help.
4. Repeat the titration with further aliquots of diluted estuary water until concordant results (titres agreeing within 0.1 mL) are obtained.
THANK YOU :D
Post by: bsdfjnlkasn on August 09, 2017, 07:04:17 pm
why is the ground based spectrometer for monitoring O3 concentrations limted by pointing directly overhead? the books all say since it is constantly changing but what does that mean?
What is an Ozone measuring instrument? why did it replace TOMS
Hey there,
I believe for that outcome we're only required to know the method of measurement and a brief description of how it works. Not so much the limitations of the different technologies/progression of their use. I don't have these in my notes and after comparison haven't needed to know the specifics for TOMS and Ground-based Dobson U.V. Spectroscopy. Would love to hear other people's opinions though! :)
Post by: Natasha.97 on August 09, 2017, 07:27:17 pm
Hey there!
I was wondering if someone could please help me find the concentration of chloride ions in the original undiluted seawater for the following method? The average titre was found to be 12mL
1. Dilute estuary water by pipetting a 20 mL sample into a 100 mL volumetric flask and making it up to the mark with distilled water.
2. Pipette a 10 mL aliquot of diluted estuary water into a conical flask and add about 50 mL distilled water and 1 mL of chromate indicator. The solution should be pale yellow, at this stage.
3. Titrate the sample with 0.1 mol L−1 silver nitrate solution. The silver chloride that forms will be a white precipitate. The endpoint of the titration is identified as the first appearance of a red-brown colour of silver chromate. It is a good idea to perform a rough titre, so you know what the end point should look like. Your teacher has put colour photos on your lab tray to help.
4. Repeat the titration with further aliquots of diluted estuary water until concordant results (titres agreeing within 0.1 mL) are obtained.
THANK YOU :D
Hi!
Not 100% sure if it's correct, attached is my working:
(http://i.imgur.com/BMwjeXy.png)
Hope this helps! :)
Post by: winstondarmawan on August 10, 2017, 11:44:52 am
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20707856_1295797373879116_1462211558_n.jpg?oh=31a935ef9d78e511705d0f372d8e9beb&oe=598DE853
TIA!
Post by: MisterNeo on August 10, 2017, 12:00:08 pm
Hello! Can someone try part b) and tell me what they got? My answer was 6.12g/100mL and not sure if correct.
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20707856_1295797373879116_1462211558_n.jpg?oh=31a935ef9d78e511705d0f372d8e9beb&oe=598DE853
TIA!
I also got 6.12g/100mL...
Was it marked wrong or something? ???
Post by: winstondarmawan on August 10, 2017, 12:18:16 pm
I also got 6.12g/100mL...Nope! Just redoing the exam. :)
Was it marked wrong or something? ???
Post by: anotherworld2b on August 11, 2017, 01:35:26 am
Post by: Natasha.97 on August 11, 2017, 08:45:21 am
I tried to do these two questions but I got them wrong and I'm not sure why
Hi :)
Correct me if I'm wrong, but I don't think that the Core contains any naming of ketones, only for alkenes, alkanes, alkanols, alkanoic acids, and esters (Unless this part is in one of the Options)
Edit:
With that being said, the first one should be correct (It's apparently called Pinacolone).
Post by: anotherworld2b on August 11, 2017, 09:22:08 pm
Hi :)
Correct me if I'm wrong, but I don't think that the Core contains any naming of ketones, only for alkenes, alkanes, alkanols, alkanoic acids, and esters (Unless this part is in one of the Options)
Edit:
With that being said, the first one should be correct (It's apparently called Pinacolone).
Post by: anotherworld2b on August 11, 2017, 09:23:52 pm
I was wondering what would be some key observable differences between alkanes, alkenes, alkynes and alcohols?
Post by: Natasha.97 on August 11, 2017, 09:41:21 pm
I was also wondering I have a prac on Wednesday that involves identifying three unknown substances.
I was wondering what would be some key observable differences between alkanes, alkenes, alkynes and alcohols?
Hi!
The only one I know of is the difference between Alkanes and Alkenes
Bromine water test:
-Alkanes = Solution still remains orange (requires time and UV light to undergo substitution reactions)
Alkenes = Solution becomes clear (Addition reaction)
Hope this helps :)
Post by: anotherworld2b on August 12, 2017, 10:40:21 am
Hi!
The only one I know of is the difference between Alkanes and Alkenes
Bromine water test:
-Alkanes = Solution still remains orange (requires time and UV light to undergo substitution reactions)
Alkenes = Solution becomes clear (Addition reaction)
Hope this helps :)
Post by: anotherworld2b on August 12, 2017, 10:44:53 am
Post by: VanillaRice on August 12, 2017, 10:58:32 am
I was wondering if anyone had tips on how to tell which organic substance has a higher melting/boiling point than the other. I heard that intermolecular forces and the length of the carbon chain plays a part in the melting/boiling point of a organic?Melting and boiling points are indeed influenced by a molecule's 1) shape and 2) the types of intermolecular bonding.
A molecule's shape will influence its ability to 'pack' close together, which will provide less opportunity to form intermolecular bonds. For example, butane would have a higher boiling point than 2-methylpropane, since the branched methyl group will prevent closer packing (unlike the straight chain of butane).
The stronger and greater (in number) the intermolecular bonding, the higher the boiling/melting point (since it will take more energy to break these intermolecular bonds). In particular, you should take note of any hydrogen bonding (esp. OH and NH-containing groups) and dipole-dipole interactions (e.g. C-Cl, C=O). You've also mentioned the length of the carbon chain - the longer the chain, the more opportunity there is for molecules to interact via dispersion forces (i.e. more intermolecular bonding).
Hope this helps :)
Post by: anotherworld2b on August 13, 2017, 12:47:34 am
Melting and boiling points are indeed influenced by a molecule's 1) shape and 2) the types of intermolecular bonding.
A molecule's shape will influence its ability to 'pack' close together, which will provide less opportunity to form intermolecular bonds. For example, butane would have a higher boiling point than 2-methylpropane, since the branched methyl group will prevent closer packing (unlike the straight chain of butane).
The stronger and greater (in number) the intermolecular bonding, the higher the boiling/melting point (since it will take more energy to break these intermolecular bonds). In particular, you should take note of any hydrogen bonding (esp. OH and NH-containing groups) and dipole-dipole interactions (e.g. C-Cl, C=O). You've also mentioned the length of the carbon chain - the longer the chain, the more opportunity there is for molecules to interact via dispersion forces (i.e. more intermolecular bonding).
Hope this helps :)
Post by: anotherworld2b on August 13, 2017, 12:49:05 am
Post by: Mathew587 on August 13, 2017, 01:04:08 pm
Post by: anotherworld2b on August 13, 2017, 01:54:12 pm
eg,
CH2=Ch2 (g) + Br2(g) -> CH3CH2CH2CH3 (?)
vs
CH2=CH2(g) + H2O(g) -> CH3CH2OH (?)
Can you give an example?
Post by: jakesilove on August 14, 2017, 02:40:57 pm
I got everything correct except for the state
eg,
CH2=Ch2 (g) + Br2(g) -> CH3CH2CH2CH3 (?)
vs
CH2=CH2(g) + H2O(g) -> CH3CH2OH (?)
By my understanding, there is no easy way to predict the state. It's just sort of 'you know it' or 'you don't'. I can't imagine you'd need to be able to predict this.
Post by: anotherworld2b on August 14, 2017, 04:05:11 pm
We have a prac where we have to identify 3 unknown compounds ( could be alkanes, esters, alcohols and ect)
I was wondering what would be some key observable changes that help identify which compound it is.
- So far I know that if you use the bromine test
Rapid discoloration of the added bromine water (orange to clear) confirms the presence of a suspected double bond. (identifies a alkene)
Whereas if it takes longer to change (it identifies a alkane)
- How would you know if a primary alcohol only oxidised to an aldehyde purely through observation? What would be some key changes?
I was also wondering how you could tell if a primary alcohol oxidized to a carboxylic acid through observations
I was also wondering what compounds a successful reaction with sodium could identify? You can test for hydrogen using a pop test I think?
By my understanding, there is no easy way to predict the state. It's just sort of 'you know it' or 'you don't'. I can't imagine you'd need to be able to predict this.
Post by: hansolo9 on August 15, 2017, 07:41:39 am
what are the limitations of the acid-base theories for laviosier davy arrhenius and B/L?
Post by: Natasha.97 on August 15, 2017, 08:20:59 am
Hey guys :)
what are the limitations of the acid-base theories for laviosier davy arrhenius and B/L?
Hi! :)
Lavoisier (Acids contained O2)
• Metal oxides tended to be basic
• Some oxides do not show any acidic/basic character, eg CO
• Some acids do not contain oxygen such as HCl
Davy (Acids contained H2)
• Many compounds containing hydrogen are not acidic (eg methane)
• Sometimes nitrogen isn't produced when acids react w/ metal, e.g when HNO3 reacts w/ metals, NO2 is produced
Arrhenius (Acids ionised in water to produce H+ ions, Bases ionise in water to produce OH- ions)
• No dissociated hydroxide ions involved in metallic oxides and carbonates
• His model required the formation of ions, yet solid carbonates can quite often neutralise acids
• He did not expand on the role of the solvent, instead focussing only on the role of the solute in acids and bases
• Why certain solutions of salts are basic or acidic. For example sodium chloride is neutral, zinc chloride is acidic
As far as I can tell, Brönsted-Lowry has no limitations (current accepted theory)
Hope this helps! ;D
Post by: MisterNeo on August 15, 2017, 02:19:24 pm
Does adding a NaOH solution shift the equilibrium to produce more hydronium (more NaOH needed to neutralise), or does the NaOH react with molecular acetic acid?
(I chose C, others chose A)
Thanks heaps :)
Post by: hansolo9 on August 15, 2017, 08:28:59 pm
I just did my Chem Trials and need clarification on this question.
Does adding a NaOH solution shift the equilibrium to produce more hydronium (more NaOH needed to neutralise), or does the NaOH react with molecular acetic acid?
(I chose C, others chose A)
Thanks heaps :)
i'm pretty sure it does shift the equilibrium, and NaOH can only react with ions since it disassociates into two ions itself. :)
someone correct me if i'm wrong
Post by: Shadowxo on August 15, 2017, 08:51:54 pm
I just did my Chem Trials and need clarification on this question.I believe NaOH can react with molecular acetic acid. A weak acid reacts with a base regardless of whether it fully reacts with water. So, it doesn't matter that acetic acid is a weak base.
Does adding a NaOH solution shift the equilibrium to produce more hydronium (more NaOH needed to neutralise), or does the NaOH react with molecular acetic acid?
(I chose C, others chose A)
Thanks heaps :)
I'm a bit rusty so someone else feel free to correct me
It's good that you're thinking about it critically though :) It's always good to think of other possibilities and why they could / couldn't work
Post by: VanillaRice on August 16, 2017, 10:08:21 am
I see now :DI don't think you can apply a general rule for observable changes when interconverting functional groups (e.g. a primary alcohol to an aldehyde), since the solution may, for example, remain colourless.
We have a prac where we have to identify 3 unknown compounds ( could be alkanes, esters, alcohols and ect)
I was wondering what would be some key observable changes that help identify which compound it is.
- So far I know that if you use the bromine test
Rapid discoloration of the added bromine water (orange to clear) confirms the presence of a suspected double bond. (identifies a alkene)
Whereas if it takes longer to change (it identifies a alkane)
- How would you know if a primary alcohol only oxidised to an aldehyde purely through observation? What would be some key changes?
I was also wondering how you could tell if a primary alcohol oxidized to a carboxylic acid through observations
I was also wondering what compounds a successful reaction with sodium could identify? You can test for hydrogen using a pop test I think?
Are you allowed to perform further tests on the compounds?
For example, I've used the Benedict's reagent test for aldehydes, base-promoted hydrolysis phenolphthalein test for esters, and the dissolve + test pH test for carboxylic acids to test for different functional groups. You could test for both aldehydes and carboxylic acids to see which has formed from the oxidation of a primary alcohol (may be a combination).
Post by: smshs2017 on August 18, 2017, 10:34:34 am
Thanks.
Post by: Natasha.97 on August 18, 2017, 12:00:51 pm
Hey guys, can someone please help me with all the equations, including word equations, for the cyclohexane/cyclohexene + bromine water prac that you need to know.
Thanks.
Hi :)
Cyclohexane + Bromine Water
- C6H12(aq) + Br2(aq) → C6H11Br(aq) + HBr(aq)
- Cyclohexane + Bromine → 1-bromohexane + Hydrogen bromide
- Observation: No change in colour (orange)
- Substitution reaction (one H is replaced with one Br)
- Requires UV light and time (slow reaction)
Cyclohexene + Bromine Water
- C6H10(aq) + Br2(aq) → C6H10Br2(aq)
- Cyclohexene + Bromine → 1,2-dibromohexane
- Observation: Orange to colourless
- Addition reaction (Br2 is added across the double bond)
- Occurs quickly under normal light
Hope this helps ;D
Post by: jakesilove on August 19, 2017, 05:13:42 pm
Hi :)
Cyclohexane + Bromine Water
- C6H12(aq) + Br2(aq) → C6H11Br(aq) + HBr(aq)
- Cyclohexane + Bromine → 1-bromohexane + Hydrogen bromide
- Observation: No change in colour (orange)
- Substitution reaction (one H is replaced with one Br)
- Requires UV light and time (slow reaction)
Cyclohexene + Bromine Water
- C6H10(aq) + Br2(aq) → C6H10Br2(aq)
- Cyclohexene + Bromine → 1,2-dibromohexane
- Observation: Orange to colourless
- Addition reaction (Br2 is added across the double bond)
- Occurs quickly under normal light
Hope this helps ;D
Such a great, succinct explanation. Love it!
Post by: smshs2017 on August 19, 2017, 05:18:13 pm
Hi :)thank you so much!!!!!!!
Cyclohexane + Bromine Water
- C6H12(aq) + Br2(aq) → C6H11Br(aq) + HBr(aq)
- Cyclohexane + Bromine → 1-bromohexane + Hydrogen bromide
- Observation: No change in colour (orange)
- Substitution reaction (one H is replaced with one Br)
- Requires UV light and time (slow reaction)
Cyclohexene + Bromine Water
- C6H10(aq) + Br2(aq) → C6H10Br2(aq)
- Cyclohexene + Bromine → 1,2-dibromohexane
- Observation: Orange to colourless
- Addition reaction (Br2 is added across the double bond)
- Occurs quickly under normal light
Hope this helps ;D
Post by: Piza on August 20, 2017, 06:27:14 pm
I just did my Chem Trials and need clarification on this question.
Does adding a NaOH solution shift the equilibrium to produce more hydronium (more NaOH needed to neutralise), or does the NaOH react with molecular acetic acid?
(I chose C, others chose A)
Thanks heaps :)
Hey! This is just my answer but feel free to correct me if I am wrong.
As we know, the ionisation of acetic acid is an equilibrium reaction. Therefore, the addition of NaOH will neutralise the hydronium ions, thereby causing equilibrium to shift to the right. This will continue to do so until all of the acetic acid has ionised, and so the answer is A (10mL).
Post by: BatBec on August 20, 2017, 08:53:35 pm
I'm having a bit of trouble with understanding half-equations for compounds. The question below is from an assignment and I can't figure out how to do it.
Describe, using half equations, what happens in the electrolysis of aqueous solutions of sodium sulfate and magnesium nitrate.
Thanks in advance for your assistance :)
Post by: peterpiper on August 20, 2017, 08:59:30 pm
Hey everyone,
I'm having a bit of trouble with understanding half-equations for compounds. The question below is from an assignment and I can't figure out how to do it.
Describe, using half equations, what happens in the electrolysis of aqueous solutions of sodium sulfate and magnesium nitrate.
Thanks in advance for your assistance :)
Isn't it just going to be water reacting with water? Because water is both the strongest reductant and oxidant than the magnesium/sodium ions. I don't know what HSC chemistry is like but with VCE chem we have a databook and what I do is I list them out like:
O2(g) + 4H+ +4e- --> <-- 2H2O(l) +1.23V
2H2O(l) + 2e- ---> <--- H2(g) + 2OH-(aq) -0.83V
Mg2+(aq) + 2e- --><--- Mg(s) -2.37V
Na+(aq) + e- ---> <--- Na(s) -2.71V
Then I highlight the ions/solids that are in the electrolysis setup, and the ones on the RHS you want the bottom-most; while for the ones on the LHS you want the uppermost. In this case it happens to be both just water as the strongest reductant/oxidant, meaning that if this were to undergo electrolysis then it won't be the actual Mg/Na ions undergoing reduction/oxidation reactions, but the water molecules themselves, as we know that the ions are in aqueous solution. As for the sulfate and nitrate ions, they're spectator ions, so they don't participate in the reaction.
Post by: BatBec on August 20, 2017, 09:13:53 pm
Isn't it just going to be water reacting with water? Because water is both the strongest reductant and oxidant than the magnesium/sodium ions. I don't know what HSC chemistry is like but with VCE chem we have a databook and what I do is I list them out like:
O2(g) + 4H+ +4e- --> <-- 2H2O(l) +1.23V
2H2O(l) + 2e- ---> <--- H2(g) + 2OH-(aq) -0.83V
Mg2+(aq) + 2e- --><--- Mg(s) -2.37V
Na+(aq) + e- ---> <--- Na(s) -2.71V
Then I highlight the ions/solids that are in the electrolysis setup, and the ones on the RHS you want the bottom-most; while for the ones on the LHS you want the uppermost. In this case it happens to be both just water as the strongest reductant/oxidant, meaning that if this were to undergo electrolysis then it won't be the actual Mg/Na ions undergoing reduction/oxidation reactions, but the water molecules themselves, as we know that the ions are in aqueous solution. As for the sulfate and nitrate ions, they're spectator ions, so they don't participate in the reaction.
Ohh thank you, that makes so much more sense :) I was over complicating it and confused myself. :)
Post by: winstondarmawan on August 22, 2017, 02:43:31 pm
Just a question about the formation of glucose and it's structural diagram.
Is there a simple way to approach and remember it or should it just be rote learned?
EDIT: Also the same question for approaching the batteries topic.
Post by: Natasha.97 on August 22, 2017, 03:07:27 pm
Hey!
Just a question about the formation of glucose and it's structural diagram.
Is there a simple way to approach and remember it or should it just be rote learned?
EDIT: Also the same question for approaching the batteries topic.
Hey!
I personally wrote it out multiple times on a blank piece of paper over and over again until I could remember it
Batteries
- Equations: I did two flashcards for each cell, one for the anode equation, and one for the cathode equation. On the other side, I would have which cell it belonged to. If I couldn't recite it from memory, I would write it out a few more times and try again.
- Cells: I drew up a table with subheadings from the dot-point, and have the information listed out as bullet points.
Tip: Teaching someone else is the best way to memorise and retain information imo!
Hope this helps ;D
Post by: kiwiberry on August 22, 2017, 03:21:06 pm
Hey!
Just a question about the formation of glucose and it's structural diagram.
Is there a simple way to approach and remember it or should it just be rote learned?
EDIT: Also the same question for approaching the batteries topic.
This might not make sense but this is the order I draw glucose in to remember it:
- draw the 5 C's and 1 O in a hexagon
- The carbon next to the O will have a CH2OH group connected on top and a H connected on the bottom
- The carbon directly under this carbon will have a OH connected on top and a H on the bottom
From there on, the OH's and H's on top alternate on each carbon.
(http://1.bp.blogspot.com/_CXSzcT4pxdk/SZMMftCVVCI/AAAAAAAAAR0/s4HNkZI1m30/s320/glucose1.JPG)
Also, remember to connect the C's to the O in the OH not the H!! Hope this helps :)
Post by: winstondarmawan on August 22, 2017, 04:08:02 pm
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/21034875_1385184934928928_389610171_o.jpg?oh=12d23e429498dc57bc697f819d77f5da&oe=599DADAE
And also, what are the benefits of using cyclo alkanes and alkenes in the bromine water test?
TIA
Post by: kiwiberry on August 22, 2017, 04:33:14 pm
Hello! Can someone please explain Q2.
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/21034875_1385184934928928_389610171_o.jpg?oh=12d23e429498dc57bc697f819d77f5da&oe=599DADAE
And also, what are the benefits of using cyclo alkanes and alkenes in the bromine water test?
TIA
For condensation polymerisation, both monomers have to have a functional group on either end - the only two here are ethylene glycol and oxalic acid.
The cyclo's are used because they liquid at room temperature and pressure, whereas ethene/ethane are gases. They're also colourless, cheap and easy to store :)
Post by: MisterNeo on August 22, 2017, 05:07:32 pm
Hello! Can someone please explain Q2.
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/21034875_1385184934928928_389610171_o.jpg?oh=12d23e429498dc57bc697f819d77f5da&oe=599DADAE
And also, what are the benefits of using cyclo alkanes and alkenes in the bromine water test?
TIA
(http://i.imgur.com/QMsO017.png)
This 5-bond carbon is really irritating me ::)
Post by: Shadowxo on August 22, 2017, 06:48:57 pm
(http://i.imgur.com/QMsO017.png)Didn't you know carbon likes having 10 electrons in its outer shell?
This 5-bond carbon is really irritating me ::)
(*Disclaimer: it's a mistake in the diagram, it's only supposed to have 8 electrons / 4 pairs in outer shell)
You know your joke is bad when the disclaimer is longer than the joke
Post by: scienceislife on August 23, 2017, 04:46:03 pm
Post by: MisterNeo on August 23, 2017, 05:19:42 pm
I have no idea how to deal with dilution and titration problems so any help with this question would be super helpful. Thank you!
Hi! ;D
This question asks for moles, so it's just stoichiometry.
-Calculate moles of K2Cr2O7.
0.0144L x 0.03 = 0.000432mol
-Calculate moles of Cr2O72-.
1:1 mole ratio, so 0.000432mol
-Calculate moles of Sn2+.
1:3 mole ratio, so 0.001296mol
-Moles of tin equals moles of tin chloride, so the answer is 1.3 x 10-3.
Post by: Mymy409 on August 24, 2017, 06:05:31 pm
Post by: Natasha.97 on August 24, 2017, 06:12:39 pm
Is it essential to know how to draw the formation of cellulose? Like showing how the monomers join together to form cellulose?
Hi!
Although the syllabus doesn't specifically say you need to know how to draw it, the dot-point is "Describe the structure of cellulose...", which is essentially an understanding of how the polymer looks like. I would memorise it :)
Post by: Mymy409 on August 24, 2017, 06:31:58 pm
Hi!
Although the syllabus doesn't specifically say you need to know how to draw it, the dot-point is "Describe the structure of cellulose...", which is essentially an understanding of how the polymer looks like. I would memorise it :)
Thanks for your reply! Also, if given a diagram of cellulose with three monomers and brackets showing the continuing chain, how many water molecules should we say are released? Are the brackets with bonds reaching out counted as links between monomers or do we only count the ones in inside the brackets - so 2 water molecules?
Post by: Natasha.97 on August 24, 2017, 06:36:40 pm
Quote
Thanks for your reply! Also, if given a diagram of cellulose with three monomers and brackets showing the continuing chain, how many water molecules should we say are released? Are the brackets with bonds reaching out counted as links between monomers or do we only count the ones in inside the brackets - so 2 water molecules?
Hi! In this case I'd say that two water molecules were released :)
Post by: hanaacdr on August 25, 2017, 09:44:03 pm
I was wondering what the fermentation of glucose diagram looks like?
thanks
Post by: Natasha.97 on August 25, 2017, 10:07:57 pm
Hi
I was wondering what the fermentation of glucose diagram looks like?
thanks
Hi!
Definition: Breakdown of glucose by yeast to form ethanol and carbon dioxide.
Equation: C6H12O6(aq) → C2H5OH(l)+2CO2(g)
The cotton wool prevents any oxygen from entering the conical flask, thus causing the yeast to respire and produce ethanol.
(http://i.imgur.com/kiJJFsd.png)
The mixture produced contains ~15% alcohol (yeast cannot tolerate higher concentrations). Distillation is required to produce high concentrations of ethanol.
In case you need the diagram for distillation:
(http://i.imgur.com/bSyxncF.png)
Hope this helps!
Post by: ilovemycat on August 26, 2017, 11:59:35 am
A mixture of oil, salt, sawdust and water are to be separated into their individual
components, each component being collected separately. What sequence of steps should
be followed to do this separation and collection?
(A) decantation filtration separating funnel evaporation
(B) filtration separating funnel distillation
(C) fractional distillation filtration decantation
(D) filtration separating funnel evaporation
Post by: MisterNeo on August 26, 2017, 01:17:54 pm
Hi everyone. I was doing a james ruse past paper for chemistry and i got this question wrong, and im not sure why.can someone help me please?
A mixture of oil, salt, sawdust and water are to be separated into their individual
components, each component being collected separately. What sequence of steps should
be followed to do this separation and collection?
(A) decantation filtration separating funnel evaporation
(B) filtration separating funnel distillation
(C) fractional distillation filtration decantation
(D) filtration separating funnel evaporation
What was the answer? ???
I think it would be B.
It can't be A or D because evaporation doesn't allow collection of water like the question says.
Post by: jakesilove on August 26, 2017, 03:09:45 pm
Hi everyone. I was doing a james ruse past paper for chemistry and i got this question wrong, and im not sure why.can someone help me please?
A mixture of oil, salt, sawdust and water are to be separated into their individual
components, each component being collected separately. What sequence of steps should
be followed to do this separation and collection?
(A) decantation filtration separating funnel evaporation
(B) filtration separating funnel distillation
(C) fractional distillation filtration decantation
(D) filtration separating funnel evaporation
I suspect it would have to be B, right? The only use of decanting would be to separate the water from the oil. However, if you've already used fractional distillation, you would have separated these components! So, I don't think C makes sense
Post by: bsdfjnlkasn on August 27, 2017, 10:07:29 am
I was just wondering what the best way to prepare for the HSC Chemistry exam was? Which schools have the 'best' papers and until which year of the HSC should we go back to?
Should we be doing anything extra? Apart from learning and possibly rewriting summaries?
Any advice would be super appreciated!!
Post by: Natasha.97 on August 27, 2017, 10:34:27 am
Hey there!
I was just wondering what the best way to prepare for the HSC Chemistry exam was? Which schools have the 'best' papers and until which year of the HSC should we go back to?
Should we be doing anything extra? Apart from learning and possibly rewriting summaries?
Any advice would be super appreciated!!
Hi!
Doing past papers are the best way to prepare imo. That way, you get more exposure to the style of questions that may be asked. In addition to checking your work with the marking criteria, also take a look at the Notes from the Marking Centre to see the areas of weaknesses that particular cohort had in terms of answering questions.
In terms of HSC/trial papers, I'd prioritise doing HSC papers, and if I run out of HSC, I'd do trial papers from selective schools e.g. JRAH/Sydney Girls + Boys etc. Not too sure about how many years to go back in terms of papers though.
In terms of studying, try and incorporate different techniques to retain information e.g. mind-maps, flashcards, notes around the house (formulas/equations), recording yourself etc. The key thing is to maintain a balance (
) between studying and relaxing: make a study timetable from the moment you wake up to the moment you go to bed (more info on that here)Hope this helps!
Post by: justwannawish on August 27, 2017, 11:23:26 am
1) If the question doesn't specify, how do you know the states of the products and reactants? Are there any other general hitns like how everything in solution is aqueous?
2) What are the most important (I know it's a subjective) concepts for prelim? Our teacher has been focusing most of our revision on molar calculations, limiting reagents, concentration, enthalpy but are there any other ideas that usually come up in prelim yearlies?
Thank you!
Post by: Natasha.97 on August 27, 2017, 11:33:56 am
A couple of prelim qs:
1) If the question doesn't specify, how do you know the states of the products and reactants? Are there any other general hinTs like how everything in solution is aqueous?
2) What are the most important (I know it's a subjective) concepts for prelim? Our teacher has been focusing most of our revision on molar calculations, limiting reagents, concentration, enthalpy but are there any other ideas that usually come up in prelim yearlies?
Thank you!
Hi!
Not too sure about the first question, sorry!
- Separating a variety of mixtures
- Intermolecular(dispersion/dipole-dipole/hydrogen) and Intramolecular(ionic/covalent/metallic) forces
- Development of the Periodic Table
- SOLUBILITY OF SALTS
- Diamond/Graphite comparison
Hope this helps!
Post by: winstondarmawan on August 27, 2017, 03:17:23 pm
So this is not a traditional question, but for my trial exams I received 3/4 for a question (Shipwrecks Option Topic). It was something along the lines of:
Account for the presence of certain bacteria in deep sea wrecks, and using relevant equations explain their effect on the corrosion of such wrecks (4)
The reason why I lost a mark was because the teacher's marking criteria said that 1 mark was allocated to the anode reaction and 1 mark to the cathode reaction.
However, I had written the overall reaction (net reaction) and had received 1 mark only. I queried with my class teacher and she said that my overall reaction was counted as the cathode reaction (???) and she was adamant about me not receiving the mark.
My main gripe with this is because the question did not specifiy that the REDOX HALF EQUATIONS were required, but just RELEVANT EQUATIONS which was what my response had.
Is this worth pursuing with the head teacher? Would appreciate any opinions/advice on this matter, even if you don't/haven't done Shipwrecks or have had any similar experiences.
Thanks in advance.
Post by: Natasha.97 on August 27, 2017, 03:21:50 pm
Hello!
So this is not a traditional question, but for my trial exams I received 3/4 for a question (Shipwrecks Option Topic). It was something along the lines of:
Account for the presence of certain bacteria in deep sea wrecks, and using relevant equations explain their effect on the corrosion of such wrecks (4)
The reason why I lost a mark was because the teacher's marking criteria said that 1 mark was allocated to the anode reaction and 1 mark to the cathode reaction.
However, I had written the overall reaction (net reaction) and had received 1 mark only. I queried with my class teacher and she said that my overall reaction was counted as the cathode reaction (???) and she was adamant about me not receiving the mark.
My main gripe with this is because the question did not specify that the REDOX HALF EQUATIONS were required, but just RELEVANT EQUATIONS which was what my response had.
Is this worth pursuing with the head teacher? Would appreciate any opinions/advice on this matter, even if you don't/haven't done Shipwrecks or have had any similar experiences.
Thanks in advance.
hmm.
My interpretation of the question is that you need equations (emphasis on the plural), so I think you lost a mark as you had put one equation instead of two
Post by: winstondarmawan on August 27, 2017, 03:24:25 pm
hmm.I had another 2 equations for the formation of rusticles.
My interpretation of the question is that you need equations (emphasis on the plural), so I think you lost a mark as you had put one equation instead of two
EDIT: The criteria was as follows:
1 - Explain the presence of anaerobic bacteria
2 - Cathode reaction
3 - Anode reaction
4 - Explains formation of rusticles (with relevant equations)
Wouldn't the overall reaction count as both anode and cathode?
Post by: Natasha.97 on August 27, 2017, 03:26:47 pm
I had another 2 equations for the formation of rusticles.
In that case, I'm not so sure. sorry!
Edit: Normally for anode/cathode reactions, the electrons are shown. Net Ionic Equations don't show them (or at least the ones I've seen), so maybe there needs to be clarification of which species is accepting/giving electrons
Post by: Mathew587 on August 27, 2017, 09:07:38 pm
Post by: A TART on August 27, 2017, 09:28:40 pm
Can someone pls explain why for q 12 2015 hsc the answer is a and not d?
The number of neutrons (not protons) have changed from Pu239 to Pu241 (If it was d, it would no longer be Pu, it'll be some other element)
Post by: MisterNeo on August 27, 2017, 10:23:31 pm
Can someone pls explain why for q 12 2015 hsc the answer is a and not d?
Hey ;D
The top number represents the mass number, which is the total protons plus neutrons. The bottom number represents atomic number, which is the total protons.
Since the plutonium-239 gains X to become plutonium-241, we know that the protons haven't changed because it's still plutonium. So it must have gained neutrons to increase its mass number by 2.
Now plutonium-241 is unstable (too many neutrons), and decays into americium-241 by changing a neutron to a proton and an electron.
Think of a neutron as a proton combined with an electron (they cancel each other). To become a proton, it kicks out the electron to become positive. (This isn't scientifically correct but it's how I remember it)
Hence, the answer is A because X is a neutron and Y is an electron.
Hope this helps :D
Post by: Mathew587 on August 28, 2017, 05:23:00 pm
Post by: justwannawish on August 29, 2017, 06:18:14 pm
Hi!
Not too sure about the first question, sorry!
- Separating a variety of mixtures
- Intermolecular(dispersion/dipole-dipole/hydrogen) and Intramolecular(ionic/covalent/metallic) forces
- Development of the Periodic Table
- SOLUBILITY OF SALTS
- Diamond/Graphite comparison
Hope this helps!
Thank you for your help!
Is there a generally standard way to set out moles questions? With or without limiting reagents
Mod edit: Formatting
Post by: Natasha.97 on August 29, 2017, 07:26:53 pm
Thank you for your help!
Is there a generally standard way to set out moles questions? With or without limiting reagents
Mod edit: Formatting
Hi!
Usually, the question gives values that allow the moles to be determined for one particular species. I would set it out with the general formula (m/M), then the molar ratio between the known substance and the unknown. Refer to the questions below:
If 15.65g of Calcium Carbonate was completely decomposed by heat, what volume of carbon dioxide gas would be produced if measured at SLC?
Spoiler
-Equation: CaCO3(s) → CO2(g) + CaO(s)
- Mass (g) = 15.65g
- Molar Mass of CaCO3 = 40.08 + 12.01+ 48 = 100.09
- Moles (n) = m/M = 15.65/100.09 = 0.1564 mol
- n(CaCO3) : n(CO2) = 1 : 1, so n(CO2) = 0.1564 mol
∴ Volume of CO2 = 0.1564 × 24.8 = 3.88L at SLC
- Mass (g) = 15.65g
- Molar Mass of CaCO3 = 40.08 + 12.01+ 48 = 100.09
- Moles (n) = m/M = 15.65/100.09 = 0.1564 mol
- n(CaCO3) : n(CO2) = 1 : 1, so n(CO2) = 0.1564 mol
∴ Volume of CO2 = 0.1564 × 24.8 = 3.88L at SLC
What volume of hydrogen gas at SLC would be produced if 10.00g of lithium metal was reacted with sulphuric acid?
Spoiler
Equation: 2Li(s) + H2SO4(s) →H2(g) + Li2SO4(aq)
- Mass (g) = 10.00g
- Molar Mass of Li = 6.941g/mol
- Moles (n) = m/M = 10.00/6.941 = 1.441 mol
- n(Li) : n(H2) = 2 : 1, so n(H2) = 1.441/2 = 0.7204
∴ Volume of H2 = 0.1564 × 24.8 = 3.88L at SLC
- Mass (g) = 10.00g
- Molar Mass of Li = 6.941g/mol
- Moles (n) = m/M = 10.00/6.941 = 1.441 mol
- n(Li) : n(H2) = 2 : 1, so n(H2) = 1.441/2 = 0.7204
∴ Volume of H2 = 0.1564 × 24.8 = 3.88L at SLC
Hydrated sodium carbonate is represented by the formula Na2CO3 ∙ xH2O, where x is an integer. When 100.00 g of this compound was heated strongly to drive off the water, the anhydrous compound left behind had a mass of 37.04g. Calculate the number of water molecules, x, in one mole of hydrated sodium carbonate.
Spoiler
Na2CO3 ∙ xH2O → Na2CO3(s) + xH2O(g)
Water
- Mass (g) = 100.00 – 37.04 = 62.96g
- M (H2O) = (2 × 1.01) + 16.00 = 18.02g/mol
- Moles (n) = m/M = 62.96/18.02 = 3.495 mol
Sodium Carbonate
- Mass (g) = 37.04g
- Molar Mass (M) = (2 × 22.99) + 12.01 + (3 × 16.00) = 105.99g/mol
- Moles (n) = m/M = 37.04/105.99 = 0.349 mol
Ratio
n(H2O) : n(Na2CO3) = 3.495 : 0.349 = 10 : 1
x = 10
∴ 10 moles of water in one mole of hydrated sodium carbonate
Water
- Mass (g) = 100.00 – 37.04 = 62.96g
- M (H2O) = (2 × 1.01) + 16.00 = 18.02g/mol
- Moles (n) = m/M = 62.96/18.02 = 3.495 mol
Sodium Carbonate
- Mass (g) = 37.04g
- Molar Mass (M) = (2 × 22.99) + 12.01 + (3 × 16.00) = 105.99g/mol
- Moles (n) = m/M = 37.04/105.99 = 0.349 mol
Ratio
n(H2O) : n(Na2CO3) = 3.495 : 0.349 = 10 : 1
x = 10
∴ 10 moles of water in one mole of hydrated sodium carbonate
Hope this helps
Post by: caprese_ on August 30, 2017, 11:51:08 am
answer is iii and iv. Why are there no hydrogen bonds between the molecules?
Ty v much
Post by: MisterNeo on August 30, 2017, 09:46:03 pm
Hi,
answer is iii and iv. Why are there no hydrogen bonds between the molecules?
Ty v much
Hey ;D
Hydrogen bonding only occurs if two molecules have a hydrogen atom bonded to a highly electronegative atom (N, O, halogens).
Ethanol has a hydrogen bonded to an oxygen, which is partially positive because the oxygen is more electronegative and will pull the electrons closer to itself.
CHF3 has its hydrogen bonded to a carbon, which has almost the same electronegativity as hydrogen. This doesn't allow hydrogen bonding since the hydrogen isn't positively charged and will not attract to ethanol's oxygen.
Basically, you look at what the hydrogen is bonded with. :)
Post by: winstondarmawan on September 01, 2017, 11:11:30 pm
For the production of LDPE, I have come across some sources which say it is created with the absence of a catalyst, and other sources which say it is created with an organic catalyst.
Which is more correct and should be used in the HSC?
TIA.
Post by: Natasha.97 on September 01, 2017, 11:22:50 pm
Hello!
For the production of LDPE, I have come across some sources which say it is created with the absence of a catalyst, and other sources which say it is created with an organic catalyst.
Which is more correct and should be used in the HSC?
TIA.
Hi!
I've asked this exact question to my teacher, and they said that there is no catalyst in LDPE production. A peroxide initiator e.g. benzoyl peroxide is used, but it is not a catalyst as it becomes part of the polymer. In the production of HDPE, a Ziegler-Natta catalyst is used e.g. Titanium Chloride.
Hope this helps
Post by: anotherworld2b on September 02, 2017, 06:45:43 pm
Post by: VanillaRice on September 02, 2017, 07:15:54 pm
I am not sure how to do this question. Can I have help please?
n(NaOH) = cV = 0.0126 x 0.390 = 0.004914 mol
n(sulfuric acid) = 1/2 x n(NaOH) = 0.002457 mol
Dilution factor is 1000/25 = 40
n(sulfuric acid)intial = 0.024578 x 40 = 0.09828 mol
m(sulfuric acid) = 98g/mol x 0.09828 mol = 9.63144g
So, there was 9.63144g of actual sulfuric acid
Total mass = 19.6mL x 1.67g/mL = 32.732g
So, % mass of sulfuric acid = 9.63144/32.732 x 100 = 29.4%
Hope this helps :)
Post by: winstondarmawan on September 03, 2017, 06:30:08 pm
Can someone go through the thought processes for answering a question like Q1.
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/21291632_1316215258503994_317323894_n.jpg?oh=658bc156d33a2b45eb8b183a5415ef5c&oe=59AE7C18
TIA.
Post by: MisterNeo on September 03, 2017, 07:37:47 pm
Hello!
Can someone go through the thought processes for answering a question like Q1.
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/21291632_1316215258503994_317323894_n.jpg?oh=658bc156d33a2b45eb8b183a5415ef5c&oe=59AE7C18
TIA.
Hey :D
For questions like these, there isn't really a specific approach but you would need to know that:
-Addition monomers are usually unsaturated (double/triple C-C bonds).
-Condensation monomers are usually saturated (single C-C bonds)
So the answer can't be C.
After that, my main approach is to do trial-and-error to see what works, like try doing a quick sketch or do it visually. Also note that the monomers alternate each time. It just takes practice. :)
Post by: winstondarmawan on September 03, 2017, 10:17:31 pm
(For hydration of ethene and dehydration of ethanol.)
Edit: Why do we need a water reaction medium for fermentation?
TIA.
Post by: MisterNeo on September 03, 2017, 10:48:06 pm
Are we required to know the structural two-step reaction which shows how sulfuric acid acts as a catalyst?
(For hydration of ethene and dehydration of ethanol.)
Edit: Why do we need a water reaction medium for fermentation?
TIA.
In the HSC course, we don't need to know how it reacts with the catalyst. All you need to know is what the catalyst is.
Water is used because:
-Moist conditions
-It dissolves glucose because yeast is a powder form and can't do anything to another solid.
-Water dilutes the ethanol produced because yeast dies in high conc. of ethanol.
Post by: Aussie1Italia2 on September 05, 2017, 04:29:21 pm
Can you please explain to me what validity is again? I'm trying to answer a past HSC paper question and its asking me to discuss the validity of a student's investigation.
Thank you!!!
Post by: Natasha.97 on September 05, 2017, 04:37:07 pm
Hi,
Can you please explain to me what validity is again? I'm trying to answer a past HSC paper question and its asking me to discuss the validity of a student's investigation.
Thank you!!!
Hi!
Validity is making sure that results obtained stem from ONLY the change in the independent variable (the one you vary), and that the experiment is testing out its hypothesis. This is ensured by keeping all variables constant (except for the independent).
Hope this helps
Post by: Aussie1Italia2 on September 05, 2017, 04:44:45 pm
Thank you!!!
Post by: Natasha.97 on September 05, 2017, 04:49:23 pm
What about reliability? Just so I don't end up accidentally talking about that.
Thank you!!!
Reliability is referring to how consistent your results are, e.g. if another person does the experiment, do the results that they obtain coincide with the results that you have obtained (how similar they are to each other). If they are similar, then the results are reliable, and if not, they are unreliable.
Hope this helps
Post by: winstondarmawan on September 05, 2017, 09:37:23 pm
Are we required to know the diagrams of various batteries for the Batteries dotpoint? (e.g. dry cell, silver button cell)
Post by: Natasha.97 on September 05, 2017, 09:42:01 pm
I haven't seen any past papers that require you to draw it, but the dot-point says "gather and present information on the structure", so I'd memorise it just in case!
Hope this helps
Post by: jakesilove on September 06, 2017, 03:49:43 pm
Hello!
Are we required to know the diagrams of various batteries for the Batteries dotpoint? (e.g. dry cell, silver button cell)
Yeah, I personally did end up learning the structure of my batteries, just because it was then easy to point to the anode/cathode/salt bridge. I don't THINK they could directly ask you to draw them out, but they would definitely help you get full marks in a longer response answer!
Post by: winstondarmawan on September 06, 2017, 08:03:13 pm
For the following question:
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/21439693_1318782871580566_2013104977_o.png?oh=8b19e7a2fb5932c0d979dc8b1f7a8adc&oe=59B2134C
Would we be required to talk about conditions in Haber Process that are not shown in the graph (e.g. 3:1 H2:N2 ratio)? Or do we just focus on temperature and pressure?
EDIT: Shipwrecks
Do sacrificial anodes donate their electrons to the metal that is to be protected? Or do they just corrode preferentially to the protected metal? Or both?
TIA.
Post by: Natasha.97 on September 06, 2017, 08:17:18 pm
For this particular question, I'd only refer to temperature and pressure, explaining each line in detail
Hope this helps
Post by: winstondarmawan on September 08, 2017, 07:18:28 pm
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/21534556_816544521861838_437814374_o.jpg?oh=a6d0cea1ffd670dc4ed57713635bc9b0&oe=59B4FD34
Post by: hansolo9 on September 08, 2017, 08:25:14 pm
Is it correct to call ammonia a catalyst in the Solvay process?
Some sources say yes, some say no.
My understanding is that it gets used up to form NH4Cl and they change it back later.
Post by: anotherworld2b on September 09, 2017, 01:35:53 am
Post by: MisterNeo on September 09, 2017, 12:13:02 pm
I was wondering what the monomer structure of Polylactide biopolymer would look like. Could someone help?
Something like this?
(http://archive.cnx.org/resources/a2f2c9f32150ba7a66f67c862586d6918145bad8/Fig5.jpg)
Post by: hazzaizmine on September 09, 2017, 03:50:30 pm
Post by: Bubbly_bluey on September 09, 2017, 03:58:26 pm
so for the 2016 mcq 11, when naming wouldn't you start alphabetically, so you would consider bromine as 1, i'm confused because i put the answer as A but in the answers its D. ( this the naming of alkanes q)Yes thats what I thought at first. But remember that there is another rule where you have to pick which one would produce least number when you addthem together.
This means that for instance option A: you have 1+1+2+2+2= 8
But for option D: 1+1+1+2+2= 7
so hence why the answer is D. So this is just another that you need for consider on top of alphabetical order. :)
Post by: Bubbly_bluey on September 09, 2017, 04:03:36 pm
Thank you :D
Post by: MisterNeo on September 09, 2017, 06:53:30 pm
Regarding to picking equations for electrolytic cells, I thought that if a solution was aqueous, you would have to consider water in a redox reaction. So in this question I thought that water would oxidise instead of the chromate because it is more easier to oxidise. Obviously my answer wasn'tin the option, but just wondering why this is the case.
Thank you :D
Hey :)
This question is referring to a galvanic cell rather than an electrolytic cell because the potentials are positive. So on the reduction table, you would look for the species closest to the top or bottom rather than in the middle.
Now why is water not oxidised in this galvanic cell?
First of all, we need to see what species are present in solution:
-Water H2O
-Potassium ion K+
-Dichromate ion Cr2O72-
-Sulfur dioxide SO2
Now we need to find out which species will be preferenced at the anode and cathode by looking for the largest Eo value.
-K+: Cannot be further oxidised. Reduction potential is non-spontaneous (-2.94V). Thus, we eliminate this.
-Cr2O72-: Can be reduced, 1.36V is spontaneous and is closer to the bottom of the reduction table than all the other species, thus this is going to be reduced at the cathode.
-H2O: Water's oxidation potential is more negative (-1.23V) than SO2 (-0.16V) hence sulfur dioxide has anode preference as it is more readily oxidised. Water cannot be reduced as dichromate has the cathode preference.
-SO2: Sulfur dioxide's potential is less negative than water's hence why it gets oxidised easier.
So we concluded that the anode preference is sulfur dioxide with a potential of -0.16V, and the cathode preference is dichromate with a potential of 1.36V. Just add these together and you will get a total potential of 1.2V, thus the answer is A.
Thanks for reading lol :D
Post by: winstondarmawan on September 09, 2017, 06:54:46 pm
Shipwrecks
Do sacrificial anodes donate their electrons to the metal that is to be protected? Or do they just corrode preferentially to the protected metal? Or both?
TIA.
Would appreciate help with the following, TIA.Just bumping my posts. :)
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/21534556_816544521861838_437814374_o.jpg?oh=a6d0cea1ffd670dc4ed57713635bc9b0&oe=59B4FD34
Post by: VanillaRice on September 09, 2017, 07:18:49 pm
EDIT: ShipwrecksThey become oxidised preferentially to the metal they are intended to protect. They will donate their electrons to whatever the oxidant is (e.g. molecular oxygen).
Do sacrificial anodes donate their electrons to the metal that is to be protected? Or do they just corrode preferentially to the protected metal? Or both?
TIA.
Would appreciate help with the following, TIA.Begin by calculating the number of moles of ZnO and Al2O3. Knowing this, can we work out how many moles of Zn and Al there are (i.e. consider n(Zn) = n(ZnO))? We are assuming all zinc and aluminium came from the alloy being studied. Once you have found the number of moles of each metal, find the masses of zinc and aluminium this equates to. Now, you will have the masses of zinc and aluminium extracted from the alloy. How can we calculate the percentage composition of each?
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/21534556_816544521861838_437814374_o.jpg?oh=a6d0cea1ffd670dc4ed57713635bc9b0&oe=59B4FD34
Hope this helps :)
Post by: anotherworld2b on September 09, 2017, 11:46:24 pm
I was wondering if someone could explain what a zwitterion is and it's importantance to me. I don't quite understand at the moment :-\
Post by: VanillaRice on September 10, 2017, 08:50:40 am
Thank you for your help MisterneoA zwitterion is a neutral molecule that has internal charges. That is, it's positive and negative charges cancel out.
I was wondering if someone could explain what a zwitterion is and it's importantance to me. I don't quite understand at the moment :-\
I assume you're learning about zwitterions in the context of amino acids? If so, think about how basic and acidic groups can cancel each other out.
One use of having a knowledge about the zwitterionic forms of an amino acid is that it allows us to predict the pHs at which the amino acid is overall neutral. This is useful for things like electrophoresis, where, if we know the charges of the amino acids at a particular pH, we can separate a sample of amino acids based on their charge, etc.
Hope this helps :)
Post by: phoebegresham on September 10, 2017, 11:18:00 am
Post by: Natasha.97 on September 10, 2017, 11:28:36 am
Hey!! If in an exam we are given a list of combustion equations and the question asks which one is incomplete combustion, what is the best way of knowing which one it is? Thanks :)
Hi!
Incomplete combustion will result in the production of Carbon Monoxide (CO)/Carbon (C)/both and water. Complete combustion will result in the production of Carbon Dioxide (CO2) and water.
Hope this helps
Post by: Bubbly_bluey on September 10, 2017, 07:01:32 pm
Thank you ;D
Post by: Natasha.97 on September 10, 2017, 07:48:46 pm
For eutrophication, is there a quantitative method to monitor the levels of phosphates and nitrate ions? I learnt about colorimetry but that seems to be a qualitative analysis. Would AAS work even though its not a trace element?
Thank you ;D
Hi!
These are the tests that could be used to monitor the levels of phosphate/nitrate ions:
Nitrates
- Brown ring test: Add Iron (II) Sulphate to conc. H2SO4. When combined with a nitrogen-containing solution, a brown ring will form at the intersection of the solutions (Qualitative)
- Kjedahl digestion: Heat sample with conc. H2SO4 (forms ammonium sulphate). Add NaOH to form ammonia. Titrate with a standardised HCl solution, then calculate the levels of ammonia. (Quantitative)
- Colorimetry: Add Nessler’s reagent, which reacts with nitrogenous compounds to form a yellow compound (Qualitative)
Phosphates
- Colorimetry: Add ammonium molybdate (forms pale yellow substance). Add solid ascorbic acid (Forms blue compound: ‘molybdenum blue’). The intensity of the blue colour is measured by a colorimeter and compared to a series of standards to determine the concentration (Quantitative)
- Adding Lead (II) nitrate : Forms a white precipitate of lead phosphate (Qualitative)
Hope this helps
Post by: anotherworld2b on September 11, 2017, 03:17:08 pm
i have a research assignment that requires me to do a flow chart with diagrams using relevant equations for the Synthesis Process but I can't find any information or equations :-\
Post by: Kle123 on September 11, 2017, 04:17:25 pm
I was wondering if anyone had information about how oil produces plastics.The flow chart would go something like this:
i have a research assignment that requires me to do a flow chart with diagrams using relevant equations for the Synthesis Process but I can't find any information or equations :-\
Crude Oil --(fractional distillation)--> ethylene (insufficient amounts to meet market demand) + high molecular weight fractions --(Thermal and Catalytic Cracking of large carbon fractions)--> ethylene {{--(substitution reactions for other plastics other than polyethylene)--> monomers such as vinyl chloride and styrene }} --(radical addition polymerisation and/or ziegler natta catalyst polymerisation)--> LDPE and/or HDPE
I don't think the syllabus asks for the production of PVC and Polystyrene plastics, which is why it is in brackets.
You could include equations of cracking stage of high molecular weight fractions such as C20H42(l)->C2H4 + C18H38
and also the stages of the polymerisation such as initiation, propagation and termination
_____________________________
1. Random(but not really) question:
Why does the carboxylic group only lie at the end of a carbon chain
2. In separating esters from reaction mixture in esterification why can we add water to the products then use a separating funnel to extract the ester on the top layer? Aren't esters polar.. aren't they soluble in water? why can they be separated like this?
Post by: Natasha.97 on September 11, 2017, 04:44:38 pm
Random(but not really) question:
Why does the carboxylic group only lie at the end of a carbon chain
Hi!
The carboxyl group is -COOH. The carbon that is involved must be bonded to two O molecules. The first oxygen shares a double covalent bond with carbon (carbonyl: C=O), whilst the second oxygen shares a single covalent bond with carbon and a hydroxyl bond with hydrogen (See diagram below). Therefore, the carboxyl group is at the end of the carbon chain as carbon only has 4 valence electrons (thus can form a maximum of 4 bonds.)
(https://i.imgur.com/NGv1MR1.png)
Hope this helps
Post by: Kle123 on September 11, 2017, 05:03:04 pm
Also,
In separating esters from reaction mixture in esterification why can we add water to the products then use a separating funnel to extract the ester on the top layer? Aren't esters polar.. aren't they soluble in water? why can they be separated like this?
Post by: VanillaRice on September 11, 2017, 05:08:26 pm
In separating esters from reaction mixture in esterification why can we add water to the products then use a separating funnel to extract the ester on the top layer? Aren't esters polar.. aren't they soluble in water? why can they be separated like this?Somewhat, but not to the same extent as the carboxyl and alcohol groups from which the ester was made. While those groups would indeed make the molecule much more soluble in water, they are lost in the condensation reaction (in the form of water molecules). Further to this, the ester linkage is located in the middle(ish) of the molecule, somewhat shielded by the hydrophobic alkyl groups in the molecule.
Hope this helps :)
Post by: MisterNeo on September 11, 2017, 05:12:40 pm
2. In separating esters from reaction mixture in esterification why can we add water to the products then use a separating funnel to extract the ester on the top layer? Aren't esters polar.. aren't they soluble in water? why can they be separated like this?
Esters are polar but not very because esters can only participate in hydrogen bonding via the negative double bonded oxygen lone pair.
The small esters are quite soluble and solubility decreases as ester size increases because the hydrocarbon chains start to get in the way of the hydrogen bonding.
Hence, big esters with around 5+ carbons start to become insoluble in water as they have weaker interactions with water.
Post by: anotherworld2b on September 11, 2017, 09:14:45 pm
Post by: winstondarmawan on September 11, 2017, 09:41:21 pm
i was wondering is oil sustainable? what effects could it have on local resources?I'm assuming you are talking about crude oil and it's fractions.
Crude oil is NOT sustainable, because it is not a renewable resource and takes millions of years to form.
This is why we are looking for alternate sources of fuel/petrochemicals, such as biomass and ethanol - which are indeed sustainable and renewable resources (however, we do not yet have the means to use them in a more commercial setting, i.e. they are not efficient enough).
By effect on local resources, I'm not sure what you mean.
Obviously the use of fossil fuels will diminish underground supply because it is not a renewable resource. It also has impacts on the environment, such as the release of CO2 which is a greenhouse gas > global warming or potentially CO, C, SOx which are air pollutants and pulminary irritants.
Hope this helps. :)
Post by: J.B on September 12, 2017, 08:23:54 am
So I understand that in the natural buffer solution, there is a weak acid and it's conjugate base, so that if acid is added, the base acts as a proton acceptor, and if base is added, the acid acts as a proton donor.
and so the equation is: H2CO3 <> H+ + HCO3-
But then in our study notes, we have been told that NaHCO3 is used for neutralising chemical spills, and when the HCO3- ion acts as a base it neutralises the H+, in an acid + carbonate > co2 + water:
H+ + HCO3- > H20 + CO2.
So I was just wondering why the reverse reaction of the natural buffer system is not the same as the neutraliser? & vise versa.
I hope that makes sense.
Thanks.
Post by: MisterNeo on September 12, 2017, 12:50:14 pm
Hi,
So I understand that in the natural buffer solution, there is a weak acid and it's conjugate base, so that if acid is added, the base acts as a proton acceptor, and if base is added, the acid acts as a proton donor.
and so the equation is: H2CO3 <> H+ + HCO3-
But then in our study notes, we have been told that NaHCO3 is used for neutralising chemical spills, and when the HCO3- ion acts as a base it neutralises the H+, in an acid + carbonate > co2 + water:
H+ + HCO3- > H20 + CO2.
So I was just wondering why the reverse reaction of the natural buffer system is not the same as the neutraliser? & vise versa.
I hope that makes sense.
Thanks.
When the carbon dioxide and water is produced by neutralisation in a closed system, it will partially dissolve to form carbonic acid.
It works in a similar way to the buffer but the carbonic acid is separated into its components.
Post by: justwannawish on September 12, 2017, 02:57:35 pm
The exothermic reaction between nitrogen and hydrogen forms ammonia with the enthaply being -100kJ/mole. The initial potential energy of the reactants was 200kJ and the activation energy is 50kJ per mole. What would be the energy change if 12.4L of nitrogen was reacted at 25 degrees and 100kPa?
Thank you!
Post by: Mymy409 on September 13, 2017, 11:27:17 am
Compare the following solutions
Soln 1: 0.25 mol/L H2SO4
Soln 2: 0.50 mol/L HCl
The H+ ion conc. and pH of soln 1 would be
A) Hydrogen ion conc - lower than soln 2 / pH - higher than soln 2
B) Hydrogen ion conc - lower than soln 2 / pH - lower than soln 2
C) Hydrogen ion conc - the same as soln 2 / pH - lower than soln 2
D) Hydrogen ion conc - the same as soln 2 / pH - the same as soln 2
_______________________________________________
The pH of a solution of CH3COOHNa was measured to be 9.05.
Which of the following best explains the measured pH?
A) Sodium ions donate protons to the water
B) Acetate ions donate protons from the water
C) Sodium ions accept protons from the water
D) Acetate ions accept protons from the water
I got this qn correct but I took a sort of guess when answering it. What's the easiest way to do it?
Mod edit: Merged posts :)
Post by: MisterNeo on September 13, 2017, 12:46:27 pm
I need help with this question:
Compare the following solutions
Soln 1: 0.25 mol/L H2SO4
Soln 2: 0.50 mol/L HCl
The H+ ion conc. and pH of soln 1 would be
A) Hydrogen ion conc - lower than soln 2 / pH - higher than soln 2
B) Hydrogen ion conc - lower than soln 2 / pH - lower than soln 2
C) Hydrogen ion conc - the same as soln 2 / pH - lower than soln 2
D) Hydrogen ion conc - the same as soln 2 / pH - the same as soln 2
The H+ concentration would be lower than Solution 2 because sulfuric acid does not fully ionise both hydrogens. The second protonation is an equilibrium and will not fully ionise, thus the 0.25M will not be doubled into a 0.5M and will be lower than Solution 2.
Since the H+ is lower, the pH will be HIGHER (don't get confused), so the answer is A.
The pH of a solution of CH3COOHNa was measured to be 9.05.
Which of the following best explains the measured pH?
A) Sodium ions donate protons to the water
B) Acetate ions donate protons from the water
C) Sodium ions accept protons from the water
D) Acetate ions accept protons from the water
So the answer is D, but why?
Firstly, cations never accept protons, and they only donate protons if it's something like the ammonium ion.
You should know that sodium acetate is a basic salt because it is made from a strong base (NaOH) and a weak acid (CH3COOH). When the salt dissociates in water, it releases the sodium ions and acetate ions.
The easiest way to do these questions is if you know the equilibriums of the acids/bases and their conjugates.
Acetate is the conjugate base of acetic acid. Bases accept protons.
When the salt is placed into water, there is no initial acetic acid concentration, so the equilibrium will immediately shift to the right according to LCP. This creates hydroxide ions and increases pH.
Post by: Mymy409 on September 13, 2017, 01:28:41 pm
The H+ concentration would be lower than Solution 2 because sulfuric acid does not fully ionise both hydrogens. The second protonation is an equilibrium and will not fully ionise, thus the 0.25M will not be doubled into a 0.5M and will be lower than Solution 2.
Since the H+ is lower, the pH will be HIGHER (don't get confused), so the answer is A.
So the answer is D, but why?
Firstly, cations never accept protons, and they only donate protons if it's something like the ammonium ion.
You should know that sodium acetate is a basic salt because it is made from a strong base (NaOH) and a weak acid (CH3COOH). When the salt dissociates in water, it releases the sodium ions and acetate ions.
The easiest way to do these questions is if you know the equilibriums of the acids/bases and their conjugates.
Acetate is the conjugate base of acetic acid. Bases accept protons.
When the salt is placed into water, there is no initial acetic acid concentration, so the equilibrium will immediately shift to the right according to LCP. This creates hydroxide ions and increases pH.
Thanks a lot, MisterNeo!
Another question:
Calculate the pH of the resultant solution when 25.0mL of 0.750mol/L HCl is added to 10.0mL of 0.500mol/L Ba(OH)2 solution.
A) 0.456
B) 0.602
C) 1.862
D) 2.058
Post by: MisterNeo on September 13, 2017, 02:05:32 pm
Thanks a lot, MisterNeo!
Another question:
Calculate the pH of the resultant solution when 25.0mL of 0.750mol/L HCl is added to 10.0mL of 0.500mol/L Ba(OH)2 solution.
A) 0.456
B) 0.602
C) 1.862
D) 2.058
To do these types of questions:
-Moles of HCl is conc. multiplied by volume equals 0.01875mol.
-Since it's a 2:1 mole ratio, Ba(OH)2 is the limiting reagent and will leave excess H+ (0.00875mol).
-Divide these excess moles by the total volume of the two solutions (0.035L), which will give you the concentration of H+ (0.25M).
-Use the pH log formula and you will get pH 0.602 (B).
Hopefully I got it right..
Post by: anotherworld2b on September 13, 2017, 08:53:54 pm
Post by: Natasha.97 on September 13, 2017, 09:07:53 pm
I was wondering what the balanced equation for copper nitrate + sodium hydroxide is. I was also wondering what effect would adding sodium hydroxide to copper nitrate in ice water surroundings do?
Hi!
Cu(NO3)2(aq) + 2NaOH(aq) = Cu(OH)2(s) + 2NaNO3(aq)
Not sure about the second question, sorry!
Post by: Mymy409 on September 13, 2017, 09:54:55 pm
To do these types of questions:
-Moles of HCl is conc. multiplied by volume equals 0.01875mol.
-Since it's a 2:1 mole ratio, Ba(OH)2 is the limiting reagent and will leave excess H+ (0.00875mol).
-Divide these excess moles by the total volume of the two solutions (0.035L), which will give you the concentration of H+ (0.25M).
-Use the pH log formula and you will get pH 0.602 (B).
Hopefully I got it right..
Yup, that's correct! Just one thing to correct: there should be a 2 before the H20 to balance both sides of the equation.
Post by: MisterNeo on September 14, 2017, 01:15:25 am
Yup, that's correct! Just one thing to correct: there should be a 2 before the H20 to balance both sides of the equation.Silly me. Thanks for that. :)
Cu(NO3)2 + 2NaOH = Cu(OH)2 + 2NaNO3 (All in solution)Isn't copper hydroxide insoluble? ???
I was also wondering what effect would adding sodium hydroxide to copper nitrate in ice water surroundings do?Not too sure about this, but solubility of salts decreases as temperature decreases. So maybe more copper hydroxide precipitate will form? Or some sodium nitrate will also precipitate because of low temperatures.
Post by: Natasha.97 on September 14, 2017, 07:24:40 am
Isn't copper hydroxide insoluble? ???
Yes it is! Nice catch, fixed :)
Post by: winstondarmawan on September 14, 2017, 09:13:05 am
They become oxidised preferentially to the metal they are intended to protect. They will donate their electrons to whatever the oxidant is (e.g. molecular oxygen).This is what I had thought originally, but after a question in my trials and the solutions from a HSC past paper I am led to believe that the electrons are donated to the metal they are trying to protect...
Hope this helps :)
Very confused by this.
Post by: Mymy409 on September 14, 2017, 07:27:00 pm
Post by: jakesilove on September 14, 2017, 08:11:57 pm
This might be a bit of a dumb question, but will I lose marks if a diagram isn't shaded correctly?
I would definitely say that you shouldn't lose marks! I don't think I've ever shaded a Chemistry diagram in my life.
Post by: beau77bro on September 15, 2017, 03:36:40 pm
Hey can someone explain this? It doesn't add up/not balanced so what's the proper - btw this is in the atarnotes
Second one btw
Post by: Shadowxo on September 15, 2017, 03:46:44 pm
(http://uploads.tapatalk-cdn.com/20170915/68cee2d9c9522789a113635f5f6fea73.jpg)I think it should just be a single H2O, then it balances correctly :)
Hey can someone explain this? It doesn't add up/not balanced so what's the proper - btw this is in the atarnotes
Second one btw
Post by: beau77bro on September 15, 2017, 04:07:53 pm
I think it should just be a single H2O, then it balances correctly :)
ohhhh ok thankyou hahaha didnt see that
whats the overall equation of the fuel cell?
Post by: Shadowxo on September 15, 2017, 08:57:04 pm
ohhhh ok thankyou hahaha didnt see thatBit rusty but I believe it would just be adding the equations together
whats the overall equation of the fuel cell?
Zn(s) + 2MnO2(aq) + 2H+(aq) + 2e- -> Zn2+(aq) + 2e- + Mn2O3(aq) + H2O(l)
Which reduces down to
Zn(s) + 2MnO2(aq) + 2H+(aq) -> Zn2+(aq) + Mn2O3(aq) + H2O(l)
Post by: necromantic on September 15, 2017, 09:42:44 pm
Post by: Natasha.97 on September 15, 2017, 09:51:25 pm
Why does the fermentation of glucose take place under anaerobic conditions? ???
Hi!
Yeast will only produce ethanol when respiring anaerobically (and as ethanol is the desired product, fermentation must take place without oxygen).
Hope this helps
Post by: MisterNeo on September 15, 2017, 09:54:25 pm
Why does the fermentation of glucose take place under anaerobic conditions? ???
If there was oxygen present during fermentation, the ethanol would get oxidised into acetic acid. Hence, it is done without oxygen.
Also respiration purposes like what Jess said :D
Post by: beau77bro on September 16, 2017, 10:12:57 am
Bit rusty but I believe it would just be adding the equations togetheryea thats the dry cell what about the fuel cell from the notes? there are waters in both equation and i'm unsure how to deal with it?
Zn(s) + 2MnO2(aq) + 2H+(aq) + 2e- -> Zn2+(aq) + 2e- + Mn2O3(aq) + H2O(l)
Which reduces down to
Zn(s) + 2MnO2(aq) + 2H+(aq) -> Zn2+(aq) + Mn2O3(aq) + H2O(l)
Post by: VanillaRice on September 16, 2017, 11:26:51 am
yea thats the dry cell what about the fuel cell from the notes? there are waters in both equation and i'm unsure how to deal with it?I think the image might be a bit cropped, so I can't exactly see that part (or it might just be my computer ::))
Either way, I assume that's the hydrogen fuel cell? I'm also going to assume that we're dealing with an alkaline electrolyte (since it involves a few more water molecules).
The equations should be:
ANODE
\(\ce{2H2 + 4OH- -> 4e- + 4H2O}\)
I've doubled the coefficients above so we can directly add it to the equation below:
CATHODE
\(\ce{O2 + 2H2O + 4e -> 4OH-}\)
Therefore adding the equations gives
\(\ce{O2 + 2H2O + 4e + 2H2 + 4OH- -> 4OH- + 4e- + 4H2O}\)
Now to directly address your question, we can simply cancel out the hydroxides and waters (similar to the way we can cancel out electrons)
\(\ce{O2 + 2H2 -> 2H2O}\)
Hope this helps :)
Post by: smshs2017 on September 16, 2017, 12:51:19 pm
thanks!!!!
Post by: MisterNeo on September 16, 2017, 01:37:04 pm
hey can you pls help me with the different type of bonds, for example hydrogen bonds, dispersion, dipole dipole and others, because i keep confusing which is which, and which one is stronger than the other and all that.
thanks!!!!
Hey :)
There are two types of bonds/forces: Intramolecular and intermolecular.
-Intramolecular is between atoms in a molecule.
-Intermolecular is between various molecules.
Intra
-Covalent:
An "electron-sharing" bond between non-metals.
Eg. H2O, O2, etc.
-Ionic:
A bond between a positively charged cation and a negatively charged anion. The anion keeps the electrons it is given, and the cation loses it. They are usually between a metal and non-metal, BUT not always because ammonium (NH4+) salts are also ionic.
Eg. NaCl, KOH, etc.
-Metallic:
Bonds between metals via delocalised electrons. This is more commonly tested in prelim rather than HSC course.
Eg. Fe, Cu, etc.
Inter
-Hydrogen bonding:
A special type of dipole-dipole bond where a slightly positive hydrogen attracts to a slightly negative atom. The slightly negative atom is usually a highly electronegative element such as oxygen or a halogen (polar), which is bonded to a hydrogen and pulls the electrons closer to itself thus creating the charges.
(https://lh3.googleusercontent.com/proxy/fmDvp0ShkFwX6iD7ap-_V__UrgUGKOaRAN3SDHaVc2Sj6gyA-IRUr5KQz7bYEgUNV1jnnSvyVDPl58k1uSvZQUDtMgKhOCe-I4fupLOKtLFNOuiLEsM0Vgri6dU9rJyw9IYJOACCV0HT2SY=w491-h299-nc)
-Dipole Dipole/Ion Dipole
Very similar to hydrogen bonding Ion dipoles form when ionic molecules are attracted to polar molecules. This is how NaCl dissolves in water. The chloride attracts to the water's positive hydrogen and vice versa with the sodium.
(https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcRF8wScwusDaGmvowT4J74EvKsZv8nk5-mLw0FHuE9O__AkTOb1ko9MZ4WuYw)
-Dispersion forces:
(https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcShS6JroAKOpkDtMV7ipLiWgc-fZGD2PeaHOR8qiFAnt3jjoNEbQA) A bit hard to explain, so this pic explains it. Hydrocarbon chains have stronger dispersion forces.
Hydrogen bond > dipoles > dispersion forces
Hope this helps ;)
Post by: beau77bro on September 16, 2017, 06:51:44 pm
I think the image might be a bit cropped, so I can't exactly see that part (or it might just be my computer ::))
Either way, I assume that's the hydrogen fuel cell? I'm also going to assume that we're dealing with an alkaline electrolyte (since it involves a few more water molecules).
The equations should be:
ANODE
\(\ce{2H2 + 4OH- -> 4e- + 4H2O}\)
I've doubled the coefficients above so we can directly add it to the equation below:
CATHODE
\(\ce{O2 + 2H2O + 4e -> 4OH-}\)
Therefore adding the equations gives
\(\ce{O2 + 2H2O + 4e + 2H2 + 4OH- -> 4OH- + 4e- + 4H2O}\)
Now to directly address your question, we can simply cancel out the hydroxides and waters (similar to the way we can cancel out electrons)
\(\ce{O2 + 2H2 -> 2H2O}\)
Hope this helps :)
THANKYOU VANILLA RICE
Post by: mjorfian on September 16, 2017, 08:44:56 pm
Post by: Natasha.97 on September 16, 2017, 08:52:04 pm
Hiii. Just revising biopolymers right now, and I was just wondering, how much of the history do I need to know? I was thinking just the date when it started being produced and whatnot but jake's notes have a whole para on them and its stressing me out because i hate remembering historical detail RIP. does anyone happen to have a short + effective summary on phbs? thanks!
Hi!
Personally, I'm not really sure if you need the historical development, but the current developments of PHB.
Here's my summary for PHB, hope this helps
(https://i.imgur.com/HT3zHO7.png)
Post by: beau77bro on September 17, 2017, 10:13:49 am
Hiii. Just revising biopolymers right now, and I was just wondering, how much of the history do I need to know? I was thinking just the date when it started being produced and whatnot but jake's notes have a whole para on them and its stressing me out because i hate remembering historical detail RIP. does anyone happen to have a short + effective summary on phbs? thanks!It does say "progress in recent development of a named biopolymer", so maybe just the basic outline would be useful to note, but definitely more focus on the potential of use based on properties. From my summary:
Post by: necromantic on September 17, 2017, 04:57:26 pm
How do you identify/explain if a salt is acidic/basic/neutral and whether it's weak/strong? (Acidic environment, dotpoint 4.4) ???
Post by: MisterNeo on September 17, 2017, 05:56:22 pm
Hi!
How do you identify/explain if a salt is acidic/basic/neutral and whether it's weak/strong? (Acidic environment, dotpoint 4.4) ???
Hey ;D
You can identify salt pH by looking at the cation and anion it is made of.
Acidic Salt
Anion is the from a strong acid, and cation is from a weak base.
Eg. NH4Cl is an acidic salt because ammonium is from a weak base (ammonia) and the chloride is from a strong acid (hydrochloric).
Basic Salt
Anion is from a weak acid, and cation is from a strong base.
Eg. CH3COONa (sodium acetate) is a basic salt because the sodium ion is from a strong base (NaOH), and the acetate ion is from a weak acid (acetic acid).
Neutral Salt
Anion is from a strong acid, and cation is from a strong base.
OR
Anion is from a weak acid, and cation is from a weak base.
Eg. NH4HCO3 OR NaCl because ammonium and bicarbonate are both from weak species (NH3 H2CO3), and sodium and chloride are from strong species (NaOH HCl).
The ions are usually a conjugate of a base or an acid. Hope this helps :)
Post by: Natasha.97 on September 17, 2017, 05:57:31 pm
Hi!
How do you identify/explain if a salt is acidic/basic/neutral and whether it's weak/strong? (Acidic environment, dotpoint 4.4) ???
Hi!
Salts
Basic salt (pH>7.0)
- Formed from a weak acid and a strong base
- Example: Sodium Acetate (acetic acid and sodium hydroxide)
○ CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l)
○ CH3COONa(aq) + H2O(l) → CH3COO–(aq) + Na+(aq)
○ CH3COO-(aq) + H2O(l) ←→ CH3COOH(aq) + OH-(aq)
○ Acetate ion accepts H+ from H2O, resulting in the formation of the OH- ion which makes the pH > 7.0
Acidic salt (pH<7.0)
- Formed from a weak base and a strong acid
- Example: Ammonium Chloride (ammonia and hydrochloric acid
○ NH4OH(aq) + HCl(aq) → NH4Cl(aq) + H2O(l)
○ NH4Cl(aq) + H2O(l) → NH4+(aq) + Cl-(aq)
○ NH4+(aq) + H2O(l) ←→ NH3(aq) + H3O+(aq)
○ Ammonium ion donates H+ to H2O, resulting in the formation of the H3O+ ion which makes pH < 7.0
Neutral salt (pH=7.0)
- Formed from either a strong acid and strong base, or a weak acid and weak base
- Example: Hydrochloric Acid and Sodium Hydroxide
○ HCl(aq) + NaOH → NaCl(aq) + H2O(l)
○ NaCl(aq) + H2O(l) → Na+(aq) + Cl-(aq)
○ Na+ stays as an ion (metal)
○ Cl- is a very weak base, so it does not accept protons
- Example: Ethanoic Acid and Ammonia
○ CH3COOH(aq) + NH3(aq) → (CH3COO)NH4(aq)
○ (CH3COO)NH4(aq) →CH3COO-(aq) + NH4+(aq)
○ CH3COO-(aq) + H2O(l) → CH3COOH(aq) + OH-(aq)
○ NH4+(aq) + H2O(l) → NH3(aq) + H3O+(aq)
○ The hydroxide ions formed by CH3COO- and the hydronium ions formed by NH3 cancel each other, so the salt is neutral
Hope this helps
Post by: Mathew587 on September 17, 2017, 08:16:26 pm
Post by: Sine on September 17, 2017, 08:19:23 pm
For the IUPAC nomenclature, is the accepted method the lowest sum or is alphabetical order?alphabetical order
Post by: Mathew587 on September 17, 2017, 09:42:43 pm
thnx
Post by: Natasha.97 on September 17, 2017, 10:38:53 pm
cos q11 from 2016 hsc had D as the answer when I would have thought it'd be A. Can someone pls explain why?
thnx
Hi!
There is a "lowest sum of locants" rule, and when calculated, the sum of locants on D is 7 (2+2+1+1+1), as opposed to 8 (1+1+2+2+2) for A
Hope this helps
Post by: RuiAce on September 17, 2017, 10:46:15 pm
alphabetical orderFor reasons unknown, they do not teach this in the HSC.
Post by: VanillaRice on September 17, 2017, 11:01:25 pm
For the IUPAC nomenclature, is the accepted method the lowest sum or is alphabetical order?
alphabetical order
Hi!
There is a "lowest sum of locants" rule, and when calculated, the sum of locants on D is 7 (2+2+1+1+1), as opposed to 8 (1+1+2+2+2) for A
Hope this helps
I feel there might be a misunderstanding here, so to clarify:
When numbering substituents, you want the lowest numbers.
When listing substituents, you want alphabetical order. :)
Post by: Mathew587 on September 17, 2017, 11:18:25 pm
Hi!
There is a "lowest sum of locants" rule, and when calculated, the sum of locants on D is 7 (2+2+1+1+1), as opposed to 8 (1+1+2+2+2) for A
Hope this helps
thanks jess. i've heard of that that but it felt dodgy in that both the locant rule and alphabetical order must be followed.
Post by: J.B on September 18, 2017, 05:08:17 pm
I just wanted to clarify that I have interpreted this correctly.
So hydrogen bonds are intermolecular forces between hydrogen and either O, F, or N.
So in this image, even though there is a possibility of molecules forming hydrogen bonds in "C" between a H of one molecule & an O of another molecule. Is it considered to not have any hydrogen bonds as intramolecularly the Hydrogen and Oxygen aren't bonded together, so they don't have strong enough positive and negative dipoles to form the H-bonds intermolecularly?
I hope this makes sense. Thank you.
Post by: VanillaRice on September 18, 2017, 05:27:53 pm
Hi,Are you referring to compound Z? If so, the oxygen indeed is not close enough to the hydrogen to have a large electron pulling effect to form a large enough delta positive region around the hydrogen for hydrogen bonding.
