Suggested Answers and Discussion for the 2016 VCE Chemistry Exam

[ciaran3105]

Also, can anyone explain why they the Substitution* reaction of Chlorine to the Alkene was an substitution* reaction AS WELL AS a redox reaction? I thought that the UV light would split the Chlorine gas and that it would become a free radical, thus interrupting the C=C double bond and hence joining the alkene. In my understanding, I don't think that there is donation+accepting of electrons occurs with this reaction.

If anything, I thought that it'd be a covalent bond between the Cl and C

Would anybody be able to back me up or give a conclusive argument against this?

Edited: I meant substitution*, not addition my bad

I believe that the oxidation number of chlorine changes as the reaction progresses, that was my thought process and i put down that it was both.

[Biology24123]

Also, can anyone explain why they the Substitution* reaction of Chlorine to the Alkene was an substitution* reaction AS WELL AS a redox reaction? I thought that the UV light would split the Chlorine gas and that it would become a free radical, thus interrupting the C=C double bond and hence joining the alkene. In my understanding, I don't think that there is donation+accepting of electrons occurs with this reaction.

If anything, I thought that it'd be a covalent bond between the Cl and C

Would anybody be able to back me up or give a conclusive argument against this?

Edited: I meant substitution*, not addition my bad

It was addition not substitution. Only 1 product produced??

[Biology24123]

I really hope 104/120 can get me a 37

[alanyin1]

lmao i fkn double rekt myself yea i meant addition, apologies for the confusion everyone. I'll edit my prior response

[Biology24123]

lmao i fkn double rekt myself yea i meant addition, apologies for the confusion everyone. I'll edit my prior response

I said only addition even though I recognised Cl changed oxidation number. Just doesnt seem right

[alanyin1]

Yeah it feels like it should be only addition, because for a redox reaction to occur, it would need to obtain a free electron from somewhere to turn into the Cl- ion. Plus it's a covalent bond between the C and Cl, which further supports this I feel

[keltingmeith]

Also, can anyone explain why they the Addition* reaction of Chlorine to the Alkene was an Addition* reaction AS WELL AS a redox reaction? I thought that the UV light would split the Chlorine gas and that it would become a free radical, thus interrupting the C=C double bond and hence joining the alkene. In my understanding, I don't think that there is donation+accepting of electrons occurs with this reaction.

If anything, I thought that it'd be a covalent bond between the Cl and C

Would anybody be able to back me up or give a conclusive argument against this?

Edited: I meant addition, not substitution, my bad

Erm... I think the way of answering this is it's just wrong? An easy assumption to make in chemistry - unless you're doing REALLY fucked up things, you won't make a radical (like, the only way I can think of is to bombard what you've made with electrons, as in mass spec).

The easiest way to see why it's a redox reaction is to count the oxidation numbers of each - initially, carbon is 2 and chlorine is 0, then carbon is 3 and chlorine is -1.

Yeah it feels like it should be only addition, because for a redox reaction to occur, it would need to obtain a free electron from somewhere to turn into the Cl- ion. Plus it's a covalent bond between the C and Cl, which further supports this I feel

Not quite - "free electrons" don't really exist, they have to come from somewhere. In this case, they come from the carbon-carbon double bond.

[Nethmini]

Can someone explain why Q8 is A? Sorry. I thought that the fact the concentration found was lower than what it should be meant that the HCL titre had to be less (so that the real concentration calculated would be more)....

[HopefulLawStudent]

Something's bugging me about that question. I saw the change in Cl, but only Cl changed oxidation number though? And it's not a half equation? So shouldn't the oxidation number of something go down and the oxidation number of something else go up for it to be redox?

[alanyin1]

Judging from the oxidation numbers, C would have been -2 initially in the Ethene (C2H4) while it would have been -1 in 1,2 DiChloroethane (C2H4Cl2). If a redox reaction were to occur, the Cl2(g) would have been reduced to 2Cl-, so the C would have increased oxidation number by 1 to balance things?

[duf0002]

Where are people looking at the exam, I want to see how I went on MC but don't even remember which question was which?!

[alanyin1]

Where are people looking at the exam, I want to see how I went on MC but don't even remember which question was which?!

A Copy of the 2016 VCE Chemistry Exam

[keltingmeith]

Something's bugging me about that question. I saw the change in Cl, but only Cl changed oxidation number though? And it's not a half equation? So shouldn't the oxidation number of something go down and the oxidation number of something else go up for it to be redox?

Except Carbon also changed, so. (granted, above I said +2 to +3? Oops, read wrong, should be -2 to -1)

how is C right? just wondering

UV-Vis obviously does concentrations, but you can also use NMR for concentrations if you dope the sample with a known amount of some other compound. It's the exact same idea as making a calibration curve, actually, wouldn't be surprised if some kids were taught this.

Judging from the oxidation numbers, C would have been -2 initially in the Ethene (C2H4) while it would have been -1 in 1,2 DiChloroethane (C2H4Cl2). If a redox reaction were to occur, the Cl2(g) would have been reduced to 2Cl-, so the C would have increased oxidation number by 1 to balance things?

Which, in its own way, it was. That gets us talking about carbocations and reaction mechanisms and such, which is uni level so you'll learn next year if you want to go that way in life, but the easiest way to think about it in terms of VCE is Cl is more electronegative, and so the electrons are more towards the chlorine than the carbon.

[Mellyboo]

Can someone PLEASE put my mind at ease I actually can't study thinking about it, I wasn't really sure if Tush meant yes consequentials are given to subsequent parts of questions. I will be specific about the error I made:
In the determination of the number of mole of CO2 volume question part a., I was out by a factor of exactly 10 because I divided the percentage by 1000 instead of 100... Very stupid..... . I've checked against Tush's answer and it seems that the only error I made was the factor of 10 in part a. Therefore my part c answer was off (0.0997 instead of 0.097). Will the examiners notice this error clearly and award consequential marks for part c. Given that part a. was incorrect clearly only due to a "small" error? Thank you all. I just need to know this so I can rest.

[asdasdasd123]

Can someone PLEASE put my mind at ease I actually can't study thinking about it, I wasn't really sure if Tush meant yes consequentials are given to subsequent parts of questions. I will be specific about the error I made:
In the determination of the number of mole of CO2 volume question part a., I was out by a factor of exactly 10 because I divided the percentage by 1000 instead of 100... Very stupid..... . I've checked against Tush's answer and it seems that the only error I made was the factor of 10 in part a. Therefore my part c answer was off (0.0997 instead of 0.097). Will the examiners notice this error clearly and award consequential marks for part c. Given that part a. was incorrect clearly only due to a "small" error? Thank you all. I just need to know this so I can rest.

yes u would still get consequentials, thushan can confirm