2021 BoS Chemistry Trial (3 Viewers)

[CM_Tutor]

The 2021 BoS Chemistry Trial is now publicly available - Trebla posted a link in the thread in the general discussion forum: https://boredofstudies.org/threads/bos-trials-maths-and-chemistry-2021.397055/page-2#post-7399485

As the person who wrote it, I'm interested in any feedback, either publicly or via DM.

I'd like to take the opportunity to see what the contributors here think is an easy or a difficult multiple-choice question. So, having looked at the papers submitted, I can say that:

• There were only 3 MCQ that were answered correctly by 70% of more of the students who did the exam.
• There were 9 MCQ that were answered correctly by between 50% and 70% of the students who did the exam.
• There were 8 MCQ that were answered correctly by less than 50% of the students who did the exam.

Which questions do you think were the 3 that most students got correct? Which were the 8 that the majority answered incorrectly?

I will post later identifying which was which, but in the meantime, you can see if your impressions of what was easy and what was not match the performance of those who took the exam earlier today.

 

[username_2]

I feel like 2, 7 and 13 were the ones most students would have got correct. 16 seemed like the least likely answer that was answered because it takes a minute realize what the question is even asking (and i don't even know what hartmann's solution is). Nice paper and definitely on the difficult side of the spectrus (especailly if it is timed.. obv didn't do the test). Overall cool paper which definitely tests niche understanding of topics and is balanced.

 

[icycledough]

Even having a look at it myself, I would think 1, 2 and 7/20 (can't decide which one) would be the ones which most people got right. Definitely agree that 16 would have individually been the hardest just because even having done chemistry myself last year, I never came across nor heard of Hartmann's Solution is. Is that something which is actually in the syllabus or not, because I don't recall studying it at all?

Just wanted to say as well that it definitely does seem quite complex compared to past HSC exams, which definitely isn't a problem at all as it just prepares students for the HSC even more. I'd say honestly that looking at the exam, a raw mark in the 80s would represent that of quite a strong candidate who could get a mid to high Band 6 in Chemistry.

 

[KCAVS]

Definitely agree that 16 would have individually been the hardest just because even having done chemistry myself last year, I never came across nor heard of Hartmann's Solution is. Is that something which is actually in the syllabus or not, because I don't recall studying it at all?

Hartmann's solution is described in question 15, so you just had to determine that sodium lactate was the organic compound.

 

[icycledough]

Hartmann's solution is described in question 15, so you just had to determine that sodium lactate was the organic compound.

Oh yh, I didn't even notice that it came up in the question before, my bad

 

[someth1ng]

Q1: A bunch of people get this one wrong for sure.
Q6: Seems like a lot of work. Just need to make sure you get the formula right and be mindful of the Ksp.
Q7: A bit ambiguous. You do form CO2 and bubbles form - I guess by "observation", you mean bubbles form but that's not necessarily indicative of a carbonate (you can get bubble formation when reacting with some metals etc).
Q8: I feel like a lot of schools don't mention that indicators will have their own pKa so they affect the solution (so you shouldn't use a lot). Good question.
Q9: There's a subtle difference between end point (indicator changes colour) and equivalence point (acid and base are fully neutralised). The end point should approximate the equivalence point (i.e. appropriate indicator choice).
Q10: Might be worth giving a zoomed in version of each peak. It's pretty hard to see the multiplicity (left one looks like a triplet, the other could be anything - it's apparently a pentet).
Q11: Might want to specify that they're in equal concentrations. If they're not, A might also be possible, even though B is clearly the "best" answer.
Q12: (I think) all of these answers are possible. I think you're going for A, but it's possible to get the carbonation (dehydration), a 1,2-hydride shift, then deprotonation. I would have made one of the answers a terminal alkene to avoid this problem.
Q13: I'm a bit confused here. The hydroxide ions are likely to react with the Fe(III) ions and precipitate...so D?
Q14: You can tell the difference with just NMR because A is chiral so the two methyl groups will have different shifts. In reality, IR and NMR would be fine, so 2?
Q15. Very nice question.
Q16. Good question, might be worth identifying which molecule is pyruvic acid and lactic acid.
Q17. Don't know much about this process. Heating increases both the forward and reverse reaction and should shift the equilibrium towards the starting materials. I'd assume the normal process involves continual removing of the products.