I just wanted to clarify that I have interpreted this correctly.
So hydrogen bonds are intermolecular forces between hydrogen and either O, F, or N.
So in this image, even though there is a possibility of molecules forming hydrogen bonds in "C" between a H of one molecule & an O of another molecule. Is it considered to not have any hydrogen bonds as intramolecularly the Hydrogen and Oxygen aren't bonded together, so they don't have strong enough positive and negative dipoles to form the H-bonds intermolecularly?
I hope this makes sense. Thank you.
Hope this helps :)
Post by: angelahchan on September 19, 2017, 04:57:49 pm
Does anyone have recent developments for polylactic acid? The online thing I found was something about improving crystallisation rates but I have no clue what it means. Where can I find more info, and should I switch to a different biopolymer if I can't find a development?
Post by: Mathew587 on September 19, 2017, 05:44:26 pm
Hi,
Does anyone have recent developments for polylactic acid? The online thing I found was something about improving crystallisation rates but I have no clue what it means. Where can I find more info, and should I switch to a different biopolymer if I can't find a development?
Use these notes:
1.2.6 Use available evidence to gather and present data from secondary sources and analyse
progress in the recent development and use of a named biopolymer. This analysis should name
the specific enzyme(s) used or organism used to synthesise the material and an evaluation of
the use or potential use of the polymer produced related to its properties
Biopolymers are naturally occurring polymers found in living organisms.
Biopol
This biopolymer is called PHBV (poly-3-hydroxybutyrate-poly-3-hydroxyvalerate).
It is a co-polymer of polyhydroxybutyrate (PHB) and polyhydroxyvalerate (PHV).
The monomer of PHB is 3-hydroxybutanoic acid (common name 3-hydroxybutyric acid).
The monomer of PHV is 3-hydroxypentanoic acid (common name 3-hydroxyvaleric acid).
Production:
- This polymer is produced industrially by bacteria (Ralstonia eutrophus or Alcaligenes eutrophus) growing in tanks with a carbon-based food source, high in glucose or valeric acid.
The bacteria manufacture the PHBV which is stored in their cell walls as granules for later us as an energy source.
The polymer is then isolated and purified.
Uses in relation to properties:
- Biopol is mainly used in medical applications (e.g. surgical stitches and tissue scaffolding) as it is biocompatible and biodegradable (products decompose naturally and no surgery is needed to remove them).
The use of biopolymers in medicine will mean more lives can be saved due to biopolymer transplants and fewer allergic reactions.
Biopol is also used to make disposable items (e.g. bottles, razors and shampoo containers) as it is biodegradable and non-toxic. However, this is uncommon now, due to the high cost of production.
Recent developments:
- Recently, the gene for producing Biopol polymer strands from the Alcaligenes eutrophus bacteria was extracted and implanted into E. coli using genetic engineering techniques. E. coli bacteria are much easier to grow than other bacteria, and thus are cheaper.
Scientists have also attempted to genetically engineer plants such as cress so that they produce biodegradable plastics rather than storing starch. This results in a much cheaper process, although it is still more expensive than petrochemical polymers.
Evaluation:
- Biopol is currently much more expensive than petrochemical plastics, so research is being carried out to genetically modify bacteria to control the plastic that they form. There is a growing demand for it where biodegradability and biocompatibility are important.
It has an advantage over petroleum-based polymers of being renewable, as it is formed from a microorganism. Biodegradable polymers may eventually replace petroleum-based polymers, reducing our dependence on fossil fuels and allowing them to last longer.
Post by: bsdfjnlkasn on September 22, 2017, 07:30:24 pm
I was just wondering if aside from knowing the conditions necessary for fermentation to occur, we were required to justify them? :)
Post by: angelahchan on September 22, 2017, 07:40:21 pm
Hey there!Is this for the option or for the rest of the chemistry course? Because if you mean the conditions for fermentation of glucose to ethanol, there are questions where you're expected to explain/justify the conditions, although I can't think of one specifically. I think I did some questions asking why the reaction has to be done anaerobically.
I was just wondering if aside from knowing the conditions necessary for fermentation to occur, we were required to justify them? :)
Here's a table of the justifications from https://dc.edu.au/hsc-chemistry-production-of-materials/#Ethanol_A_renewable_resource
(https://i.imgur.com/XArTm8O.png)
Post by: arunasva on September 22, 2017, 10:58:23 pm
Hey there!
I was just wondering if aside from knowing the conditions necessary for fermentation to occur, we were required to justify them? :)
I have seen a question where they askedwhy did the rate of fermentation drop after an hour (or some period of time). Then you gotta recognize the condition affecting it and justify the statement - Alkanol increased and the poor yeast died off. Other an' that I haven't seen any, where you justify the actual conditions.
Post by: mary123987 on September 23, 2017, 09:45:34 am
http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2005exams/pdf_doc/chemistry_05.pdf
Regards
Post by: Natasha.97 on September 23, 2017, 10:03:15 am
Hey could someone please explain why the volume of gas never exceeds 380 ml for part question 22(b) , it is clear from the graph but im not sure what calculations to use to infer that Hcl is the limiting factor and that Zn is in excess.
http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2005exams/pdf_doc/chemistry_05.pdf
Regards
Hi!
No calculations are required for this question. In this case, hydrogen gas was produced when the amount of Zinc reached 1.22g. However, as more Zinc is added, the volume of gas remains the same. This is because all HCl has been used up; any additional Zinc that is added to the mixture does not have any HCl to react with. Therefore, HCl is the limiting factor (as it limits the volume of H2 produced), and Zn is in excess (in this case, in excess of 0.71g).
Hope this helps
Post by: mary123987 on September 23, 2017, 10:11:13 am
Hi!Hey thanks alot ,just wondering if you weren't given the table how would you know when all the HCl was used up? and how did you calculate that Zn was in excess by 0.73 g thanks heaps
No calculations are required for this question. In this case, hydrogen gas was produced when the amount of Zinc reached 1.22g. However, as more Zinc is added, the volume of gas remains the same. This is because all HCl has been used up; any additional Zinc that is added to the mixture does not have any HCl to react with. Therefore, HCl is the limiting factor (as it limits the volume of H2 produced), and Zn is in excess (in this case, in excess of 0.73g).
Hope this helps
Post by: angelahchan on September 23, 2017, 10:37:38 am
Hey thanks alot ,just wondering if you weren't given the table how would you know when all the HCl was used up? and how did you calculate that Zn was in excess by 0.73 g thanks heaps
Hey thanks alot ,just wondering if you weren't given the table how would you know when all the HCl was used up? and how did you calculate that Zn was in excess by 0.73 g thanks heaps
The zinc is in excess because by 1.22 g the vol of gas doesn't increase, thus when 1.93 g of zinc is added in, there's 0.71 g of excess (I think she corrected it)
I don't think you're able to calculate when it would be used up if you were only given data up to 1.22g, because we aren't given the concentration of the HCl. This means we don't know if the moles of HCl are insufficient to react with the moles of zinc in a 2:1 ratio.
Mod edit: ratio changed as requested :)
Post by: Natasha.97 on September 23, 2017, 10:39:36 am
Hey thanks alot ,just wondering if you weren't given the table how would you know when all the HCl was used up? and how did you calculate that Zn was in excess by 0.73 g thanks heaps
Hi!
If the graph was given, it should look something like this:
(https://i.imgur.com/cKsXWCV.png)
The HCl is used up when the graph starts to level off, which shows that the volume of hydrogen gas doesn't increase when more Zinc is being added to the mixture. Excess should be 0.71g :)
Post by: mary123987 on September 23, 2017, 10:42:35 am
Hi!
If the graph was given, it should look something like this:
(https://i.imgur.com/cKsXWCV.png)
The HCl is used up when the graph starts to level off, which shows that the volume of hydrogen gas doesn't increase when more Zinc is being added to the mixture. Excess should be 0.71g :)
The zinc is in excess because by 1.22 g the vol of gas doesn't increase, thus when 1.93 g of zinc is added in, there's 0.71 g of excess (I think she corrected it)Thanks so much it makes alot of sense im glad you cant be expected to calculate when it is used up
I don't think you're able to calculate when it would be used up if you were only given data up to 1.22g, because we aren't given the concentration of the HCl. This means we don't know if the moles of HCl are insufficient to react with the moles of zinc in a 1:1 ratio.
Post by: angelahchan on September 23, 2017, 10:55:30 am
Thanks so much it makes alot of sense im glad you cant be expected to calculate when it is used up
idk why the "modify message" thing isn't working, just wanted to correct myself that it's a 1:2 ratio of zinc:HCl, not 1:1
Post by: mylinh-nguyen on September 23, 2017, 09:52:23 pm
(b) Determine the concentration of citric acid in the lemon cordial.
I'm so confused please help
Post by: Eric11267 on September 23, 2017, 09:59:14 pm
(a) Why is the calculated concentration of the standardised NaOH solution different 2 from the concentration calculated using the mass given, assuming no human error occurred?I do VCE Chemistry so please take what I say with a grain of salt.
(b) Determine the concentration of citric acid in the lemon cordial.
I'm so confused please help
To my knowledge, NaOH reacts with carbon dioxide in the air so the amount you put in isn't the same as the amount in solution.
For part b, I'm assuming you use the results of titrating HCl to determine the concentration of the NaOH solution and use those results to determine the concentration of citric acid
Post by: MisterNeo on September 23, 2017, 10:18:46 pm
(a) Why is the calculated concentration of the standardised NaOH solution different 2 from the concentration calculated using the mass given, assuming no human error occurred?
(b) Determine the concentration of citric acid in the lemon cordial.
I'm so confused please help
Sodium hydroxide in solid form is hygroscopic and will absorb moisture from the air. This may be why the calculations are off.
For Part B calculations: :)
(https://i.imgur.com/Pm2m6YQ.jpg)
Post by: mylinh-nguyen on September 23, 2017, 11:46:40 pm
Sodium hydroxide in solid form is hygroscopic and will absorb moisture from the air. This may be why the calculations are off.thank youu soo much!!
For Part B calculations: :)
(https://i.imgur.com/Pm2m6YQ.jpg)
Post by: Fahim486 on September 24, 2017, 12:43:24 pm
Post by: Natasha.97 on September 24, 2017, 12:59:56 pm
Hey so I just wanted to know why the answer is D and not A. Thanks!
Hi!
There is a "lowest sum of locants" rule when naming these compounds. The sum of locants on D is 7 (2+2+1+1+1), as opposed to 8 (1+1+2+2+2) for A.
Hope this helps
Post by: mary123987 on September 24, 2017, 09:25:04 pm
like for example a question is asking :how many isomers are there for C3H6Cl2
these are the combinations i found (1,1), (1,2) ,(1,3),(2,2) ,(2,3)
the options are as listed below:
a) 3
b) 4
c) 5
d) 6
the answer is b!
Post by: arunasva on September 25, 2017, 01:42:07 am
Hey guys, just wondering is there like any formula or something to calculate the number of isomers?if you can derive one :')
like for example a question is asking :how many isomers are there for C3H6Cl2
these are the combinations i found (1,1), (1,2) ,(1,3),(2,2) ,(2,3)
the options are as listed below:
a) 3
b) 4
c) 5
d) 6
the answer is b!
Post by: Shadowxo on September 25, 2017, 11:00:52 am
Hey guys, just wondering is there like any formula or something to calculate the number of isomers?I don't know of any formulas, for questions like this it's really guess-and-check
like for example a question is asking :how many isomers are there for C3H6Cl2
these are the combinations i found (1,1), (1,2) ,(1,3),(2,2) ,(2,3)
the options are as listed below:
a) 3
b) 4
c) 5
d) 6
the answer is b!
You have to remember that (1,2) is the same as (2,3) as you can rotate the first to get the second.
I'd approach this question by approaching it like you have, finding where the two Cl atoms could be. They could both be on the first carbon, or both be on the second/middle, but both being on the third is the same as both being on the first. So two options if they're both on the same carbon atom. If they're not, they could be one on either side, or one on the middle and one on the side, so two options (as having one on the middle and one on the other side is the same as the second option). This would give a total of 4.
You just need to figure out which options aren't redundant, if there's a formula I don't know of it
Post by: angelahchan on September 25, 2017, 11:09:17 am
Hey guys, just wondering is there like any formula or something to calculate the number of isomers?
like for example a question is asking :how many isomers are there for C3H6Cl2
these are the combinations i found (1,1), (1,2) ,(1,3),(2,2) ,(2,3)
the options are as listed below:
a) 3
b) 4
c) 5
d) 6
the answer is b!
Was this in an HSC paper? Because I don't you need to know how many isomers random organic compounds have (I think the syllabus point is more about naming haloalkanes) . I did a google, and no there isn't a formula for organic compounds in general, but there is one for alkanes if you're interested: (it looks incredibly complicated)
http://pubs.acs.org/doi/abs/10.1021/ci00027a019?journalCode=jcics1&
Post by: mary123987 on September 25, 2017, 02:30:02 pm
I don't know of any formulas, for questions like this it's really guess-and-checkYes !!! that's what i forgot thankyou soooo much i completely forgot about how 2,3 is the same as 1,2 (when you look from the oppsote side) wow good spot thanks a lot
You have to remember that (1,2) is the same as (2,3) as you can rotate the first to get the second.
I'd approach this question by approaching it like you have, finding where the two Cl atoms could be. They could both be on the first carbon, or both be on the second/middle, but both being on the third is the same as both being on the first. So two options if they're both on the same carbon atom. If they're not, they could be one on either side, or one on the middle and one on the side, so two options (as having one on the middle and one on the other side is the same as the second option). This would give a total of 4.
You just need to figure out which options aren't redundant, if there's a formula I don't know of it
Was this in an HSC paper? Because I don't you need to know how many isomers random organic compounds have (I think the syllabus point is more about naming haloalkanes) . I did a google, and no there isn't a formula for organic compounds in general, but there is one for alkanes if you're interested: (it looks incredibly complicated)Hey yes this is actually from the 2006 paper q5
http://pubs.acs.org/doi/abs/10.1021/ci00027a019?journalCode=jcics1&
Haha that formula looks intense like 6 unit maths!!http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2006exams/pdf_doc/chemistry_06.pdf
Post by: winstondarmawan on September 25, 2017, 03:59:10 pm
Yes !!! that's what i forgot thankyou soooo much i completely forgot about how 2,3 is the same as 1,2 (when you look from the oppsote side) wow good spot thanks a lotHey yes this is actually from the 2006 paper q5
Haha that formula looks intense like 6 unit maths!!http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2006exams/pdf_doc/chemistry_06.pdf
The best bet for the amount of isomers I think is to draw them out. It's long and tedious but ensure you don't skip one or have an extra one. Do this at the end of the exam.
Anyways, a question.
When ammonia is added to water, does the pH increase or decrease?
Thanks in advance.
Post by: Fahim486 on September 25, 2017, 04:05:47 pm
Post by: mary123987 on September 25, 2017, 04:20:54 pm
Hey so for question 4 the answer is C but I got B and don't understand why, for question 10 why is the answer A and not D and finally for question 20 I got B but the answer is D and can't figure out why and I'm thinking maybe I jumbled my calculations or something. Thanks!Hey so im currently answering q 20 i got D
so the molar heat of combustion is equal to q/n
q=mcΔT
n=m/M
mplar heat of combustion =mc(ΔT)/(m/M)
firstly q=m(4.18)(45-20)
n=1/116.204
now heptanol has 7 carbons and so
4638=m((4.18)(25)) /(1/116.204)
therefore m(4.18(25) =4638(1/116.204)
therefore m=0.3819384455kg
therefore d
let me know if you are confused
Post by: MisterNeo on September 25, 2017, 04:24:30 pm
Hey so for question 4 the answer is C but I got B and don't understand why, for question 10 why is the answer A and not D and finally for question 20 I got B but the answer is D and can't figure out why and I'm thinking maybe I jumbled my calculations or something. Thanks!
For question 4, oxidation is when something loses electrons to become more positive. Since the Fe2+ becomes a more positive charge to Fe3+, it becomes oxidised by losing the electrons to be more positive. :)
For question 10, answer D is unbalanced.
Post by: mary123987 on September 25, 2017, 04:31:33 pm
The best bet for the amount of isomers I think is to draw them out. It's long and tedious but ensure you don't skip one or have an extra one. Do this at the end of the exam.HEY from my understanding ammonia is basic thus by adding ammonia to water the pH should increase (remember acids decrease pH and bases increase pH)
Anyways, a question.
When ammonia is added to water, does the pH increase or decrease?
Thanks in advance.
this is apparent by its reaction with water NH3 + H2O --> NH4+ + OH-
its clear OH- is a product thus making it basic
let me know if your confused
Post by: Fahim486 on September 25, 2017, 04:54:20 pm
Hey so im currently answering q 20 i got D
so the molar heat of combustion is equal to q/n
q=mcΔT
n=m/M
mplar heat of combustion =mc(ΔT)/(m/M)
firstly q=m(4.18)(45-20)
n=1/116.204
now heptanol has 7 carbons and so
4638=m((4.18)(25)) /(1/116.204)
therefore m(4.18(25) =4638(1/116.204)
therefore m=0.3819384455kg
therefore d
let me know if you are confused
It's sort of starting to kind of make sense but I originally thought you sub in the temperature and heat of combustion into the q=mcAt formula and just rearrange it to make m the subject which is probably why I ended up getting B as the answer but one thing I wanted to ask how did you know you had to use the molar heat of combustion thing. Thanks!
Post by: mary123987 on September 25, 2017, 05:06:59 pm
It's sort of starting to kind of make sense but I originally thought you sub in the temperature and heat of combustion into the q=mcAt formula and just rearrange it to make m the subject which is probably why I ended up getting B as the answer but one thing I wanted to ask how did you know you had to use the molar heat of combustion thing. Thanks!Hey so like you i always thought that you just use q=mcΔT but i found out that q refers to the amount of heat realeased , when asked for the molar heat of combustion or heat of combustion(really the same thing ) they are asking for the amount of energy(in the form of heat) given off by 1 mol of the substance now how i knew this is by looking at the table as the units are kJ/mol.
Since they have already given you the value of ΔHc/mol it is just a matter of manipulating the formula to get what you want in this case m (remember they dont give you something unless you need it ).
If you are still confused about anything please let me know ,hope it makes sense
Post by: Fahim486 on September 25, 2017, 05:11:16 pm
Hey so like you i always thought that you just use q=mcΔT but i found out that q refers to the amount of heat realeased , when asked for the molar heat of combustion or heat of combustion(really the same thing ) they are asking for the amount of energy(in the form of heat) given off by 1 mol of the substance now how i knew this is by looking at the table as the units are kJ/mol.
Since they have already given you the value of ΔHc/mol it is just a matter of manipulating the formula to get what you want in this case m (remember they dont give you something unless you need it ).
If you are still confused about anything please let me know ,hope it makes sense
Ahh okay that makes a lot more sense now. Thanks!
Post by: bridie_2345 on September 25, 2017, 07:35:26 pm
I'm confused about how the micelles come into it aswell!
Thanks heaps!
Post by: MisterNeo on September 25, 2017, 07:54:48 pm
Hi there for the industrial chemistry option in saponification, I'm confused about explaining how soap, water and oil act together as an emulsion?
I'm confused about how the micelles come into it aswell!
Thanks heaps!
Hey! ;)
Water and oil are usually immiscible (don't mix) but can be made misicble using an emulsifying agent or emulsifier, which is soap.
An emulsion is basically two immiscible liquids dispersed throughout each other with the help of an emulsifier. Examples of emulsions include: milk, mayo, butter, etc.
You would need to know the role of soap's chemical structure.
(https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcRYyeKitKlNtRYQfqlh471JaCVvahgldi8GCKCPTmU6LLZP5zONJtSxswhm)
The hydrophobic tail "adsorbs" to the oil to escape the water (it hates water), whereas the hydrophilic head stays out in the water (loves water). This forms a micelle.
(https://qph.ec.quoracdn.net/main-qimg-864d5f830cc6281aea42d8eb64e75486-c)
The tiny droplets of oil are trapped in all these micelles. Notice how the heads are on the outside? The micelle is negatively charged, and repel each other to disperse themselves throughout the solution. Hence, becoming an emulsion and able to be washed away with water.
Hope this helps :D
Post by: clarence.harre on September 25, 2017, 08:09:12 pm
Can you please let me know if I got it right? If I did, how could I condense it? If it's wrong, please point out where :)
(2007.29.d) 4 marks
Question:
The work of early scientists has increased our understanding of electron transfer
reactions. Describe the impact of this work on society.
Response:
Galvani's work catalysed Volta's development of the Galvanic Cell, leading to Davy's discovery of electrolysis and creation of stronger batteries, prompting Faraday's quantitative laws relating the charge transfer to the products produced.
This work has resulted in the ability to protect and conserve artefacts, benefiting society.
Electrolysis (electron transfer reactions) cleans and stabilises iron, copper and lead artefacts. They are placed in a dilute, alkaline solution (2% NaOH) which acts an electrolyte between the cathode (artefact) and anode (inert steel). Using iron as an example:
(anode): 4OH- <-> 2H2O + O2 + 4e-
(cathode): Fe(OH)Cl(s) + 2e- -> Fe(s) + OH- + Cl-
Post by: bsdfjnlkasn on September 25, 2017, 08:50:34 pm
2007 doesn't have any marking guidelines.
Can you please let me know if I got it right? If I did, how could I condense it? If it's wrong, please point out where :)
(2007.29.d) 4 marks
Question:
The work of early scientists has increased our understanding of electron transfer
reactions. Describe the impact of this work on society.
Response:
Galvani's work catalysed Volta's development of the Galvanic Cell, leading to Davy's discovery of electrolysis and creation of stronger batteries, prompting Faraday's quantitative laws relating the charge transfer to the products produced.
This work has resulted in the ability to protect and conserve artefacts, benefiting society.
Electrolysis (electron transfer reactions) cleans and stabilises iron, copper and lead artefacts. They are placed in a dilute, alkaline solution (2% NaOH) which acts an electrolyte between the cathode (artefact) and anode (inert steel). Using iron as an example:
(anode): 4OH- <-> 2H2O + O2 + 4e-
(cathode): Fe(OH)Cl(s) + 2e- -> Fe(s) + OH- + Cl-
Hey sorry, just wondering which topic this is relevant to because I don't recall learning about any of these chemists this year? :O
Post by: Natasha.97 on September 25, 2017, 08:52:49 pm
Hey sorry, just wondering which topic this is relevant to because I don't recall learning about any of these chemists this year? :O
It's from the Shipwrecks option :)
Post by: winstondarmawan on September 25, 2017, 09:02:05 pm
2007 doesn't have any marking guidelines.
Can you please let me know if I got it right? If I did, how could I condense it? If it's wrong, please point out where :)
(2007.29.d) 4 marks
Question:
The work of early scientists has increased our understanding of electron transfer
reactions. Describe the impact of this work on society.
Response:
Galvani's work catalysed Volta's development of the Galvanic Cell, leading to Davy's discovery of electrolysis and creation of stronger batteries, prompting Faraday's quantitative laws relating the charge transfer to the products produced.
This work has resulted in the ability to protect and conserve artefacts, benefiting society.
Electrolysis (electron transfer reactions) cleans and stabilises iron, copper and lead artefacts. They are placed in a dilute, alkaline solution (2% NaOH) which acts an electrolyte between the cathode (artefact) and anode (inert steel). Using iron as an example:
(anode): 4OH- <-> 2H2O + O2 + 4e-
(cathode): Fe(OH)Cl(s) + 2e- -> Fe(s) + OH- + Cl-
Good answer, however I think you need more than one application for the four marks.
Maybe include the hydrolysis of water for obtain elemental Hydrogen and Oxygen?
Or more particularly, you can reference Davy's separation of salts from their compounds and how this is still used in society today to reconstitute these salts into new compounds and their uses. (e.g. KCl into KOH).
Post by: bsdfjnlkasn on September 25, 2017, 09:38:16 pm
It's from the Shipwrecks option :)
omg phew hahaha :)
Post by: marshmallow.on.fire on September 26, 2017, 11:43:26 am
Post by: Natasha.97 on September 26, 2017, 11:54:59 am
How should i go about working this out? The answer is B.
Hi! I've attached the solution below:
(https://i.imgur.com/SOsbIrA.png)
Hope this helps
Post by: clarence.harre on September 26, 2017, 12:40:50 pm
Good answer, however I think you need more than one application for the four marks.
Maybe include the hydrolysis of water for obtain elemental Hydrogen and Oxygen?
Or more particularly, you can reference Davy's separation of salts from their compounds and how this is still used in society today to reconstitute these salts into new compounds and their uses. (e.g. KCl into KOH).
Thank you winstondarmawan :)
I hadn't even considered that.
Post by: bsdfjnlkasn on September 26, 2017, 05:31:13 pm
Post by: Natasha.97 on September 26, 2017, 05:36:34 pm
How would you write the equation for the oxidation of ammonia?
Hi!
4NH3(g) + 5O2(g) → 6H2O(g) + 4NO(g)
Post by: J.B on September 26, 2017, 05:46:40 pm
Post by: bsdfjnlkasn on September 26, 2017, 07:33:33 pm
In the HSC course do we need to know about carbon branching (-yl) groups?
Hey! The only thing I know about alkyl groups is that they function as chain stiffeners in polymers. The resulting plastic (if they have chain stiffeners) will be hard and rigid because of the intermolecular forces that act. For example, the additional Cl in PVC is electronegative, which then causes for dipole-dipole forces to act (i.e. attraction). This leads to a stiffer plastic as the polymer chains are held more tightly together.
Post by: MisterNeo on September 26, 2017, 08:11:47 pm
In the HSC course do we need to know about carbon branching (-yl) groups?
In terms of branching groups, you would probably only need to know how to name them when given a diagram. Like: methyl, ethyl, etc.
Other than that, there isn't much on them except for LDPE vs HDPE, and soaps in the industrial topic. :)
Post by: justwannawish on September 27, 2017, 07:06:40 am
1. Are saturated substances necessarily pure?
2. And with groups on polymer chains, why do they have to be on every second one?
Post by: MisterNeo on September 27, 2017, 08:07:15 am
A few random chemistry questions
1. Are saturated substances necessarily pure?
2. And with groups on polymer chains, why do they have to be on every second one?
It depends on what "pure" is. Saturated substances can include alkanes such as methane in natural gas, which is a mixture. Mixtures contain multiple pure substances mixed. The definition of impure can be physical or chemical. Like, iron ore is impure, and dirty water is also impure.
For the polymer question, do you have a specific polymer? Something like PVC has the chlorine on every second carbon because the monomer is 2 carbons long. :)
Post by: hansolo9 on September 27, 2017, 08:40:46 am
can someone explain the different types of detergents and their strengths/weaknesses?
Post by: MisterNeo on September 27, 2017, 08:53:47 am
Hi
can someone explain the different types of detergents and their strengths/weaknesses?
Hey ;)
So there are 3 types of detergents: Anionic, cationic, and non-ionic.
Normal soap is bad at cleaning in hard or acidic water because of the forming of scum.
Anionic
-Doesn't form precipitates in hard of acidic water like soap.
-It's effectiveness is low because of the positive ions disturbing the sulfonate head's interaction with water.
-Phosphate builders are usually added to get rid of the ions, but this causes eutrophication.
Cationic
-Very effective in hard or acidic water because the positive charge repels positive ions.
-Bad in alkaline water because it attracts OH-.
-Non-biodegradable, as cationic detergents are antimicrobial and kill the bacteria responsible for decomposition.
-Sucks at cleaning glass because of glass having a slight negative charge, which causes cationic molecules to cling.
Non-ionic
-Good at cleaning both acidic, basic, and hard water because it has no charge. However, it is still weak in acidic/hard water because all the oxygens in the structure cause it to have a slight negative charge that can attract positive ions.
-Doesn't form scum.
High branching reduces biodegradability of all detergents.
Hope this helps :)
Post by: justwannawish on September 27, 2017, 11:24:47 am
It depends on what "pure" is. Saturated substances can include alkanes such as methane in natural gas, which is a mixture. Mixtures contain multiple pure substances mixed. The definition of impure can be physical or chemical. Like, iron ore is impure, and dirty water is also impure.
For the polymer question, do you have a specific polymer? Something like PVC has the chlorine on every second carbon because the monomer is 2 carbons long. :)
Thank you for your answer! I was wondering about polystyrene but also in general. So is it always based on the number of carbons in the monomer?
2. What are the conditions for two compounds go form a condensation polymer?
3. How do we deconstruct polymers into their respective monomers?
Post by: bsdfjnlkasn on September 27, 2017, 10:51:25 pm
Thank you for your answer! I was wondering about polystyrene but also in general. So is it always based on the number of carbons in the monomer?
2. What are the conditions for two compounds go form a condensation polymer?
3. How do we deconstruct polymers into their respective monomers?
Hey there!
So since polystyrene has the addition of a benzene ring (a.k.a group) to the carbon chain, it is not pure.
A condensation polymer is one that upon polymerisation, releases a small molecule (usually water). So it's crucial that the monomers that go into forming a condensation polymer chain contain functional groups that can eject these small molecules (NH3 is another example) upon joining. Some functional groups involved in condensation polymerisation include(where R is some molecular structure)
• Alkanol: R – OH
- When it joins with another alkanol, a water molecule is released, causing the remaining oxygen to become apart of the polymer
• Carboxylic acid: R – COOH
• Amine: R – NH2
• Ester: R – COO – R
3. In terms of 'deconstructing polymers' to find what the original monomer was, we need to consider whether it initially underwent condensation or addition polymerisation. If it was condensation, we have to consider what molecule was released and what functional group that hints at being present in the monomer. Then by becoming familiar with the list above, we can pick which it was most likely to be (especially if we know the compound).
On the other hand, if it was addition polymerisation, we would simply need to look at a diagram for repetition, from which we can section off the different monomers and eventually identify theme. For example, PVC will have a Cl on every alternating carbon and so because that repeats, we can deduce that the monomer is just chloroethene. We know it's ethene (i.e. presence of a double bond) because during addition polymerisation, we break the double bond of the unsaturated molecule and pair monomers across the newly released bonds (free electrons)
Post by: winstondarmawan on September 27, 2017, 11:03:12 pm
Thank you for your answer! I was wondering about polystyrene but also in general. So is it always based on the number of carbons in the monomer?I think for the HSC, the polymers we focus on are simple and usually an alkene of some variation, which is why the ones we encounter you see the side chain on 'every second one'. Don't need to worry about this too much.
2. What are the conditions for two compounds go form a condensation polymer?
However, you should note that the benzene ring (for polystyrene) actually contains more carbons. So you shouldn't say 'carbons in the monomer'.
2. Monomers must have a functional group each.
Hope this helps!
Anyways, I have a couple of questions:
a) Compare the process of polymerisation of ethylene and glucose. Include relevant chemical equations. (3)
b) Explain the relationship between the structures and properties of three different polymers from ethylene and glucose, and their uses. (5)
For this question, I chose to talk about LDPE, HDPE and cellulose. I could talk about relationship between LDPE and HDPE and their uses but I'm stumped for cellulose - in terms of the relationship between LDPE and HDPE.
And also this question:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22054551_1336533976472122_2117348658_n.jpg?oh=35cfe5c35729edbffc00a89dce11c179&oe=59CDE1E2
Thanks in advance!
Post by: bsdfjnlkasn on September 27, 2017, 11:55:27 pm
Anyways, I have a couple of questions:
a) Compare the process of polymerisation of ethylene and glucose. Include relevant chemical equations. (3)
b) Explain the relationship between the structures and properties of three different polymers from ethylene and glucose, and their uses. (5)
For this question, I chose to talk about LDPE, HDPE and cellulose. I could talk about relationship between LDPE and HDPE and their uses but I'm stumped for cellulose - in terms of the relationship between LDPE and HDPE.
And also this question:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22054551_1336533976472122_2117348658_n.jpg?oh=35cfe5c35729edbffc00a89dce11c179&oe=59CDE1E2
Thanks in advance!
Hey! Let's have a look at these questions :)
I would structure the first one in a table format to ensure that you're directly showing differences and similarities (i.e. comparing). So check out the screenshot to help you clarify :)
Post by: MisterNeo on September 27, 2017, 11:57:24 pm
Anyways, I have a couple of questions:
a) Compare the process of polymerisation of ethylene and glucose. Include relevant chemical equations. (3)
b) Explain the relationship between the structures and properties of three different polymers from ethylene and glucose, and their uses. (5)
For this question, I chose to talk about LDPE, HDPE and cellulose. I could talk about relationship between LDPE and HDPE and their uses but I'm stumped for cellulose - in terms of the relationship between LDPE and HDPE.
And also this question:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22054551_1336533976472122_2117348658_n.jpg?oh=35cfe5c35729edbffc00a89dce11c179&oe=59CDE1E2
Thanks in advance!
So for the polymerisation of the ethylene and glucose, you would talk about the fact that polyethylene is produced via addition, whereas cellulose is produced via condensation. You would draw the monomer units in the equation, showing the water being a byproduct.
For the second question, cellulose has a lot of hydroxyl groups in its monomer units that create very strong bonds between polymer chains. This doesn't allow water to interact with the cellulose, hence cellulose is insoluble in water and really good as plant material.
For the last question:
Zn Eo= -0.76V
Fe Eo= +0.44V
Ca Eo= +2.87V
Cu Eo= -0.34V
Mn Eo= +1.18V
A potential would need to be positive since it needs to be spontaneous. Just add them up for each one, and the Ca/Mn are positive values with Mn being the lowest. :)
Post by: clarence.harre on September 28, 2017, 06:46:35 am
Question:
Which term best describes the role of the chlorine free radical in this process?
(A) Anion
(B) Catalyst
(C) Initiator
(D) Oxidant
Response:
I lean towards B and C. Since we're taught catalysts just lower the activation energy and not actually participate in the reaction, and given that in polymerisation the initiator series joins in, why is the answer B and not C?
So, I figure it must be C - in polymerisation the initiator series catalyses the reaction and joins in. But the answer is B. Why?
Post by: justwannawish on September 28, 2017, 07:04:19 am
I think for the HSC, the polymers we focus on are simple and usually an alkene of some variation, which is why the ones we encounter you see the side chain on 'every second one'. Don't need to worry about this too much.
However, you should note that the benzene ring (for polystyrene) actually contains more carbons. So you shouldn't say 'carbons in the monomer'.
2. Monomers must have a functional group each.
Hope this helps!
Anyways, I have a couple of questions:
a) Compare the process of polymerisation of ethylene and glucose. Include relevant chemical equations. (3)
b) Explain the relationship between the structures and properties of three different polymers from ethylene and glucose, and their uses. (5)
For this question, I chose to talk about LDPE, HDPE and cellulose. I could talk about relationship between LDPE and HDPE and their uses but I'm stumped for cellulose - in terms of the relationship between LDPE and HDPE.
And also this question:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22054551_1336533976472122_2117348658_n.jpg?oh=35cfe5c35729edbffc00a89dce11c179&oe=59CDE1E2
Thanks in advance!
Hey there!
So since polystyrene has the addition of a benzene ring (a.k.a group) to the carbon chain, it is not pure.
A condensation polymer is one that upon polymerisation, releases a small molecule (usually water). So it's crucial that the monomers that go into forming a condensation polymer chain contain functional groups that can eject these small molecules (NH3 is another example) upon joining. Some functional groups involved in condensation polymerisation include(where R is some molecular structure)
• Alkanol: R – OH
- When it joins with another alkanol, a water molecule is released, causing the remaining oxygen to become apart of the polymer
• Carboxylic acid: R – COOH
• Amine: R – NH2
• Ester: R – COO – R
3. In terms of 'deconstructing polymers' to find what the original monomer was, we need to consider whether it initially underwent condensation or addition polymerisation. If it was condensation, we have to consider what molecule was released and what functional group that hints at being present in the monomer. Then by becoming familiar with the list above, we can pick which it was most likely to be (especially if we know the compound).
On the other hand, if it was addition polymerisation, we would simply need to look at a diagram for repetition, from which we can section off the different monomers and eventually identify theme. For example, PVC will have a Cl on every alternating carbon and so because that repeats, we can deduce that the monomer is just chloroethene. We know it's ethene (i.e. presence of a double bond) because during addition polymerisation, we break the double bond of the unsaturated molecule and pair monomers across the newly released bonds (free electrons)
Thank you so much! It really means a lot to me!
Post by: angelahchan on September 28, 2017, 08:40:58 am
This question is from the 2007 paper. I still don't agree with the answer. If someone could please explain why I'm wrong, that'd be much appreciated.
Question:
Which term best describes the role of the chlorine free radical in this process?
(A) Anion
(B) Catalyst
(C) Initiator
(D) Oxidant
Response:
I lean towards B and C. Since we're taught catalysts just lower the activation energy and not actually participate in the reaction, and given that in polymerisation the initiator series joins in, why is the answer B and not C?
So, I figure it must be C - in polymerisation the initiator series catalyses the reaction and joins in. But the answer is B. Why?
This question is from the 2007 paper. I still don't agree with the answer. If someone could please explain why I'm wrong, that'd be much appreciated.
Question:
Which term best describes the role of the chlorine free radical in this process?
(A) Anion
(B) Catalyst
(C) Initiator
(D) Oxidant
Response:
I lean towards B and C. Since we're taught catalysts just lower the activation energy and not actually participate in the reaction, and given that in polymerisation the initiator series joins in, why is the answer B and not C?
So, I figure it must be C - in polymerisation the initiator series catalyses the reaction and joins in. But the answer is B. Why?
The initiator, e.g. in making polyethylene isn't a catalyst because it gets incorporated into the reaction, whereas chlorine is a catalyst because as you said, it doesn't get used up
(https://i.imgur.com/6KTVhpp.png)
(so it's attached along the chain as it propogates)
Post by: MisterNeo on September 28, 2017, 08:49:26 am
Question:
Which term best describes the role of the chlorine free radical in this process?
(A) Anion
(B) Catalyst
(C) Initiator
(D) Oxidant
From my understanding, initiators are the species that creates the free radicals to be used in the initiation stage and is used up, which would be the CFC itself (? kinda like the peroxide in LDPE). The chlorine radical does in a way act as a catalyst because it provides an alternate reaction pathway.
Overall equation is:
A catalyst does react with the species involved but it reforms itself, as that is how it lowers activation energy since it is easier using chlorine path rather than ozone + oxygen radical. :)
Post by: anotherworld2b on September 28, 2017, 05:26:36 pm
I was wondering how can you compare the pH of solutions of these compounds? (ethanol, ethanoic acid, enthanamine)?
Q2. Which one of the following compounds is least likely to be one of the amino acids that undergo a condensation reaction to form a protein?
a) H2NCH(CH2OH)COOH
b) H2NCH(CH2OOH)COOH
c) H2NCH2(CH2SH)COOH
d) H2NCH(CH2CH2CONH2)COOH
Q3. Which of the following substances will not act as a soap?
a) CH3(CH2)16COOK
b) CH3(CH2)16COOH
c) CH3(CH2)14COONa
d) CH3(CH2)12C6H4SO3Na
Post by: MisterNeo on September 28, 2017, 05:40:04 pm
I was wondering how can you compare the pH of solutions of these compounds? (ethanol, ethanoic acid, enthanamine)?In increasing order: ethanoic acid, ethanol, ethylamine.
Acid is pretty straight forward. Ethanol is fairly neutral with a very slight basic pH because of its -OH group. Most amines are based because they are derived from ammonia.
Which of the following substances will not act as a soap?Answer B will not act as a soap because it is insoluble scum. One of the limitations of carboxylate soaps is that it sucks in acidic water because the soap ion gains a H+ ion to form scum. The other answers are soaps, and D is an anionic detergent.
a) CH3(CH2)16COOK
b) CH3(CH2)16COOH
c) CH3(CH2)14COONa
d) CH3(CH2)12C6H4SO3Na
Not sure about the second question though since without a structural diagram, it's a bit difficult. :)
Post by: phoebegresham on September 29, 2017, 12:31:38 pm
Mod Edit: Post merge, use the 'Modify' post button to add to your previous question to avoid chain posting :)
Post by: Daniyahasan on September 29, 2017, 12:45:14 pm
Post by: anotherworld2b on September 29, 2017, 02:10:08 pm
Post by: MisterNeo on September 29, 2017, 03:28:01 pm
Hey! Could I please get some help with these questions? Thanks :)
Hey :D
For Q1, write out the equation first:
Then you need to find the limiting reagent of the two reactants by finding moles of both.
n(HCl) = 0.2 x 0.01 = 0.002mol
n(Ba(OH2) = 0.12 x 0.005 = 0.0006mol
The HCl will be in excess since it needs 0.001mol of barium hydroxide to react fully (2:1 ratio)
Hence, the excess HCl will be 0.002mol - 0.0012mol = 0.0008mol.
HCl is monoprotic, so the H+ concentration is the same as HCl, which is 0.0008mol / 0.1L = 0.008M since it is diluted to 100mL.
Using the log formula, you get pH 2.10. :)
For Q2, you would write out the equation for the combustion of ethane:
7mol of oxygen is needed for 2mol of ethane. Just halve 7 to get 3.5mol per 1mol of ethane.
For Q3, write out the equation of the carbonic acid titration:
n(NaOH) = 0.0083L x 0.0095M = 0.00007885mol
n(acid) = 0.00003942mol (1:2 ratio)
n(H+) = 0.00007885mol (diprotic, and assuming full protonation)
c(H+) = 0.00007885mol / 0.25L = 0.0003154M
Use the log formula, and it should be 3.50. That is if it were full protonation of the weak acid, unless there was something I missed. :)
Do we need to know how to draw the structure of glucose?Yes. They may ask you to draw the cellulose polymerisation, which would require glucose monomer units.
i was wondering if anyone knows the the solubility trend of the functional groups from lowest solubility to highest solubility.-All alkanes/alkenes/alkynes are insoluble.
-Carboxylic acids are more soluble than alkanols of a similar molar mass.
Post by: bsdfjnlkasn on September 29, 2017, 03:50:02 pm
Do we need to know how to draw the structure of glucose?
I think it's useful, especially as you should know how to draw cellulose (which is a polymer made from B-glucose monomers) :)
Post by: Piza on September 29, 2017, 05:03:44 pm
For the Saponification dot point, do I have to memorise the entire structure of a specific fat/oil, or would it be sufficient to put an R at the end of each carbon chain?
Post by: Zainbow on September 29, 2017, 05:46:45 pm
Post by: bsdfjnlkasn on September 29, 2017, 06:22:18 pm
I was just wondering if I could have the screenshotted notes explained to me?
I'm not too sure how the hydronium ion is working as an oxidant (nor do I now what that means)
Also how are the reactive metals being oxidised by the sulphuric acid?
Like i'm really not understanding the vocabulary and so am feeling really lost when looking at the given equation and trying to make sense of it all...
Could I also get some help with:
a) Why does temperature only affect the equilibrium constant? If it's a measure of concentration, then why don't changes in ion concentration (which is the most obvious in my mind) and changes in pressure/volume change K? Because when these factors are changed, don't they change the position of the equilibrium and so the relative concentrations of the products/reactants?
b) I've just read that an explanation for the exothermic nature of sulphuric acid's ionisation is because the energy released by the bond forming is greater than the total energy required to ionise the molecule and to overcome the hydrogen bonding that holds the water molecules together.
What bond formation is being referred to? How is it different to the ionisation being referred to (and the hydrogen bonding for that matter - why is it even relevant?)
c) Lastly, I would like some clarification for these notes:
Are these just general rules/blanket statements that apply to all equilibriums? I've never heard of them before and can't really understand them. Could someone possibly illustrate the point with an example? :)
When H2O is added the concentration of all species decreases. So the reactants are favoured to increase the concentration of dissolved ions
When H2O is removed, concentration of all species increases. So the products are favoured to minimise total amount of dissolved ions in the system.
Thank you so much!!
Post by: Mounica on September 29, 2017, 06:52:14 pm
Post by: Daniyahasan on September 29, 2017, 06:54:09 pm
does anyone have notes or knows where i can get notes for forensics (option topic)?
forensic notes are soo hard to find but im pretty sure they have one in the Atarnotes notes section lol, ill try finding it and ill dm you the link :)
I think it's useful, especially as you should know how to draw cellulose (which is a polymer made from B-glucose monomers) :)
ohh okay. Thanks!
Mod Edit: Posts merged, use the 'Modify' post button to avoid chain posting
Post by: MisterNeo on September 29, 2017, 09:19:06 pm
Hi! This is a question regarding the option Industrial Chemistry.They won't expect you to memorise a specific triglyceride, and will give you the formula if they ask for a specific fat. Otherwise, they may ask you to draw the general equation for saponification, which you would put the "R" like what Zainbow said. :)
For the Saponification dot point, do I have to memorise the entire structure of a specific fat/oil, or would it be sufficient to put an R at the end of each carbon chain?
I was just wondering if I could have the screenshotted notes explained to me?An oxidant is a species that causes oxidation of another but gets reduced itself. If you look on the Data Sheet, the H+ (same as hydronium) has a value of 0.00V. It will oxidise any metal above it from Pb to K. This is because it must be a spontaneous reaction with a positive Eo value.
I'm not too sure how the hydronium ion is working as an oxidant (nor do I now what that means)
Also how are the reactive metals being oxidised by the sulphuric acid?
Like i'm really not understanding the vocabulary and so am feeling really lost when looking at the given equation and trying to make sense of it all...
For example, let's oxidise zinc:
To oxidise Zn into Zn2+, it is 0.76V.
To reduce H+, it is 0.00V.
Remember back in galvanic cells, you add these two together to get 0.76V total, and it is spontaneous. If it were negative, the metal will be unreactive to the hydrogen ions under the same conditions.
The H+ will get reduced as a result of oxidising a metal by gaining its electrons. Hence why acid + metal releases hydrogen gas, because the hydrogen ions get reduced into hydrogen gas. :) Dilute sulfuric acid oxidises metal because it provides the hydorgen ions.
a) Why does temperature only affect the equilibrium constant? If it's a measure of concentration, then why don't changes in ion concentration (which is the most obvious in my mind) and changes in pressure/volume change K? Because when these factors are changed, don't they change the position of the equilibrium and so the relative concentrations of the products/reactants?I was once confused by this too. Remember that the equilibrium constant is the RATIO of products to reactants. Once you add more moles, the equlibrium will shift, yes. BUT, the equilibrium will shift back into that special ratio, hence the K constant remains unchanged.
Temperature changes only affect the equilibrium because it moves the whole system to one side and keeps it there as long as that temperature is held constant. Once the system stays on one side, the entire RATIO changes because the products may have permanently increased or decreased depending on the heat of reaction.
b) I've just read that an explanation for the exothermic nature of sulphuric acid's ionisation is because the energy released by the bond forming is greater than the total energy required to ionise the molecule and to overcome the hydrogen bonding that holds the water molecules together.The dilution of sulfuric acid is actually a two-step reaction.
What bond formation is being referred to? How is it different to the ionisation being referred to (and the hydrogen bonding for that matter - why is it even relevant?)
The dissociation is endothermic, absorbs a bit of heat because bonds are broken.
The actual hydronium ion forming is exothermic, releases much more heat as bonds are formed. Hence, ionisation is exothermic because it produces hydronium ions (that's the relevant part).
c) Lastly, I would like some clarification for these notes:This isn't something I've seen, but I wouldn't use it as a general rule. Equilibriums can be liquid or gaseous. A gaseous equilibrium, unless steam is involved, would not change if water/steam were added. If you added water to a liquid equilibrium, that doesn't have water involvement, it wouldn't have much of an effect
Are these just general rules/blanket statements that apply to all equilibriums? I've never heard of them before and can't really understand them. Could someone possibly illustrate the point with an example? :)
When H2O is added the concentration of all species decreases. So the reactants are favoured to increase the concentration of dissolved ions
When H2O is removed, concentration of all species increases. So the products are favoured to minimise total amount of dissolved ions in the system.
An example of an equilibrium that will be affected is the dilution of weak acids:
Adding water to this will shift the equilibrium to the right.
Hope this helps :D
Post by: winstondarmawan on September 29, 2017, 09:30:05 pm
Can someone please go over the uses of starch and cellulose in relation to the structure. Confused about this part of the syllabus.
Also, are we required to be able to draw starch/cellulose?
TIA.
Post by: justwannawish on September 30, 2017, 10:39:06 am
Hello!
Can someone please go over the uses of starch and cellulose in relation to the structure. Confused about this part of the syllabus.
Also, are we required to be able to draw starch/cellulose?
TIA.
Hello!
Can someone please go over the uses of starch and cellulose in relation to the structure. Confused about this part of the syllabus.
Also, are we required to be able to draw starch/cellulose?
TIA.
Not entirely sure about the first one but I believe you're meant to be able to draw cellulose (which is made out of beta-glucose) and starch is made out of alpha glucose.
Cellulose exists in a ring structure with multiple hydroxyl groups (aka the O-H). The carbon atom of one ring is joined to the C atom of the next and these polymer chains are held by hydrogen bonding, which gives the fibres strength. As it is a naturally occurring condensation polymer, it can be be broken down into petrochemicals through multiple reactions due to its carbon structure.
As you can see in this photo, the linking oxygen atoms alternate with one Ch2OH for each ring for b-glucose like cellulose. Between chains they also change the setting out. For a-glucose, the individual rings alternate the oxygen but the different rings combined don't vary like that in a set pattern.
Post by: winstondarmawan on September 30, 2017, 12:25:47 pm
Post by: bsdfjnlkasn on September 30, 2017, 12:45:00 pm
What are the disadvantages of biopol, other than being expensive and inefficient to produce at a large scale?
Hey there,
PHB is rarely used commercially, as it costs about 10x as much to produce as petrochemical polymers. However, it is particularly effective for uses where biodegradability is important. So I do agree that it's an issue of cost primarily but, by looking at the developments we can see how the current issues are being worked on (and so, deduce it's limitations with more specific examples which is what a question is likely to ask for :) ).
So by growing the PHB in a valerie acid medium, the copolymer PHBV is produced which is stronger and much more flexible. This allows for greater variety in use, making it more desirable in today's market.
Other advances include genetically engineering cress so that it produces biopol instead of starch which reduces costs. However, petrochemical-derived plastics are still cheaper, emphasising cost to be a major limitation of wide-spread use.
Finally, the gene for producing Biopol polymer strands from the Alcaligenes eutrophus bacteria was extracted and implanted into E. coliusing genetic engineering techniques. E. coli bacteria are much easier to grow than other bacteria, and thus are cheaper.
Post by: justwannawish on September 30, 2017, 12:47:30 pm
What are the disadvantages of biopol, other than being expensive and inefficient to produce at a large scale?
Also it has poorer properties, such as low thermal stability and it's brittle.
Post by: josephmalicdem on September 30, 2017, 04:19:26 pm
Requesting help for Questions 15-16. I got both wrong.
Mod Edit: Use the "modify" button to add more than one post. :)
Post by: anotherworld2b on September 30, 2017, 06:22:18 pm
Post by: MisterNeo on September 30, 2017, 07:31:01 pm
How do I do Question 14: HSC Chemistry 2012?(https://i.imgur.com/bMozHCn.png)
Requesting help for Questions 15-16. I got both wrong.
From the previous question, you would have identified dichromate as the oxidising agent that undergoes reduction. The iron (II) undergoes oxidation. Using the data sheet values, you can find the Eo value. Remember that you do not multiply these data values according to stoichiometric moles, and use them as they are on the sheet. :)
For Q15, a pH increase of 1 indicates a tenfold decrease in H+ concentration, according to the pH formula. The question has a pH increase of 2, so the concentration must have been reduced by 100x. So the original volume plus the added water must equal 100x the original volume. The only one there is C because 10+990 is 1000, which is 100x 10. :D
For Q16, the answer is B because H3O+ has a coordinate bond between that extra hydrogen with oxygen's lone pair.
(https://i.imgur.com/32PTnPs.png) For these questions, I'd memorise the common compounds that have these bonds.
Post by: josephmalicdem on September 30, 2017, 07:52:16 pm
(https://i.imgur.com/bMozHCn.png)
From the previous question, you would have identified dichromate as the oxidising agent that undergoes reduction. The iron (II) undergoes oxidation. Using the data sheet values, you can find the Eo value. Remember that you do not multiply these data values according to stoichiometric moles, and use them as they are on the sheet. :)
For Q15, a pH increase of 1 indicates a tenfold decrease in H+ concentration, according to the pH formula. The question has a pH increase of 2, so the concentration must have been reduced by 100x. So the original volume plus the added water must equal 100x the original volume. The only one there is C because 10+990 is 1000, which is 100x 10. :D
For Q16, the answer is B because H3O+ has a coordinate bond between that extra hydrogen with oxygen's lone pair.
(https://i.imgur.com/32PTnPs.png) For these questions, I'd memorise the common compounds that have these bonds.
Thank you so much. Those are really great explanations. 👍🏻
Post by: justwannawish on September 30, 2017, 07:59:50 pm
Advantages of either, the types of questions, anything is good :)
Post by: MisterNeo on September 30, 2017, 08:18:42 pm
Can someone please compare industrial chemistry and shipwrecks and corrosion? Our teacher wants to do the latter but our school has usually done Industrial chem, and we have to vote on what to do before the break's over.I'm currently doing Industrial, and I have to say that it is pretty much a revision of the 3 core topics.
Advantages of either, the types of questions, anything is good :)
It's mainly equilibriums, maximising yields, electrochemistry (galvanic/electrolysis), environmental issues.
Common questions include:
-Calculate the K-constant of this equilibrium (very easy).
-What is the role of this chemical in this process.
-Draw the diagram of soap and explan why it cleans stuff.
-Outline a prac you did in school about this process.
-Assess the environmental risks of these two processes. (mini essay)
The first few questions are very easy, but the last few are quite time consuming because you need to write a mini essay and/or diagrams included. There is only every 1-2 calculation questions about "If X-moles of this is used, how much of Y is produced?" and "Calculate the equilibrium concentration of chemical Y if chemical X was 1M initially".
Industrial is the most common choice for option, and I'd recommend it since it's basically revision. :)
I'm not sure about Shipwrecks, so you may want to ask someone who does. (Jake does)
Post by: winstondarmawan on September 30, 2017, 08:35:53 pm
Can someone please compare industrial chemistry and shipwrecks and corrosion? Our teacher wants to do the latter but our school has usually done Industrial chem, and we have to vote on what to do before the break's over.Shipwrecks is basically a superstudy of the Redox topic in Preliminary.
Advantages of either, the types of questions, anything is good :)
- The earlier stuff is content dumping - no need to worry much about that.
- Electrolytic cells, basically the opposite of galvanic cells.
- Corrosion of metals, particularly iron. Just a redox reaction between iron and water+oxygen.
- Artefacts topic at the end is also memorisation and content dumping.
ALSO THERE IS A SHITLOAD OF EQUATIONS YOU NEED TO MEMORISE.
Overall, from what I've heard from peers, Industrial seems easier.
Post by: Arvacado on September 30, 2017, 09:28:39 pm
Can someone help me with this question: What is the process of AAS?
1.How does soap act as an emulsifier?
2.What is the Solvay process?
Thank you! ;D ;D
Mod Edit: Posts merged, use the 'Modify' post button to avoid chain posting :)
Post by: bsdfjnlkasn on September 30, 2017, 09:38:36 pm
Hiii!
Can someone help me with this question: What is the process of AAS?
Hey there!
So in addition to the method below, I recommend you memorise how to draw the diagram (it's not too difficult :) )
Process of AAS
1. Several standard solutions of the ion being tested for are prepared (i.e. solutions of which the concentration is known).
• Range of concentrations of the standard solutions must include the anticipated concentration of the sample being tested, this is important for when we graph the absorbance values
2. Sample of the element being tested for is burnt in a hollow cathode ray lamp. Its electrons are momentarily excited and then fall back to ground state, emitting a unique pattern of light frequencies.
3. One by one, the standard solutions are pumped into a nearby flame, so that they vaporise into a mist that the lamplight must passes through. Ions absorb the same frequencies of light that they are able to emit
• The more ions there are in the sample that match the burning element, the more light will be absorbed and the less intense the beam will become.
4. After passing through the flame, the light beam is focused by a lens, and a chosen wavelength of it is refracted through prism into the photomultiplier. (The light beam is restricted to one wavelength to improve accuracy).
5. The photomultiplier detects the light intensity, and uses this to calculate the absorbance of the sample (in units A).
The absorbances of the different standard solutions are plotted against their concentration, to form a calibration curve. This should be a straight increasing line and so its equation can be found as A = kc
6. Steps 3-5 are repeated using the sample being tested, to find an absorbance value.
7. The point with the given absorbance value is located on the graph, and its concentration co-ordinate is identified as the concentration of the sample.
1.How does soap act as an emulsifier?
2.What is the Solvay process?
Thank you! ;D ;D
Soap acts as an emulsifier (i.e. it is an emulsifying agent) because it has both non-polar and polar parts. Looking at the example of water and oil (and why when you add soap, the two can suddenly mix):
The non-polar tails of soap molecules gather around oil droplets, so that the negatively charged polar ends face outwards. This creates a negative charge on the droplet of oil which repels other negatively charged soap and oil droplets, preventing the formation of large clumps of oil. Now, water can interact with the oil i.e. they can mix.
Mod edit: Merged posts :)
Post by: MisterNeo on September 30, 2017, 11:10:46 pm
1.How does soap act as an emulsifier?To add onto bsdfjnlkasn's answer, soap acts as an emulsifying agent (emulsifier) by allowing two usually immiscible liquids to become miscible.
If that were a question in an exam, you must define what an emulsifying agent is, then address soap's structure like in the previous post :)
2.What is the Solvay process?The Solvay process is the industrial method of producing Na2CO3 from brine (NaCl) and limestone (CaCO3) with the use of ammonia.
Post by: justwannawish on October 01, 2017, 12:11:21 pm
Shipwrecks is basically a superstudy of the Redox topic in Preliminary.
- The earlier stuff is content dumping - no need to worry much about that.
- Electrolytic cells, basically the opposite of galvanic cells.
- Corrosion of metals, particularly iron. Just a redox reaction between iron and water+oxygen.
- Artefacts topic at the end is also memorisation and content dumping.
ALSO THERE IS A SHITLOAD OF EQUATIONS YOU NEED TO MEMORISE.
Overall, from what I've heard from peers, Industrial seems easier.
I'm currently doing Industrial, and I have to say that it is pretty much a revision of the 3 core topics.
It's mainly equilibriums, maximising yields, electrochemistry (galvanic/electrolysis), environmental issues.
Common questions include:
-Calculate the K-constant of this equilibrium (very easy).
-What is the role of this chemical in this process.
-Draw the diagram of soap and explan why it cleans stuff.
-Outline a prac you did in school about this process.
-Assess the environmental risks of these two processes. (mini essay)
The first few questions are very easy, but the last few are quite time consuming because you need to write a mini essay and/or diagrams included. There is only every 1-2 calculation questions about "If X-moles of this is used, how much of Y is produced?" and "Calculate the equilibrium concentration of chemical Y if chemical X was 1M initially".
Industrial is the most common choice for option, and I'd recommend it since it's basically revision. :)
I'm not sure about Shipwrecks, so you may want to ask someone who does. (Jake does)
Thank you both for your answers. I think I'm going to go for industrial, it seems like it's more related to the rest of the syllabus :)
Post by: anotherworld2b on October 01, 2017, 12:53:21 pm
Post by: J.B on October 01, 2017, 01:09:30 pm
For eg.
Hydration of ethene, is the water and ethanol gas or liquid?
Thank you
Post by: Wordswurth on October 01, 2017, 01:18:29 pm
For states in chemical reactions, are they for the chemical at standard conditions or state in the reaction?
For eg.
Hydration of ethene, is the water and ethanol gas or liquid?
Thank you
State in the reaction. Not standard condition I would say. I'd have thought water is liquid and ethene gas. Depends on the question.
Post by: MisterNeo on October 01, 2017, 01:33:27 pm
For states in chemical reactions, are they for the chemical at standard conditions or state in the reaction?It depends on the conditions in which the reaction takes place.
For eg.
Hydration of ethene, is the water and ethanol gas or liquid?
Thank you
For the hydration of ethene, it is done at 300oC. This means that the water and ethanol will be gases in the equation.
For combustion, it is usually done at room temperature. This means that the water product will be written as a liquid. (Combustion heat is a product, not a condition.) :)
Post by: anotherworld2b on October 01, 2017, 03:21:53 pm
Post by: J.B on October 01, 2017, 03:36:26 pm
It depends on the conditions in which the reaction takes place.
For the hydration of ethene, it is done at 300oC. This means that the water and ethanol will be gases in the equation.
For combustion, it is usually done at room temperature. This means that the water product will be written as a liquid. (Combustion heat is a product, not a condition.) :)
Ok, Thank you.
Also I was just wondering for the Industrial production of LDPE is the process called Free Radical polymerisation, and for the production of HDPE is the process called Zieglar-Natta Process?
Thank you
Post by: Shadowxo on October 01, 2017, 03:45:27 pm
How do you do this question 7?You know the initial mass was 8.2g including the water, and when the water was removed (resulting in anhydrous zinc sulphate) you ended up with 4.6g. So, the mass of water in the original sample was (8.2-4.6) = 3.6g
Mass ZnSO4 : Mass H2O
4.6 : 3.6
mol ZnSO4 : mol H2O
4.6/161 : 3.6/18
0.0286 : 0.2
1 : 0.2/0.0286
1 : 7
So x=7, as there's 7 H2O molecules for every ZnSO4 molecule
Post by: bsdfjnlkasn on October 01, 2017, 03:51:33 pm
Ok, Thank you.
Also I was just wondering for the Industrial production of LDPE is the process called Free Radical polymerisation, and for the production of HDPE is the process called Zieglar-Natta Process?
Thank you
Hey there!
This is what I have :)
LDPE: Free radical process
HDPE: Ziegler Natta Process (or Catalysis)
Post by: anotherworld2b on October 01, 2017, 07:09:04 pm
You know the initial mass was 8.2g including the water, and when the water was removed (resulting in anhydrous zinc sulphate) you ended up with 4.6g. So, the mass of water in the original sample was (8.2-4.6) = 3.6g
Mass ZnSO4 : Mass H2O
4.6 : 3.6
mol ZnSO4 : mol H2O
4.6/161 : 3.6/18
0.0286 : 0.2
1 : 0.2/0.0286
1 : 7
So x=7, as there's 7 H2O molecules for every ZnSO4 molecule
Thank you for your help :)
I was also working on this question but I got stuck on part a. This is what I have so far
Mod edit: Merged posts :)
Post by: MisterNeo on October 01, 2017, 08:39:53 pm
I was also working on this question but I got stuck on part a. This is what I have so farMake sure you write the formulas correctly. :)
For this question, you'd find moles of the ammonium h-phosphate produced and work backwards to find the calcium phosphate contained in the original 60kg.
(https://i.imgur.com/Y5eDLBb.jpg)
Post by: J.B on October 02, 2017, 02:08:43 pm
Thanks
Post by: MisterNeo on October 02, 2017, 03:09:00 pm
I was just wondering if membrane filters be used to reduce eutrophication?Reverse osmosis probably can prevent eutrophication because it can remove small molecules from water, such as nitrates and phosphates. :)
Thanks
Post by: J.B on October 02, 2017, 03:20:14 pm
Reverse osmosis probably can prevent eutrophication because it can remove small molecules from water, such as nitrates and phosphates. :)
Thank you :)
I also have a couple more questions:
I was also wondering what is the difference between Dissolved ions and TDS? As they are two separate things in the syllabus?
And in the industrial chemistry option my class did not do the Sulfuric acid oxidising practical. So I was just wondering what observations are made between the reaction of hot concentrated sulfuric acid and copper? And what exact concentration of Sulfuric acid is needed?
Also are these the correct states in the equation?
Copper Oxidises: Cu(s) -> Cu2+(aq) + 2e-
Sulfuric acid reduces: 2H2SO4 (aq) + 2e- -> SO2 (g) + 2H2O(l) + SO42-(aq)
Overall Reaction: Cu(s) + 2H2SO4 (aq) -> CuSO4 (aq) + SO2 (g) + 2H2O(l)
I have also read that Conc. Sulfuric Acid can oxidised only those below lead of standard potentials producing SO2, but then was told that Conc. Sulfuric Acid can oxidise those above lead as well producing SO2. I'm just wondering which one is correct?
And in a soap molecule, in water does the Na+ and Negative fatty acid chain separate (dissolve)?
Thanks
Post by: arunasva on October 02, 2017, 11:46:11 pm
Is corrosion faster in Neutral or Alkaline environment considering everything else is constant in the environment ? Ik acidic is the fastest.
Post by: MisterNeo on October 03, 2017, 01:25:58 am
Thank you :)Dissolved ions is exactly what the name suggests...cations and anions in water. Total dissolved solids, however, include everything dissolved in water that used to be a solid, such as soluble salts, fine dirt particles, sugar, cow shit, etc.
I also have a couple more questions:
I was also wondering what is the difference between Dissolved ions and TDS? As they are two separate things in the syllabus?
And in the industrial chemistry option my class did not do the Sulfuric acid oxidising practical. So I was just wondering what observations are made between the reaction of hot concentrated sulfuric acid and copper? And what exact concentration of Sulfuric acid is needed?
Also are these the correct states in the equation?
Copper Oxidises: Cu(s) -> Cu2+(aq) + 2e-
Sulfuric acid reduces: 2H2SO4 (aq) + 2e- -> SO2 (g) + 2H2O(l) + SO42-(aq)
Overall Reaction: Cu(s) + 2H2SO4 (aq) -> CuSO4 (aq) + SO2 (g) + 2H2O(l)
I have also read that Conc. Sulfuric Acid can oxidised only those below lead of standard potentials producing SO2, but then was told that Conc. Sulfuric Acid can oxidise those above lead as well producing SO2. I'm just wondering which one is correct?
And in a soap molecule, in water does the Na+ and Negative fatty acid chain separate (dissolve)?
Thanks
Observations from the prac would include the release of sulfur dioxide gas (bubbles), which you must prove was actually sulfur dioxide using moist litmus paper. The brown copper will become a bluish-white copper sulfate. (Anhydrous is white, pentahydrate is blue. The acid will dehydrate some of it.) The sulfuric acid is very concentrated at around ~98%.
Your equations must have sulfuric acid as a liquid when used on copper or to dehydrate something. Other than that, the rest looks fine. I'm not sure about the reduction table question since you wouldn't need to go into that much detail. But, I would say above lead towards potassium (I may be wrong).
Soaps do dissociate in solution to release the sodium/potassium ion. :)
Post by: J.B on October 03, 2017, 09:35:19 am
Dissolved ions is exactly what the name suggests...cations and anions in water. Total dissolved solids, however, include everything dissolved in water that used to be a solid, such as soluble salts, fine dirt particles, sugar, cow shit, etc.
Observations from the prac would include the release of sulfur dioxide gas (bubbles), which you must prove was actually sulfur dioxide using moist litmus paper. The brown copper will become a bluish-white copper sulfate. (Anhydrous is white, pentahydrate is blue. The acid will dehydrate some of it.) The sulfuric acid is very concentrated at around ~98%.
Your equations must have sulfuric acid as a liquid when used on copper or to dehydrate something. Other than that, the rest looks fine. I'm not sure about the reduction table question since you wouldn't need to go into that much detail. But, I would say above lead towards potassium (I may be wrong).
Soaps do dissociate in solution to release the sodium/potassium ion. :)
Thank you :)
And would the copper metal dissolve or does the copper sulfate become a solid?
Post by: MisterNeo on October 03, 2017, 09:46:43 am
The resulting copper sulfate would be aqueous because water is also produced and the conc. acid has 2% water in it. So the equation would have aqueous copper sulfate. ;)
Thank you :)
And would the copper metal dissolve or does the copper sulfate become a solid?
Post by: Aaron12038488 on October 03, 2017, 10:14:15 am
Post by: Natasha.97 on October 03, 2017, 10:28:53 am
I need help with the steps involved LDPE such as Initiation, Propagation, termination. I'm confused with diagrams to support the description of each step.
Hi!
(https://i.imgur.com/2s1aloZ.png)
Hope this helps
Post by: bluecookie on October 04, 2017, 01:10:09 am
[H+][OH-]=1.0*10^-14
Log10[H+][OH-]=log101*10^-14
Log10[H+][OH-]=-14
Log10H+ + log10OH- = -14
PH-log10OH-=14
PH=14+log10OH-
pH=-log10[H+]
POH = -log10[OH]
PH+pOH = 14
I think some elements of it are familiar from the pH/acids and bases module whatever it was called lol, but I'm confused about the conceptual understanding of it. Could someone explain the concept behind the first line? Why does the concentration of hydrogen and hydroxide ions multiply to 1*10^-14
Post by: Somedudelmaoy on October 04, 2017, 02:32:46 am
Okay, I have this scribbled in the side of my note taking book and I have no idea what it means:
[H+][OH-]=1.0*10^-14
Log10[H+][OH-]=log101*10^-14
Log10[H+][OH-]=-14
Log10H+ + log10OH- = -14
PH-log10OH-=14
PH=14+log10OH-
pH=-log10[H+]
POH = -log10[OH]
PH+pOH = 14
I think some elements of it are familiar from the pH/acids and bases module whatever it was called lol, but I'm confused about the conceptual understanding of it. Could someone explain the concept behind the first line? Why does the concentration of hydrogen and hydroxide ions multiply to 1*10^-14
At 25 degrees Celsius, the product of the concentration of hydronium and hydroxide ions equals 10^-14. Maybe there is a reason why but I think you just need to know that they do. This means that if given the concentration of hydroxide ions, you can find the concentration of hydronium ions by doing 10^-14/[OH-]. In reality, the only formula you need to know is pH=-log10[H+]. If given the H+ concentration and asked to find the pH, you use the formula. If given the OH- concentration you can do -log10[OH-] to get pOH and then minus it from 14 to get pH. Or you could do 10^-14/[OH-] to get H+ concentration and then do -log10[H+]. It's the same thing but just done in a different order.
Post by: justwannawish on October 04, 2017, 11:28:11 pm
Just wondering how do we do sig figs in the sciences? Is a number like 2800 considered to be in 2sf or 4?
Post by: MisterNeo on October 04, 2017, 11:31:19 pm
Hey, guys!
Just wondering how do we do sig figs in the sciences? Is a number like 2800 considered to be in 2sf or 4?
According to maths, 2800 is 2 sig figs. If it were 2800.0, that would be 5 sig figs. In chem, your answer should be written with the smallest amount of sig figs used in the question.
Post by: justwannawish on October 04, 2017, 11:41:52 pm
According to maths, 2800 is 2 sig figs. If it were 2800.0, that would be 4 sig figs. In chem, your answer should be written with the smallest amount of sig figs used in the question.
Thank you so much for such a quick answer! I think I get it now :)
A couple more questions: what's the relationship between the anode/cathode, oxidation/reduction, and activity of metals?
What I currently interpret it as is: the negative anode contains the oxidisation reaction, which is reduction agent, and is generally made out of the more active metals. At the anode, solid metal will be lost as it transforms into a metal ion. And the cathode is pretty much the opposite.
Is this correct? And is there anything else to know regarding anodes and their properties in redox experiments.
But I don't understand entirely the concept of galvanic cells, particularly how the salt bridge helps the electrode reaction. Can anyone please help by explaining the experiment once more? Thank you :)
Post by: Somedudelmaoy on October 04, 2017, 11:44:07 pm
According to maths, 2800 is 2 sig figs. If it were 2800.0, that would be 4 sig figs. In chem, your answer should be written with the smallest amount of sig figs used in the question.
would 2800.0 not be 5 sig figs?
Post by: MisterNeo on October 04, 2017, 11:48:13 pm
would 2800.0 not be 5 sig figs?Yes, sorry my bad.
2800. is 4, 2800.0 is 5
Post by: f_tan on October 05, 2017, 01:54:03 pm
(https://i.imgur.com/eFAk7gS.png)
Post by: Azurelies on October 05, 2017, 02:26:54 pm
I have no idea where to even start :(
Sorry for the blurry photo!
Post by: MisterNeo on October 05, 2017, 03:44:23 pm
Can anyone help explain this question please? Thank you!(https://i.imgur.com/KW2Yh17.jpg)
(https://i.imgur.com/eFAk7gS.png)
The absorption is proportionate to the concentration, so you can just divide 10ppm by 4 then multiply by 5 to get the absorbance for 0.500.
Parts per million is the same as mg/L, and you can find mol/L using copper molar mass. Then divide the mol/L by 10 to get the moles in 100mL, and it’s just stoichiometry from there. :)
Hi, can I have some help on this question please?(https://i.imgur.com/ST5FrKh.jpg)
I have no idea where to even start :(
Sorry for the blurry photo!
This is what I got, not sure if correct so answers would be nice. Since they give you the NaCl data, I’d assume it plays a role in the calculations because Na+ initial concentrations would be increased. You would find the initial concentrations, then work backwards with the K values to find the concentration of bicarbonate and carbonate. The given pH can be converted into H+ using 10^(-pH). :)
Post by: anotherworld2b on October 05, 2017, 06:28:47 pm
Also when you decrease volume wouldn't
Pressure increase? So every thing except NoBr decreases? (NoBr increases?)
I understand the principles but I get confused when there is a solid involved.
I got confused doing the second table how do you know there isn't effect for the reactants/products? I understand the reaction rates and I have put what the answers are in the tables
Post by: MisterNeo on October 05, 2017, 08:38:44 pm
I had some question about equilibrium questions. For the first table wouldn't [NoBr] increase when you remove NoBr?So for [NOBr], it has decreased its concentration compared to what it was prior to any changes. Whenever you remove some of a species, it will shift to recover it, but not all of it. The equilibrium shifts to minimise the change.
Also when you decrease volume wouldn't
Pressure increase? So every thing except NoBr decreases? (NoBr increases?)
I understand the principles but I get confused when there is a solid involved.
I got confused doing the second table how do you know there isn't effect for the reactants/products? I understand the reaction rates and I have put what the answers are in the tables
The volume decrease does increase pressure. Gas pressure is like the concentration of gas because you fit more particles into a given space. Same concept with the thing above, the concentration will increase but it won’t go back to its orginal concentration, as the equilibrium will minimise the change.
[H2O] is a pure liquid and is always equal to 1. :)
Post by: anotherworld2b on October 05, 2017, 09:48:00 pm
I was also wondering what would be the easiest and quickest way to identify the nature of a salt solution?
So for [NOBr], it has decreased its concentration compared to what it was prior to any changes. Whenever you remove some of a species, it will shift to recover it, but not all of it. The equilibrium shifts to minimise the change.
The volume decrease does increase pressure. Gas pressure is like the concentration of gas because you fit more particles into a given space. Same concept with the thing above, the concentration will increase but it won’t go back to its orginal concentration, as the equilibrium will minimise the change.
[H2O] is a pure liquid and is always equal to 1. :)
Post by: MisterNeo on October 05, 2017, 11:07:48 pm
I get confused when there is a solid involvements. An example is the second table. Do I treat as just a reactant\product?Solids and pure liquids are still treated as reactants and products because they shift like every other chemical species. But, the [X] brackets specifically measure the concentration of the species at equilibrium. In the second table, the water will be used up if the system shifts to the right, but the concentration of that water is unchanged because it remains constant, it's still 100% pure water but just less of it.
I was also wondering what would be the easiest and quickest way to identify the nature of a salt solution?
Aqueous and gaseous species do change in concentration, however.
When an aqueous species gets used up, the water stays there but the solute gets used up. This reduces its concentration. When a gas species gets used up, the volume remains unchanged but moles of gas are used up. This reduces its concentration too.
Hope this explanation is clear, it used to baffle me too. :-\
For the salt question, the easiest way to tell if a salt solution is acidic/neutral/basic is to look at the cation and anion of the salt.
Anions are derived from acids (Cl-, CH3COO-, etc), whereas cations are derived from bases (Na+, NH4+, etc). Strong-Strong, Weak-Weak make neutral salts, whereas Strong-Weak and Weak-Strong make basic and acidic salts respectively. The only thing you should know is which is a strong acid/base and which is a weak acid/base. :)
Post by: anotherworld2b on October 06, 2017, 12:09:10 am
Post by: winstondarmawan on October 06, 2017, 12:12:42 am
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22264927_1342875159171337_923228709_n.jpg?oh=e208756ef1b1436ff5f53d49a6168d71&oe=59D82712
Thanks in advance.
Post by: bsdfjnlkasn on October 06, 2017, 12:47:46 am
Would appreciate help with the following (MC 2011 HSC)
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22264927_1342875159171337_923228709_n.jpg?oh=e208756ef1b1436ff5f53d49a6168d71&oe=59D82712
Thanks in advance.
Hey there!
Remember when you did the bromine water experiment? This question is very similar, only this time it is for propane instead of (cyclo)hexane which you would have used for the experiment in class. The question is asking you to recall that when an alkene and bromine water are reacted, the alkene breaks down into a single bonded alkane with the two bromines from the bromine water (Br2) adding to the two new free electrons (which were before tied up in the double bond).
In the case of propene, it becomes propane allowing the bromine to add to any of the carbon atoms. So that leaves us with 1, 1 - dibromopropane or 1,2 - dibromopropane. The "di" indicates how many bromines there are. The answer is the latter one as the first one doesn't follow correct naming conventions. If the Br's were on the same carbon for instance, then you would have 1, - dibromopropane as that 1 specifies the number of the carbon atom to which the bromines are attached. If the bromines are attached to the first and third carbon, you would have to name the compound: 1, 3 - dibromopropane not 1, 1 - dibromopropane as you need to make sure you count from the carbon that you assign number one (you can't have two number ones). You always number the carbons so that you will have the lowest numbers out the front of the name.
Hopefully this is clear :)
Post by: MisterNeo on October 06, 2017, 01:17:57 am
I was also wondering if I can have help with electrolysis. I am not sure how you identify the possible reactions. Also how do know water is involved and which equation it is? ( there are numerous water equations)The first one is dilute NaCl, which is the same as electrolysis of water because the salt just acts as an electrolyte to conduct electricity.
