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Author Topic: HSC Chemistry Question Thread  (Read 1040776 times)  Share 

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RuiAce

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Re: Chemistry Question Thread
« Reply #75 on: February 27, 2016, 09:00:17 am »
0
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)

katherine123

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Re: Chemistry Question Thread
« Reply #76 on: February 27, 2016, 10:11:37 am »
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[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?

RuiAce

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Re: Chemistry Question Thread
« Reply #77 on: February 27, 2016, 02:01:49 pm »
+1

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.

amandali

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Re: Chemistry Question Thread
« Reply #78 on: February 28, 2016, 07:10:19 am »
0
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

katherine123

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Re: Chemistry Question Thread
« Reply #79 on: February 28, 2016, 08:34:24 am »
0
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  :/

RuiAce

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Re: Chemistry Question Thread
« Reply #80 on: February 28, 2016, 08:56:51 am »
+2
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.)

RuiAce

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Re: Chemistry Question Thread
« Reply #81 on: February 28, 2016, 09:02:19 am »
+1
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).
« Last Edit: February 28, 2016, 09:04:12 am by RuiAce »

jakesilove

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Re: Chemistry Question Thread
« Reply #82 on: February 28, 2016, 09:22:46 am »
+1
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
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Chemystery

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Re: Chemistry Question Thread
« Reply #83 on: February 28, 2016, 05:19:32 pm »
0
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!
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Happy Physics Land

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Re: Chemistry Question Thread
« Reply #84 on: February 28, 2016, 06:59:37 pm »
+4
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
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jakesilove

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Re: Chemistry Question Thread
« Reply #85 on: February 28, 2016, 07:16:36 pm »
+2
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
ATAR: 99.80

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Studying a combined Advanced Science/Law degree at UNSW

RuiAce

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Re: Chemistry Question Thread
« Reply #86 on: February 28, 2016, 07:17:42 pm »
+1
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...

Happy Physics Land

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Re: Chemistry Question Thread
« Reply #87 on: February 28, 2016, 07:30:10 pm »
0
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
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RuiAce

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Re: Chemistry Question Thread
« Reply #88 on: February 28, 2016, 08:10:07 pm »
0
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.

ProfLayton2000

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Re: Chemistry Question Thread
« Reply #89 on: February 29, 2016, 04:56:05 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?