VCE Chemistry Exam Discussion and Solutions (in progress)

[keltingmeith]

Alright, now that we have an exam to work off of, solutions should slowly start to appear in the first post - keep your eyes peeled, and I'll make another post when every question has an answer. I don't have the time to go through the entire topic to check any contentions, so if you have one with of the suggested answers, post in here and a mod will change the front if need be and let you know if you're right or not.

[The Bus]

Lol already made 3 careless mistakes it seems..

I’m so dependent on excess time to look over my work to do well, it’s sad..

[flozza00]

My comments on the suggested MC answers are: (see page 7 for my choices)
Q2. What logic is used to arrive at C? the question is not asking for optimum operating conditions but for maximum number of recharges. This particular cell is not mandated in the study design and students do not have access to the internet while doing the exam.
Q3 I chose D as some definitions of zwitterion include no overall charge.
Q12 I thin k we are looking for the most unsaturated fatty acid here, so A
Q15. Sadly alcohol is actually toxic to cells. Alcohol lowers there polarity of the surrounding solution (from the usual watery environment) so Hydrogen bonding arrangement can change leading to denaturation. So B.
Q18. Increase in temp will increase both forward and back reaction rates and the forward reaction is exothermic co increase in temp will lead to a shift to reactants so a decrease in the conc of NH3 . B.
Q25. Amylopectin is more readily converted to glucose than amylose so it leads to a higher GI. The quest asks for lowest GI so we are looking for the highest amylose content which is A.
Q28 I found tricky but I chose C. because the lowest E0 reaction is still V2+ to V3+ but now as V3+ forms it could go on to be converted to VO2+ as the E0 for this is only +.34 so still low enough if the concentration of V3+ was high enough. I think B is unlikely as you are reversing an equation with the second highest E0 value given.
Q30. We need to consider charge as well as atomic mass here. The gold ion needs 3 electrons to make one atom whereas the silver ion needs only one. Use n=it/F to get the mole of e- used to plate the Pb then use the same moles of e- to work out the mass of each metal that will be plated, taking care of mole ratios, I think you get silver as the winner so B.

[VanillaRice]

Here are my suggested answers to the multi choice questions that keltingmeith has highlighted in their answers. Note that these are not anymore correct than the current solutions, they are my opinion only, so feel free to tell me where I'm wrong

3 D
I agree that D is the only reasonable answer, although the question can indeed be interpreted in different ways.

10 D
I'm pretty sure that zwitterions are by definition uncharged molecules (with internal charges), and I recall this being referenced somewhere in an examiner's report a few years ago.

13 A
C and D aren't correct (petrodiesel is neither renewable, nor does it have the lowest CO₂ emissions). I'm saying A because it takes the least amount of fuel to travel the same distance as the other fuels.

15 B
Going from keltingmeith's response, I agree that C is not completely correct (since not all bonds are broken). I think that without the question specifically stating "denaturation of a protein in the body, B can be the correct answer.

16 A
This question seems to require some very detailed knowledge about the two vitamins (which is fair game, considering that it's in the study design and all ). Vitamin D is not water-soluble, so C isn't correct. B is for vitamin D only, so isn't right either. D could be true (not actually sure if leafy vegetables are a good source of vitamin C), but "without wearing sun protection" seems  to go against the "safely" term in the question.

19 D
B is not quantitative. I don't C is the answer, since viscosity would be the best point of comparison. D seems to be the best answer. Not sure if A is actually qualitative.

25 A
Lowest GI would imply it takes the longest to be digested and absorbed into the body. Amylose is less branched and compact than amylopectin, so low GI would imply high amylose.

My comments on the suggested MC answers are: (see page 7 for my choices)
Q2. What logic is used to arrive at C? the question is not asking for optimum operating conditions but for maximum number of recharges. This particular cell is not mandated in the study design and students do not have access to the internet while doing the exam.
Q3 I chose D as some definitions of zwitterion include no overall charge.
Q12 I thin k we are looking for the most unsaturated fatty acid here, so A
Q15. Sadly alcohol is actually toxic to cells. Alcohol lowers there polarity of the surrounding solution (from the usual watery environment) so Hydrogen bonding arrangement can change leading to denaturation. So B.
Q18. Increase in temp will increase both forward and back reaction rates and the forward reaction is exothermic co increase in temp will lead to a shift to reactants so a decrease in the conc of NH3 . A.
Q25. Amylopectin is more readily converted to glucose than amylose so it leads to a higher GI. The quest asks for lowest GI so we are looking for the highest amylose content which is A.
Q28 I found tricky but I chose C. because the lowest E0 reaction is still V2+ to V3+ but now as V3+ forms it could go on to be converted to VO2+ as the E0 for this is only +.34 so still low enough if the concentration of V3+ was high enough. I think B is unlikely as you are reversing an equation with the second highest E0 value given.
Q30. We need to consider charge as well as atomic mass here. The gold ion needs 3 electrons to make one atom whereas the silver ion needs only one. Use n=it/F to get the mole of e- used to plate the Pb then use the same moles of e- to work out the mass of each metal that will be plated, taking care of mole ratios, I think you get silver as the winner so B.