The second one has chlorine gas, hydrogen gas, and sodium hydroxide products because the higher concentration of salt allows the chloride to be oxidised over water.
Would appreciate help with the following (MC 2011 HSC)Bromine water has bromine as Br2. When the bonds open up, there are two possible additions on the separate carbons. Refer to the attachment below. :)
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22264927_1342875159171337_923228709_n.jpg?oh=e208756ef1b1436ff5f53d49a6168d71&oe=59D82712
Thanks in advance.
Post by: arton123 on October 06, 2017, 09:07:15 am
Post by: mary123987 on October 06, 2017, 10:17:02 am
Post by: MisterNeo on October 06, 2017, 11:12:22 am
I just have a question for when combustion. When they ask you why combustion is monitored, are we allowed to use ethanol as the fuel in our equations? Most use octane, but ethanol is easier for me as I remember the molar ratio of the reactants and products, while with octane it is a bit harder. So will I be penalised if I use ethanol in the complete and incomplete combustion questions?I’m pretty sure you can use either, as long as you show why complete combustion is preferred over incomplete combustion. I used octane for both of mine. :)
Hey does anyone know like how to know the states in general for an equation ? or is it all dependent on memory?The states of an equation depends on the conditions in which the reaction takes place.
Hydration of ethylene occurs at 300’C, so the water and ethanol will be gases. Combustion occurs at room temperature, so the water will be liquid. (The heat is a product of combustion, not a condition)
Fermentation occurs in an aqueous solution of glucose, so the ethanol produced will also be aqueous. You can tell what state it is by the temperature or aqueous/non-aqueous condition of the reaction ;)
Post by: winstondarmawan on October 06, 2017, 11:50:31 am
Hey does anyone know like how to know the states in general for an equation ? or is it all dependent on memory?
Adding onto this, how would you know the states of various hydrocarbons?
And also would appreciate help with the following:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22291667_1343257242466462_1759583141_n.jpg?oh=3696c9107bddde65553435bbdcbb3c25&oe=59D90D9F
Thanks in advance!
Post by: MisterNeo on October 06, 2017, 01:20:35 pm
Adding onto this, how would you know the states of various hydrocarbons?For alkanes and alkenes, the 1-4 carbons are gases whereas the rest are liquids.
And also would appreciate help with the following:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22291667_1343257242466462_1759583141_n.jpg?oh=3696c9107bddde65553435bbdcbb3c25&oe=59D90D9F
Thanks in advance!
For the galvanic cell question, reaction with acid is being oxidised by the H+ ion.
You can tell by the table that Metal Q reacts with everything whereas Metal R doesn't. This means that Metal Q must be higher than the H2O + e- on the Data Sheet (around potassium) and wants to oxidise. Metal R would be below the H+ 0.00V value on the Data Sheet (around copper/silver) and wants to reduce. Since Q is on the top of the Data Sheet and R is on the bottom, there would be a big Eo value when they are set up in a galvanic cell. Hence, the answer would be C. :)
Post by: winstondarmawan on October 06, 2017, 02:27:28 pm
Post by: MisterNeo on October 06, 2017, 02:42:17 pm
For bromine water test equations, would it be better to use the reaction with Br2 or HOBr?HOBr is technically the correct method, but the HSC accepts the Br2 method as well. Personally, I’d use the Br2 because it’s easier. That previous HSC question from the bromine water + propene uses the Br2 method.
Post by: angelahchan on October 06, 2017, 03:09:09 pm
thanks
Post by: MisterNeo on October 06, 2017, 04:03:27 pm
Hi, could someone please help me with the question "explain why sodium hydroxide produced by the mercury cell is not contaminated with salt"The mercury cell features negatively charged liquid mercury as the cathode to reduce sodium ions to form an amalgam of Na/Hg. An amalgam is an alloy mixture of a metal and mercury, and NaCl is insoluble in mercury, which prevents contamination.
thanks
The liquid amalgam of sodium and mercury flows into a separate chamber in the cell where it is reacted with water to form sodium hydroxide where no NaCl is present.
Whereas, the other methods of NaOH production rely on a porous diaphragm/membrane to stop chloride from migrating but allow sodium ions to cross, which does not always stop contamination.
Post by: f_tan on October 06, 2017, 04:35:27 pm
Post by: MisterNeo on October 06, 2017, 05:06:27 pm
If sulphuric acid is a strong diprotic acid and citric is weak but triprotic, which one is the stronger acid?Sulfuric acid.
A strong acid is an acid that has its protonations (giving hydrogen ions) lying well to the right, whereas a weak acid lies well to the left and is denoted by an equilibrium arrow.
For example, you have 100 molecules of sulfuric acid and 100 molecules of citric acid in water.
Sulfuric acid will give out 99 of their first proton, whereas citric acid will only give out 3 because it is weak.
Post by: bridie_2345 on October 06, 2017, 06:22:16 pm
Post by: MisterNeo on October 06, 2017, 08:00:48 pm
Hey there I was wondering if this is how you represent rhthe ionisation of phosphoric acid? The answer in the back of the text book isn't clear. Thanks heaps! 😊Phosphoric acid is a weak acid, so the first two arrows should be an equilibrium like the third.
Make sure you write states on all the species and some of the hydronium are missing plus charges. Also on the second and third line, it reacts with water, not hydronium.
But it looks like you have a good understanding of acid protonation. ;D
Post by: mylinh-nguyen on October 06, 2017, 08:13:50 pm
Post by: MisterNeo on October 06, 2017, 08:57:20 pm
I don't get how to do this(v/v%) is equal to volume of ozone divided by volume of solution, then multiplied by 100. To calculate ppm, divide 0.000002% by 100 to get 0.00000002L per 1L of air. Multiply this by 1000000 to get the litres of ozone for every million litres of air, which is 0.02ppm. :)
Post by: mylinh-nguyen on October 06, 2017, 09:13:55 pm
(v/v%) is equal to volume of ozone divided by volume of solution, then multiplied by 100. To calculate ppm, divide 0.000002% by 100 to get 0.00000002L per 1L of air. Multiply this by 1000000 to get the litres of ozone for every million litres of air, which is 0.02ppm. :)thank you!
Post by: justwannawish on October 06, 2017, 10:16:31 pm
Are the main advantages it is renewable, biodegradable and can be made out of waste products of cellulose?
The disadvantages: expensive to manufacture, fuel inefficient, uses a lot of energy and land to produce in large quantities?
Post by: winstondarmawan on October 06, 2017, 11:12:34 pm
How should we structure an extended response about the use of ethanol to society?Start with your criteria, which is a restatement of the question in your own words.
Are the main advantages it is renewable, biodegradable and can be made out of waste products of cellulose?
The disadvantages: expensive to manufacture, fuel inefficient, uses a lot of energy and land to produce in large quantities?
Then you would start with advantages:
1.
2.
3.
etc. Then the disadvantages.
After that, a judgement is required. So basically here you will sum up your points and say that 'Due to the advantages and disadvantages discussed above, ethanol has high potential in benefiting society as a fuel source, however at the current stage the disadvantages outweigh the advantages in terms of completely replacing crude oil. However, it is still effective as a fuel extender thus minimising the strain on natural reserves of crude oil and with greater production efficiency in the future will definitely see a rise in usage.
Hope this helps. :)
Post by: winstondarmawan on October 07, 2017, 12:46:48 am
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22290341_1343710039087849_1169313217_n.jpg?oh=38c0aa9bb7f45af267b33b47963eceea&oe=59D96F7D
Can't find enough to write for 7 marks... would appreciate help with this.
Thanks in advance!
Post by: justwannawish on October 07, 2017, 08:16:10 pm
Start with your criteria, which is a restatement of the question in your own words.
Then you would start with advantages:
1.
2.
3.
etc. Then the disadvantages.
After that, a judgement is required. So basically here you will sum up your points and say that 'Due to the advantages and disadvantages discussed above, ethanol has high potential in benefiting society as a fuel source, however at the current stage the disadvantages outweigh the advantages in terms of completely replacing crude oil. However, it is still effective as a fuel extender thus minimising the strain on natural reserves of crude oil and with greater production efficiency in the future will definitely see a rise in usage.
Hope this helps. :)
Thanks :)
How many advantages/disadvantages do we need for a response for 5 marks vs something that's 7?
Post by: Natasha.97 on October 07, 2017, 08:21:57 pm
Thanks :)
How many advantages/disadvantages do we need for a response for 5 marks vs something that's 7?
Hi!
Not 100% sure on this but I would allocate 1 mark for the judgement, so that leaves 4 marks for the first scenario (2 advantages, 2 disadvantages) and 6 marks for the second scenario (3 advantages, 3 disadvantages) :)
Post by: justwannawish on October 07, 2017, 08:28:28 pm
Hi!
Not 100% sure on this but I would allocate 1 mark for the judgement, so that leaves 4 marks for the first scenario (2 advantages, 2 disadvantages) and 6 marks for the second scenario (3 advantages, 3 disadvantages) :)
Ooh, thank you for such a quick reply! I always end up giving the same amount of reasons for 5-8 mark questions because I don't want to lose marks on not writing down things I did know because I underestimated the question's worth!
Post by: Henry Kocatekin on October 08, 2017, 11:26:10 am
And the same for 2 markers of the same type.
Post by: justwannawish on October 08, 2017, 11:50:48 am
For 1 Mark chemical equations, do you need the states?
And the same for 2 markers of the same type.
Not sure but include it just in case. You don't want to be marked down by such a simple thing
Post by: MisterNeo on October 08, 2017, 12:15:41 pm
For 1 Mark chemical equations, do you need the states?Always have states in all your equations. I lost quite a few marks for this in Prelims. The only species that don’t need states are electrons (in galvanic questions) and free radicals (in ozone and polymerisation).
And the same for 2 markers of the same type.
Post by: bdobrin on October 08, 2017, 12:34:54 pm
I just went to your chemistry HSC lecture last week - i thought it was great.
Im just a little confused on what is a polymer, monomer and a radical.
Thanks,
Ben
Post by: Aaron12038488 on October 08, 2017, 01:16:06 pm
Hi jake,Monomer: A relatively simple molecule from which a polymer can be made.
I just went to your chemistry HSC lecture last week - i thought it was great.
Im just a little confused on what is a polymer, monomer and a radical.
Thanks,
Ben
Polymer: A large molecule in which one or more monomers are repeated.
a radical (more precisely, a free radical) is an atom, molecule, or ion that has an unpaired valence electron.
Post by: MisterNeo on October 08, 2017, 01:39:48 pm
Im just a little confused on what is a polymer, monomer and a radical.
A monomer is the repeating unit of a polymer. A polymer is a really long chain made of repeating monomers.
(http://www.davis.com/assets/techinfo/images/Zeus_Chem_Resistance_img_2.jpg)
A free radical is an atom that has an unpaired electron. For example, carbon will always want to have 4 bonds with other atoms because of the octet rule (it thinks it has 8 electrons). If carbon had only 3 bonds, then that molecule will be a free radical and is very reactive, as carbon will want get that bond to obtain stability. It is often written in chemical equations with a dot after it, like: CH3•
Post by: roygbivmagic on October 09, 2017, 11:27:35 am
cellulose -> acid hydrolysis -> glucose -> fermentation -> ethanol
Or are you supposed to talk about the whole sugar cane process? (i.e. the industrial production of ethanol from sugar cane dot point?)
And in general, when are you supposed to talk about the sugar cane process of forming ethanol? Is it only if the question specifically asks for 'sugar cane'?
Thanks!
Post by: MisterNeo on October 09, 2017, 12:30:36 pm
Hi! If a question asks "describe the formation of ethanol for fuel from biomass", can you just answer it by going through the steps:
cellulose -> acid hydrolysis -> glucose -> fermentation -> ethanol
Or are you supposed to talk about the whole sugar cane process? (i.e. the industrial production of ethanol from sugar cane dot point?)
And in general, when are you supposed to talk about the sugar cane process of forming ethanol? Is it only if the question specifically asks for 'sugar cane'?
Thanks!
hey :D
I would mention the sugar cane process in the extraction of cellulose step because the question has the key word "biomass", so you must specifically address how it can be obtained from biomass by providing examples. There are a wide range of plants that produce cellulose (corn, sugar canes, rice, etc), and by mentioning sugar cane specifically, it shows the marker that you know your stuff.
If the question asks about the production of ethanol from biomass or its suitability as an alternative to fossil fuels, then I would definitely mention the sugar cane process.
Post by: Azurelies on October 09, 2017, 12:50:46 pm
(https://i.imgur.com/KW2Yh17.jpg)
The absorption is proportionate to the concentration, so you can just divide 10ppm by 4 then multiply by 5 to get the absorbance for 0.500.
Parts per million is the same as mg/L, and you can find mol/L using copper molar mass. Then divide the mol/L by 10 to get the moles in 100mL, and it’s just stoichiometry from there. :)(https://i.imgur.com/ST5FrKh.jpg)
This is what I got, not sure if correct so answers would be nice. Since they give you the NaCl data, I’d assume it plays a role in the calculations because Na+ initial concentrations would be increased. You would find the initial concentrations, then work backwards with the K values to find the concentration of bicarbonate and carbonate. The given pH can be converted into H+ using 10^(-pH). :)
Thank you!
Post by: itssona on October 09, 2017, 06:04:31 pm
kinda confused; so basically we use cracking to get ethylene? but in some places it says that we crack to get petrol but it can't just be petrol...
can we form petrol from ethylene? which fractions do we crack? I know we cracked heavy molecules but which fractions of crude oil??
thank you :)
Post by: Natasha.97 on October 09, 2017, 06:21:26 pm
heeya
kinda confused; so basically we use cracking to get ethylene? but in some places it says that we crack to get petrol but it can't just be petrol...
can we form petrol from ethylene? which fractions do we crack? I know we cracked heavy molecules but which fractions of crude oil??
thank you :)
Hi!
Yes, cracking is process used to get ethylene. Fractionally distillation of petroleum results in fractions such as Refinery gas, gasoline, naptha, LPG, and heavy fuel oils. Most of these fractions aren't in as high demand as ethylene, so cracking (steam/thermal and catalytic) of some of the fractions (namely naptha and LPG) would produce ethylene.
Hope this helps
Post by: itssona on October 09, 2017, 06:27:23 pm
Hi!ohh that makes sense, thank you so much!!!
Yes, cracking is process used to get ethylene. Fractionally distillation of petroleum results in fractions such as Refinery gas, gasoline, naptha, LPG, and heavy fuel oils. Most of these fractions aren't in as high demand as ethylene, so cracking (steam/thermal and catalytic) of some of the fractions (namely naptha and LPG) would produce ethylene.
Hope this helps
Post by: Iminschool on October 09, 2017, 10:07:54 pm
Thanks
Post by: itssona on October 09, 2017, 10:16:37 pm
so do I draw butene and a double bond in the middle and then i add CH3 to the second and third carbon? can someone show a pic please
Post by: Natasha.97 on October 09, 2017, 10:24:56 pm
Hey guys, could somebody please share their best tree diagram for the identification of cations and anions.
Thanks
This is how I remember them :)
(https://i.imgur.com/OvVxZtP.png)
heey if i wanna draw 2 3 dimethyl 2 butene
so do I draw butene and a double bond in the middle and then i add CH3 to the second and third carbon? can someone show a pic please
Yep! Here is a pic:
(https://i.imgur.com/9w3C8Gg.png)
Hope this helps
Post by: winstondarmawan on October 09, 2017, 11:10:36 pm
Would appreciate help with the MC questions:
2. https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22396488_1346167522175434_333891129_o.png?oh=f2bf961b52984c47ee8bb3d9cda05a4a&oe=59DD9713
4. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22385082_1346167608842092_1024465893_n.png?oh=b619ab38e782503b5f93464ac538d3ec&oe=59DE027C
18. https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22396679_1346168335508686_1796698340_o.png?oh=f9b1e9dff4bac82883afc9bc7fea8200&oe=59DDA77F
Thanks in advance!
Post by: Iminschool on October 10, 2017, 12:09:10 pm
Hello!For Q.2, the answer is D as all those substances are acidic
Would appreciate help with the MC questions:
2. https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22396488_1346167522175434_333891129_o.png?oh=f2bf961b52984c47ee8bb3d9cda05a4a&oe=59DD9713
4. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22385082_1346167608842092_1024465893_n.png?oh=b619ab38e782503b5f93464ac538d3ec&oe=59DE027C
18. https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22396679_1346168335508686_1796698340_o.png?oh=f9b1e9dff4bac82883afc9bc7fea8200&oe=59DDA77F
Thanks in advance!
For Q.4 the answer is D as it is unsaturated
Mod edit: Merged posts, please use the “Modify” button at the top right to avoid chain posting :)
Post by: Savas_P on October 10, 2017, 10:56:27 pm
Hello!
Would appreciate help with the MC questions:
2. https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22396488_1346167522175434_333891129_o.png?oh=f2bf961b52984c47ee8bb3d9cda05a4a&oe=59DD9713
4. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22385082_1346167608842092_1024465893_n.png?oh=b619ab38e782503b5f93464ac538d3ec&oe=59DE027C
18. https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22396679_1346168335508686_1796698340_o.png?oh=f9b1e9dff4bac82883afc9bc7fea8200&oe=59DDA77F
Thanks in advance!
for question 18 have no idea but I am guessing you would add HCl to make the conditions acidic and shift it to the right.
Post by: Natasha.97 on October 10, 2017, 11:22:23 pm
Hello!
18. https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22396679_1346168335508686_1796698340_o.png?oh=f9b1e9dff4bac82883afc9bc7fea8200&oe=59DDA77F
Thanks in advance!
Hi!
The answer would be D (HCl and H2O):
- The addition of HCl, as Savas_P pointed out, would make the conditions acidic. According to LCP, the reaction would shift to the right to favour the forward reaction (and the formation of HOCl) as the OH- will neutralise the HCl
- The addition of H2O (one of the reactants) will also favour the formation of HOCl: The equilibrium will shift to minimise the change, which in this case, will favour the forward reaction ('uses up' the additional water)
Hope this helps
Post by: bsdfjnlkasn on October 11, 2017, 11:40:43 am
Can I get some help with this question? :D
(https://scontent-syd2-1.xx.fbcdn.net/v/t1.0-9/22365603_857579451067777_6961522703438222238_n.jpg?oh=3cbfaae54a921abe6285432de77af504&oe=5A7365FC)
Post by: winstondarmawan on October 11, 2017, 12:25:01 pm
Hi!The answer provided was B. My friend's explanation was that the solution is already in a swimming pool so adding water would have basically no effect. How relevant is this explanation?
The answer would be D (HCl and H2O):
- The addition of HCl, as Savas_P pointed out, would make the conditions acidic. According to LCP, the reaction would shift to the right to favour the forward reaction (and the formation of HOCl) as the OH- will neutralise the HCl
- The addition of H2O (one of the reactants) will also favour the formation of HOCl: The equilibrium will shift to minimise the change, which in this case, will favour the forward reaction ('uses up' the additional water)
Hope this helps
Post by: MisterNeo on October 11, 2017, 12:42:40 pm
The answer provided was B. My friend's explanation was that the solution is already in a swimming pool so adding water would have basically no effect. How relevant is this explanation?From my understanding, equilibriums will only shift if the concentration of any of the species changes. Adding water doesn't shift the equilibrium because water is in liquid state, which is pure and cannot change concentration (1L of water is the same conc. as 1000L of water). However, by adding water, the other aqueous species will decrease in concentration. But since both sides have aqueous species, the concentrations of both sides will decrease in concentration, maintaining that special ratio of products over reactants.
Hey there!None of them would be correct, unless there was a typo where pH 2 was supposed to be pH 3. In that case, the answer would be B since a weak acid would need much higher concentration to obtain the same pH as a strong acid.
Can I get some help with this question? :D
(https://scontent-syd2-1.xx.fbcdn.net/v/t1.0-9/22365603_857579451067777_6961522703438222238_n.jpg?oh=3cbfaae54a921abe6285432de77af504&oe=5A7365FC)
Post by: winstondarmawan on October 11, 2017, 01:12:39 pm
From my understanding, equilibriums will only shift if the concentration of any of the species changes. Adding water doesn't shift the equilibrium because water is in liquid state, which is pure and cannot change concentration (1L of water is the same conc. as 1000L of water). However, by adding water, the other aqueous species will decrease in concentration. But since both sides have aqueous species, the concentrations of both sides will decrease in concentration, maintaining that special ratio of products over reactants.
So just confirming, the answer is indeed B?
Post by: itssona on October 11, 2017, 04:23:40 pm
Post by: Savas_P on October 11, 2017, 05:02:31 pm
heyy so I'm kinda lost rn, isn't steam and thermal cracking the same thing? we learnt that they are different and that we need to know ALL 3??? :/ wjat should i know
yeah thermal cracking is also called steam cracking
Post by: itssona on October 11, 2017, 05:21:22 pm
yeah thermal cracking is also called steam cracking
Ah okay thank you!!
Post by: limtou on October 11, 2017, 06:16:56 pm
Post by: MisterNeo on October 11, 2017, 06:50:46 pm
Can someone please explain this? Thanks!
Hey :)
So the answer is A, citric acid. This is because weak acids form a buffer solution that minimises the changes in pH when sodium hydroxide is added. Out of the two weak acids, citric acid is triprotic and would require more base than acetic acid to use up all the H+.
Post by: limtou on October 11, 2017, 07:30:12 pm
Hey :)
So the answer is A, citric acid. This is because weak acids form a buffer solution that minimises the changes in pH when sodium hydroxide is added. Out of the two weak acids, citric acid is triprotic and would require more base than acetic acid to use up all the H+.
Thanks! Though can you clarify what you meant by buffer solution?
Post by: Savas_P on October 11, 2017, 09:45:36 pm
Can someone please explain this? Thanks!
this question is easy because the acids are the same concentration and volume. also it doesn't matter if they are weak or strong, the NaOH will completely ionise all the solutions regardless. this means we just have to look at how many Hydrogen ions there is in the acid solutions. the HCl and acetic acid requires equal amount of NaOH, the sulphuric acid because diprotic requires twice as much and the citric acid will require three times as much NaOH.
Post by: anotherworld2b on October 11, 2017, 11:45:37 pm
Post by: clarence.harre on October 12, 2017, 06:31:11 am
Could someone please explain how to do this?
Post by: MisterNeo on October 12, 2017, 08:23:27 am
I've attached an extract from the 2009 past paper. I honestly had no idea what to do when I encountered this question.You’d start by finding the amount of oxygen dissolved in 1L at 25’C (8mg), then multiply it by 10 to get amount in 10L (80mg). Divide this by 1000 to get the mass of dissolved oxygen in grams (0.08g). Divide 0.08g by 32g/mol to get the moles of oxygen dissolved (0.0025mol). Multiply this by 24.79L/mol to get the volume of the oxygen that can dissolve (0.062L), which is A (62mL). :)
Could someone please explain how to do this?
Post by: J.B on October 12, 2017, 12:33:01 pm
In industrial Chemistry for the head of an anionic detergent, is it sulfate or sulfanoate? and how do these look structurally?
Also for esterification my school used a bunsen burner, which I understand we should not use for safety reasons. I was just wondering when drawing a diagram for this prac, how do we draw the heating mantle? Like does it also need a water bath, and boiling chips, and a retort stand etc?
Thanks.
Post by: MisterNeo on October 12, 2017, 01:48:17 pm
HI,In most textbooks, they have sulfonate heads. Sulfates have 4 oxygen, and the alkyl chain is bonded to an oxygen instead of the sulfur. I would use the sulfonate or benzyl sulfonate in the HSC.
In industrial Chemistry for the head of an anionic detergent, is it sulfate or sulfanoate? and how do these look structurally?
Also for esterification my school used a bunsen burner, which I understand we should not use for safety reasons. I was just wondering when drawing a diagram for this prac, how do we draw the heating mantle? Like does it also need a water bath, and boiling chips, and a retort stand etc?
Thanks.
(https://upload.wikimedia.org/wikipedia/commons/thumb/4/46/Sulfonate.png/220px-Sulfonate.png)(http://www.quirkyscience.com/wp-content/uploads/2012/04/Sodium-lauryl-sulfate.jpg)
When drawing the esterification reflux setup:
-If drawn with Bunsen burner, must have a water bath (safety).
-If drawn with a heating mantle, doesn't need a water bath (it has a curved base).
I've always drawn it with a Bunsen + water bath. Bunsens must not be used against the glass directly in case it cracks. it's acceptable to use a water bath with Bunsens. The retort stand holds it upright in the water bath. It also needs boiling chips.
Post by: J.B on October 12, 2017, 07:25:08 pm
In most textbooks, they have sulfonate heads. Sulfates have 4 oxygen, and the alkyl chain is bonded to an oxygen instead of the sulfur. I would use the sulfonate or benzyl sulfonate in the HSC.
(https://upload.wikimedia.org/wikipedia/commons/thumb/4/46/Sulfonate.png/220px-Sulfonate.png)(http://www.quirkyscience.com/wp-content/uploads/2012/04/Sodium-lauryl-sulfate.jpg)
When drawing the esterification reflux setup:
-If drawn with Bunsen burner, must have a water bath (safety).
-If drawn with a heating mantle, doesn't need a water bath (it has a curved base).
I've always drawn it with a Bunsen + water bath. Bunsens must not be used against the glass directly in case it cracks. it's acceptable to use a water bath with Bunsens. The retort stand holds it upright in the water bath. It also needs boiling chips.
Thank you :)
Post by: Mathew587 on October 12, 2017, 08:54:31 pm
Thank you :)you also need to draw and specify boiling chips
Post by: Iminschool on October 12, 2017, 09:29:53 pm
Post by: Anjally on October 12, 2017, 09:49:36 pm
Post by: Savas_P on October 12, 2017, 09:56:41 pm
Hi why does the dehydration process of ethene need a catalyst?
It needs a powerful acid catalyst (sulphuric acid) to break off the C-OH and C-H bonds on the ethanol molecules.
Post by: Iminschool on October 12, 2017, 09:57:02 pm
Hi why does the dehydration process of ethene need a catalyst?Btw its the dehydration of ethanol to produce ethene.
The dehydration of ethanol is the chemical process where the water molecule is removed from the ethanol. A strong catalyst in sulfuric acid is required as the hydroxyl functional group (OH-) is bonded strongly to the (CH3CH2) chain. Therefore the sulfuric acid is used to lower the activation energy as it provides an alternative pathway for the reaction to occur.
Post by: Natasha.97 on October 12, 2017, 10:00:15 pm
Hi why does the dehydration process of ethene need a catalyst?
Hi!
A catalyst (namely concentrated H2SO4) is used to lower the activation energy of the reaction. As the forward reaction is exothermic, the temperature cannot be too high (Le Chatelier's Principle: the reverse reaction will be favoured), otherwise the overall yield is reduced. The catalyst also acts as a 'dehydrating agent' by absorbing water (formed alongside ethene), the forward reaction will be favoured, promoting the formation of ethene.
Hope this helps
Brown: Covered in Module 2 (The Acidic Environment)
Post by: MisterNeo on October 12, 2017, 10:13:12 pm
For Industrial Chem, does the equilibrium constant (K) have an arbitrary unit? Or is it mol/L?They don't have units because it's a ratio of products to reactants. :)
Post by: Iminschool on October 12, 2017, 10:22:27 pm
They don't have units because it's a ratio of products to reactants. :)I thought so, for some reason Success One had mol/L in one of their solutions
Post by: necromantic on October 13, 2017, 09:15:27 am
in the cathode reaction for a Lead Acid Battery, the valency of Lead changes from 4+ to 2+. How/why does this happen?
Post by: Natasha.97 on October 13, 2017, 09:31:15 am
Hi,
in the cathode reaction for a Lead Acid Battery, the valency of Lead changes from 4+ to 2+. How/why does this happen?
Hi!
The total oxidation number of a compound is 0. In this case, the oxidation number of O2 is -4 , so to balance it out, the oxidation state of Pb = +4.
In the compound PbSO4, SO4 has a charge of -2 (derived from the formula H2SO4), so the oxidation state of Pb = +2
Hope this helps
Post by: Dragomistress on October 14, 2017, 01:52:17 pm
2Al(s) + 6HCL(aq) --> 3AlCl3(aq) + 3H2(g)
Post by: Natasha.97 on October 14, 2017, 02:06:05 pm
What is the net ionic equation for
2Al(s) + 6HCL(aq) --> 3AlCl3(aq) + 3H2(g)
Hi!
Balanced equation: 2Al(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2(g)
Next, write it out in terms of its constituent ions:
2Al(s) + 6H+(aq) + 6Cl-(aq) → 2Al3+(aq) + 6Cl-(aq) + 3H2(g)
Cancel out the spectator ions (in this case, Chlorine has not been changed). This results in:
2Al(s) + 6H+(aq) → 2Al3+(aq) +3H2(g)
Hope this helps
Post by: mxrylyn on October 15, 2017, 10:19:55 am
So i'm taking notes on halogenation and the addition of halogens to ethylene, and in the formula (ethylene + bromine ----> 1,2-dibromoethane) , I don't know what the "1,2" in the dibromoethane means or is referring to.
Post by: MisterNeo on October 15, 2017, 10:25:56 am
Hey,The “1,2-“ refers to which carbon the bromine is located on the ethane molecule.
So i'm taking notes on halogenation and the addition of halogens to ethylene, and in the formula (ethylene + bromine ----> 1,2-dibromoethane) , I don't know what the "1,2" in the dibromoethane means or is referring to.
(https://upload.wikimedia.org/wikipedia/commons/4/42/1,2-Dibromoethane2.svg)
One of the bromine is one the 1st carbon, whereas the other is on the 2nd, hence 1,2-. :)
Post by: skullcandy on October 17, 2017, 11:09:30 pm
(http://i66.tinypic.com/23ix7oj.png)
Post by: kylesara on October 18, 2017, 11:08:15 am
Post by: MisterNeo on October 18, 2017, 12:35:13 pm
Hey guys... two simple questions apparently, im having a hard time doing...For the first question, 9% of the ONCl dissociates in a 1L vessel, so the equilibrium concentration is 0.91M. The change in concentration is 0.09M. You would construct a RICE table to show these changes. The ratio of ONCl to NO is 2:2, so NO would be 0.09M at equilibrium. The ratio of ONCl to Cl2 is 2:1, so Cl2 would be 0.045M at equilibrium. Then, just sub these values into the K formula.
(http://i66.tinypic.com/23ix7oj.png)
The second question is probably missing something because you can't really get anything from that. Unless, it is a trick question where the concentrations of both gases are 0.01mol/L in the 1L vessel.
Hi I have a question for PE but that forums dead so ill post here i guess its similar. Are you allowed to answer an option that the school doesn't run? ThanksI believe you can do it in the HSC, but not in Internals.
Post by: skullcandy on October 18, 2017, 08:31:35 pm
For the first question, 9% of the ONCl dissociates in a 1L vessel, so the equilibrium concentration is 0.91M. The change in concentration is 0.09M. You would construct a RICE table to show these changes. The ratio of ONCl to NO is 2:2, so NO would be 0.09M at equilibrium. The ratio of ONCl to Cl2 is 2:1, so Cl2 would be 0.045M at equilibrium. Then, just sub these values into the K formula.
The second question is probably missing something because you can't really get anything from that. Unless, it is a trick question where the concentrations of both gases are 0.01mol/L in the 1L vessel.I believe you can do it in the HSC, but not in Internals.
Sorry did you get 24 as your answer?
Post by: MisterNeo on October 19, 2017, 08:18:22 am
Sorry did you get 24 as your answer?
Post by: winstondarmawan on October 19, 2017, 07:44:15 pm
What are some of the impacts on society and environment of the dry cell and silver oxide cell?
And also, what exactly are we required to know for recently discovered transuranic elements?
TIA.
Post by: roygbivmagic on October 19, 2017, 08:53:28 pm
For example:
Pb + 2AgNO3 -> Pb(NO3)2 + 2Ag+
0.1 mol Pb and 0.1 mol AgNO3 react. How much Pb is left over in the final solution?
Thanks!
Post by: Victoria-Lee.Inthavong on October 19, 2017, 09:05:30 pm
This is a very basic question, and quite frankly I'm embarrassed that I'm getting so confused on such a minor detail but- in a galvanic cell, what causes the flow of electrons from the anode to the cathode? I understand that the less active metal ions will 'attract' the electrons from the neutral atoms in the anode- but how? And why? They aren't physically connected. Does this mean this attraction will only work within a certain distance?
How do i accept that there is 'just' a force of attraction between the less active metal ions and the electrons from a metal solid that is not physically connected with it?
Please don't burn me for being this dumb :(
Post by: MisterNeo on October 20, 2017, 01:13:09 am
Hello!For silver oxide cells, they are non-rechargeable and must be discarded, which can cause potential harm to the environment because the KOH electrolyte is caustic and can leak. Silver is also quite expensive, so the batteries are somewhat expensive. But their small size has allowed them to be used in small appliances like toys and remotes. Dry cells are pretty much the same advantages and disadvantages (small size, non-rechargeable, can leak, etc).
What are some of the impacts on society and environment of the dry cell and silver oxide cell?
And also, what exactly are we required to know for recently discovered transuranic elements?
TIA.
I believe that for the transuranic elements, all you need is its characteristics (how it is made, half life, discovery date, emission). You just need to know the basics about it.
For example:So, Pb is in excess and AgNO3 is limiting. To find amount of excess remaining, you get the number of moles of the limiting reagent (0.1mol) and convert that into the moles of Pb reacted (0.05mol) using stoichiometry. Then subtract this 0.05mol from the 0.1mol of the initial Pb because that is how much reacted away. So the excess Pb would be 0.05mol. Basically, you convert limiting into excess that has been used, then subtract that from excess. :)
Pb + 2AgNO3 -> Pb(NO3)2 + 2Ag+
0.1 mol Pb and 0.1 mol AgNO3 react. How much Pb is left over in the final solution?
Hey guys! I'm a new user so I hope this question goes right :-[Hey, welcome to the AN forums! ;)
This is a very basic question, and quite frankly I'm embarrassed that I'm getting so confused on such a minor detail but- in a galvanic cell, what causes the flow of electrons from the anode to the cathode? I understand that the less active metal ions will 'attract' the electrons from the neutral atoms in the anode- but how? And why? They aren't physically connected. Does this mean this attraction will only work within a certain distance?
How do i accept that there is 'just' a force of attraction between the less active metal ions and the electrons from a metal solid that is not physically connected with it?
When a less active metal is connected to a more active metal, in a way that allows electron flow, the less active metal's ions will want to be reduced (gain electrons), which has a positive reduction potential on the Data Sheet.
The more active metal will want to oxidise (lose electrons), which has also has a positive oxidation potential on the Data Sheet (flip the reactants/products around). A positive potential indicates spontaneous reaction where the chemical species will want to move to a lower energy state, hence the electrons move to the cathode. In the HSC, you’d just have to accept that electrons flow from anode to cathode because oxidation (at the anode) loses electrons to reduction (at the cathode).
ANODE: Mg(s) —> Mg2+ + 2e-
CATHODE: Cu2+ + 2e- —> Cu(s)
Hope this helps :D
Post by: roygbivmagic on October 20, 2017, 09:44:44 am
Just another quick question - how can you tell whether a reaction occurs spontaneously?
From HSC chem 2013 q29:
"Consider this chemical equation. 2Cl- (aq) + Br2 (l) -> 2Br- (aq) + Cl2 (g). Will the reaction occur spontaneously? Justify your response."
Post by: MisterNeo on October 20, 2017, 10:15:27 am
Thank you so much, I finally understand! :)A spontaneous redox reaction occurs when the total Eo value is positive. You find these on the Data Sheet.
Just another quick question - how can you tell whether a reaction occurs spontaneously?
From HSC chem 2013 q29:
"Consider this chemical equation. 2Cl- (aq) + Br2 (l) -> 2Br- (aq) + Cl2 (g). Will the reaction occur spontaneously? Justify your response."
Since it is negative, the reaction is not spontaneous, and would require electrical energy to drive the reaction (electrolysis). :)
Post by: goodsj on October 20, 2017, 10:21:58 am
Q26 Petroleum and sugar cane are both raw materials used for the production of ethanol.
Construct seperate flow diagrams for the production of ethanol from each raw material
I'm a bit confused on what exactly needs to go in each flow chart and each resource of information I look at has a different flow chart. How complex or how simple does the answer have to be?
Post by: Graceful on October 20, 2017, 12:02:51 pm
My scientist for 4.1.2 (in chemical managing) is Richard Payne who artificially develops drugs and vaccines out of proteins and peptides. The website said his technology builds them 'one amino acid at a time'. What chemical principles would this use? He's technically a physical chemist but I'm having trouble finding principles which apply to him.
Thanks
Post by: angelahchan on October 20, 2017, 02:37:26 pm
Hey so I came across this question in the Hsc 2012 exam
Q26 Petroleum and sugar cane are both raw materials used for the production of ethanol.
Construct seperate flow diagrams for the production of ethanol from each raw material
I'm a bit confused on what exactly needs to go in each flow chart and each resource of information I look at has a different flow chart. How complex or how simple does the answer have to be?
here's a flowchart from the success one hsc chem book:
sorry if you've already seen this flowchart before when you mentioned you've looked at different resources, but I think this flowchart covers everything pretty well
According to marker's comments: flowchart should be clear and comprehensive, showing processes and conditions. Flowcharts showed a number of steps, placed in the correct sequence, identified physical and chemical processes, including appropriate catalysts
[img width=2000 height=1500]http://i.imgur.com/WQDGv6a.jpg[/img]
Post by: goodsj on October 20, 2017, 04:48:32 pm
here's a flowchart from the success one hsc chem book:
sorry if you've already seen this flowchart before when you mentioned you've looked at different resources, but I think this flowchart covers everything pretty well
According to marker's comments: flowchart should be clear and comprehensive, showing processes and conditions. Flowcharts showed a number of steps, placed in the correct sequence, identified physical and chemical processes, including appropriate catalysts
[img width=2000 height=1500]http://i.imgur.com/WQDGv6a.jpg[/img]
Thankyou!
Post by: skullcandy on October 20, 2017, 09:55:59 pm
Could any of you help me answer these two questions:
(http://i65.tinypic.com/fd4x04.png)
For question 9, I believe the atoms may tend to cool as the H20 would reach equilibrium
Thanks <3
Post by: JuliaPascale123 on October 20, 2017, 10:02:34 pm
(http://i65.tinypic.com/14nk6qu.png)
Post by: kemi on October 20, 2017, 10:29:09 pm
I feel retarted as I cant answer this..
(http://i65.tinypic.com/14nk6qu.png)
Given the mass, you can find the no. of moles of K2O:
n = m/M
n(K2O) = 100/[2(39.10)+16]
= 1.061571125...
But the no. of moles of potassium is actually twice the number of moles of potassum oxide (two potassium ions per molecule)
Hence,
n(K) = 2 x 1.061571125...
= 2.123142251...
Now we apply the formula
n = N/NA, where N is the number of particles, and NA is Avogadro's constant (6.022 x 10^23)
Rearrange:
N = n x NA
N(K) = 2.123142251... x (6.022 x 10^23)
= 1.278556263... x 10^24
= 1.28 x 10^24 (3.s.f.)
Therefore, there are 1.28 x 10^24 potassium ions in the sample.
Post by: winstondarmawan on October 20, 2017, 10:39:03 pm
TIA.
Post by: angelahchan on October 20, 2017, 11:11:37 pm
Hey Guys,For Q9 I think you're meant to talk about Le Chatelier's principle. e.g. Increase in temp would shift equilibrium to the right favouring the forward reaction since it's endothermic, as predicted by le chat's principle system in equilibrium shifts to counteract change, thus the shift would increase amount of CO and H2O produced.
Could any of you help me answer these two questions:
(http://i65.tinypic.com/fd4x04.png)
For question 9, I believe the atoms may tend to cool as the H20 would reach equilibrium
Thanks <3
Hello! Can someone please go over the advantages and disadvantages of scientific models?Advantages: allows us to observe a concept we can't see with the naked eye/difficult to show in school lab, easy to understand complex concepts with visual representation
TIA.
Disadvantages: oversimplifies processes, you can say it misses out a certain part of the thing it's representing (I know this sounds really vague, but it depends on what model you're talking about- there's usually always something the model skips out on and you can pretty much make it up)
Post by: JuliaPascale123 on October 21, 2017, 03:29:18 am
Hey Guys,
Could any of you help me answer these two questions:
(http://i65.tinypic.com/fd4x04.png)
For question 9, I believe the atoms may tend to cool as the H20 would reach equilibrium
Thanks <3
This looks like a really good question anyone know how to answer it?
Post by: toasted on October 21, 2017, 11:41:59 am
I'm really struggling to find information about the following dotpoints from the syllabus- they're under the option shipwrecks:
-Identify the use of electrolysis as a means of cleaning and stabilising iron, copper and lead artefacts
-Discuss the range of chemical procedures which can be used to clean, preserve and stabilize artefacts from wrecks and, where possible, provide an example of the use of each procedure
I've looked through textbooks and they're all so brief- so im unsure if there's any more info on them. Anything would be amazing :D
Post by: clarence.harre on October 22, 2017, 10:24:47 am
To expand:
I don't mean how to answer in sig fig, but when to answer in sig fig and to how many we're meant to do.
My understanding so far is the smallest number of significant figures in the question is the number of sig fig we're meant to answer to, but often I'll do that and be marked wrong because my rounded answer isn't what the teacher has on their marking sheet - although I do put the non-rounded answer before it i.e. Answer = 1.4356623.... = 1.44 (3 sig fig)
It's just extremely irritating that the marking guidelines sometimes specify sig fig and if I don't do it then I'll be marked down, so I've just been doing it for every question, but it seems that if it isn't specified then for some reason I'm also marked down.
Calculations are correct though. Maybe my teachers are just religious when it comes to marking.
Post by: clarence.harre on October 22, 2017, 10:57:58 am
Hey!!!Identify the use of electrolysis as a means of cleaning and stabilising iron, copper and lead artefacts
I'm really struggling to find information about the following dotpoints from the syllabus- they're under the option shipwrecks:
-Identify the use of electrolysis as a means of cleaning and stabilising iron, copper and lead artefacts
-Discuss the range of chemical procedures which can be used to clean, preserve and stabilize artefacts from wrecks and, where possible, provide an example of the use of each procedure
I've looked through textbooks and they're all so brief- so im unsure if there's any more info on them. Anything would be amazing :D
If a question asks how to clean or stabilise any metallic artefact, you just need to go "oh yeah, electrolysis".
Pretty much you just make the metal artefact the cathode by attaching it to the negative terminal and an inert (must be inert) steel electrode as the anode. Place it in a dilute alkaline solution (i.e. 2% NaOH) and pump a small current through it. This will draw out the chlorine and sulfate ions, effectively cleaning and stabilising it. If you want to get rid of the concretions, pump a high current through it. You'll know it's working when you get hydrogen gas bubbling up near the cathode. The concretions should break away.
Discuss the range of chemical procedures which can be used to clean, preserve and stabilize artefacts from wrecks and, where possible, provide an example of the use of each procedure
I take it this refers to the process of finding something, going 'yep, we gotta fix that'.
1. First would be getting rid of anything keeping it submerged, using a drill or in the case of the Endeavour Canons, using explosives.
2. Now we pack up the artefact, keeping it wet with water or sand from the environment it was found in - this stops it drying out. If it dries out the salts saturated inside will expand into their crystal form and damage both the structural integrity and the aesthetic of the artefact. It is sent to the lab for cleaning and such
3. Remove the concretions (calcium carbonate) using either a hammer, electrolysis or acid
Example: Hammer was used in the Endeavour Canons
4. Get rid of salts using electrolysis (the first point you talked about). Constantly measure the chloride concentration in the solution. When the it seems to be plataeuing, replace the solution with a new one. Keep doing this until the concetration reaches 25ppm (some sources say as high as 50ppm, but less is always better for the artefact).
Example: The Sirius anchor spent 8 weeks doing it (well that's what the guy at the national maritime museum said)
5. Coat the artefact - either clear polyurethane polymer or micro crystalline wax
Example: Verner Anchor - displayed outside the National Maritime Museum
Fun fact - kids kept playing on them and eventually rubbed the coating off. So they'd been corroding again for a bit until people realised what was going on.
As they're also an outdoor 24/7 display item, meaning they are exposed to heaps of oxygen and water and all the good stuff that accelerates corrosion, they stand on a metallic mesh that acts as a sacrificial anode.
Think that's about it. If I've missed anything, definitely add to the list, correct my errors (bet there's at least 3) and expand on points.
Hope it helps
Post by: kemi on October 22, 2017, 04:26:16 pm
Can someone please explain responding in significant figures?
To expand:
I don't mean how to answer in sig fig, but when to answer in sig fig and to how many we're meant to do.
My understanding so far is the smallest number of significant figures in the question is the number of sig fig we're meant to answer to, but often I'll do that and be marked wrong because my rounded answer isn't what the teacher has on their marking sheet - although I do put the non-rounded answer before it i.e. Answer = 1.4356623.... = 1.44 (3 sig fig)
It's just extremely irritating that the marking guidelines sometimes specify sig fig and if I don't do it then I'll be marked down, so I've just been doing it for every question, but it seems that if it isn't specified then for some reason I'm also marked down.
Calculations are correct though. Maybe my teachers are just religious when it comes to marking.
What you're doing is correct - always round to sig fig, because in the HSC there is ONE question that tests students on sig fig, but nobody knows which one it is. So just play it safe - round to the smallest number of sig figs, as given in the question.
Post by: michelleh on October 22, 2017, 08:32:33 pm
Just a quesstion: Is the radioisotope, Cobalt-60, used to detect pipe leakages? CAN they detect pipe leakages (e,g flushed through pipes and emits gamma radiation which is picked up by a gamma detector/camera etc.)?
Thanks everyone! :)
Post by: Natasha.97 on October 22, 2017, 08:36:57 pm
Hi guys!
Just a quesstion: Is the radioisotope, Cobalt-60, used to detect pipe leakages? CAN they detect pipe leakages (e,g flushed through pipes and emits gamma radiation which is picked up by a gamma detector/camera etc.)?
Thanks everyone! :)
Yes, Co-60 can be used to detect cracks/breaks in pipes :)
Post by: bsdfjnlkasn on October 23, 2017, 08:03:33 pm
I know this has been answered already, but any ideas on how to solve this? Wouldn't mind getting linked to the place to save someone the time of solving it (again) :D
Post by: kauac on October 23, 2017, 08:12:13 pm
What are the differences in the names used for lactic acid - specifically, 2-hydroxypropanoic acid and 2-hydroxypropionic acid?? The first one is the IUPAC name, though I can't seem to find any information on the second one, and what its purpose is.
Any help greatly appreciated. Thanks!
Post by: angelahchan on October 23, 2017, 08:32:31 pm
Hey there!
I know this has been answered already, but any ideas on how to solve this? Wouldn't mind getting linked to the place to save someone the time of solving it (again) :D
c1v1= c2v2
c1= 0.1
v1= 0.09 (converting it into liters)
c2 = 10^-2 = 0.01
0.1 * 0.09 = 0.01 *x
therefore (0.1*0.09)/0.01 = x
x= 0.9
which is 900mls. But this is the final volume including the initial 90 mls.
Therefore 900-90 = 810. Answer is C
Post by: kauac on October 23, 2017, 08:45:36 pm
Hey there!
I know this has been answered already, but any ideas on how to solve this? Wouldn't mind getting linked to the place to save someone the time of solving it (again) :D
Saw this and thought I'd give it a try!
I'm assuming from my calculations that C is the correct answer... If not, let me know!
To start off with, if you have a solution with a pH of 1, then changing the pH to 2 will require a 10x decrease in the concentration of hydrogen ions, as the pH scale is a logarithm. As HCl is a strong acid and completely dissociates in water, you can assume that the 0.1 molarity given in the questions is also the hydrogen ion concentration.
Decreasing the hydrogen ion concentration by 10 would give it a value of 0.01 - you can check this using the pH=-log[H+] formula. Hence this is the molarity to aim for in the following calculation:
C1V1 = C2V2
0.1 x 0.09 = 0.01V2
0.009 = 0.01V2
V2 = 0.009 / 0.01
= 0.9 L or 900mL
Using the original concentration and volume, as well as the intended final concentration, 900mL is found to be the final volume of solution needed to obtain a pH of 2. Since the solution is already 90ml, then you can simply subtract 90 from 900 to give you the amount of water need for the dilution, which is: 900 - 90 = 810mL (option C).
Hope this helps!
Post by: winstondarmawan on October 23, 2017, 11:35:38 pm
Can someone please go over the formation of ozone in the troposphere and the associated formula? I'm missing it in my notes..
TIA.
Post by: toasted on October 24, 2017, 09:55:54 am
Hello!
Can someone please go over the formation of ozone in the troposphere and the associated formula? I'm missing it in my notes..
TIA.
Formation:
O2 --UVc--> 2O. note O. denotes a free oxygen radical. UVc is 'Catastrophic' and hence its absorption is beneficial
O.+O2---->O3
Degradation:
O3--UVb--> O2 + O. UVb is 'Bad'
Hope that helps! :D
Post by: bsdfjnlkasn on October 24, 2017, 10:23:29 am
I thought the answer was C for this, I can't see how the change in oxidation states is what is given in B..
I knew it was ethanol, but what calculations would we be expected to calculate to find this out?
For pH, I thought it was B? Or is it A because there's been a 100 fold increase in volume as opposed to 10 which is what the 2.4 pH would indicate ?
Also, aren't neutralisation reactions exothermic? So if temperature increases, the reactants will be favoured i.e. less ionisation and so pH increases? (answer is A)
Post by: Natasha.97 on October 24, 2017, 11:03:09 am
Hey there!Hi!
I thought the answer was C for this, I can't see how the change in oxidation states is what is given in B..
C is incorrect: A decrease in the oxidation state means that there is a gain in electrons. In this case, Zn gives electrons to Oxygen, so the oxidation state of Zn changes from 0 to +2.
B is correct: The total oxidation number of a compound is 0. In this case, the oxidation state of O2 is -4 (2 x -2), so to balance it out, the oxidation state of Mn is +4. In the compound Mn2O3, the oxidation number of O2 is -6 (3x-2) so the oxidation state of Mn is 6÷2, which is +3.
Hope this helps
Post by: MisterNeo on October 24, 2017, 11:43:24 am
Hey there!
I thought the answer was C for this, I can't see how the change in oxidation states is what is given in B..
I knew it was ethanol, but what calculations would we be expected to calculate to find this out?-Find how many moles of every fuel is present in 1000 grams using m/MM.
-Multiply each fuels' moles with their respective heat of combustion values to obtain the amount of energy released from 1kg.
-The smallest value would be the answer.
For pH, I thought it was B? Or is it A because there's been a 100 fold increase in volume as opposed to 10 which is what the 2.4 pH would indicate ?
Also, aren't neutralisation reactions exothermic? So if temperature increases, the reactants will be favoured i.e. less ionisation and so pH increases? (answer is A)They should be exothermic. For this question, it requires you to interpret the graph to see if you can understand a graph. So, the answer would be A since pH decreases as more H+ are ionised. ;)
Post by: bsdfjnlkasn on October 24, 2017, 06:21:05 pm
Just wanted to clarify a few things to do with nuclear chem as a part of the production topic. Do we have to know anything for this outcome: "Process information from secondary sources to describe recent discoveries of elements"? I sort of just assumed that they would give us data to interpret or something, has anyone seen any questions that might relate to this outcome where they haven't given you data? Because should I then memorise some examples of recent element discoveries?
Do we have to know the specifics of the different radiation detectors? How many is everyone learning?
What sort of questions can they ask us to answer with regards to interpreting the screen-shotted graph?
Any help would be super appreciated!
Post by: f_tan on October 24, 2017, 11:42:47 pm
I came across this equation: CuSO4.5H2O --> CuSO4 + 5H2O for the dehydration of copper sulphate crystals, and i'm not sure what it means.
Thanks!
Post by: MisterNeo on October 25, 2017, 01:23:05 am
Can anyone help explain what a dot means in an equation?The dot shows a number of water molecules attached to a hydrated salt. It’s chemically bonded to the salt but shown in a special way using the dot. Concentrated sulfuric acid will remove those water molecules from the blue hydrated salt to form a white anhydrous salt of copper sulfate.
I came across this equation: CuSO4.5H2O --> CuSO4 + 5H2O for the dehydration of copper sulphate crystals, and i'm not sure what it means.
Thanks!
Hey there!I haven’t seen a question on recent discoveries, but I’m not particularly sure if they will. It’s best to prepare one just in case. All you would need is a brief description of how it is produced using what particles/technology, and half-life/emissions. For radiation detectors, most people are doing Geiger counter or Scintillation counter, but I don’t think they’ll ask for more than one. From the graph, they would expect you to know that too many neutrons causes -beta decay, too little causes +beta decay, and too heavy nucleus causes alpha/gamma. They wouldn’t ask you something very specific to the graph unless it was supplied. :)
Just wanted to clarify a few things to do with nuclear chem as a part of the production topic. Do we have to know anything for this outcome: "Process information from secondary sources to describe recent discoveries of elements"? I sort of just assumed that they would give us data to interpret or something, has anyone seen any questions that might relate to this outcome where they haven't given you data? Because should I then memorise some examples of recent element discoveries?
Do we have to know the specifics of the different radiation detectors? How many is everyone learning?
What sort of questions can they ask us to answer with regards to interpreting the screen-shotted graph?
Any help would be super appreciated!
Post by: arunasva on October 25, 2017, 05:50:35 am
The dot shows a number of water molecules attached to a hydrated salt. It’s chemically bonded to the salt but shown in a special way using the dot. Concentrated sulfuric acid will remove those water molecules from the blue hydrated salt to form a white anhydrous salt of copper sulfate.I haven’t seen a question on recent discoveries, but I’m not particularly sure if they will. It’s best to prepare one just in case. All you would need is a brief description of how it is produced using what particles/technology, and half-life/emissions. For radiation detectors, most people are doing Geiger counter or Scintillation counter, but I don’t think they’ll ask for more than one. From the graph, they would expect you to know that too many neutrons causes -beta decay, too little causes +beta decay, and too heavy nucleus causes alpha/gamma. They wouldn’t ask you something very specific to the graph unless it was supplied. :)
well it came up in the cssa trials.
Post by: mary123987 on October 25, 2017, 10:13:48 am
Post by: MisterNeo on October 25, 2017, 10:15:51 am
HEY guys for AAS does anyone knnow wether or not the graph is a curve or line of best fit as i have been told from numerous sources varying things from my understanding for HSC level it is a line of best fit but in reality it is actually a curve of best fit is that true ?The graph should have a straight line (connecting the dots) because the absorbance and concentration are proportionate to each other, and the plotted points should be collinear. ;)
Post by: mary123987 on October 25, 2017, 10:19:01 am
Post by: roygbivmagic on October 25, 2017, 10:44:02 am
'What volume of 0.005 mol L-1 KOH is required to neutralise 15mL of 0.0005 mol L-1 H2SO4?'
The equation is H2SO4 + 2KOH -> K2SO4 + 2H2O so isn't the number of moles of H+ already equal to the number of moles of OH-? In the answers they found the number of moles of H2SO4 and multiplied that by 2 to find number of moles of H+ and found how many mL of KOH gives this many moles. But what I don't understand is that aren't H+ and OH- already in a 1:1 ratio? Why do you have to multiply H+ by 2? Do you just disregard molar ratios for pH calcs?
Hopefully that made sense! Thank you :D
Post by: MisterNeo on October 25, 2017, 10:55:49 am
Hi! Please can you explain how to do this question?Was the answer 3mL? To find the moles of KOH, you need to multiply moles of H2SO4 by 2 stoichiometrically, which would be the same as multiplying by 2 to get moles of H+. Doing it either way would get you the same answer because both make you multiply by 2. :)
'What volume of 0.005 mol L-1 KOH is required to neutralise 15mL of 0.0005 mol L-1 H2SO4?'
The equation is H2SO4 + 2KOH -> K2SO4 + 2H2O so isn't the number of moles of H+ already equal to the number of moles of OH-? In the answers they found the number of moles of H2SO4 and multiplied that by 2 to find number of moles of H+ and found how many mL of KOH gives this many moles. But what I don't understand is that aren't H+ and OH- already in a 1:1 ratio? Why do you have to multiply H+ by 2? Do you just disregard molar ratios for pH calcs?
Hopefully that made sense! Thank you :D
Post by: winstondarmawan on October 25, 2017, 12:18:49 pm
What's the term for when a salt in solution is in such high concentration that it won't dissolve anymore?
I can't seem to remember..
Post by: MisterNeo on October 25, 2017, 12:21:48 pm
Hello!Saturated
What's the term for when a salt in solution is in such high concentration that it won't dissolve anymore?
I can't seem to remember..
Post by: beau77bro on October 25, 2017, 01:36:36 pm
What does it mean "if no precipitate forms is ismade of alkaline"?
Towards the bottom^
Mod Edit: Merged post. :)
Post by: angelahchan on October 25, 2017, 01:42:10 pm
Hi! Please can you explain how to do this question?I thought I"d give it a go
'What volume of 0.005 mol L-1 KOH is required to neutralise 15mL of 0.0005 mol L-1 H2SO4?'
The equation is H2SO4 + 2KOH -> K2SO4 + 2H2O so isn't the number of moles of H+ already equal to the number of moles of OH-? In the answers they found the number of moles of H2SO4 and multiplied that by 2 to find number of moles of H+ and found how many mL of KOH gives this many moles. But what I don't understand is that aren't H+ and OH- already in a 1:1 ratio? Why do you have to multiply H+ by 2? Do you just disregard molar ratios for pH calcs?
Hopefully that made sense! Thank you :D
First we work out the moles of H2SO4 .
Moles = concentration * volume = 0.0005*0.015= 0.0000075
Since this is a 1:2 ratio of H2SO4 to KOH
Multiply 0.0000075 by 2 = 0.000015 mols of KOH
Volume = moles/concentration = 0.000015/0.005= 0.003
which is 3 ml
I'm not sure if I did it correctly though....tell me if I'm wrong because I totally could be , and I'm also not sure how they calculated H+
I just worked out the moles of H2SO4 rather than the moles of H+, and then multiplied by 2 to find moles of KOH, since there's a 1:2 ratio.
And then from the moles of KOH I worked out the volume via volume = moles/concentration.
Maybe (idk tho) try to work with the moles of the substances rather than H+ and OH- in this case...? I have no clue, maybe you can post the answers for someone else to have a look?
whoops i was looking at 2nd last page rather than last, didn't notice someone already posted a reply
Post by: winstondarmawan on October 25, 2017, 01:46:06 pm
What are some sources of sulfate in deep ocean wrecks? Not sure if we are required to know this because I can't seem to find it in my notes. :(
CORE TOPICS
1. Are concentrated and dilute acids defined by >1M and <1M respectively?
2. Also, is there an equation for photochemical smog that we are advised to know?
3. When water is added to a weak acid, does pH increase or decrease?
2012 HSC:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22811213_1360059024119617_5363852_n.jpg?oh=30d80c4c98a69709c377d13fc9d5ba6b&oe=59F1FF4E
Not sure how filter funnels are used in the preparation of standard solutions.
TIA.
Post by: MisterNeo on October 25, 2017, 03:24:25 pm
(http://uploads.tapatalk-cdn.com/20171025/d65087b563b214ef78126f60aa0184f8.jpg)Sulfide reacts with hydrogen ions to form aqueous hydrogen sulfide, which doesn’t bond with those metal ions to form precipitates. Hence, the solution is made alkaline to get rid of the hydrogen ions so that sulfide can have a chance to bond with the metal ions rather than the hydrogen ion. Through this, we can tell if those cations are present due to sulfide precipitates being formed. ;D
What does it mean "if no precipitate forms is ismade of alkaline"?
Towards the bottom^
Post by: bridie_2345 on October 25, 2017, 04:58:53 pm
also how in depth should i be about the enivornmental impacts of soaps and detergents?
thanks so much :)
Post by: aryak on October 25, 2017, 05:56:16 pm
Post by: MisterNeo on October 25, 2017, 06:21:52 pm
Heyy, I am having a little bit of trouble in solving this question. I dont know how to do the question itself. Is it ok if you could explain to me the steps in solving this question, cause I dont understand how the answer is C.
Hi there for industrial chemistry, i was wondering if someone could please explain the how soap, water and oil together form an emulsion with the soap acting as an emulsifier?An emulsion is when a two usually immiscible liquids are dispersed in solution, such as soap making water and oil mix. Soap is made of a hydrophillic (water-loving) negatively charged head and a hydrophobic (water-fearing) hydrocarbon tail. The hydrocarbon tail will want to get out of the water due because it is non-polar, and will adsorb into oil particles. Upon scrubbing, the oil particles will be agitated and break up into smaller droplets, then more soap will adsorb into the oil droplets to form micelles. These micelles have the hydrophillic negative head on the outside, so they repel each other and disperse themselves along with the oil throughout the solution.
also how in depth should i be about the enivornmental impacts of soaps and detergents?
thanks so much :)
(https://qph.ec.quoracdn.net/main-qimg-6c80b8c9b4f169b8b9dc591b2dcb595b)
Post by: beau77bro on October 25, 2017, 06:47:41 pm
Sulfide reacts with hydrogen ions to form aqueous hydrogen sulfide, which doesn’t bond with those metal ions to form precipitates. Hence, the solution is made alkaline to get rid of the hydrogen ions so that sulfide can have a chance to bond with the metal ions rather than the hydrogen ion. Through this, we can tell if those cations are present due to sulfide precipitates being formed. ;D
ohh ok so it said "it is made alkaline" thanks neo - the explanation really helps this late late study
Post by: aryak on October 25, 2017, 06:53:48 pm
I can't seem to do AAS calculation questions. Is it ok if you could explain to me this question?
A sample of fish was analysed to determine its mercury concentration. A 2.00g of sample was digested and made upto 100 ml of solution. This was then analysed using an atomic absorption spectrometer, and the standard Hg concentration was 0.3 ppm for the dilute sample? Calculate the concentration of mercury in the orignal sample in ppm?
Also, do you by any chance have a list of AAS questions of this type by any chance?
Thank you
Post by: ilovemycat on October 25, 2017, 07:34:37 pm
i should probs know this but why exactly does celluloses carbon chain structure make it a good alternative for petrochemicals? is it simply the fact that it already contains carbon chains, so its easy to add stuffs to it..?..?
and also in your slides jake, (a true blessing and gift to this planet - god/other bless your soul!!) you include the history, production, properties and life story of phb but do we need to know alllllllll of that? because its a helluva lot of info
thanks!
mish
Post by: Baylsskool on October 25, 2017, 07:40:48 pm
Post by: kauac on October 25, 2017, 08:13:24 pm
Is the whole point behind why we titrate, to get to 7pH and produce water or to reach equivalence point??
Titrations are usually used to find the concentration of an unknown solution. So in most cases, the aim is to reach the equivalence point, as this allows the ratio of moles of acid and base to match the stoichiometric ratio of the equation. The equivalence point is not always at a pH of 7, if the titration involves a strong base and weak acid, or vice versa.
Hope this is alright?
hii :)
i should probs know this but why exactly does celluloses carbon chain structure make it a good alternative for petrochemicals? is it simply the fact that it already contains carbon chains, so its easy to add stuffs to it..?..?
and also in your slides jake, (a true blessing and gift to this planet - god/other bless your soul!!) you include the history, production, properties and life story of phb but do we need to know alllllllll of that? because its a helluva lot of info
thanks!
mish
As for your first question, what you have said is correct, as far as I know... Cellulose has a long carbon chain, and can be broken down into glucose by acid hydrolysis, which then ferments with yeast to produce ethanol, which is dehydrated to produce ethene. I think that cellulose having the ability to be turned into ethene is the key point here, as like you said, ethene can then be used to create a range of monomers, thus polymers.
Mod edit: Merged, use the “Modify” button to add replies :)
Post by: J.B on October 26, 2017, 10:23:19 am
I really need to practice back titration calculations.
I was wondering if anyone had a bunch of questions they could send me?
I was also wondering for the electrolysis of NaCl school practical, if using a concentrated solution, would 200mL of 6.0mol/L be suitable for a Hoffman's Voltameter on 6 volts?
Thanks.
Post by: winstondarmawan on October 26, 2017, 11:09:25 am
Shipwrecks
What are some sources of sulfate in deep ocean wrecks? Not sure if we are required to know this because I can't seem to find it in my notes. :(
CORE TOPICS
1. Are concentrated and dilute acids defined by >1M and <1M respectively?
2. Also, is there an equation for photochemical smog that we are advised to know?
3. When water is added to a weak acid, does pH increase or decrease?
2012 HSC:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22811213_1360059024119617_5363852_n.jpg?oh=30d80c4c98a69709c377d13fc9d5ba6b&oe=59F1FF4E
Not sure how filter funnels are used in the preparation of standard solutions.
TIA.
Bump
Post by: angelahchan on October 26, 2017, 12:32:07 pm
Bump1) I don't think there's a definition for a specific concentration that means an acid is concentrated, e.g. concentrated sulfuric acid is around 18M. Here's a definition I found: "A dilute acid is that in which the concentration of the water mixed in the acid is higher than the concentration of the acid itself."
2) Not sure, I don't have anything in my notes for this
3) pH increases- even though equilibrium is shifted towards increasing dissociation of the weak acid, the acid is still more dilute than it was before despite this
Filter funnel: i think after you've dissolved the substance to make the solution with water in a beaker, the filter funnel is used to transfer the solution from the beaker to the volumetric flask?
Post by: bridie_2345 on October 26, 2017, 04:10:29 pm
thanks!
Post by: mbdtHSC on October 26, 2017, 05:17:41 pm
Hi there was just wondering how to figure this out? the answer is B but i got A
thanks!
Just missing one step where you divide by the grams you found (answer A) by 0.025 to make it in terms of concentration
Post by: MisterNeo on October 26, 2017, 05:45:03 pm
Hi there was just wondering how to figure this out? the answer is B but i got A
thanks!
Post by: Shadowfox42 on October 26, 2017, 06:03:19 pm
Do you think we need to address the fact that a substitution reaction technically takes place between the cyclohexane and bromine water? But because only one bromine atom takes place in forming the larger molecule, the brown colour still remains. Would we lose marks if we didn't mention that in the actual exam?
Thanks
Post by: bsdfjnlkasn on October 26, 2017, 06:28:00 pm
Was just wondering if we need to know an example of a saponification reaction? What would be a good one to remember?
Post by: mylinh-nguyen on October 26, 2017, 06:52:42 pm
Post by: Sine on October 26, 2017, 07:10:52 pm
you use c1v1=c2v2 right? I got 90ml but the answer is C what did I do wrong??pH = 2 = -log_10([H+])
10^-2 = [H+]
0.01 = [H+]
C1V1 = C2V2
0.1 x 90 = 0.01 x V2
9 = 0.01 x V2
V2 = 900
Water added = V2 - V1 = 900 - 90 = 810mL
Post by: mylinh-nguyen on October 26, 2017, 07:51:53 pm
pH = 2 = -log_10([H+])
10^-2 = [H+]
0.01 = [H+]
C1V1 = C2V2
0.1 x 90 = 0.01 x V2
9 = 0.01 x V2
V2 = 900
Water added = V2 - V1 = 900 - 90 = 810mL
pH = 2 = -log_10([H+])oh that makes sense thanksss!
10^-2 = [H+]
0.01 = [H+]
C1V1 = C2V2
0.1 x 90 = 0.01 x V2
9 = 0.01 x V2
V2 = 900
Water added = V2 - V1 = 900 - 90 = 810mL
Post by: kauac on October 26, 2017, 08:06:30 pm
Post by: bridie_2345 on October 26, 2017, 08:28:25 pm
ah farout completely forgot about converting it to concentration rip haha
thanks :)
Post by: MisterNeo on October 26, 2017, 10:48:14 pm
Hi quick question for Production of Materials, regarding the cyclohexane/cyclohexene prac.It depends on what the question is. Personally, I would include it if there were enough lines, so I'd definitely know it because it adds much more depth and understanding. :)
Do you think we need to address the fact that a substitution reaction technically takes place between the cyclohexane and bromine water? But because only one bromine atom takes place in forming the larger molecule, the brown colour still remains. Would we lose marks if we didn't mention that in the actual exam?
Thanks
Hey there,You don't really need to know the specific names of saponification reactions. Usually, they would give you the name and formula of the triglyceride, which you have to draw. It wouldn't hurt to know one anyway, so glyceryl tristearate is the most common one people would use. The rest of the equation would be the same but with a stearate salt. ;D
Was just wondering if we need to know an example of a saponification reaction? What would be a good one to remember?
Post by: liya1234 on October 27, 2017, 11:12:37 am
Post by: J.B on October 27, 2017, 12:50:06 pm
I really need to practice back titration calculations.
I was wondering if anyone had a bunch of questions they could send me?
In industrial chemistry:
I was wondering for the electrolysis of NaCl school practical, if using a concentrated solution, would 200mL of 6.0mol/L be suitable for a Hoffman's Voltameter on 6 volts?
And in the modelling of the solvay process practical, my class modelled CaCO3 -> Co2 + CaO, and CaO + H2O -> Ca(OH)2
I have one limitation being we can't perform it at the 900 degrees, but I'm struggling to come up with another one? Any Suggestions?
Also, in soap is the anionic head considered to be a hydrogen bond with water molecules?
Thanks.
Post by: bridie_2345 on October 27, 2017, 02:04:04 pm
perform a first-hand investigation to determine the concentration of a domestic acidic substance using computer-based technologies
thanks!
Post by: angelahchan on October 27, 2017, 03:20:55 pm
Hey there i was wondering if anyone could help me address this dot point in the acidic environment?Honestly, I forgot about this one so I did a quick summary- it's just a titration but with a pH probe and data logger, there's sometimes questions on interpreting titration graphs based on this. (https://i.imgur.com/XJab83K.jpg) This is a similar graph to what the prac should've looked like: I don't think my chem class actually did it with a data logger but I guess we pretended to?
perform a first-hand investigation to determine the concentration of a domestic acidic substance using computer-based technologies
thanks!
(https://i.imgur.com/Kl9sWhr.png)
Post by: Nick Seb on October 27, 2017, 03:59:19 pm
~Nick
Post by: winstondarmawan on October 27, 2017, 04:58:12 pm
In regards to significant figures, I have been taught it is best to round off the the lowest significant figure that is in the question. However, in some molar calculations involving volume, would values such as 25 degrees have any bearing on the value of SF that we should be rounding off to?
Post by: roygbivmagic on October 27, 2017, 08:06:53 pm
"A student set up the following galvanic cell as shown [zinc anode, copper cathode, ZnSO4 and CuSO4 electrolyte solutions, KCl salt bridge.] Both electrodes were weighed before beginning the experiment and each beaker contained 200mL of a suitable electrolyte at 0.100molL-1 concentration. After a short period of time, the student reweighed the copper electrode and found that it had increased in mass by 0.435g. What was the final concentration of the zinc electrolyte?"
(Answer=0.134molL-1)
Post by: liya1234 on October 27, 2017, 08:13:09 pm
Post by: naezeroth on October 27, 2017, 08:35:38 pm
this is probably a kinda stupid question but i've somehow forgotten when units i should be using when putting values into formulas (n=m/M and n=cv) - is mass always in grams for the first one? and is volume in L or ml for the second?? thanks in advance!!
It is always grams for m and litres for volume
Post by: Nick Seb on October 27, 2017, 09:32:30 pm
Hi, how do you solve this question?
"A student set up the following galvanic cell as shown [zinc anode, copper cathode, ZnSO4 and CuSO4 electrolyte solutions, KCl salt bridge.] Both electrodes were weighed before beginning the experiment and each beaker contained 200mL of a suitable electrolyte at 0.100molL-1 concentration. After a short period of time, the student reweighed the copper electrode and found that it had increased in mass by 0.435g. What was the final concentration of the zinc electrolyte?"
(Answer=0.134molL-1)
Copper cathode oxidised, hence its mass increased as it pulled copper ions out of solution which precipitated on the metal surface. As the cathode gained 0.435g of copper, then 0.435g of copper must have been pulled out of solution. Given the molar mass of copper is 63.55, by n=mass/Molarmass, n=0.435/63.55,n=0.00684500393... moles. Therefore, the copper cathode gained 0.00684500393... moles of copper.
Cu2+(aq) + 2e- --> Cu(s) and Zn(s) --> Zn2+(aq) + 2e-, which can be written as Cu2+(aq) + Zn(s) --> Cu(s) + Zn2+(aq)
Given that the molar ratio is 1:1, you can determine that the zinc anode lost 0.00684500393... moles of zinc, meaning that the electrolyte solution gained the same moles of zinc ions by the same equation.
Now find the initial moles of Zn2+ in solution, which you can do by n=cv, n=0.1*0.2 (remembering to convert 200ml to 0.2L because of SI units), n=0.02 moles.
From here you simply add the initial moles of the solution with the moles which the zinc anode added to its electrolyte solution (0.02 + 0.00684500393... = 0.02684500393... moles).
Then just solve c=n/v using the new moles of zinc ions in solution and the same volume, n=0.02684500393.../0.2 = 0.1342250197mol/L.
Hope this helps,
~Nick
Post by: angelahchan on October 28, 2017, 09:47:52 am
http://educationstandards.nsw.edu.au/wps/wcm/connect/e76232cd-1d84-4d3e-92ce-ea41546810f6/2016-hsc-chemistry.pdf?MOD=AJPERES&CACHEID=ROOTWORKSPACE-e76232cd-1d84-4d3e-92ce-ea41546810f6-lDY2cMn
Post by: J.B on October 28, 2017, 10:05:55 am
Could someone explain Question 15 from 2015 HSC?
Answer is A.
Thanks
Post by: angelahchan on October 28, 2017, 10:33:13 am
Hi,
Could someone explain Question 15 from 2015 HSC?
Answer is A.
Thanks
We can rule out B and C because they both have low turbidity which is not likely due to having passed through farmland and a village. We can rule out D because of it's low total dissolved solids - since X is close to the ocean (estuarine), there would be high TDS due to all the minerals dissolved in the ocean.
This leaves out A by elimination- high turbidity and high TDS.
Although someone else's opinion on the pH value would be great- since I don't really understand why the pH is lower at A (since farmland can sometimes have fertiliser run-off that makes water more alkaline).
Post by: julies on October 28, 2017, 12:21:52 pm
are there any similarities between the lead acid cell and the button cell?
Post by: angelahchan on October 28, 2017, 12:35:38 pm
I never really understood how it worked, but how do carbon isotopes in trees and calcium carbonate in coral suggest an acidic oxide increase over the past two centuries [taken from the first edition of the ATARNotes HSC Chemistry complete course notes]?They are evidence for increasing CO2 levels in the atmosphere.
~Nick
Since the dot point is "Assess the evidence which indicates increases in atmospheric concentration of oxides of sulfur and nitrogen." I think this was mentioned (I have no idea what the atarnotes book says since I don't have it), to compare how it's relatively easier to find evidence for increasing concentration of CO2 in comparison to sulfur and nitrogen oxides.
Basically, since carbonates stay in inert forms, past concentration levels can be observed to observe the increase in CO2 levels. In contrast most sulfates and all nitrates are water soluble, which means they're washed out of air by the rain and so it's harder to measure significant build up of these gases over time.
Sorry if this doesn't explain how because I don't really know, and the focus of the dot point is increasing sulfur and nitrogen oxide levels rather than carbon
Post by: winstondarmawan on October 28, 2017, 03:33:48 pm
Can someone please explain the first limitation:
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22883625_1443073785806709_699782289_o.jpg?oh=e373a670b3d43488ce77d26f619ea8a4&oe=59F6FCF8
TIA.
Post by: J.B on October 28, 2017, 04:34:15 pm
We can rule out B and C because they both have low turbidity which is not likely due to having passed through farmland and a village. We can rule out D because of it's low total dissolved solids - since X is close to the ocean (estuarine), there would be high TDS due to all the minerals dissolved in the ocean.
This leaves out A by elimination- high turbidity and high TDS.
Although someone else's opinion on the pH value would be great- since I don't really understand why the pH is lower at A (since farmland can sometimes have fertiliser run-off that makes water more alkaline).
Thank you :)
I was also wondering if someone could show me how to do this? (I can do a, but not b and I don't have the answers)
a)
A Hydrochloric acid solution was standardised by titrating 10ml of it against a 0.106 mol/L sodium hydroxide solution. The average titration volume was 35.6ml. Calculate the concentration of the HCl.
b)
A sample of Magnesium Oxide was found to be contaminated with the sodium chloride. Magnesium oxide is not very soluble in water but can be dissolved in an excess of the standardisted HCl
In order to determine the purity of the magnesium oxide, 3.86g of the sample was dissolved in 500ml of Hcl and then 50ml of the resulting solution was titrated against the NaOH solution of known concentration. The average titration volume was found to be 10.4ml.
Calculate the percentage of magnesium oxide in the concentration sample.
Thanks :)
Post by: toasted on October 28, 2017, 05:35:19 pm
Thank you :)
I was also wondering if someone could show me how to do this? (I can do a, but not b and I don't have the answers)
a)
A Hydrochloric acid solution was standardised by titrating 10ml of it against a 0.106 mol/L sodium hydroxide solution. The average titration volume was 35.6ml. Calculate the concentration of the HCl.
b)
A sample of Magnesium Oxide was found to be contaminated with the sodium chloride. Magnesium oxide is not very soluble in water but can be dissolved in an excess of the standardisted HCl
In order to determine the purity of the magnesium oxide, 3.86g of the sample was dissolved in 500ml of Hcl and then 50ml of the resulting solution was titrated against the NaOH solution of known concentration. The average titration volume was found to be 10.4ml.
Calculate the percentage of magnesium oxide in the concentration sample.