Also, I appreciate that the paper was written in perfect ACS style haha

 

[CM_Tutor]

I'm not commenting while I wait for others to offer suggestions, but I am interested in what was seen as difficult too.

And, just as a teaser, and considering answers I've received by DM as well, two of the three questions with the highest correct rates have yet to be mentioned.

 

[CM_Tutor]

Q1: A bunch of people get this one wrong for sure.
Q6: Seems like a lot of work. Just need to make sure you get the formula right and be mindful of the Ksp.
Q7: A bit ambiguous. You do form CO2 and bubbles form - I guess by "observation", you mean bubbles form but that's not necessarily indicative of a carbonate (you can get bubble formation when reacting with some metals etc).
Q8: I feel like a lot of schools don't mention that indicators will have their own pKa so they affect the solution (so you shouldn't use a lot). Good question.
Q9: There's a subtle difference between end point (indicator changes colour) and equivalence point (acid and base are fully neutralised). The end point should approximate the equivalence point (i.e. appropriate indicator choice).
Q10: Might be worth giving a zoomed in version of each peak. It's pretty hard to see the multiplicity (left one looks like a triplet, the other could be anything - it's apparently a pentet).
Q11: Might want to specify that they're in equal concentrations. If they're not, A might also be possible, even though B is clearly the "best" answer.
Q12: (I think) all of these answers are possible. I think you're going for A, but it's possible to get the carbonation (dehydration), a 1,2-hydride shift, then deprotonation. I would have made one of the answers a terminal alkene to avoid this problem.
Q13: I'm a bit confused here. The hydroxide ions are likely to react with the Fe(III) ions and precipitate...so D?
Q14: You can tell the difference with just NMR because A is chiral so the two methyl groups will have different shifts. In reality, IR and NMR would be fine, so 2?
Q15. Very nice question.
Q16. Good question, might be worth identifying which molecule is pyruvic acid and lactic acid.
Q17. Don't know much about this process. Heating increases both the forward and reverse reaction and should shift the equilibrium towards the starting materials. I'd assume the normal process involves continual removing of the products.

Also, I appreciate that the paper was written in perfect ACS style haha

Thanks for the comments.

I'll refrain from commenting on them all until I publish which were the easiest and hardest questions, but for now:

Q1: I was pleased that this did not trick as many students as I expected.

Q7: Anion testing in module 8 is restricted to a set of specified anions and cations, so the interpretation is valid within the confines of the syllabus. The question was certainly testing whether students distinguish observation from inference. I can observe a gas being formed without knowing its identity... attributing an identification to the gas is properly an inference drawn from some other observation - such as bubbling the gas into limewater and looking for a milky precipitate - or some other information.

Q10: Can be solved without examining splitting or looking at chemical shifts. The quintet appears clear on my original, but I will consider a better image for the final version.

Q11: The concentrations are deliberately not specified and a 1:1 mixture would not produce a buffer.

Q14: The HSC does not introduce chirality. What difference in IR would you expect that was simple to interpret at a qualitative level?

Q16: The student is meant to deduce which is which from the information given.

 

[someth1ng]

Q27: Very cool question. A few points: A is effectively the environmental background because it precedes the zinc smelter. However, the calibration curve does not include a non-zero data point below A, so the linearity data is questionable at this range (i.e. 0-2 ppb).

In terms of data provided, calibration curves normally have an R2 value (usually >0.99), but R2 isn't HSC so...

Q29: Seems like a good, safe question. Solution should have sodium ions, carbonate ions, hydrogencarbonate ions, carbonic acid (in very small amounts), dissolved carbon dioxide, hydroxide, hydronium (in very low concentration), and water.