2) I think it should A be as well - since batteries have a longer life under colder conditions.

18) Do you mean B?

[Tknight]

Iv'e lost 5 marks on MC ( what will be the cut off this year for A+ any predictions?

[barryexo]

Where is the answers, i cant access it

[VanillaRice]

Where is the answers, i cant access it

They're being added to the original (first) post

[lovebiology]

I chose C for Q23 of MC, because I think taking samples from three sources will improve accuracy? Can someone explain why the answer is A?

[bentennason]

I chose C for Q23 of MC, because I think taking samples from three sources will improve accuracy? Can someone explain why the answer is A?

To take from different sources means you are not keeping the crude oil a constant variable. You may get variable results if you collect a sample from the tank of a truck, 500m underground, etc.
If you use a thermometer,
1) the accuracy is questionable if you're stirring one corner of the beaker
2) you may read it differently to somebody else

[waterangel82]

Hi guys,

I have checked the both the mc answer short answer questions, but I need a bit more of checking with my answers to short answer questions.

For question 1, I did write H2O and MnO4/H+, but do we absolutely have to write states - I didn't for these? Like for the reaction between propanol and ethanoic acid, I wrote all of the reactants and products in liquid state? And also, do we have to absolutely include the H2SO4 catalyst for the formation of ester? (I forgot this)

For question 6ai, would it be okay for one mark to say that the electrolyte helps to carry ions between the electrodes so that they could react to form the products required?

For question 6bi, when it says 'state two advantages of PEM fuel cell', I mentioned that the fuel cell is more energy efficient compared to diesel engine, and also that it is more environmental friendly as no CO2 is produced. Would this be okay? (by the way, there is such thing as a closed system diesel engine in submarines)

For question 8d, when it says that why carbon anode cannot be interchanged with iron, I did say that iron is a stronger reducing agent and oxidises in preference to fl-, however, my half equation had the wrong state for Fe2+ cos I said that it was 'aq' instead of 'hf'. Would I get penalised for this?

For question 9b, would concentration of NaOH be an appropriate controlled variable - cause I said that higher concentrations of NaOH will result in less titre being delivered?

For question 9f, I wrote for preparing the solution you needed 'safety glasses, gloves and gas mask' and for conducting titration you needed 'safety glasses, gloves'. Would this be correct?

[dk1234]

What do u guys think about mcq 15. I thought it was C, whilst not all bonds are disrupted I thought it was a better option than alcohol, as we do digest and consume alcohol. 

What do you guys also think a 100/120 and a 85 percent sac average in a strong cohort will get?

[The Bus]

@waterangel

Yes you would lose marks for wrong states, as mentioned in many examiners’ reports.

Yes the concentration of NaOH is one of the controlled variables in the experiment.

[bentennason]

What do u guys think about mcq 15. I thought it was C, whilst not all bonds are disrupted I thought it was a better option than alcohol, as we do digest and consume alcohol. 

doesn't denaturation affect secondary and quaternary too? so the statement that all bonds are disrupted in tertiary is a bit vague

how did y'all interpret MCQ28?
I went with excess O2 means that O2 is always being reduced at the cathode
V(II) -> V(III) + e     (-0.26V)     
V(III) + H2O -> VO(II) + 2H + 2e    (0.34V)
then theoretically VO(II) should keep oxidising to VO2(I) but theres no answer for that
So V(II) -> V(III) intermediate -> VO(II)

any thoughts?

[Willba99]

then theoretically VO(II) should keep oxidising to VO2(I) but theres no answer for that

my thoughts exactly. I got that far, then guessed the answer

[keltingmeith]

On my phone plz forgive short responses, just ceebs.