Thanks :)
This is an incredibly hard back titration question with a lot of nuances, i could be way off but here's how i would go about it.
a)
HCl + NaOH ---> NaCl+ H2O
n(HCl)=n(NaOH)
n(NaOH)=0.0356 x 0.106
=0.0037736
n(HCl)=0.0037736
concentration(HCl)= 0.0037736/0.01\
=0.37736 mol/L
b)
always start with the titration in these questions bc we essentially have to find the moles left over and subtract it from initial to find the moles used in the reaction
NaOH+HCl--->NaCl+H2O
n(NaOH)=n(HCl)
=0.0104x0.106 (using n=CV)
=0.0011024
n(HCl) in the 500ml = n(HCl) in the 50ml x 10 (C1V1=C2V2) (kinda)
= 0.0011024 x 10
=0.011024
n(HCl) initial = 0.5 x 0.37736 (because its our standardised hcl solution) (n=CV)
=0.18868
n(HCl) used to react with MgO = 0.18868-0.011024
=0.177656
MgO + 2HCl---> MgCl2 + H2O
n(MgO) =1/2 n(HCl)
=1/2 (0.177656)
=0.088828
mass(MgO)=0.088828 x (24.31+16) (mass=nxmm)
=3.58g
% mass = 3.58/3.86 x 100
=92.76%
=93% (2 s.f.)
I'm not 100% sure this is right but i feel like this is how you would approach the question. Also, if you want some practise on these types of questions try 2016 hsc 29b)
Hope that helps! :)
Post by: J.B on October 28, 2017, 05:55:22 pm
This is an incredibly hard back titration question with a lot of nuances, i could be way off but here's how i would go about it.
a)
HCl + NaOH ---> NaCl+ H2O
n(HCl)=n(NaOH)
n(NaOH)=0.0356 x 0.106
=0.0037736
n(HCl)=0.0037736
concentration(HCl)= 0.0037736/0.01\
=0.37736 mol/L
b)
always start with the titration in these questions bc we essentially have to find the moles left over and subtract it from initial to find the moles used in the reaction
NaOH+HCl--->NaCl+H2O
n(NaOH)=n(HCl)
=0.0104x0.106 (using n=CV)
=0.0011024
n(HCl) in the 500ml = n(HCl) in the 50ml x 10 (C1V1=C2V2) (kinda)
= 0.0011024 x 10
=0.011024
n(HCl) initial = 0.5 x 0.37736 (because its our standardised hcl solution) (n=CV)
=0.18868
n(HCl) used to react with MgO = 0.18868-0.011024
=0.177656
MgO + 2HCl---> MgCl2 + H2O
n(MgO) =1/2 n(HCl)
=1/2 (0.177656)
=0.088828
mass(MgO)=0.088828 x (24.31+16) (mass=nxmm)
=3.58g
% mass = 3.58/3.86 x 100
=92.76%
=93% (2 s.f.)
I'm not 100% sure this is right but i feel like this is how you would approach the question. Also, if you want some practise on these types of questions try 2016 hsc 29b)
Hope that helps! :)
Thank you so so much!
Post by: winstondarmawan on October 28, 2017, 06:56:46 pm
Hey,
In regards to significant figures, I have been taught it is best to round off the the lowest significant figure that is in the question. However, in some molar calculations involving volume, would values such as 25 degrees have any bearing on the value of SF that we should be rounding off to?
Hello!
Can someone please explain the first limitation:
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22883625_1443073785806709_699782289_o.jpg?oh=e373a670b3d43488ce77d26f619ea8a4&oe=59F6FCF8
TIA.
Bump :-)
Hi, can someone please help me with Q20 of the 2016 paper? I don't understand how AAS works in general... thanks!
http://educationstandards.nsw.edu.au/wps/wcm/connect/e76232cd-1d84-4d3e-92ce-ea41546810f6/2016-hsc-chemistry.pdf?MOD=AJPERES&CACHEID=ROOTWORKSPACE-e76232cd-1d84-4d3e-92ce-ea41546810f6-lDY2cMn
Hello!
So this is a bait question because they gave you lots of unnecessary information that you do not need.
The only piece of theory that you need to know that relates to this question is that the wavelength that is emitted from the lamp must be the same that is detected, as you are measuring the ABSORPTION of the atomised sample.
So, the answer is A. :)
Mod Edit: Merged posts. ;)
Post by: RuiAce on October 28, 2017, 07:44:57 pm
Hey,The 25 degrees does not influence your rounding.
In regards to significant figures, I have been taught it is best to round off the the lowest significant figure that is in the question. However, in some molar calculations involving volume, would values such as 25 degrees have any bearing on the value of SF that we should be rounding off to?
However, the 24.79 L mol^-1 bit certainly does and you are restricted to at most 4SF as a result of it.
Post by: kauac on October 28, 2017, 08:26:46 pm
Post by: toasted on October 28, 2017, 10:15:51 pm
On average how many calculation questions (marks) are allocated per hsc chemistry exam?
Post by: J.B on October 29, 2017, 08:11:39 am
What are the differences in definitions of 'ionise' and 'dissociate'? And when it is appropriate to use each, when referring to acids and solutions?
Don't quote me but I believe acids ionise in aqueous solution, whereas salts dissociate into their ions in aqueous solution?
Post by: liya1234 on October 29, 2017, 10:23:50 am
Hey!
On average how many calculation questions (marks) are allocated per hsc chemistry exam?
In the recent ones that I've gone through there's usually about 9/10 marks for calculations in total, not including the option (i.e. in multiple choice and core short answer only)
Mod Edit: Merged posts. :)
Post by: mylinh-nguyen on October 29, 2017, 10:41:27 am
Post by: roygbivmagic on October 29, 2017, 01:03:08 pm
'A student prepared a standard solution of Na2CO3 for titration. The student filled the 250mL volumetric flask to the level shown (below the graduation mark) and recorded its concentration as 0.20molL-1. 25mL of this solution was transferred to a conical flask and titrated with an unknown HCl acid solution. The volume of acid delivered for neutralisation was 21.30mL. Identify the mistake the student made and explain the effect this would have had on the calculated concentration of HCl.'
I understand that the mistake was that they filled the volumetric flask to below the graduation mark, but what would the effect be?
Post by: kauac on October 29, 2017, 02:19:52 pm
Hi, can you please explain how to do this question?
'A student prepared a standard solution of Na2CO3 for titration. The student filled the 250mL volumetric flask to the level shown (below the graduation mark) and recorded its concentration as 0.20molL-1. 25mL of this solution was transferred to a conical flask and titrated with an unknown HCl acid solution. The volume of acid delivered for neutralisation was 21.30mL. Identify the mistake the student made and explain the effect this would have had on the calculated concentration of HCl.'
I understand that the mistake was that they filled the volumetric flask to below the graduation mark, but what would the effect be?
Hey... I just did a very similar question in a past paper today!
When filling a solution into the volumetric flask, the bottom of the meniscus should line up with the calibration/graduation line. Hence if the solution was filled just to below the calibration mark, then the volume would have been less than what it was recorded to be. So this would have increased the number of moles of sodium carbonate, and therefore the number of moles of HCl. So HCl would end up having a calculated concentration higher than its actual concentration.
Post by: MisterNeo on October 29, 2017, 02:27:20 pm
If oxidation always occurs at the anode and reduction at the cathode in both galvanic and electrolytic cells, how come the anode is negatively charged (and the cathode positively charged) in galvanic cells while in electrolytic cells the anode is positively charged (and the cathode is negatively charged)??It's got to do with the flow of electrons.
In a galvanic cell, electrons want to flow from anode to cathode, which means that the cathode is positive and "attracts" the electrons.
In an electrolytic cell, electrons don't want to go from anode to cathode, which means that the cathode is negative and "repels" the electrons, hence an external power source is used to overcome this. You don't need to know why they are, just accept that it is. :P
please help I don't understand this question
Bump :-)Arrhenius couldn't explain why acids/bases weren't acidic or basic in non-water solvents even though he said they release H+ and OH- ions in solution. He also couldn't explain why ammonia was basic because of the role of water in the basic nature of ammonia.
Post by: ACCC on October 29, 2017, 02:39:22 pm
I was doing practice paper 2015 and I got stuck on question 20. I don't know what I'm doing wrong! Any help would be great! Thank you!
Post by: Natasha.97 on October 29, 2017, 03:10:21 pm
Hi guys,
I was doing practice paper 2015 and I got stuck on question 20. I don't know what I'm doing wrong! Any help would be great! Thank you!
Hi!
Hope this helps
Post by: roygbivmagic on October 29, 2017, 03:29:05 pm
So this would have increased the number of moles of sodium carbonate, and therefore the number of moles of HCl. So HCl would end up having a calculated concentration higher than its actual concentration.Thanks!! I don't quite get the last part though - so the sodium carbonate actually used has a higher concentration. So wouldn't more HCl be needed to titrate it and reach the end point? Meaning HCl calculated concentration is lower than its actual concentration?
Post by: ACCC on October 29, 2017, 03:34:51 pm
Post by: toasted on October 29, 2017, 04:47:31 pm
Thanks!! I don't quite get the last part though - so the sodium carbonate actually used has a higher concentration. So wouldn't more HCl be needed to titrate it and reach the end point? Meaning HCl calculated concentration is lower than its actual concentration?
If more moles of HCl are needed to titrate it. The concentration will increase as C=n/V where V will stay constant and the number of moles calculated would increase. Hence, the concentration will be higher than its actual concentration. Hope that made sense! :D
Hey guys!
For those titration questions where they give the titres in a table format, do you eliminate the first titre (assuming its a test run), and also what defines an outlier in those types of questions?
Thanks!
Mod edit: Merged posts :)
Post by: kauac on October 29, 2017, 05:01:42 pm
Hey guys!
For those titration questions where they give the titres in a table format, do you eliminate the first titre (assuming its a test run), and also what defines an outlier in those types of questions?
Thanks!
An outlier is a result that is largely different to the others. If there is an outlier in a titration, it will generally be the first one. However, if the volume used for the first titration does not differ much from the rest of the titres, then I would still count it, even though it is the first one... That's just my opinion though...
Post by: MisterNeo on October 29, 2017, 05:12:58 pm
Hey guys!
For those titration questions where they give the titres in a table format, do you eliminate the first titre (assuming its a test run), and also what defines an outlier in those types of questions?
Thanks!
Mod edit: Merged posts :)
(https://i.imgur.com/7JLG0fd.png)
Don't include rough titres, and exclude anything doesn't match with the other titres, then find the average. :)
Post by: Noorijaz on October 29, 2017, 08:29:15 pm
i have a quick question
can someone plz explain the difference between polar and non-polar substances in the most easiest way possible.
thanks
Post by: kauac on October 29, 2017, 08:54:15 pm
Do we need to write the states of ions when writing equations in the exam?
Post by: RuiAce on October 29, 2017, 09:52:45 pm
gdayDepends on what you're aiming for.
i have a quick question
can someone plz explain the difference between polar and non-polar substances in the most easiest way possible.
thanks
Oversimplified explanation of what actually happens: A pair of electrons have a greater tendency to just hang around a corner/side of the atom. In water, there is an electron pair that just hangs on the opposite side of O, which makes it \(\delta- \).
Oversimplified explanation of what the outcome is: As a rough example, a polar molecule is basically behaves like a charged molecule, but the charge is something like +0.01 instead of +1 (if not less).
Hi...With ions, I've skipped writing states and never got penalised for them, so that may be food for thought. I also do vaguely recall my teacher saying it doesn't matter.
Do we need to write the states of ions when writing equations in the exam?
Simple reason - if it's charged, it's assumed to be (aq).
(Of course, everything else - obviously yes.)
Post by: MisterNeo on October 29, 2017, 10:06:59 pm
gday
i have a quick question
can someone plz explain the difference between polar and non-polar substances in the most easiest way possible.
thanks
Basically, polar substances usually have hydrogen bonded to a highly electronegative element, which pull the hydrogen's electron towards itself and makes the hydrogen slightly positive.
C-H are non-polar because carbon isn't very electronegative.
O-H or N-H are polar (in alkanols and carboxylic acids, ammonia, etc).
(Halogen)-H are also polar.
Ozone is also polar.
Hi...I always write them just to be safe even though they really only exist in aqueous form. I did lose a mark in Prelims for not writing aqueous. ::)
Do we need to write the states of ions when writing equations in the exam?
Post by: Noorijaz on October 30, 2017, 02:36:28 am
how do i cram all of chem most effectively in two days??
plz provide advice
thanks
Post by: ACCC on October 30, 2017, 10:12:29 am
Thank you!!
Post by: angelahchan on October 30, 2017, 10:12:51 am
hi
how do i cram all of chem most effectively in two days??
plz provide advice
thanks
Go through the syllabus and highlight everything you don't know( e.g. red = know nothing, yellow = need to revise more, green = good)- next go through everything in red and try to memorize it (e.g. look, cover, write, use mnemonics etc.). Then do past papers, skipping the questions which you know how to do (do most of the multichoice since it won't take as long as short answer) , and mark, look at exemplar responses. It really depends on how much content you have left which you have to go over again, though
Post by: RuiAce on October 30, 2017, 10:20:08 am
Hey guys can someone please help me with this question??
Thank you!!
which best rounds to 55g.
Post by: MisterNeo on October 30, 2017, 10:21:16 am
Hey guys can someone please help me with this question??
Thank you!!
hi
how do i cram all of chem most effectively in two days??
plz provide advice
thanks
I usually go through past HSC papers to see what I know and don't know. The notes section is what I recommend if you wanted to cram the entire syllabus, and go through them with a highlighter or something to memorise the key points of each core topic. ;)
Post by: J.B on October 30, 2017, 01:49:49 pm
I was told that 50000 is considered 5 significant figures in chemistry. Is this true?
Also can someone explain to me avogrado's number and how we use it?
Thanks
Post by: angelahchan on October 30, 2017, 02:50:57 pm
Hi,Yes. A 0 becomes included as a significant figure once any number besides 0 is in front of it.
I was told that 50000 is considered 5 significant figures in chemistry. Is this true?
e.g. 0.0327 to 2 s.f. = 0.033
but 0.3027 to 2 s.f. = 0.30. The 3 = the first significant figure, and the 0 behind it is the second.
Post by: MisterNeo on October 30, 2017, 03:16:18 pm
Also can someone explain to me avogrado's number and how we use it?
Thanks
In chemistry, we usually work with moles. Avogadro's Number is the amount of particles in one mole, 6.022 x 1023.
-One mole of carbon has 6.022 x 1023 atoms of carbon, which weigh 12.01 grams because that's one mole of its 12.01g/mol molar mass.
-One mole of fluorine has 6.022 x 1023 atoms of fluorine, which weighs 19 grams. However, there is fluorine GAS, which is 6.022 x 1023 of F2 molecules, which weigh 38 grams because there are twice as many fluorine atoms.
-One mole of ethanol has 6.022 x 1023 molecules of ethanol, which weigh 46.068 grams because it's made of 2 x 6.022 x 1023 atoms of carbon, 6 x 6.022 x 1023 atoms of hydrogen, and 6.022 x 1023 atoms of oxygen.
Avogadro's Number is most prevalent in Prelims. In HSC, it's all done in moles (1mol, 2mol, etc) and they may occasionally ask "How many particles are there in..." for multiple choice. :)
Post by: J.B on October 30, 2017, 03:28:39 pm
In chemistry, we usually work with moles. Avogadro's Number is the amount of particles in one mole, 6.022 x 1023.
-One mole of carbon has 6.022 x 1023 atoms of carbon, which weigh 12.01 grams because that's one mole of its 12.01g/mol molar mass.
-One mole of fluorine has 6.022 x 1023 atoms of fluorine, which weighs 19 grams. However, there is fluorine GAS, which is 6.022 x 1023 of F2 molecules, which weigh 38 grams because there are twice as many fluorine atoms.
-One mole of ethanol has 6.022 x 1023 molecules of ethanol, which weigh 46.068 grams because it's made of 2 x 6.022 x 1023 atoms of carbon, 6 x 6.022 x 1023 atoms of hydrogen, and 6.022 x 1023 atoms of oxygen.
Avogadro's Number is most prevalent in Prelims. In HSC, it's all done in moles (1mol, 2mol, etc) and they may occasionally ask "How many particles are there in..." for multiple choice. :)
Thank you :)
Do you know a year that they asked this?
Post by: Fahim486 on October 30, 2017, 03:29:39 pm
Post by: angelahchan on October 30, 2017, 03:49:40 pm
https://www.boardofstudies.nsw.edu.au/hsc_exams/2015/exams/2015-hsc-chemistry.pdf
Post by: toasted on October 30, 2017, 04:49:49 pm
Hi, could someone please help me with question 19 of the 2015 paper? thanks!
https://www.boardofstudies.nsw.edu.au/hsc_exams/2015/exams/2015-hsc-chemistry.pdf
Hey!
This question is tricky in the sense that they give us more information than required to solve the question.
Pb+2NaCl ---> PbCl2 + 2Na
through mole ratios n(Pb)=n(PbCl2)=n/2(Nacl) however Pb can be the limiting agent and this means that n(pb) does NOT necessarily equal n/2(NaCl)
n(Pb)= n(PbCl2)
=0.13/(207.2+35.45x2)
=0.000467
n(Pb)= 0.000467
conc(Pb)~0.000467/0.05
~ 9.3x10^-3
=C
Hope that helped! :)
Mod Edit: Removed link. :)
Post by: angelahchan on October 30, 2017, 04:59:20 pm
Hey!thanks so much! the thing I'm confused about is the moles of lead ions being the same as moles of PbCl2, is this the case for all molecules?
This question is tricky in the sense that they give us more information than required to solve the question.
Pb+2NaCl ---> PbCl2 + 2Na
through mole ratios n(Pb)=n(PbCl2)=n/2(Nacl) however Pb can be the limiting agent and this means that n(pb) does NOT necessarily equal n/2(NaCl)
n(Pb)= n(PbCl2)
=0.13/(207.2+35.45x2)
=0.000467
n(Pb)= 0.000467
conc(Pb)~0.000467/0.05
~ 9.3x10^-3
=C
Hope that helped! :)
Post by: toasted on October 30, 2017, 05:07:10 pm
thanks so much! the thing I'm confused about is the moles of lead ions being the same as moles of PbCl2, is this the case for all molecules?
Yes it would be true for all molecules, due to mole ratios. However, you have to be incredibly careful of limiting reagents for the reactants. :D
Hey so I'm having trouble understanding this question so could someone pls explain it. Thanks!
Hey similar to the question above, this is one where nesa is simply trying to confuse you with useless information.
Essentially, what it's testing is whether you know that the wavelength of light released from the lamp matches the wavelength of the light at the detector. This is necessary because AAS essentially measures the absorption, and if the wavelengths were not the same, it would be unreliable. Hence the answer is A.
Mod Edit: Merged posts. :)
Post by: winstondarmawan on October 30, 2017, 05:42:38 pm
When asked to draw glucose, is it okay to use the condensed structural formula?
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/23113018_1364885916970261_476464333_o.jpg?oh=f3edc646ed78d2cdbaab29b8e7893f92&oe=59F8A60B
Also, how recent does the recently discovered elements have to be?
TIA.
Post by: bridie_2345 on October 30, 2017, 08:14:35 pm
thanks :)
Post by: angelahchan on October 30, 2017, 08:40:55 pm
Hey there do we need to know the complete structural formula for a soap, eg. sodium tristearate or just the condensed structural formula?
thanks :)
I think it's better if you memorise the full structural formula just in case- i did a past question today on saponification which asked for including an equation, and the sample answer included the complete structural formula
Post by: bsdfjnlkasn on October 30, 2017, 08:48:47 pm
n(Pb)= 0.000467
conc(Pb)~0.000467/0.05
~ 9.3x10^-3
=C
Hope that helped! :)
Hey! how did you go between these steps?
Post by: toasted on October 30, 2017, 09:55:01 pm
Hey! how did you go between these steps?
Hey!
Im assuming its from the moles to the concentration? So we're given in the question that the sample collected from the pond was 50ml. We know that C=n/V where V is in litres so we get C=0.000467/0.05.
Hope that makes sense now :)
Post by: _denise on October 30, 2017, 09:58:49 pm
I've been having a lot of trouble with calculation questions, mainly because I'm not sure which formulae to use and which steps I'm supposed to take to get to the answer. Just wondering if anyone has any tips on how to approach them?
Thanks! :)
Post by: Mymy409 on October 30, 2017, 11:29:19 pm
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2009exams/pdf_doc/2009-hsc-exam-chemistry.pdf
Post by: f_tan on October 30, 2017, 11:59:03 pm
(https://i.imgur.com/za9CsE2.png)
Post by: winstondarmawan on October 31, 2017, 12:01:27 am
Hello!
When asked to draw glucose, is it okay to use the condensed structural formula?
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/23113018_1364885916970261_476464333_o.jpg?oh=f3edc646ed78d2cdbaab29b8e7893f92&oe=59F8A60B
Also, how recent does the recently discovered elements have to be?
TIA.
Bump.
Post by: Baylsskool on October 31, 2017, 12:19:16 am
Post by: MisterNeo on October 31, 2017, 01:58:09 am
How do you do questions 14 and 15 from 2009 HSC? TIA! :)
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2009exams/pdf_doc/2009-hsc-exam-chemistry.pdf
Can anyone please explain this question? Thank you!For AAS to work, you need a specific wavelength of light. It's the same wavelength from the lamp to the detector, where the only difference is the intensity of the light. Hence, the answer is A because it is the only one that has the same wavelength (615.2nm) for both intensity readings. ;)
(https://i.imgur.com/za9CsE2.png)
Bump.In questions that require a drawing, it would always be the structural formula ie. diagram showing shape of glucose. The other equation you’d use is in short response as C6H12O6. For recent discoveries, you can use the most recent you can find such as Flerovium (1998), which can be from 1950-2017 since that seems pretty recent. :)
Post by: toasted on October 31, 2017, 08:36:15 am
why are the metal acetates known as CH3COOA rather than ACH3COO where A is a metal. eg. CH3COOK
Shouldn't the cation come before the anion?
Post by: MisterNeo on October 31, 2017, 09:07:40 am
Hey!(https://upload.wikimedia.org/wikipedia/commons/thumb/f/fd/Acetic-acid-2D-flat.png/800px-Acetic-acid-2D-flat.png)
why are the metal acetates known as CH3COOA rather than ACH3COO where A is a metal. eg. CH3COOK
Shouldn't the cation come before the anion?
The hydrogen that is ionised comes from the -COOH group, which is at the right-hand side of the molecule. Since we write it as CH3COOH, the ionised form is CH3COO-. The positive cation attracts to the negative ion, and forms CH3COONa because that's where the negative charge is from where hydrogen has been ionised. You can get away with writing NaCH3COO, but CH3COONa is more correct since it actually shows ionic attraction. ;)
Post by: ACCC on October 31, 2017, 09:29:22 am
40 mL of 0.10 mol L–1 NaOH is mixed with 60 mL of 0.10 mol L–1 HCl.
What is the pH of the resulting solution?
(A) 7.0 (B) 1.7 (C) 1.4 (D) 1.2
Post by: RuiAce on October 31, 2017, 09:44:25 am
Hi! Can someone pls show me how to work out this question? Thank you
40 mL of 0.10 mol L–1 NaOH is mixed with 60 mL of 0.10 mol L–1 HCl.
What is the pH of the resulting solution?
(A) 7.0 (B) 1.7 (C) 1.4 (D) 1.2
i.e. the mole ratio is 1-1.
_____________________________
Post by: winstondarmawan on October 31, 2017, 10:46:39 am
In questions that require a drawing, it would always be the structural formula ie. diagram showing shape of glucose. The other equation you’d use is in short response as C6H12O6. For recent discoveries, you can use the most recent you can find such as Flerovium (1998), which can be from 1950-2017 since that seems pretty recent. :)
Thank you!
I was just wondering specifically about condensed structural formula (like the link I posted).
Post by: angelahchan on October 31, 2017, 10:55:19 am
"A solution is prepared by using equal volumes and concentrations of acetic acid
and sodium acetate.
Explain how the pH of this solution would be affected by the addition of a small
amount of sodium hydroxide solution. Include an equation in your answer."
Post by: MisterNeo on October 31, 2017, 11:30:24 am
Hi, can someone please explain how a buffer is made? I know it consists of a weak acid and its conjugate base but for example in the 2015 paper q24b, I don't understand how the buffer is actually formed. Here's the question:
"A solution is prepared by using equal volumes and concentrations of acetic acid
and sodium acetate.
Explain how the pH of this solution would be affected by the addition of a small
amount of sodium hydroxide solution. Include an equation in your answer."
Post by: mary123987 on October 31, 2017, 11:32:17 am
Post by: MisterNeo on October 31, 2017, 12:27:54 pm
HEY does anyone know why you cant detect anions using AAS ????????
Technically, you can for some anions. Remember that AAS functions by using a single cathode lamp that emits one specific wavelength of light unique to a specific element. The problem is that some anions are polyatomic, such as CO32-. This means that multiple lamps are required to detect each element, but AAS doesn't function if more than one wavelength is present, hence the inability to detect anions. However, some anions can be detected by AAS, such as Cl-, because they are single elements.
In the HSC, AAS is only used for metal ions since that was its original purpose to detect things like heavy metals (lead in water) and trace elements (copper in soil). ;)
Post by: bananna on October 31, 2017, 12:51:40 pm
is 1 each enough?
so like turbidity for quality and acidity for quantity?
thanks
also what are some other qualitative tests?
Post by: kauac on October 31, 2017, 12:52:41 pm
When drawing structural formulae for molecules in exam, do we need to draw individual bonds between atoms in a functional group? E.g. a bond between a oxygen and hydrogen atom in a hydroxyl group. Or can we just draw the groups in condensed form, such as OH, as long as they are in the appropriate order for bonding in the molecule?
Post by: bananna on October 31, 2017, 12:58:28 pm
Hi...
When drawing structural formulae for molecules in exam, do we need to draw individual bonds between atoms in a functional group? E.g. a bond between a oxygen and hydrogen atom in a hydroxyl group. Or can we just draw the groups in condensed form, such as OH, as long as they are in the appropriate order for bonding in the molecule?
my teacher said when drawing the structural formula of citric acid, you must draw out all bonds, including O-H
so yep, I think they are strict with structural formula and drawing out all necessary bonds. When they specifically ask for structural formula, that is.
otherwise, its fine to draw the condensed form
Post by: winstondarmawan on October 31, 2017, 12:59:07 pm
For example:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/23146248_1365590483566471_2141030131_n.jpg?oh=25754963e74fcebb116b007aaf0d0367&oe=59F9EB63
Post by: MisterNeo on October 31, 2017, 01:11:20 pm
Hi was just wondering for chem monitoring : how many quantitative /qualitative tests do we need to know practical-wiseFor the water quality tests (hardness, turbidity, TDS, etc), one qualitative and quantitative for each of them would be fine. So for turbidity, you can talk about Nephelometric Units as quantitative, and the appearance of water as qualitative. Or for hardness, precipitating calcium and magnesium would be qualitative to prove hardness, whereas soap lather height or gravimetric analysis would show the extent of hardness. There are also the cation/anion tests, where flame and precipitate tests are qualitative, and gravimetric analysis is quantitative. :)\
is 1 each enough?
so like turbidity for quality and acidity for quantity?
thanks
also what are some other qualitative tests?
Hi...I would draw out all bonds. Especially for ethanol solvent questions, where you would need to show the polar nature of ethanol on the -OH group. The only exception I would make is for drawing benzene rings on styrene and the -OH groups on cellulose/glucose.
When drawing structural formulae for molecules in exam, do we need to draw individual bonds between atoms in a functional group? E.g. a bond between a oxygen and hydrogen atom in a hydroxyl group. Or can we just draw the groups in condensed form, such as OH, as long as they are in the appropriate order for bonding in the molecule?
Post by: statues on October 31, 2017, 02:12:30 pm
Why is the Iupac name
2-bromo-2-chloro-1,1,1-triflouroethane (D)
preferable over
1-bromo-1-chloro-2,2,2-trifluoroethane (A)
Post by: angelahchan on October 31, 2017, 02:24:39 pm
This is from the 2016 MC Q11There's this rule called sum of smallest locants or something. D is 2-bromo-2chloro-1,1,1,trifluoroethane not 2-bromo-2-chloro-2,2,2-triflouroethane (so maybe you read over the wrong answer)
Why is the Iupac name
2-bromo-2-chloro-2,2,2-triflouroethane (D)
preferable over
1-bromo-1-chloro-2,2,2-trifluoroethane (A)
So the sum of the numbers for A is 1+1+2+2+2= 8
and the sum of numbers for D is 2+2+1+1+1 = 7
You have to choose the naming that gives the smallest sum- (I have no clue why, but that's a rule in the naming system), which is why the answer is D
Post by: statues on October 31, 2017, 02:47:59 pm
There's this rule called sum of smallest locants or something. D is 2-bromo-2chloro-1,1,1,trifluoroethane not 2-bromo-2-chloro-2,2,2-triflouroethane (so maybe you read over the wrong answer)
So the sum of the numbers for A is 1+1+2+2+2= 8
and the sum of numbers for D is 2+2+1+1+1 = 7
You have to choose the naming that gives the smallest sum- (I have no clue why, but that's a rule in the naming system), which is why the answer is D
ah cool beans thanks:)
Post by: liya1234 on October 31, 2017, 03:01:05 pm
Post by: MisterNeo on October 31, 2017, 03:04:16 pm
For the bromine water prac equations, is it okay to refer to bromine water as Br2(aq)? Will we lose marks if we don't use BrOHI've always used Br2(aq). In the HSC, they will accept it even though BrOH is the scientifically correct method. :P
Post by: Baylsskool on October 31, 2017, 03:05:26 pm
Post by: MisterNeo on October 31, 2017, 03:08:38 pm
For the dot point, name a synthetic replacment for natural product in industrial chemistry, does guano being replaced by superphosphate as a fertiliser count???I wouldn't do a fertiliser. I've seen questions in Trial papers (maybe a HSC paper) where they can specify not to use a fertiliser or a fossil fuel (it's to stop people from using Haber process). Most people do Styrene Butadiene, if you need an alternative. :)
Post by: J.B on October 31, 2017, 04:17:26 pm
In Titration Questions I also get confused which is in the Burette, and which is pipetted into the conical flask. So I was wondering what terminology they use to let you know?
Thank you
Post by: Dante1091 on October 31, 2017, 04:25:53 pm
HI,
In Titration Questions I also get confused which is in the Burette, and which is pipetted into the conical flask. So I was wondering what terminology they use to let you know?
Thank you
Remember that the Base in Burette, while the Acid + (indicator) are in the conical flask
Remember B for Burette and Base
Post by: J.B on October 31, 2017, 04:27:25 pm
Remember that the Base in Burette, while the Acid + (indicator) are in the conical flask
Remember B for Burette and Base
Thank you!
I was just very confused, because we did it both ways at my school.
Post by: beau77bro on October 31, 2017, 04:30:11 pm
Post by: mbdtHSC on October 31, 2017, 04:59:40 pm
ok for the adding H2SO4 step - what if there is Ca, how do u know if the precipitate means there is Ca alone or Ca and Ba cations?
Add NaF to the sample, if it's Ca2+ then CaF2 white ppt forms
Post by: winstondarmawan on October 31, 2017, 05:11:40 pm
[ https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/23023624_1365754790216707_196707540_n.png?oh=679c254c90b7ff9a64ffcdf9fec28b28&oe=59F9F75E
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/23113784_1365754990216687_594565583_n.png?oh=8d23c08682923b5849368a2267c647dc&oe=59F9CD8F ]
Part b).
TIA!
Post by: aryak on October 31, 2017, 05:19:32 pm
Post by: bsdfjnlkasn on October 31, 2017, 05:34:12 pm
Is it because U.V is absorbed in the formation of oxygen? Doesn't that deplete ozone though? Don't we need ozone in the stratosphere?
Post by: Mathew587 on October 31, 2017, 05:51:11 pm
Why is ozone good in the stratosphere?Yeah.
Is it because U.V is absorbed in the formation of oxygen? Doesn't that deplete ozone though? Don't we need ozone in the stratosphere?
Ozone assists in the maintenance of the ozone layer. CFCs result in the depletion of the ozone layer not oxygen.
Post by: ACCC on October 31, 2017, 05:52:38 pm
Quick question, for this equation
C6H8O7 + 3NaOH → C6H5O7Na3 + 3H2O
how is there 8 hydrogens in citric acid in reactants but only 7 in the products (combined)?
Mod edit: Merged :)
Post by: Natasha.97 on October 31, 2017, 06:44:03 pm
Hi is it ok if you could explain how to answer question 19 of the 2015 hsc multiple choice? I dont get why the answer is C.
toasted has answered this here :)
Quote from: ACCC
Just double checking, is the Q the same as delta H (in DeltaH = mc deltaT) or completely different? Thank you!
Both Qs are the same :)
Quote from: ACCC
Quick question, for this equation
C6H8O7 + 3NaOH → C6H5O7Na3 + 3H2O
how is there 8 hydrogens in citric acid in reactants but only 7 in the products (combined)?
On the reactant side, there are 11 Hydrogens (8 from citric acid, 3 from sodium hydroxide), and on the product side, there are also 11 Hydrogens (5 in sodium citrate and 6 in water) :)
Post by: ACCC on October 31, 2017, 06:48:35 pm
Post by: MisterNeo on October 31, 2017, 07:27:45 pm
Would appreciate help with this Q (2007 HSC):
[ https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/23023624_1365754790216707_196707540_n.png?oh=679c254c90b7ff9a64ffcdf9fec28b28&oe=59F9F75E
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/23113784_1365754990216687_594565583_n.png?oh=8d23c08682923b5849368a2267c647dc&oe=59F9CD8F ]
Part b).
TIA!
Hi is it ok if you could explain how to answer question 19 of the 2015 hsc multiple choice? I dont get why the answer is C.
Why is ozone good in the stratosphere?
Is it because U.V is absorbed in the formation of oxygen? Doesn't that deplete ozone though? Don't we need ozone in the stratosphere?
Oh! Thank you can't believe I missed that. But in citric acid the H8 changed to H5, does that always happen?Citric acid can donate one proton at a time like how sulfuric acid does. In the HSC, we just assume full protonation whenever it reacts with something, so all 3 hydrogens are used ;)
Post by: bsdfjnlkasn on October 31, 2017, 07:32:46 pm
Post by: angelahchan on October 31, 2017, 07:51:37 pm
https://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-hsc-chemistry.pdf
Post by: erinyyyyy on October 31, 2017, 08:02:42 pm
Post by: MisterNeo on October 31, 2017, 08:17:48 pm
Why can temperature only affect the equilibrium constant? :) thanks!The K constant is the ratio of products to reactants at a specific temperature.
When concentration/pressure changes, the ratio is out of balance, so the equilibrium will shift to minimise the change and re-establish that special ratio of products to reactants. When temperature changes, the equilibrium will shift to minimise that change in temperature and it will stay there, hence the K ratio is now changed as long as that temperature remains constant.
For example:
I have an exothermic reaction, and I increase the temperature from 400'C to 700'C and keep it there. The equilibrium will shift to the left to minimise the change but the position will stay there because it cannot forward react to the right since that's exothermic. hence, K is now changed.
Hope this explanation helps :)
Hi, can someone please help me with Q17 of the 2014 paper? thanks
https://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-hsc-chemistry.pdf
For the production of sulfuric acid why can’t water be added straight to SO3, why is oleum produced first?The reaction between sulfur trioxide and water is very exothermic and will create a fine mist of sulfuric acid everywhere, which is both dangerous and hard to clean up. The reaction between oleum and water is less exothermic and is safer to use. :P
Post by: liya1234 on October 31, 2017, 09:03:41 pm
Post by: ellybubble on October 31, 2017, 09:07:22 pm
For the production of sulfuric acid why can’t water be added straight to SO3, why is oleum produced first?
if water is added directly, its a very exothermic reaction, and risks the production of acid mist instead of liquid sulfuric acid, which is what we want. Acid mist is firstly very dangerous and secondly hard to convert back to liquid sulfuric acid. Thus, oleum is produced first, and then diluted to form sulfuric acid (l)
Post by: winstondarmawan on October 31, 2017, 09:10:57 pm
Hello, I'm struggling with knowing when exactly a line of best fit should be linear or a curve.
For example:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/23146248_1365590483566471_2141030131_n.jpg?oh=25754963e74fcebb116b007aaf0d0367&oe=59F9EB63
Bump. :) :) :)
Post by: MisterNeo on October 31, 2017, 09:15:54 pm
when doing calculations involving heats of combustion and specific heat capacities, should we always use J/kg/K because that is what is given? and how do you convert something that is in kJ/g/K to J/kg/K? thanks so much in advance, I really appreciate all the help here!For me, ΔH is always in joules, and for specific heat capacity of water (C) I just plug in 4.18 without any units (it's 4.18J/degree).
If your C is in kJ/degree (0.00418kJ), your ΔH must be in kJ. You just divide/multiply by 1000 since there are 1000J in 1kJ. :P
Post by: liya1234 on October 31, 2017, 09:29:10 pm
For me, ΔH is always in joules, and for specific heat capacity of water (C) I just plug in 4.18 without any units (it's 4.18J/degree).
If your C is in kJ/degree (0.00418kJ), your ΔH must be in kJ. You just divide/multiply by 1000 since there are 1000J in 1kJ. :P
okay thank you!! also then do we not need to consider whether it is g or kg as long as we use the same unit throughout?
Post by: bsdfjnlkasn on October 31, 2017, 10:02:07 pm
Post by: MisterNeo on October 31, 2017, 10:11:15 pm
Help please :)
Post by: winstondarmawan on October 31, 2017, 10:52:54 pm
For membrane filters, is it necessary to know the three types and their specifics (i.e. micro, ultra and nano)?
Post by: MisterNeo on October 31, 2017, 11:21:18 pm
Hello!Yeah, i would know them because they may ask something like “explain the use of membrane filters...”, which would require the 3 filters. Also note their pore sizes and what things they block (pathogens, suspended solids, etc). :)
For membrane filters, is it necessary to know the three types and their specifics (i.e. micro, ultra and nano)?
Post by: Noorijaz on October 31, 2017, 11:29:45 pm
is it the minimum number of sig figs in queation or max number of sig fig in question , which we address our answer to.???
Post by: Natasha.97 on November 01, 2017, 06:38:07 am
for calcs
is it the minimum number of sig figs in queation or max number of sig fig in question , which we address our answer to.???
You would change your answer to the least number of significant figures, e.g. if they gave you a question that had two values, one with 4 and one with 2 sig figs, change it to 2 :)
Post by: ilovemycat on November 05, 2017, 08:26:04 pm
Thank you :)
Post by: Natasha.97 on November 05, 2017, 08:53:26 pm
Hi, could someone please help me with this question on heat of combustion? Im getting a ridiculously small number (0.000001287) and im sure its not right . i always get confused when they ask "x% is lost to the surroundings" with these questions.. :(
Thank you :)
Hi!
Hope this helps
Edit: Fixed latex
Post by: ilovemycat on November 06, 2017, 06:21:30 am
Post by: jakesilove on November 07, 2017, 02:55:17 pm
hey, thanks so much for that. just wondering where you got the "0.0835" from?
Hey! To find the number of moles of Ethanol, Jess has divided the Energy that was RELEASED by the energy that WOULD BE RELEASED if one mole of Ethanol was burned. ie. 57.057/683.5= 0.0835. It's definitely a weird step, but one that you'll get super used to :)
Jake
Post by: itssona on November 07, 2017, 08:26:27 pm
CH3CHOHCH3
is that 2-propanol?
CH3CH2CHOHCH2CH2CH2OH
1,4-hex diol??
how do we draw this
also how do we do q4 from the 2005 hsc
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2005exams/pdf_doc/chemistry_05.pdf
Mod Edit: Post merge. :)
Post by: mxrylyn on November 07, 2017, 08:56:23 pm
So I'm looking at the syllabus dot point which states Explain what is meant by a condensation polymer
I understand that a condensation polymer is a polymer (long chain molecule) formed when pairs of monomers join and a small molecule (usually water) is eliminated.
but I don't understand the statement "Condensation polymers are made from monomers that have two different groups of atoms that can join together"
*I put the bit I don't understand in bold
also, does
"One or two types of monomers can be involved and no double bond is required – the bond forms where the molecule is eliminated."
mean that there cannot be three monomers? Also does it also mean that alkanes can form condensation polymers?
I'm also not really sure what is being addressed in this statement
"The end product of condensation polymerisation depends on the number of functional end groups of the monomer that can react. If there is only one reactive group the growing chian will terminate. With two reactive end groups a linear polymer if formed. with three or more reactive end groups a three-dimensional, cross0-linked polymer can be made"
^This is a statement I have memorised, but I have no idea what it means. My main problem is that I don't understand/am not used to the terminology.
Do we need to know about the formation of Nylon and Kevlar in the syllabus dot point Describe the reaction involved when a condensation polymer is formed ?When they say that, do they mean we need to be able to describe where the small molecule splits out and where the new bonds are formed?
Mod Edit: Post merge. :)
Post by: itssona on November 08, 2017, 02:41:32 pm
Post by: bdobrin on November 08, 2017, 05:22:49 pm
I was wondering if anyone could tell me what it means for a biopolymer to synthesize?
i.e. "douglas dennis was able to transfer genes from alcalegenes eutrophus that control PHB synthesis.
thanks,
ben
Post by: MisterNeo on November 08, 2017, 10:33:44 pm
Hi,It just means that the monomer units join together into a polymer. Synthesis is combining things, so in this instance it means the same as "polymerise".
I was wondering if anyone could tell me what it means for a biopolymer to synthesize?
i.e. "douglas dennis was able to transfer genes from alcalegenes eutrophus that control PHB synthesis.
thanks,
ben
would someone be able to illustrate using an example, how we use activity series to figure half equations of displacement redox stuff? thank you :)Hey ;)
A redox reaction always has a species being oxidised (lose electrons) and a species being reduced (gain electrons). In a displacement reaction, you need to determine if the solid metal will oxidise and the cations reduce.
Let's say you placed a piece of zinc into copper nitrate in a displacement reaction.
Will the zinc oxidise? Will the copper ions reduce?
This is when the Data Sheet is your best friend.
The voltages are potentials, or how willing zinc is to lose electrons, or how willing copper is to gain electrons. The larger the potential, the more spontaneous the reaction is. To see if a displacement reaction will actually occur, you need to find the total potential (Eo value) by adding zinc's oxidation potential (0.76V) with copper ions reduction potential (0.34V), which is 1.1V. Since it is positive, a reaction will occur because the displacement is willing to occur (spontaneous). The above equations are your half-equations.
If the total potential were negative, such as copper metal in zinc ions, there would be no half-equations because nothing happens.
Hope this helps :)
Hey!Esterification is a condensation reaction, but cannot polymerise because the 'monomers' only have one functional group each.
So I'm looking at the syllabus dot point which states Explain what is meant by a condensation polymer
I understand that a condensation polymer is a polymer (long chain molecule) formed when pairs of monomers join and a small molecule (usually water) is eliminated.
but I don't understand the statement "Condensation polymers are made from monomers that have two different groups of atoms that can join together"
*I put the bit I don't understand in bold
also, does
"One or two types of monomers can be involved and no double bond is required – the bond forms where the molecule is eliminated."
mean that there cannot be three monomers? Also does it also mean that alkanes can form condensation polymers?
I'm also not really sure what is being addressed in this statement
"The end product of condensation polymerisation depends on the number of functional end groups of the monomer that can react. If there is only one reactive group the growing chain will terminate. With two reactive end groups a linear polymer if formed. with three or more reactive end groups a three-dimensional, cross0-linked polymer can be made"
^This is a statement I have memorised, but I have no idea what it means. My main problem is that I don't understand/am not used to the terminology.
(https://i.imgur.com/41OiHoV.png)
Cellulose is a condensation polymer because each glucose monomer has multiple functional groups.
(https://i.imgur.com/leQwQn0.png)
For the 3D polymers, they're not in the syllabus. :P
Also, for the "describe condensation", you don't necessarily have to include where the water comes from. As long as you include "Monomers with two functional groups each polymerise to form a polymer and a small molecule such as water." then you'll get the marks. Give an example if you have enough lines, and talk about cellulose. :)
also how do we do q4 from the 2005 hscThe question states that the polymer is a condensation polymer, so the monomers must have two functional groups each (the two can be the same type). You can immediately eliminate A and C because of those H-benzene-H molecules, which have no functional groups (-H are not functional groups). All the other monomers have two hydroxyl groups that allow for condensation, however 1st one of Option B has a double bond, which would undergo addition on the carbons rather than condensation on its two hydroxyl groups. Hence, the answer is D. I would do a bit of trial-and-error tbh. ;D
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2005exams/pdf_doc/chemistry_05.pdf
can someone pls check my answers :/Yep, they're both correct. :)
CH3CHOHCH3
is that 2-propanol?
CH3CH2CHOHCH2CH2CH2OH
1,4-hex diol??
how do we draw this
2-propanol can also be propan-2-ol
Admire my fabulous Paint skills...
(https://i.imgur.com/VB4rYlj.png)
Post by: tina1395 on November 10, 2017, 08:33:38 pm
For my assessment task we have to research an Australian practising chemist. I decided to write about Maria Skyllas-Kazacos, who is an electrochemist and pioneered the Vanadium Redox Battery. However, most of her work was done in the 1980/90s, and there isnt too much information about what she is doing now. Would she still be considered as a practising chemist if she has retired?
Thanks :)
Post by: itsangiej101 on November 12, 2017, 04:07:35 pm
2. There are three main processes involved for the industrial production of ethanol; filtering, fermentation and distillation. For each process answer the following;
i) Physical or chemical reaction involved?
ii) Why is this procedure used?
iii) Outline the procedure.
This section is worth 3 marks
Any help is appreciated
Post by: daniel.hu3 on November 13, 2017, 11:05:26 am
Hey I have a chemistry research portfolio I have to finish and revise for an exam in like three days. Almost done but I have a section I'm not quite sure about
2. There are three main processes involved for the industrial production of ethanol; filtering, fermentation and distillation. For each process answer the following;
i) Physical or chemical reaction involved?
ii) Why is this procedure used?
iii) Outline the procedure.
This section is worth 3 marks
Any help is appreciated
I'll attempt to make this as concise as possible since this is a 3 mark question.
FILTERING (physical reaction): Before fermentation occurs, the sugar cane needs to be processed after it has been grown, harvested and transported. You are left with two main products- sugar and molasses. The high sucrose sugar cane juices are extracted via crushing. Sucrose is crystallised out and refined into table sugar. Molasses, a thick syrupy liquid still containing significant amounts of sucrose remains. Molasses is usually separated from the sugar by centrifuging, as it is denser than sugar, it falls to the bottom.
FERMENTATION (chemical reaction): The glucose molasses is fermented anaerobically by yeast to produce ethanol and carbon dioxide.(would be a good idea to include the fermentation equation here)
Conditions under which this reaction occurs: warm temperature of 37 degrees, anaerobic, presence of yeast, containing zymase enzymes, alcohol concentration of less than 15% v/v
DISTILLATION (physical reaction): The ethanol then undergoes distillation to produce purer ethanol of 95% v/v (this procedure is just to ensure you have ethanol of higher purity- additional info: afterwards, ethanol is also dehydrated to obtain 99.7% v/v, even higher purity!)
Please correct me if i made any mistakes, and hopefully this helps!
D.H.
Post by: itsangiej101 on November 13, 2017, 01:08:34 pm
I'll attempt to make this as concise as possible since this is a 3 mark question.
FILTERING (physical reaction): Before fermentation occurs, the sugar cane needs to be processed after it has been grown, harvested and transported. You are left with two main products- sugar and molasses. The high sucrose sugar cane juices are extracted via crushing. Sucrose is crystallised out and refined into table sugar. Molasses, a thick syrupy liquid still containing significant amounts of sucrose remains. Molasses is usually separated from the sugar by centrifuging, as it is denser than sugar, it falls to the bottom.
FERMENTATION (chemical reaction): The glucose molasses is fermented anaerobically by yeast to produce ethanol and carbon dioxide.(would be a good idea to include the fermentation equation here)
Conditions under which this reaction occurs: warm temperature of 37 degrees, anaerobic, presence of yeast, containing zymase enzymes, alcohol concentration of less than 15% v/v
DISTILLATION (physical reaction): The ethanol then undergoes distillation to produce purer ethanol of 95% v/v (this procedure is just to ensure you have ethanol of higher purity- additional info: afterwards, ethanol is also dehydrated to obtain 99.7% v/v, even higher purity!)
Please correct me if i made any mistakes, and hopefully this helps!
D.H.
Thank you so much daniel! I think the information you put up would be consise enough for the marks allocated
Post by: jakesilove on November 13, 2017, 01:39:59 pm
Hi,
For my assessment task we have to research an Australian practising chemist. I decided to write about Maria Skyllas-Kazacos, who is an electrochemist and pioneered the Vanadium Redox Battery. However, most of her work was done in the 1980/90s, and there isnt too much information about what she is doing now. Would she still be considered as a practising chemist if she has retired?
Thanks :)
Hey!
I would probably recommend using a chemist that is currently working in some field in Australia. Not that you'll ever be assessed on this particular dot-point, but I believe the chemist ought to be 'practising'; Maria probably doesn't count as practising anymore.
Post by: itsangiej101 on November 13, 2017, 07:54:17 pm
Thanks!
Post by: MisterNeo on November 13, 2017, 10:25:12 pm
I have an exam on wednesday thats both prac and research based. Does anyone know specifically what kind of questions exams usually ask relating to ethanol as fuel, production of ethanol (fermenation), comparison of battery cells and galvanic cell experiment?Hey :)
Thanks!
From the top of my head, possible questions include:
-Calculate the mass of X produced if Y was used, sometime it gives you a graph to extract data. (Fermentation)
-Outline the conditions of fermentation and its limitations (the ethanol concentration).
-Advantages and disadvantages of ethanol as a alternative to octane fuel, environmentally and societal.
-Calculate X if this mass of ethanol was burned. This would involve heat of combustion and some variable of the -mCΔT formula, ie they might give you everything except mass of water and then ask you to calculate mass of water based on ethanol’s HoC.
-Battery cell questions are mostly always “Compare the impact in society and the environment...” which you would compare two batteries.
-Galvanic cell questions can give you two metals and ask you to draw a galvanic cell with the electrodes labelled (Data Sheet). Then they can ask you to write half equations/net ionic equation and calculate total E value. Also you may be asked to justify the use of a salt bridge.
Hope this helps, good luck with your exam ;)
Post by: mxrylyn on November 15, 2017, 06:07:05 am
I'm very confused about the temperature In which ethanol gets dehydrated.
My chemistry contexts 2nd edition textbook says it happens at 350 degrees, but my Atarnotes chemistry poster says it happens at 170 degrees.
For the hydration of ethylene, my the aforementioned text bok says it happens at 300 degrees, but my Excel chem study guide says it happens at 70-100 degrees
Post by: justwannawish on November 15, 2017, 07:04:07 pm
Any advice on practicals about titration, dilutions and indicator testing?
Post by: MisterNeo on November 15, 2017, 08:02:03 pm
Hey.I’ve always used 300 for hydration and 170 for dehydration without any issues. :)
I'm very confused about the temperature In which ethanol gets dehydrated.
My chemistry contexts 2nd edition textbook says it happens at 350 degrees, but my Atarnotes chemistry poster says it happens at 170 degrees.
For the hydration of ethylene, my the aforementioned text bok says it happens at 300 degrees, but my Excel chem study guide says it happens at 70-100 degrees
Hey guys,An error I once made in titration was not reading the miniscus properly. Make sure it is read from the bottom of the concave at eye level. Don’t force out the liquid at the tip of the pipette because it has been calibrated to account for the lost volume. Rinse everything with the solution that is going to be used in it except for the conical flask (water). Don’t use a beaker to measure volume in titrations because its markings are estimated, use that tall beaker. Do the titration on top of a white surface like paper so that changes of the indicator colour are easier to see. Don’t put too much indicator (I did this lol) because they can skew the titration results if you overdo it as they’re slightly acidic/basic themselves. When the titration is approaching the end point, open the burette drop by drop and swirl the flask after every drop. If the drop of titre is clinging onto the top of the burette, don’t risk opening the burette anymore because you might end up adding multiple drops, which could exceed the end point. Instead, spray the droplet off with distilled water from one of those plastic bottles with the long tube. It doesn’t matter if concentration of the conical flask changes because the moles are still the same. The end point is reached when the colour change stays when you swirl the beaker. For example, titration NaOH with a weak acid you would use phenolphthalein. When the phenolphthalein keeps a slight pink shade even after you swirl the flask, that would be the end point.
Any advice on practicals about titration, dilutions and indicator testing?
Hope this helps ;)
Post by: LaraC on November 15, 2017, 08:10:45 pm
Sorry I'm new on this site so hope I'm not messing things round - not really sure how everything works yet! But Jake, you seem like the person I need to talk to atm! ;) Getting rather confused about a few things in my HSC chemistry......
Wondering if you could help me with the following qu:
Carbowax can also be considered as an addition polymer of ethylene oxide {as well asn a condensation polymer formed from ethylene glycol} . Explain how this can be so.
Thanks so much!!
Post by: MisterNeo on November 16, 2017, 12:36:03 am
Hello,
Sorry I'm new on this site so hope I'm not messing things round - not really sure how everything works yet! But Jake, you seem like the person I need to talk to atm! ;) Getting rather confused about a few things in my HSC chemistry......
Wondering if you could help me with the following qu:
Carbowax can also be considered as an addition polymer of ethylene oxide {as well asn a condensation polymer formed from ethylene glycol} . Explain how this can be so.
Thanks so much!!
Hey! Welcome to ATAR Notes! ;)
An addition polymerisation occurs when monomers join to form a polymer and nothing else. This usually occurs when some part of the monomer is quite reactive or unstable. The pyramid-looking oxygen in ethylene oxide is quite unstable and will break. This also occurs in ethylene when forming polyethylene because the double bond is reactive.
(https://i.imgur.com/BlPAtqI.jpg)
A condensation polymerisation forms a small molecule along with the polymer, which is usually water. This occurs when the monomer itself is fairly stable but it has two functional groups (in this case, hydroxyl) that can break off to form the small water molecule. Ethylene glycol has those two -OH groups that allow condensation reactions to occur. The free radicals of the detached atoms then join together to form the carbowax polymer.
(https://i.imgur.com/CT6KSpO.jpg)
Hope this helps :)
Post by: LaraC on November 16, 2017, 09:38:43 am
Post by: mxrylyn on November 19, 2017, 07:18:07 am
So I know how to write a cyclohexene + Bromine Water reaction equation, but I'm not sure how I would write the same reaction equation with cyclohexane is apposed to cyclohexene.
Post by: Natasha.97 on November 19, 2017, 07:30:09 am
Hey,
So I know how to write a cyclohexene + Bromine Water reaction equation, but I'm not sure how I would write the same reaction equation with cyclohexane is apposed to cyclohexene.
Hi!
Even though BrOH is technically more scientifically correct, Br2(aq) is accepted. This would result in the following equations:
Cyclohexane + Bromine Water
- C6H12(aq) + Br2(aq) → C6H11Br(aq) + HBr(aq)
- Cyclohexane + Bromine → 1-bromohexane + Hydrogen bromide
Cyclohexene + Bromine Water
- C6H10(aq) + Br2(aq) → C6H10Br2(aq)
- Cyclohexene + Bromine → 1,2-dibromohexane
Hope this helps :)
Post by: justwannawish on November 19, 2017, 09:55:00 am
I’ve always used 300 for hydration and 170 for dehydration without any issues. :)An error I once made in titration was not reading the miniscus properly. Make sure it is read from the bottom of the concave at eye level. Don’t force out the liquid at the tip of the pipette because it has been calibrated to account for the lost volume. Rinse everything with the solution that is going to be used in it except for the conical flask (water). Don’t use a beaker to measure volume in titrations because its markings are estimated, use that tall beaker. Do the titration on top of a white surface like paper so that changes of the indicator colour are easier to see. Don’t put too much indicator (I did this lol) because they can skew the titration results if you overdo it as they’re slightly acidic/basic themselves. When the titration is approaching the end point, open the burette drop by drop and swirl the flask after every drop. If the drop of titre is clinging onto the top of the burette, don’t risk opening the burette anymore because you might end up adding multiple drops, which could exceed the end point. Instead, spray the droplet off with distilled water from one of those plastic bottles with the long tube. It doesn’t matter if concentration of the conical flask changes because the moles are still the same. The end point is reached when the colour change stays when you swirl the beaker. For example, titration NaOH with a weak acid you would use phenolphthalein. When the phenolphthalein keeps a slight pink shade even after you swirl the flask, that would be the end point.
Hope this helps ;)
Thank you so much! I totally missed this. But could you please explain how to get the appropriate indicator for each standard solution and acid? I don't really get the conjugate acid/ base theorem
Post by: Natasha.97 on November 19, 2017, 09:41:01 pm
Thank you so much! I totally missed this. But could you please explain how to get the appropriate indicator for each standard solution and acid? I don't really get the conjugate acid/ base theorem
Hi!
The indicator is chosen by predicting the acidity/basicity of the produced salt. There are 3 probabilities:
- Strong acid + Strong base = Neutral [Use litmus (purple) or bromothymol blue(green)]
- Strong acid + Weak base = Acidic [Use methyl orange(red)/bromothymol blue (yellow)]
- Weak acid + Strong base = Basic [Use phenolphthalein(pink)]
Hope this helps :)
Post by: mxrylyn on November 20, 2017, 08:53:11 pm
Hi!
Even though BrOH is technically more scientifically correct, Br2(aq) is accepted. This would result in the following equations:
Cyclohexane + Bromine Water
- C6H12(aq) + Br2(aq) → C6H11Br(aq) + HBr(aq)
- Cyclohexane + Bromine → 1-bromohexane + Hydrogen bromide
Cyclohexene + Bromine Water
- C6H10(aq) + Br2(aq) → C6H10Br2(aq)
- Cyclohexene + Bromine → 1,2-dibromohexane
Hope this helps :)
Thank you!
How would I then show the reaction between cyclohexane and cyclohexene with acidified potassium permanganate (KMnO4)?
Post by: Natasha.97 on November 20, 2017, 09:15:12 pm
Thank you!
How would I then show the reaction between cyclohexane and cyclohexene with acidified potassium permanganate (KMnO4)?
Hi!
Cyclohexene will decolourise the potassium permanganate solution, while cyclohexane will not. The permanganate ions are reduced to manganese (II) ions (Mn2+). The reaction is as follows:
- 3C6H10 + 2KMnO4 + 4H2O → 3C6H12O2 + 2MnO2 + 2KOH
- Cyclohexene + potassium permanganate + water → 1,2-cyclohexanediol + manganese dioxide + potassium hydroxide
Hope this helps :)
Post by: Mate2425 on November 20, 2017, 11:51:34 pm
Post by: Natasha.97 on November 21, 2017, 08:45:00 am
Hi, Jake I was just wondering if you could please help me with explaining the manufacturing of the radioisotope "Technetium - 99m" and the process required to obtain the Technetium 99m for medical use. Thanks Heaps :)
Hi!
Firstly, Molybdenum-98 undergoes neutron bombardment in a nuclear reactor to produce the Molybdenum-99:
(https://i.imgur.com/uC2JD47.png)
Then, Molybdenum-99 (half life of 66 hours) is transported to hospitals where it undergoes beta decay to form Technetium-99m:
(https://i.imgur.com/ZjDyLH3.png)
Technetium-99m decays to form Technetium-99, releasing gamma rays in the process:
(https://i.imgur.com/qZxJ49u.png)
Hope this helps :)
Post by: Mate2425 on November 21, 2017, 02:22:10 pm
Hi!
Firstly, Molybdenum-98 undergoes neutron bombardment in a nuclear reactor to produce the Molybdenum-99:
(https://i.imgur.com/uC2JD47.png)
Then, Molybdenum-99 (half life of 66 hours) is transported to hospitals where it undergoes beta decay to form Technetium-99m:
(https://i.imgur.com/ZjDyLH3.png)
Technetium-99m decays to form Technetium-99, releasing gamma rays in the process:
(https://i.imgur.com/qZxJ49u.png)
Hope this helps :)
Thank you it helps alot 😃
Post by: cnimm2000 on November 21, 2017, 03:02:24 pm
In the chemistry syllabus there is a dot point which asks you to identify a practising male and female Australian chemist. I was struggling finding an Australian female research chemist. it would be great if you had any suggestions.
thanks
Post by: mary123987 on November 22, 2017, 08:21:16 pm
Hey,Hey welcome to the forums!
In the chemistry syllabus there is a dot point which asks you to identify a practising male and female Australian chemist. I was struggling finding an Australian female research chemist. it would be great if you had any suggestions.
thanks
did a quick search you can use :
Professor Michelle Coote
This website has some good info http://rsc.anu.edu.au/~mcoote/
But quite frankly if you are looking for more info use a male chemist.
Anyways goodluck if you need help let me know
Post by: justwannawish on November 25, 2017, 10:19:25 am
Calculate the pH of the solution produced by mixing 50ml of 0.1M HCl with 20mL of 0.05M NaOH
Post by: Natasha.97 on November 25, 2017, 11:54:25 am
How do I do this question:Hi!
Calculate the pH of the solution produced by mixing 50ml of 0.1M HCl with 20mL of 0.05M NaOH
1. Write out the balanced equation between the reactants
2. Calculate the moles of H+ and OH- by using the formula c=nv (make sure that the volume is in L!)
3. Calculate the difference between the two values to determine the excess H+ moles
4. Calculate the concentration of H-: divide moles by the total volume
5. Use the formula pH = -log10[H+] to find the pH.
I’ve attached my working in case you get stuck, hope this helps :)
Edit: Fixed superscript
Post by: JaredMayo on November 28, 2017, 10:04:47 am
Post by: Natasha.97 on November 28, 2017, 10:52:22 am
Hey do any of you think you could describe the processes of complete ionisation (dissociation) and incomplete ionisation (dissociation) (using balanced equations with states of matter)
Hi!
Strong acids such as Hydrochloric Acid (HCl) undergo complete ionisation (all acid molecules have ionised to hydronium ions).
HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq)
Weak acids such as Acetic Acid (CH3COOH) undergo incomplete ionisation (some acid molecules have ionised to hydronium ions).
CH3COOH(aq) + H2O(l) ⇌ H3O+(aq) + CH3COO-(aq)
Here is a diagram to illustrate:
(https://i.imgur.com/CzLEzkl.png)
Hope this helps :)
Edit: Added states
Post by: JaredMayo on November 28, 2017, 11:25:47 am
Hi!
Strong acids such as Hydrochloric Acid (HCl) undergo complete ionisation (all acid molecules have ionised to hydronium ions).
HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq)
Weak acids such as Acetic Acid (CH3COOH) undergo incomplete ionisation (some acid molecules have ionised to hydronium ions).
CH3COOH(aq) + H2O(l) ⇌ H3O+(aq) + CH3COO-(aq)
Here is a diagram to illustrate:
(https://i.imgur.com/CzLEzkl.png)
Hope this helps :)
Edit: Added states
Hi, its 6 marks do you think i could add anything else
Post by: nin.jas17 on November 28, 2017, 12:07:23 pm
What are like the key chemical and physical process in this production?
Post by: jakesilove on November 28, 2017, 02:51:20 pm
Hi! I'm doing a research task on ethanol and I'm confused on the 'INDUSTRIAL production of ethanol from sugar cane.' I've done like initial research but websites keep telling me different things.
What are like the key chemical and physical process in this production?
Hey! Here's a brief summary of the production of ethanol from Sugar Cane.
Sugar Cane
We use sugar cane to produce ethanol because it contains biomass. Biomass makes up 50% of dry plant matter. Chemically, we are interested in the cellulose, which is a 'polymer' often made of over 10,000 glucose base units. A huge amount of biomass is produced each year; over 5*10^11 tones of the stuff! This makes biomass incredibly renewable.
First, we smash up the sugar cane to extract the biomass.
Cellulose to Glucose
Next, we want to turn the cellulose into glucose. Remember, glucose is the 'monomer' of cellulose. To do this, we either use acid electrolysis or specific enzymes which digest cellulose. You don't need to understand how either of these processes work; just that they do, and that they are expensive and time consuming.
Glucose to ethanol
Next, we want to turn our glucose into ethanol. To do this, we ferment the glucose. Successful fermentation requires a number of conditions to be met;
- pH neutral water
- Yeast
- Low oxygen concentrations
- Nutrients (eg. glucose)
- Alcohol concentrations of <14%
- Warm temperatures (around 30 degrees)
The chemical equation which describes fermentation is
Ethanol purification
Now, we have a mixture of ethanol, glucose, yeast, and a bunch of other stuff that we don't want. To purify the ethanol, we use a technique called fractional distillation. This exploits the difference in boiling points between different substances to extract a particular substance. Ethanol has a low boiling point (as it is an alcohol), and so readily boils!
Hope this helps!
Jake
Post by: MisterNeo on November 28, 2017, 04:20:24 pm
Hi, its 6 marks do you think i could add anything else
Acids exhibit their properties through ionisation to form the H+ ion (proton). However, not all acids ionise to the same extent as they can be either strong or weak in nature.
Hydrochloric acid is an example of a strong ionisation acid in water, as it is 'willing' to donate its proton to a water molecule to form H3O+, and it's equation lies well to the right.
Acetic acid is an example of a weak ionisation acid in water, as it isn't 'willing' to donate its proton to water, and lies well to the left, forming an equilibrium.
As a result, HCl is more acidic than acetic acids as it has more H+ ions in solution.
A diagram would be helpful too. Maybe talk about bases if you have enough room. :)
Post by: itssona on November 28, 2017, 05:37:08 pm
stuck on this
write the half equations, overall reaction and shorthand representation for the following electrochemical cell:
an electrode consists of a piece of silver dipping into a silver nitrate solution. Its connected by a salt bridge to another electrode consisting of a piece of platinum dipping into a chloride solution with chlorine gas bubbling through the solution over the inert platinum electrode. The platinum is the positive electrode
so
not sure, because they are dipped in different solutions
am I right by saying that platinum is the cathode and thus is "reducing"
how does this cell work? :(
Post by: MisterNeo on November 29, 2017, 07:17:13 am
Hii
stuck on this
write the half equations, overall reaction and shorthand representation for the following electrochemical cell:
an electrode consists of a piece of silver dipping into a silver nitrate solution. Its connected by a salt bridge to another electrode consisting of a piece of platinum dipping into a chloride solution with chlorine gas bubbling through the solution over the inert platinum electrode. The platinum is the positive electrode
so
not sure, because they are dipped in different solutions
am I right by saying that platinum is the cathode and thus is "reducing"
how does this cell work? :(
Hey ;)
It would be correct to say that platinum 'acts as a cathode', however it's not reducing itself but the chlorine gas (watch out for wording). The cell works like any other galvanic cell where there are two half-cells. This one uses a platinum electrode because chlorine gas can't have a solid chlorine electrode, hence it's bubbled through.
Since the chlorine is going to be reduced (positive cathode), its half equation would be:
The silver would be the anode and is being oxidised:
Overall:
Hope this helps :)
Post by: justwannawish on November 29, 2017, 03:43:38 pm
How do you do questions when you're finding the [H+] concentration and how does that vary with finding the concentration of a solution itself? Do you have to consider diprotic acids for both or just for the H+ concentration
Post by: RustyWasTaken on December 02, 2017, 02:43:38 pm
Post by: Natasha.97 on December 02, 2017, 10:04:38 pm
Hello please help me"Outline the chemistry of the fermentation process and using flow charts, compare the process of the industrial production of ethanol from both sugarcane and petroleum. Label and state the purpose of each step in the process. (5 MARKS). Not sure how to answer properly
Hi!
For petroleum, you would have the following steps:
- Fractional distillation: Separates crude oil into its constituent fractions
- Thermal cracking of long-chained hydrocarbons (750°C) OR catalytic cracking (Zeolite, 500°C) to form ethylene
- Catalytic hydration of ethylene (H2SO4/H3PO4) into ethanol
For sugarcane (cellulose), you would have the following steps
-Enzymatic breakdown by zymase/maltase/invertase to form glucose
-Fermentation of glucose to form ethanol: List out essential conditions such as an anaerobic environment, water (for the reaction to take place in), alcohol tolerant yeast (maximum is 15%), and a moderately warm temperature (~30°C).
-Distillation: The ethanol produced is at 15%. Fractional distillation can be used to produce concentrated ethanol (~96%)
Hope this helps :)
Post by: Aaron12038488 on December 08, 2017, 01:57:40 pm
use an equation to support this
Post by: MisterNeo on December 09, 2017, 04:34:04 pm
help. Describe how transuranic isotopes are produced in particle accelerators?
use an equation to support this
Hey ;)
Firstly, I would state that particle accelerators can linear or cyclotrons and are used to create transuranic elements such as Hassium-264 by bombarding Lead-208 with Iron-58 at high speeds achieved using electromagnetic fields.
20882Pb + 5826Fe → 264108Hs + 11p
Post by: Dragomistress on December 26, 2017, 12:47:25 pm
Post by: MisterNeo on December 26, 2017, 05:30:29 pm
How do I do this. Draw the polymer for ethylene.
Hey ;)
Ethylene can act as a monomer (repeating unit) and polymerise into polyethylene. There are two types of polyethylene, low-density and high-density. For most drawing questions, you would draw the high-density version with at least 3 monomers polymerised (you can lose marks for this). And make sure you put "[ ]" square brackets at the end of the polymer to show that it continues.
(https://i.imgur.com/nna81JX.png)
Hope this helps ;)
Post by: ilovemycat on December 28, 2017, 01:30:17 pm
Post by: MisterNeo on December 28, 2017, 01:56:59 pm
hey! anyone know how to do this heat of combustion question? im stuck :(
Post by: mxrylyn on January 01, 2018, 11:43:33 am
On Process information from secondary sources to summarise the processes involved in the industrial production of ethanol from sugar cane
My excel study guide has process which goes sugarcane > fermentation >distillation > bioethanol,
Whereas my AN Chemistry poster goes sugar cane >cellulose >glucose>ethanol (>Fractional distillation > ethylene)
Are these processes The same? Are they different? If so, what is the difference between them? And which one is better to memorise for HSC.
Also, I haven't gotten up to this point in my notes yet, but I know I'm gonna have trouble with it so I would like to ask now. What is the balanced equation for the fermentation of glucose to ethanol?
Post by: Natasha.97 on January 01, 2018, 11:55:37 am
Hey. Happy new years everybody.
On Process information from secondary sources to summarise the processes involved in the industrial production of ethanol from sugar cane
My excel study guide has process which goes sugarcane > fermentation >distillation > bioethanol,
Whereas my AN Chemistry poster goes sugar cane >cellulose >glucose>ethanol (>Fractional distillation > ethylene)
Are these processes The same? Are they different? If so, what is the difference between them? And which one is better to memorise for HSC.
Also, I haven't gotten up to this point in my notes yet, but I know I'm gonna have trouble with it so I would like to ask now. What is the balanced equation for the fermentation of glucose to ethanol?
Hi!
The excel version has simplified the process (They didn't include the smashing of sugarcane to extract cellulose, nor the acid digestion of cellulose to form glucose [specific details aren't needed]). In the second process, fermentation (yeast) is how glucose is turned into ethanol. Fractional distillation allows the ethanol to increase its purity (from ~15% to ~95%).
Balanced equation:
C6H12O6(aq) --> 2C2H5OH(aq) + 2CO2(g)
Hope this helps :)
Post by: mxrylyn on January 01, 2018, 01:49:11 pm
solve problems, plan and perform a first hand investigation to carry out the fermentation of glucose and monitor mass changes
What do the equations, C6H12O6 ---yeast--->2CH3CH2OH +2CO2 and Ca(OH)2 + CO2 ------> CaCO3 + H2O, represent in the syllabus dotpoint
solve problems, plan and perform a first hand investigation to carry out the fermentation of glucose and monitor mass changes
I just realised that the first one is the fermentation of Glucose to ethanol. I still don't know what the second one is :)
Mod edit: Merged double post. Please edit your existing post instead of posting two in a row.
Post by: Natasha.97 on January 01, 2018, 02:08:31 pm
What do the equations, C6H12O6 ---yeast--->2CH3CH2OH +2CO2 and Ca(OH)2 + CO2 ------> CaCO3 + H2O, represent in the syllabus dotpoint
solve problems, plan and perform a first hand investigation to carry out the fermentation of glucose and monitor mass changes
I just realised that the first one is the fermentation of Glucose to ethanol. I still don't know what the second one is :)
Mod edit: Merged double post. Please edit your existing post instead of posting two in a row.
Hi!
The second equation is a confirmation test for carbon dioxide: calcium hydroxide is soluble. Upon reaction with carbon dioxide, it forms calcium carbonate, which is insoluble (forms a white precipitate: it appears milky). It's relevant to the dot-point as the mass change is due to the carbon dioxide formed.
Post by: itssona on January 01, 2018, 08:32:14 pm
what about for formation of polystyrene? not sure how to write the benzene ring in the equation
thank you!!
Post by: Natasha.97 on January 01, 2018, 08:41:12 pm
hii so for the formation of PVC I would say the equation is nCH2CHCl--> (CH2CHCl)n
what about for formation of polystyrene? not sure how to write the benzene ring in the equation
thank you!!
Hi!
I have yet to see a HSC question asking about the formation of these two polymers. Styrene is commonly written as C8H8, so the equation would be nC8H8 --> (C8H8)n.
Post by: itssona on January 02, 2018, 02:55:29 pm
thank youu
Post by: kiwiberry on January 02, 2018, 03:13:18 pm
does ethanol have a high molar heat of combustion? is this a good thing for its use as a fuel?
thank youu
Compared to petrol, ethanol has a lower molar heat of combustion, meaning that it will release less energy and be less efficient - this is a disadvantage of using ethanol as an alternative fuel :)
Post by: itssona on January 03, 2018, 01:08:41 pm
I know zinc is oxidising and copper is reducing but how do I write the equations for copper reducing?
Cu2So4..
thank you
also, when copper is dipped in silver nitrate solution, what is formed? and if copper loses electrons, what does it become?
Post by: RuiAce on January 03, 2018, 01:11:26 pm
When zinc metal is placed in copper sulfate solution, the zinc will displace the copper ions, forming zinc sulfate and solid copper
I know zinc is oxidising and copper is reducing but how do I write the equations for copper reducing?
Cu2So4..
thank you
Recall that sulfate is just a spectator ion
Post by: itssona on January 03, 2018, 05:29:56 pm
thank you
Post by: Natasha.97 on January 03, 2018, 05:37:58 pm
if I have a galvanic cell in which copper electrode is in CuSO4 and zinc is in ZnSO4 then why does the cathode electrode (copper) become bigger due to a deposit? is it because it is reducing and thus the solution of CuSO4 becomes an atom instead of remaining as an ion?
thank you
Hi!
In the galvanic cell, electrons flow from the Zinc anode to the Copper cathode as Zn is more reactive than Cu. The Zinc anode is oxidised (loses electrons, AN OX) and the Copper cathode is reduced (gains electrons, RED CAT). The Zinc anode loses mass as it forms ions, and the Copper cathode gains mass as the copper ions from the CuSO4 solution are displaced.
Hope this helps :)
Post by: swordkillz on January 03, 2018, 05:50:33 pm
Post by: Natasha.97 on January 03, 2018, 06:13:04 pm
Can someone clarify whether Cobalt-60 is a medical or industrial radioisotope? I see many people say it is used industrially as a food stain removal/irradiation function while others say that it is used medically for killing cancer cells deposited across the human body. Can someone explain benefits and problems of both?
Hi :)
Cobalt-60 can be used for both medical and industrial purposes! However, if you're using it as a medical isotope, know a different industrial isotope e.g. Mg-24, and if you're using it as an industrial isotope, know a different medical isotope e.g. Tc-99m.
Industrial
(1) Benefits:
- Low energy emitting (reduced radiation exposure)
- Gamma ray emitter (able to penetrate metals)
(2) Problems:
- Health of workers is still at risk (low gamma radiation can still cause harm, expensive to implement safety measures)
- Relatively short half-life (5 years), frequent equipment replacement required [/list]
Medical
Benefits:
- Able to attack malignant tumours (can penetrate body tissue)
Hope this helps :)
Post by: itssona on January 03, 2018, 06:46:10 pm
Post by: Natasha.97 on January 03, 2018, 06:57:02 pm
heey to calculate the potential E, do we subtract the cathode metal from the anode always?
Hi!
I'll use the previous galvanic cell as an example (Zinc and Copper).
The reaction at the Cu cathode is Cu2+ + 2e- --> Cu, with a cell potential of +0.34V. For the Zn anode, reverse the standard reduction equation and cell potential on the data sheet: Zn --> Zn2+ + 2e-, with a cell potential of +0.76V (was -0.76V). Then add the two cell potentials to get 1.1V.
If we calculated it using your method, it would result in the same potential, but I find that it is more beneficial to go through the "longer" method of thinking about what is happening at each electrode and why we reverse the reduction equation for the anode (in case a really tricky question comes up in the HSC).
Hope this helps :)
Post by: itssona on January 04, 2018, 03:18:07 pm
thank you :)
Post by: RuiAce on January 04, 2018, 04:10:19 pm
does alpha decay occur when the nucleus is too large? what about beta decay - unstable nucleus or too many neutrons?Yeah. I don't believe this is "literally" what goes on but you can think of things this way for some understanding.
thank you :)
When your nucleus is just too large, you just want to get rid of a bit of everything. Because if you only get rid of one thing, you're gonna do weird things to the neutron:proton ratio. So in alpha decay, you just eject a whole helium nucleus (2 protons, 2 neutrons) to make life easier.
