You can't polymerise vinyl alcohol because it exists almost entirely as acetaldehyde, as the preceding question / calculation proved. However, you can polymerise vinyl acetate and then hydrolyse the ester groups to produce polyvinyl alcohol.
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2021 BoS Chemistry Trial (2 Viewers)
- Thread starter CM_Tutor
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oh thats pretty neat, was wondering why i got like 0.001% remnants of the alcohol.
There were quite a few places where one part of a question would provide support for the next... perhaps not as well as I intended, though
cassicowfan
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was the boric acid one meant for that as well?
Also for 32 (d) the question states that the silver mirror test works by re-dissolving the silver precipitate and in q18 since a ppt forms via the addition of HCl but doesnt re-dissolve with ammonia, doesnt this tell students that its not the Ag+ ion present. Is a minor thing ik but i just noticed it towards the end of the exam.
Yes, but I am looking for the issue being raised being recognised in the next part, and that can be done with or without considering boric acid itself
Good point, it does mean that there was a reminder that Q18 could not include silver as one of the three cations... but I don't think it advantaged too many students, given that Q18 was one of the 8 questions answered incorrectly by the majority of students who sat the exam.
For those interested in the hardest / easiest MCQ, you might be surprised to learn that, if I look at questions 1 and 2, one of them was answered correctly by almost twice as many students as the other... so one of them was one of three easiest questions, and the other was in the five most difficult.
someth1ng
Retired Nov '14
Yeah, in the industry (which I'm adjusting to), a single data point (at each concentration) is not sufficient for linearity. Instead, they expect precision data for several data points. For example, at a very low concentration, you'd run the 2 ppb test 6 times and do the same at the top and middle. It turns out, it's possible to get high linearity (R2>0.99) when the data near zero (i.e. the 0-2 ppb region) is effectively random. There are a lot of considerations for linearity/precision of instruments (a lot of which is covered by various regulatory bodies like the EMA's ICH Q2). As far as this one goes, I think it's fair to say there's a tonne of cadmium coming out, but you can't necessarily say the cadmium levels are at safe drinking levels before the smelter.
Not gonna lie, I felt like a smartass writing all of those...it is a bit excessive, but I'd accept any mixture of them.
I only mention it because it's not uncommon to expand the view for peaks of interest to make things clearer. It's more of a resolution issue but still doable without it.
Yeah, I just prefer answers that are more clearly incorrect (I've definitely seen HSC answers be more blatantly wrong) - there's a small number of students that may study beyond the HSC syllabus (for olympiads etc) that may get more confused and I think that can be a bit unfair.
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Gloucestershire
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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 pKa. 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 pKa 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.
There are a few perspectives on this. Personally, I think the correctness of answer should be syllabus-agnostic (for the sciences and maths), and that the questions should be what is syllabus-dependent. The level of explanation demanded by a short answer response can also, of course, vary depending on the nature of the exam concerned, but my opinion is that the final results should not differ. To illustrate what I mean, consider the following made-up question
Ethanal is the major product that would be formed (probably nearly exclusively). However, HSC reactions would suggest that a mixture of ethenol and ethandiols would be formed. My perspective would be that such a question shouldn't be included, as to obtain the correct answer the student must know of reactions beyond those taught in the HSC syllabus.
On the other hand, consider Question 19 of the paper, asking for a statement concerning the number of hydrogen and carbon environments. I would consider this to be syllabus-agnostic, as if you ran the spectra on a NMR machine, you would likely find the results indicated by the answer as there is no effect e.g chirality, or unexpected proton equivalence, that could cause disparate spectra.
someth1ng
Retired Nov '14
I agree with this but I understand that it's difficult to achieve without a big review panel with a broad knowledge base. There's a stupid amount of chemistry that I don't know much about (including some in this trial...either that, or I just forgot it lol) and even then, it's easy to know without realizing other possibilities (like that dehydration question).
Based on what I've seen (and I've come back to BoS for a bit just to see how things have changed), there's still a bit of a disconnect between how things are taught and what's actually useful in chemistry...and it's not because current techniques are new - they're often decades old. For example, doing little reactions to distinguish compounds/salts is not useful (and I've never seen it be used irl) because there are simply better ways of identifying compounds. For organic chemistry, you just need NMR, MS, IR, and maybe elemental analysis.
