H2 Chemistry 2013 'A' Levels P2 & P3 & P1

last SG · 2013-11-25T17:34:48+00:00

what's the transition planning about? any stunner questions?

lolol233431432 SG

understandard conditions, H2 must be 1atm, if it is not dry h2 will not be pure and that 1atm of gas may consist of other gases.

anyone can explain to me how does NO2 catalyse So2 in question12?

Edification. SG

Uhh, dont you find it quite presumptuous, since they never limit the amount of gas found in the environment, so for all we know H2 could still be at 1 atm?

UltimaOnline SG

ah ok peace peace. so for 12, do you feel th answer should be D instead? since for B NO2 catalyses th reaction, but in fact it does not right? it jus forms acid rain

It's true that NO2 is a greenhouse gas, but it's not one of the most common or one of the most damaging greenhouse gases. Acid rain from both HNO3 and H2SO4 (for which NO2 catalyzes the oxidation of SO2 to SO3, which hydrolyzes to H2SO4) is probably a greater concern than global warming, in the context of NO2.

UltimaOnline SG

NO2 catalyzes oxidation of SO2 to SO3, which undergoes hydrolysis to form H2SO4.

UltimaOnline SG

Uhh, dont you find it quite presumptuous, since they never limit the amount of gas found in the environment, so for all we know H2 could still be at 1 atm?

Of course, the hydrogen needs to be pure and dry. But it still needs to be dry.

lolol233431432 SG

uhm even without oxidation so2 can also form acid rain already.

UltimaOnline SG

uhm even without oxidation so2 can also form acid rain already.

Yes of course, but it doesn't negate the fact that NO2 can catalyze oxidation of SO2.

All 4 options are not very good, but this seems to be the best answer.

AJQZC SG

Hi, care to explain why Q15 is B and not D? Isn't it just Fe2+ being oxidised to Fe3+ ?

Mapleguy1995 SG

Hi, care to explain why Q15 is B and not D? Isn't it just Fe2+ being oxidised to Fe3+ ?

because u add KMnO4 in excess and purple is clearly darker than pale yellow of Fe3+ so it will cloud the pale yellowness of Fe3+

UltimaOnline SG

You don't need to get all correct to get your A grade, which should be about 73-75% this year. You can do an estimate based on your SPA, P1, P2, P3 performance combined.

The most agonizing estimates and trepidatious waiting for the results, of course, is when you're at the boderline threshold of each grade, and whether you get (for instance) A or B grade, is impossible to say for sure, and no matter which grade you eventually get, you know (either with relief or with even more agony) that it was an extremely close call, either way.

UltimaOnline SG

Q24. I gave Cambridge too much credit and misread that the qn was asking "arrange in order of boiling points" which would have been a far more intelligent and discriminating qn, for which the answer would be C - Q, R, P since OH is protic while CN is aprotic, even though CN has a higher Mr. But nooooo, the silly qn was just asking "arrange in order of Mr", for which the answer is obviously B - Q, P, R.

I guess I was in my typical teacher mode when reading the qn, thinking, "if I were to set this qn, how would I make it more tricky?" and in the end, end up tricking myself. Sheesh.

BadMetEvil SG

Hi, I just wondering why 15 is B. I chose A.

When u slowly add KMnO4 (say, drop by drop) to Fe2+, the purple KMnO4 quickly decolorised and Fe3+ is produced right? So now, the mixture containing only two coloured species: Fe3+ and unreacted Fe2+, that should explain the "darker green" in A. Only when there is no more Fe2+ left, the excess KMnO4 will stay as purple, which is the finally colour of the mixture.

UltimaOnline SG

Hi, I just wondering why 15 is B. I chose A.

Fe3+(aq) is pale yellow, and unreacted Fe2+(aq) is pale green, there is no reason why such a mixture would be dark green.

Mapleguy1995 SG

Fe3+(aq) is pale yellow, and unreacted Fe2+(aq) is pale green, there is no reason why such a mixture would be dark green.

yeah and plus Mn2+ is pale pink in colour which is darker than pale yellow and pale green so will cloud out the colour of these 2. S

Yit SG

Hi sir,
Just want to ask for qs 37, the qs stated that the reaction is prepared by slowly adding conc H2SO4, so am I right to assume that conc H2SO4 is not added in excess? And that is the reason why I exclude (1) which is an alkene as my ans since to form alkene from alcohol u need excess conc H2SO4 at high temp.

UltimaOnline SG

Adding conc H2SO4 in excess would ensure a high yield of the dehydration product, the alkene. But adding it slowly (to keep the temperature low), would still yield a small amount of the alkene. Moreover the question specifies, "what by-products are possible if the temperature is allowed to rise?"

MCQs are often problematic due to such ambiguity. Every year since 2011, there are always a couple of questions from the A level H2 Chemistry papers (especially P1), that school JC teachers disagree with each other, and/or disagree with Cambridge, on what the most correct answers should be.

This year 2013, is no exception.

Mapleguy1995 SG

well if u exclude 1, then its impossible to form 2..and ur ans will oni be left with 3..and plus conc H2SO4 will eventually be in excess after adding lots of drops to form 1 and eventually form 2 from 1

UltimaOnline SG

Correct. Even though the reaction conditions are more ideal for generating alkyl halides instead of alkenes, in industrial practice, there's always a mixture of competing reaction pathways, eg. substitution vs elimination. Which is why further separation and purification of the desired products must always be done.

Yit SG

@UltimaOnline & Mapleguy1995, I understand ardy, thanks for clearing my doubts! (: Well this paper proved to be tricky for me and with some careless here and there, I only got a total of 28/40, well there goes my A D:

UltimaOnline SG

Perhaps your P2 and P3 will pull your overall score up.

But at the very least, in the worst case scenario, looks like your B grade should be secure. And if you do well for your other subjects, with As and Bs, you should still be able to get into a decent Uni course, eg. Chemistry @ NUS or Chemistry @ NTU. I've heard of students with B for H2 Chem, still successfully getting into Chemistry @ NUS / NTU, as long as their/your overall score is good enough.

And worst case scenario, you can always retake as a private candidate (it's not as bad as it sounds).

P.S.
Actually it's not that bad, your 28/40 = 70% exactly. You just need to do slighty better than 70% for your SPA, P2 and P3, and it's still possible to get an A grade, which is most likely to be around 73% or so this year.

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