But when your n:p ratio is what's too large, you would want to have less neutrons and more protons to balance things out. In beta decay, an electron is ejected from the nucleus. That electron is pretty much being ejected from a neutron, which decays into a proton. So what have you really done? Turned a neutron to a proton, which makes the n:p ratio smaller.
In both cases, the nucleus could be said to be unstable. But the instability is caused by different factors.
Post by: shellmekler on January 07, 2018, 11:23:13 am
I need to know the properties, growth conditions and recent developments which led to making PHBV - please any help would be much appreciated.
Also, the formula for PHB !
Thanks in advance,
Shelley
Post by: itssona on January 07, 2018, 02:24:35 pm
Hi,Biopol (PHBV) is actually a co-polymer of two other biopolymers, polyhydroxybutyrate and polyhydroxyvalerate.
I need to know the properties, growth conditions and recent developments which led to making PHBV - please any help would be much appreciated.
Also, the formula for PHB !
Thanks in advance,
Shelley
Its development began after the development of PHB (since PHB makes PHBV)
During 1923-1927, Maurice Lemoigne researched about using bacteria to produce polyesters, and discovered PHB. But it was forgotten since need for this biopolymer at the time was unneccesary. In 1957, PHB was rediscovered in Britain and USA. The mass production of PHB began after the 1990's because of the growing need of a environmentally friendly plastic alternative.
It is produced by the bacteria Alcaligenes Eutrophus. When this bacteria is placed in conditions with an excess of carbon (glucose) and the deficiency of another nutrient (usually nitrogen) . Under these conditions the organism is no longer able to increase its population but instead produces PHBV as a storage reservoir of the carbon. PHBV is produced in cytoplasmic granules (30-80% dry weight). The biopolymer is then purified by dissolving it in a chlorinated hydrocarbon (such as trichloromethane), centrifuging to remove solid waste and precipitating the final product solution and drying it to form powder.
Biopol has similar properties to common synthetic plastics; thermoplastic, elastomeric, insoluble to water, strong and resistant to chemical attack. It also has the added advantage of being biodegradable. This allows it to be used as disposable nappies, wrapping film and plastic bags.
Its applications within the medical industry have been comparatively successful, as the nontoxic and biodegradable nature of PHB removes the need for follow-up surgeries to remove medical sutures, which will now decompose over time.
However, in general, such success is constrained by its low usage. However continued research is required to overcome the economic disadvantages and improve the properties of Biopol, extending its potential for usage. Certainly, its impact upon the environment will be significant.
What is the relationship between dissolved oxygen and biochemical oxygen demand?
Mod edit: Merged posts :)
Post by: Natasha.97 on January 07, 2018, 09:04:26 pm
What is the relationship between dissolved oxygen and biochemical oxygen demand?
Hi!
Let's start off with some definitions:
- Dissolved Oxygen (DO): Pretty self explanatory (amount of oxygen dissolved in water)
- Biochemical Oxygen Demand (BOD): The amount of oxygen consumed by microorganisms which break down organic waste
These indicators of water quality are inversely related. Take the following two scenarios for example:
(1) If there is a large amount of organic waste, microorganisms will consume more oxygen to break it down. This means that the BOD is high. At the same time, as the microorganisms are depleting the supply of oxygen in the water, the DO level decreases.
(2) If there is a low level of organic waste, microorganisms will not consume as much oxygen (BOD is low). Thus, the level of DO increases.
Hope this helps :)
Post by: itssona on January 08, 2018, 12:14:42 pm
Hi!that was a really nice explanation, thank you so much jess :3
Let's start off with some definitions:
- Dissolved Oxygen (DO): Pretty self explanatory (amount of oxygen dissolved in water)
- Biochemical Oxygen Demand (BOD): The amount of oxygen consumed by microorganisms which break down organic waste
These indicators of water quality are inversely related. Take the following two scenarios for example:
(1) If there is a large amount of organic waste, microorganisms will consume more oxygen to break it down. This means that the BOD is high. At the same time, as the microorganisms are depleting the supply of oxygen in the water, the DO level decreases.
(2) If there is a low level of organic waste, microorganisms will not consume as much oxygen (BOD is low). Thus, the level of DO increases.
Hope this helps :)
Post by: cnimm2000 on January 08, 2018, 12:15:14 pm
I wanted to know if i answered the dot point correctly:
T.1.2 Identify that ethylene, because of the high reactivity of its double bond, is readily transformed into many useful products
Alkenes are unsaturated thereby giving the possibility of further atoms or molecules joining the existing hydrocarbon chain. Alkenes e.g. CH2=CH2 (ethylene) are more reactive molecules than alkanes due to the high electron density of the double bond thus attracting electronegative species such as halogens to readily react with it. The presence of double bond allows ethylene to readily undergo addition reactions by opening out the double bond to form two single bonds.
Therefore because of the high reactivity of the double bond of ethylene, it means that it can be easily converted into a range of very useful products and the starting materials for several important plastics (polymers).
Also would we need to know the reactions of ethylene to make other things such as ethanol
Post by: RuiAce on January 08, 2018, 02:11:03 pm
Also would we need to know the reactions of ethylene to make other things such as ethanolThat one - absolutely.
However, on top of it:
- Being able to write the equations for the polymerisation of ethene to polyethylene is looked highly upon.
- Most of the other ones can be worked out on the spot so long as you know the ethanol one (e.g. ethene -> ethane, bromination)
Post by: cnimm2000 on January 10, 2018, 01:48:07 pm
If you were asked to describe the dehydration of ethanol is this all you have to know?
Ethylene is made from ethanol by dehydration. Dehydration is a chemical reaction in which water is removed from a compound.
Ethanol is dehydrated by heating it with concentrated sulfuric acid which acts as a catalyst and a dehydrating agent and is used to remove water from the ethanol molecule.
Post by: RuiAce on January 10, 2018, 02:16:09 pm
Hey guys,When there’s chemistry involved, you should always specify the relevant chemical equation as well. Also, I’m not too keen on the idea of water “being removed”, because it’s not a reactant. Rather, water is just a product in the reaction.
If you were asked to describe the dehydration of ethanol is this all you have to know?
Ethylene is made from ethanol by dehydration. Dehydration is a chemical reaction in which water is removed from a compound.
Ethanol is dehydrated by heating it with concentrated sulfuric acid which acts as a catalyst and a dehydrating agent and is used to remove water from the ethanol molecule.
Fairly sure your catalyst is correct and nothing else important is missing.
Post by: cnimm2000 on January 10, 2018, 04:19:14 pm
- Indirect hydration
- Catalytic hydration
If a question popped up in an exam would we also be required to write both?
Also i wanted to clarify if I understood both processes correctly:
Indirect:
The reaction is facilitated by dilute sulfuric acid which acts as a catalyst.
The reaction of ethylene to produce ethanol occurs in two steps.
1. Ethylene is reacted with 98% sulfuric acid in absorption columns held at 55-80C and 1-3.5 MPa pressure to produce ethyl hydrogen sulfate.
2. The ethyl hydrogen sulfate is then reacted with water until the sulfuric acid concentration is about 50% and then heated to 70-100C. This generates ethanol, the sulfuric acid is reformed.
The ethanol that forms is distilled off and has a concentration of 70% (v/v)
Catalytic:
Alternative method includes passing ethylene and steam over a silica gel or zeolite surface impregnated with phosphoric acid at 300C and at high pressure. The O—H bond of the water breaks, and a H atom and an OH group add across the double bond, causing it to break. The final product is ethanol and the double bond has been destroyed.
Post by: itssona on January 12, 2018, 12:45:33 pm
Are we required to know both of the process of hydration:As long as you understand indirect hydration well (which looking from what you wrote, you do :) ) The main thing I think, is that sulphuric acid acts as a catalyst and dehydrating agent (the fact that it is a dehydrating agent means it absorbs water and this is necessary to drive the equilibrium towards the ethylene product.
- Indirect hydration
- Catalytic hydration
If a question popped up in an exam would we also be required to write both?
Also i wanted to clarify if I understood both processes correctly:
Indirect:
The reaction is facilitated by dilute sulfuric acid which acts as a catalyst.
The reaction of ethylene to produce ethanol occurs in two steps.
1. Ethylene is reacted with 98% sulfuric acid in absorption columns held at 55-80C and 1-3.5 MPa pressure to produce ethyl hydrogen sulfate.
2. The ethyl hydrogen sulfate is then reacted with water until the sulfuric acid concentration is about 50% and then heated to 70-100C. This generates ethanol, the sulfuric acid is reformed.
The ethanol that forms is distilled off and has a concentration of 70% (v/v)
Catalytic:
Alternative method includes passing ethylene and steam over a silica gel or zeolite surface impregnated with phosphoric acid at 300C and at high pressure. The O—H bond of the water breaks, and a H atom and an OH group add across the double bond, causing it to break. The final product is ethanol and the double bond has been destroyed.
Hope this helped
Post by: itssona on January 12, 2018, 12:46:55 pm
Thank you guys :D
Post by: MisterNeo on January 12, 2018, 04:17:29 pm
just wondering if for Le Chatelier's Principle, it is enough to know the direction of equilibrium for pressure change, temp change, and concentration change? Is that pretty much the applications of knowing Le Chatelier's Principle?
Thank you guys :D
Hey :)
Depending on which option you do, Le Chatelier's Principle can also be used in Industrial questions where they ask to predict any changes in colour of a solution when a certain change occurs. It is also used in calculations involving the K constant. The questions in the core topics are pretty straight forward with LCP like in Haber process compromises, etc.
Knowing the changes in equilibrium would be enough however you should be able to explain why it shifts that particular direction such as when an exothermic reaction is cooled, the products are favoured to produce more heat to minimise that lowered temperature. ;)
Post by: cnimm2000 on January 13, 2018, 11:57:32 am
I was struggling to do this 2010 HSC Question:
HSC Question:
In which of the following reactions does he metal atom show the greatest change in oxidation state?
a) MnO4- to Mn2+
b) MnO2 to Mn(OH)3
c) PbO2 to PbSO4
d) VO2+ to VO2+
Could someone please explain it. I know the rules and stuff but i get confused on how to find the oxidation states for particular compounds such PbSO4.
Post by: itssona on January 13, 2018, 12:20:02 pm
Hey guys,sure so we would need to find the oxidation numbers and most important thing to remember is that the sum of the oxidation states of all the atoms or ions in a neutral compound is zero.
I was struggling to do this 2010 HSC Question:
HSC Question:
In which of the following reactions does he metal atom show the greatest change in oxidation state?
a) MnO4- to Mn2+
b) MnO2 to Mn(OH)3
c) PbO2 to PbSO4
d) VO2+ to VO2+
Could someone please explain it. I know the rules and stuff but i get confused on how to find the oxidation states for particular compounds such PbSO4.
So to find PbO2 oxidation state, we first know that O has an oxidation number of -2. Since we have 2 O atoms, the total oxidation state for oxygen is 2 x -2 which is -4. To make the element sum add to zero, Pb has to have an ox number of 4.
Similarly for PbSO4. We see that the polyatomic ion SO4 is there and it has a charge of 2-. So that means Pb is 2.
Does this help? :)
Post by: itssona on January 13, 2018, 12:21:44 pm
excess HCL is added to 5g Copper. What is the volume of gas emitted at 0 degrees C and 100kPA.
so do I find the moles and multiply by constant? In the answer, they found the moles of copper but how is that the same as the moles for HYDROGEN?
Post by: vcestressed on January 13, 2018, 12:23:02 pm
so basically oxidation is loss of electrons. I got the answer A, because it has a difference of oxidation number of -5. If someone finds any mistake with my working out, please let me know. Try to find out the difference in oxidation numbers for c and d by yourself.
(Attached a picture for a and b).
Post by: MisterNeo on January 13, 2018, 12:40:39 pm
hey guys I was looking at this question
excess HCL is added to 5g Copper. What is the volume of gas emitted at 0 degrees C and 100kPA.
so do I find the moles and multiply by constant? In the answer, they found the moles of copper but how is that the same as the moles for HYDROGEN?
The moles of copper are of a 1:1 ratio to the moles of hydrogen. The answers probably just assumed that the reader would know it’s a 1:1 ratio even though you should show that ratio step in an exam.
Post by: itssona on January 13, 2018, 01:46:40 pm
CO2 + H2 --> H2CO3
I understand that if temperature decreases, the equation would move forwards so that products are favoured to produce more heat.
The answer said that that is correct but also said that : as a result the concentration of the H2CO3 would increase and concentrations of the CO2 and H2O would decrease. (How the freak do we know that??)
This change would also effect reaction (2)
H2CO3 + H2O --> H3O+ + HCO3 -
as the concentration of the H2CO3 has increased. According to Le Chatelier’s principle the system will want to reduce the concentration and this can be achieved by favouring the forward reaction. This will increase the concentration of H3O+ and HCO3-. H3O+ gives the favourable acidic taste and hence a cooler temperature is desired.
I dont understand how they know about the concentration.. any input is greatly appreciated :)
Post by: Lear on January 13, 2018, 02:00:51 pm
Since there is more in the same amount of space, their concentrations have increased.
Likewise, we have used up more H2CO3 and there is less of H2CO3 in the space than before = decrease in concentration.
Feel free to correct me. This is just my understanding of what I’ve read on Le chatelier so far.
Post by: itssona on January 13, 2018, 02:05:54 pm
Concentration is basically how much of something there is in a certain amount of space. Now, if the reaction moves forward it is obvious that the products are favoured over the reactants meaning more products are formed (which uses up reactants). Therefore there exists MORE h3o+ and HCO3 now.ohh this makes sense!! seems right, thank you so much!!!! :D
Since there is more in the same amount of space, their concentrations have increased.
Likewise, we have used up more H2CO3 and there is less of H2CO3 in the space than before = decrease in concentration.
Feel free to correct me. This is just my understanding of what I’ve read on Le chatelier so far.
Post by: RuiAce on January 13, 2018, 06:02:01 pm
HiFor any such cell, a suitable electrolyte for the salt bridge should be anything that will not be reactive. The salt bridge should only facilitate the transfer of ions, and not affect the chemistry of the cell itself.
I was going through the 2009 HSC chemistry paper:
An electrochemical cell is constructed using two half cells. One half cell consists of
an inert platinum electrode and a solution of Fe2+ and Fe3+. The other half cell consists
of a lead electrode and a solution of Pb2+.
Identify an appropriate electrolyte to use in the salt bridge.
How would we know if what electrolyte is appropriate?
So one that will work in every case is KNO3, because both the K+ and NO3 ions are very unreactive.
Post by: itssona on January 15, 2018, 05:37:47 pm
Calculate the pH of 0.05 mol L-1 citric acid if it has an ionisation rate of 50%
Post by: RuiAce on January 16, 2018, 09:56:48 am
how do I account for the ionisation rate?
Calculate the pH of 0.05 mol L-1 citric acid if it has an ionisation rate of 50%
Recall: [something] = Csomething, i.e. it's concentration.
I just used H+ for convenience; of course it should actually be H3O+
Post by: rozz.godly on January 16, 2018, 07:33:00 pm
Thanks
Post by: cnimm2000 on January 16, 2018, 11:25:28 pm
Hi, I was wondering if we need to know the process of *thermal (steam) cracking* and *catalytic cracking* from the production of materials topic.Hi,
Thanks
I would definitely know both as they are the two methods used to break long-chained hydrocarbons into smaller chains. i would also recommend a chemical equation as an example for both of these methods.
Here is a summary for both to help you :)
Catalytic Cracking (carried out in absence of air and pressures somewhat above atmospheric):
The heavy crude oil is heated in the presence of a catalyst composed of a zeolite (crystalline aluminium silicate).
Hydrogen atoms are removed from the hydrocarbons onto the catalyst’s surface. The positive hydrocarbon ion that forms undergoes further reactions leading to the formation of the final products: one alkane of shorter chain length (used for petrol) and one small alkene, until the desired product such as ethylene is created.
Thermal cracking:
2. Thermal cracking (in the Naphtha- fraction)
It is a non-catalytic process in which a mixture of alkanes with steam passes through very hot metal tubes and at just above atmospheric pressure. Long pyrolysis coils are used to heat the hydrocarbon feedstocks and steam to about 750–900C inside the furnace
There are three main steps in the process:
a) Initiation:
Free radicals are produced when the hydrocarbon chains are split into fragments at high temperatures. These radicals are reactive, as they have an unpaired electron.
E.g. Decane is split into two carbon chains: 2 penty radicals (C5H11•)
C10H22 --> 2C5H11•
b) Propagation:
The free radicals decompose to produce smaller free radicals and release alkenes such as ethylene.
E.g. Each penty radical decomposes into a propyl radical (C3H7•) and ethylene
C5H11• --> C3H7• + C6H14
c) Termination:
Hydrocarbon molecules can reform when free radicals react with other free radicals. These hydrocarbons can be recycled for further cracking
E.g. The two propyl radicals formed in the propagation process undergo termination to form hexane and this hydrocarbon can be recycled for further cracking
2C3H7• --> C6H14
For thermal cracking its best to create a flowchart to help you to remember the process
Post by: cnimm2000 on January 17, 2018, 05:44:57 pm
For this dot point:
T.2.4 Identify factors which can affect the equilibrium in a reversible reaction
Im struggling to understand what the effect of pressure is
i know that an increase in pressure will favour the side of the fewer moles but i dont understand why?
Also what happens if there isa decrease in temperature
Really appreciate the help
Post by: kiwiberry on January 17, 2018, 10:55:28 pm
Hi guys
For this dot point:
T.2.4 Identify factors which can affect the equilibrium in a reversible reaction
Im struggling to understand what the effect of pressure is
i know that an increase in pressure will favour the side of the fewer moles but i dont understand why?
Also what happens if there isa decrease in temperature
Really appreciate the help
Hey!! Pressure is caused by gas molecules hitting the side of the container - so the more molecules in the container, the higher the pressure will be. So, when there is an increase in pressure, equilibrium will shift to favour the side that reduces the pressure (by Le Chatelier). This will be the side that produces fewer moles, as this will reduce the number of molecules in the reaction vessel, thus reducing pressure.
When there is a decrease in temperature, equilibrium will shift to favour the side which increases the temperature (by Le Chatelier) - this will be the exothermic reaction. If the equation has an exothermic forward reaction, it will shift right; if it has an endothermic forward reaction, it will shift left.
Let me know if any of that doesn't make sense!! :)
Post by: cnimm2000 on January 17, 2018, 11:08:10 pm
Hey!! Pressure is caused by gas molecules hitting the side of the container - so the more molecules in the container, the higher the pressure will be. So, when there is an increase in pressure, equilibrium will shift to favour the side that reduces the pressure (by Le Chatelier). This will be the side that produces fewer moles, as this will reduce the number of molecules in the reaction vessel, thus reducing pressure.
When there is a decrease in temperature, equilibrium will shift to favour the side which increases the temperature (by Le Chatelier) - this will be the exothermic reaction. If the equation has an exothermic forward reaction, it will shift right; if it has an endothermic forward reaction, it will shift left.
Let me know if any of that doesn't make sense!! :)
This is a really great explanation.
Thank you so much. This really helped me to understand the concept better.
Post by: riya.bhavsar on January 20, 2018, 11:08:48 am
I am was writing my practical report. Can someone please explain to me what is reliability, accuracy and validity in an experiment, how can we improve that reliability, accuracy and validity.
Thanks
Post by: RuiAce on January 20, 2018, 11:10:38 am
Hi,This is explained quite well here.
I am was writing my practical report. Can someone please explain to me what is reliability, accuracy and validity in an experiment, how can we improve that reliability, accuracy and validity.
Thanks
Post by: riya.bhavsar on January 20, 2018, 11:12:08 am
This is explained quite well here.
Thank you that helps
Post by: Calley123 on January 20, 2018, 11:17:11 am
In the 2013 Exam it said: Construct separate flow diagram to show the steps used in the production of polyethylene and those used in the production of a recently developed biopolymer (5 marks)
If tho the example answer and the marking criteria didn't include structural equations, is it a good idea to include them anyway or it is just a waste of time?
Thanks :)
Post by: RuiAce on January 20, 2018, 11:22:23 am
Hey,First note that the answers BOSTES/NESA provide are not necessarily of band 6 level. They will just grab any random answer for their sample response.
In the 2013 Exam it said: Construct separate flow diagram to show the steps used in the production of polyethylene and those used in the production of a recently developed biopolymer (5 marks)
If tho the example answer and the marking criteria didn't include structural equations, is it a good idea to include them anyway or it is just a waste of time?
Thanks :)
I would say it is a great idea, because it makes the whole process a lot clearer. At the same time, where possible you could just replace it with some chemical equations.
Post by: cnimm2000 on January 20, 2018, 08:07:55 pm
I was doing this dot point:
P. 3.6 Identify data sources, choose resources and perform a first-hand investigation to determine and compare heats of combustion of at least three liquid alkanols per gram and per mole
And i came across a question asking calculate the heat of combustion per gram and per mole of each fuel.
To calculate the heat of combustion per gram I've heard that we divide the molar heat of combustion by the molar mass of the alkanol.
i just wanted to confirm if thats correct or if we divide by the mass of the alkanol burnt
Thanks in advance.
Post by: Natasha.97 on January 20, 2018, 09:15:22 pm
Hey guys:
I was doing this dot point:
P. 3.6 Identify data sources, choose resources and perform a first-hand investigation to determine and compare heats of combustion of at least three liquid alkanols per gram and per mole
And i came across a question asking calculate the heat of combustion per gram and per mole of each fuel.
To calculate the heat of combustion per gram I've heard that we divide the molar heat of combustion by the molar mass of the alkanol.
i just wanted to confirm if thats correct or if we divide by the mass of the alkanol burnt
Thanks in advance.
Yes! Dividing kj/mol by g/mol will result in kJ/g, which is the HoC per gram :)
Post by: kaustubh.patel on January 21, 2018, 01:47:32 pm
Why do we use specific indicators like phenolphthalein (only indicates from pH 8.6 to 10) and why not a universal indicator.
Post by: kauac on January 21, 2018, 02:09:15 pm
Hey chem livers I have a question about titration.
Why do we use specific indicators like phenolphthalein (only indicates from pH 8.6 to 10) and why not a universal indicator.
Phenolphthalein has 1 obvious colour change as [H+] changes, making it easy to find the end point of the titration. The colour/shade of universal indicator constantly changes as [H+] is altered, and is therefore hard to observe a distinct end point when the mole ratio is balanced. Hope this helps! :)
Post by: cnimm2000 on January 23, 2018, 10:10:15 pm
Yes! Dividing kj/mol by g/mol will result in kJ/g, which is the HoC per gram :)Sorry i don't quite understand what you mean dividing it by grams per mole - are you referring to the molar mass?
Post by: RuiAce on January 23, 2018, 10:20:40 pm
Sorry i don't quite understand what you mean dividing it by grams per mole - are you referring to the molar mass?Yeah, because g/mol is basically the SI units that correspond to the molar mass.
She's taking advantage of the fact that in formulas in science, the units actually cancel out in the numerator and denominator. In your case, you initially had the molar heat of combustion, measured in kJ/mol. If you divide the molar heat of combustion by the molar mass, then units wise you divide kJ/mol by g/mol, which lands you to kJ/g. Incidentally, kJ/g is the units used for heat of combustion per gram
Post by: cnimm2000 on January 23, 2018, 10:23:07 pm
Yeah, because g/mol is basically the SI units that correspond to the molar mass.
She's taking advantage of the fact that in formulas in science, the units actually cancel out in the numerator and denominator. In your case, you initially had the molar heat of combustion, measured in kJ/mol. If you divide the molar heat of combustion by the molar mass, then units wise you divide kJ/mol by g/mol, which lands you to kJ/g. Incidentally, kJ/g is the units used for heat of combustion per gram
Ok thank you
Post by: Calley123 on January 24, 2018, 09:01:21 am
When I was writing half-equations for galvanic cells, I noticed that in the answers they stated that 1/2 Cl(g) {on data sheet} is the same thing as Cl2 (G). Please explain me how or why that is true.
Thanks
Post by: RuiAce on January 24, 2018, 09:15:07 am
Hey,It's not.
When I was writing half-equations for galvanic cells, I noticed that in the answers they stated that 1/2 Cl(g) {on data sheet} is the same thing as Cl2 (G). Please explain me how or why that is true.
Thanks
You don't have 1/2 Cl(g). You have 1/2 Cl2(g), which is half a mole of the chlorine element in the gaseous phase. Note that in the chlorine element molecule, you always have two chlorine atoms, not one.
And then when it gets reduced (i.e. gains electrons), you end up with Cl-(aq), which is one mole of the chloride ion. You only have one chloride ion appearing at a time.
Post by: Calley123 on January 24, 2018, 10:04:28 am
In the 2009 Exam, there was a question on galvanic cells which stated that " One half cell consists of an inert platinum electrode and a solution of Fe (2+) and Fe(3+). The other half cell consists of a lead electrode and a solution of Pb (2+).
The sample answer stated that oxidation occurred with Pb(s) to Pb(2+) and Reduction with Fe(3+) to Fe(2+)
1. Does the solution of Fe(3+) ions and Fe(2+) exist because the inert platinum just sits there?
2. I know that the more reactive metal is supposed to displace the ions of the less reductive ions but in this case how was I suppose to tell which would displace which?
Sorry for the spam before -it was an accident :/
tHnaks
First note that the answers BOSTES/NESA provide are not necessarily of band 6 level. They will just grab any random answer for their sample response.
I would say it is a great idea, because it makes the whole process a lot clearer. At the same time, where possible you could just replace it with some chemical equations.
THANK YOU!
It's not.
You don't have 1/2 Cl(g). You have 1/2 Cl2(g), which is half a mole of the chlorine element in the gaseous phase. Note that in the chlorine element molecule, you always have two chlorine atoms, not one.
And then when it gets reduced (i.e. gains electrons), you end up with Cl-(aq), which is one mole of the chloride ion. You only have one chloride ion appearing at a time.
Thanks :)
Merged posts - J41. :)
Post by: Aaron12038488 on January 24, 2018, 01:29:08 pm
Post by: Lear on January 24, 2018, 01:32:13 pm
2003 hsc chem- 8. of MC, my answer was 6.18 close to 6.12. When i checked the solutions instead of multilplying the moles with 24.79 (RTP) they used 24.47L?
If i’m not mistaken these constants change over time due to new definitions. Since the exam in from 2003, the value could have been different.
Post by: RuiAce on January 24, 2018, 09:11:12 pm
Hey,1. The platinum just sits there but I wouldn't say that the solution of Fe3+ and Fe2+ exist purely because of that. As dumb as it might seem, I'd say it exists because whoever made the cell poured some of both of the ions in there to begin with.
In the 2009 Exam, there was a question on galvanic cells which stated that " One half cell consists of an inert platinum electrode and a solution of Fe (2+) and Fe(3+). The other half cell consists of a lead electrode and a solution of Pb (2+).
The sample answer stated that oxidation occurred with Pb(s) to Pb(2+) and Reduction with Fe(3+) to Fe(2+)
1. Does the solution of Fe(3+) ions and Fe(2+) exist because the inert platinum just sits there?
2. I know that the more reactive metal is supposed to displace the ions of the less reductive ions but in this case how was I suppose to tell which would displace which?
Sorry for the spam before -it was an accident :/
tHnaks
THANK YOU!
Thanks :)
Merged posts - J41. :)
As for why it stays there (instead of reacting with the Pt for example), well yep that is because platinum is inert.
2. Use the data sheet. In the table of standard reduction potentials, whichever half-equation is higher up will need to be flipped (as it represents oxidation), whereas whichever is lower will represent reduction. the lead one lies above the iron one, so the former will be the one involved in the oxidation process.
2003 hsc chem- 8. of MC, my answer was 6.18 close to 6.12. When i checked the solutions instead of multilplying the moles with 24.79 (RTP) they used 24.47L?
If i’m not mistaken these constants change over time due to new definitions. Since the exam in from 2003, the value could have been different.Yeah this should be the cause. A lot of minor details got changed in 2010 in the HSC sciences.
_______________________________________
Not too worried this time because both the above questions were fairly brief and clear. In the future, please provide the actual question, or at least link to the past paper itself for convenience.
Post by: Calley123 on January 25, 2018, 08:14:52 am
1. The platinum just sits there but I wouldn't say that the solution of Fe3+ and Fe2+ exist purely because of that. As dumb as it might seem, I'd say it exists because whoever made the cell poured some of both of the ions in there to begin with.
As for why it stays there (instead of reacting with the Pt for example), well yep that is because platinum is inert.
2. Use the data sheet. In the table of standard reduction potentials, whichever half-equation is higher up will need to be flipped (as it represents oxidation), whereas whichever is lower will represent reduction. the lead one lies above the iron one, so the former will be the one involved in the oxidation process.Yeah this should be the cause. A lot of minor details got changed in 2010 in the HSC sciences.
_______________________________________
Not too worried this time because both the above questions were fairly brief and clear. In the future, please provide the actual question, or at least link to the past paper itself for convenience.
It kinda makes sense now except how would I have know whether it was supposed to be Fe+2 (lies above lead) or Fe 3+ ( lies below lead) that was involved in reduction process?
Post by: RuiAce on January 25, 2018, 02:13:06 pm
Post by: Calley123 on January 25, 2018, 03:52:46 pm
The equation \(Fe^{2+} + 2e^- \rightleftharpoons Fe_{(s)} \) was not of interest of us as we do not produce or react iron metal in our cell.
I get it :)
However, does this mean that there is no mass gained on the Pt(s) cathode since there is no solid formed on it? I always thought there was mass gain on the cathode in a galvanic cell...
:)
Post by: RuiAce on January 25, 2018, 04:32:08 pm
I get it :)Yes, there will be no mass gain.
However, does this mean that there is no mass gained on the Pt(s) cathode since there is no solid formed on it? I always thought there was mass gain on the cathode in a galvanic cell...
:)
It's fair enough to say that "usually" there is a gain in mass on the electrodes (and also loss in mass for the other electrode). But this isn't something that always occurs.
Post by: Calley123 on January 25, 2018, 04:51:24 pm
Yes, there will be no mass gain.
It's fair enough to say that "usually" there is a gain in mass on the electrodes (and also loss in mass for the other electrode). But this isn't something that always occurs.
Thank you so much !!!!
Post by: cnimm2000 on January 25, 2018, 08:01:27 pm
For the nuclear chemistry section, i wanted to know how strontium-90 is produced
thanks in advance.
Post by: kauac on January 25, 2018, 08:39:49 pm
Hey guys,
For the nuclear chemistry section, i wanted to know how strontium-90 is produced
thanks in advance.
Hi...
Not 100% sure on this, but I think strontium-90 is produced by bombarding a lighter strontium isotope with neutrons. (I worked this out as the neutron:proton ratio is very high, meaning that this is what made it unstable). The neutrons for this reaction are sourced from a nuclear fission reaction where uranium nuclei are split into neutrons + protons. Hope this is somewhat helpful!
Post by: cnimm2000 on January 25, 2018, 09:14:39 pm
Hi...Thanks,
Not 100% sure on this, but I think strontium-90 is produced by bombarding a lighter strontium isotope with neutrons. (I worked this out as the neutron:proton ratio is very high, meaning that this is what made it unstable). The neutrons for this reaction are sourced from a nuclear fission reaction where uranium nuclei are split into neutrons + protons. Hope this is somewhat helpful!
Also how do you know if the neutron: proton ratio is high?
Post by: RuiAce on January 25, 2018, 09:16:35 pm
Thanks,As a rule of thumb, the n:p ratio should be 1:1 for atoms with smaller atomic number, and as you go all the way to Uranium it should become 1.5:1.
Also how do you know if the neutron: proton ratio is high?
There isn't really any rule of thumb as to when the transition from 1:1 to 1.5:1 starts to occur but you can consider the diagram on Wikipedia as a reference point.
Post by: Mate2425 on January 28, 2018, 11:09:35 pm
I was just wondering if you could please help with what the best way to go about calculating the volume of CO2 released during the fermentation process of mass change 3.33g would be and also what the process of calculations needed to answer this question would look like.
Thank you,
Mate2425 😃
Post by: kiwiberry on January 30, 2018, 08:21:40 pm
Hi could i please have some help with a question in regard to the fermentation of glucose and monitoring mass changes dotpoint.
I was just wondering if you could please help with what the best way to go about calculating the volume of CO2 released during the fermentation process of mass change 3.33g would be and also what the process of calculations needed to answer this question would look like.
Thank you,
Mate2425 😃
Hi!! So the equation for fermentation is
Post by: Mate2425 on January 30, 2018, 10:40:34 pm
Hi!! So the equation for fermentation isThe mass change is due to the CO2 released, so 3.33g of CO2 was released. Now that we have the mass, we can find moles using n=m/M:
and then the volume using n=v/MV (assuming this is at 0oC so MV=22.71, if it's at 25oC then MV=24.79)
So the volume of CO2 released is 1.72L. Hope that helps! :)
Post by: Gkumar on February 04, 2018, 06:57:09 pm
Post by: Lear on February 04, 2018, 08:10:38 pm
a) N (g) + O2 (g) ---> NO2 (g)
b) n (N) = m/M
= 25 / 14.07
= 1.78... moles
since stoick ratio is 1:1:1
n (NO2) = m /m
m = nM
= 25 x 1.78...
= 44.42 g
c) n (NO2) = V / Vm
1.78... = V / 24.79
V = 44.05 L
I believe nitrous oxide is actually N20 but might be incorrect.
Post by: clovvy on February 05, 2018, 08:13:25 pm
I believe nitrous oxide is actually N20 but might be incorrect.You are right it is N2O, the whole calculations got messed up because it is a wrong compound and NO2 is nitrogen dioxide..
Therefore the calculations should be:
a) 2(N2)(g) + O2(g) --> 2N2O (nitrogen element don't exist by itself, just like other halogens)
b) ratio 2:1->2
nitrogen: 25/14.007=1.78482.... mols...
therefore the mass of N2O to be produced are: 1.78482 x (16 + 14.007 x 2) = 78.557 grams
c) At 25°C and 100 kPa, the molar volume of all gases is 24.79 L
therefore V= 1.78482 x 24.79 = 44.22 L (2 d.p.)...
Anyway that is how the question is done
Post by: clovvy on February 06, 2018, 10:14:28 pm
Hi, I need some help figuring out my independent variable (not sure if more than one is allowed) for my chemistry experiment.
Heading: Identifying and comparing the properties of particular household emulsions.
Aim: To compare the differences in properties for different types of emulsions (oil-in-water emulsions and water-in-oil emulsions) in everyday household products.
So I'm testing five properties on five different emulsions (yes, that's 25 trials without repetition yet) at home (so no fancy chemistry lab equipment).
Emulsions: Mayonnaise, Salad dressing, Ice-cream, Butter, Hand cream (Nivea)
Property 1 is just the physical feel of the emulsion on the skin... so I'm basically feeling the coolness or greasiness of the emulsion (dependent variable) which helps me differentiate between o/w and w/o emulsions. But what would be my independent variable here?
Skipping down to property 3 regarding the stability of the emulsions. I put equal amounts of each emulsion onto separate plates and left it in the sun for a few hours. Oil-in-water emulsions are less stable since some of their higher water proportion evaporates off first. Anyway, what would be my independent variable here?
Also, I'm not sure if I'm meant to do an independent variable for each property or one independent variable applicable to all properties (if it works for these 2 properties, it should work for the rest).
[PS: Sorry this post doesnt have that much structure.. I guess it represents my confusion about this task..]
Thanks in advance. :)
Sorry to say this but the way you described the experiment is all over the place and it is very confusing to read, generally for sciences you need to write down your procedures very clearly (what are your steps for the experiment, what is your hypothesis, records of results).... Clearly yours is lacking this... What is property 1,2 and 3? Please lay it out properly next time... (Title, aim- that part is ok until the next bit- hypothesis, experiment procedures, results, conclusion, discussions i.e. risk assessments etc)
Anyway straight to the point, for property 1, I would say the substance you are using (oil and water) and I don't know what your aim is because you are very very vague with your explanations.... Property 3 I would say the time is the independent variable because you are looking at change in something overtime (again I don't understand what you are doing)... What are you refering to with property 2???? If you don't know what independent variable is, in science (not just chem) it is basically a variable that is controlled or changed to test the effects of the dependent variable
I would suggest that you try to change your lay-out for the experiment and make it neater and clearer so you know what you need to do, what to do etc so all the information is properly set instead of having them all over the place which makes it confusing... If unsure feel free to ask anyone in this forum (the forum exist for that reason anyway so don't stress too much)
Post by: arii on February 06, 2018, 11:49:43 pm
Sorry to say this but the way you described the experiment is all over the place and it is very confusing to read, generally for sciences you need to write down your procedures very clearly (what are your steps for the experiment, what is your hypothesis, records of results).... Clearly yours is lacking this... What is property 1,2 and 3? Please lay it out properly next time... (Title, aim- that part is ok until the next bit- hypothesis, experiment procedures, results, conclusion, discussions i.e. risk assessments etc)
Anyway straight to the point, for property 1, I would say the substance you are using (oil and water) and I don't know what your aim is because you are very very vague with your explanations.... Property 3 I would say the time is the independent variable because you are looking at change in something overtime (again I don't understand what you are doing)... What are you refering to with property 2???? If you don't know what independent variable is, in science (not just chem) it is basically a variable that is controlled or changed to test the effects of the dependent variable
I would suggest that you try to change your lay-out for the experiment and make it neater and clearer so you know what you need to do, what to do etc so all the information is properly set instead of having them all over the place which makes it confusing... If unsure feel free to ask anyone in this forum (the forum exist for that reason anyway so don't stress too much)
Hi clovvy, thanks so much for the fast reply. I wrote up a thorough explanation of what I was doing so hopefully that helps. If it still doesn't, I might just paste in my complete method on the docs. https://docs.google.com/document/d/1EPvqE_DqAa8UYyYg60mEwfdktmx-J8V8HWU-BmDWDBQ/edit?usp=sharing
Post by: clovvy on February 07, 2018, 07:11:01 am
Hi clovvy, thanks so much for the fast reply. I wrote up a thorough explanation of what I was doing so hopefully that helps. If it still doesn't, I might just paste in my complete method on the docs. https://docs.google.com/document/d/1EPvqE_DqAa8UYyYg60mEwfdktmx-J8V8HWU-BmDWDBQ/edit?usp=sharing
Much better than before, however still have a lot to improve (it's ok, I have failed a prac task before so personally I know the pain)... You can't just jump into results without showing the procedures of the prac because it kinda look like you don't really know what you are doing.... You are not quite there yet but I can see some improvements which is good... I know that you are afraid of plagiarism so it make sense if you do not want to post your actual work there (I know)... To give you a rough idea on what your report should look like, read this https://www.adelaide.edu.au/writingcentre/docs/new_practical_report_guide.pdf
or https://sjesci.wikispaces.com/file/view/prac_report_proforma.png/187658559/682x1048/prac_report_proforma.png
Post by: arii on February 07, 2018, 09:04:35 am
Much better than before, however still have a lot to improve (it's ok, I have failed a prac task before so personally I know the pain)... You can't just jump into results without showing the procedures of the prac because it kinda look like you don't really know what you are doing.... You are not quite there yet but I can see some improvements which is good... I know that you are afraid of plagiarism so it make sense if you do not want to post your actual work there (I know)... To give you a rough idea on what your report should look like, read this https://www.adelaide.edu.au/writingcentre/docs/new_practical_report_guide.pdf
or https://sjesci.wikispaces.com/file/view/prac_report_proforma.png/187658559/682x1048/prac_report_proforma.png
I'm planning out the experiment (which is the bit before actually doing the experiment and writing up the report). It needs me to figure out my variables, so what do you think my independent variable would be? I'm not sure if it is time or the ratio (of oil and water in the emulsions tested).
(By the way, this is in the option topic. Industrial Chem)
Post by: clovvy on February 07, 2018, 01:09:44 pm
I'm planning out the experiment (which is the bit before actually doing the experiment and writing up the report). It needs me to figure out my variables, so what do you think my independent variable would be? I'm not sure if it is time or the ratio (of oil and water in the emulsions tested).
(By the way, this is in the option topic. Industrial Chem)
I am not doing Industrial Chem yet (your school did it a different way which is fine)... After reading your stuff over and over again I think the independent variable would be the time and you will need different values or graphs to record your result. Ratios and amounts of items you put in I would call that a 'controlled variable'....
(If anyone could pop in and help out here it would be greatly appreciated thanks)
Post by: meditatingwalrus on February 07, 2018, 05:45:19 pm
14. Calculate the mass of carbon dioxide formed when 74.4 L of oxygen gas at 100kPa and 25 oC completely combust a sample of ethanol.
(A) 44 g
(B) 50 g
(C) 88 g
(D) 132 g
thanks :)
Post by: arii on February 07, 2018, 05:57:13 pm
Finding the mass of CO2(g):
n(O2) = 74.4/24.79 = 3.00121... (So you were correct until here)
n(CO2) = 2/3 * n(O2) = 2/3 * 3.00121... = 2.000806... moles
m(CO2) = (2.000806)(12.01+2*16.00) = 88.0555014...grams.
Therefore, your closest answer would be (C).
Post by: ARCHITECT on February 07, 2018, 06:08:39 pm
If this mass of carbon dioxide (2.03 Kg) dissolves in 5 L of water, what is the concentration, in grams per litre, of the solution
produced?
CO2+H2O--->H2CO3
thanks
Mod edit: merged double post - if you need to fix up your post or want to add something, just click the modify button on the post :)
Post by: clovvy on February 07, 2018, 10:07:45 pm
Hey Could someone help with the following,
If this mass of carbon dioxide (2.03 Kg) dissolves in 5 L of water, what is the concentration, in grams per litre, of the solution
produced?
CO2+H2)--->H2CO3
thanks
You are told that you have 2030 grams of CO2 dissolved in 5L of water.. all you need to do is convert 2030g/5L to 406g/L
Post by: RuiAce on February 08, 2018, 09:59:11 am
You are told that you have 2030 grams of CO2 dissolved in 5L of water.. all you need to do is convert 2030g/5L to 406g/LI don't think it's that simple. When CO2 dissolves it doesn't just do what NaCl does and dissociate, but rather it reacts (in accordance to the equation given).
Having said that,
Hey Could someone help with the following,What I'm doing here is assuming that ALL of the CO2 given dissolved.
If this mass of carbon dioxide (2.03 Kg) dissolves in 5 L of water, what is the concentration, in grams per litre, of the solution
produced?
CO2+H2O--->H2CO3
thanks
Mod edit: merged double post - if you need to fix up your post or want to add something, just click the modify button on the post :)
Of course, at the same time I don't know if this passes a sensibility check.
Post by: meditatingwalrus on February 08, 2018, 06:20:51 pm
1. Calculate the pH of the solution when 25.0ml of 0.750mol L-1 hydrochloric acid solution is added to 10ml of 0.500mol L-1 barium hydroxide solution.
(A) 0.456
(B) 0.602
(C) 1.862
(D) 2.058
thanks so much :D
Post by: meditatingwalrus on February 08, 2018, 06:24:10 pm
You applied the ratio thing in the wrong part.
Finding the mass of CO2(g):
n(O2) = 74.4/24.79 = 3.00121... (So you were correct until here)
n(CO2) = 2/3 * n(O2) = 2/3 * 3.00121... = 2.000806... moles
m(CO2) = (2.000806)(12.01+2*16.00) = 88.0555014...grams.
Therefore, your closest answer would be (C).
oh wow im an idiot ::)
thank you so much - you're such a legend!! :) :D ;D
Post by: RuiAce on February 08, 2018, 06:33:43 pm
hey! im really struggling with this question - i keep getting a humongous answer? can someone help me out if possible? it means a lot,
1. Calculate the pH of the solution when 25.0ml of 0.750mol L-1 hydrochloric acid solution is added to 10ml of 0.500mol L-1 barium hydroxide solution.
(A) 0.456
(B) 0.602
(C) 1.862
(D) 2.058
thanks so much :D
Be careful to make sure you converted all the millilitre measurements to litres.
__________________________________
Post by: meditatingwalrus on February 08, 2018, 06:56:49 pm
Be careful to make sure you converted all the millilitre measurements to litres.
__________________________________
YOU . ARE. AMAZING. that was actually the clearest explanation ever dude
thank you so much!
Post by: slinkybench on February 10, 2018, 09:30:38 pm
I'm a preliminary chemistry student and don't understand the Pauli's Exclusion Principle and Hund's Rule.
Can someone please explain? ???
Thanks so much!
Post by: clovvy on February 10, 2018, 09:50:26 pm
Hi there,
I'm a preliminary chemistry student and don't understand the Pauli's Exclusion Principle and Hund's Rule.
Can someone please explain? ???
Thanks so much!
Personally I never did the new syllabus and your year will be the very first ones to do it... I never studied it in depth but I liked it way more than the ones that I did.... Anyway enough about myself and I'll try my best to explain
Paul's expulsion rule stated that in any atom no two electrons can have all four quantum numbers the same.
Hund's rule describes the electronic configuration of degenerate orbitals in the ground state. The electronic configuration will have the maximum number of unpaired electrons. The ground state will have the maximum orbital angular momentum....
Since I never studied the new prelim, this is the best way I could explain so if it doesn't make sense than ask your teacher
Post by: sweetiepi on February 10, 2018, 09:57:35 pm
Hi there,Hey! Sorry if this is fairly brief, as I only barely touched this in uni chem, so bear with me. :)
I'm a preliminary chemistry student and don't understand the Pauli's Exclusion Principle and Hund's Rule.
Can someone please explain? ???
Thanks so much!
Pauli's exclusion rule is where no two electrons in an atom can have the same quantum numbers. As an orbital can contain a maximum of only two electrons, the two electrons must have opposing spins- an up-spin and a down-spin. :)
(I.e. a p-shell orbital is degenerate- there are actually 3 orbitals of the same energy level that carry two electrons each :) )
Hund's rule is where each orbital is occupied by only one electron before filling up with a second electron. (I.e. 2px1 2py1 2pz1 for 3 electrons).
Edit: was beaten but clovvy is also right :)
Post by: arii on February 12, 2018, 11:00:01 pm
For a first hand investigation that I'm doing, one of the questions say: "How do you know the information you have collected is relevant?"
Is the word "relevant" the same meaning as the word "valid"? If so, do I simply say everything not tested was kept constant (i.e. just talk about controlled variables)?
Post by: clovvy on February 14, 2018, 01:26:31 pm
I just want to confirm something to ensure I don't do it wrong...
For a first hand investigation that I'm doing, one of the questions say: "How do you know the information you have collected is relevant?"
Is the word "relevant" the same meaning as the word "valid"? If so, do I simply say everything not tested was kept constant (i.e. just talk about controlled variables)?
I may be wrong but as far as my understanding goes, the word 'relevant' implies the close connection of the data you gather to the prac. for example, if you want to use n=m/M then speed is irrelevant as it is NOT connected to what you are trying to find? I think validity and relevance are different because with validity you are testing consistency of your results, if say your results always vary then the prac is unreliable therefore it is invalid. Do that make any sense?
Post by: arii on February 14, 2018, 08:09:42 pm
I may be wrong but as far as my understanding goes, the word 'relevant' implies the close connection of the data you gather to the prac. for example, if you want to use n=m/M then speed is irrelevant as it is NOT connected to what you are trying to find? I think validity and relevance are different because with validity you are testing consistency of your results, if say your results always vary then the prac is unreliable therefore it is invalid. Do that make any sense?
I think validity is whether or not the experiment actually tests the what you intended it to test. Whilst, the consistency of results is reliability. (There's 3 parts schools normally teach: Reliability, Accuracy and Validity).
Post by: cocopops201 on February 14, 2018, 09:48:09 pm
I'm looking for chem notes regarding the Ostwald process.
More specifically the reaction conditions required at each step vis-a-vis Le Chatelier's principle.
It would be greatly appreciated!
Thank you for anyone who replies. :)
Just tryna graduate
Post by: arii on February 15, 2018, 12:28:35 am
Hello!
I'm looking for chem notes regarding the Ostwald process.
More specifically the reaction conditions required at each step vis-a-vis Le Chatelier's principle.
It would be greatly appreciated!
Thank you for anyone who replies. :)
Just tryna graduate
The internet is a wonderful source of information.
If you just want stuff on the Ostwald process, check out these links:
http://www.worldofchemicals.com/449/chemistry-articles/manufacturing-of-nitric-acid-by-ostwald-process.html
http://www.bbc.co.uk/bitesize/standard/chemistry/plasticsandothermaterials/fertilisers/revision/7/
The first link is a bit more wordy but there is some good info whilst I find the 2nd one to be more condensed and summarised. You should also utilise YouTube if you don't want to read or scan reputable textbooks (i.e. Jacaranda) for what you want.
Post by: beeangkah on February 17, 2018, 04:45:28 pm
"What is the pH of 0.1mol/L of carbonic acid if only 8% ionises?"
So far I've found mole ratio but I'm confused with how to continue with the degree of ionisation part...
H2CO3(aq) --> 2H+(aq) + CO32-(aq)
Therefore the calculations involve 2[H+]...
Thanks!
Also, the answers say 2.1
Post by: RuiAce on February 17, 2018, 04:48:37 pm
Hii, just need help with this question:
"What is the pH of 0.1mol/L of carbonic acid if only 8% ionises?"
So far I've found mole ratio but I'm confused with how to continue with the degree of ionisation part...
H2CO3(aq) --> 2H+(aq) + CO32-(aq)
Therefore the calculations involve 2[H+]...
Thanks!
Also, the answers say 2.1
Post by: beeangkah on February 17, 2018, 05:02:13 pm
Oh I see, thanks!
So the mole ratio is irrelevant? Or is that only for fully ionised equations?
Post by: kaustubh.patel on February 17, 2018, 05:07:24 pm
And i suppose the mole ratio is still necessary to look at cause it shows only one proton is donated to make hydronium.
Post by: RuiAce on February 17, 2018, 05:18:32 pm
Oh I see, thanks!It's not needed here because we didn't care about the carbonate ions produced in the ionisation process
So the mole ratio is irrelevant? Or is that only for fully ionised equations?
Ummmm just to be sure does that mean carbonic acid is a monoprotic acid cause in biology I learnt that as CO2 dissolves in blood it dissolutes into bicarbonate ion and a proton.The degree of ionisation automatically caters for the diprotic nature of the acid. In general, unless otherwise specified, the degree of ionisation measures how much all of the hydrogen atoms get ionised.
And i suppose the mole ratio is still necessary to look at cause it shows only one proton is donated to make hydronium.
This is also the reason why sulfuric acid has a degree of ionisation OVER 100% (somewhere around 154%)
The stuff you learnt in biology (which is also a good example of a buffer to use in chemistry) is not incorrect. The equation provided just combined two steps into the one, since technically one hydrogen atom comes off at a time, not both at once.
Post by: cnimm2000 on February 18, 2018, 06:16:46 pm
I was doing this dot point:
T.4.6 Identify amphiprotic substances and construct equations to describe their behaviour in acidic and basic solutions.
I noticed in the textbook some equations use equilibrium arrows and some do not. I wanted to ask if we are required to include equilibrium arrows for writing equations for amphiprotic substances. If so, could you please provide examples of cases where we do include equilibrium arrows?
Post by: RuiAce on February 18, 2018, 06:20:45 pm
Hi guys,The equilibrium arrow is used when the reaction does not go to completion. When it does, we use the normal arrow.
I was doing this dot point:
T.4.6 Identify amphiprotic substances and construct equations to describe their behaviour in acidic and basic solutions.
I noticed in the textbook some equations use equilibrium arrows and some do not. I wanted to ask if we are required to include equilibrium arrows for writing equations for amphiprotic substances. If so, could you please provide examples of cases where we do include equilibrium arrows?
For acids, essentially the reaction goes to completion provided we have either a strong acid or a strong base (or both). All the strong acids you need to know in this course are HCl, H2SO4 and HNO3, and a strong base is essentially any group I metal - hydroxide (e.g. NaOH)
Post by: kaustubh.patel on February 24, 2018, 01:05:47 pm
I saw this question on a khan academy quiz and have nooo idea how to solve for it. If anyone know how to o it please lend me a hand i'd deeply appreciate it.
the link to the quiz (https://www.khanacademy.org/test-prep/mcat/chemical-processes/titrations-and-solubility-equilibria/e/titration-questions)
Post by: itssona on February 24, 2018, 04:18:27 pm
Post by: Natasha.97 on February 24, 2018, 04:41:11 pm
hey guys, is it true that basicity of oxides decreases from left to right as well?
Yep!
Group 1 would form basic oxides, whereas Group 7 would form acidic oxides :)
Post by: itssona on February 24, 2018, 05:32:03 pm
thank you!! :)
Post by: kiwiberry on February 24, 2018, 05:59:02 pm
the answer is D which makes sense but.. cant it also be C? since its endothermic and increase in temperature will want to shift to the right?
thank you!! :)
Delta H is negative which means that the reaction is exothermic, so an increase in temperature will cause a shift to the left :)
Post by: itssona on February 24, 2018, 06:39:37 pm
so the reaction should shift left since theres a decrease in pressure but how does this yield the answer A since this is exothermic?? thank you so much ahh
Post by: Sine on February 24, 2018, 06:49:21 pm
sorry I have a similar question:I think the forward reaction is exothermic so the backward reaction is endothermic
so the reaction should shift left since theres a decrease in pressure but how does this yield the answer A since this is exothermic?? thank you so much ahh
Post by: Rajan_ on February 25, 2018, 03:56:54 pm
I was wondering how you are supposed to determine a suitable indicator when doing titrations. I know it's to do with the pH of the salt produced in the reaction, but doesn't the pH of that salt differ depending on the concentration of the unknown solution, and therefore the indicator required will differ too?? Really confused.
Thanks!!
Post by: RuiAce on February 25, 2018, 04:10:54 pm
Hello!The pH of the salt is primary dependent on whether or not the acid and base are strong or weak. The concentration simply measures just how much acid there is (relative to the volume of water); it has no say in the degree of ionisation of the acid/base.
I was wondering how you are supposed to determine a suitable indicator when doing titrations. I know it's to do with the pH of the salt produced in the reaction, but doesn't the pH of that salt differ depending on the concentration of the unknown solution, and therefore the indicator required will differ too?? Really confused.
Thanks!!
Strong acid + Strong base -> Neutral salt, and therefore bromothymol blue (or litmus)
Strong acid + Weak base -> Acidic salt, and therefore methyl orange
Weak acid + Strong base -> Basic salt, and therefore phenolphthalein
Post by: beeangkah on March 01, 2018, 03:52:05 pm
What volume of 0.99 mol/ L sulphuric acid is required to neutralise 500mL of a solution containing 10.5 g of sodium hydroxide?
Post by: kiwiberry on March 02, 2018, 01:32:05 pm
I need help with this question :)Hello!! First let’s write out the equation
What volume of 0.99 mol/ L sulphuric acid is required to neutralise 500mL of a solution containing 10.5 g of sodium hydroxide?
Post by: ushna on March 02, 2018, 08:35:08 pm
Investigate the properties of unstable isotopes using natural and human made radioisotopes as examples, including but not limited to:
-types of radiation
-types of balanced nuclear reactions
-industrial and medical uses
Post by: RuiAce on March 02, 2018, 08:36:29 pm
Hey. I am a year 11 student. I have an exam coming up and I need help on the question,Which radioisotopes are you studying?
Investigate the properties of unstable isotopes using natural and human made radioisotopes as examples, including but not limited to:
-types of radiation
-types of balanced nuclear reactions
-industrial and medical uses
(Jake's notes has info on Tc-99m and Co-60. But not everyone will be studying these radioisotopes.)
Post by: ushna on March 02, 2018, 08:43:21 pm
Which radioisotopes are you studying?My teacher said that we will need to research by ourselves. We are not going to be learning this at my school.
(Jake's notes has info on Tc-99m and Co-60. But not everyone will be studying these radioisotopes.)
Post by: RuiAce on March 02, 2018, 08:53:50 pm
My teacher said that we will need to research by ourselves. We are not going to be learning this at my school.Then you should pick two that you can find a lot of information on.
The two in Jake's notes tend to appear a lot. I'll start you off with a brief sketch. For Tc-99m the radiation involved is gamma decay, Wikipedia gives quite detailed information regarding its uses, but you can probably find more simplified versions in random websites. (And of course, in Jake's notes.)
Note that Wikipedia also has the nuclear equation for the decay of Tc-99m. I will just provide you the equation for its production myself:
Step 1. \( ^{98}_{42}Mo + ^1_0n \to ^{99}_{42}Mo \)
Step 2. \( ^{99}_{42}Mo \to ^{99m}_{43}Tc + ^0_{-1}e \) (Note that beta decay is involved in the production of Tc-99m)
Post by: beeangkah on March 02, 2018, 09:53:36 pm
1. Just wondering what the key esters are needed to be memorised/familiar with (including their properties e.g. apple smell/flavour)
2. Also, any tips for if given the name of a salt, how you can tell if its acidic/basic/neutral?
Thankss
Post by: RuiAce on March 02, 2018, 10:38:49 pm
2. Also, any tips for if given the name of a salt, how you can tell if its acidic/basic/neutral?By looking at the cation and anion that make up the salt.
Thankss
Acidic cation + Neutral anion = Acidic salt
Neutral cation + Neutral anion = Neutral salt
Neutral cation + Basic anion = Basic salt
Acidic cations include ammonium and, really just that for the HSC. (Hydronium doesn't appear in a salt.)
Neutral cations include pretty much all the group 1 metal cations (essentially the cations of a strong base)
Neutral anions include chloride, sulfate and nitrate (essentially the anions of a strong acid)
Basic anions include pretty much every other anion
(Acidic cation + Basic anion is a bit of a hard one. That is usually done on a case-by-case basis. They also tend to appear very very rarely in the course.)
Post by: mxrylyn on March 05, 2018, 06:12:49 pm
I'm just reading, "The oxidation number of an atom in an elementary substance is zero. (e.g in Cl2 the oxidation number of cholrine is zero"
What does elementary substance mean? Does it just mean an atom with a full shell?
Post by: RuiAce on March 05, 2018, 07:01:53 pm
Hello,That just means pure elements. These are either single atoms, or molecules that are made up by the same atom. (Cl2 is an example of this because it's a molecule only made up by chlorine atoms.)
I'm just reading, "The oxidation number of an atom in an elementary substance is zero. (e.g in Cl2 the oxidation number of cholrine is zero"
What does elementary substance mean? Does it just mean an atom with a full shell?
Post by: chloefrecs on March 10, 2018, 09:26:43 am
I'm having trouble on what to do for the second part of this question (from Conquering Chem):
22. 25 mL of sulfuric acid from a car battery was accurately diluted to 500 mL. 25 mL of the diluted solution was titrated with 0.206 mol/L sodium hydroxide. It required 38.8 mL. Calculate the molarity of the original battery acid and its concentration in grams per litre. If 50 mL of this acid were spilt on the bench, how much anhydrous sodium carbonate would be needed to neutralise it?
I found the molarity of sulfuric acid = 3.2 mol and 314 g/L
Thanks in advance!!! :D
Post by: RuiAce on March 10, 2018, 09:51:11 am
Hello!
I'm having trouble on what to do for the second part of this question (from Conquering Chem):
22. 25 mL of sulfuric acid from a car battery was accurately diluted to 500 mL. 25 mL of the diluted solution was titrated with 0.206 mol/L sodium hydroxide. It required 38.8 mL. Calculate the molarity of the original battery acid and its concentration in grams per litre. If 50 mL of this acid were spilt on the bench, how much anhydrous sodium carbonate would be needed to neutralise it?
I found the molarity of sulfuric acid = 3.2 mol and 314 g/L
Thanks in advance!!! :D
Post by: LaraC on March 10, 2018, 06:19:16 pm
I'm having trouble with the question below. I do Chemistry via DE and have had about 3 actual lessons in the past year and a half....so I'm struggling quite a bit. I don't understand diprotic acids or calculation of acid dissociation constants properly. Can someone please help me??
Carbonic acid is a diprotic acid. Write TWO equations to show its stepwise ionisation in water.
Sorry I know its so basic but I'm pretty stupid! :-\ :'( :-[
Post by: Sine on March 10, 2018, 06:25:42 pm
Hello,H2CO3 + H20 ---------> HCO3- + H30+
I'm having trouble with the question below. I do Chemistry via DE and have had about 3 actual lessons in the past year and a half....so I'm struggling quite a bit. I don't understand diprotic acids or calculation of acid dissociation constants properly. Can someone please help me??
Carbonic acid is a diprotic acid. Write TWO equations to show its stepwise ionisation in water.
Sorry I know its so basic but I'm pretty stupid! :-\ :'( :-[
HCO3- + H20 ---------> CO3(2-) + H30+
diportic acids have two acidic protons which may be donated from the species.
Each of the equation above will have different acid dissociation constants (Ka) but you will need extra info to calculate them
Post by: RuiAce on March 10, 2018, 06:29:41 pm
H2CO3 + H20 ---------> HCO3- + H30+Just make sure to note that \( K_a\) is not in the HSC course. Also, at times like this, the reversible arrow is preferred (because this is not a strong acid).
HCO3- + H20 ---------> CO3(2-) + H30+
diportic acids have two acidic protons which may be donated from the species.
Each of the equation above will have different acid dissociation constants (Ka) but you will need extra info to calculate them
Anyway,
Hello,The equations above are correct. Essentially, to demonstrate its diprotic nature, you should be able to demonstrate how it can give away one proton, and then give away another one, so effectively speaking giving away 2 protons. Recall from B-L acid theory that an acid is a proton donor. The "di-" prefix indicates that it should be able to give two away, which is what Sine has shown.
I'm having trouble with the question below. I do Chemistry via DE and have had about 3 actual lessons in the past year and a half....so I'm struggling quite a bit. I don't understand diprotic acids or calculation of acid dissociation constants properly. Can someone please help me??
Carbonic acid is a diprotic acid. Write TWO equations to show its stepwise ionisation in water.
Sorry I know its so basic but I'm pretty stupid! :-\ :'( :-[
Post by: LaraC on March 10, 2018, 06:44:09 pm
Very much appreciated! :)
Post by: LaraC on March 10, 2018, 07:08:14 pm
What is this percentage for Chemistry? Or is it only the HSC course in the actual HSC exam?
Thanks!
Post by: RuiAce on March 10, 2018, 07:11:29 pm
Just while I think of it, I know that Mathematics can include up to 20% of the prelim coursework in the HSC.0%.
What is this percentage for Chemistry? Or is it only the HSC course in the actual HSC exam?
Thanks!
That 20% rule only applies to mathematics
Post by: cnimm2000 on March 12, 2018, 01:38:31 pm
For this dot point:
P.3.3 Gather and process information from secondary sources to write ionic equations to represent the ionisation of acids
Since citric acid is a triprotic acid are we required to know how to write equations for every ionisation step, or just the first one.
If so, have i dont the ionisation steps correctly for citric acid:
C6H8O7 (aq) + H2O (l) H3O+ (aq) + C6H7O7- (aq)
C6H7O7- (aq) + H2O (l) H3O+(aq) + C6H6O72-
C6H6O72- (aq) + H2O (l) H3O+ (aq) + C6H5O73- (aq)
Post by: MisterNeo on March 13, 2018, 09:23:00 pm
hey guys,
For this dot point:
P.3.3 Gather and process information from secondary sources to write ionic equations to represent the ionisation of acids
Since citric acid is a triprotic acid are we required to know how to write equations for every ionisation step, or just the first one.
If so, have i dont the ionisation steps correctly for citric acid:
C6H8O7 (aq) + H2O (l) H3O+ (aq) + C6H7O7- (aq)
C6H7O7- (aq) + H2O (l) H3O+(aq) + C6H6O72-
C6H6O72- (aq) + H2O (l) H3O+ (aq) + C6H5O73- (aq)
Hey :)
In the HSC, we assume polyprotic acids fully ionise when reacting with a base. In the ionisation of the citric acid in water, it may be necessary to write out each protonation because they can ask for you to show why the acid is weak or why it forms a buffer with some chemical.
Your equations look good :)
I'd only write out each step if it were to ionise in water, otherwise reactions with base/metals we assume all 3 are donated at once.
Post by: cnimm2000 on March 15, 2018, 12:16:08 pm
i just wanted to clarify:
that Element 114, Uuq (ununquadium) is now known as flerovium?
thanks in advance
Post by: RuiAce on March 15, 2018, 03:38:36 pm
hey guys,Yeah. That's been the case for a while.
i just wanted to clarify:
that Element 114, Uuq (ununquadium) is now known as flerovium?
thanks in advance
113, 115, 117 and 118 are the new ones
Post by: SpanishPear on March 15, 2018, 04:21:42 pm
Post by: kiwiberry on March 15, 2018, 11:41:29 pm
Are Back Titration calculations assessable in the HSC Exam?
Yes!! There was one in the 2016 HSC I believe :)
Post by: arii on March 16, 2018, 08:33:31 pm
Post by: MisterNeo on March 17, 2018, 11:30:02 am
I'm not sure how to do this question. Refer to attachment :)
Post by: hilaryl on March 17, 2018, 01:35:08 pm
Post by: LaraC on March 19, 2018, 08:25:59 pm
Wondering if someone could help me with the balanced neutralisation reaction for nitric acid + ammonia solution? Will it have H2O in the products or not? I'm a little confused how to balance it with this....
Thanks.
Post by: RuiAce on March 19, 2018, 08:32:31 pm
Hello :)
Wondering if someone could help me with the balanced neutralisation reaction for nitric acid + ammonia solution? Will it have H2O in the products or not? I'm a little confused how to balance it with this....
Thanks.
In general, water doesn't always appear when ammonia is the base involved.
(Edit: Broken code)
Post by: LaraC on March 19, 2018, 08:37:36 pm
Post by: Mate2425 on March 19, 2018, 09:10:22 pm
Equ. 1 CO2 (g) = CO2 (aq)
Equ. 2 CO2 (aq) + H2O (L) = H^+ (aq) + HCO3^- (aq)
Equ. 3 HCO3^-1 (aq) + H20 (L) = H3O^+ (aq) + CO3 ^-2 (aq)
Where '=' is equilibrium.
Thank you very much :) :)
Post by: 3.14159265359 on March 19, 2018, 10:18:05 pm
Post by: kiwiberry on March 20, 2018, 03:45:30 am
Hi i was just wondering would the following equilibrium's satisfy the solubility of CO2 in water.
Equ. 1 CO2 (g) = CO2 (aq)
Equ. 2 CO2 (aq) + H2O (L) = H^+ (aq) + HCO3^- (aq)
Equ. 3 HCO3^-1 (aq) + H20 (L) = H3O^+ (aq) + CO3 ^-2 (aq)
Where '=' is equilibrium.
Thank you very much :) :)
Yep :)
Heyy can someone please check if I did this correctly. Tyy
The image isn’t appearing for me!!
Post by: Mate2425 on March 20, 2018, 11:48:13 am
Yep :)Thank you
The image isn’t appearing for me!!
Post by: 3.14159265359 on March 20, 2018, 06:28:01 pm
The image isn’t appearing for me!!
is there a way to post the picture. its not letting me send it. I tried everything
Post by: RuiAce on March 20, 2018, 06:32:28 pm
is there a way to post the picture. its not letting me send it. I tried everythingLooking at your attachment you posted an image file that's 0kb, which would imply that it's a blank image. So potentially the forum is messing up but some problem is existing between your image and the servers.
The alternative is to upload it to imgur and then manually link it.
Post by: 3.14159265359 on March 20, 2018, 06:34:24 pm
Post by: RuiAce on March 20, 2018, 06:37:11 pm
oh wait I got itIs there something missing (or in an earlier part)? You subbed 0.25L in for \(V_2\) but I don't know why
Post by: 3.14159265359 on March 20, 2018, 06:40:10 pm
Is there something missing (or in an earlier part)? You subbed 0.25L in for \(V_2\) but I don't know why
oh because I was going to prepare that in a 250 ml volumetric flask
Post by: RuiAce on March 20, 2018, 06:42:56 pm
oh because I was going to prepare that in a 250 ml volumetric flaskIn that case,
As you correctly calculated, you should have 2.5 mL of HCl at the start. Using the dilutions formula, the combined volume of everything at the end will be 250 mL.
Therefore, you need to add \( 250 - 2.5 = 247.5\text{ mL}\) of water.
Post by: LaraC on March 20, 2018, 06:56:27 pm
The salt, potassium hydrogen carbonate (KHCO3) hydrolyses in water to produce a basic solution. Write a balanced equation to explain why the solution is basic.
Post by: 3.14159265359 on March 20, 2018, 06:57:40 pm
In that case,
As you correctly calculated, you should have 2.5 mL of HCl at the start. Using the dilutions formula, the combined volume of everything at the end will be 250 mL.
Therefore, you need to add \( 250 - 2.5 = 247.5\text{ mL}\) of water.
am I supposed to do a method explaining what to do? like how to perform a hcl standard? or something along those lines? but the thing is hcl can't be a standard because it doesn't meet the criteria.
btw this was the marking guidelines for it
Post by: 3.14159265359 on March 20, 2018, 07:02:46 pm
Hello, can someone please help me out with this question?
The salt, potassium hydrogen carbonate (KHCO3) hydrolyses in water to produce a basic solution. Write a balanced equation to explain why the solution is basic.
hco3^- +h2o <--> h2co3 +oh^-
the solution is basic as hydroxide was produced which is an indication of a base present. also when you write the equation you disregard the potassium (k) because its a neutral as it doesn't react with water because it comes from the strong base KOH
Post by: RuiAce on March 21, 2018, 07:09:50 pm
am I supposed to do a method explaining what to do? like how to perform a hcl standard? or something along those lines? but the thing is hcl can't be a standard because it doesn't meet the criteria.At first glance, the marking guidelines given don't mention anything about 250 mL HCl at the start.
btw this was the marking guidelines for it
If anything, I'd use titration against something like anhydrous \(Na_2CO_3\) instead. HCl certainly cannot be a primary standard, but I see no problem with it being a secondary standard.
Post by: LaraC on March 23, 2018, 12:03:58 pm
Also, I'm not sure if there is such a thing as a brief rundown/explanation on titrations, but if there is, would anyone please mind helping me out? I'm pretty confused by the whole deal! Thanks :)
Post by: LaraC on March 24, 2018, 09:39:00 pm
I'm really stuck with this and if someone could possibly help me that would be awesome!! :D
The sulfate concentration in a sample of water can be determined gravimetrically by:
- first adding acid to the sample to react with carbonate ions
- then adding excess barium ions
- filtering off, drying and weighing the BaSO4 precipitate formed.
Barium carbonate is insoluble. Adding acid before adding barium ions prevents BaCO3 precipitating with the BaSO4. Write a balanced equation for the reaction in which H+ ions destroy a carbonate ion releasing a gas.
Thanks so much!!
Post by: fireives1967 on March 24, 2018, 11:02:26 pm
I've tried to use Ethene(s) monomers --> Polyethylene through
n(C2H4) --> (C2H4)n but I'm not sure if it's quite correct.
Can someone please provide a valid equation????
Much appreciated!!!! ;D
Post by: beeangkah on March 25, 2018, 08:46:36 am
If the question does not specify, do we draw a line of best fit or join the dots? I've gotten mixed messages across the years so I'm not entirely sure.
And also same with curves of best fit.
Thanks!
Post by: kiwiberry on March 26, 2018, 03:03:10 am
Qu: is a solution made using equal volumes of sulfuric acid and sodium sulfate a buffer solution and why?
Also, I'm not sure if there is such a thing as a brief rundown/explanation on titrations, but if there is, would anyone please mind helping me out? I'm pretty confused by the whole deal! Thanks :)
Hey, sorry for the wait!!
A solution made with sulfuric acid and sodium sulfate would not be a buffer solution because 1) a buffer must be made using a weak acid and 2), sulfate is not the conjugate base of sulfuric acid.
This is a brief outline of the steps in titration, does this help a bit? :)
(https://i.imgur.com/rNFUygk.png)
Maybe I'll try with a different question ;)
I'm really stuck with this and if someone could possibly help me that would be awesome!! :D
The sulfate concentration in a sample of water can be determined gravimetrically by:
- first adding acid to the sample to react with carbonate ions
- then adding excess barium ions
- filtering off, drying and weighing the BaSO4 precipitate formed.
Barium carbonate is insoluble. Adding acid before adding barium ions prevents BaCO3 precipitating with the BaSO4. Write a balanced equation for the reaction in which H+ ions destroy a carbonate ion releasing a gas.
Thanks so much!!
The equation will be
Post by: kiwiberry on March 26, 2018, 03:16:58 am
What is a valid equation to explain addition polymerisation?
I've tried to use Ethene(s) monomers --> Polyethylene through
n(C2H4) --> (C2H4)n but I'm not sure if it's quite correct.
Can someone please provide a valid equation????
Much appreciated!!!! ;D
That equation is perfectly fine to use to describe the overall process!! If you want, you can also use these equations for initiation and propagation
(https://i.imgur.com/UwkIMKQ.png)
Post by: kiwiberry on March 26, 2018, 03:19:17 am
Hi, just a quick question on graphing.
If the question does not specify, do we draw a line of best fit or join the dots? I've gotten mixed messages across the years so I'm not entirely sure.
And also same with curves of best fit.
Thanks!
I would say draw a line/curve of best fit if they don't specify
Post by: LaraC on March 26, 2018, 03:04:56 pm
Post by: mxrylyn on March 29, 2018, 03:03:46 pm
Does Polystyrene polymerisation need a free radical/ initiator?
Post by: kiwiberry on March 29, 2018, 05:37:54 pm
Hey!
Does Polystyrene polymerisation need a free radical/ initiator?
I believe it does - but you don’t need to know about this :)
Post by: kiwiberry on March 29, 2018, 05:38:46 pm
Hey!
Does Polystyrene polymerisation need a free radical/ initiator?
I believe it does - but you don’t need to know about this :)
Post by: arii on March 29, 2018, 10:24:43 pm
Post by: Calley123 on March 30, 2018, 04:56:12 pm
Can someone please explain Q28 c) of 2008 to me??
- why is the titre ( 0.02345L ) used to calculate the moles of Na2CO3 ( i think this is standard solution- thus, the known conc.) rather than 0.025 L ????
Im confused about entire question since I always thought that the unknown conc. of solution was the titrant and known conc.. the standard solution???
http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2008exams/pdf_doc/chemistry-notes-08.pdf
Thanks
Post by: Mate2425 on March 31, 2018, 11:53:38 am
Many thanks :)
Post by: RuiAce on March 31, 2018, 03:51:33 pm
Hi could someone please tell me the cathode (reduction) and anode (oxidation) half equations for the Dry Cell and the Fuel Cell.I found the dry cell one with a Google search. But I have no clue with the fuel cell because I swear there's more than one of them.
Many thanks :)
Hey,0.025L was the volume of the acid, not the base.
Can someone please explain Q28 c) of 2008 to me??
- why is the titre ( 0.02345L ) used to calculate the moles of Na2CO3 ( i think this is standard solution- thus, the known conc.) rather than 0.025 L ????
Im confused about entire question since I always thought that the unknown conc. of solution was the titrant and known conc.. the standard solution???
http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2008exams/pdf_doc/chemistry-notes-08.pdf
Thanks
In this experiment, presumably 25 mL of acid was in the conical flask, and it was the Na2CO3 base that was in the burette. (Else, the words "average titration volume" would be bs.) So whereas you use 0.025L to calculate the volume of HCl, you'd need 0.02345L for the volume of the Na2CO3
Post by: beeangkah on March 31, 2018, 05:32:01 pm
Q:
A student tests some solid metal oxides. All 4 oxides react with sulphuric acid. Only one of the oxides reacts with sodium hydroxide. This oxide could be:
a) Al2O3
b) MgO
c) Fe2O3
d) BaO
Thanks!
Post by: kiwiberry on March 31, 2018, 06:04:26 pm
Hi could someone explain how to get the answer for this?
Q:
A student tests some solid metal oxides. All 4 oxides react with sulphuric acid. Only one of the oxides reacts with sodium hydroxide. This oxide could be:
a) Al2O3
b) MgO
c) Fe2O3
d) BaO
Thanks!
Hey!!
For the oxide to react with NaOH, it has to be either an acidic oxide (reacts with bases) or amphoteric oxide (reacts with both acids and bases). MgO, Fe2O3 and BaO are all basic oxides (left side of the periodic table), so won't react with NaOH. Al2O3, however, is an amphoteric oxide, meaning it will react with NaOH, so the answer is A :)
Post by: RuiAce on March 31, 2018, 10:27:08 pm
I'm not sure about part (ii) of both. I'm pretty sure I did the identification right in (i).Taking a stab at this, for part a) you could use an acetic acid/acetate ion buffer, whilst for b) you could use a hydrogen carbonate ion/carbonate ion buffer. Because I reckon the former would be acidic by nature whilst the latter is basic by nature, yet you're still putting in an acid with its conjugate base.
(The idea is that for B, both \(HCO_3^-\) and \(CO_3^{2-} \) are basic, so they'd show some resistance over the basic range. But you'd still need an acid-base conjugate pair. Whereas for A, presumably \(CH_3COOH\) is more acidic than \(CH_3COO^-\) is basic, so it'd probably be more stable on the acidic side of things. Although tbh, I asked the chemistry team and we really thought that this wouldn't be necessary for the HSC.)
Post by: Calley123 on April 01, 2018, 12:50:50 pm
I found the dry cell one with a Google search. But I have no clue with the fuel cell because I swear there's more than one of them.
0.025L was the volume of the acid, not the base.
In this experiment, presumably 25 mL of acid was in the conical flask, and it was the Na2CO3 base that was in the burette. (Else, the words "average titration volume" would be bs.) So whereas you use 0.025L to calculate the volume of HCl, you'd need 0.02345L for the volume of the Na2CO3
Thanks !
Post by: Mate2425 on April 02, 2018, 01:04:44 am
Does anyone know where i can find a link to the reasoning behind multiple choice questions in the HSC Chemistry paper 2015.