I am hoping to see comment on the linearity at high concentration, but also noting that there is a large amount of cadmium being detected, even if the accuracy of the calculated concentration is open to question. One of the possible areas for comment on improving the investigation will be what can be done to address the issue. The other validity question is whether the conclusion that the smelter is the source of the cadmium is the demonstrated by the evidence presented. I agree that comment on the safe drinking level is expected, along with some reference to the given coastal water data. There's a lot in there that can be addressed and I don't expect everything to be addressed, but I do expect an answer to match the requirements for an "assess" question.
Your list matched mine exactly. I am looking for:
- recognition of the sodium and hydrogencarbonate ions from dissociation of the salt
- the solvent being water which assures that there must be water, hydronium cations, and hydroxide anions
- recognition of the amphiprotism of the hydrogencarbonate ion, adding carbonic acid molecules and carbonate anions to the mixture
- I won't penalise not recognising the presence of dissolved carbon dioxide
You are correct, of course. It's valuable feedback, I would have made the quintet clearer if I had thought it was unclear.
I strongly agree that there is a great danger of disadvantaging stronger students, or those who have further studies, with questions containing flaws. There was a question many years ago that caused just that problem: it presented a table with the properties of 5 substances, a pair of immiscible liquids at RT, and three solids, one soluble in liquid A, one soluble in liquid B, and one soluble in neither. Students were asked for a procedure to separate a mixture of the five substances, but gave no quantities. One student taking the exam recognised that not knowing the quantities meant that the mixture could have the residue at the bottom having a mixture of the three solids due to the solutions being saturated, and then tried to outline a procedure that could account for any combination of quantities and wasted a lot of time dealing with an issue that no one else recognised.
As another example, I have criticised some recent trial questions where permanganate has been used in contexts such as in an MCQ that included cyclohexane, cyclohexene, and cyclohexanol, where the answer given by a student varies with whether they are aware that permanganate oxidises alkenes, despite that not being included in the syllabus. Sadly, questions can be written by teachers whose knowledge of chemistry is insufficient to spot flaws in questions. Consider, for example, this question from a 2021 trial:
The labels have fallen off six bottles. The labels are:
- Aqueous sodium acetate
- Aqueous lead(II) nitrate
- Ethanamide
- Ethanamine
- Ethanoic acid
- Ethanol
I'm not going to comment further so that students reading this can ponder the question and what they might do with it in an exam, but I am nearly certain that my answer would not have been well-received by the teacher marking it.
While I do take on board and welcome the comments on Q12 of the BoS paper, I don't think it suffers from a flaw anywhere near as serious as some others we are discussing. Treating the given alcohol with concentrated sulfuric acid will promote dehydration / elimination of water, and without looking at mechanisms, this can be done by taking an H from one of the three alpha carbons, leading to three immediately anticipated products. Students struggle enough with the idea of major and minor products in the organic chemistry topic so recognising the three ways to eliminate and thus identify which of the four products offered cannot arise by that process is a reasonable, though challenging, expectation. If I wrote the question as a short answer and someone did note other possibilities, that could be credited, but I don't see why someone with knowledge beyond the syllabus would find identifying (A) as the product that is not expected from dehydration disadvantaging. Indeed, (A) was chosen by twice as many respondents in the exam than chose the next most popular option.
In general, I agree.
I don't mind asking questions on unfamiliar chemistry - I asked about the exact tautomerisation that is relevant to your example, after all - but I wouldn't ask the question seeking the incorrect answer. Hydration of alkynes can't reasonably be asked unless tautomerism has been studied, or unless it is given (i.e. ethyne undergoes hydration to form a product which rearranges to ethanal, the major product, ....).
Perhaps I should tweak the question to something like "Which of the following substances would XXX, an HSC chemistry student, not expect to be found in the product mixture ...", to make it clear that the question is meant to be considering standard HSC knowledge? Or, perhaps "XXX has been told that organic chemistry reactions often produce a mixture of products, and that the reaction of this alkanol with concentrated sulfuric acid is an example. Using your knowledge of this reaction, which one of these products would you predict to not be found in the product mixture"?
I don't entirely agree because I see what is "useful" in chemistry as much broader than the specific methods and techniques that a practicing chemist carries out on a day-to-day basis.