Second part is good thinking. Vinegar/acetic acid, sulfuric acid are good options and sodium hydroxide (drain cleaner). I'd imagine deploying the solid is better to reduce volume and prevent further spread.

Q30b: huge bait. solubility still goes up with temperature because entropy increases with dissolution.
Q30d: Interesting way of doing titration.

Q31a: Is the term "tautomer" in HSC?

 

[Gloucestershire]

Q1: A bunch of people get this one wrong for sure.
Q6: Seems like a lot of work. Just need to make sure you get the formula right and be mindful of the Ksp.
Q7: A bit ambiguous. You do form CO2 and bubbles form - I guess by "observation", you mean bubbles form but that's not necessarily indicative of a carbonate (you can get bubble formation when reacting with some metals etc).
Q8: I feel like a lot of schools don't mention that indicators will have their own pKa so they affect the solution (so you shouldn't use a lot). Good question.
Q9: There's a subtle difference between end point (indicator changes colour) and equivalence point (acid and base are fully neutralised). The end point should approximate the equivalence point (i.e. appropriate indicator choice).
Q10: Might be worth giving a zoomed in version of each peak. It's pretty hard to see the multiplicity (left one looks like a triplet, the other could be anything - it's apparently a pentet).
Q11: Might want to specify that they're in equal concentrations. If they're not, A might also be possible, even though B is clearly the "best" answer.
Q12: (I think) all of these answers are possible. I think you're going for A, but it's possible to get the carbonation (dehydration), a 1,2-hydride shift, then deprotonation. I would have made one of the answers a terminal alkene to avoid this problem.
Q13: I'm a bit confused here. The hydroxide ions are likely to react with the Fe(III) ions and precipitate...so D?
Q14: You can tell the difference with just NMR because A is chiral so the two methyl groups will have different shifts. In reality, IR and NMR would be fine, so 2?
Q15. Very nice question.
Q16. Good question, might be worth identifying which molecule is pyruvic acid and lactic acid.
Q17. Don't know much about this process. Heating increases both the forward and reverse reaction and should shift the equilibrium towards the starting materials. I'd assume the normal process involves continual removing of the products.

Also, I appreciate that the paper was written in perfect ACS style haha

Q10 looks OK, you can tell what the answer is from the shifted triplet. The quintet peak is probably just poorly resolved - an issue with the NMR machine, not how much the spectra is zoomed in.
Q11 - a buffer solution implies that there is reasonable buffering capacity. An ethanoic acid/ethanoate buffer will not have reasonable buffering capacity at pH > 7, so I think the statement is OK
Q12 - is a tertiary to tertiary hydride shift likely? I don't know the answer - perhaps you are more familiar with these things?
Q13 - yes, that's what I expect
Q14 - Is the reason you think permanganate won't work because of an oxidative cascade type thing, where you get acetone and acetic acid? It does seem like a perfect substrate for that sort of reaction. Shouldn't A and B just have different NMR peaks? Don't see why you'd need to look at chirality. Zn/HCl should work in this situation, though
Q16 - you can tell from the description which one is which

 

[someth1ng]

Q10 looks OK, you can tell what the answer is from the shifted triplet. The quintet peak is probably just poorly resolved - an issue with the NMR machine, not how much the spectra is zoomed in.
Q11 - a buffer solution implies that there is reasonable buffering capacity. An ethanoic acid/ethanoate buffer will not have reasonable buffering capacity at pH > 7, so I think the statement is OK
Q12 - is a tertiary to tertiary hydride shift likely? I don't know the answer - perhaps you are more familiar with these things?
Q13 - yes, that's what I expect
Q14 - Is the reason you think permanganate won't work because of an oxidative cascade type thing, where you get acetone and acetic acid? It does seem like a perfect substrate for that sort of reaction. Shouldn't A and B just have different NMR peaks? Don't see why you'd need to look at chirality. Zn/HCl should work in this situation, though
Q16 - you can tell from the description which one is which