Thanks 8)
Post by: RuiAce on April 02, 2018, 07:06:29 am
Post by: Mate2425 on April 02, 2018, 01:07:03 pm
Post by: RuiAce on April 02, 2018, 01:29:39 pm
Hey could i please have some help with the calculations for this question. Thanks :DWhere's the question?
Post by: Mate2425 on April 02, 2018, 02:02:07 pm
https://educationstandards.nsw.edu.au/wps/wcm/connect/cb65ca25-fc28-4e21-8f2d-4886bf176ddd/2017-hsc-chemistry.pdf?MOD=AJPERES&CACHEID=ROOTWORKSPACE-cb65ca25-fc28-4e21-8f2d-4886bf176ddd-lZ.o-ZW
Post by: RuiAce on April 02, 2018, 02:10:41 pm
___________________________________________________
Hence C.
Post by: Mate2425 on April 02, 2018, 02:19:04 pm
Thank You very much :) :)
___________________________________________________
Hence C.
Post by: 3.14159265359 on April 02, 2018, 07:09:38 pm
Post by: kiwiberry on April 02, 2018, 07:29:11 pm
hey, what would the answer to this be?
Hey!!
When bromine water is added to hexane, no reaction will occur, and the bromine water will float above it in another layer. Hexene will be more reactive than hexane due to its double bond, and when bromine water is added to it, a reaction occurs, decolourising the bromine water. Hence the answer is C :)
Post by: 3.14159265359 on April 02, 2018, 07:36:08 pm
Hey!!
When bromine water is added to hexane, no reaction will occur, and the bromine water will float above it in another layer. Hexene will be more reactive than hexane due to its double bond, and when bromine water is added to it, a reaction occurs, decolourising the bromine water. Hence the answer is C :)
that makes so much sense! thank you
Post by: 3.14159265359 on April 02, 2018, 09:28:29 pm
Post by: RuiAce on April 02, 2018, 09:37:56 pm
Hey, you see when we calcualte E° we add the E(oxidation) And E(reduction) and for the oxidation if it’s -ve we flip the sign to be positive because we flip the eqn. How about if E(oxidation) was already positive, do we still change like above (so from positive to negative) ???Yeah. The sign change always occurs regardless.
If you don't flip the sign then you'll get an E° value that's too high (when they're both +'ve). The cell actually produces relatively small voltage when they're both positive.
Post by: 3.14159265359 on April 02, 2018, 09:47:13 pm
Yeah. The sign change always occurs regardless.
If you don't flip the sign then you'll get an E° value that's too high (when they're both +'ve). The cell actually produces relatively small voltage when they're both positive.
Hmmm makes sense. Thanks!!
Post by: Mate2425 on April 02, 2018, 10:17:40 pm
http://boardofstudies.nsw.edu.au/hsc_exams/2016/exams/2016-hsc-chemistry.pdf
Thank you.
Post by: 3.14159265359 on April 03, 2018, 07:10:08 am
Hi could someone please help me with wondering why the answer to Q11 2016 MC is D and not A.
http://boardofstudies.nsw.edu.au/hsc_exams/2016/exams/2016-hsc-chemistry.pdf
Thank you.
Hey, I think it’s because you have to add all the numbers and you pick the one with the lowest numbers.
Like for a:
There is a 1+1+2+2+2= 8
And for b:
There is a 2+2+1+1+1= 7
So that’s why it’s d because 7<8.
I’m not too sure tho, so just wait for one of the moderators to confirm
Post by: RuiAce on April 03, 2018, 08:27:30 am
Yeah this is fine.
Hey, I think it’s because you have to add all the numbers and you pick the one with the lowest numbers.
Like for a:
There is a 1+1+2+2+2= 8
And for b:
There is a 2+2+1+1+1= 7
So that’s why it’s d because 7<8.
I’m not too sure tho, so just wait for one of the moderators to confirm
________________________
Will add a comment though. From what I now know, this technically is no longer how IUPAC does things. According to the actual IUPAC rules, you should just go by alphabetical, which would give you A. But we just have to suck it up
Post by: 3.14159265359 on April 03, 2018, 04:10:30 pm
Yeah this is fine.
________________________
Will add a comment though. From what I now know, this technically is no longer how IUPAC does things. According to the actual IUPAC rules, you should just go by alphabetical, which would give you A. But we just have to suck it up
If that was true, then what would be different from choosing A or D???
Post by: RuiAce on April 03, 2018, 04:16:45 pm
Post by: Abdul_k on April 03, 2018, 06:28:23 pm
https://drive.google.com/file/d/1Kq0lbrCQJ2AluVs3kdTnQaZpkyK9-1FK/view?usp=drivesdk
Post by: MisterNeo on April 04, 2018, 12:00:17 pm
Hi, I'm just unsure for this question and there are no answers. Thanks in advanced :)
https://drive.google.com/file/d/1Kq0lbrCQJ2AluVs3kdTnQaZpkyK9-1FK/view?usp=drivesdk
Hey :)
Sodium acetate forms a basic solution in water because of the acetate ion forming an equilibrium with acetic acid.
Initially, when the sodium acetate is dissolved in water, there is no acetic acid in the products of the equilibrium, hence it will shift to the right to maintain that special ratio (K constant). This shift to the right will produce hydroxide ions in the process and thus increase the pH of the solution and become basic. We can leave out Na+ in the equation because it's a spectator ion.
For the Part B, I would start off by defining a buffer solution and how sodium acetate and acetic acid form a buffer. A buffer is a solution made from equal amounts of a weak acid/base and its conjugate base/acid, which experiences small changes in pH when a strong acid/base is added.
When NaOH is added, it neutralises the H3O+ ions, causing the buffer equilibrium to shift to the right (Le Chat's Principle) to replace the lost ions, hence the pH will only experience small changes when the strong base is added. ;)
Post by: skisso on April 06, 2018, 03:24:36 pm
Thanks!
Post by: moxiao402 on April 07, 2018, 08:30:21 pm
Post by: emily_p on April 08, 2018, 12:01:41 am
Hello guys, I’ve got a titration question. It says a standard solution of Na2CO3 is prepared by dissolving 1.208g and 250ml water up the volumetric flask. 25ml aliquot of the Na2CO3 solution is titrated against HCl, and the average titration result is 22.1 ml of HCl. What would the concentration be? I got 0.103molL^-1. Please help!
Yep! I also got 0.103 M :)
n(Na2CO3) = 1.208/2(22.99)+12.01+3(16)
= 0.011397...
C(standard solution) = 0.011397.../0.25
= 0.045589206...
Na2CO3 + 2HCl => 2NaCl + CO2 +H2O
n(Na2CO3) = 0.45589206...x 0.025
= 0.0011397...
n(HCl) = 2 x 0.0011397... (stoichiometric ratio)
= 0.00227946...
C(HCl) = 0.00227946.../0.0021
= 0.10314300...
= 0.103 M (3 sf)
Post by: 3.14159265359 on April 08, 2018, 10:39:00 pm
Hi guys, not too sure how to work this one out :)
Thanks!
Hey, basically what you would do is look at the question and figure what it’s asking. They question is asking which of the 4 acids that were tested is the weakest acid? In other terms which of them has the highest ph. From the table we can see that Acid X has a ph of 3.8 which is the highest ph meaning weakest acid. We know the ph “graph” is 0-7 acidic, 7 neautral and 7-14 basic. And since all options have a ph<7 we know they’re all acidic, but acid X is the weakest acid as it has the highest ph. Therefore the answer is b.
Let me know if I don’t make any sense, I’ll try explaining in different way. And I’m pretty sure I repeated myself a few times so excuse that.
Post by: KT Nyunt on April 09, 2018, 11:22:33 am
Hi guys, not too sure how to work this one out :)
Thanks!
I just had a look at the answer above me, I think there is more to answering this question, especially since they also give you the concentration of each of the monoprotic acids. We need to remember the fact that weak acids only partially ionise, whereas strong acids ionise completely, and we also need to recall that pH is a measure of Hydrogen ion concentration. Thus (as described in the answer above me), weaker acids have a higher pH as there are fewer Hydrogen ions but also the calculated pH (using the formula -log[H+]) should also be much lower than the actual pH (the pH in the table) as there aren't as many hydrogens ionised.
If we used the ph formula and substitute the concentrations from the table into [H+] we'll see that W and Z are strong acids as the calculated pH is either the same (or very close) to the the actual pH, meaning all the hydrogens ionised.
However, if X were to be a strong acid, its pH should be 2 (using the formula). Likewise, if Y were to be a strong acid, its pH should be 1. As the pH given in the table is much higher than this, this indicates that these acids have only partially ionised and thus are weak acids.
Since there is a greater difference in pH in Y than X, I believe the answer should be Y.
Hope this helps! Let me know if this clarifies things.
Post by: alisoneom on April 13, 2018, 11:25:49 pm
A household cleaning agent contains a weak base of general formula NaX. 1.00 g of this compound was dissolved in 100.0 mL of water. A 20.0 mL sample of the solution was titrated with 0.1000 mol L–1 hydrochloric acid and required 24.4 mL of the acid for neutralisation.
(a) What is the Brönsted–Lowry definition of a base? [1 mark]
(b) What is the molar mass of this base? [3 marks]
Post by: meditatingwalrus on April 19, 2018, 11:16:14 am
Post by: kauac on April 19, 2018, 11:45:15 am
heyy i having some trouble with this MC. can someone help me out? thanks and much appreciated :)
Hi...
Answer would be A, I assume.
Using flame colour, you can eliminate calcium (a red flame) and iron (no characteristic colour).
Then by referring to the solubility rules, you will find that all sulfates are soluble, except for barium, calcium, silver and lead.
Therefore, because the ion forms a ppt with Na2SO4, Barium2+ is the only feasible option left.
I'm sure there are other ways of working it out too. :)
Post by: meditatingwalrus on April 21, 2018, 12:36:21 pm
A gas is produced when 10.0 g of zinc is placed in 0.50 L of 0.20 mol L−1 nitric acid.
Calculate the volume of gas produced at 25°C and 100 kPa. Include a balanced chemical equation in your answer.
why is it that we use the moles from HNO3 and not Zn? couldnt i go v = vc x n , v = 0.153 x 24.79?
Post by: KT Nyunt on April 22, 2018, 09:27:37 pm
heyy im asking wayy too many questions on this thread hahah! but,, can someone help me out with this question?
A gas is produced when 10.0 g of zinc is placed in 0.50 L of 0.20 mol L−1 nitric acid.
Calculate the volume of gas produced at 25°C and 100 kPa. Include a balanced chemical equation in your answer.
why is it that we use the moles from HNO3 and not Zn? couldnt i go v = vc x n , v = 0.153 x 24.79?
Hi!
So they give you this information because we need to find the number of moles of nitric acid (HNO3) in this reaction as well as the number of moles of Zinc (Zn) we have, in order to see which one is the limiting reagent...
From the answers:
n(HNO3) = 0.1 mol [we find this by doing n = Cv]
n(Zn) = 0.153 mol [we find this by doing mass/molar mass]
As there are fewer moles of nitric acid than Zinc, this is the limiting reagent, meaning only 0.1 moles of HNO3 reacted with 0.1 moles of Zn and some Zn was left in excess.
From the equation, we can deduce the molar ratios, which will help us find the volume of the gas produced, H2. Now that we know 0.1 moles of HNO3 reacted, and the molar ratio between HNO3 and H2 is 2:1, the number of moles of H2 produced is 0.05 mol.
Then we put this into the equation n x 24.79, giving us the answer: 1.24L as required.
Hope this helps!
Post by: itssona on April 23, 2018, 06:16:12 pm
Post by: KT Nyunt on April 24, 2018, 11:09:06 am
anyone know how Q5 from the 2014 paper is C?? Where is the chlorine atom really from? thanks :)
An addition reaction is when an unsaturated molecule (a molecule with a double bond) reacts with another molecule, making it saturated (all the possible bonds are used up) without the loss of any atoms. So, the reactant must have a double bond (hence A and B are out) and the product must be saturated (hence D is out and the answer is C).
The chlorine atom comes from HCl: where there was once a double bond, one H joined to one CH (they showed this by having a CH2) and the Cl joined the other.
:)
Post by: cnimm2000 on April 26, 2018, 12:15:56 pm
for module 3 section 2: maximising production and ensuring quality and control, do we need to know the haber process in depth (if so could i get a quick explanation on it), or just how to increase the yield and rate of reaction
thanks in advance :)
Post by: Victorious on April 28, 2018, 02:31:17 pm
Could anybody please explain the production of LDPE and HDPE???
I'm so confused about the two seperate processes.
Thank you in advanced!
Post by: RuiAce on April 28, 2018, 02:45:51 pm
Hi,The two separate processes follow the same steps. The only difference between the production of LD and HD PE is that a Ziegler-Natta catalyst replaces the peroxide initiator for the latter. So if you ever require the production of HDPE, you just write the exact same thing for LDPE, except with a Ziegler-Natta catalyst instead.
Could anybody please explain the production of LDPE and HDPE???
I'm so confused about the two seperate processes.
Thank you in advanced!
In short:
0. Activation of the peroxide initiator
1. Initiation - the peroxide initiator attacks the first ethene molecule (which produces the radical)
2. Propagation - the radical now attacks another ethene, and so on. (We don't know how many ethene atoms will be attacked, we just care that this happens)
3. Termination - eventually, it'll collide with another ethene radical and then you have your polyethylene.
(Remark: I did this off memory. Might've mixed up the words activation and initiation other way around.)
Hey guys,It's more of those highly-recommended band 6 pieces of knowledge. What's good about understanding the Haber process is that if you properly understand its method, then you can potentially figure out "why we do this, why we do that" on the spot.
for module 3 section 2: maximising production and ensuring quality and control, do we need to know the haber process in depth (if so could i get a quick explanation on it), or just how to increase the yield and rate of reaction
thanks in advance :)
I haven't done this in ages so this explanation is very loose. Essentially, we pump a mixture of nitrogen and hydrogen into the reaction chamber, and we note that the N:H ratio is roughly 3:1 (why?).
We want to rely on LCP to drive the forward reaction, without breaking the delicate glassware. So we make the pressure around 250atm. (Why 250 and not something higher like 1000?)
We want to rely on LCP to drive the forward reaction, but we still leave the temperatures relatively high. (Why?)
The reaction chamber involves a catalyst. We need to maintain the purity of this catalyst. (What is the catalyst and why do we maintain its purity? Is the catalyst the only purity that we want to maintain; what if argon or even worse oxygen creeps into the reaction chamber?)
Of course, most of the important stuff happens there. The last bit is relatively easy to remember. Essentially, we also note that ammonia has a lower condensation (boiling) point in contrast to nitrogen and oxygen. Because the gas will go up by itself, we set the temperature somewhere high enough in the pipe (or whatever) so that ammonia will condense into the liquid state (and thus get collected).
Post by: KT Nyunt on April 29, 2018, 11:53:31 am
Hi,
Could anybody please explain the production of LDPE and HDPE???
I'm so confused about the two seperate processes.
Thank you in advanced!
If you wanted any more information, I have some screenshots from my notes for production of materials: (the full notes are also up on the ATAR notes site)
:)
Post by: meditatingwalrus on May 01, 2018, 09:27:06 pm
Hi!
So they give you this information because we need to find the number of moles of nitric acid (HNO3) in this reaction as well as the number of moles of Zinc (Zn) we have, in order to see which one is the limiting reagent...
From the answers:
n(HNO3) = 0.1 mol [we find this by doing n = Cv]
n(Zn) = 0.153 mol [we find this by doing mass/molar mass]
As there are fewer moles of nitric acid than Zinc, this is the limiting reagent, meaning only 0.1 moles of HNO3 reacted with 0.1 moles of Zn and some Zn was left in excess.
From the equation, we can deduce the molar ratios, which will help us find the volume of the gas produced, H2. Now that we know 0.1 moles of HNO3 reacted, and the molar ratio between HNO3 and H2 is 2:1, the number of moles of H2 produced is 0.05 mol.
Then we put this into the equation n x 24.79, giving us the answer: 1.24L as required.
Hope this helps!
legend! thanks so much :)
Post by: cocopops201 on May 07, 2018, 06:26:28 pm
I recently received my chemistry mid-course exam mark and honestly I'm not that too happy with it due to external factors effecting my proactivity during the exam. *internally cries* AND a true genuine question I have is how do I find my strengths and weaknesses to later fix, before any upcoming exams in Term 2 turn up? As well as, how would you specifically study for chemistry? I understand practice papers and revising syllabus notes are core elements to succeed but how does one know what to do? i.e calculation questions (step-by-step guide), production of [ethanol] using flowcharts etc.
Thanks,
Fellow struggling year 12 student in chem
Post by: KT Nyunt on May 07, 2018, 06:43:25 pm
Hello,
I recently received my chemistry mid-course exam mark and honestly I'm not that too happy with it due to external factors effecting my proactivity during the exam. *internally cries* AND a true genuine question I have is how do I find my strengths and weaknesses to later fix, before any upcoming exams in Term 2 turn up? As well as, how would you specifically study for chemistry? I understand practice papers and revising syllabus notes are core elements to succeed but how does one know what to do? i.e calculation questions (step-by-step guide), production of [ethanol] using flowcharts etc.
Thanks,
Fellow struggling year 12 student in chem
Hi!
The way that I've found to really work with studying for my science subjects - and this is a little different to normal methods of study, but hey it may work for you too! - is to teach.
Basically what I would do is:
1) create study notes (often handwritten is the way to go but this is up to you, but handwriting may improve your writing speed in exams)\
2) use these notes as 'prompters' when I teach someone (I would normally teach like a parent or a fellow classmate - setting up a study group is really good for this - this is just so they can ask you questions or give feedback)
3) You can measure how well you understand a topic by how clearly and succinctly you were able to communicate this
4) taking note, revising, then re-teaching topics that you struggled with is another way to improve understanding and this will also help to make content stick!
I still suggest working through practice papers as more of a final step, to practice writing responses and applying your knowledge. But the idea is, if you can verbally communicate your ideas, you are more than likely able to write your ideas as well.
when it comes to how to answer types of questions in exams:
- When a question says 'compare' - a quick way to do this is with a table
- when a question says 'outline a process' - a flowchart is great! (if it were to say 'describe a process' I would go in further depth than just a flowchart)
- Whenever you know a diagram for something - use it! (it shows you really know your stuff)
- calculations - when studying for this, it's always good to expose yourself to as many examples and then write yourself like a step by step guide on how to approach these types of questions (ie. a step-by-step for molar heat of combustion question, a step-by-step for concentrations in titrations etc)
- always have a look at the criteria in the answers (if it is provided) so that you can look on how to improve responses - or if they give a sample answer, add what you didn't have in your response into your own notes.
This is just my way of studying and I find it works well for me, but this may vary depending on the person. Nonetheless, I hope this was helpful! :)
Post by: cocopops201 on May 11, 2018, 12:41:11 pm
This is just my way of studying and I find it works well for me, but this may vary depending on the person. Nonetheless, I hope this was helpful! :)Thank you so much!
Post by: Dragomistress on May 12, 2018, 06:32:45 pm
Post by: KT Nyunt on May 13, 2018, 01:14:25 pm
What do I have to memorise for the catalyst used for the Haber process? Do I have to just remember that it is Fe3O3 and not the other small powders?
Yes or more accurately, just say iron (II/III) oxide :)
Post by: cnimm2000 on May 14, 2018, 12:11:59 am
I’m struggling with ppm calculations. If anyone has any tricks on how to do these questions, or how to begin to calculate it would be great!
Post by: KT Nyunt on May 14, 2018, 09:46:04 pm
Hey guys,
I’m struggling with ppm calculations. If anyone has any tricks on how to do these questions, or how to begin to calculate it would be great!
Hi!
This website helped me a lot with understanding ppm: http://www.ausetute.com.au/partspm.html
It goes through a lot of the types of ppm questions you may encounter and goes through them step by step.
To summarise it:
Here are the units you'd be working with when it comes to parts per million...
In weight/volume (w/v) terms,
1 ppm = 1g m-3 = 1 mg L-1 = 1 μg mL-1
In weight/weight (w/w) terms,
1 ppm = 1 mg kg-1 = 1 μg g-1
A lot of sources will use this formula:
mass of the solute/volume of the solution x 1000000
This is fine and dandy but another trick is just taking any information they give you (mass of solutes and volumes or mass of solution) and convert it into one of the units above (i.e. if they give you a solute mass in grams and a solution volume in millilitres, convert it into milligrams and litres). Then your final step is to divide the mass of the solute with the volume or mass of the solution ( e.g. mass of solute/volume or mass of solution).
Essentially, how this works is... if we look at an example of a ppm unit : mg kg-1
A milligram is a millionth of a kilogram. So by converting any information into these units first then we can really see how many "parts per million" we have.
I'm not too sure about how well I explained that... Let me know if this makes sense :)
Post by: itssona on May 18, 2018, 07:40:16 am
how do I write an equation for the incomplete combustion of ethyne (c2h2) that produces a ratio of soot to CO2 of 5:1
Post by: KT Nyunt on May 18, 2018, 09:47:12 am
hey:)
how do I write an equation for the incomplete combustion of ethyne (c2h2) that produces a ratio of soot to CO2 of 5:1
(https://atarnotes.com/Users/ktnyunt/Desktop/Screen Shot 2018-05-18 at 9.40.02 am.png)
^ I've attached this in case this doesn't work :)
A tip on balancing combustion equations: always balance the oxygen last
- Some schools would prefer not to have the fraction but that's an easy fix (just times everything by two)
- Also, some schools would say water is a liquid - H20 (l) - and that's acceptable as well... my school argues that at those high temperatures in combustion, H20 would be in a gaseous state.
Hope the helps!
Post by: itssona on May 18, 2018, 12:26:29 pm
(https://atarnotes.com/Users/ktnyunt/Desktop/Screen Shot 2018-05-18 at 9.40.02 am.png)thank you so much!!
^ I've attached this in case this doesn't work :)
A tip on balancing combustion equations: always balance the oxygen last
- Some schools would prefer not to have the fraction but that's an easy fix (just times everything by two)
- Also, some schools would say water is a liquid - H20 (l) - and that's acceptable as well... my school argues that at those high temperatures in combustion, H20 would be in a gaseous state.
Hope the helps!
Post by: Mate2425 on May 24, 2018, 10:36:07 pm
Post by: cocopops201 on May 26, 2018, 08:58:26 pm
My question is,
how do you find the trend? and how do you properly answer the question so it is clear and concise.
The questions are:
a) the pH of the carbonated water would also vary with the pressure of carbon dioxide gas. Indicate this trend by sketching a graph for pH on the grid above.
b)Using equilibrium principles, account for both relationships shown on the graph.
[p.s. I had to crop the photo, bc it was too large in KB]
thank you!!!!
Post by: KT Nyunt on May 27, 2018, 10:02:27 am
Hi all, i was wondering if someone had any strategies/ideas that could help to boost and maximise my marks in the HSC Chemistry Course.
Hey here are some tips I think help me greatly: hopefully some of these may be of use to you
Note making
I often collate information from many different places and then put them together in my notes (or just pick which site explains something best and then add that to my notes)
Places where I get most of my information:
class notes
this ATAR notes book: https://atarnotes.com/product/hsc-chemistry-notes/
https://easychem.com.au
and this Excel chemistry book: https://www.booktopia.com.au/excel-hsc-chemistry-excel/prod9781741252996.html?source=pla&gclid=CjwKCAjwi6TYBRAYEiwAOeH7GVgANB_bGYaYOWK2a42otUMTiX7QAibgc3Y05Z-bWpXW_3-52GIKwhoCApcQAvD_BwE
Other people's notes on ATAR notes: https://atarnotes.com/notes/?pag=1&state=3719&subject=4855&unit=0&key=&sort=date
I find for me, going through at least more than one explanation helps to consolidate my understanding of a concept.
Teaching as a tool for revision
I often teach my parents and I have a study group at school for biology and chemistry... essentially I use my notes as 'prompters' and teach whoever would listen. This is a really quick way to go over content, a good way to measure how well you understand something (how clearly you can explain it/ teach it = how well you understand it) and it really helps to make any content stick. Since Chem is pretty content heavy, this may be a very helpful strategy to improve your general knowledge and understanding of the topic.
Practice questions
To help maximising marks in the exam, definitely practice questions are the way to go.
These are some useful places to get some past paper questions:
http://www.quarkology.com/12-chemistry/12-chemistry-home.html
https://thsconline.github.io/s/yr12/Chemistry/
These are particularly useful for calculations. This is a big part of them exams and mastery of these questions just comes with practice and exposure to a lot of these questions.
In the exam
With Chem, I often find I finish the test with about 15-30 mins to spare because I've rushed through it. However, this is normally also very detrimental to my marks as I don't check over my answers as well I would like to be able to. (ie. I would look over at a question and be like 'yeahhh I answered that right' but when I recieved my marks, I had left out one piece of information that cost me getting full marks on a question. In saying that, I believe it's better to give a bit of time to really think about what the question is asking and what might be in the criteria in order to formulate a more cohesive answer before you even start writing. This may take more time to go through the test, but it's better to get it right the first time rather then rely on picking up on the mistake when you check over it at the end (if that makes sense).
Another trick I've learnt is for those moments when you come across a 4-5 mark question and you're not too sure of what to put down. This happened to me in my HY on a question about buffers. The question was more convoluted with a little story about acid rain and a swimming pool and what the caretaker should do to the pool and in the exam I just stared at it thinking 'BUT WHAT DO YOU WANT FROM ME?'. I then resorted to regurgitating any information that I think might be in the criteria based on what I've seen with previous buffer questions (ie. reference to Le Chatelier, how a buffer is made, an equation etc). I wrote it out in dotpoint form, hoping some of it would get me some marks and I ended up getting full marks for that question. So when in doubt, just say what you know, and it might get you some marks!
Hope this helps!
Post by: KT Nyunt on May 27, 2018, 10:30:15 am
Hi, I need help with this carbonated water LCP question ASAP!!
My question is,
how do you find the trend? and how do you properly answer the question so it is clear and concise.
The questions are:
a) the pH of the carbonated water would also vary with the pressure of carbon dioxide gas. Indicate this trend by sketching a graph for pH on the grid above.
b)Using equilibrium principles, account for both relationships shown on the graph.
[p.s. I had to crop the photo, bc it was too large in KB]
thank you!!!!
Hi :)
For a) I'm not too sure on how you get the values for the pH graph. I initially thought you could pick a value for pCO2, and then find it's corresponding concentration of carbonate ions and then use that concentration and plug it into the pH formula (-log[H+]). But the values I got were weird so hopefully someone else might have a better idea.
Although for b) essentially this is your average Equilibrium question with Le Chatelier
criteria for this question would most likely be something like:
- includes equation (I included a picture of this equation if you need it) - remember to include states!
- Explains how increase in pressure causes both equilibriums to shift to the right (as per Le Chatelier's principle - references moles of gases and increasing concentration of reactants)
- Explains how an increase in pressure leads to increase in carbonate ion concentration, which lowers the pH as this is an acid.
If you want, you could post your response and I'd be happy to give feedback :)
Post by: cocopops201 on May 27, 2018, 10:49:14 am
If you want, you could post your response and I'd be happy to give feedback :)
Wow!! thanks so much :)
Post by: aadharmg on May 30, 2018, 06:56:06 pm
Post by: RuiAce on May 30, 2018, 08:11:59 pm
What is the indicator used for a H2SO4 vs NaOH titration? Some places it says Bromothymol blue as they are strong acids and bases, but other places say phenolpthalein for reasons I don't know.Titration with H2SO4 is usually a nuisance, and probably why it never comes up in HSC chemistry. The fact that H2SO4 is diprotic causes a lot of problems.
H2SO4 works like this. It has 2 protons it can donate. The first proton, is that of a strong acid, so we have no problem. However, the second proton, is that of a weak acid. Because of the second proton not behaving in a same way like the first, strong one, the endpoint starts slipping into the basic range. This is why in the real world, you'd go for phenolphthalein instead.
Post by: LaraC on May 30, 2018, 08:21:01 pm
Can anyone please help me out with reasons why the pH of our lake (local catchment area) is slightly higher than our home tap water? There isn't much agriculture near the lake to cause increases in nitrates/nitrites etc, so I'm a little unsure as to the reason for this measure in my first-hand experiment?
Also, can anyone give reasons behind why the hardness of the tap water would be significantly higher than the hardness of the lake water?
Thanks in advance!!
Post by: aadharmg on May 30, 2018, 08:28:52 pm
Titration with H2SO4 is usually a nuisance, and probably why it never comes up in HSC chemistry. The fact that H2SO4 is diprotic causes a lot of problems.That makes sense. Say I was to use Bromothymol, which theoretically corresponds to such a titration (strong acid - strong base), would it have a major difference on my results or would my results (Bromothymol/Phenolphthalein) be the same, i.e. the amounts of whatever substance required to neutralise the other?
H2SO4 works like this. It has 2 protons it can donate. The first proton, is that of a strong acid, so we have no problem. However, the second proton, is that of a weak acid. Because of the second proton not behaving in a same way like the first, strong one, the endpoint starts slipping into the basic range. This is why in the real world, you'd go for phenolphthalein instead.
Post by: lalall on May 30, 2018, 11:14:10 pm
Can someone pls help?
In a titration prac, does rinsing the glasswares correctly improve the validity or accuracy? What are the other factors affecting validity and? And is reliability just affected by the repetition and consistancy of the result?
Thanks in advance!
Post by: varun.amin on June 01, 2018, 07:13:34 pm
Hello!
Can anyone please help me out with reasons why the pH of our lake (local catchment area) is slightly higher than our home tap water? There isn't much agriculture near the lake to cause increases in nitrates/nitrites etc, so I'm a little unsure as to the reason for this measure in my first-hand experiment?
Also, can anyone give reasons behind why the hardness of the tap water would be significantly higher than the hardness of the lake water?
Thanks in advance!!
Hi LaraC!
I believe the increased pH could be due to levels of CO2 absorption. Basically photosynthesis uses up dissolved carbon dioxide, which acts like carbonic acid (H2CO3) in water. CO2 removal, in effect, reduces the acidity of the water and so pH increases. Perhaps if there is a large quantity of aquatic plants this could lead to a slightly higher than average pH. Not too sure on the second question :(
Post by: RuiAce on June 01, 2018, 07:19:39 pm
That makes sense. Say I was to use Bromothymol, which theoretically corresponds to such a titration (strong acid - strong base), would it have a major difference on my results or would my results (Bromothymol/Phenolphthalein) be the same, i.e. the amounts of whatever substance required to neutralise the other?Someone can correct me but I feel like it would, which is concerning.
Which sources are telling you bromothymol blue and which are telling you phenolphthalein?
Post by: varun.amin on June 01, 2018, 07:26:29 pm
Hi!
Can someone pls help?
In a titration prac, does rinsing the glasswares correctly improve the validity or accuracy? What are the other factors affecting validity and? And is reliability just affected by the repetition and consistancy of the result?
Thanks in advance!
Hi Lalall!,
1. I'm inclined to say that correct rinsing of glassware improves accuracy, as this would help you get a result closer to the actual value. But at the same time, validity is also a measure of how well your variables are controlled, the variable being the pH of the solutions. I think it impacts both.
2. Some of the other things impacting validity are :
-Volume of unknown solution
-Type of indicator used
-Concentration of standard solution
-Equipment used
-Use of a control, that is, the aliquot with indicator untitrated.
3. Reliability is determined through repetition of titration and then the evaluation of the concordance of results across the three titres. By repeating it multiple times, outliers can be to identified and excluded.
Post by: lalall on June 01, 2018, 08:05:01 pm
Hi Lalall!,Makes perfect sense! Thank you
1. I'm inclined to say that correct rinsing of glassware improves accuracy, as this would help you get a result closer to the actual value. But at the same time, validity is also a measure of how well your variables are controlled, the variable being the pH of the solutions. I think it impacts both.
2. Some of the other things impacting validity are :
-Volume of unknown solution
-Type of indicator used
-Concentration of standard solution
-Equipment used
-Use of a control, that is, the aliquot with indicator untitrated.
3. Reliability is determined through repetition of titration and then the evaluation of the concordance of results across the three titres. By repeating it multiple times, outliers can be to identified and excluded.
Post by: lalall on June 01, 2018, 08:11:37 pm
Someone can correct me but I feel like it would, which is concerning.Most of the sources online use phenolphthalein, however when we did it in class our teacher said that we should use bromothymol blue because the equivalance point is around 7 :P. What you said earlier makes sense as well.
Which sources are telling you bromothymol blue and which are telling you phenolphthalein?
Post by: RuiAce on June 01, 2018, 09:44:08 pm
Most of the sources online use phenolphthalein, however when we did it in class our teacher said that we should use bromothymol blue because the equivalance point is around 7 :P. What you said earlier makes sense as well.It's really hard to say, because I'm not sure if the equivalence point actually is around 7 at all for this one. I wouldn't be surprised if it were, but there's definitely evidence to deter that thought
Post by: preliminary17hsc18 on June 03, 2018, 06:48:44 pm
- When trying to identify an anion or cation if there's only ONE present in the solution, do we just conduct each test with a new sample or do we still have to filter everything?
- When there are multiple anions or cations present, do we have to filter at EVERY step?
Thanks in advance :)
Post by: cocopops201 on June 03, 2018, 10:00:02 pm
How do I explain the conversion of chemical energy to electrical energy in a dry cell?
Do I just include the anode, cathode and overall equation on how certain chemicals react with each other generating a 1.5 Voltage of the battery?
Thank you to anyone who responds. :)
Post by: KT Nyunt on June 03, 2018, 11:29:58 pm
Couple questions on identifying ions...Hello :)
- When trying to identify an anion or cation if there's only ONE present in the solution, do we just conduct each test with a new sample or do we still have to filter everything?
- When there are multiple anions or cations present, do we have to filter at EVERY step?
Thanks in advance :)
1) Filtering is more for separating mixed solutions and for gravimetric analysis. So for identifying which ion is present in a solution where only one ion is present, this wouldn't call for any filtering. Although, I believe you would have to use a new sample for each test, especially if a precipitate is formed in a test. Technically, if no precipitate is formed in a test, you can add another solution into that same test and see if a precipitate forms (but if you're testing for cations, your anions would always have to be soluble - e.g. nitrates - and vice versa for anions). But to keep things simple and clear on what solutions are placed in a test, use a new sample for each test.
2) Yes, now we filter. Maybe not at every step (e.g. to test the presence of carbonate ions in a mixture: you would add acid like HCl. If gas bubbles form then you can confirm carbonate ions were present in the solution and this also removes the carbonate ions from the solution, thus this doesn't call for filtering) but you would definitely filter whenever you've formed a precipitate.
Hi!
How do I explain the conversion of chemical energy to electrical energy in a dry cell?
Do I just include the anode, cathode and overall equation on how certain chemicals react with each other generating a 1.5 Voltage of the battery?
Thank you to anyone who responds. :)
Hi! I think just including the anode and cathode half questions and the 1.5V is fine (I don't think you even need the overall equation). This should be sufficient in explaining the chemistry behind the dry cell. The transfer of electrons in the redox reaction between the anode and cathode produces 1.5 V.
Hope this helps! ;D
Post by: clovvy on June 05, 2018, 05:17:01 pm
Post by: varun.amin on June 06, 2018, 11:53:48 am
Hi!
How do I explain the conversion of chemical energy to electrical energy in a dry cell?
Do I just include the anode, cathode and overall equation on how certain chemicals react with each other generating a 1.5 Voltage of the battery?
Thank you to anyone who responds. :)
Hi cocopops201,
This is just a rough guide on explaining the conversion process:
I'd first give a basic outline of what the cell consists of (the anode, the cathode, the electrolyte). Then I'd go on to explain each step with a relevant chemical equation. Start at the anode and give the anodic half equation, explaining how the zinc atoms of the anode donate their electrons to form Zn2+ ions. Then as the dry cell is connected to a device/circuit, the excess electrons from the anode flow through the circuit, travelling to the carbon rod. This movement of electrons is what forms the electrical current which powers the device. After progressing through the entire circuit, the electrons enter the carbon rod cathode. This is where they combine with manganese dioxide and water. This reacts to produce manganese (III) hydroxide. Then I'd give the cathode half equation.
Hope this helps :)
Post by: varun.amin on June 06, 2018, 12:05:22 pm
If anyone can explain how the titration curve really works I will appreciate greatly.. I have been looking through textbooks and notes but I still don't get how the graphs work and I have never been able to do HSC and regular questions on them T_T.. I like equivalence points and stuff I find very confusing
Hi Clovvy,
Basically, a titration curve is the plot of the pH of the solution you're testing versus the volume of the titrant added as the titration progresses. When you carry out a simple acid-base titration, you use an indicator to tell you when you have the acid and alkali mixed in exactly the right proportions to "neutralise" each other.
Simply put, the equivalence point, is the exact point in a titration when moles of one titrant equal the moles of the substance being titrated. It is theoretical point.
The end point, is the point at which an indicator changes color. It’s actual point not theoretical which you can determine it in lab.
I found this link helpful in understanding the concept, hopefully it helps :)
https://www.khanacademy.org/test-prep/mcat/chemical-processes/titrations-and-solubility-equilibria/a/acid-base-titration-curves
Post by: cocopops201 on June 06, 2018, 06:34:04 pm
I was just wondering, what have been the affects of the haber process' chemistry on society and the environment? It's for a 6 marker question that says to include 'TWO examples you have studies, analyse the impact of applications of chem on society & environment'. Where I'll be doing radioisotopes for the other example.
THANKS :)
Post by: KT Nyunt on June 07, 2018, 11:29:39 pm
Hello :D
I was just wondering, what have been the affects of the haber process' chemistry on society and the environment? It's for a 6 marker question that says to include 'TWO examples you have studies, analyse the impact of applications of chem on society & environment'. Where I'll be doing radioisotopes for the other example.
THANKS :)
Hi,
Here are some things I'd include about the Haber process:
Applications of the Haber Process include: cleaning products, producing nylon and dyes etc. But the most impactful contributions would have to be the production of fertilisers and explosives.
The Haber Process [ N2(g) + 3H2(g) <--> 2NH3(g) ] {as they ask for the chemistry, definitely make sure you include the equations} allowed for ammonia to be industrially produced. This process was developed by Fritz Haber at the time of the 1st world war, where countries involved were on the brink of starvation and his country, Germany, was losing the war. Ammonia is required for plant growth. By developing the Haber process, ammonia could be industrially produced and used in fertilisers, immensely increasing crop growth and diminishing world hunger. Today, the use of fertiliser is still very prevalent in increasing crop growth. Furthermore, the Haber process could also be used to create explosives. (side note: Fritzy was a bit of a mad fella. Look it up - he practically caused the deaths of masses of Jews in WWII holocaust - Hitler who?) The production of explosives for Germany led to the prolonging of WWI.
Thus, the Haber Process greatly impacted the environment and society as it increased crop growth, diminishing world hunger, however it's use in making explosives also prolonged the 1st world war.
There's definitely more you could say about the Haber process and the history behind it but this should help you get started :)
Hope this helps ;D
Post by: cocopops201 on June 09, 2018, 11:53:07 am
There's definitely more you could say about the Haber process and the history behind it but this should help you get started :)
Hi, thanks so much for helping!
I was also wondering how I could incorporate the Haber process' impact on todays overpopulation due to its contribution of fertilizers for food production?
Thanks ;D ;D
Post by: KT Nyunt on June 09, 2018, 01:08:03 pm
Hi, thanks so much for helping!
I was also wondering how I could incorporate the Haber process' impact on todays overpopulation due to its contribution of fertilizers for food production?
Thanks ;D ;D
Yeah I guess you could argue that! As it is used to produce ammonia for fertilisers, it increased crop growth, solving world hunger, thus feeding the world, hence increasing populations ;D
Post by: fallenorigins on June 09, 2018, 02:08:13 pm
I just had a couple of questions regarding testing for ions
1. When there are more than one ion in the sample, should you always follow the flowchart process or can you test each individually (or is that only for one ion in the sample)?
2. Do you only filter the solution when there is a precipitate formed, or even when there is no prepcipitate if there are more than one ion in the sample?
Thanks! :)
Post by: RuiAce on June 09, 2018, 02:10:39 pm
Hi,1. A flow chart is designed so that it's very explicit what to do first, if you need to do multiple tests at once. The flow chart itself should be able to guide you which to do first, without impacting on everything else.
I just had a couple of questions regarding testing for ions
1. When there are more than one ion in the sample, should you always follow the flowchart process or can you test each individually (or is that only for one ion in the sample)?
2. Do you only filter the solution when there is a precipitate formed, or even when there is no prepcipitate if there are more than one ion in the sample?
Thanks! :)
2. Only when there's precipitation, because if no precipitate (i.e. no solid) forms, there's nothing to filter out. Filtration exists to split a solid-liquid mixture.
Post by: kojijo on June 12, 2018, 12:08:44 pm
Why is it that for precipitation tests the minimum ion concentration should be 0.1 molar? Is it just standard solution?
Thank you!
Post by: Mate2425 on June 12, 2018, 10:36:24 pm
Thank you. :)
Post by: KT Nyunt on June 17, 2018, 03:59:29 pm
justify the appropriateness of a particular investigation plan
To justify your method you want to think about
- Why use these reagents in this order? The answer would always be: because [insert element here] is the most reactive and would give a positive result for all tests, confusing results, thus this element is tested for first.... (and so on - going to the next element reactive, finishing with the least reactive element)
- Why use certain equipment? For precipitation reactions, the main thing would be: using test tubes infant of a black background - allowing colour changes to be clearly identified.
Evaluate the accuracy and reliability that this could be improved investigation ... relation to the Prac test and AAS.
To evaluate the accuracy of your investigation: (for this you would have to think about how sensitive/precise are you measuring equipment at ever step)
- consider having precipitates in front of a black background to ensure the correct colour is viewed
- consider using test tubes allow the sample to be larger and therefore the result can more effectively be determined
- consider if the first tests match up with the expected results from the confirmatory tests it is likely to be accurate
- consider if equal amounts of reagent were used in each test to determine whether a precipitate formed
To evaluate the reliability of your investigation:
- consider if experiment was repeated
- consider if results were consistent among tests
Accuracy and reliability of AAS:
- accuracy: measures amount of specific metals present in ppm / ppb (hence, it is very sensitive way to measure metals in a substance quantitatively)
- reliability: it is a fast a simple procedure, thus it can be repeated multiple times, an average can be taken - thus it is a procedure that can produce reliable results
Hope this was helpful! ;D (sorry it was a bit late)
Post by: cocopops201 on June 18, 2018, 09:32:10 pm
How can I answer this question:
'Justify the procedure you used to prepare an ester in a school laboratory. Include relevant chemical equations in your answer.'
Thank you :)
Post by: MisterNeo on June 18, 2018, 10:39:44 pm
Hi,
How can I answer this question:
'Justify the procedure you used to prepare an ester in a school laboratory. Include relevant chemical equations in your answer.'
Thank you :)
Hi
Esterification is a slow equilibrium reaction between a carboxylic acid and an alkanol. (Include an equation here to show the reversible properties of the reaction) The conditions to produce esters requires high temperatures and a sulfuric acid catalyst to force the equilibrium to the right. However, the alkanols used in the esterification process are volatile and flammable, which means that we cannot use a Bunsen burner to directly heat the chemicals. Hence a hot plate or a water bath should be used to reduce risk. A reflux stand is also used in the experiment because of volatile nature of the chemicals which ensures the chemicals are not lost and allows the equilibrium to take place under higher temperatures to speed up the process.
Hope this helps :)
Post by: jaredmayo01 on June 23, 2018, 03:47:38 pm
Post by: mxrylyn on June 27, 2018, 06:39:42 pm
I'm not sure how to identify the concentration of hydrogen ions and hydroxide ions in pure water at 25 degrees Celsius
And part 2 of the question is to "Define the ionic product"
Post by: cocopops201 on June 27, 2018, 07:22:35 pm
Post by: mxrylyn on June 27, 2018, 08:26:37 pm
Finding forensic notes are soo hard, does anyone know where to find them or have any?? would appreciate so much :-[Heyy, I don't have any notes for Forensic Chem, but I did a quick google search for some.
http://www.acehsconline.net/topic-forensic-chemistry/
https://www.scribd.com/doc/7727247/Forensic-Chemistry
https://easychem.com.au/forensic-chemistry/ (click on the dot points and topic areas you want to learn) - a very good source of HSC Chem information
https://sites.google.com/site/chem4hsc/forensic-chemistr
Post by: cocopops201 on June 27, 2018, 08:38:51 pm
Heyy, I don't have any notes for Forensic Chem, but I did a quick google search for some.THANK YOU :)
Post by: clovvy on July 01, 2018, 05:45:04 pm
- The tablet was crushed and then placed in a beaker
- A pipette was used to add 25.0mL of 0.600mol/L HCl to the crushed tablet
- Once the reaction between HCL and calcium carbonate had stopped, phenolphthalein indicator was added to the reaction mixture
- A teflon-coated burette was then used to add 0.100 mol/L sodium hydroxide to the beaker to neutralise the excess HCl
- The phenolphthalein changed from colourless to pink after 14.2 mL of the sodium hydroxide solution has been added
Calculate the mass of calcium carbonate in the original antacid tablet.
Post by: Kate Science on July 02, 2018, 03:47:47 am
An antacid tablet is known to contain calcium carbonate. To determine the mass of calcium carbonate in the tablet, the following procedure was used.
- The tablet was crushed and then placed in a beaker
- A pipette was used to add 25.0mL of 0.600mol/L HCl to the crushed tablet
- Once the reaction between HCL and calcium carbonate had stopped, phenolphthalein indicator was added to the reaction mixture
- A teflon-coated burette was then used to add 0.100 mol/L sodium hydroxide to the beaker to neutralise the excess HCl
- The phenolphthalein changed from colourless to pink after 14.2 mL of the sodium hydroxide solution has been added
Calculate the mass of calcium carbonate in the original antacid tablet.
I hope this is a useful model answer to use in future. Just remember that it's always best to identify the exact step in a calculation where you get stumped because then you get help on that bit in particular. These questions are the hardest types of calculation you will encounter in the HSC because it's a combination of gravimetric analysis and titration so there are lots of fiddly steps where it can understandably be easy to make errors. Let me know if there's a section in there that doesn't make sense still or if you need more clarification! :D
CaCO3 + 2HCl CaCl2 + CO2 + H2O
mol HCl initial = 0.025 L * 0.600 mol L-1
= 0.015 mol
HCl + NaOH H2O + NaCl
mol HCl titrated = 0.0142 L * 0.100 mol L-1
= 0.00142 L
mol HCl reacted with CaCO3 = initial – titrated
= 0.015 – 0.00142
= 0.01358 mol
therefore, mol CaCO3 in tablet = 0.5 * mol HCl reacted (molar ratio)
= 0.5 * 0.01358
= 0.00679 mol
mass CaCO3 = mol * molar mass
= 0.00679 mol * (40.09 + 12.01 + 3 * 16.00) mol g-1
= 0.00679 * 88.10
= 0.598199 g
= 0.598 g (3 sigfig)
Post by: lexi24216 on July 02, 2018, 02:11:37 pm
Thanks :)
Post by: justwannawish on July 02, 2018, 11:38:47 pm
For the first dotpoint of Chemical monitoring the one about a chemist in a named industry, do we need the name of a real-life chemist or is it just a hypothetical role? I had my notes around what an analytical chemist at a consulting company and a production chemist at an ammonia producing factory would do but can't find any real life people to use! Any ideas? Thanks :)
Post by: KT Nyunt on July 03, 2018, 08:32:10 pm
Hello, everyone!
For the first dotpoint of Chemical monitoring the one about a chemist in a named industry, do we need the name of a real-life chemist or is it just a hypothetical role? I had my notes around what an analytical chemist at a consulting company and a production chemist at an ammonia producing factory would do but can't find any real life people to use! Any ideas? Thanks :)
Unfortunately I don't know a named chemist for those roles but yes, you'd need a names chemist. A classic example almost everyone uses is Burhan Gemiknalki and there's info about him on pretty much any HSC site. But I have heard that you can make up a chemist because the main idea is that you name a profession/branch within the world of chemistry and you describe the role accurately - don't take my word for it though, I suggest waiting for a moderator to confirm or just use a real chemist just to be safe :)
Post by: Kate Science on July 06, 2018, 01:13:28 pm
Hi I'm not sure how to work out the heat produced when 1.0L of ethanol undergoes complete combustion, given the molar heat of combustion as 1370KJ/mol and its density 0.80g/mL
Thanks :)
Hiya,
So here are the steps:
1. 1.0L of ethanol = 0.8 g/mL * 1000mL = 800g ethanol burned (mass)
2. mol ethanol burned = (mass)/(molar mass ethanol) = X
3. heat produced = (molar heat)*(mol) = 1370KJ/mol * X = answer in kilojoules
You should be able to fill in the numbers okay from there.
If that's not clear, just chuck a reply and I can help you fill it in further :)
Cheers
Post by: Mate2425 on July 07, 2018, 02:10:35 pm
Thank you.
Post by: mxrylyn on July 09, 2018, 12:25:54 pm
Im doing some practice papers for the option "Shipwrecks and Corrosion" and im having a lot of trouble doing questions where I have to relate or compare things that I didn't lern in tademn at school such as
"Identify the ions normally dissolved in the ocean and relate these findings to the corrosion of metals.
"Explain why the concentrations af selected gasses chnages with increasing depth in the ocean and relate these findings to the corrosion of metals at different depths"
"Identify the effects of changes in temperature with depth on the rate of corrosion of metals" and
"Identify the chemical reactions involved in corrosion of metals at great depth and explain how these reactions differ from corrosion in shallow water."
Does anyone have an idea as the correct answer to these questions as well as tips on how to answer questions like this :)
Thank you
Post by: clovvy on July 09, 2018, 07:36:30 pm
Hey, quick question, does any one know of any good Websites/ resources to understand titration calculations, i am finding it hard to understand the concept/ formulative way of approaching these sorts of question.
Thank you.
Titration is just one of those topics where you just need to be strong with your chem skills(interpreting data and info given) and basics on moles calculations like limiting reagents, stoic ratios from yr 11.... Titration is basically calculating amounts of acid or bases needed to neutralise a solution (like 100mL of 0.1 mols of HCL, how much 0.05 mol of NaOH needed to neutralise it).... The best way to study for these type of stuff is just practice questions.... If you need help with questions just ask here, either me or someone stronger than me may be able to answer them (or at least try to guide you through them)....
Post by: Mate2425 on July 11, 2018, 12:05:40 am
Titration is just one of those topics where you just need to be strong with your chem skills(interpreting data and info given) and basics on moles calculations like limiting reagents, stoic ratios from yr 11.... Titration is basically calculating amounts of acid or bases needed to neutralise a solution (like 100mL of 0.1 mols of HCL, how much 0.05 mol of NaOH needed to neutralise it).... The best way to study for these type of stuff is just practice questions.... If you need help with questions just ask here, either me or someone stronger than me may be able to answer them (or at least try to guide you through them)....Thank you, much appreciated!
Post by: varun.amin on July 11, 2018, 06:17:45 pm
Hey, quick question, does any one know of any good Websites/ resources to understand titration calculations, i am finding it hard to understand the concept/ formulative way of approaching these sorts of question.
Thank you.
Hi Mate2425,
Usually I like to structure titration calculations using a set of steps to find the answer. Just be careful using this method based on what the question is asking.
1. Write out a balanced equation from the given information (this helps later with molar ratios)
2. Write out the following, usually one of the concentrations will be missing and is usually what you need to find:
Volume(base)= Concentration(base)= Volume(acid)= Concentration(acid)=
3. Calculate the moles of the known acid/base (this will be the one where you are given both the concentration and the volume using the formula n=cv)
4. See if you need to adjust the moles of the unknown concentration of acid/base based on the molar ratio equation above (say for example you might have twice as many moles of the unknown acid/base, at this stage you would have to double the known moles of the acid/base, to get the moles of the unknown acid/base)
5. Then finally you need to calculate the concentration of the unknown substance using the formula c=n/v where we know both n as the amount of moles and v as the volume
Hope this helps!
Post by: cnimm2000 on July 14, 2018, 09:28:28 pm
Why do we need to do sequencing when testing cations in mixtures
it would be great if you could help!
Post by: RuiAce on July 14, 2018, 09:42:04 pm
Hey guys,Don't remember the flow chart off the top of my head anymore but the main culprit is the solubility rules.
Why do we need to do sequencing when testing cations in mixtures
it would be great if you could help!
Thanks to solubility rules, cation tests actually exist. But unfortunately, some anions will precipitate with more than one cation. (A big one is the carbonate ion, which precipitates with almost everything; only exclusions being ammonium and group I metals as far as I know of.) We want to design our cation tests so that a particular sequence of steps can only lead to one possible cation being what's present, as opposed to multiple.
Post by: earlgrey1803 on July 18, 2018, 07:09:57 pm
"A student wanted to determine the percentage of calcium carbonate in sticks of coloured chalk.
1. A sample of coloured chalk weighing 0.235g was reacted with 40.00mL of 1.00 mol L-1 HCl (excess).
2. After the reaction was complete the solution was transferred to a 250mL volumetric flask and filled up to the mark with distilled water.
3. 25.00mL portions of this solution were then titrated against 0.150 mol L-1 sodium hydroxide using methyl orange as the indicator.
Calculate the percentage purity of the chalk."
The average amount of NaOH used is 23.96mL
Any help would be greatly appreciated!!
Post by: emily_p on July 18, 2018, 09:53:37 pm
Hey guys - was wondering if I could grab some help on a titration question I've been struggling with - whenever I do it I always seem to end up with 1.72 grams of calcium carbonate which doesn't quite work...
"A student wanted to determine the percentage of calcium carbonate in sticks of coloured chalk.
1. A sample of coloured chalk weighing 0.235g was reacted with 40.00mL of 1.00 mol L-1 HCl (excess).
2. After the reaction was complete the solution was transferred to a 250mL volumetric flask and filled up to the mark with distilled water.
3. 25.00mL portions of this solution were then titrated against 0.150 mol L-1 sodium hydroxide using methyl orange as the indicator.
Calculate the percentage purity of the chalk."
The average amount of NaOH used is 23.96mL
Any help would be greatly appreciated!!
Hello!! I’ve attached an image of calculations I’ve gone through. Since the question is a back titration these are some of the steps I’ve gone through:
1. Calculate moles of NaOH reacted
2. Calculate moles of HCl reacted
3. Calculate moles of HCl in beaker (25ml —> 250ml)
4. Calculate moles of HCl used in reaction with chalk from initial - excess
5. Calculate moles of CaCO3 used, calculate mass and then find percentage
The percentage I got was 86.5% to 3 sf, if you have the answer could you check with this and confirm?
Hope this helps! Let me know if I’ve stuffed anything up or if something doesn’t make sense :)
(http://i64.tinypic.com/33wnkom.jpg)
Post by: key to success on July 18, 2018, 11:47:39 pm
Just in your working can explain the step where the work out the number of moles of the HCL in excess- what formula did you use and what does it even mean by 'in excess'?
Thanks!
Post by: emily_p on July 19, 2018, 09:18:41 am
Hey, sorry for intervening, the question looked good and so did the solution so thought i'll check it out!
Just in your working can explain the step where the work out the number of moles of the HCL in excess- what formula did you use and what does it even mean by 'in excess'?
Thanks!
Definitely! By HCl(excess) I was referring to the total remaining HCl after the reaction - ie all the moles of HCl in the beaker, probably should have made this clearer whoops! Since only a 25 ml sample was taken from a 250 ml beaker, I multiplied the moles in the sample by 10 to get the moles in the beaker.
Alternatively, you could use C=n/V twice:
C(titrated HCl) = 0.003594/0.025
= 0.14376 mol/litre
C(beaker) = n/0.25 ==> sub in C of titrated HCl (conc of sample will be same as conc of whole thing)
0.14376= n/0.25
n(beaker) = 0.14376 x 0.25
= 0.03594
So the answer would be the same as multiplying by 10 but in case you're a bit unsure of it C=n/V works too!!
Post by: earlgrey1803 on July 19, 2018, 02:07:55 pm
Definitely! By HCl(excess) I was referring to the total remaining HCl after the reaction - ie all the moles of HCl in the beaker, probably should have made this clearer whoops! Since only a 25 ml sample was taken from a 250 ml beaker, I multiplied the moles in the sample by 10 to get the moles in the beaker.
Alternatively, you could use C=n/V twice:
C(titrated HCl) = 0.003594/0.025
= 0.14376 mol/litre
C(beaker) = n/0.25 ==> sub in C of titrated HCl (conc of sample will be same as conc of whole thing)
0.14376= n/0.25
n(beaker) = 0.14376 x 0.25
= 0.03594
So the answer would be the same as multiplying by 10 but in case you're a bit unsure of it C=n/V works too!!
Thanks for all your help! The answer looks right but I don't have the answers so I can't confirm...
All my working matched up with yours except for this step which I completely forgot which is why I was getting a wildly incorrect answer!
Thanks :D
Post by: sarra.mourad2000 on July 20, 2018, 02:05:13 pm
"Explain why alkanes and corresponding alkenes have similar properties, but very different chemical properties" comes up all the time but I can never get around to figuring out how to answer it... HELPPP :'( :'(
Post by: Ace2018 on July 20, 2018, 05:54:44 pm
This question
"Explain why alkanes and corresponding alkenes have similar properties, but very different chemical properties" comes up all the time but I can never get around to figuring out how to answer it... HELPPP :'( :'(
I presume you meant "Explain why alkanes and corresponding alkenes have similar PHYSICAL properties, but very different chemical properties" :
When explaining physical properties, you would talk about the intermolecular bonds i.e. the bonds BETWEEN molecules. So, we know that both alkanes and alkenes can only form weak dispersion forces with other molecules, therefore they both are insoluble in water, have a low boiling point and low melting point.
Whereas with explaining chemical properties, you would talk about the intramolecular bonds, i.e. the bonds WITHIN molecules. So, we know that alkanes have single bonds whilst alkenes have double bonds. This means that the alkenes' double is highly reactive and can readily undergo addition reaction.
I hope that help.
Post by: iimooncabbageii on July 20, 2018, 08:14:16 pm
Having some trouble with this question
Spoiler
(https://snag.gy/u7vlJU.jpg)
Post by: KT Nyunt on July 21, 2018, 12:23:18 pm
Hi~
Having some trouble with this questionApparently the answer is B. I do understand why Mn would have a lower oxidation state, but wouldn't it have lost electrons instead of gaining them?Spoiler(https://snag.gy/u7vlJU.jpg)
Hi there! Welcome to ATAR notes. Yes the answer is B, but Mn does gain electrons.
In (MnO4-)... Mn has an oxidation state of +7
(this is because the O always has an oxidation state of -2, we times this by 4 so the O4 has an oxidation state of -8. The molecule as a whole has a valency of -1 so... +7 -8 = -1)
So Mn7+ goes to Mn2+
This means the oxidation state is lowered (7 to 2)
And Mn has gained electrons because it needs to gain negative charges for it's positive valency to decrease.
It might help to think of it this way:
Remember: OIL RIG
Oxidation - It Loses (electrons)
Reduction - It Gains (electrons)
Lowering in oxidation state means Mn has undergone reduction, so it has gained electrons
Hope this helps :)
Post by: sarra.mourad2000 on July 21, 2018, 07:42:49 pm
I presume you meant "Explain why alkanes and corresponding alkenes have similar PHYSICAL properties, but very different chemical properties" :
When explaining physical properties, you would talk about the intermolecular bonds i.e. the bonds BETWEEN molecules. So, we know that both alkanes and alkenes can only form weak dispersion forces with other molecules, therefore they both are insoluble in water, have a low boiling point and low melting point.
Whereas with explaining chemical properties, you would talk about the intramolecular bonds, i.e. the bonds WITHIN molecules. So, we know that alkanes have single bonds whilst alkenes have double bonds. This means that the alkenes' double is highly reactive and can readily undergo addition reaction.
I hope that help.
Thanks heaps, I find that question so odd for some reason, but that really helped :)
Post by: deprezj on July 26, 2018, 05:51:50 pm
When using the equation for fermentation of glucose is it enough to just say yeast for the catalyst or do you need to specifically say zymase, and in responses when they ask for the conditions for this process do you label yeast as a catalyst or a condition?
Thank you lifesaver.
Jen
Post by: deprezj on July 26, 2018, 05:53:01 pm
When using the equation for fermentation of glucose is it enough to just say yeast for the catalyst or do you need to specifically say zymase, and in responses when they ask for the conditions for this process do you label yeast as a catalyst or a condition (like temperature)?
Thank you lifesaver.
Jen
Post by: amelia20181 on July 28, 2018, 09:36:26 pm
Post by: clovvy on July 29, 2018, 09:55:18 am
Hi Jake,Write the equation out and describe the conditions after that.... To be fancy yes feel free to add yeast on the arrow... However yeast is NOT a catalyst... A catalyst does not react directly towards the reactants to produce the required products, but merely create the required environment so to speak....
When using the equation for fermentation of glucose is it enough to just say yeast for the catalyst or do you need to specifically say zymase, and in responses when they ask for the conditions for this process do you label yeast as a catalyst or a condition (like temperature)?
Thank you lifesaver.
Jen
Post by: KT Nyunt on July 29, 2018, 03:21:07 pm
is it possible to cram the whole syllabus in 2 weeks??
I did it once in two days - Although I really don't recommend doing this! This is a wayyy too stressful process that you shouldn't go through. Space yourself out as much as you can, try teaching the content to someone - this is a really fast way to make content stick. Sometimes cramming is even the way to go. It's not going to help you learn and remember in the long run, but it does mean everything will be fresh in your head by the time the exam comes around. You got this! :)
Post by: key to success on July 31, 2018, 07:58:08 pm
Any help will be appreciated! :)
Thanks.
Post by: KT Nyunt on July 31, 2018, 08:08:33 pm
Please please help! im incredibly stuck on this attached question.
Any help will be appreciated! :)
Thanks.
Hey I think you may have forgotten to attach the question :)
Post by: Katie-E on August 01, 2018, 09:38:10 pm
6. 14 g of coal undergoes complete combustion in 58 g of oxygen gas.
What volume of gas (measured at 25 C and 100 kPa) is present when the reaction is
complete?
(A) 16 L
(B) 29 L
(C) 45 L
(D) 74 L
Post by: clovvy on August 01, 2018, 10:18:47 pm
Hey, I was wondering whether anyone could give me a hand on these two calculations and how to do them. This question and the attached one as well please. Cheers ;)
6. 14 g of coal undergoes complete combustion in 58 g of oxygen gas.
What volume of gas (measured at 25 C and 100 kPa) is present when the reaction is
complete?
(A) 16 L
(B) 29 L
(C) 45 L
(D) 74 L
I will guide you through the first question you ask, I will start off with reaction:
Post by: Katie-E on August 01, 2018, 10:29:00 pm
Post by: Dragomistress on August 03, 2018, 10:37:36 am
Post by: Fergus6748 on August 03, 2018, 10:54:37 am
Hey, I am not quite sure how to do this. How do I know?So the Zinc electrode is oxidising so is called the reductant, you can see this as the Zinc comes off the electrode and reacts with the chlorine to make the ZnCl2 solution. To answer the question, the Platinum electrode is undergoing reduction.
Post by: Bells_123 on August 03, 2018, 05:06:37 pm
Hey, I was wondering whether anyone could give me a hand on these two calculations and how to do them. This question and the attached one as well please. Cheers ;)
6. 14 g of coal undergoes complete combustion in 58 g of oxygen gas.
What volume of gas (measured at 25 C and 100 kPa) is present when the reaction is
complete?
(A) 16 L
(B) 29 L
(C) 45 L
(D) 74 L
For the second screenshot question, you have to find the mass of the petrol using the density formula (mass/volume --> make sure you convert 45L to mL!). From this mass you can find moles which you would multiply by the molar heat of combustion (5460) to find how many kilojules is released from that amount of petrol.
You should get something like 1487131 kJ that you divide by 1370 (the heat of combustion) to find the number of moles of ethanol needed to liberate that amount of energy. From this you find the mass of ethanol using the mole formula, and then use the density formula again and rearrange to find the volume (which you convert to litres). So the answer should end up being A.
Hope this helps :D
Post by: Bells_123 on August 03, 2018, 05:18:29 pm
So the Zinc electrode is oxidising so is called the reductant, you can see this as the Zinc comes off the electrode and reacts with the chlorine to make the ZnCl2 solution. To answer the question, the Platinum electrode is undergoing reduction.
The zinc is the site of oxidation, but I think the platinum is an inert electrode that doesn't take part in the chemical reaction and is just responsible for electron transfer. So the chlorine that's bubbled into the solution should be the one undergiong reduction as it's reduced to chlorine ions.
Post by: itssona on August 03, 2018, 05:59:44 pm
A chemist analysed aspirin tablets. Initially they had to determine the molarity of a NaOH solution. Three 25mL samples of a 0.1034mol/L solution of standardised HCl were titrated with the NaOH solution.
The average volume of the standardised NaOH required for neutralisation was 25.75mL
(a) Calculate the molarity of the NaOH solution
Post by: Bells_123 on August 03, 2018, 06:09:02 pm
hey help with this please
A chemist analysed aspirin tablets. Initially they had to determine the molarity of a NaOH solution. Three 25mL samples of a 0.1034mol/L solution of standardised HCl were titrated with the NaOH solution.
The average volume of the standardised NaOH required for neutralisation was 25.75mL
(a) Calculate the molarity of the NaOH solution
With titration calculations I always like to write a balanced equation first to determine the mole ratios. This neutralisation reaction should be in a 1:1 mole ratio between NaOH and HCl, so you can find the moles of HCl by using the concentration equation (moles/volume in litres) which also equals the moles of the NaOH. Then you can find the molarity/concentration of the NaOH by plugging it in the concentration equation again :)
Post by: fkkiwi on August 05, 2018, 12:00:15 pm
Which of the following is the best oxidising agent?
A
A+
D2+
D
given A+ + e- --> A = 1.8V and D2+ + 2e- --> D = -1.7V
Post by: Fergus6748 on August 05, 2018, 01:59:30 pm
Anyone have an explanation to this?Well the oxidising agent is what is being reduced and if you look at the standard potential table, the voltage values increase from negative to positive. The oxidising agent is usually the lower element on the table so I would say A+ (B). Pretty sure that's right
Which of the following is the best oxidising agent?
A
A+
D2+
D
given A+ + e- --> A = 1.8V and D2+ + 2e- --> D = -1.7V
Post by: amelia20181 on August 05, 2018, 07:12:57 pm
Post by: Bells_123 on August 06, 2018, 12:02:07 am
how do you do this question from the 2005 paper
If it's a condensation polymer it most likely means that a water molecule comes off when the monomers join. By looking at the polymer you can see that 4 monomers are joined together (2 of each type and they alternate). If you circle around the first monomer from the first oxygen to the second oxygen on either side of the ring (counting left to right), you can see it repeats again after the second monomer, so where the monomers join must be at the second oxygen (right side of ring). The answer looks like D because when the monomers join you can see it will leave one oxygen when a water molecule forms.
Hopefully that made sense and helped you :)
Post by: clovvy on August 06, 2018, 06:11:42 pm
When 1.635g of ethanol was burnt, the temp of water was increased to 47. Calculate the energy released, in kJ, when 1.50g of ethanol was burnt
a) 19.2kJ
b) 5.89kJ
c) 5.44kJ
d) 18.8kJ
Post by: Fergus6748 on August 06, 2018, 08:50:16 pm
Volume of water: 200mL, initial temp: 22Hey, so what I understand is that I would find the heat of combustion (mcat) and then divide by the mass of fuel burnt (the original 1.635g) to give the heat of combustion per gram of ethanol:
When 1.635g of ethanol was burnt, the temp of water was increased to 47. Calculate the energy released, in kJ, when 1.50g of ethanol was burnt
a) 19.2kJ
b) 5.89kJ
c) 5.44kJ
d) 18.8kJ
DeltaH = mcdeltat
= (200)(4.18)(25)
= 20900
= 20900/1.635
= 12782.87462
Then multiply by the new mass of fuel burnt (1.50g):
= 12782.87462 x 1.50
= 19174.31193 J
= 19.17431193 kJ
= 19.2 kJ (A)
Post by: Dragomistress on August 08, 2018, 12:52:35 pm
Just curious, how do I know what compound is the most volatile?
For example, which is the most volatile out of propane, propan-1-ol, propanoic acid and methyl propanoate. (It is propane)
Post by: KT Nyunt on August 08, 2018, 08:06:37 pm
Hey,
Just curious, how do I know what compound is the most volatile?
For example, which is the most volatile out of propane, propan-1-ol, propanoic acid and methyl propanoate. (It is propane)
Since a volatile substance means that it easily becomes a gas. To determine which one of these are the most volatile, I think we have to looks of the intermolecular forces they form. Since propanol, propanoic acid and methyl propanoate all can form some hydrogen bonds, whereas propane can only form weak dispersion forces, it is more likely to be found as a gas than propanol, propanoic acid and methyl propanoate. Hence, propane is the most volatile.
Post by: Dragomistress on August 09, 2018, 04:43:30 pm
How do I complete this question?
A lawn fertiliser lists the sulfate content as 38.5%(w/w). What mass of barium sulfate precipitate would be expected to form if a 1.50g sample of the fertiliser were analysed by reacting the sample with excess barium nitrate solution? (0.238g, 0.578g, 1.40g or 3.64g)? (Answer is 1.40g)
Thank you!
Post by: jazcstuart on August 09, 2018, 06:37:02 pm
Hello again!Hi, this is how I would do it (I did it on paper first, I attached a picture if that is easier to read):
How do I complete this question?
A lawn fertiliser lists the sulfate content as 38.5%(w/w). What mass of barium sulfate precipitate would be expected to form if a 1.50g sample of the fertiliser were analysed by reacting the sample with excess barium nitrate solution? (0.238g, 0.578g, 1.40g or 3.64g)? (Answer is 1.40g)
Thank you!
You know the weight of the sample, therefore you can find the mass of sulfate in the sample using the sulfate content % given:
38.5% x 1.50 = 0.5775g
Now you can work out the moles of sulfate:
n = m/MM
n(SO4) = 0.5775/(32.07+4x16.00)
=0.0060112418...mol
Using the mole ratio, n(SO4) = n(BaSO4), therefore n(BaSO4) = 0.0060112418...mol
From here you can work out the mass os BaSO4 precipitate using n = m/MM again:
0.0060112418 = m/(137.3+32.07+4x16.00)
m = 0.0060112418 x 233.37
= 1.402843499...g
= 1.40 (3 significant figures) (yay)
Hope this makes sense!
Post by: StephTol on August 09, 2018, 08:59:16 pm
I'm really confused with this question and I don't understand the solution
Thanks in advance
Post by: amelia20181 on August 11, 2018, 07:24:05 pm
If it's a condensation polymer it most likely means that a water molecule comes off when the monomers join. By looking at the polymer you can see that 4 monomers are joined together (2 of each type and they alternate). If you circle around the first monomer from the first oxygen to the second oxygen on either side of the ring (counting left to right), you can see it repeats again after the second monomer, so where the monomers join must be at the second oxygen (right side of ring). The answer looks like D because when the monomers join you can see it will leave one oxygen when a water molecule forms.
Hopefully that made sense and helped you :)
But doesn't B also leave one oxygen when a water molecule forms
Post by: dermite on August 11, 2018, 10:02:51 pm
if anyone can help me with this Question on the Equilibrium Constant, it would be much appreciated.
also, what's the general method of approaching equilibrium constant questions?
Post by: radnan11 on August 12, 2018, 12:27:40 pm
Is ammonia considered a raw material for the Solvay Process? Reading from two textbooks with one saying yes,the other saying "sometimes, depending on the situation"
Post by: KT Nyunt on August 12, 2018, 02:55:02 pm
Hi there, im struggling with how to do part (ii)
if anyone can help me with this Question on the Equilibrium Constant, it would be much appreciated.
also, what's the general method of approaching equilibrium constant questions?
Ok so the method I like to use to approach equilibrium constant questions is through a RICE table: Ratio (mole ratio) , Initial Concentration, Change in concentration and Equilibrium concentration, as shown in the photo attached. By filling out the information they give us (fyi we have to convert everything into concentrations first) we can find the change in concentration of H2(g). This change would be proportional to the change in concentration for the other reagents, in their molar ratios. From this, we get all the change in concentrations from the initial concentrations in CO and CH3OH as well, allowing us to calculate the final equilibrium concentration.
Then we use the equilibrium concentrations and plug that into the K expression.
Unfortunately, I'm a bit unsure about the answer I got. I'm a bit confused since the question says the volume was increased (ie. pressure was decreased). According to Le Chatelier's principle, this would cause the equilibrium to shift to the side where there are more gas moles. This would be the reactants side. But my calculations show that K>1, meaning the equilibrium has shift to the right.
Can someone please explain if there's something I'm not getting :'(
Post by: dermite on August 12, 2018, 06:49:27 pm
Ok so the method I like to use to approach equilibrium constant questions is through a RICE table: Ratio (mole ratio) , Initial Concentration, Change in concentration and Equilibrium concentration, as shown in the photo attached. By filling out the information they give us (fyi we have to convert everything into concentrations first) we can find the change in concentration of H2(g). This change would be proportional to the change in concentration for the other reagents, in their molar ratios. From this, we get all the change in concentrations from the initial concentrations in CO and CH3OH as well, allowing us to calculate the final equilibrium concentration.
Then we use the equilibrium concentrations and plug that into the K expression.
Unfortunately, I'm a bit unsure about the answer I got. I'm a bit confused since the question says the volume was increased (ie. pressure was decreased). According to Le Chatelier's principle, this would cause the equilibrium to shift to the side where there are more gas moles. This would be the reactants side. But my calculations show that K>1, meaning the equilibrium has shift to the right.
Can someone please explain if there's something I'm not getting :'(
you subtracted 0.32 instead of 0.36. Is there a reason for this?
Post by: KT Nyunt on August 12, 2018, 07:45:11 pm
you subtracted 0.32 instead of 0.36. Is there a reason for this?
I subtracted 0.32 because that was difference in the concentration between the initial concentration of the H2 and the final concentration I calculated. But, I did misread the question and thought 0.36 were the final amount of moles in the vessel at equilibrium. Therefore, the final equilibrium concentration I calculated was incorrect.
I'm still unsure on how to fix this though, no matter what I try I'm getting K>1.
I just retried the RICE table after I realised my mistake but got a K value of 15 (2sf). Do you have the answers on you? Maybe we can work backwards.
Post by: dermite on August 13, 2018, 03:26:20 pm
I subtracted 0.32 because that was difference in the concentration between the initial concentration of the H2 and the final concentration I calculated. But, I did misread the question and thought 0.36 were the final amount of moles in the vessel at equilibrium. Therefore, the final equilibrium concentration I calculated was incorrect.
I'm still unsure on how to fix this though, no matter what I try I'm getting K>1.
I just retried the RICE table after I realised my mistake but got a K value of 15 (2sf). Do you have the answers on you? Maybe we can work backwards.
here's the answers:
i dont understand why they added instead of subtracted the change.
Post by: KT Nyunt on August 16, 2018, 01:57:29 pm
here's the answers:
i dont understand why they added instead of subtracted the change.
Ok so they used moles instead of concentrations, which I guess is easier for this question. Then at the end they converted this into concentrations.
So the question says the number of moles of H2 'changes' by 0.36 moles. As the question also informs us that the volume is increased, we know from le chatelier's principle that this will cause the equilibrium to shift to the reactants where there are more gas moles.
Hence, the number of moles of H2 will increase by 0.36 moles. Using molar ratios, we can also deduce that CO will change by 0.18 moles, but since it is also a reactant it will increase by 0.18. Whereas, CH3OH is a product, so it will decrease by 0.18
:)I hope that makes sense...
I may still be confused as to why K is >1 but at this point I'm just accepting this lol.
Post by: amelia20181 on August 16, 2018, 06:47:39 pm
how do you memorise lots of equations
Post by: KT Nyunt on August 16, 2018, 07:01:14 pm
do you need an equation or diagram for a question that is 3 marks or more
how do you memorise lots of equations
Generally it is the 4+ mark questions where you'd consider including a diagram or equation, unless the question specifies otherwise. As a general rule, if it is a 4+ mark question and you know an equation that is related to the question, always include that equation.
As for memorising lots of equations... Either memorise it using flash cards and writing it out a couple times or just remember what reactants and products are involved and then just remember you might need to balance it.
Hope this helps! :)
Post by: amelia20181 on August 18, 2018, 04:35:49 pm
Post by: varun.amin on August 18, 2018, 06:14:07 pm
is a 6 mark question usually a band 6 type of question
Usually, it's the extended response questions that seperate Band 6 students from Band 5 students. Both know their content and can pick up marks in MCQ and Shorter response questions (1-4 marks). The level of detail you put into an extended response and how well you answer it can be the difference between a 3/6 and 6/6. Most Band 6 students I've talked to have practiced the extended response questions quite a few times and have developed a refined way to answer them. From my own experience, they tend to use the same topics in extended response questions, so if you practice them enough you'll nail them.
Post by: amelia20181 on August 21, 2018, 05:12:55 pm
what is the chemical equation for the reaction when octyl ethanoate is made by reacting 1-octanol with ethanoic acid using the catalyst, concentrated sulfuric acid in a refluxing mixture
and what property of esters allows them to be used in industry as solvents or thinners
whats the equilibrium reaction for the incomplete ionization of the weak acid acetic acid
is sulfate or hydrogen carbonate amphoteric
Post by: flacko on August 28, 2018, 04:57:08 pm
I have been confused about bond breaking and endo/exothermic reactions.
Everywhere I look it states that when bonds break energy is required and when bonds form energy is released. If for example the energy required to break a bond is lower than the energy released by making a bond the reaction is exothermic as heat/energy is released. I understand this, however, doesn't this defy the law of conservation of energy? The elements require a certain amount of energy to break their bonds and release even more energy when bonds are formed. Where is this extra energy coming from? Thanks
Post by: fun_jirachi on August 28, 2018, 07:34:43 pm
Hey
I have been confused about bond breaking and endo/exothermic reactions.