My comments on the suggested MC answers are: (see page 7 for my choices)
Q2. What logic is used to arrive at C? the question is not asking for optimum operating conditions but for maximum number of recharges. This particular cell is not mandated in the study design and students do not have access to the internet while doing the exam.
Q3 I chose D as some definitions of zwitterion include no overall charge.
Q12 I thin k we are looking for the most unsaturated fatty acid here, so A
Q15. Sadly alcohol is actually toxic to cells. Alcohol lowers there polarity of the surrounding solution (from the usual watery environment) so Hydrogen bonding arrangement can change leading to denaturation. So B.
Q18. Increase in temp will increase both forward and back reaction rates and the forward reaction is exothermic co increase in temp will lead to a shift to reactants so a decrease in the conc of NH3 . A.
Q25. Amylopectin is more readily converted to glucose than amylose so it leads to a higher GI. The quest asks for lowest GI so we are looking for the highest amylose content which is A.
Q28 I found tricky but I chose C. because the lowest E0 reaction is still V2+ to V3+ but now as V3+ forms it could go on to be converted to VO2+ as the E0 for this is only +.34 so still low enough if the concentration of V3+ was high enough. I think B is unlikely as you are reversing an equation with the second highest E0 value given.
Q30. We need to consider charge as well as atomic mass here. The gold ion needs 3 electrons to make one atom whereas the silver ion needs only one. Use n=it/F to get the mole of e- used to plate the Pb then use the same moles of e- to work out the mass of each metal that will be plated, taking care of mole ratios, I think you get silver as the winner so B.

2. Chose that using the logic that those temperatures are more likely to mimic everyday life. If you think there's something being taught that would better apply to this question than my simple logic, more than happy to change the answer.

12. I'm actually fairly confident here - they're all the same length, so more saturation = more spots to oxidise, hence my decision to pick the most saturated.

15. Yeah, it was close between the two. The only thing is enzymes typically exist in water, with alcohols being a weaker hydrogen bond donor and acceptor, so I kinda can't see how they'd compete with the water for interactions with the protein? Unless I'm thinking about it the wrong way, but I'm assuming in the body and they might be implying a benchtop environment.

18. Loooool can you tell it was like 1 in the morning when I did this? Changing to B.

25. Cool, I figured amylopectin is absorbed faster, I must've interpreted GI itself wrong, hence having the exact opposite answer. will change

28. Yeah, I didn't like picking B for that reason. I do like your logic for C though, will change.

30. Reckon you could step through those calcs? Because I tried to go pure logic (you don't want to know how long it's been since I've done this kind of electrochem, it's the reason I don't tutor VCE), and that logic says gold should still return more mass than silver.

Here are my suggested answers to the multi choice questions that keltingmeith has highlighted in their answers. Note that these are not anymore correct than the current solutions, they are my opinion only, so feel free to tell me where I'm wrong

3 D
I agree that D is the only reasonable answer, although the question can indeed be interpreted in different ways.

10 D
I'm pretty sure that zwitterions are by definition uncharged molecules (with internal charges), and I recall this being referenced somewhere in an examiner's report a few years ago.

13 A
C and D aren't correct (petrodiesel is neither renewable, nor does it have the lowest CO₂ emissions). I'm saying A because it takes the least amount of fuel to travel the same distance as the other fuels.

15 B
Going from keltingmeith's response, I agree that C is not completely correct (since not all bonds are broken). I think that without the question specifically stating "denaturation of a protein in the body, B can be the correct answer.

16 A
This question seems to require some very detailed knowledge about the two vitamins (which is fair game, considering that it's in the study design and all ). Vitamin D is not water-soluble, so C isn't correct. B is for vitamin D only, so isn't right either. D could be true (not actually sure if leafy vegetables are a good source of vitamin C), but "without wearing sun protection" seems  to go against the "safely" term in the question.

19 D
B is not quantitative. I don't C is the answer, since viscosity would be the best point of comparison. D seems to be the best answer. Not sure if A is actually qualitative.

25 A
Lowest GI would imply it takes the longest to be digested and absorbed into the body. Amylose is less branched and compact than amylopectin, so low GI would imply high amylose.

10. Sweet, I preferred D, anyway. More than happy to go off of an examiner's report.

13. Glad some of my logic is good. :')

15. Another B rooter. I might change that one, then. Still seems weird to me, but I'm not a big protein chemist, so. :S

16. Wow. The study design really has changed...

I chose C for Q23 of MC, because I think taking samples from three sources will improve accuracy? Can someone explain why the answer is A?

Not a fan of bentennason's logic here - however, C isn't true in the sense that sampling from different places isn't going to improve accuracy. It'll give you a measure of precision. It's not a bad idea, you're right, it just won't improve accuracy.

Meanwhile, the thermometer will definitely improve precision, hence the choice for that.

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Thanks to everyone checking my answers - keep them coming if you have concerns about something I've written.