Q10: There are several factors involved that can indicate what molecule it is (e.g. the two hydrogen environments are indicative of a symmetrical molecule). I'm just saying multiplicity is an important part of NMR and that should be made reasonably clear.
Q11: A buffer is just a mixture of an acid/base and its conjugate - a specific concentration is not required. If you had a 10:1 mixture of weak base to strong acid, it's still a buffer but pH isn't necessarily below 7. I clearly didn't read the question lol.
Q12: At higher temperatures, it's likely because even when you form the alkene, you can protonate and rearrange again (i.e. the product isn't inert)
Q14. I totally tunnel-visioned when I saw the chiral centre and could identify with NMR easily (the number of H environments should also be different), then forgot to answer the question lol. You could use potassium permanganate to oxidise the alcohol (secondary vs tertiary) but you'd still need to identify the product? TBH, it's a bit hard to answer now because I'd never bother using these tests except NMR.

 

[someth1ng]

Thanks for the comments.

I'll refrain from commenting on them all until I publish which were the easiest and hardest questions, but for now:

Q1: I was pleased that this did not trick as many students as I expected.

Q7: Anion testing in module 8 is restricted to a set of specified anions and cations, so the interpretation is valid within the confines of the syllabus. The question was certainly testing whether students distinguish observation from inference. I can observe a gas being formed without knowing its identity... attributing an identification to the gas is properly an inference drawn from some other observation - such as bubbling the gas into limewater and looking for a milky precipitate - or some other information.

Q10: Can be solved without examining splitting or looking at chemical shifts. The quintet appears clear on my original, but I will consider a better image for the final version.

Q11: The concentrations are deliberately not specified and a 1:1 mixture would not produce a buffer.

Q14: The HSC does not introduce chirality. What difference in IR would you expect that was simple to interpret at a qualitative level?

Q16: The student is meant to deduce which is which from the information given.

Q11. Yeah right, I just re-read the question and it's A lol. B will give a pH below 7. I guess the question was designed for someone like me haha.

Q14. Yeah, really depends on what information is available (I'm normally have reference samples, so these questions can be confusing for me in their context) but the spectra would certainly be different.

 

[Gloucestershire]

Q10: There are several factors involved that can indicate what molecule it is (e.g. the two hydrogen environments are indicative of a symmetrical molecule). I'm just saying multiplicity is an important part of NMR and that should be made reasonably clear.
Q11: A buffer is just a mixture of an acid/base and its conjugate - a specific concentration is not required. If you had a 10:1 mixture of weak base to strong acid, it's still a buffer but pH isn't necessarily below 7.
Q12: At higher temperatures, it's likely because even when you form the alkene, you can protonate and rearrange again (i.e. the product isn't inert)
Q14. I totally tunnel-visioned when I saw the chiral centre and could identify with NMR easily (the number of H environments should also be different), then forgot to answer the question lol. You could use potassium permanganate to oxidise the alcohol (secondary vs tertiary) but you'd still need to identify the product? TBH, it's a bit hard to answer now because I'd never bother using these tests except NMR.

Q10: I agree with you on this. I ran the simulated 1H spectra and the quintet shows five distinct peaks, so it's not a complex phenomena causing peak overlap, which means a clearer graph would also match with what you'd get in reality.
Q11: Do you have a reference for that definition? Wikipedia says "A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small amount of strong acid or base is added to it.", chemguide says "A buffer solution is one which resists changes in pH when small quantities of an acid or an alkali are added to it." so there seems to be a mention of resistance to pH change.
Q12: You're right on this. I forgot that rehydration wasn't necessary for bond shifting - all you need is protonation, which can occur in the presence of concentrated sulfuric acid.
Q14: The idea is that the potassium permanganate gets decolorized when it oxidises something - so if the purple colour remains, it didn't react (at least significantly). However, potassium permanganate has a tendency to react with many things that are not alcohols, so it isn't a simple matter to determine if a particular substrate will react with it.