I believe that learning chemistry is about far more than knowledge of chemical facts and techniques. It is about learning to approach problems in a scientific way, to think as a chemist would, and ultimately to become enculturated within the community of practice to which chemists belong.
Take a simple example like the bromine water test for alkenes. It's true that I have only ever done this as a student or as an educator, but never in any of my research work. However, in carrying it out, I need to think about quantities - which reagent do I need in excess, how much do I need, what will the result look like (students who haven't done it tend to forget that there are usually two layers) - I need to think about states (I've lost count of the number of times I've seen someone suggest testing ethene as if it was a liquid at RT), I need to think about conditions and the potential for false results with too much light around. I need to consider whether I can draw a valid conclusion if I add 5 drops of cyclohexene to 5 mL or bromine water and why the proper procedure would reverse these. If taught properly, I need to be aware that the addition can involve Br2 and HOBr and to be aware of equilibrium - and also equilibrium between immiscible phases. I need to consider safety and appropriate disposal of organic wastes. My point is that there is a lot of good chemistry that can be built into exploration of these simple tests, even if the technique I would actually use in a professional laboratory would be different.
These sorts of tests were a major part of the development of analytical techniques and the historical journey of discovery can also be highly instructive. What assumptions am I making in doing (say) cation and anion analysis, and why do they matter? Many analyses start with attempting to dissolve an ionic material in water, but that is a problem if my assumptions about what it could be has not excluded something like lithium aluminium hydride, for example.
Also, on the word "useful", this points to another part of scientific understanding that the HSC is trying (perhaps unsuccessfully) to explore... Science is full of models and theories and most students start by wanting to know which is "right" and which is "wrong", and struggle to develop the more nuanced perspective that a model may be useful in one circumstance but not suited to another. Acids and bases is a classic in this regard. For example, Arrhenius notions are useful in limited circumstances and still appear in places (how many organic mechanisms with arrow pushing use H+ despite us all knowing that that species is essentially non-existent in a liquid phase). HSC students learn Bronsted-Lowry, not so much as a replacement for Arrhenius, but as a model with much broader applications. They touch on Lewis theory, but it is not as if Lewis is "true" and BL "untrue", it's just that Lewis is applicable to many organic chemistry circumstances where BL is not, but that doesn't render BL any less useful in the contexts in which it applies. Physics tries the same idea with classical motion and its limitation in relativistic circumstances. It's not that relativistic equations don't apply to driving to the local supermarket, it's that they are needlessly complex when a simpler model applies just as well.
What a student needs to learn (in terms of facts) is far more important as a vehicle for developing their skills and thinking processes. Equipping them with the ability to analyse a problem, to recognise a need to consult references, to source them and apply what is self-taught from them, is far more important than making sure they know technique X or fact Y. The facts are used as a vehicle and a context to develop skills and understanding. Not all develop in this way, of course... some will learn the simple tests as facts to regurgitate and not recognise the deeper ideas and concepts that are embedded within... and certainly there can be a lively debate about how to achieve defined goals and best ways to teach / learn / educate / enculturate... but, in my opinion, the fact that a particular area of information is not directly used in practice is not sufficient a reason to not explore it in an educational context.
By the way, on enculturation... we tend to think of the idea of an apprenticeship as a means of developing skills in a trade. It's actually the basis for our most advanced education. A PhD is very much an apprenticeship to a skilled professional (one or more academic supervisors), working in a team with others of varying skill levels, and ultimately been judged by other members of the community of practice to be granted acceptance / entry into that community by the granting of the degree. At an earlier (undergraduate) level, the analogy is weaker, but lectures / lab instructors, and at school, teachers, are fulfilling similar roles but with the instruction and support much more top down than the collaborative mode that typically develops when undertaking research degrees. My big issue with science teachers with inadequate content knowledge is that they may not be enough within the broadest community of scientific practice to be modelling scientific thinking and reasoning, applying knowledge, and seeking out information when needed. A teacher who insists they s/he is "right" in the face of questioning is asserting their correctness by reference to authority rather than demonstrating the evidence that confirms their position... and if they are wrong and won't consult sources to check or admit it, they are showing their own inadequacies as a science practitioner. But that's another topic...
someth1ng
Retired Nov '14
Yeah, it'd be nice if the HSC actually taught a few different approaches to oxidation/reduction reactions in the HSC (e.g. LiAlH4, NaBH4) so they can actually ask questions that aren't just adding potassium permanganate.