Everywhere I look it states that when bonds break energy is required and when bonds form energy is released. If for example the energy required to break a bond is lower than the energy released by making a bond the reaction is exothermic as heat/energy is released. I understand this, however, doesn't this defy the law of conservation of energy? The elements require a certain amount of energy to break their bonds and release even more energy when bonds are formed. Where is this extra energy coming from? Thanks
someone correct me if i'm wrong, im using my prelim knowledge here :D
from my understanding to break bonds you need an energy source ie. activation energy, and the energy released is the energy used to break the bonds + the energy stored in the bonds, idk :-\ :-\ :-\
Post by: 3.14159265359 on August 29, 2018, 08:13:48 pm
Post by: beeangkah on August 31, 2018, 11:20:55 am
Hey, does anyone know which equation needs to be memorised for the sulphate-reducing bacteria?
Is it reduction of sulphate in water or sulphate reacting with H+ ions?
Thanks!
Post by: KT Nyunt on August 31, 2018, 09:15:34 pm
hey for industrial, how many natural replacements should you know?
pretty much only one in a lot of depth...
You wanna know:
- An exhaustible natural resource, what it is used for and why is it running out (think of increasing demand due to increasing populations etc.)
- A synthetic material produced industrially to replace this resource
- any relevant equations for how its produced and/or a good description of the industrial process
- any further research and progress being made
Post by: 3.14159265359 on September 02, 2018, 09:11:50 pm
pretty much only one in a lot of depth...
You wanna know:
- An exhaustible natural resource, what it is used for and why is it running out (think of increasing demand due to increasing populations etc.)
- A synthetic material produced industrially to replace this resource
- any relevant equations for how its produced and/or a good description of the industrial process
- any further research and progress being made
I just saw this, thank you so much!
Post by: bhavs1 on September 02, 2018, 10:09:46 pm
Question regarding Industrial Chem:Hi perhaps you don't need this anymore but a raw material is a necessary reagent, as such ammonia is a raw material. However, it should also be noted that it is theoretically recycled so it isn't replenished as often and perhaps that's why textbooks may not be clear??
Is ammonia considered a raw material for the Solvay Process? Reading from two textbooks with one saying yes,the other saying "sometimes, depending on the situation"
Post by: Dragomistress on September 03, 2018, 07:52:59 pm
During saponification, why do we use water, oil, sodium hydroxide and ethanol? What is the point of ethanol in the process?
Post by: 3.14159265359 on September 03, 2018, 08:06:18 pm
Hey,
During saponification, why do we use water, oil, sodium hydroxide and ethanol? What is the point of ethanol in the process?
hey I might be wrong. but we don't need ethanol. we need sodium hydroxide, fat/oil (ester--> which is a combination of an alcohol and alkanoic acid), and excess sodium chloride (brine). maybe the ethanol issued to make the ester?
Post by: jazcstuart on September 03, 2018, 09:49:22 pm
OPTION TOPIC: SHIPWRECKS
Hey, does anyone know which equation needs to be memorised for the sulphate-reducing bacteria?
Is it reduction of sulphate in water or sulphate reacting with H+ ions?
Thanks!
Just going to quote this again because I would also like to know the answer, anyone know? :)
Post by: 3.14159265359 on September 03, 2018, 09:54:25 pm
Just going to quote this again because I would also like to know the answer, anyone know? :)
sorry I dont do shipwrecks as my option, otherwise I wouldve helped! :(
Post by: dermite on September 10, 2018, 06:54:50 pm
i'd like some help on the following qn: (Industrial Chem)
Progress in chemical engineering has been driven by improved outcomes for society and the environment.
Assess this statement with reference to the production methods of sodium hydroxide and surfactants.
(8 Marks)
Post by: Mate2425 on September 13, 2018, 11:20:21 am
Thank you.
Post by: jazcstuart on September 13, 2018, 05:54:36 pm
Hi guys which types of detergent; anionic, cationic or non-ionic detergents lather in hard water?
Thank you.
Hi, I'm guessing this is from Industrial chem, I don't actually do this option topic but I was curious (it kind of relates to chem monitoring and management, plus I like chemistry!).
I would imagine that anionic soaps don't lather because the anions are attracted to the Ca and Mg cations in hard water. Not sure about non-ionic detergents.
Have a look in the notes section (https://atarnotes.com/notes/?state=3719), you should be able to find a summary!
Post by: aadharmg on September 13, 2018, 07:09:02 pm
Hi guys which types of detergent; anionic, cationic or non-ionic detergents lather in hard water?Anionic detergents (other than alkyl sulfates and alkyl phosphates) DO lather in hard water. In case of soaps, the fatty acid anions combine with those cations of hard water to precipitate and form the insoluble scum. The fact that anionic detergents lather is one of the reasons they were made, as a response to the problem with soaps.
Thank you.
Post by: Dragomistress on September 17, 2018, 01:29:50 pm
How do I know if a salt is acidic or basic?
Post by: jazcstuart on September 17, 2018, 05:28:17 pm
Hey,
How do I know if a salt is acidic or basic?
One way to tell is to figure out which acid/base neutralisation reaction it comes from, for example NaCl is formed in the reaction between HCl and NaOH, NaCH3OO from NaOH and CH3OOH etc. The acidity/basicity of the salt depend o the strength of the acid and base:
- a strong acid and strong base form a neutral salt
- a weak acid and weak base of equal strength form a neutral salt
- a strong acid and weak base form a slightly acidic salt
- a weak acid and strong base form a slightly basic salt
However if you are asked to show whether a salt is acidic or basic, you have to split it into its ions and show whether it reacts with water (I then just use the above method to check). For example using NaCH3OO again:
Na+ + H2O ---> No reaction
CH3OO- + H2O <---> CH3OOH +OH-
This produces OH- ions which make the salt solution basic.
It's not too hard, you may just need to practice a few times working out whether the ions in the salt would react with water. Hope this makes sense, feel free to ask if it doesn't and I'll see if I can explain :)
Post by: Mate2425 on September 22, 2018, 04:55:05 pm
I had a look at Matrix answers on their website and am not sure where they plucked their numbers from.
Thank you!
Also Q24b HSC 2015 please.... so confused thank you https://www.boardofstudies.nsw.edu.au/hsc_exams/2015/guides/2015-hsc-mg-chemistry.pdf
Post by: jazcstuart on September 22, 2018, 06:31:10 pm
Hey guys, I was wondering how you answer HSC 2017 Chemistry Multiple Choice Q20. http://educationstandards.nsw.edu.au/wps/wcm/connect/cb65ca25-fc28-4e21-8f2d-4886bf176ddd/2017-hsc-chemistry.pdf?MOD=AJPERESI have attached my solution, hope it makes sense.
I had a look at Matrix answers on their website and am not sure where they plucked their numbers from.
Thank you!
Essentially you have to find the number of moles remaining of the substance in excess (in this case HCl) after neutralisation, then find the concentration using the volume of both solutions. You can then use this to find pH.
If you don't understand my explanations feel free to ask :)
Post by: Mate2425 on September 22, 2018, 06:49:42 pm
I have attached my solution, hope it makes sense.
Essentially you have to find the number of moles remaining of the substance in excess (in this case HCl) after neutralisation, then find the concentration using the volume of both solutions. You can then use this to find pH.
If you don't understand my explanations feel free to ask :)
Thank you so much for your help, makes much more sense!!
If possible could you please also help me with my new edit on the 2015 Question above, i would greatly appreciate it!! :) :)
Post by: jazcstuart on September 22, 2018, 07:51:27 pm
Yeah sure! (I love answering questions because it helps me clarify my understanding ;D)
Thank you so much for your help, makes much more sense!!
If possible could you please also help me with my new edit on the 2015 Question above, i would greatly appreciate it!! :) :)
So this is a buffer solution, which is a really important concept to understand. This is because acetate ions (from the sodium acetate) is the conjugate base of acetaic acid. We have to use Le Chatlier's Principle because the solution exists in equilibrium:
CH3COOH + H2O <---> CH3COO- + H3O+
The OH- ions added from the sodium hydroxide solution react with hydronium ions (H3O+), reducing their concentration. Therefore according to Le Chatlier's Principle, the equilibrium shifts to the left, minimising the change in pH (as more H3O+ ions are produced to counteract the addition of OH-)
Although this question doesn't ask for it, it is similar when an acid is added. The concentration of H3O+ is increased, shifting to the equilibrium to the right according to Le Chatlier's Principle, again minimising the change in pH.
Post by: Dragomistress on September 24, 2018, 09:31:11 pm
What is the net ionic equation of 2NaCl+H2O-->Cl2+H2+2NaOH?
Post by: jazcstuart on September 25, 2018, 11:33:41 am
Hi,
What is the net ionic equation of 2NaCl+H2O-->Cl2+H2+2NaOH?
Hi, first of all to balance the chemical equation you need a 2 infront of H2O.
Essentially when you write net ionic equations you are determining the chemical species that undergo a reaction, ie. they change oxidation state. I think you can only split substances up into their individual ions if they are in solution. H2O is a covalent molecule so it remains the same, so you should get:
2Na+ + 2Cl- + 2H2O ---> Cl2 + H2 + 2Na+ + 2OH-
Then you remove the spectator ion (Na+, doesn't change oxidation state) to get:
2Cl- + 2H2O ---> Cl2 + H2 + 2OH- (hope this is right)
Hope this makes sense :)
Post by: Dragomistress on September 26, 2018, 02:33:20 pm
What is the purpose of using concentrated sulfuric acid in esterification reactions?
Post by: Dragomistress on September 26, 2018, 02:41:06 pm
In a reaction, 30.0mL of a 0.106mol/L NaOH is added to 25.0 mL of 0.100 mol/L HCl solution.
Post by: Bri MT on September 26, 2018, 08:09:56 pm
Hello again!
What is the purpose of using concentrated sulfuric acid in esterification reactions?
The sulfuric acid acts as a catalyst for the reaction.
if you think about the equation, it's: alcohol + carboxylic acid ---> ester + water
but this is actually a reversible reaction! so if you have lots of water, you're going to favour the reverse reaction and therefore have less ester made.
That's why the sulfuric acid needs to be concentrated, as otherwise you're adding too much extra water
Hope this helps :)
(also, in future please edit the post you already have rather than posting twice in a row)
Post by: Dragomistress on September 27, 2018, 02:43:00 pm
I am wondering what is the structural formula for ethanol and butanoic acid?
Post by: Bri MT on September 27, 2018, 03:01:47 pm
Hello again!
I am wondering what is the structural formula for ethanol and butanoic acid?
Eth indicates that you have 2 carbons, and -ol indicates that you have a OH group. From this you get the formula C2H5OH. Everything will be connected to the carbons.
But indicates that there are 4 carbons, oic acid indicates that one of those carbons is part of a COOH (both oxygens are bonded to the carbon, one by a double bond and one by a single bond)
I've made VCE chemistry notes on this topic that might also be useful to you (search for organic nomenclature in the notes section if you want to find it)
Post by: Dragomistress on September 27, 2018, 03:14:34 pm
Post by: Bri MT on September 27, 2018, 07:44:11 pm
Oh, sorry. I forgot to write the question. How am I meant to draw the corresponding ester and what is the name of that ester?
The alcohol loses its OH and the acid loses a H (which form water).
The bond connecting the two occurs where these atoms were lost.
The alcohol gets an "yl" ending (which replaces the ol) and the acid gets an "oate" ending (which replaces the oic acid). Don't put a space between the two
Post by: Mate2425 on September 27, 2018, 09:22:18 pm
Also with HSC 2015 past paper wouldn't you be correct in saying for Q6MC that CH4 is the most suitbale replacement to CFC's as they have no c-cl and c-F bonds thus no free radicals and hence no Ozone depletion.They remark the answer as B??
Thank you.
HSC 2017: http://educationstandards.nsw.edu.au/wps/wcm/connect/cb65ca25-fc28-4e21-8f2d-4886bf176ddd/2017-hsc-chemistry.pdf?MOD=AJPERES
HSC 2016: https://educationstandards.nsw.edu.au/wps/wcm/connect/e76232cd-1d84-4d3e-92ce-ea41546810f6/2016-hsc-chemistry.pdf?MOD=AJPERES&CACHEID=ROOTWORKSPACE-e76232cd-1d84-4d3e-92ce-ea41546810f6-lHPDE.v
Post by: blasonduo on September 28, 2018, 04:58:25 pm
snip
I doubt you'd lose a mark for it as the marking feedback didn't state that better responses did not extrapolate, but a good rule of thumb is unless you know for CERTAIN that the relationship is linear all the way to infinity (generally by an equation), then don't extrapolate.
Here's an example of why extrapolating when unsure could give you very wrong results.
(https://i.imgur.com/g8JE6vC.png)
(This is a physics example and not a chemistry example, I know, sorry, but it conveys my point)
As you can see, when we plot the electric magnetic field magnitude with distance of a conducting sphere, it starts out very linear, but once the distance is greater than the radius of the conductor (R), it decreases exponentially. So, if you were given 5 points that all lied between 0 and R, you'd believe that it was linear, however, since we aren't CERTAIN of the equation, we shouldn't extrapolate, because in this case, we would be very wrong. This is similar to why we don't do it in the chemistry question.
As for Q20 in 2016, AAS works by comparing the same wavelength before,and after to see how much of the intensity was absorbed, you wouldn't get any proper results comparing the intensities of 2 different wavelengths, because well, they are different, and different wavelengths absorb differently.
Lastly, HFC's are the offical way for improving, and yeah you are correct, however the equation would be CH4 + Cl2 -> CH3Cl + HCl. This is producing hydrochloric acid! As the question wants the one that reduces the environmental impact the most, I'd go with B.
Hope this helps! :))
Post by: justwannawish on October 01, 2018, 12:41:00 pm
1. Place 2g of Sodium hydrogen carbonate precipitate into a large test tube and fit with a gas delivery tube and stopper. Clamp the test tube to a retort stand and half fill another test tube with limewater and place gas delivery tube in it
2. The carbon dioxide produced was bubbled through limewater to ensure the reaction was progressing
3. Light a Bunsen burner below the test tube such the tip of the flame just touches the base of the test tube. Heat the NaHCO3 strongly to convert it to Na2CO3. Record any observations
Post by: SanaBanana on October 02, 2018, 11:13:07 am
what i would do is define condensation polymerisation and then explain why it fits that crtieria, is that just a waste of words? is chem a subject where more info is preferred or is it the more direct the better?
Post by: Dragomistress on October 03, 2018, 10:05:07 am
Why doesn't the net ionic equation of water get separated into ions? For example, 2NaCl+2H2O->Cl2+h2+2NaOH is 2Cl-+2H2O->Cl2+H2+2OH-
Post by: jazcstuart on October 03, 2018, 11:01:55 am
hi i have a question about answering chem questions. i generally answer my chem questions much like how i answer my bio questions. so, say you get a question that says for example, explain why cellulose is classified as a condensation polymerI'm not totally sure, however I do know for chem your answers need to be succinct and logically structured. For the example question you gave, I agree you would define condensation polymerisation then talk about why cellulose is a condensation polymer. It would probably even be good to include the chemical equation.
what i would do is define condensation polymerisation and then explain why it fits that crtieria, is that just a waste of words? is chem a subject where more info is preferred or is it the more direct the better?
For other questions, it is great if you can use different methods, for example use a table (particularly for compare questions) or use diagrams.
Hi,In net ionic equations you are determining which ions undergo a reaction. I you just look at water, in reacts to form new products:
Why doesn't the net ionic equation of water get separated into ions? For example, 2NaCl+2H2O->Cl2+h2+2NaOH is 2Cl-+2H2O->Cl2+H2+2OH-
2H2O ---> H2 + 2OH-
you can't split H2O into ions because it doesn't exist as ions, instead H and O are covalently bonded, which is why it remains as H2O on the left side of the net ionic equation.
Hope this answers your question
Post by: Dragomistress on October 03, 2018, 11:08:01 am
Why is the answer C?
Post by: jazcstuart on October 03, 2018, 12:04:08 pm
Hello again!Total dissolved solids is a measure of the ions in the solution. A higher soncentration of ions increases conductivity and vice versa, so C is the correct answer. A and B are incorrect because not all dissolved solid affect pH, for example NaCl is neutral. D is also incorrect because AAS only measures the exact concentration of one specific ion (and one with small concentrations), but we need to find the total concentration of all dissolved solids.
Why is the answer C?
Post by: aadharmg on October 04, 2018, 11:17:51 pm
Post by: Mate2425 on October 06, 2018, 11:34:38 pm
I doubt you'd lose a mark for it as the marking feedback didn't state that better responses did not extrapolate, but a good rule of thumb is unless you know for CERTAIN that the relationship is linear all the way to infinity (generally by an equation), then don't extrapolate.
Here's an example of why extrapolating when unsure could give you very wrong results.
(https://i.imgur.com/g8JE6vC.png)
(This is a physics example and not a chemistry example, I know, sorry, but it conveys my point)
As you can see, when we plot the electric magnetic field magnitude with distance of a conducting sphere, it starts out very linear, but once the distance is greater than the radius of the conductor (R), it decreases exponentially. So, if you were given 5 points that all lied between 0 and R, you'd believe that it was linear, however, since we aren't CERTAIN of the equation, we shouldn't extrapolate, because in this case, we would be very wrong. This is similar to why we don't do it in the chemistry question.
As for Q20 in 2016, AAS works by comparing the same wavelength before,and after to see how much of the intensity was absorbed, you wouldn't get any proper results comparing the intensities of 2 different wavelengths, because well, they are different, and different wavelengths absorb differently.
Lastly, HFC's are the offical way for improving, and yeah you are correct, however the equation would be CH4 + Cl2 -> CH3Cl + HCl. This is producing hydrochloric acid! As the question wants the one that reduces the environmental impact the most, I'd go with B.
Hope this helps! :))
Many thanks, your help has made an impact! :)
Post by: horse9996 on October 07, 2018, 08:40:15 am
I'm stuck on the second part. I know its about not repeating it so its not reliable, but how do I write that for 3 marks?
Post by: clovvy on October 07, 2018, 08:58:52 am
(Q27b from the 2003 exam)Hey there, I am by no means expert at HSC chem but I'll try my best... To get 3 marks, I believe you need to explain the relationship between repetition and reliability and make judgements based on your explanation of the matter. You might want to talk about how reliability depends on repetition of the procedure.
I'm stuck on the second part. I know its about not repeating it so its not reliable, but how do I write that for 3 marks?
So I'll just start you off with a few dot-points:
- fertiliser may not have been completely soluble in water, therefore mixtures must be filtered
- Acid should have been used to help dissolve and remove carbonate ions and prevent phosphate ions from precipitating
- Use Ba_2Cl to precipitate sulfates (explain why)
- Discuss the unreliability of the experiments:- technique used to collect the precipitate that can cause precipitates of formed by other impurities, lack of proof that 50mL of Barium Chloride was sufficient to precipitate all sulfate ions..
- Then discuss how to make the experiment more valid and increase the reliability by averaging out results from repeated experiment (also note that if the procedure is not done properly, the results are not valid)..
hopefully that helps
Post by: horse9996 on October 07, 2018, 09:29:55 am
Hey there, I am by no means expert at HSC chem but I'll try my best... To get 3 marks, I believe you need to explain the relationship between repetition and reliability and make judgements based on your explanation of the matter. You might want to talk about how reliability depends on repetition of the procedure.
So I'll just start you off with a few dot-points:
- fertiliser may not have been completely soluble in water, therefore mixtures must be filtered
- Acid should have been used to help dissolve and remove carbonate ions and prevent phosphate ions from precipitating
- Use Ba_2Cl to precipitate sulfates (explain why)
- Discuss the unreliability of the experiments:- technique used to collect the precipitate that can cause precipitates of formed by other impurities, lack of proof that 50mL of Barium Chloride was sufficient to precipitate all sulfate ions..
- Then discuss how to make the experiment more valid and increase the reliability by averaging out results from repeated experiment (also note that if the procedure is not done properly, the results are not valid)..
hopefully that helps
I read a sample answer that had some of this stuff like adding acid, but I thought that was validity when this question is just reliability?
Post by: clovvy on October 07, 2018, 10:00:34 am
I read a sample answer that had some of this stuff like adding acid, but I thought that was validity when this question is just reliability?If the experiment is invalid, how could the results be reliable than? This question requires you to make judgement... Your results will be more reliable through repetition of a valid experimental procedure if they consistently produce similar results...
Post by: horse9996 on October 07, 2018, 10:44:52 am
If the experiment is invalid, how could the results be reliable than? This question requires you to make judgement... Your results will be more reliable through repetition of a valid experimental procedure if they consistently produce similar results...
So basically an experiment can't be reliable unless its valid, therefore discussion on validity is relevant here?
Post by: clovvy on October 07, 2018, 10:49:43 am
So basically an experiment can't be reliable unless its valid, therefore discussion on validity is relevant here?I believe yes.. Because you can have a lot of other crap in that fertiliser that also precipitates with sulfate ions... So if your procedure is invalid, your results is more likely to be inconsistent and it will be unreliable.. While validity and reliability is different, they do relate to each other... This is based on using judgement
Post by: horse9996 on October 07, 2018, 11:47:43 am
I believe yes.. Because you can have a lot of other crap in that fertiliser that also precipitates with sulfate ions... So if your procedure is invalid, your results is more likely to be inconsistent and it will be unreliable.. While validity and reliability is different, they do relate to each other... This is based on using judgement
Yea that makes sense, thanks :)
Post by: DamnDhruv on October 10, 2018, 01:58:15 pm
how much heat is required to raise the temperature of 68.0g of AlF3 from 25.0C
BTW I am new to the forum do please do tell if I did something wrong (posted on the wrong place or didnt ask the question properly.
thanks :)
Post by: horse9996 on October 10, 2018, 03:21:16 pm
Hello Just wondering if anyone can answer this question of mine (they have not given me the specific heat capacity so I dobt know how to figure it out)
how much heat is required to raise the temperature of 68.0g of AlF3 from 25.0C
BTW I am new to the forum do please do tell if I did something wrong (posted on the wrong place or didnt ask the question properly.
thanks :)
Where did you get that question from?
Post by: Dragomistress on October 10, 2018, 05:11:33 pm
I am confused about why for K, they did not divide by the concentration of NH4HS. Or am I just wrong?
Isn't for aA+bB-->cC+dD going to K=([C]^c*[D]^d)/([A]^a^b)?.
Post by: dermite on October 10, 2018, 05:57:01 pm
Hi,
I am confused about why for K, they did not divide by the concentration of NH4HS. Or am I just wrong?
Isn't for aA+bB-->cC+dD going to K=([C]^c*[D]^d)/([A]^a^b)?.
NH4HS is a solid; you dont consider solids in K calculations
Post by: DamnDhruv on October 13, 2018, 05:12:41 pm
Where did you get that question from?
my teacher gave it to me.
Post by: horse9996 on October 14, 2018, 09:45:37 am
my teacher gave it to me.
I'm not too sure, I think you need the specific heat capacity. Then you can solve for the change in heat (delta H) where delta T = 1
Post by: SpanishPear on October 16, 2018, 03:20:34 pm
Hello Just wondering if anyone can answer this question of mine (they have not given me the specific heat capacity so I dobt know how to figure it out)
Hey!
Welcome to the forums!
I must say, I've never experienced a question like this, I don't actually think this is in the HSC Chemistry syllabus!
Unless they give you the specific heat capacity and are testing your knowledge of the formula: delta H = -mCdeltaT.
Although, there could be a really niche application of this that I'm missing, so someone correct me if I'm wrong!
My reccomendation would be to email your teacher for solutions and share it with the forums!
Post by: SpanishPear on October 16, 2018, 03:31:09 pm
So basically an experiment can't be reliable unless its valid, therefore discussion on validity is relevant here?I actually disagree with this.
Reliable is essentially is the experiment getting similar results result every time , akin to consistency.
Think about an a archery target. An experiment is:
Valid: If the aim was to hit the center of the target, and the center of the target was hit.
Reliable: If the arrow was shot several times, would it hit the same area?
Here's an infographic:
https://www.google.com.au/search?q=validity+vs+accuracy+chemistry&safe=strict&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiD-ey2j4reAhUHd94KHVYIBkIQ_AUIDigB&biw=1440&bih=821#imgrc=kGgM-_U6otnKcM:
In regards to your question, I would talk about how repeating and avereging values would increase reliability. BUT this only works if the experiment is conducted exactly the same each trial, hence you should talk about the steps taken and how you can make sure each step is done precisely (eg using exact measurements rather than "a few drops" )
Accuracy is how does the experimental value compare to the accepted value.
TLDR: An experiment CAN be reliable and invalid.
Post by: DamnDhruv on October 16, 2018, 05:16:16 pm
Hey!
Welcome to the forums!
I must say, I've never experienced a question like this, I don't actually think this is in the HSC Chemistry syllabus!
Unless they give you the specific heat capacity and are testing your knowledge of the formula: delta H = -mCdeltaT.
Although, there could be a really niche application of this that I'm missing, so someone correct me if I'm wrong!
My reccomendation would be to email your teacher for solutions and share it with the forums!
Got it thanks !!
Post by: catkittenz on October 26, 2018, 04:25:29 pm
im very confused with evaluating current usage
thankuu
Post by: jazcstuart on October 26, 2018, 05:01:23 pm
process information from secondary sources to summarise the use of ethanol as an alternative car fuel, evaluating the success of currenT usageHi, welcome to Atar Notes!!!
im very confused with evaluating current usage
thankuu
Essentially you need to evaluate how ethanol is being used as an alternative fuel, and how successfully. The main current use is for ethanol/petrol mixes, eg. E10 which is 10% ethanol. This has not been used overly successfully in Australia, due to misconceptions about the damage this can cause to car engines. However ethanol/petrol blends have been used more successfully in other countries, for example in Sweden 85% ethanol blends are common, and Brazil requires all cars to be able to accept at least 25% ethanol blends. Brazil and Argentina have 100% hydrous ethanol fuels (so 4% water), which is a very clean fuel, however existing car engines must be modified.
Therefore the evaluation would be that the use of has great potential to replace dwindling petroleum resources and reduce CO2 emissions, however it is currently restricted to petrol blends with 5-25% ethanol. This is because existing car engines must be modified to accept higher proportions of ethanol, and more research is needed to make ethanol more cost effective and efficient as an alternative fuel.
Hope this helps :)
Post by: sofichu on October 26, 2018, 10:34:54 pm
Explain the role of the conjugate acid/base pair, H2PO4 ^- / HPO4 ^ 2-, in maintaining the pH of living cells. Include chemical equations in your answer ( including one that represents equilibrium)
Thank you!!!!
Post by: Jakeybaby on October 26, 2018, 11:00:00 pm
Hey :) Can anyone help me with this questionHello!
Explain the role of the conjugate acid/base pair, H2PO4 ^- / HPO4 ^ 2-, in maintaining the pH of living cells. Include chemical equations in your answer ( including one that represents equilibrium)
Thank you!!!!
The conjugate acid/base pairing of H2PO4- and HPO42- can neutralise both and acid or a base within a cell, meaning that this pairing acts as a buffer system within the cell. As it is able to neutralise an acid or a base within the cell, it can maintain the pH.
The following equation would be sufficient:
HPO42- + H3O+ <-> H2PO4- + H2O
I hope this helps!
Post by: sofichu on October 26, 2018, 11:15:11 pm
Stuck on another question this time. Any help would be appreciated !
Describe how you would prepare a H2PO4 ^- / HPO4 ^ 2- buffer solution with a pH of 7.40 from the sodium salts of both the H2PO4 ^ - and HPO4 ^ 2- ions .
Ka = 6.2 x 10^-8
Post by: Jakeybaby on October 26, 2018, 11:45:27 pm
Hey again :’)What are your thoughts so far?
Stuck on another question this time. Any help would be appreciated !
Describe how you would prepare a H2PO4 ^- / HPO4 ^ 2- buffer solution with a pH of 7.40 from the sodium salts of both the H2PO4 ^ - and HPO4 ^ 2- ions .
Ka = 6.2 x 10^-8
I would be using the Henderson-Hasselbalch equation to solve this problem.
Post by: RuiAce on October 27, 2018, 08:43:56 am
What are your thoughts so far?I don't see that anywhere in the new syllabus
I would be using the Henderson-Hasselbalch equation to solve this problem.
Post by: Jakeybaby on October 27, 2018, 06:18:33 pm
I don't see that anywhere in the new syllabusMy apologies, not familiar with the syllabus - just assumed that it would be included.
Post by: emiq on October 29, 2018, 06:34:47 pm
I found solutions but I don't understand where the equation: H3X(aq) + 3NaOH(aq) -> Na3X(aq) + 3H2O(l)
I understand the balancing of the equation, but more specifically, where did they get H3X(aq) ?
Post by: Jakeybaby on October 29, 2018, 06:40:04 pm
HSC 2011. Question 26 (b).H3X(aq) represents the triprotic acid (citric acid, in this case) - 3 acidic hydrogens
I found solutions but I don't understand where the equation: H3X(aq) + 3NaOH(aq) -> Na3X(aq) + 3H2O(l)
I understand the balancing of the equation, but more specifically, where did they get H3X(aq) ?
Post by: emiq on October 29, 2018, 08:04:45 pm
H3X(aq) represents the triprotic acid (citric acid, in this case) - 3 acidic hydrogens
Not sure if I'll need it for this year's exam, but I'm just curious, which 3 hydrogens did you mean? (Circle it or something please)
Post by: Jakeybaby on October 29, 2018, 09:47:22 pm
Not sure if I'll need it for this year's exam, but I'm just curious, which 3 hydrogens did you mean? (Circle it or something please)Not something that I think you'd be required to know; but it's the 3 hydrogens which are within the carboxylic acid functional groups.
Post by: belle12345 on October 31, 2018, 12:23:33 pm
I'm not sure if this is the right place to ask this question but I shall ask anyway:
With a couple days until the 2018 Chemistry HSC, would you recommend to NOT study the questions/topics in last years (2017) paper?
For example, in Industrial Chemistry, the last question was about Sodium Hydroxide (the Mercury, Diaphragm, Membrane Process etc) so is there any chance it will show up in this years paper? (not as the last question obviously, but maybe a short answer)
In previous years have they ever had a topic (or parts of a topic) tested 2 years in a row?
Post by: emily_wuuuuu on October 31, 2018, 09:38:48 pm
Post by: michelle_mcl on October 31, 2018, 10:29:54 pm
Post by: Jakeybaby on October 31, 2018, 10:43:30 pm
hi everyone, can someone explain to me why it is Cr2O7 that is involved in the reaction with SO2 and not K?This is just due to the fact that K+ is a common spectator ion and does not take any active part in reactions.
Post by: Bells_123 on October 31, 2018, 11:44:55 pm
- Why can the electrodes in electrolytic cells be in the same electrolyte (unlike most galvanic cells)?
- What are the main reduction equation(s) for anaerobic bacteria reducing sulfate to sulfide (I've seen quite a few variations of similar equations but wasn't sure which ones were correct)?
Thanks!
Post by: jazcstuart on November 01, 2018, 11:31:46 am
Hi Everyone,Yep this is a good place to ask :) I think it could be asked again, as you said maybe as a shorter answer rather than a longer response or vice versa. I doubt you would get a question exactly the same as last year, but you could get one using similar principles. I would revise it to be safe, but I guess it's up to you, if you don't feel like you have enough time to study everything, that could be one of the things you leave until last because it would be less likiely to come up. I don't really know about previous years though.
I'm not sure if this is the right place to ask this question but I shall ask anyway:
With a couple days until the 2018 Chemistry HSC, would you recommend to NOT study the questions/topics in last years (2017) paper?
For example, in Industrial Chemistry, the last question was about Sodium Hydroxide (the Mercury, Diaphragm, Membrane Process etc) so is there any chance it will show up in this years paper? (not as the last question obviously, but maybe a short answer)
In previous years have they ever had a topic (or parts of a topic) tested 2 years in a row?
Can anyone suggest easy ways to remember the anion and cation tests flow chart + results? Currently still struggling :((((( Thanks in advance!I struggle with this as well, I think everyone does :-\ I have 2 strategies, one is using flashcards (I find flashcards good for chemistry for things like polyatomic ions and solubility rules, as well as this). My other strategy is writing it out as a process/flowchart, because I find it is easyier to remember one longer process rather than the individual results. I think you need to remember this anyway so you know in which order the tests need to be conducted. So I have just written it out a few times, trying to do as much of it from memory as I can each time (I find just copying it doesn't help me remember as much). Hope this helps!
Hi everyone, I'm doing the 'Shipwrecks and Salvage' option topic for chemistry and have been trying to have a clearer understanding of electrolytic cells and anaerobic bacteria in particular:For your first question I thought they can be in the same electrolyte, for example if you are using intert electrodes to electrolyse a salt solution. Where did you hear that they can't be in the same electrolyte?
- Why can the electrodes in electrolytic cells be in the same electrolyte (unlike most galvanic cells)?
- What are the main reduction equation(s) for anaerobic bacteria reducing sulfate to sulfide (I've seen quite a few variations of similar equations but wasn't sure which ones were correct)?
Thanks!
For sulfate reducing bacteria there are 2 different equations depending on if it is occurring in an acidic or non-acidic microclimate, so this might have caused your confusion? I think you only need to remember 1 though. This is the one I am using for an acidic microclimate:
oxidation: Fe (s) ---> Fe2+ (aq) + 2e-
reduction: SO42- (aq) +10H+ (aq) + 8e- ---> H2S (aq) + 4H2O
overall: 4Fe (s) + SO42- (aq) +10H+ (aq) ---> 4Fe2+ (aq) + H2S (aq) + 4H2O
Hope this helps :)
Post by: horse9996 on November 01, 2018, 12:17:05 pm
Can anyone suggest easy ways to remember the anion and cation tests flow chart + results? Currently still struggling :((((( Thanks in advance!
Just remember to solubility rules! (+ general knowledge about acid carbonate reactions to detect carbonate)
Always soluble: NAGSAG
nitrates, acetates, group 1, sulfates, ammonium, group 7
EXCEPTIONS: PMS, CaStroBar
PMS (sulfates and group 7): lead, mercury, silver
CaStroBar (sulfates only): calcium, strontium, barium
Make sure you know about flame colours as well, and distinguishing between Fe2/3+
Post by: varun.amin on November 01, 2018, 05:57:31 pm
What are the main chemical you need to know equations for the Shipwrecks and Salvage option topic? If anyone has a list that would be much appreciated. Thank you in advance :)
Post by: sunshine32323123123123q3 on November 01, 2018, 10:16:38 pm
"Evaluate how environmental issues are addressed in the Solvay Process"
How would u go about answering this quesiton?
Post by: horse9996 on November 02, 2018, 09:20:59 am
A 2001 HSC INDUSTRIAL CHEM QUESTION:
"Evaluate how environmental issues are addressed in the Solvay Process"
How would u go about answering this quesiton?
I would recommend planning your answer first. Consider: what are the issues? How do they impact the environment? How are they addressed? Make sure to include a judgement.
Some examples:
Carbon dioxide (greenhouse gas) is recycled
Calcium chloride (waste cause there are limited uses) is pumped into oceans or the solid can be put in landfill (but this can pollute groundwater through leaching if the rock is porous so specific criteria must be met)
Waste water must be cooled (thermal pollution)
Ammonia is recycled but some is lost to the atmosphere (pollutant_
Solids were previously put in rivers and waterways but this is ugly and blocks rivers. How used for things like fertiliser, bricks or landfill
Overall, as most reactants/products are natural, good waste management means that there is minimal environmental issues.
Post by: Bells_123 on November 02, 2018, 11:50:58 am
Yep this is a good place to ask :) I think it could be asked again, as you said maybe as a shorter answer rather than a longer response or vice versa. I doubt you would get a question exactly the same as last year, but you could get one using similar principles. I would revise it to be safe, but I guess it's up to you, if you don't feel like you have enough time to study everything, that could be one of the things you leave until last because it would be less likiely to come up. I don't really know about previous years though.
I struggle with this as well, I think everyone does :-\ I have 2 strategies, one is using flashcards (I find flashcards good for chemistry for things like polyatomic ions and solubility rules, as well as this). My other strategy is writing it out as a process/flowchart, because I find it is easyier to remember one longer process rather than the individual results. I think you need to remember this anyway so you know in which order the tests need to be conducted. So I have just written it out a few times, trying to do as much of it from memory as I can each time (I find just copying it doesn't help me remember as much). Hope this helps!
For your first question I thought they can be in the same electrolyte, for example if you are using intert electrodes to electrolyse a salt solution. Where did you hear that they can't be in the same electrolyte?
For sulfate reducing bacteria there are 2 different equations depending on if it is occurring in an acidic or non-acidic microclimate, so this might have caused your confusion? I think you only need to remember 1 though. This is the one I am using for an acidic microclimate:
oxidation: Fe (s) ---> Fe2+ (aq) + 2e-
reduction: SO42- (aq) +10H+ (aq) + 8e- ---> H2S (aq) + 4H2O
overall: 4Fe (s) + SO42- (aq) +10H+ (aq) ---> 4Fe2+ (aq) + H2S (aq) + 4H2O
Hope this helps :)
Thanks for the reply! For the first question I was wondering why the electrodes can be in the same electrolyte for electrolytic cells unlike a lot of galvanic cells which are in different electrolytes - does it have something to do with the flow of ions in the solution when an electric current is applied?
Post by: jazcstuart on November 02, 2018, 12:51:12 pm
Thanks for the reply! For the first question I was wondering why the electrodes can be in the same electrolyte for electrolytic cells unlike a lot of galvanic cells which are in different electrolytes - does it have something to do with the flow of ions in the solution when an electric current is applied?Oh sorry, I must have read your question wrong!
Tbh I don't really know why this is usually the case (maybe someone else can clarify?). I think it depends on what you are trying to achieve with the cell, for example with electrolytic cells often you are trying to separate a solution, which is why they have the same electrolyte.
Electrolytic cells can be in different electrolytes if you want to reverse a typical galvanic cell, and galvanic cells can be in the same electrolyte (for example the corrosion of iron in seawater acts as a galvanic cell). So it's not a case of electrolytic cells can't be in separate electrolytes or vice versa, I think it's just how they are usually written.
Sorry that wasn't super helpful, if anyone else has a better answer that would be great.
Post by: Bri MT on November 02, 2018, 01:24:19 pm
Thanks for the reply! For the first question I was wondering why the electrodes can be in the same electrolyte for electrolytic cells unlike a lot of galvanic cells which are in different electrolytes - does it have something to do with the flow of ions in the solution when an electric current is applied?
The reductant wants to give its electrons to the oxidant right?
So the reductant will donate its electrons and they go through the external circuit to reach the oxidant & you have current! But if the reductant and oxidant are in contact with each other, you're not going to push electrons through the external circuit - you'll just directly transfer them.
So if they aren't separated, you'll have a spontaneous reaction with an associated enthalpy change but no electricity generation.
With an electrolytic cell, you're forcing movement of electrons that would not spontaneously occur - so you wouldn't have a spontaneous reaction occurring between the oxidant and reductant when they're in contact.
Little bit of an oversimplification, but I hope this helps & addresses your question :)
Post by: preliminary17hsc18 on November 02, 2018, 01:28:51 pm
Need some help with this absolutely horrendous question for shipwrecks:
How does iron corrode in acidic conditions, because:
Oxidation: Fe --> Fe2+ 2e-
Reduction: O2 + 4H+ + 4e- --> 2H20
So in the end you are left with iron ions and water so how does the iron form the hydroxide that's the actual rust?
Post by: Mate2425 on November 02, 2018, 02:29:08 pm
Thanks to you all! :) :)
Post by: cocopops201 on November 02, 2018, 02:57:43 pm
Evaluate the implications of the use of DNA as an identification molecule for society
Post by: horse9996 on November 02, 2018, 03:01:24 pm
Hey guys, i always seem to not get full marks in heat of combustion questions mainly due to not knowing what equations to specifically use. If possible could someone please send the equations we need to answer these questions and also the general process i should apply in order to gain a better mark in these questions.
Thanks to you all! :) :)
1. Calculate the change in heat of the water (convert to kJ by dividing by 1000, delta H = mc delta T)
2. Calculate the mass and following that the moles of the alkanol which was combusted (number of moles = mass/molar mass)
3. Heat of combustion = change in heat (part 1) divided by number of moles of the alkanol (part 2)
Post by: jazcstuart on November 02, 2018, 03:21:13 pm
1. Calculate the change in heat of the water (convert to kJ by dividing by 1000, delta H = mc delta T)Just to add to this, the m in the first formula is the mass of the liquid being heated, NOT the mass of the alkonol burned. I always used to get confused by this. The c is the specific heat capacity (generally of water, which is on the back of the periodic table). Make sure everything is in the same units, so if you use grams for the mass you must use grams for the specific heat capacity as well.
2. Calculate the mass and following that the moles of the alkanol which was combusted (number of moles = mass/molar mass)
3. Heat of combustion = change in heat (part 1) divided by number of moles of the alkanol (part 2)
So essentially in the first step you are finding delta H, which is just the heat of combustion, and in the third step you are finding (delta H)/mol, which is the molar heat of combustion.
Need some help with this absolutely horrendous question for shipwrecks:Not sure about this sorry, the only thing I can think of is that the Fe2+ reacts with OH- already in the water, but I have no idea if this is right.
How does iron corrode in acidic conditions, because:
Oxidation: Fe --> Fe2+ 2e-
Reduction: O2 + 4H+ + 4e- --> 2H20
So in the end you are left with iron ions and water so how does the iron form the hydroxide that's the actual rust?
Post by: Mate2425 on November 02, 2018, 11:16:15 pm
Just to add to this, the m in the first formula is the mass of the liquid being heated, NOT the mass of the alkonol burned. I always used to get confused by this. The c is the specific heat capacity (generally of water, which is on the back of the periodic table). Make sure everything is in the same units, so if you use grams for the mass you must use grams for the specific heat capacity as well.
So essentially in the first step you are finding delta H, which is just the heat of combustion, and in the third step you are finding (delta H)/mol, which is the molar heat of combustion.
Not sure about this sorry, the only thing I can think of is that the Fe2+ reacts with OH- already in the water, but I have no idea if this is right.
Thanks jazcstuart!
When you said the specific heat capacity must be in the same units e.g grams does that mean i have to times the value from formula sheet by 10^3.
Post by: horse9996 on November 03, 2018, 08:07:07 am
Thanks jazcstuart!
When you said the specific heat capacity must be in the same units e.g grams does that mean i have to times the value from formula sheet by 10^3.
I'm not sure, I always use 4.18 and get the right answer. Its more grams vs kilograms and joules vs kilojoules I think
Post by: horse9996 on November 03, 2018, 08:54:11 am
"Chemists can assist in reversing or minimising the environmental problems caused by technology and the human demand for products and services.
With reference to this statement, assess the need for chemists to collaborate when monitoring the environmental impact of a named electrochemical cell."
Thoughts?
Post by: Dragomistress on November 03, 2018, 09:04:13 am
How do I know which the anode and which is the cathode in an electrochemical cell?
Post by: Bri MT on November 03, 2018, 09:09:15 am
Hey,
How do I know which the anode and which is the cathode in an electrochemical cell?
The anode is always the site of oxidation and the cathode is always the site of reduction
Post by: jazcstuart on November 03, 2018, 09:11:29 am
Thanks jazcstuart!Like horse9996 I usually just use 4.18J g-1 K-1 because usually the information given is in jules and grams. The one on the formula sheet is 4.18J kg-1 K-1, so to get it in grams you actually have to divide it by 103 (since it is ^-1). Don't worry about it too much, I think it's easiest to just use 4.18J g-1 K-1 and make sure all your other information is in jules and grams.
When you said the specific heat capacity must be in the same units e.g grams does that mean i have to times the value from formula sheet by 10^3.
2012 q33Yeah this is a really hard question, I've had a go at it in class. Essentially you need to choose a cell and talk about it's environmental impact, an talk about the different skills chemists need at different stages in the use of the battery (eg. production, use and disposal) to minimise this impact. Have a look at the sample answer if you haven't already, I think it will give you a better idea :)
"Chemists can assist in reversing or minimising the environmental problems caused by technology and the human demand for products and services.
With reference to this statement, assess the need for chemists to collaborate when monitoring the environmental impact of a named electrochemical cell."
Thoughts?
Post by: Dragomistress on November 03, 2018, 09:18:46 am
The anode is always the site of oxidation and the cathode is always the site of reduction
How do I know which is the site of oxidation and the site of reduction?
Post by: jazcstuart on November 03, 2018, 09:27:24 am
How do I know which is the site of oxidation and the site of reduction?For a galvanic cell you figure out which undergoes oxidation and reduction using the list of standard potentials, which tells you the reduction potential. The metal with the higher reduction potential will undergo reduction (of course :D). In an electrolytic cell this is reversed; the negative terminal supplies electrons, therefore forcing reduction, so the metal with the lower reduction potential will undergo reduction (I think you only need to know about electrolytic cells if you are doing Shipwrecks).
Hope this makes sense
Post by: Fergus6748 on November 03, 2018, 12:41:52 pm
How do I know which is the site of oxidation and the site of reduction?
For a galvanic cell you figure out which undergoes oxidation and reduction using the list of standard potentials, which tells you the reduction potential. The metal with the higher reduction potential will undergo reduction (of course :D). In an electrolytic cell this is reversed; the negative terminal supplies electrons, therefore forcing reduction, so the metal with the lower reduction potential will undergo reduction (I think you only need to know about electrolytic cells if you are doing Shipwrecks).I've always been thinking of the Standard Potential as the higher metal on the chart oxidises and thus the anode, Aha. Thus, the lower one reduces (cathode). I've never hheard about it being told the other way. Anox Redcat is a popular mnemonic to remember that the anode oxidises and the Cathode reduces. Electrolytic cells are in industrial chemistry as well. Dot point 5, for the production of sodium hydroxide.
Hope this makes sense
Post by: horse9996 on November 03, 2018, 01:48:57 pm
Post by: Fergus6748 on November 03, 2018, 01:55:43 pm
For saponification, you know how you add oil, sodium hydroxide and a water/ethanol mixture into a beaker which is heated to make the soap? Why is the ethanol added, what's its purpose?Hey, from what I've been told the ethanol is just there to help the reaction between the oil and the sodium hydroxide. It doesn't have any other purpose from that. Hope this helps!!
Post by: horse9996 on November 03, 2018, 02:33:15 pm
Hey, from what I've been told the ethanol is just there to help the reaction between the oil and the sodium hydroxide. It doesn't have any other purpose from that. Hope this helps!!
Does it act as a catalyst?
Post by: Fergus6748 on November 03, 2018, 04:13:11 pm
Does it act as a catalyst?Not too sure to be honest, aha. All I was told is that it assists with the reaction. I think it essentially is, as it doesn't take part in the reaction but helps the reaction.
Post by: horse9996 on November 03, 2018, 04:19:31 pm
Not too sure to be honest, aha. All I was told is that it assists with the reaction. I think it essentially is, as it doesn't take part in the reaction but helps the reaction.
Ok, so if they ask what the purpose of the ethanol is I just say it assists in the saponification process while the mixture is being heated. Thanks for the help :)
Post by: jazcstuart on November 03, 2018, 04:27:04 pm
Ok, so if they ask what the purpose of the ethanol is I just say it assists in the saponification process while the mixture is being heated. Thanks for the help :)I don't do industrial chem, but remeber that ethanol is a solvent which can dissolve both polar and non-polar substances. So if the oil is non-polar, it won't dissolve in water but it will dissolve in ethanol, then the ethanol dissolves in water. I would say this is why ethanol is used, hope this helps.
(Ps you should look at the shoutouts and appreciation thread if you haven't already ;))
Post by: horse9996 on November 03, 2018, 04:44:44 pm
I don't do industrial chem, but remeber that ethanol is a solvent which can dissolve both polar and non-polar substances. So if the oil is non-polar, it won't dissolve in water but it will dissolve in ethanol, then the ethanol dissolves in water. I would say this is why ethanol is used, hope this helps.
(Ps you should look at the shoutouts and appreciation thread if you haven't already ;))
Ohh that makes heaps of sense, also with why the soap has to be seperated at the end (soap is similar with a polar head and non polar hydrocarbon tail, so it allows water so dissolve oils/grease, I think of it like a tadpole)
Post by: Dragomistress on November 04, 2018, 11:08:55 am
Post by: Fergus6748 on November 04, 2018, 11:29:20 am
Hey, how am I meant to find the isomer for this question?Hey, so isomers are just the same molecule with a different structure, it's a bit hard to describe without diagrams but for example, both 1-bromo-1-chloropentane and 1-bromo-2-chloropentane are both isomers of that molecule. So, the isomers of C3H6BrCl are:
1-bromo-1-chloropentane
1-bromo-2-chloropentane
2-bromo-2-chloropentane
2-bromo-1-chloropentane
Those are the only isomers of C3H6BrCl so the answer is 4.
With the other q, the first thing you need to do is make an equation:
HCl + NaX --> NaCl + HX
With that you need to find the moles of hydrochloric acid that was needed to neutralise the solution so:
Moles = Concentration x Volume
= 0.100 x 0.0244
= 0.00244 moles
From above, we know that the moles of HCl equals the moles of NaX so:
nHCl=nNaX
=0.00244 moles
We know that Moles = Mass/Molar Mass so Molar Mass = Mass/Moles
MNaX= (1)/(0.00244)
= 409.8360656....g/mol
Therefore, the molar mass of NaX is 410 g/mol, I think, pretty sure what was the answer? Hope this helps!!
Post by: horse9996 on November 04, 2018, 11:29:43 am
Hey, how am I meant to find the isomer for this question?
Draw out the carbon chain and count the possible arrangements (remember naming conventions eg 1-chloropropane is the same as 3-chloropropane). I don't think there's any other way really
Also see working for the titration question attached.
Post by: Dragomistress on November 04, 2018, 11:55:25 am
Hey, so isomers are just the same molecule with a different structure, it's a bit hard to describe without diagrams but for example, both 1-bromo-1-chloropentane and 1-bromo-2-chloropentane are both isomers of that molecule. So, the isomers of C3H6BrCl are:
1-bromo-1-chloropentane
1-bromo-2-chloropentane
2-bromo-2-chloropentane
2-bromo-1-chloropentane
Those are the only isomers of C3H6BrCl so the answer is 4.
With the other q, the first thing you need to do is make an equation:
HCl + NaX --> NaCl + HX
With that you need to find the moles of hydrochloric acid that was needed to neutralise the solution so:
Moles = Concentration x Volume
= 0.100 x 0.0244
= 0.00244 moles
From above, we know that the moles of HCl equals the moles of NaX so:
nHCl=nNaX
=0.00244 moles
We know that Moles = Mass/Molar Mass so Molar Mass = Mass/Moles
MNaX= (1)/(0.00244)
= 409.8360656....g/mol
Therefore, the molar mass of NaX is 410 g/mol, I think, pretty sure what was the answer? Hope this helps!!
Sadly, they are both wrong
I should have posted the answer but for 14 it is C and for 18 it is A.
Post by: horse9996 on November 04, 2018, 11:58:33 am
Sadly, they are both wrong
I should have posted the answer but for 14 it is C and for 18 it is A.
My one is the solution to a short answer in the 2001 paper, I assume that multiple choice is a similar method
What paper is q14 from?
Post by: Bri MT on November 04, 2018, 12:05:23 pm
Hey, so isomers are just the same molecule with a different structure, it's a bit hard to describe without diagrams but for example, both 1-bromo-1-chloropentane and 1-bromo-2-chloropentane are both isomers of that molecule. So, the isomers of C3H6BrCl are:
1-bromo-1-chloropentane
1-bromo-2-chloropentane
2-bromo-2-chloropentane
2-bromo-1-chloropentane
"pent" is for 5 carbons in the chain, "prop" is for 3
1 bromo 3 chloro
Thankyou for having a go at the question and explaining isomerism :)
Post by: 3.14159265359 on November 04, 2018, 01:42:15 pm
Hey, so isomers are just the same molecule with a different structure, it's a bit hard to describe without diagrams but for example, both 1-bromo-1-chloropentane and 1-bromo-2-chloropentane are both isomers of that molecule. So, the isomers of C3H6BrCl are:
1-bromo-1-chloropentane
1-bromo-2-chloropentane
2-bromo-2-chloropentane
2-bromo-1-chloropentane
Those are the only isomers of C3H6BrCl so the answer is 4.
hey, going back to what @Fergus6748 said, he was actually missing one more isomer. so the correct answer should be
1-bromo-1-chloropropane
1-bromo-2-chloropropane
2-bromo-2-chloropropane
2-bromo-1-chloropropane
as well as 1-bromo-3-chloropropane
therefore there are 5 isomers hence c
I hope this helps
Post by: horse9996 on November 04, 2018, 01:45:27 pm
Post by: 3.14159265359 on November 04, 2018, 01:53:39 pm
For saponification, you know how you add oil, sodium hydroxide and a water/ethanol mixture into a beaker which is heated to make the soap? Why is the ethanol added, what's its purpose?
again, adding on to what @Fergus6748 said, basically the "equation" for saponification is
ESTER + NAOH <----> SODUM CARBOXYLATE + ALKANOL
an example would be an ester being ETHYL METHANOATE
SO IT WOULD BE ETHYL METHANOATE + NAOH <----> SODUM METHANOATE + ETHANOL
so the use of ethanol, is for making the ester along with an alkaloid acid which on this case is methanol acid. and being. byproduct of the reaction
I hope this makes sense
Post by: Fergus6748 on November 04, 2018, 03:47:37 pm
Does anyone have any predictions for tomorrow's paper?It's going to be hard?? aha
Post by: horse9996 on November 04, 2018, 04:14:27 pm
It's going to be hard?? aha
Haha yea, based on the other exams so far this year
Post by: InnererSchweinehund on November 05, 2018, 08:11:15 pm
I was wondering if anyone could give advice on the best way to study for a practical exam - specifically a titration exams.
Also does anyone know of any past papers / practice exams I could do for preparation for ^^ and where to access these?? I just want to know what the common questions are sort of thing.
Thanks!!
Post by: Bri MT on November 05, 2018, 11:02:46 pm
Hey,
I was wondering if anyone could give advice on the best way to study for a practical exam - specifically a titration exams.
Also does anyone know of any past papers / practice exams I could do for preparation for ^^ and where to access these?? I just want to know what the common questions are sort of thing.
Thanks!!
I didn't do HSC but here are some things to watch out for:
- what are you rinsing the glassware with and why? What would happen if you didn't rinse correctly?
- what's the difference between the end point and equivalence point?
- why is the indicator suitable for this reaction? What would be the consequence of using an unsuited indicator?
- what would the pH curve look like?
- how did you read a burette and to how many decimal places?
- what are concordant titres and why are they important?
- why is it good to use a white background?
- what are the characteristics of a good standard?
Post by: charlottemchenry on November 05, 2018, 11:11:54 pm
Hey,
I was wondering if anyone could give advice on the best way to study for a practical exam - specifically a titration exams.
Also does anyone know of any past papers / practice exams I could do for preparation for ^^ and where to access these?? I just want to know what the common questions are sort of thing.
Thanks!!
My class did this for the exam and pretty much exactly what miniturtle said. We had 30 marks of questions at it was made up of those things.
Just know your equations ie like weak strong all that. Know which indicator and how to rinse glassware.
Not sure if your school allowed this but my school did it so like we did it in class and then could go at lunch a week before exam for another practice. So we did it twice before the prac in groups so maybe ask if you could do that.
Just try make your measurements as accurate as possible.
Good luck!
Post by: david.wang28 on November 06, 2018, 12:05:56 pm
I'm just here to ask about question 10 b,c and d about my chemistry hw. May I please get a detailed reply ASAP? Thanks! :)
Post by: Fergus6748 on November 06, 2018, 12:21:02 pm
Hello,Heya, is the new syllabus, I will give it a go!
I'm just here to ask about question 10 b,c and d about my chemistry hw. May I please get a detailed reply ASAP? Thanks! :)
(10.b) So at point A, Q will increase as the system reaches equilibrium as the equilibrium is shifting to the right, producing more products, and because we know that:
Q = Products/Reactants
Since the concentration of the products is increasing while the concentrations of the reactants is decreasing as the system reaches equilibrium. Thus, you can say that Q is going to increase.
(10.c) So what has happened is that the concentration of lead (Pb) has increased sharply because excess lead has been added to the chamber. From there, the equilibrium is shifting to the right to produce more products to counteract the sharp rise in lead, according to Le Chatelier's Principle. Le Chatelier's Principle states that a system at equilibrium will adjust itself to reduce any change in the system. So in this case, the change is the increase in lead that has been added, thus the system has produced more products (PbNO3) to counteract the change. It then reaches a new equilibrium.
(10.d) that is literally the exact same question as (10.c) so is that a mistake?
Hope this helps!!
Post by: david.wang28 on November 06, 2018, 12:29:44 pm
Post by: david.wang28 on November 07, 2018, 11:52:06 am
I'm here to ask about Question 9, 10 and 11 as shown in the attachment. I've attempted them, but can anyone please give me corrections if need be? Thanks :)
Post by: InnererSchweinehund on November 07, 2018, 08:39:10 pm
I didn't do HSC but here are some things to watch out for:
- what are you rinsing the glassware with and why? What would happen if you didn't rinse correctly?
- what's the difference between the end point and equivalence point?
- why is the indicator suitable for this reaction? What would be the consequence of using an unsuited indicator?
- what would the pH curve look like?
- how did you read a burette and to how many decimal places?
- what are concordant titres and why are they important?
- why is it good to use a white background?
- what are the characteristics of a good standard?
My class did this for the exam and pretty much exactly what miniturtle said. We had 30 marks of questions at it was made up of those things.
Just know your equations ie like weak strong all that. Know which indicator and how to rinse glassware.
Not sure if your school allowed this but my school did it so like we did it in class and then could go at lunch a week before exam for another practice. So we did it twice before the prac in groups so maybe ask if you could do that.
Just try make your measurements as accurate as possible.
Good luck!
Thanks so much!!
Post by: jamonwindeyer on November 11, 2018, 03:32:19 pm
Keen to see this thread be just as much of a collaborative resource for the new course as it was for the old :)
Post by: g.xzhu on November 12, 2018, 10:30:33 pm
Could I please have some help with this question?
Calculate the molar solubility of magnesium phosphate (in mol/L), given that its Ksp is 1.04 x 10-24.
I keep getting 6.26 x 10-6 as my answer...
Thanks!
Post by: horse9996 on November 14, 2018, 12:48:18 pm
Hello!
Could I please have some help with this question?
Calculate the molar solubility of magnesium phosphate (in mol/L), given that its Ksp is 1.04 x 10-24.
I keep getting 6.26 x 10-6 as my answer...
Thanks!
https://www.chemguide.co.uk/physical/ksp/calculations.html
Post by: david.wang28 on November 14, 2018, 10:11:14 pm
I need help with the questions in the pictures I sent Exercise 3 (14 c and d and question 15 a and b and 16). Can anyone please give me a detailed reply ASAP? Thanks :)
Post by: g.xzhu on November 17, 2018, 07:43:47 pm
Hello,
I need help with the questions in the pictures I sent Exercise 3 (14 c and d and question 15 a and b and 16). Can anyone please give me a detailed reply ASAP? Thanks :)
Hopefully these help! Please tell me if any of my answers are wrong
[Q15 a, b]
Post by: g.xzhu on November 17, 2018, 07:54:12 pm
Hello,
I need help with the questions in the pictures I sent Exercise 3 (14 c and d and question 15 a and b and 16). Can anyone please give me a detailed reply ASAP? Thanks :)
[Q14, Q16]
Post by: david.wang28 on November 17, 2018, 10:25:15 pm
Hopefully these help! Please tell me if any of my answers are wrongThanks for the answers! :)
[Q15 a, b]
Post by: SebastianHabibi on November 28, 2018, 08:33:06 pm
For my chemistry assessment this term we have to write a depth study regarding module 5. The aim of my investigation is to "determine the effect of concentration and temperature on the position of the equilibrium, and therefore yield of iron thiocyanate.
I want to determine which will be most effective for maximising yield, concentration or temperature. To do this, I conducted two experiments one with varying concentrations and one with varying temperatures. I'm not exactly sure how to write my variables table because I have two experiments. What would my independent variable be? OR do I have to make two variables tables? Help I'm confused
Post by: kauac on November 28, 2018, 09:17:56 pm
Soooooo,
For my chemistry assessment this term we have to write a depth study regarding module 5. The aim of my investigation is to "determine the effect of concentration and temperature on the position of the equilibrium, and therefore yield of iron thiocyanate.
I want to determine which will be most effective for maximising yield, concentration or temperature. To do this, I conducted two experiments one with varying concentrations and one with varying temperatures. I'm not exactly sure how to write my variables table because I have two experiments. What would my independent variable be? OR do I have to make two variables tables? Help I'm confused
Hi...
You will have two independent variables in this set-up, since there are technically two different experiments. Concentration is one independent variable, and temperature is another independent variable, as independent variables can be identified as the one that is altered through the course of the experiment. Yield is your dependent variable, as its results will change according to concentration and temperature.
It would be best to have two separate variable tables - as 1 table with 2 independent variables could make the experiment look invalid. Hope this makes sense. :)
Post by: jazcstuart on November 28, 2018, 09:41:17 pm
Soooooo,I would agree with kauac, you would need to think about them as 2 separate experiments, one where you are varying concentration and one varying temperature. You could use 2 separate tables, but the dependent and controlled variables should still be the same, so another option could be to use one table and label the independent variables for each experiment (if you are concerned about having 2 tables). So for example your independent variable column would say:
For my chemistry assessment this term we have to write a depth study regarding module 5. The aim of my investigation is to "determine the effect of concentration and temperature on the position of the equilibrium, and therefore yield of iron thiocyanate.
I want to determine which will be most effective for maximising yield, concentration or temperature. To do this, I conducted two experiments one with varying concentrations and one with varying temperatures. I'm not exactly sure how to write my variables table because I have two experiments. What would my independent variable be? OR do I have to make two variables tables? Help I'm confused
Experiment 1: concentration
Experiment 2: temperature
Good luck!
Post by: mirakhiralla on December 03, 2018, 08:42:03 pm
how do I show it to you?
Post by: sweetiepi on December 03, 2018, 08:43:37 pm
hey I have a q but it won't let me upload the photo of it because its too big,Hey there, you can upload it to an image-sharing site such as http://www.imgur.com and share the link with us! :))
how do I show it to you?
Post by: scienceislife on December 12, 2018, 02:51:57 pm
Post by: david.wang28 on December 15, 2018, 05:17:47 pm
Given the concentration of ethanoic acid through an esterification reaction, how do you find the corresponding concentrations of ethanol when you aren't given a volume?Do you have the number of moles given to you? If you do, you can find the volume and then go on and find the concentration of ethanol. I'm not exactly sure if this is the right answer, but from what you said, that is how I tried to get my answer out.
Post by: jasn9776 on December 22, 2018, 01:09:05 pm
Post by: kauac on December 22, 2018, 03:00:29 pm
DIY bridging course question: i understand why certain atoms form certain cations. e.g. Na forms 1+ cations to gain the same electron structure as its closest noble gas(Neon) by losing an electron. However what about Silver ions and stuff? Is there a reason i have to know or is it just how it is? (see attached 'common additional cations')
Hi...Just going to give a little bit of an explanation of behind the formation of ions, so that it might make sense for silver.
So you will find that the formation of of both cations and anions works by the number of electrons in the valency shell (outermost shell) of an atom. Valency works according to the periodic table, so that Group I has a valency of 1, Group II has a valency of 2, Group III has a valency of 3, and so on. Thus, elements in Group I will form 1+ cations, Group II: 2+ cations, Group III: 3+ cations, and for the otherside of the table, Group VIII: -1 anion, Group VII: -2 anions, etc.
Since silver is a transitional metal (i.e. it is not in one of the 8 specified groups), it does not have a set ion that it forms. It can form both cation Ag+ and Ag 2+. However, the Ag 2+ is quite rare and only forms under certain conditions, so therefore it is most common to use the Ag+ cation when referring to the ion for silver.
Hope this helps! :)
Post by: david.wang28 on December 27, 2018, 09:34:11 pm
I'm having trouble with Q 17, Q 19 b) and Q 20, but I have shown my working out in the attachments. May I get a detailed answer please? Thanks :)
Post by: fun_jirachi on December 27, 2018, 11:21:51 pm
For 19, I think you should use an ICE table. What you've done there is add some ammonia that doesnt exist by subbing directly into the equilibrium constant equation, and that doesnt work. Using an ICE table, you eventually get to an expression like this:
I'm going to trust your maths skills here, but if you need any help with this part ask again :). Solve for x, which is roughly 8.28 by 10 to the -3. Since the equilibrium constant is zero at the start (since ammonia has zero concentration at the start), and now it has a value, then it follows that equilibrium shifts right.
For 20, a) is pretty much spot on. At the start, Q is 150, so at equilibrium, there will be more reactants and less products. So that part's great :D With b) you have to notice what's implied by this. As the temperature increases, there are more products (implied by increase in constant). This tells you that when you increase the temperature, by LCP the system shifts right to minimise the change, which would be to absorb it. Therefore the forward reaction is endothermic, and therefore the enthalpy change is positive.
Hope this helps :)
Post by: david.wang28 on December 28, 2018, 09:56:11 pm
For question 17, I think you've understood quite well what you're doing. Just stick a zero in for the initial concentration of nitrogen gas, since it doesn't say anything about nitrogen at the beginning of the question. The assumption is that if it isn't mentioned, it doesnt exist at the start. Because what's given is added to an empty vessel, nothing was in the vessel to begin with and nothing more or less than what is added is added. Crunch the numbers and you should get thatFor Q19, I seemed to have trouble finding x. I've tried to expand the equation using binomial theorem (it seems surreal, I know), but that has led me to nowhere. Can you please give me guidance on how to find x please? Thanks. :)
For 19, I think you should use an ICE table. What you've done there is add some ammonia that doesnt exist by subbing directly into the equilibrium constant equation, and that doesnt work. Using an ICE table, you eventually get to an expression like this:
I'm going to trust your maths skills here, but if you need any help with this part ask again :). Solve for x, which is roughly 8.28 by 10 to the -3. Since the equilibrium constant is zero at the start (since ammonia has zero concentration at the start), and now it has a value, then it follows that equilibrium shifts right.
For 20, a) is pretty much spot on. At the start, Q is 150, so at equilibrium, there will be more reactants and less products. So that part's great :D With b) you have to notice what's implied by this. As the temperature increases, there are more products (implied by increase in constant). This tells you that when you increase the temperature, by LCP the system shifts right to minimise the change, which would be to absorb it. Therefore the forward reaction is endothermic, and therefore the enthalpy change is positive.
Hope this helps :)
Post by: fun_jirachi on January 09, 2019, 09:39:12 pm
Firstly please note my answer of 8.28 x 10^-3 is erroneous, ignore that please! made a calculation error. It's more like 7.75x10^-3. This question is a bit of a tough one because you're given a quartic to solve. There might be an easier method, but I found that Newton's method of finding a root approximation was relatively okay. I subbed in values for f(x) which happens to be 1.404x^4-0.7722x^3-3.8479x^2-0.01235+0.000325 (best expanded with binomial theorem, saves a lot of work). I just looked for a value that had 5 zeroes after the decimal point before trying newton's method (for reference i started with x1 being 0.0078, going down in small increments). Finding f'(x), you use this method quite a few times till I guess you get a good enough approximation for our standards (i chose something pretty small when subbing into f(x)). I guess with this method it's up to your own discretion where you stop, but as for this, I'm not too sure. That's how I did it anyway, you may have a different better method!
Hope this helps :D
Post by: david.wang28 on January 11, 2019, 11:56:06 am
Sorry I didn't reply, I was away for quite a while!This is what I did for Q19.
Firstly please note my answer of 8.28 x 10^-3 is erroneous, ignore that please! made a calculation error. It's more like 7.75x10^-3. This question is a bit of a tough one because you're given a quartic to solve. There might be an easier method, but I found that Newton's method of finding a root approximation was relatively okay. I subbed in values for f(x) which happens to be 1.404x^4-0.7722x^3-3.8479x^2-0.01235+0.000325 (best expanded with binomial theorem, saves a lot of work). I just looked for a value that had 5 zeroes after the decimal point before trying newton's method (for reference i started with x1 being 0.0078, going down in small increments). Finding f'(x), you use this method quite a few times till I guess you get a good enough approximation for our standards (i chose something pretty small when subbing into f(x)). I guess with this method it's up to your own discretion where you stop, but as for this, I'm not too sure. That's how I did it anyway, you may have a different better method!
Hope this helps :D
Post by: fun_jirachi on January 11, 2019, 12:29:05 pm
Good job! :D
Post by: david.wang28 on January 11, 2019, 12:39:15 pm
Well, should've thought of that one. Looking at that now, it seems pretty obvious :) (should really stop overthinking things)Thanks man! Usually chemistry questions shouldn't be too hard, or at least not to a 3u level. Though the knowledge you gain in 3u and 4u should be beneficial to u
Good job! :D
Post by: louisaaa01 on January 14, 2019, 10:09:52 am
The question is asking to write an equation for the reaction between propyl methanamine and hydrochloric acid and name all organic products.
I said the products would be N-chloromethanamine and propane (really more a guess than anything) but I wasn't sure whether this was right or not. What are the products of this reaction?
Post by: louisaaa01 on January 14, 2019, 10:13:00 am
Post by: janeaustin on January 14, 2019, 08:10:12 pm
For example, the reaction between phosphoric acid and sodium hydrogen carbonate:
H3PO4 (aq) + 3NaHCO3 (s) → Na3PO4 (aq) + 3CO2 (g) + 3H2O (l)
Why is NaHCO3 a solid? According to the solubility rules, all Group 1 metals (in this case, Na) are soluble, so isn't this carbonate meant to exist in aqueous form?
Post by: Jessthemess on January 14, 2019, 10:55:24 pm
analyse examples of non-equilibrium systems in terms of the effect of entropy and enthalpy, for example:
– combustion reactions
– photosynthesis
I thought combustion was a static equilibrium reaction rather than a non-equilibrium one.
Post by: myopic_owl22 on January 16, 2019, 03:54:25 pm
For Acid + Metal Carbonate reactions, is the state of the metal carbonate reactant always a solid?
For example, the reaction between phosphoric acid and sodium hydrogen carbonate:
H3PO4 (aq) + 3NaHCO3 (s) → Na3PO4 (aq) + 3CO2 (g) + 3H2O (l)
Why is NaHCO3 a solid? According to the solubility rules, all Group 1 metals (in this case, Na) are soluble, so isn't this carbonate meant to exist in aqueous form?
Here, sodium bicarbonate (NaHCO3) is added in its solid form - it can be in aqueous form if you add water and dissolve it. Solubility rules only apply when the ions inside dissociate into its ions in the presence of water!
In an exam or somewhere you're asked to write down the states of an equation, they should give you the states of the reactants as you wouldn't know otherwise. But as there's water involved (phosphoric acid is aqueous and therefore dilute), any soluble products will be aqueous to some degree (remember that precipitates can form as according to each compound's ksp values!)
hope this helps :)
Post by: janeaustin on January 16, 2019, 04:08:05 pm
For example, for the reaction FeCl2 (s) ⇌ Fe2+ (aq) + 2Cl- (aq)
Why is an equilibrium arrow used and not just a forward arrow? How do we know when to use which arrow?
Post by: janeaustin on January 16, 2019, 04:09:51 pm
Here, sodium bicarbonate (NaHCO3) is added in its solid form - it can be in aqueous form if you add water and dissolve it. Solubility rules only apply when the ions inside dissociate into its ions in the presence of water!
In an exam or somewhere you're asked to write down the states of an equation, they should give you the states of the reactants as you wouldn't know otherwise. But as there's water involved (phosphoric acid is aqueous and therefore dilute), any soluble products will be aqueous to some degree (remember that precipitates can form as according to each compound's ksp values!)
hope this helps :)
Yes this helped, thank you so much!
Post by: myopic_owl22 on January 16, 2019, 05:20:20 pm
When writing ionic equations for the dissociation of a compound, are we to always use a reversible arrow?
For example, for the reaction FeCl2 (s) ⇌ Fe2+ (aq) + 2Cl- (aq)
Why is an equilibrium arrow used and not just a forward arrow? How do we know when to use which arrow?
Depends on whether the reaction is reversible or not. If the reaction is reversible, the arrow goes both forwards and backwards. Reactions like combustion or many precipitation reactions will not be reversible, as the enthalpy/ entropy is too varied for ΔG to = 0 (which happens when a reaction is in equilirbium). I'm assuming that everything is in a closed system, by the way.
Sometimes, the single forwards arrow also denotes an equilibrium reaction that has gone to completion, but it isn't used too often. Let me explain what I mean...
Iron (II) chloride, like most compounds, are soluble to some degree. At one point, though, the solution will become saturated (no more water molecules can attach to the dissociated ions) and the stuff will precipitate as solid. How much you need to add in to achieve this effect is determined by each compunds' solubility/ ksp value. FeCl2 is pretty soluble given it's a halide salt. In a large amount of water, a small amount of it will be easily dissolved and the equation above will achieve completion. Of course, we can't guarantee that this situation will always be the case, so the double arrow covers all possibilities of amounts of water/ compound, etc. This also includes other factors like increasing the temperature. In short, all dissolution reactions will be reversible and should be written with a double arrow :)
Post by: myopic_owl22 on January 16, 2019, 05:33:14 pm
So, I thought this dot point was simple but now I'm confused.
analyse examples of non-equilibrium systems in terms of the effect of entropy and enthalpy, for example:
– combustion reactions
– photosynthesis
I thought combustion was a static equilibrium reaction rather than a non-equilibrium one.
Combustion really isn't in an equilibrium at all, since equilibrium reactions must inherently be reversible, and combustion releases too much energy (very low (i.e. negative) enthalpy and high entropy) to be reversible.
Static equilibrium exists in only reversible reactions, where the energy/ pressure requirements are too high to convert reactants to products. Therefore, the rates of each reaction are pretty much zero. The only example I've ever been taught is C(graphite) ⇌ C(diamond)
For the record, photosynthesis will also reach completion, but the higher activation energy required (as per endothermic reaction energy profiles) means it needs a catalyst - UV light.
A lot of people (myself included) will remember respiration to be the opposite of photosynthesis - respiration being a form of combustion. It's important to note that while the reactants and products appear inverted, these are two separate reactions and are not in equilibrium with each other. However, you can use this fact to deduce the relative entropy and enthalpy of each reaction - which is essentially what the dot-point is asking. Photosynthesis is endothermic (+ve enthalpy) reaction with a -ve entropy (given the large molecular size of its products), which is the opposite of all combustion reactions. In either case, ΔG will never be 0, so there'll be no equilibrium involved. Hope this answers your question! :)
Post by: david.wang28 on January 24, 2019, 04:03:26 pm
I have trouble with Question 3 in the link below. I have also posted my working out in the link below. Can anyone please help me with this question? Thanks :)
Post by: fun_jirachi on January 24, 2019, 05:13:37 pm
Your first step is perfect! You are using 0.01mol of phosphorus pentoxide. This also corresponds to 0.02mol of phosphoric acid by molar ratios. Subbing this into the molar ratio of the second equation, you need 0.06mol of sodium hydroxide. Using n=cV, sub in n=0.06, c=0.3 to get that v= 0.2. So the answer is C. Not quite sure where you got the 3 from. :)
Hope this helps :)
EDIT: in red
Post by: david.wang28 on January 24, 2019, 05:29:11 pm
Hey there!I checked the answer and it said C. Not sure why?
Your first step is perfect! You are using 0.01mol of phosphorus pentoxide. This also corresponds to 0.02mol of phosphoric acid by molar ratios. Subbing this into the molar ratio of the second equation, you need 0.06mol of sodium hydroxide. Using n=c/v, sub in n=0.06, c=0.3 to get that v= 5. So the answer is D. Not quite sure where you got the 3 from. :)
Hope this helps :)
Post by: sweetcheeks on January 24, 2019, 05:39:10 pm
Hey there!
Your first step is perfect! You are using 0.01mol of phosphorus pentoxide. This also corresponds to 0.02mol of phosphoric acid by molar ratios. Subbing this into the molar ratio of the second equation, you need 0.06mol of sodium hydroxide. Using n=c/v, sub in n=0.06, c=0.3 to get that v= 5. So the answer is D. Not quite sure where you got the 3 from. :)
Hope this helps :)
Great explanation. You've made a little error in the formula. It should be volume = moles/concentration, which would give 0.2 L rather than 5 L.
Post by: fun_jirachi on January 24, 2019, 05:44:19 pm
Post by: myopic_owl22 on January 28, 2019, 03:55:04 pm
"Explain why we cannot compare ksp values to determine the relative solubility between two compounds."
Is this because ksp is dimensionless? I always thought that we'd use ksp to judge if something was good at dissolving or not...
Thanks!
Post by: szetotess on January 28, 2019, 04:39:53 pm
i was wondering why insoluable and sparingly soluable salts reach equilibrium and not soluable salts??
:)
Post by: fun_jirachi on January 28, 2019, 04:47:30 pm
Hi there, a quick textbook question:
"Explain why we cannot compare ksp values to determine the relative solubility between two compounds."
Is this because ksp is dimensionless? I always thought that we'd use ksp to judge if something was good at dissolving or not...
Thanks!
