H2 Chemistry 2013 'A' Levels P2 & P3 & P1
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Originally posted by lolol233431432:
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. -
NO2 catalyzes oxidation of SO2 to SO3, which undergoes hydrolysis to form H2SO4.
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Originally posted by Edification.:
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. -
uhm even without oxidation so2 can also form acid rain already.
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Originally posted by lolol233431432:
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.
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Hi, care to explain why Q15 is B and not D? Isn't it just Fe2+ being oxidised to Fe3+ ?
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Originally posted by AJQZC:
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+
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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.
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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.
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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.
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Originally posted by BadMetEvil:
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.
Fe3+(aq) is pale yellow, and unreacted Fe2+(aq) is pale green, there is no reason why such a mixture would be dark green. -
Originally posted by UltimaOnline:
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
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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. -
Originally posted by Yit:
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.
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.
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Originally posted by Yit:
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.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
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Originally posted by Mapleguy1995:
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
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. -
@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:
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Originally posted by Yit:
@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:
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. -
I posted :
Considering the difficulty of all 3 papers, as well as how all the Singapore JCs have primed their students for a super killer paper, it's most likely around 75%. But 73% for A grade is still very possible, and I don't want to overly dishearten the many students who scored between above 70%, but less than 75%. It's actually still possible that Cambridge might still allow an A grade for 71% or 72% this year. Though my final guess is between 73-75%.
PizzaTeddy posted :
Well our college heard that HCI told their students 80-85%, which definitely scared us out of our wits. Hopefully its just a rumour haha
I posted :
That's just BS. Don't trust those who exaggerate and sensationalize.
As long as you have approximately 75%, your A is secure.
As long as you have above 70%, you still have a good chance at getting A.
Enjoy your Dec holidays, results should be released on 3rd March 2014.
Bryannn posted :
WOAH 80% is insane! I don't know how it can go up that high and how you guys calculate the projected A grade but from what I know(from friends and in this forum), only paper 2 is tough and others are very easy but with lots of carelnessso should it be safe to assume to get an A,candidates should score above 75% for ALL papers except paper 2? If so,it should be something like (0.75 X 65 + 0.7 X 35) = 73.5%.
That's like wayyy below 80%,even 75% would be alittle too farfetched. Haha.I posted :
Interesting that your estimate of 73.5% is curiously close to mine, between 73% and 75%.
75% isn't as far fetched as some might like to think (or desperately hope for), but 85% (supposedly HCI's estimate) is just plain BS.
And I already said in an earlier post, 80% is the upper possible limit, but even then, very unlikely for H2 Chemistry to ever hit this threshold on the bell-curve.
And as I said earlier, there's still a chance that Cambridge might still allow an A grade at 71% or 72%, so those of you at the boderline threshold, take heart, you still stand a chance to get your A grade.
Apply realistically for a variety of Uni courses within your reach; take the time now these couple of months, to find out more about all the Uni courses. And as a last resort, if your overall UAS falls short of all your desired courses, retaking your A levels isn't as bad as you might think. -
Do you think HCI said is 80% because of spa? Cause I thought normally almost every student would get the full 15% for spa, thus making 80% realistic and possible?
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Originally posted by CaiHongRainx:
Do you think HCI said is 80% because of spa? Cause I thought normally almost every student would get the full 15% for spa, thus making 80% realistic and possible?
SPA is problematic, and to some extent, skewed differently in different JCs, abit akin to PW, but not quite as biased and unfair.Still, majority of students score closer to 10% rather than the full 15%, and there are always failures in every JC, even for SPA.
80% for A grade for H2 Chemistry is not impossible, but not realistic and very unlikely, especially after considering the difficulty level of the 2013 H2 Chem papers, each of which did contain a very fair range of easy, standard, and difficult questions.
I still stick to my estimate of 73-75% for A grade this year.
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Err really seriously people, why are you guys debating about the answers and all!? Whats done is already done! Just move on and study for your next paper and wait for the results to be out next year! Lol is there any use predicting how many % you need to get A n stuff!!!? Its all up to cambridge! Comparing answers will do you no good except drive you into depression.
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Hi. Could anyone kindly tell me why the answer to 8 is not A? The temperature falls from 25 to 0 degrees, isn't it less disorder, hence negative?
Also, why is it radius of atom is the answer to 13 and not electrical conductivity? And lastly question 19, I cannot locate the fifth chiral centre.
Thank you!
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Originally posted by alycx:
Hi. Could anyone kindly tell me why the answer to 8 is not A? The temperature falls from 25 to 0 degrees, isn't it less disorder, hence negative?
Also, why is it radius of atom is the answer to 13 and not electrical conductivity? And lastly question 19, I cannot locate the fifth chiral centre.
Thank you!
Entropy change is linked, but not necessary directly the same, as temperature change. For instance, a reaction could be endothermic, but result in a greater number of species on the RHS (relative to LHS), and so more species = more entropy.Since the enthalpy change is endothermic (ie. thermodyanically unfavourable), as evidenced by the fall in temperature, and yet the Gibbs free energy change is negative (ie. thermodyanically favourable), it means that the entropy change must have been sufficiently positive (thermodyanically favourable) and large in magnitude, to have outweighed the thermodyanically unfavourable endothermic enthalpy change, such that the overall Gibbs free energy change was negative and thermodyanically favourable.
Electrical conductivity increases from Grp II to Grp III (since more delocalized valence electrons to participate in electrical conductivity) before dropping. Not consistently decreasing.
5th chiral carbon? It's there all right. Treat it as a fun puzzle and keep looking for Wally.
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36/40 Well it is not that bad considering I have 3 careless mistakes and 1 conceptual error
Anyway Ultima for question 34 why does the anion have 18 electrons? I took the anion to be O- so there should be 9 electrons? I didn't know they counted both oxygen ionsThe anion in this question, refers to the peroxide anion :