For the one in purple, this is an example of a question I don't like because it's way too broad and you should consider physical properties...you don't necessarily need many chemicals to distinguish (which few people may consider). I would:
1. Ethanamide is the only solid (but you'd need to know this).
2. Only lead acetate will precipitate when added to NaCl solution.
3. Distillation will show which one is sodium acetate since it will leave a lot of residue (after identifying lead)...or just add it to chloroform and see which one left is biphasic (or find which one is biphasic first, then NaCl to test them)
4. Ethyl amine will evaporate at rt, so leave it outside for a bit and see.
5. Ethanol, acetic acid...add water, pH test or put it in the freezer (acetic acid should freeze).
As for Q12, I don't think the question is bad or anything, A is clearly the one that's furthest, but I'd recommend a product that requires going through a primary or secondary carbocation to be sure.
Gloucestershire
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I agree with what you have written. The italicized portion was the intent of the example.
The issue being discussed about Q12 falls on the relatively minor end, all things considered. However, I think it is always useful to seek to reduce ambiguity, where possible. In this case, a simple change would be to make option A a terminal alkene which is not the thermodynamic product and probably not the kinetic product, 3,4,5-trimethyl-1-hexene, and to adjust the wording from "Which of the following substances would not be present in the product mixture formed when this alkanol is treated with concentrated sulfuric acid?" to "A product mixture is formed when this alkanol is treated with concentrated sulfuric acid. Of the following substances, which would present in the lowest quantity in the mixture?"
Disadvantage can come in subtle ways. Sometimes when the "obvious" answer is not the chemically correct one, those with more experience (than the syllabus) will not see the answer clearly, even if the intent (i.e of simple dehydration) should seem clear. For example, the formation of option D is expressly disfavored by Zaitsev's rule. A student aware of this would then be faced with the decision to choose between a disfavored product, and an alkene shift that can easily occur under the conditions mentioned. The solution you propose, which is to make reference to the HSC student, goes some way to reduce ambiguity and may well be the only solution sometimes, however, I find it to be rather inelegant since it is addressing the syllabus and not the chemistry.
Discussion on Q12 aside, Q14 is the one I think presents ambiguity, as it is not made clear whether the student has access to reference spectra for the IR spectroscopy. Discussion of the procedures conducted may go someway to reducing this ambiguity. E.g
I think the teacher asking the question thought all six would be liquids. I think the teacher didn't consider just how dangerous ethanoic acid that is water-free or even glacial actually is, nor that it will solidify if cooled down to about 10 degrees C. Ethylamine is meant to be a liquid in the question, so is it meant to be an aqueous solution?
I'd be tempted to test conductivity as that would give a lot of information, but the results would differ for genuinely pure ethanoic acid v. glacial v. any sensible concentration in a lab. Similarly for ethylamine, which would conduct (though poorly) in aqueous state, though not as a pure liquid (if it was one at RT, which it isn't).
Yes, if you had the reference spectra for A and B, then IR certainly would be useful. I didn't think from that perspective as identification by comparison to reference spectra is not covered at all, but it is widely used beyond the HSC. Perhaps another issue to clarify, that I mean you are given a sample and collect the spectrum and need to decide from it alone.
someth1ng
Retired Nov '14
Yeah, that question is just poorly designed. I know a few people that got blisters from glacial acetic acid which appear instantly. In contrast, I've had 37% HCl on my skin but some running water and there are no issues. It's surprising how much more dangerous some weak acids can be.
There's a lot of ways to test for this. You could just get CDCl3 and test every solution and from the salts, you can do a precipitation test.
I agree that there are plenty of approaches, which actually makes marking more difficult to. What astounds me is that a teacher at HSC level would not see any of the problems with it, and nor did anyone notice in the checking process. I wouldn't let first year uni students near glacial acetic acid, let alone genuinely anhydrous acetic acid! So why would a teacher contemplate a question on it without considering "hmmm... this is too dangerous to let them near in reality, maybe I should ask something else". Maybe they considered oleum or anhydrous perchloric acid...