You can actually use Ksp to compare the relative solubility between compounds, but if and only if they dissociate to produce the same ratio of ions. For example, you can compare the relative solubility of potassium chloride and sodium hydroxide, or even calcium carbonate, because they dissociate to produce the same ratio of ions as can be seen here:
KCl(aq)⇌K+(aq)+Cl-(aq)
NaOH(aq)⇌Na+(aq)+OH-(aq)
CaCO3(aq)⇌Ca2+(aq)+CO32-(aq)
Another example of this would be say magnesium hydroxide and calcium chloride:
Mg(OH)2(aq)⇌Mg2+(aq)+2OH-(aq)
CaCl2(aq)⇌Ca2+(aq)+2Cl-(aq)
What you can't do is compare two compounds that when ionised, form different ratios of ions. This is fundamentally because of the mathematical definition of Ksp, because for example in the compound AB⇌A++B-
If we now consider the compound AB2⇌A++2B-, our Ksp becomes
One of the reasons we can't is that when considering the units of the former Ksp, it is mol L-1, while for the latter it is mol 2 L-2, but also because you have an extra value when considering the solubility product. However, in addition to this, in the former case, when you're looking at the actual solubility, you get that Ksp=s2, while in the latter case you get that Ksp=4s3 simply because of ion ratios. Thus, even if they had really similar Ksp values, the actual solubility is heavily skewed. What you need to compare is not the Ksp, but the actual solubility s. Note that this doesn't occur with the compounds with the same ion ratios, since it'll just be s2 for both, or 4s3 or something or other. Hence, this is why you (usually) can't compare Ksp to compare relative solubility between two compounds.
Hope this helps :D
EDIT:
helloo,
i was wondering why insoluable and sparingly soluable salts reach equilibrium and not soluable salts??
:)
Hey :)
Soluble salts will not reach equilibrium generally because they will just dissolve completely and dissociate into their ions, hence leaving none of the compound. The system will just go to completion instead of equilibrium. The opposite is true for insoluble and sparingly soluble salts; they don't have as much compound dissociating, so they will be 'stuck' in equilibrium. To put it in terms of Ksp, we know that if Ksp is small, the substance is not as soluble, and if it is large, it is more soluble. eg. For a soluble salt, more ions will be produced at equilibrium, producing a more 'top-heavy' solubility product which is larger --> more soluble. In your case, this is when the substance is completely soluble, and there are no reactants left, leading to a Ksp value that tends to infinity. For an insoluble or sparingly soluble salt, the Ksp value will be zero or close to zero respectively, indicating that at equilibrium, the solubility product is 'bottom-heavy' with a lot more reactants, and thus that the salt is not as soluble, thus forming an equilibrium.
Hope this explanation makes sense :)
Post by: myopic_owl22 on January 29, 2019, 01:22:11 pm
Post by: mirakhiralla on January 31, 2019, 10:45:50 pm
Calculate the volume of CO2 needed to produce 100ml of saturated solution of carbon dioxide at 25 degrees. (assume that the solution contains 100g of water)
Post by: szetotess on February 06, 2019, 06:27:44 pm
i was wondering what the acid base reaction equation for
1. phosphoric acid + ammonia -->
2. acetic acid + sodium oxide -->
3. calcium oxide + ammonium nitrate -->
thanks :) :)
Post by: sweetcheeks on February 06, 2019, 06:39:22 pm
hi all,
i was wondering what the acid base reaction equation for
1. phosphoric acid + ammonia -->
2. acetic acid + sodium oxide -->
3. calcium oxide + ammonium nitrate -->
thanks :) :)
What are your thoughts? Look at the molecules, which one is acidic? which one is basic? What happens during an acid-base reaction?
Post by: szetotess on February 06, 2019, 06:51:04 pm
What are your thoughts? Look at the molecules, which one is acidic? which one is basic? What happens during an acid-base reaction?
in the answers the answer for the first one is (NH4)3.PO4
but is there not meant to be a water molecule if it is acid base
Post by: david.wang28 on February 07, 2019, 06:18:57 pm
I was wondering what to do for Q6 and 2.1 a). Can anyone please help me out with these questions? Thanks :)
Post by: sweetcheeks on February 07, 2019, 06:45:40 pm
in the answers the answer for the first one is (NH4)3.PO4
but is there not meant to be a water molecule if it is acid base
Not all acid-base reactions result in the formation of water. Ammonia, NH3, has a lone pair of electrons which is able to accept a proton from a donor acid, forming the ammonium cation, NH4+. Many negatively charged compounds (e.g. carboxylate anions) can react with acids, without there being the formation of water.
Hello,
I was wondering what to do for Q6 and 2.1 a). Can anyone please help me out with these questions? Thanks :)
For question 6, you are given the moles of the substance and the mass of the substance. What other information can gain? (hint, think Mw)
For 2.1, what are your difficulties? You are on the right track, I suggest drawing out the structure of the molecule and looking at where the electrons are located.
Post by: david.wang28 on February 07, 2019, 07:33:35 pm
Not all acid-base reactions result in the formation of water. Ammonia, NH3, has a lone pair of electrons which is able to accept a proton from a donor acid, forming the ammonium cation, NH4+. Many negatively charged compounds (e.g. carboxylate anions) can react with acids, without there being the formation of water.I know how to draw the structure of the molecule, it's just the fact that I don't know where the electrons are located.
For question 6, you are given the moles of the substance and the mass of the substance. What other information can gain? (hint, think Mw)
For 2.1, what are your difficulties? You are on the right track, I suggest drawing out the structure of the molecule and looking at where the electrons are located.
Post by: keltingmeith on February 07, 2019, 08:29:46 pm
I know how to draw the structure of the molecule, it's just the fact that I don't know where the electrons are located.
So think about fact that carbon needs four bonds, hydrogen can only have one bond, and go from there. Where can you add electrons to make this possible?
Post by: david.wang28 on February 07, 2019, 09:12:42 pm
So think about fact that carbon needs four bonds, hydrogen can only have one bond, and go from there. Where can you add electrons to make this possible?Ahhh, I get it now. Thanks for the help guys! :)
Post by: david.wang28 on February 08, 2019, 04:41:00 pm
I have more problems on chemistry in the links below. Can anyone help me with 2.2 b), al of 2.5, and 2.6 b) please? Thanks :)
Post by: Bri MT on February 08, 2019, 08:13:20 pm
Hello,
I have more problems on chemistry in the links below. Can anyone help me with 2.2 b), al of 2.5, and 2.6 b) please? Thanks :)
Remember m/M = n
You can find M using the periodic table, so if you have m you can then find n.
Then, using the equation you wrote out in a, you can use n of each reactant to find m for each reactant
Remember that you don't have to stick to single bonds between the carbons :)
Post by: david.wang28 on February 08, 2019, 08:18:47 pm
Remember m/M = nCan you help me with the other parts please?
You can find M using the periodic table, so if you have m you can then find n.
Then, using the equation you wrote out in a, you can use n of each reactant to find m for each reactant
Remember that you don't have to stick to single bonds between the carbons :)
Post by: Bri MT on February 09, 2019, 11:25:08 am
Can you help me with the other parts please?
Sorry! Completely missed the diagram based questions
I've got notes in the VCE chem section that might be easier to understand because they use diagrams but I'll try to explain here.
First you need to identify the longest chain, eg where you've identified an ethyl functional group that should actually be part of the parent chain. This means that there are 5 carbons (pent) in the parent chain and a methyl group on the 3rd carbon
When naming a cyclohexane, you start at the highest priority functional group and number around the ring in the direction that gives the lowest numbers for the functional groups.
For the bottom right diagram include the double bond in the parent chain & remember to indicate the number with the -ene
Have a shot at it and let us know if you want more help :)
Post by: david.wang28 on February 15, 2019, 04:28:34 pm
I'm stuck on a chem question (Q6), and this is my working out below. Can anyone please help me with that question? Thanks :)
Post by: fun_jirachi on February 15, 2019, 05:10:28 pm
Just as a side thing, lines given don't mean you have to fill them all out or write twice as much; it's a good indicator of how much you should be writing. The fact that you've gone and written up a whole page of mathematical calculations should ring a few warning bells (ie. it's all well and good, but am I doing something wrong here?). In addition, you're missing the key word in explain. You can calculate the pH like your life depends on it, but if you don't answer the question you could only get a maximum of 1 with maximum leniency. Most times, you'd get zero because you haven't answered the question. The while part of the question is also pretty important as it indicates that you need a comparison of some sort, but that's basically covered in the explain part.
What you need to be doing is writing two key equations:
HCl(aq)→H+(aq)+Cl-(aq)
CH3COOH(aq)⇌CH3COO-(aq)+H+(aq)
Note that the second equation uses an equilibrium arrow, while the first doesn't. Here you can also throw in your Ka values if you a) want to be super fancy or b) somehow remember them or c) both, but it's not completely necessary. Relate these two things if you have them, otherwise relate the equations to the strength of dissociation in water. Say that HCl is a strong acid, and (for our purposes) dissolves completely in water (ie. 1 mol L-1 of HCl will produce an equal amount of hydrogen ions, and then by subbing into the pH equation, gives you that pH of zero.) In contrast, acetic acid doesn't dissociate completely as a weak acid, and will not produce 1 mol of hydrogen ions, thus resulting in a corresponding higher pH. The hole here is the 2.6, but you can do the math if you really want to and prove it to them, but it's totally unnecessary, and in an exam situation, a total waste of time given time pressure.
Hope this helps :)
Post by: david.wang28 on February 15, 2019, 05:38:42 pm
First off, it's great to see you understand all these concepts and how to calculate pH! But that for the most part is irrelevant to the question.So what exactly do I do for the acetic acid part again?
Just as a side thing, lines given don't mean you have to fill them all out or write twice as much; it's a good indicator of how much you should be writing. The fact that you've gone and written up a whole page of mathematical calculations should ring a few warning bells (ie. it's all well and good, but am I doing something wrong here?). In addition, you're missing the key word in explain. You can calculate the pH like your life depends on it, but if you don't answer the question you could only get a maximum of 1 with maximum leniency. Most times, you'd get zero because you haven't answered the question. The while part of the question is also pretty important as it indicates that you need a comparison of some sort, but that's basically covered in the explain part.
What you need to be doing is writing two key equations:
HCl(aq)→H+(aq)+Cl-(aq)
CH3COOH(aq)⇌CH3COO-(aq)+H+(aq)
Note that the second equation uses an equilibrium arrow, while the first doesn't. Here you can also throw in your Ka values if you a) want to be super fancy or b) somehow remember them or c) both, but it's not completely necessary. Relate these two things if you have them, otherwise relate the equations to the strength of dissociation in water. Say that HCl is a strong acid, and (for our purposes) dissolves completely in water (ie. 1 mol L-1 of HCl will produce an equal amount of hydrogen ions, and then by subbing into the pH equation, gives you that pH of zero.) In contrast, acetic acid doesn't dissociate completely as a weak acid, and will not produce 1 mol of hydrogen ions, thus resulting in a corresponding higher pH. The hole here is the 2.6, but you can do the math if you really want to and prove it to them, but it's totally unnecessary, and in an exam situation, a total waste of time given time pressure.
Hope this helps :)
Post by: sav07 on February 23, 2019, 06:14:20 pm
How would dilution affect the equilibrium in a system? And what would be explanation be in terms of the collusion theory?
Post by: carlasilvia on February 26, 2019, 12:29:25 pm
Post by: Jefferson on March 01, 2019, 06:08:40 pm
Post by: r1ckworthy on March 01, 2019, 06:46:36 pm
HI GUYS ITS ME AGAIN!
How would dilution affect the equilibrium in a system? And what would be explanation be in terms of the collusion theory?
Dilution simply means you are decreasing the molarity/ concentration of a solution. So, if you think about an equilibrium system as a whole, the concentration of all reactants and products decreases when it is diluted, as more water is added. This would decrease the pressure, resulting in the system favouring the reaction with the most number of moles, in order to increase the pressure and minimise the change (le Chatelier's principle).
Now let's analyse this in terms of the collision theory. The collision theory states that particles must be colliding with sufficient kinetic energy and at correct orientation in order for a reaction to occur. When you dilute a solution, the particles will spread out as there is more volume. The pressure will obviously decrease, as there are less particles colliding as they have spread out. Le Chatelier's principle states that a system will try to minimise this change, so the system will favour the reaction will the most number of moles (as this will ensure the formation of the most number of particles), in order to increase the pressure/ amount of particles colliding with eachother. This is where equilibrium will shift.
I think the best way for you to understand would be through an example. Let's go through this pretend equation:
2A + 3B --> 4C + 3D ( the letters represent the molecules- I just made them up)
The solution is at equilibrium. We dilute the solution (add more water to increase volume), thereby decreasing the concentration of all reactants and products (as they have now spread out). The pressure of the system has decreased. The system, due to Le Chatelier's principle, will favour the reaction with the most number of moles, which is the forward reaction. Equilibrium will shift to the right, as more product particles will be made to increase the pressure).
Hopefully that made sense. If not, reply on what part confuses you the most, and I or someone else will try to help out.
Post by: david.wang28 on March 01, 2019, 07:09:24 pm
Why is the answer (B) in this question (from Nesa).The reason why the answer is B is because the OH ion is a base, meaning H is a conjugate acid. CO2 is an acid, meaning HCO3 is a conjugate base. Hence, this is a Bronsted-Lowry reaction. (Please note: I cannot guarantee this is the correct answer, but I gave it a try!) :)
Post by: Jefferson on March 01, 2019, 08:12:09 pm
The reason why the answer is B is because the OH ion is a base, meaning H is a conjugate acid. CO2 is an acid, meaning HCO3 is a conjugate base. Hence, this is a Bronsted-Lowry reaction. (Please note: I cannot guarantee this is the correct answer, but I gave it a try!) :)yeah, but that doesn't quite eliminate A and D.
Post by: sarrahbarodawala on March 02, 2019, 01:41:46 pm
I was working through the chemistry topic test book and got confused on how to answer this question about combustion reactions. The question is
"Combustion reactions are typically considered to be irreversible reaction. To what extent is this conception a genuine description of the system, as opposed to simply a useful way of thinking about the reactions?"
I understood why they aren't reversible, but the answer at the back said something about the increase in entropy, which i didn't get. Someone please explain how they are related.
Thank you :)
Post by: annabeljxde on March 02, 2019, 03:45:28 pm
Hello!
I was working through the chemistry topic test book and got confused on how to answer this question about combustion reactions. The question is
"Combustion reactions are typically considered to be irreversible reaction. To what extent is this conception a genuine description of the system, as opposed to simply a useful way of thinking about the reactions?"
I understood why they aren't reversible, but the answer at the back said something about the increase in entropy, which i didn't get. Someone please explain how they are related.
Thank you :)
Recall what entropy means. Entropy is the degree of disorder in a system and generally refers to the number of possible arrangements. Entropy tends to increase in reactions where:
- number of particles increase
- number of gaseous particles increase
Combustion reactions are always spontaneous, meaning they do not require a continuing supply of energy for the reaction to occur. Recall also that the symbol used to represent this fact is ΔG < 0, or ΔH - TΔS < 0. Since combustion reactions are exothermic, there is a release of energy and therefore, ΔS of the surroundings increase (release of gaseous particles increases the number of possible arrangements). Putting this into the equation:
ΔH = negative
ΔS = positive
ΔH - TΔS --> (negative) - (positive) which will always be negative. Therefore ΔG < 0 and the reaction is spontaneous.
ΔS = positive
ΔH - TΔS --> (negative) - (positive) which will always be negative. Therefore ΔG < 0 and the reaction is spontaneous.
The question asks you to discuss whether the fact that knowing combustion reactions are irreversible is a sufficient description of the nature of the system, and in short, it is. The high activation energy of the reverse reaction (since combustion reactions are exothermic, therefore, Ea of reverse is greater than the forward reaction) reduces the chance that the products will recombine to form reactants. And since they occur in an open system, the products cannot recombine and therefore an equilibrium is impossible. (Here you can talk about entropy)
I hope this helps :) (I'm also answering you to help myself as well!)
Post by: sarrahbarodawala on March 02, 2019, 04:04:18 pm
Recall what entropy means. Entropy is the degree of disorder in a system and generally refers to the number of possible arrangements. Entropy tends to increase in reactions where:
- number of particles increase
- number of gaseous particles increase
Combustion reactions are always spontaneous, meaning they do not require a continuing supply of energy for the reaction to occur. Recall also that the symbol used to represent this fact is ΔG < 0, or ΔH - TΔS < 0. Since combustion reactions are exothermic, there is a release of energy and therefore, ΔS of the surroundings increase (release of gaseous particles increases the number of possible arrangements). Putting this into the equation:ΔH = negative
ΔS = positive
ΔH - TΔS --> (negative) - (positive) which will always be negative. Therefore ΔG < 0 and the reaction is spontaneous.
The question asks you to discuss whether the fact that knowing combustion reactions are irreversible is a sufficient description of the nature of the system, and in short, it is. The high activation energy of the reverse reaction (since combustion reactions are exothermic, therefore, Ea of reverse is greater than the forward reaction) reduces the chance that the products will recombine to form reactants. And since they occur in an open system, the products cannot recombine and therefore an equilibrium is impossible. (Here you can talk about entropy)
I hope this helps :) (I'm also answering you to help myself as well!)
Thank you very much!!!!
Post by: annabeljxde on March 02, 2019, 05:42:32 pm
I'm super confused about what refers to what in statements like: strong acid titrated against a weak base. Is the strong acid the aliquot and the weak base the standardised solution? Or is it the other way around?
How do you know which is which when you are given questions like this in an exam?
Post by: milie10 on March 13, 2019, 09:33:30 pm
I'm currently doing my year11 chemistry depth study, and I'm struggling with ideas- our teachers say it has to be more complicated than a year 8-10 experiment, but we have to be able to buy the chemicals ourselves. It's based on consumer chemistry. Any thoughts on what I could do?
Thanks!! :D
Post by: myopic_owl22 on March 16, 2019, 02:58:43 pm
Hey!
I'm super confused about what refers to what in statements like: strong acid titrated against a weak base. Is the strong acid the aliquot and the weak base the standardised solution? Or is it the other way around?
How do you know which is which when you are given questions like this in an exam?
Hello,
I've always considered the 'against' as to mean 'with reference to'. You're right about the acid being the aliquot (or analyte or titrand - whatever's in the conical flask) and the base being the standardised solution for the burette. If you like, you can remember that if A is titrated against B, B is the burette and A is the analyte.
However, for most calculation questions you'd be able to tell which one goes where as, typically, you'd be given a known concentration for whatever is in the burette (standardised solution) and the analyte will just have a volume. If you're feeling brave, though, it is possible to switch them around (i.e. put the analyte in the burette and standard solution in the flask) and still get the right answer.
Hope this helps!
Post by: david.wang28 on March 18, 2019, 05:50:23 pm
I have trouble with question 2.6 and 2.1 in the link below. Can anyone please help me out? Thanks :)
Post by: InnererSchweinehund on March 26, 2019, 10:21:35 am
Fe3+(aq) + SCN–(aq) ⇋ FeSCN2+(aq)
1. Why do you think the equilibrium constant remained virtually constant, within experimental error, even though you were changing the concentrations?
2. Beer's Law (the linear relationship between concentration and absorbance) is accurate to about A = 1.5. How would you modify the experiment if the absorbance readings were higher than 1.5?
Finally, if someone could explain how to write a conclusion for the a practical investigation, that would be great!!
Thanks!!
Post by: e2503 on March 27, 2019, 11:49:44 pm
I was just wondering whether enthalpy has a significant effect on the thermal stability of compounds ( specifically metal CO3 ). Ultimately, I'm trying to figure out whether the thermal decomposition temperatures of metal carbonates are affected by enthalpy but I am a little lost. :'( If anyone could send some help, it would be very much appreciated. Thank you in advance :)
Post by: e2503 on March 28, 2019, 12:07:28 am
Hi! I was wondering if anyone could help me with the following questions, based off a student investigation to determining the Equilibrium Constant using Colorimetry.
Fe3+(aq) + SCN–(aq) ⇋ FeSCN2+(aq)
1. Why do you think the equilibrium constant remained virtually constant, within experimental error, even though you were changing the concentrations?
2. Beer's Law (the linear relationship between concentration and absorbance) is accurate to about A = 1.5. How would you modify the experiment if the absorbance readings were higher than 1.5?
Finally, if someone could explain how to write a conclusion for the a practical investigation, that would be great!!
Thanks!!
Hi there :)
The conclusion of your practical investigation should consist of a few sentences which summarise your experiment. You should re-address your hypothesis to either accept or reject it providing clear reasons based on the implications made from your major findings. The key is to keep the conclusion concise. I hope this was helpful :D :D
Post by: InnererSchweinehund on March 28, 2019, 08:01:31 am
Hi there :)
The conclusion of your practical investigation should consist of a few sentences which summarise your experiment. You should re-address your hypothesis to either accept or reject it providing clear reasons based on the implications made from your major findings. The key is to keep the conclusion concise. I hope this was helpful :D :D
Thank you!!!
Post by: maxinenicole on April 04, 2019, 06:22:11 pm
NaOH solution has a pH of 12.0. a 60.00mL volume of NaOH solution is added to 4.0 x 10-3 mol in HCl in water. What volume of 0.200mol L-1 KOH must be added to bring the pH of the solution to 7.0?
Thanks
Post by: InnererSchweinehund on April 11, 2019, 09:09:43 am
I was wondering if someone could tell me if my explanation to this question is correct, and if not, could you please explain the concept??
Q: Beer's Law (the linear relationship between concentration and absorbance) is accurate to about A = 1.5. How would you modify the experiment if the
absorbance readings were higher than 1.5?
A: According to Beer’s Law, if absorbance readings are higher than 1.5 they are usually inaccurate, as it means that most of the light was absorbed by the sample, and only a small amount of light was detected by the detector. When this occurs, the experiment should be modified by diluting each of the samples equally, usually by a factor of 10. This will enable the new concentrations to be accurately re-calculated, and can be repeated until the absorbance measured produces more accurate readings. It is important that the final concentrations and their measured absorbance have a linear relationship in order for the equilibrium constant to be calculated.
Thanks!!
Post by: avocadinq on April 15, 2019, 06:39:15 pm
What is the difference between primary, secondary and tertiary alcohols (organic chemistry)? Thanks
Post by: Rom_Dog on April 15, 2019, 06:55:40 pm
Structurally there is a clear difference between primary, secondary and tertiary alcohols. The carbon atom attached to the hydroxyl (-OH) group of a primary alcohol is only attached to one carbon atom, in secondary alcohols this carbon is attached to 2 other carbon atoms and in tertiary alcohols this carbon is attached to 3 other carbon atoms.
Physically there are also differences. Primary, secondary and tertiary alcohols with the same molecular formula have different melting and boiling points, this is because their structure has an effect on the intermolecular bonds between alcohol molecules.
There is also a chemical difference. When oxidised primary alcohols form carboxylic acids (with an aldehyde as an intermediate state), secondary alcohols are oxidised into ketones and tertiary alcohols cannot be oxidised!
These are the keys differences that you need to know for VCE chemistry, I'm not sure which of these points are relevant for HSC chem but I hope that this helps!
- Rom ;D
Post by: 006896 on April 23, 2019, 09:35:25 am
Could you help confirm the following statements, regarding equilibrium reactions.
If adding an inert gas to a system while keeping the volume the same, no change occurs.
- How would you keep the volume the same while adding an inert gas? Does it mean the volume of the reacting gases, or the volume of the system?
If adding an inert gas to a system while keeping the pressure the same, the volume must increase and the equilibrium shifts to the side with more gas volume.
- I don't really understand this statement.
Thanks
Post by: emmajb37 on April 23, 2019, 12:30:52 pm
I am really confused because the answers say that potassium chloride is soluble because of the PMS. But I thought the P was for lead.
As well as that potassium is a group 1 ion which is always soluble. Just really confused and would love some clarification.
Thanks,
Emma
Post by: emmajb37 on April 23, 2019, 12:33:38 pm
Hey, I was just doing Question 7, Section2 of the Solution Equilibria Test 1 in the new chemistry topic test book.And also this is just part a) and the rest of the question parts b) and c) continue to use KCl as a solid.
I am really confused because the answers say that potassium chloride is soluble because of the PMS. But I thought the P was for lead.
As well as that potassium is a group 1 ion which is always soluble. Just really confused and would love some clarification.
Thanks,
Emma
Post by: iktimal on April 24, 2019, 09:50:39 pm
Ksp= 1.41 x 10^-12
Thanks!
Post by: myopic_owl22 on April 25, 2019, 06:06:14 pm
Hi,
Could you help confirm the following statements, regarding equilibrium reactions.
If adding an inert gas to a system while keeping the volume the same, no change occurs.
- How would you keep the volume the same while adding an inert gas? Does it mean the volume of the reacting gases, or the volume of the system?
If adding an inert gas to a system while keeping the pressure the same, the volume must increase and the equilibrium shifts to the side with more gas volume.
- I don't really understand this statement.
Thanks
Hey there,
I know it's a little old, but this thread explains the issue of inert gases much better than I ever could (it's to do with the gas not affecting the partial pressures of the reagent gases): https://atarnotes.com/forum/index.php?topic=7350.0
And here's another site which addresses your questions specifically: https://chemistry.stackexchange.com/questions/18567/what-would-be-the-effect-of-the-addition-of-an-inert-gas-to-a-reaction-at-equili
For your statements:
1) is correct. Adding the inert gas won't change the number of moles of the reagents of the vessel, and therefore nothing happens to the equilibrium. Assuming the reaction vessel is rigid, there won't be any change in volume anyway. If you meant to say pressure, then yes, the total pressure of the system does change, but we're looking at partial pressures so it's irrelevant.
2) From the stackexchange website, the equilibrium position will change if the volume changes when adding an inert gas (which will happen if the pressure is kept constant - Boyle's law) - in the specific case where the reaction vessel is not rigid - e.g. a balloon. Increasing the volume of the system will affect the partial pressures of the reagents - they will decrease as the inert gas is added (as there's more volume for the same moles of reagent) and thus the equilibrium will shift to increase the pressure of the vessel by increasing the moles it contains.
Hopefully this was a little useful! Might I add, this whole concept is really weird - it's one of those things you just accept and move on, I guess ;)
Post by: myopic_owl22 on April 25, 2019, 06:40:56 pm
Hey, Can someone please explain to me how to do part D.
Ksp= 1.41 x 10^-12
Thanks!
Hey there,
There are a few steps involved, but hopefully I'm making some sense!
1. Write an equation (as always)
Note the molar ratios - the OH has 2.
2. Write a Ksp expression:
That's the solubility product, considering the OH's molar ratio. We don't divide by the magnesium hydroxide as that's a solid when it isn't dissolved.
3. Replace the concentration of Mg2+ with x. We know this to be the same concentration as that of magnesium hydroxide (what we're trying to figure out).
4. Solving for x gives: 7.06 *10-5M. This is the molar solubility of Mg(OH)2 - the amount of moles that can be dissolved in a litre of water.
An aside: regular solubility is measured in g/L. To convert, just multiply 7.06 *10-5 by the molar mass of Mg(OH)2
Post by: mani.s_ on April 26, 2019, 09:59:58 am
Thanks :)
Post by: sweetiepi on April 26, 2019, 10:25:18 am
I just had a question for the chemistry notes, why is Carbon 12 considered a isotope when it is the normal/natural form of carbon, having 6 protons, 6 neutrons and 6 electrons.Isotopes are forms of the same element- they have the same number of protons, but each isotope has a different number of neutrons. Let's look at Carbon-12 and Carbon-14. They both have 6 protons. However, Carbon-12 has 6 neutrons whilst Carbon-14 has 8 neutrons. By definition this means that Carbon-12 and Carbon-14 are both isotopes of the carbon atom.
Thanks :)
Carbon-12 is the most abundant isotope of carbon, whereas Carbon-13 and Carbon-14 are less abundant. :)
Post by: mani.s_ on April 26, 2019, 11:09:53 am
Isotopes are forms of the same element- they have the same number of protons, but each isotope has a different number of neutrons. Let's look at Carbon-12 and Carbon-14. They both have 6 protons. However, Carbon-12 has 6 neutrons whilst Carbon-14 has 8 neutrons. By definition this means that Carbon-12 and Carbon-14 are both isotopes of the carbon atom.Does that mean every element is an isotope of that element???
Carbon-12 is the most abundant isotope of carbon, whereas Carbon-13 and Carbon-14 are less abundant. :)
Post by: myopic_owl22 on April 27, 2019, 02:12:48 pm
Does that mean every element is an isotope of that element???
Sure does :)
-----------------------------------------------------------------------------------
Also, I've got a question from the NESA sample paper: Can someone explain why the answer would be C?
(https://i.ibb.co/S0nmrZT/Capture.png)
Thanks a lot!
Post by: Bookdragon_202 on May 08, 2019, 10:26:28 am
Post by: stella_atarnotes on May 08, 2019, 06:24:24 pm
I have a titration assessment coming up and am feeling a little overwhelmed. What is the best way to study for this assessment? It is a practical with some theory questions.
Hey! With titrations, just understand what solutions you are putting into the burette, pipette, volumetric flask and know the common indicators (e.g. phenolphthalein) and the colour change. Know what to wash your instruments with really well. Also work on some practice titration questions from textbooks or past exams to make sure you know how to write up your results. Also try to know some sources of errors (e.g. washing the burette with water will dilute the solution you put into it) Other than that, just keep calm and trust your knowledge. You'll do fine!
Post by: stella_atarnotes on May 20, 2019, 07:10:10 pm
Sure does :)
-----------------------------------------------------------------------------------
Also, I've got a question from the NESA sample paper: Can someone explain why the answer would be C?
(https://i.ibb.co/S0nmrZT/Capture.png)
Thanks a lot!
You can eliminate options A and B because these are acids and have a relatively low pH. With C and D, they have used the acids given to you in the question stem and formed a salt with it. Remember that weak acids and weak bases combine to form a neutral salt, but if you have weak acids combining with strong bases (NaOH in this case), the salt will be slightly alkalinic. Since Nitrous acid already has a higher pKa, the salt formed when it is combined with a strong base will be more alkalinic than a salt formed with chlorous acid. Thus it is C
Post by: david.wang28 on May 31, 2019, 04:11:53 pm
I am stuck on two questions, 2.1 and 2.2. Can anyone help me out please? Thanks :)
Post by: mirakhiralla on June 09, 2019, 08:39:52 pm
Dumb q but,
do different types of detergents behave differently in hard water?
Post by: horse9996 on June 10, 2019, 02:36:23 pm
Dumb q but,
do different types of detergents behave differently in hard water?
No. All detergents behave the same with a head and a hydrocarbon tail which allows it to interact with water and grease.
Post by: annabeljxde on June 12, 2019, 08:32:15 pm
Could anyone briefly explain why longer hydrocarbons have a tendency to burn in incomplete combustion?
Post by: r1ckworthy on June 12, 2019, 11:48:07 pm
Hi!Edit: There is a limited amount of oxygen (20% in atmosphere) in the air, which will not react with ALL the hydrocarbons in the long hydrocarbon chain. I'll keep my answer up as it is kind of relevant, but don't rely too much on it. Incomplete combustion will always occur when there is an inefficient amount of oxygen. I thought it was too simple at first, but just confirmed it with my teacher ;D
Could anyone briefly explain why longer hydrocarbons have a tendency to burn in incomplete combustion?
Okay I'm gonna take a shot at explaining this. I got my information from this page.
Combustion is essentially the process of gaseous oxygen reacting with the surface of the hydrocarbon. So if a hydrocarbon is easily volatile (turn into gas), the oxygen will react with the surface (which turns into gas) and then with the next molecule (which turns into gas) and then the next molecule (which turns into gas) and so on with ease. This occurs for smaller hydrocarbons that are easy volatile.
For bigger hydrocarbons however, their volatility decreases (accumulation of dispersion forces increases strength/ bonding), and so it will be much harder for oxygen to "grab" the surface molecule and then the next molecule and so on. Oxygen will react much more readily with gaseous hydrocarbons and get used up. But because of the increased strength of the longer hydrocarbon, the oxygen will react only with the surface molecules, resulting in incomplete combustion.
Super dodgy explanation, but that is kind of the whole premise. I'll probably edit my answer tomorrow and add some more stuff to make it more clear.
Post by: annabeljxde on June 13, 2019, 05:12:28 pm
Edit: There is a limited amount of oxygen (20% in atmosphere) in the air, which will not react with ALL the hydrocarbons in the long hydrocarbon chain. I'll keep my answer up as it is kind of relevant, but don't rely too much on it. Incomplete combustion will always occur when there is an inefficient amount of oxygen. I thought it was too simple at first, but just confirmed it with my teacher ;D
Okay I'm gonna take a shot at explaining this. I got my information from this page.
Combustion is essentially the process of gaseous oxygen reacting with the surface of the hydrocarbon. So if a hydrocarbon is easily volatile (turn into gas), the oxygen will react with the surface (which turns into gas) and then with the next molecule (which turns into gas) and then the next molecule (which turns into gas) and so on with ease. This occurs for smaller hydrocarbons that are easy volatile.
For bigger hydrocarbons however, their volatility decreases (accumulation of dispersion forces increases strength/ bonding), and so it will be much harder for oxygen to "grab" the surface molecule and then the next molecule and so on. Oxygen will react much more readily with gaseous hydrocarbons and get used up. But because of the increased strength of the longer hydrocarbon, the oxygen will react only with the surface molecules, resulting in incomplete combustion.
Super dodgy explanation, but that is kind of the whole premise. I'll probably edit my answer tomorrow and add some more stuff to make it more clear.
This is such an amazing explanation, I can't thank you enough!!! Seriously thank you thank you! ヽ(•‿•)ノ
Ahhh, I see... that explains why my teacher told us to justify why assuming complete combustion for longer hydrocarbons is invalid..
My assessment for chemistry is due today and again, I can't stress how much you've saved me!
Post by: annabeljxde on June 13, 2019, 05:14:59 pm
Hi everyone!
Can anyone please explain to me how you would find the amount of CO2 produced from complete combustion in terms of energy production (i.e. in kJ/mol). My teacher told me to use the energy output of the hydrocarbon (e.g. 5470kJ/mol) to work this out but I'm a little stuck..
Post by: fun_jirachi on June 13, 2019, 09:41:10 pm
C3H8(aq) + 5O2(g) --> 3CO2(g) + 4H2O(g) enthalpy = -2220.0 kJ mol-1
For every mole of propane burnt, you 'expel' -2220.0 kJ of energy.
For every mole of CO2 produced, you get -740.0 kJ of energy.
So say some experiment or whatever 'expelled' 14712 kJ of energy. Then, 14712/740 moles of CO2 would be produced, and hence you have the amount of CO2 produced in the chemical reaction. You need the secondary value, otherwise, you have nothing to work off :)
Hope this helps :)
Post by: emmawazza on July 24, 2019, 04:02:21 pm
Just have a question about Module 7.
In addition reactions of alkynes, do you express it as two equations, or one?
e.g. for halogenation, do you do equations:
(1) alkyne -> di-halogenated alkene
(2) di-halogenated alkene -> tetrahalogenated alkane
or jump straight from alkyne -> alkane in 1 equation?
Post by: fun_jirachi on July 24, 2019, 05:16:34 pm
Hey Hey!
Just have a question about Module 7.
In addition reactions of alkynes, do you express it as two equations, or one?
e.g. for halogenation, do you do equations:
(1) alkyne -> di-halogenated alkene
(2) di-halogenated alkene -> tetrahalogenated alkane
or jump straight from alkyne -> alkane in 1 equation?
Hey there!
I think that you should definitely show the steps (just to show the marker where you're going, even though it's pretty obvious). I'd say it's a similar thing to skipping steps in maths; do it at your own peril/up to your own discretion. In this case, it shouldn't be too much work to add in one extra line, just to show your superior understanding :)
Hope this helps :)
Post by: classof2019 on July 30, 2019, 10:17:32 pm
Post by: fun_jirachi on July 31, 2019, 07:21:33 pm
Post by: classof2019 on July 31, 2019, 07:37:31 pm
Upon searching for the question online (for anyone else searching for the question, it's from the NESA Sample Chemistry Questions), the reason the answer is B is because you've got the question wrong! The question should actually be 'Which of the following is NOT a Bronsted-Lowry reaction?' in which case the answer is in fact B.
Ha, very strange! The question was screenshotted directly from the NESA document entitled "Chemistry Additional Sample Examination Questions". They must have made a typo!
Thank you for clarifying.
Post by: 006896 on August 06, 2019, 02:41:48 pm
Could someone help with this attached question?
Thanks
Post by: ^^^111^^^ on August 06, 2019, 02:49:01 pm
Hi!Use the Henderson-hasselbalch equation . It is given like this :
Could someone help with this attached question?
Thanks
pH = pKA + log base 10 of (conjugate base concentration/ conjugate acid concentration).
Since we have the pH, manipulate the equation to find the concentration of acetic acid and ion.
Correct me if I am wrong.
Post by: SynX on August 06, 2019, 03:45:08 pm
Use the Henderson-hasselbalch equation . It is given like this :Wow, that’s great! I just have a hard time memorizing “Henderson-hasselbalch equation” all I know about this is the pH=-log(aH+), by doing some research I found out that pKa=-log(Ka), which is really similar to the pH equation, as well as pOH equation. But the definition of Ka is “where "Ka" is the “equilibrium constant for the ionization of the acid”. Seems the pH and pOH value of a substance adds up to a constant around 14. Just learning these at school. But the concept of pKa is confusing!
pH = pKA + log base 10 of (conjugate base concentration/ conjugate acid concentration).
Since we have the pH, manipulate the equation to find the concentration of acetic acid and ion.
Correct me if I am wrong.
Plus, I just figured out the function of the log button by playing the calculator a few weeks ago...
Post by: horse9996 on August 12, 2019, 06:18:53 pm
Wow, that’s great! I just have a hard time memorizing “Henderson-hasselbalch equation” all I know about this is the pH=-log(aH+), by doing some research I found out that pKa=-log(Ka), which is really similar to the pH equation, as well as pOH equation. But the definition of Ka is “where "Ka" is the “equilibrium constant for the ionization of the acid”. Seems the pH and pOH value of a substance adds up to a constant around 14. Just learning these at school. But the concept of pKa is confusing!
Plus, I just figured out the function of the log button by playing the calculator a few weeks ago...
Ka is literally just any old equilibrium constant, the a just means that its for an acid. pKa is the same as pH, except with Ka rather than [H+]. This is to make values easier to work with and more comparable. Also, pKa + pKb = 14 (pKb is the same thing but the dissociation of a weak base rather than acid - denoted by b rather than a). To remember the Henderson-Hasselbalch equation, remember 'aha' - since the log is [A-]/[HA] = [base]/[acid]
Post by: louisaaa01 on September 04, 2019, 02:56:58 pm
For 17 - I've just assumed that the acetic acid completely dissociates. I know that isn't the case, but they haven't given any Ka values - are we expected to just know Ka for acetic acid? (But even if it only partially dissociates, shouldn't the mass required be greater than what I've calculated?)
Thank you.
Post by: horse9996 on September 08, 2019, 03:27:16 pm
Can someone please help out with these questions?
For 17 - I've just assumed that the acetic acid completely dissociates. I know that isn't the case, but they haven't given any Ka values - are we expected to just know Ka for acetic acid? (But even if it only partially dissociates, shouldn't the mass required be greater than what I've calculated?)
Thank you.
For 17 you need the Ka value - I would assume it should be given to you? Check the data sheet. Form a RICE table. The equilibrium concentration of H+ ions can be determined from the pH and then let the initial concentration of acetic acid be x. By using the equilibrium constant expression and the Ka value, you should be able to calculate x and therefore the mass of acetic acid needed.
For 18 it is as follows:
- Not A - cellulose is a polymer which requires glucose as its monomer so this doesn't make sense as coming from ethylene
- Not C - glucose can't be formed from ethylene in 1 step as far as I know
- Not D - ethanol can't undergo polymerisation and doesn't form any polymers
- B - styrene can be formed from ethylene (one of the hydrogens is replaced with a benzene) and this undergoes addition polymerisation to form a polymer (polystyrene)
Hence the answer must be B.
I hope that makes sense
Post by: Katie-E on September 13, 2019, 08:51:41 pm
Does anyone know of any tutors in the Bankstown/ 2200 postcode area who are available for some year 12 HSC Chemistry tutoring?
Thanks sooooooooooo much in advance ;)
Post by: DamnDhruv on October 11, 2019, 07:55:31 pm
here is the image for reference:
Post by: Erutepa on October 11, 2019, 08:14:16 pm
hello I have the atar notes topic notes ook for chemistry wherein i found out that they wrote potassium chloride is not soluble. Now is this a genuine mistake or am just missing something?Potassium chloride is definitely soluble. Unfortunately, you will come across the odd mistake/typo in pretty much all resources from textbooks to study notes like these, so just be wary.
here is the image for reference:
Post by: DamnDhruv on October 11, 2019, 11:52:25 pm
Wow, that’s great! I just have a hard time memorizing “Henderson-hasselbalch equation” all I know about this is the pH=-log(aH+), by doing some research I found out that pKa=-log(Ka), which is really similar to the pH equation, as well as pOH equation. But the definition of Ka is “where "Ka" is the “equilibrium constant for the ionization of the acid”. Seems the pH and pOH value of a substance adds up to a constant around 14. Just learning these at school. But the concept of pKa is confusing!
Plus, I just figured out the function of the log button by playing the calculator a few weeks ago...
its on the fomula sheet i think
Post by: RACHEL1111 on October 29, 2019, 09:35:44 am
when the answer says ' sextet' , I understand that it refers there is carbon next door attaching 5 proton. But as far as I can see, there isn't any such carbon.
Also, I am a bit confused about N+1 rule because the next door carbon could refer to either left or right? So, which side is it?
Thanks in advance!
Post by: louisaaa01 on October 29, 2019, 09:50:17 am
Hi, Could anyone help me with this questions, question 8 part b.
when the answer says ' sextet' , I understand that it refers there is carbon next door attaching 5 proton. But as far as I can see, there isn't any such carbon.
Also, I am a bit confused about N+1 rule because the next door carbon could refer to either left or right? So, which side is it?
Thanks in advance!
Hi!
Since carbon forms a maximum of four bonds (as it is tetravalent), a single carbon actually can't bond with five protons. The N+1 rule refers to the carbons on both sides of the carbon you're thinking of.
For instance, if we consider the diagram on the left, the second carbon from the left is a CH2 group, bonded to a CH3 group on the left and a CH2 group on the right. An arrangement like this will produce a sextet in the H-NMR spectrum - there is a carbon atom attached to two carbon atoms with five protons in total that are directly attached, so using the n+1 rule, there is a sextet.
The other sextet from the diagram on the left, I think, comes from the carbon attached to the hydroxyl group. Note that it is also attached to a CH3 and a CH2 group (five identical hydrogens on either side), so a sextet is present.
When we do the N+1 rule, it's also worth noting that a hydroxyl group will show a singlet (it is not attached to an atom directly attached to any other hydrogens).
Also, when using the N+1 rule on adjacent hydrogens, if an -OH group is present on the adjacent carbon, you don't add this. For instance, if we consider the third carbon on the left diagram, we see that it is attached to a CH2 group, and a carbon attached to a H and an OH. In this case, we look only at the hydrogens directly attached to adjacent carbons - so the third carbon in this diagram is attached to a carbon with 2 hydrogens directly attached and another carbon with only one hydrogen attached (3 in total). This produces a quartet by the N+1 rule.
Similar logic is applied to the structural formula on the right.
I hope this helps! If you have any more questions, don't hesitate to ask.
Post by: RACHEL1111 on October 29, 2019, 02:49:03 pm
Hi!
Since carbon forms a maximum of four bonds (as it is tetravalent), a single carbon actually can't bond with five protons. The N+1 rule refers to the carbons on both sides of the carbon you're thinking of.
For instance, if we consider the diagram on the left, the second carbon from the left is a CH2 group, bonded to a CH3 group on the left and a CH2 group on the right. An arrangement like this will produce a sextet in the H-NMR spectrum - there is a carbon atom attached to two carbon atoms with five protons in total that are directly attached, so using the n+1 rule, there is a sextet.
The other sextet from the diagram on the left, I think, comes from the carbon attached to the hydroxyl group. Note that it is also attached to a CH3 and a CH2 group (five identical hydrogens on either side), so a sextet is present.
When we do the N+1 rule, it's also worth noting that a hydroxyl group will show a singlet (it is not attached to an atom directly attached to any other hydrogens).
Also, when using the N+1 rule on adjacent hydrogens, if an -OH group is present on the adjacent carbon, you don't add this. For instance, if we consider the third carbon on the left diagram, we see that it is attached to a CH2 group, and a carbon attached to a H and an OH. In this case, we look only at the hydrogens directly attached to adjacent carbons - so the third carbon in this diagram is attached to a carbon with 2 hydrogens directly attached and another carbon with only one hydrogen attached (3 in total). This produces a quartet by the N+1 rule.
Similar logic is applied to the structural formula on the right.
I hope this helps! If you have any more questions, don't hesitate to ask.
That's very helpful! Thank you very much!
Post by: RACHEL1111 on October 29, 2019, 03:02:19 pm
I lose track of the second part of solution.
1. For the highly shifted (around 4.2ppm) peak which represents those two functional group: is because that they are unshielded that they have high ppm?
2. And I am completely confused from the sentence: the second group will experience a higher peak... For example: what does height of the peak mean? what does alkyl portion mean? After all, please help me with the way to eliminate option 2.
Thanks in advance!
Post by: classof2019 on October 29, 2019, 04:17:38 pm
A 25.0mL sample of acetic acid has a pH of 1.30. It is mixed with 20.0mL of 0.130mol/L calcium hydroxide. What is the pH of this mixture?
A. 2.4
B. 2.9
C. 11.1
D. 11.6
I'm consistently getting a pH of about 12.9, which isn't even one of the options! So I'm not sure whether I'm making a mistake or the question itself is flawed. Any assistance is greatly appreciated. Cheers.
Post by: InnererSchweinehund on October 30, 2019, 08:29:25 am
Hi, can someone please help answer this?
A 25.0mL sample of acetic acid has a pH of 1.30. It is mixed with 20.0mL of 0.130mol/L calcium hydroxide. What is the pH of this mixture?
A. 2.4
B. 2.9
C. 11.1
D. 11.6
I'm consistently getting a pH of about 12.9, which isn't even one of the options! So I'm not sure whether I'm making a mistake or the question itself is flawed. Any assistance is greatly appreciated. Cheers.
Hi there!
I did the question (working below) and got the answer of pH = 11.1
I'm not 100% sure if this is correct so if someone could check it and let me (and classof2019) know, that would be great!
Thanks!
:)
(ps. Sorry if the image quality is bad!)
Post by: RACHEL1111 on October 30, 2019, 08:47:50 am
Hi there!
I did the question (working below) and got the answer of pH = 11.1
I'm not 100% sure if this is correct so if someone could check it and let me (and classof2019) know, that would be great!
Thanks!
:)
(ps. Sorry if the image quality is bad!)
Hi, I might be wrong. There are some point in your solution that I don't understand.
Since the ratio between calcium hydroxide and acetate acid is 1:2, 1.25*10-3 mols of CH3COOH should react with 6.25*10-4 mols of Ca(OH)2. So,Ca(OH)2 left would be 1.975*10-3 mols.
And I think we should do 1.975*10-3 / volume which is 4.5*10-2 to get concentration of OH- first and substitute it into -log formula.
But I still did not get the answer though...
Post by: classof2019 on October 30, 2019, 08:48:47 am
Hi there!
I did the question (working below) and got the answer of pH = 11.1
I'm not 100% sure if this is correct so if someone could check it and let me (and classof2019) know, that would be great!
Thanks!
:)
(ps. Sorry if the image quality is bad!)
Hey,
I really appreciate your help, however, I've just realised that the correct answer is D. 11.6. I'm not sure how they would have gotten this.
I've attached my working (which still doesn't align with their answer) in the spoiler - let me know what you think.
Spoiler
25.0mL acetic acid has pH - 1.30, therefore [H+]=10-1.30
So n(H+) = 0.025 x 10-1.30
20mL Ca(OH)2 --> n(Ca(OH)2) = 0.130 x 0.020 = 0.0026
So n(OH-) = 0.0026 X 2 = 0.0052 moles
H+(aq) + OH-(aq)--> H2O(l)
So n(OH-) excess = 0.0052 - (0.0025 x 10-1.30) = 0.00394703191 ...
So [OH-] = 0.00394703191 ... / 0.045 = 0.08771182035
Therefore, pOH = -log (0.08771182035...) = 1.056941876 ...
So pH = 12.94
I think this may be problematic in the sense that it doesn't account for any further dissociation of acetic acid (which decreases pH) - but I don't really know any alternative.
Thanks.
Post by: classof2019 on October 30, 2019, 08:53:11 am
Hi, I might be wrong. There are some point in your solution that I don't understand.
Since the ratio between calcium hydroxide and acetate acid is 1:2, 1.25*10-3 mols of CH3COOH should react with 6.25*10-4 mols of Ca(OH)2. So,Ca(OH)2 left would be 1.975*10-3 mols.
And I think we should do 1.975*10-3 / volume which is 4.5*10-2 to get concentration of OH- first and substitute it into -log formula.
But I still did not get the answer though...
Hm, I think if you multiply the [Ca(OH)2] you found by 2 to find [OH-], you eventually end up with a result of 12.94.
Perhaps the question itself has an error.
Post by: worldno1 on October 30, 2019, 02:32:43 pm
Hm, I think if you multiply the [Ca(OH)2] you found by 2 to find [OH-], you eventually end up with a result of 12.94.hi, i just did the question independently (without looking at your solution) and i landed on the same answer of 12.94. i'm pretty sure that is the correct answer! :) not sure how 11.6 came about.
Perhaps the question itself has an error.
Post by: classof2019 on October 30, 2019, 04:41:59 pm
hi, i just did the question independently (without looking at your solution) and i landed on the same answer of 12.94. i'm pretty sure that is the correct answer! :) not sure how 11.6 came about.
Awesome, thanks for that!
Just another q (for anyone to answer) - how many carbon environments does hexane have? Because I always thought it was 2 but a spectrum I saw had three peaks.
Post by: studyingg on October 30, 2019, 04:58:40 pm
Awesome, thanks for that!
Just another q (for anyone to answer) - how many carbon environments does hexane have? Because I always thought it was 2 but a spectrum I saw had three peaks.
I think It would be 3, if you draw a hexane structure and rule a line of symmetry there are 3 carbons on each side of the line, hence, 3 C environments I believe.
Post by: InnererSchweinehund on October 30, 2019, 08:40:09 pm
Hi, I might be wrong. There are some point in your solution that I don't understand.
Since the ratio between calcium hydroxide and acetate acid is 1:2, 1.25*10-3 mols of CH3COOH should react with 6.25*10-4 mols of Ca(OH)2. So,Ca(OH)2 left would be 1.975*10-3 mols.
And I think we should do 1.975*10-3 / volume which is 4.5*10-2 to get concentration of OH- first and substitute it into -log formula.
But I still did not get the answer though...
Hi!
I checked with a chemistry teacher and she seemed to think the way I did it would be correct, if the answer I got was correct.
She also did the question and couldn't get pH 11.6
Not really sure what's going on...
:o
Post by: worldno1 on October 30, 2019, 11:07:44 pm
Hi!
I checked with a chemistry teacher and she seemed to think the way I did it would be correct, if the answer I got was correct.
She also did the question and couldn't get pH 11.6
Not really sure what's going on...
:o
hey there! when you were calculating pOH, you didn't convert the number of moles of Ca2+ to concentration (by dividing by 0.045 L), since pOH = log [OH-]. additionally, the number of moles of OH- is twice the number of moles of Ca(OH)2, so you need to multiply by 2. :)
Post by: Hawraa on November 02, 2019, 12:08:56 pm
With the haber process equilibrium graph:
N2+3H2<>2NH3
If the reaction vessel is doubled (pressure decreased), isn't like we show that as a sharp decrease in the concentration of all species (reactants and products) and then there will be a decrease in the product curve and increase in the reactants curve?
Also would this be different if we graph it as (moles vs. time) or (concentration vs. time) graphs? Thanks.
Post by: fun_jirachi on November 02, 2019, 01:29:53 pm
You're 100% correct! If you're given a graph of concentration/time with an actual scale though, don't forget the sharp decrease ends when each substance has its concentration halved ie. if there were 6 mol/L of nitrogen, after the halved pressure, the sharp drop would drop the concentration to 3 mol/L. Afterwards, there would be a shift towards the left side which has more moles of gas, by LCP as you say :) Make sure the curve shows proportional increases/decreases as well ie. in a 1:3:2 ratio.
The above is for concentration versus time. When we change the pressure or volume, the number of moles of gas shouldn't change; consider the equation PV=nRT - by increasing/decreasing pressure, we increase/decrease volume by the inverse of that factor, and vice versa ie. nRT remains constant (all this means is that provided everything is at a constant temperature, changes in volume or pressure have no effect on the no. of moles of gas!). However, after equilibrium shifts, you should note a change in the number of moles, as the system shifts one way or another to adjust to the change in volume or pressure :)
Hope this helps :)
Post by: worldno1 on November 02, 2019, 06:15:56 pm
Hi there!
You're 100% correct! If you're given a graph of concentration/time with an actual scale though, don't forget the sharp decrease ends when each substance has its concentration halved ie. if there were 6 mol/L of nitrogen, after the halved pressure, the sharp drop would drop the concentration to 3 mol/L. Afterwards, there would be a shift towards the left side which has more moles of gas, by LCP as you say :) Make sure the curve shows proportional increases/decreases as well ie. in a 1:3:2 ratio.
The above is for concentration versus time. When we change the pressure or volume, the number of moles of gas shouldn't change; consider the equation PV=nRT - by increasing/decreasing pressure, we increase/decrease volume by the inverse of that factor, and vice versa ie. nRT remains constant (all this means is that provided everything is at a constant temperature, changes in volume or pressure have no effect on the no. of moles of gas!). However, after equilibrium shifts, you should note a change in the number of moles, as the system shifts one way or another to adjust to the change in volume or pressure :)
Hope this helps :)
so in other words, when you're graphing moles vs time, there'd be no immediate drop (or sharp decrease as you said) but afterwards, there'd be a gradual increase/decrease of moles in whichever direction equilibrium moves?
Post by: RACHEL1111 on November 03, 2019, 11:12:14 am
I have seen this definition of oxidation reaction: ' Oxidation of alcohols will create a double bond from the carbon that the alcohol was originally on to another carbon molecule by taking away two hydrogen atoms- 1 per carbon'. However, it contradicts to my understanding of oxidation reaction. In my drawing, when the alcohol is oxidised, two circled hydrogen from the same carbon, rather than one per carbon, are removed to form hydrogen gas.
So, which one is correct? Any help is appreciated!
Post by: Hawraa on November 03, 2019, 03:52:30 pm
Is ammonium a strong or weak acid? So when we write its ionisation equation to produce hydrogen ion and ammonia (conjugate base) do we use the normal arrow or the equilibrium arrow?
The same thing with the floride ion and the hydrofluoric acid, is it strong or weak and which arrow do we use?
Thanks.
Post by: r1ckworthy on November 03, 2019, 04:01:16 pm
Hi everyone,
Is ammonium a strong or weak acid? So when we write its ionisation equation to produce hydrogen ion and ammonia (conjugate base) do we use the normal arrow or the equilibrium arrow?
The same thing with the floride ion and the hydrofluoric acid, is it strong or weak and which arrow do we use?
Thanks.
From researching, it seems that while ammonium is a stronger acid than ammonia (which is a weak base), it is not totally strong (does not fully dissociate). Check out this link for a more in-depth explanation.
So I would say, still use the equilibrium arrow when writing these reactions.
Hydrofluoric acid is a weak acid. While it dissociates in water, the hydroxide ions form a strong bond to the dissociated fluoride ion, which limits it's strength. Check out this link for a better explanation.
Hope that helps!
Post by: Hawraa on November 04, 2019, 01:24:45 pm
Can someone please explain this question. I thought it would be A but the answer is C.
Post by: louisaaa01 on November 04, 2019, 01:48:03 pm
Hi everyone,
Can someone please explain this question. I thought it would be A but the answer is C.
Hi Hawraa,
We can rule out option A because nitrous acid will partially dissociate to produce H+ ions and NO2- ions. The presence of H+ ions will reduce pH (noting that pH = -log10[H+], so greater [H+] means lower pH). Option B is ruled out for a similar reason.
Now, to discern between C and D we look at pKa. Since nitrous acid has a higher pKa than chlorous acid, this means that nitrous acid is a weaker acid - so its conjugate base (NO2-) is a stronger base. This means a solution containing NO2- ions has a greater pH than a solution containing the same concentration of ClO2 ions, so C must be the answer. Note also that Na+ ions don't really react with water - it's the anion in this instance that governs pH.
So to sum up, C has a higher pH than A simply because there are no H+ ions in its structure. C has the highest pH since its anion is the strongest base present.
Post by: Hawraa on November 04, 2019, 02:02:02 pm
Hi Hawraa,
We can rule out option A because nitrous acid will partially dissociate to produce H+ ions and NO2- ions. The presence of H+ ions will reduce pH (noting that pH = -log10[H+], so greater [H+] means lower pH). Option B is ruled out for a similar reason.
Now, to discern between C and D we look at pKa. Since nitrous acid has a higher pKa than chlorous acid, this means that nitrous acid is a weaker acid - so its conjugate base (NO2-) is a stronger base. This means a solution containing NO2- ions has a greater pH than a solution containing the same concentration of ClO2 ions, so C must be the answer. Note also that Na+ ions don't really react with water - it's the anion in this instance that governs pH.
So to sum up, C has a higher pH than A simply because there are no H+ ions in its structure. C has the highest pH since its anion is the strongest base present.
That's great thanks a lot. Just another question please. You know how you said that nitrous acid is weak and so its conjugate base is strong, I read something similar on the internet but didn't really understand it. So is it like whenever we have a weak acid, its CB is strong? And vice versa? And is the same thing applies for bases, so a weak base would have a strong CA?
Thanks
Post by: louisaaa01 on November 04, 2019, 02:21:26 pm
That's great thanks a lot. Just another question please. You know how you said that nitrous acid is weak and so its conjugate base is strong, I read something similar on the internet but didn't really understand it. So is it like whenever we have a weak acid, its CB is strong? And vice versa? And is the same thing applies for bases, so a weak base would have a strong CA?
Thanks
Yes, that's exactly right!
We can come to this conclusion based on how the various acids and bases dissociate. For instance, HCl is a strong acid, completely dissociating into H+ and Cl-. Cl- is the conjugate base of HCl - since HCl dissociates so readily, Cl- is highly unlikely to accept that H+ back to form HCl, so it's a very weak base (by the Bronsted-Lowry theory). A similar scenario occurs for strong bases - since a strong base will readily accept a proton, once that proton has been accepted and the conjugate acid is formed, it won't readily 'donate' it back.
A weak acid, on the other hand, will form an equilibrium in water. Take, for example, the fairly weak ethanoic acid, CH3COOH, which dissociates in water as per the following equilibrium:
CH3COOH (aq) + H2O (l) <--> CH3COO- (aq) + H3O+ (aq)
The conjugate base of CH3COOH, being CH3COO-, is a moderately strong base since it has the tendency to accept a proton back from the hydronium ion and form CH3COOH again (reverse reaction). A similar thing occurs for a weak base and its conjugate acid.
Hope this clarifies the concept a little more!
Post by: Aaron Lillis on November 04, 2019, 07:42:32 pm
Could someone please provide a complete solution to this question? I'm finding it extremely confusing - particularly the equation given and how it helps, how the molarity of sodium ethanoate relates to anything mathematically and which dissociation equation you're meant to use for Ka. This is my worst type of question in chemistry, any help would be appreciated :)
Post by: louisaaa01 on November 04, 2019, 08:09:31 pm
Hey all!!
Could someone please provide a complete solution to this question? I'm finding it extremely confusing - particularly the equation given and how it helps, how the molarity of sodium ethanoate relates to anything mathematically and which dissociation equation you're meant to use for Ka. This is my worst type of question in chemistry, any help would be appreciated :)
I have attached my attempt at this question - hope this helps! If you're still confused with anything, don't hesitate to ask.
EDIT: x may also be solved using the quadratic formula - however, since x is small, it is generally safe to assume that, when subtracted from the molarity of the solution, the difference is negligible.
Post by: Aaron Lillis on November 04, 2019, 08:55:24 pm
I have attached my attempt at this question - hope this helps! If you're still confused with anything, don't hesitate to ask.
EDIT: x may also be solved using the quadratic formula - however, since x is small, it is generally safe to assume that, when subtracted from the molarity of the solution, the difference is negligible.
Thank you so much, oh my lord I didn't realise it was the base dissociation equation given. Thank you x100
Post by: RACHEL1111 on November 05, 2019, 11:22:26 pm
Could anyone help me with this confusion : when the volume of the equilibrium reaction increases, would there be a sudden drop of the curve representing concentration of chemical species in the concentration vs. time graph? or the concentration just slowly increases or decrease?
I saw these two different versions on different textbooks and it really confuses me. Any help is appreciated!!
Post by: Hawraa on November 05, 2019, 11:54:34 pm
Hi!
Could anyone help me with this confusion : when the volume of the equilibrium reaction increases, would there be a sudden drop of the curve representing concentration of chemical species in the concentration vs. time graph? or the concentration just slowly increases or decrease?
I saw these two different versions on different textbooks and it really confuses me. Any help is appreciated!!
Hey,
When the volume of the vessel increase then the pressure decrease, therefore, the concentration of the species decreases represnted by a sharp decrease on the graph. I've seen this also in some past papers sample answers. Check 2009 Q23 and also 2005 Q27(this one is the other case with reduced volume).
Hope this clarifies the confusion.
Post by: RACHEL1111 on November 06, 2019, 06:14:55 am
Hey,
When the volume of the vessel increase then the pressure decrease, therefore, the concentration of the species decreases represnted by a sharp decrease on the graph. I've seen this also in some past papers sample answers. Check 2009 Q23 and also 2005 Q27(this one is the other case with reduced volume).
Hope this clarifies the confusion.
Yes! It does! Thank you very much!
Post by: Hawraa on November 06, 2019, 01:35:43 pm
Just regarding this graph question from ATAR Notes chemistry topic test. So it says that the trend line(line of best fit) should not be linear but should go near most points. For me it looks like the dotted line has been connected by hand. Is it wrong to draw it with a ruler ( like I did, if you can see the line in the middle) because I'm confused. My teacher said it should be linear but the answer here says it should not. So do we connect the dots or just draw a line approximately through them.
Can someone please explain this to me? Thanks.
Post by: Hawraa on November 06, 2019, 09:31:15 pm
Urgent question please.
Ethanol is widely used fuel that can be made from both renewable and non renewable resources. Using relevant chemical equations, outline two methods that can be used to produce Ethanol.
The methods I would use are fermentation (equation of glucose turning into Ethanol and CO2) and hydration of ethene. Is the second method wrong? Because the answer I have talks about fermentation and the substitution of halogenated alkanes with water? Are they both correct?
Any help would be appreciated guys, thanks.
Post by: keltingmeith on November 18, 2019, 09:27:28 pm
Hi everyone,
Just regarding this graph question from ATAR Notes chemistry topic test. So it says that the trend line(line of best fit) should not be linear but should go near most points. For me it looks like the dotted line has been connected by hand. Is it wrong to draw it with a ruler ( like I did, if you can see the line in the middle) because I'm confused. My teacher said it should be linear but the answer here says it should not. So do we connect the dots or just draw a line approximately through them.
Can someone please explain this to me? Thanks.
Would help if we could see the question
Hi everyone,
Urgent question please.
Ethanol is widely used fuel that can be made from both renewable and non renewable resources. Using relevant chemical equations, outline two methods that can be used to produce Ethanol.
The methods I would use are fermentation (equation of glucose turning into Ethanol and CO2) and hydration of ethene. Is the second method wrong? Because the answer I have talks about fermentation and the substitution of halogenated alkanes with water? Are they both correct?
Any help would be appreciated guys, thanks.
No need to worry, hydration of ethene is still a correct answer
Post by: mani.s_ on November 22, 2019, 07:48:30 pm
Post by: louisaaa01 on November 22, 2019, 07:58:21 pm
Hi, how would I find the pH of 0.020 mol/L citric acid without using Ka values??? Thanks
Hi,
Citric acid is a weak triprotic acid, so to accurately find pH you would have to use Ka. However, if Ka isn't provided, the question could be solved (though somewhat erroneously) relying on the assumption that citric acid is a strong acid.
Since citric acid is triprotic, assuming it is strong and thus completely dissociates, a 0.020 mol/L solution of citric acid will have [H+] = 3 x 0.020 = 0.060 mol/L. You can then use the formula, pH = -log[H+] to determine pH.
Hope this helps!
Post by: milie10 on January 30, 2020, 06:22:37 pm
I tried doing some of the past HSC questions on acids and bases, and I'm having a bit of trouble doing these two.
(https://i.imgur.com/XU5OIea.jpg)
For this, I tried using c1v1=c2v2 but ended up with 10L :( Is this the right method to use here?
(https://i.imgur.com/nXaNi3k.jpg)
I'm still confused about limiting reagent type questions- in this question, when I use n=cxv, does this find n(2HCl), or does it only find n(HCl)? Also, is there a quicker way to do this question?
Thanks so much! :D
Post by: r1ckworthy on January 30, 2020, 06:59:02 pm
Hi!You are right! 10L is the answer, so you choose the answer that is closest to this value, which is C!
I tried doing some of the past HSC questions on acids and bases, and I'm having a bit of trouble doing these two.
(https://i.imgur.com/XU5OIea.jpg)
For this, I tried using c1v1=c2v2 but ended up with 10L :( Is this the right method to use here?
(https://i.imgur.com/nXaNi3k.jpg)Here is my working out of the question (click on the image to make it larger):
I'm still confused about limiting reagent type questions- in this question, when I use n=cxv, does this find n(2HCl), or does it only find n(HCl)? Also, is there a quicker way to do this question?
Thanks so much! :D
(https://i.imgur.com/VnEPxQS.png)
When you use \( n=c \times v \), it only finds n(HCl). I find limiting reagent questions confusing, so I like to write statements as I did in my working. In those statements, I calculated how many moles a substance will react with/ use up (according to stoichiometric ratios). I compare the two statements and pretty quickly find out which substance is the limiting reagent.
I am not sure of a quicker way to do this question (hopefully someone can chip in!).
Let me know if you need any more help!
Post by: milie10 on January 30, 2020, 10:37:40 pm
Hey!You are right! 10L is the answer, so you choose the answer that is closest to this value, which is C!
Here is my working out of the question (click on the image to make it larger):
(https://i.imgur.com/VnEPxQS.png)
When you use \( n=c \times v \), it only finds n(HCl). I find limiting reagent questions confusing, so I like to write statements as I did in my working. In those statements, I calculated how many moles a substance will react with/ use up (according to stoichiometric ratios). I compare the two statements and pretty quickly find out which substance is the limiting reagent.
I am not sure of a quicker way to do this question (hopefully someone can chip in!).
Let me know if you need any more help!
Thanks heaps, that clears things up!! Writing the limiting reagent statements really helps- I'll start doing that. :D
* Noticed a trivial mistake in your working- HCl leftover = 0.002- 0.0008 =0.0012
Post by: r1ckworthy on January 31, 2020, 09:22:34 am
Thanks heaps, that clears things up!! Writing the limiting reagent statements really helps- I'll start doing that. :D
* Noticed a trivial mistake in your working- HCl leftover = 0.002- 0.0008 =0.0012
Oops! Not surprised though, did rush a lot ;D Glad it was of help to you!
Post by: milie10 on February 02, 2020, 12:58:59 am
I'm a bit confused about why CaCO3 is a basic salt instead of neutral. Could someone explain this please?
Thanks! :)
Post by: r1ckworthy on February 02, 2020, 11:34:02 am
Hi!Hey!
I'm a bit confused about why CaCO3 is a basic salt instead of neutral. Could someone explain this please?
Thanks! :)
It all has to do with how \( \text{CaCO}_3 \) dissociates in water. \( \text{CaCO}_3 \) dissociates in water to produce \( \text{Ca}^\text{2+} \) and \( \text{CO}_3^\text{2-} \) ions. The \( \text{CO}_3^\text{2-} \) ions react with water to produce \( \text{HCO}_3^- \) and hydroxide ions, which leads the solution to be basic.
Post by: BlackFrost on February 15, 2020, 04:06:49 pm
Post by: milie10 on February 15, 2020, 07:56:38 pm
In an experiment of separating sand from salt water (which involves filtration and evaporation), what could be the sources of errors that could eventually cause substances to be lost ?
Hi!
- Some of the salt water could be left on the filter paper or the filter funnel while filtrating
- Over evaporating the water with a Bunsen Burner can result in the salt 'spitting out' from the evaporating basin
There can also be errors that causes an increased mass:
- Under evaporating the solvent could result in leftover water in the resultant mass of salt
- Evaporating the water overnight (to try and eliminate the error of over evaporating with a Bunsen Burner) can lead to dust particles and other contaminants getting into the evaporating basin.
Hope that helps :)
Post by: Einstein_Reborn_97 on March 02, 2020, 09:35:30 pm
Post by: milie10 on March 08, 2020, 12:26:14 am
(https://i.imgur.com/ZMVktVQ.jpg)
(https://i.imgur.com/h0zqLe9.jpg)
Did I do q2 correctly? I'm also not sure what the difference between q2 and q3 is.
thanks :D
Post by: Erutepa on March 08, 2020, 05:21:03 pm
If the equivalence point of a reaction is between the pH range of colour change for the indicator in use, does the change in colour happening at the maximum or minimum pH within that range or at the equivalence point? E.g. phenolphtalein has a pH range of colour change of around 8.3 to 10.0; if the equivalence point of the reaction is 9.1, will you observe the colour change from colourless to red-pink at about a pH of 8.3 or 9.1?Sorry for the late reply!
The colour change of the indicator occurs somewhat gradually within its range. An indicator doesn't change instantly at a precise pH, but rather changes over a pH interval. For your example, phenolphtalein will begin turning slightly pink at a pH of 8.3 and will become darker as you reach pH of 10 If you want to read more about this you can do so here :)
hi!For question 2, the c value you are using is 4.18J/g/K, meaning your mass should be expressing in grams, however it seems you might have expressed your mass in kg. Also note that you are calculating the temperature change of the final (100ml) solution, as such the mass in your energy calculation should be 100g (since 1ml of water = 1g of water).
(https://i.imgur.com/ZMVktVQ.jpg)
(https://i.imgur.com/h0zqLe9.jpg)
Did I do q2 correctly? I'm also not sure what the difference between q2 and q3 is.
thanks :D
When you are calculating the change in temperature you don't need to be converting it to kelvin as the change in degrees celcius is equal to the change in kelvin. If you do want to convert to kelvin, then you need to be adding 273 to both the initial and the final temperatures (to convert both into kelvin) as shown here for example:
you cannot just add 273 to the final change as you have done, you must convert both the initial and final temps before finding the change in temperature. Note that if you simplify the above equation as such:
which shows that when calculating the change in temperature, you do not need to convert anything into kelvin
Hopefully by taking on these tips you can fix up your answers for question 2
Question 3 is a similar question, however is not quite the same thing. Question 2 asks you to calculate the change in energy of the total reaction, however question 3 wants you to calculate the change in energy for just 1 mol of reaction. For reaction A this would be calculating the energy change for 1 mol of HCl to react with 1M of NaOH.
Hopefully this helps :)
Post by: Einstein_Reborn_97 on March 16, 2020, 07:20:53 pm
Keq = 3.80 x 10-4
Identify the strongest acid and the strongest base in this system.
I know HF is a weak acid and thus it's conjugate base will be a strong base. I also know that the equilibrium constant indicates that there are more reactants than products. I'm just not sure where to go from there.
Post by: BlackFrost on April 16, 2020, 08:24:55 pm
I was just wondering if anyone has any resources on Preliminary Chemistry past papers (2015 onwards) and any topic tests on Module 1 and 2.
Thanks
Post by: Einstein_Reborn_97 on April 17, 2020, 06:34:43 pm
Hey guys,
I was just wondering if anyone has any resources on Preliminary Chemistry past papers (2015 onwards) and any topic tests on Module 1 and 2.
Thanks
The NESA website has tons of previous past papers (exams and marking guidelines) for pretty much every subject, which you can download and print. Here's the link: https://educationstandards.nsw.edu.au/wps/portal/nesa/11-12/resources/hsc-exam-papers
Post by: stels on May 27, 2020, 05:14:40 pm
Post by: Einstein_Reborn_97 on May 27, 2020, 08:16:36 pm
You add 25 milliliters of 2.0 mM HCl solution to 75 milliliters of 1.0 mM Ca (OH) 2 solution. What is the pH of the resulting solution?Hi stels,
Step 1: Balanced chemical equation ALWAYS:
\(2HCL_{(aq)}+Ca(OH)_{2(aq)}→CaCl_{2 (aq)}+2H_2O_{(l)}\)
Step 2: Find the limiting reagent and the solution in excess (and how much)
\(n(HCl)=cV=(2.0\times10^{-3})(25\times10^{-3})=50\times10^{-6} mol\)
\(n(Ca(OH)_2)=cV=(1.0\times10^{-3})(75\times10^{-3})=75\times10^{-6} mol\)
Reaction stoichiometry = 2:1
∴ HCl is the limiting reagent (all of it will be neutralised) and \(Ca(OH)_2\) is in excess.
\(Ca(OH)_2\) required (used up) = \(50\times10^{-6} \div 2 = 25\times10^{-6} mol\)
Excess \(Ca(OH)_2\) = \(75\times10^{-6} - 25\times10^{-6} = 50\times10^{-6} mol\)
Step 3: Work out the concentration of the solution in excess, then the concentration of \(H^+\) or \(OH^-\) ions and finally the pH
Total volume of resulting solution = 25 + 75 = 100 mL = 0.10 L
c(\(Ca(OH)_2\)) = \(\frac{n}{V}\) = \(\frac{50\times10^{-6}}{0.10}\) = 0.0005 mol/L
\([OH^-]\) = 2\(\times\)0.0005 = 0.001 mol/L
\(pOH = -log_{10}(0.001) = 3\)
pH + pOH = 14
∴ pH = 14 - 3 = 11
Hope that helps! ;)
Post by: 006896 on June 28, 2020, 12:37:51 pm
Could someone help me with this question about redox reactions and galvanic cells?
Thanks!
Post by: 006896 on June 28, 2020, 01:34:58 pm
Could someone help me with part b i) and ii) of this question about galvanic cell? part bi and ii are pasted in below, but the question stem is attached.
part b
After a period of time, a solid deposit that had formed on the copper electrode was removed and dried. The deposit had a mass of 0.395 g.
i) Calculate the final mass of the nickel electrode.
ii) Calculate the final concentration of the nickel (II) nitrate solution.
Thank you!
Post by: eliza.langer on November 16, 2020, 08:05:23 pm
I just started the HSC Chemistry course and we have our first assessment task at which we have to research a gaseous equilibrium system that IS NOT the Haber Process. Does anyone have a really interesting system that I could research?
Thanks!
Post by: keltingmeith on November 16, 2020, 08:24:26 pm
Hi!
I just started the HSC Chemistry course and we have our first assessment task at which we have to research a gaseous equilibrium system that IS NOT the Haber Process. Does anyone have a really interesting system that I could research?
Thanks!
Absolutely - Haber sucks anyway, too energy inefficient, too much greenhouse gas by-product.
A very common one is the decomposition of dinitrogen tetroxide, \(\ce{N2O4 (g)\to 2NO2 (g)}\). This one is nice because one of the two is coloured, and the other isn't. I personally prefer the sublimation of iodine, though it might not count because it's not entirely gaseous, \(\ce{I2 (s)\to I_2 (g)}\). Another cool one is the synthesis of some halide acids, of general form, \(\ce{X2 (g) + H2 (g)\to 2HX (g)}\), usually iodine or bromine since chlorine has an equilibrium that highly favours the products, but I can't seem to find videos of those anywhere...
Post by: neha.singh4 on December 03, 2020, 01:13:13 pm
I have recently received my first Chemistry assessment task for Year 12. It requires us to use your understanding of dissolution of ionic compounds in water and Le Chatelier’s principle research to remove toxins from native indigenous foods. Although I understand the nature of the task, I'm struggling to find some information on it since it's apart of the new Chemistry syllabus and there is limited resources available.
The topic is 'Indigenous Chemistry'. I'm struggling to:
1) Find a toxic Aboriginal food (except for cycads since we're not allowed to do that)
2) The detoxification process of the food
3) The correlation the food has to solution equilibria, Le Chatelier's principle and dissolution of ionic compounds.
If anyone can provide any sort of information to get the ball rolling for my assessment, I would greatly appreciate it!!!
Post by: Bri MT on December 03, 2020, 01:36:36 pm
species to look at if you stay really stuck
Dioscorea bulbifera, Beilschmiedia bancroftii and Castanospermum australe
I hope this helps!
Post by: neha.singh4 on December 11, 2020, 08:45:15 pm
This is the toxin- C8H15NO4 (Castanospermine)
It should be a reversible reaction.
Also I'm a little unsure as to what states it would be too (eg. gas, liquid, aqueous, solid)
Post by: blackcoffee2.718281828 on August 22, 2021, 11:18:53 am
Equal volumes of two 0.04 molL−1 solutions were mixed together. Which pair of solutions would give the greatest mass of precipitate?
A. Ba(OH)2 and MgCl2
B. Ba(OH)2 and MgSO4
C. Ba(OH)2 and NaCl
D. Ba(OH)2 and Na2SO4
PS this is my first post ever let's hope all goes well:)
Post by: fun_jirachi on August 22, 2021, 01:59:39 pm
You can instantly rule out C since it won't produce any precipitate.
Writing out the chemical equations and using solubility rules will tell you that the relevant precipitates are magnesium hydroxide and barium sulfate. Since this is a multiple choice question, it's okay to cut corners here and just realise that A and D produce only one of the previously mentioned precipitates while B produces both (the key here is equal volumes of solutions, and writing out the equations will show you that for each option the precipitates are produced in equal ratios). Hence, the answer is B.