 

[someth1ng]

Q10: I agree with you on this. I ran the simulated 1H spectra and the quintet shows five distinct peaks, so it's not a complex phenomena causing peak overlap, which means a clearer graph would also match with what you'd get in reality.
Q11: Do you have a reference for that definition? Wikipedia says "A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small amount of strong acid or base is added to it.", chemguide says "A buffer solution is one which resists changes in pH when small quantities of an acid or an alkali are added to it." so there seems to be a mention of resistance to pH change.
Q12: You're right on this. I forgot that rehydration wasn't necessary for bond shifting - all you need is protonation, which can occur in the presence of concentrated sulfuric acid.
Q14: The idea is that the potassium permanganate gets decolorized when it oxidises something - so if the purple colour remains, it didn't react (at least significantly). However, potassium permanganate has a tendency to react with many things that are not alcohols, so it isn't a simple matter to determine if a particular substrate will react with it.

Q11. Yeah, I didn't read the question properly (and also wasn't really thinking lol). A buffer doesn't require any specific concentration or ratio as long as both conjugates are present in decent quantities (I think the definition is deliberately left vague).
Q14. This is why I'm not a fan of chemical test questions - they're often a bit...vague?

 

[Gloucestershire]

Q11. Yeah, I didn't read the question properly (and also wasn't really thinking lol). A buffer doesn't require any specific concentration or ratio as long as both conjugates are present in decent quantities (I think the definition is deliberately left vague).
Q14. This is why I'm not a fan of chemical test questions - they're often a bit...vague?

Q11. We can actually do the calculation to determine the ratio required: The Ka of acetic acid is 1.78e-5 and the H+ concentration is 1e-7 so the acetate/acetic acid ratio is about 178:1. I suppose you are saying that if you have a large quantity of this 178:1 ratio solution, it'll be able to resist pH change of some small amount of NaOH or HCl added. So there's a small, but not zero buffering capacity. Ultimately it depends on the exact definition of a buffer.
Q14. I think that's the nature of reactions You don't know what you'll get until you perform the reaction.

 

[someth1ng]

Q11. We can actually do the calculation to determine the ratio required: The Ka of acetic acid is 1.78e-5 and the H+ concentration is 1e-7 so the acetate/acetic acid ratio is about 178:1. I suppose you are saying that if you have a large quantity of this 178:1 ratio solution, it'll be able to resist pH change of some small amount of NaOH or HCl added. So there's a small, but not zero buffering capacity. Ultimately it depends on the exact definition of a buffer.
Q14. I think that's the nature of reactions You don't know what you'll get until you perform the reaction.

Yeah, something like that. I already got the question wrong because I can't read so. There's that lol. Happens to all of us xD

Q14. Yeah, it's just that...what you see is very dependent on how the reaction is set up and the techniques aren't really used anymore.

Overall though, the trial was actually excellent and it's insanely difficult to write an exam like that while staying within the syllabus.

 

[CM_Tutor]

First, it's great to see an active discussion of the paper.

To pick up on a few topics...

Q11. I've seen different guidelines on what is needed for effective buffering. If you take the view that you need the ratio of the conjugates to be between 10:1 and 1:10 then buffering is present only in 1 pH unit on either side of the pK_a. I usually advise my students not to push beyond 20:1 at most (so plus or minus 1.3), and I have seen sources arguing for a ratio between 1:5 and 5:1. Under any reasonable definition, an acetic acid / sodium acetate buffer will not have significant buffering capacity at a pH above 7 as it is far too far from its pK_a of 4.76. I included it as an option since many students only ever see a couple of examples and I suspected many would not be able to recognise that a buffer can be made with a suitable addition of acid or base to any starting material which is weak and which has a weak conjugate. As it happens, option B was selected over option A at approaching a rate of 2:1.

