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March 29, 2024, 05:10:07 am

Author Topic: HSC Chemistry Question Thread  (Read 1040612 times)  Share 

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amandali

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Re: Chemistry Question Thread
« Reply #90 on: February 29, 2016, 05:45:12 pm »
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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

jakesilove

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Re: Chemistry Question Thread
« Reply #91 on: February 29, 2016, 05:53:53 pm »
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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
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jakesilove

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Re: Chemistry Question Thread
« Reply #92 on: February 29, 2016, 05:57:44 pm »
+1
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
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ProfLayton2000

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Re: Chemistry Question Thread
« Reply #93 on: February 29, 2016, 06:31:34 pm »
+1

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.

jakesilove

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Re: Chemistry Question Thread
« Reply #94 on: February 29, 2016, 10:47:02 pm »
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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
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katherine123

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Re: Chemistry Question Thread
« Reply #95 on: March 02, 2016, 07:16:09 am »
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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 :/

RuiAce

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Re: Chemistry Question Thread
« Reply #96 on: March 02, 2016, 08:10:58 am »
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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.

jakesilove

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Re: Chemistry Question Thread
« Reply #97 on: March 02, 2016, 08:20:54 am »
+2
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
[1.5]

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
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William3558

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Re: Chemistry Question Thread
« Reply #98 on: March 02, 2016, 02:18:32 pm »
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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...
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katherine123

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Re: Chemistry Question Thread
« Reply #99 on: March 02, 2016, 02:51:34 pm »
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i dont get why it is B

jakesilove

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Re: Chemistry Question Thread
« Reply #100 on: March 02, 2016, 03:33:20 pm »
+1
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
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jakesilove

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Re: Chemistry Question Thread
« Reply #101 on: March 02, 2016, 03:37:55 pm »
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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
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William3558

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Re: Chemistry Question Thread
« Reply #102 on: March 02, 2016, 04:17:21 pm »
+1
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
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Re: Chemistry Question Thread
« Reply #103 on: March 03, 2016, 05:51:11 pm »
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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! :)

jakesilove

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Re: Chemistry Question Thread
« Reply #104 on: March 03, 2016, 06:27:30 pm »
+2
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 to be 0.1mol/L for a strong acid (and therefore the pH to be 1). If you test this experimentally, and find the pH to be higher, you know its a weak acid!

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 :)
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