Q12: Hydride shifts, rearrangements on protonation, and related topics are not addressed within the HSC syllabus. While I agree that (A) can be formed, this is not covered by HSC chemistry unless one of the other products is isolated, rehydrated, and a second elimination / dehydration process occurs. It is true that the yield of thermodynamically disfavoured products will be poor if the system is heated with excess acid for an extended period, such considerations are also beyond HSC chemistry.

Q14: Organic chemists have an enormous variety of reagents to use for oxidation and reduction. The HSC, unfortunately, treats oxidation as a process involving acidified dichromate or permanganate and does not explore that these are powerful oxidants ill-suited to many tasks. The present syllabus does not cover the reactions of permanganate with alkenes to diols or to initiate bond cleavage. It does teach that oxidation of primary alcohols goes to the aldehyde which is then further oxidised to the carboxylic acid, but does not explore the difficulty in obtaining the aldehyde from such a system before the carboxylic acid is formed. The use of chemical tests does not properly explore their potential limitations in many cases. In this question, I was looking for students to recognise that bromine water would not be decolourised (and so was not useful as it would react with neither isomer), that testing for the acid group would be similarly unhelpful as both isomers would give the same positive result, and that the spectroscopic differences would be obvious in the ¹H NMR spectra but not in the IR. Since the IR analysis only goes as far as a hydroxyl is somewhere around X, a carbonyl around Y, etc, I can't see any difference in the IR that would be meaningful from an HSC perspective.

Q10: I agree on the importance of multiplicity. I thought it was clear. I have looked at altering the graphic used to make the quintet structure clearer. There was a need to use multiplicity in the question at the end of the paper, though, so it was covered.

I do appreciate all the feedback, its constructive and gratifying.

 

[CM_Tutor]

Q27: Very cool question. A few points: A is effectively the environmental background because it precedes the zinc smelter. However, the calibration curve does not include a non-zero data point below A, so the linearity data is questionable at this range (i.e. 0-2 ppb).

In terms of data provided, calibration curves normally have an R2 value (usually >0.99), but R2 isn't HSC so...

Q29: Seems like a good, safe question. Solution should have sodium ions, carbonate ions, hydrogencarbonate ions, carbonic acid (in very small amounts), dissolved carbon dioxide, hydroxide, hydronium (in very low concentration), and water.

Second part is good thinking. Vinegar/acetic acid, sulfuric acid are good options and sodium hydroxide (drain cleaner). I'd imagine deploying the solid is better to reduce volume and prevent further spread.

Q30b: huge bait. solubility still goes up with temperature because entropy increases with dissolution.
Q30d: Interesting way of doing titration.

Q31a: Is the term "tautomer" in HSC?

On Q27, that's an excellent point about linearity in the 0-2 ppb range. I am looking for a comment on linearity as far as extrapolating to give the highest result, but I hadn't noticed that the same issue may arise at the low concentrations. The question was meant to stimulate consideration of the validity of the concentrations (in light of an assumption of linearity for higher concentrations) but also for the validity of using them to make claims about the smelter as the origin. I'm looking forward to seeing what issues are raised in the responses.

On Q29, I'm anticipating a significant number to identify hydronium but not hydroxide, and to miss carbonic acid. I won't be surprised if some miss water, too. I'm not going to penalise missing carbon dioxide.

On Q31, no, the term "tautomer" is not covered, which is why I avoided using it. The synthesis of polyvinyl alcohol question is pushing the boundaries of the syllabus, but I hope that I gave enough support for people to recognise the alternative.

 

[tito981]

I didnt understand 32 (d), i just put both the formation of vinyl acetate via esterification and i also put the addition polymerisation reaction. If someone can tell me what the question is asking that would be great.

 

[CM_Tutor]

Overall though, the trial was actually excellent and it's insanely difficult to write an exam like that while staying within the syllabus.

Thank you. I wouldn't say "insanely difficult" but it is certainly a challenge. It does help to have a very strong knowledge of chemistry, the HSC, and education.