2015 A Level H2 Chemistry Paper 1, 2, 3
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Originally posted by UltimaOnline:
You've followed my past year threads (plural) on the after-paper discussions for H2 Chemistry, even when you were in Sec 4 / IP Yr 4, and JC 1? Or you dug up these old threads to read them? Either way, good to know you really love Chemistry ;)Ahahaha sorry for the poor command of English. I dug up those old threads to read on some of your answers (and explanation) on certain discussed topics and questions for which the TYS provided answers did not do a good job in explaining. I must say your threads are one of the most reliable, consistent and trustable around.
(I really think that we should not derail the direction of the thread :) )
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Originally posted by LemonRay:
Ahahaha sorry for the poor command of English. I dug up those old threads to read on some of your answers (and explanation) on certain discussed topics and questions for which the TYS provided answers did not do a good job in explaining. I must say your threads are one of the most reliable, consistent and trustable around.
(I really think that we should not derail the direction of the thread :) )
I pointed out you said "threads" (plural) because it means you really love Chemistry and dug up my post-paper discussions for more than 1 year (eg. 2013 + 2014 and now 2015), which shows your passion for Chemistry, especially if it turned out you were already following my threads back in 2013 and 2014 when you were only in Sec 4 or JC1.
No worries about derailing the thread. The thread is still young, there's still P3 and P1 to go. You can look forward to these papers (and how they might affect the bell-curve), and (all JC students do feel free to) share your thoughts on them in this thread after each paper! :) -
Hi for distinguishing HCl and HBr, i added Pb(NO3) to both solutions, is that okay?
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Originally posted by DUbPie:
Hi for distinguishing HCl and HBr, i added Pb(NO3) to both solutions, is that okay?
Cannot, because unlike AgX where the precipitates are white, cream and yellow, for PbX2, the precipitates are white, white and bright yellow. Cambridge isn't a 3 year old kid (Chinese saying), they already sneakily took into consideration that students would simply replace Ag+ with Pb2+ and use the (otherwise) same method. -
I thought this paper wasn't easy. Screwed up big time. Lost at least 6 marks for planning and 11 marks for written (Probably even more due to insufficiently rigorous answers or careless mistakse). It's pretty much impossible to get an A grade already right?
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Originally posted by hallopanda:
I thought this paper wasn't easy. Screwed up big time. Lost at least 6 marks for planning and 11 marks for written (Probably even more due to insufficiently rigorous answers or careless mistakse). It's pretty much impossible to get an A grade already right?
Impossible is nothing. If you can get a high A grade score for P3 and P1, you can still pull up your overall to an A grade. So prepare well for P3 and P1, and be careful not to make any careless mistakes. -
Hi is there error carried forward for chem? i wrote Ag+ + e --> Ag for the half equation and the next part requires calculation of mass of Ag2O :/
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Originally posted by Kaboom97:
Hi is there error carried forward for chem? i wrote Ag+ + e --> Ag for the half equation and the next part requires calculation of mass of Ag2O :/
For *some* of the questions involving calculations, yes. For this question, there should be, but no guarantee. -
Hi , for the question about distiguishing between Br- and Cl-, can I use the displacement method by adding Cl2 (aq) at r.t.p?
If Br- is present, a reddish-brown Br2 solution is seen, but if Cl- is present, no reddish brown solution is seen.
Also, for the very first question of planning, which requires us to give the displayed formula for the ester, do we have to show the shape of the shape (trigonal planar shape around C atom for R-COO-R')?
Thanks!
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Originally posted by shiningrays:
Hi , for the question about distiguishing between Br- and Cl-, can I use the displacement method by adding Cl2 (aq) at r.t.p?
If Br- is present, a reddish-brown Br2 solution is seen, but if Cl- is present, no reddish brown solution is seen.
Also, for the very first question of planning, which requires us to give the displayed formula for the ester, do we have to show the shape of the shape (trigonal planar shape around C atom for R-COO-R')?
Thanks!
Yes, can use Cl2(g).
No need to show the shape (ie. using dash-&-wedge), unless specifically required by the question.
No prob. -
Hi , for the question about distiguishing between Br- and Cl-, I used the displacement method by adding Cl2. However for the observation, I stated that reddish brown gas of Br2 will be seen instead of a reddish brown solution. How many marks will I get? Thanks!
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Originally posted by MadForChemistry:
Hi , for the question about distiguishing between Br- and Cl-, I used the displacement method by adding Cl2. However for the observation, I stated that reddish brown gas of Br2 will be seen instead of a reddish brown solution. How many marks will I get? Thanks!
Was it a 1 or 2 mark question? If 2 marks, you get 1. If 1 mark, you get 0. -
It was a 3 mark question.
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Originally posted by MadForChemistry:
It was a 3 mark question.
Then you should get 2 marks (assuming you wrote everything else correctly). -
ultima this year you still taking as private candidate?
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Originally posted by SBS n SMRT:
ultima this year you still taking as private candidate?
Why u ask leh? U also want ah? -
Feel free to discuss the P3 questions here in this thread after the paper tomorrow! As always, I'll only comment on your answers, instead of giving out full model answers. Because the question paper for P3 can be taken back, within hours many JC teachers and private tutors will be releasing their own solutions. You can always check against their answers, and any questions or concerns, you can ask me here.
Have fun with P3 tomorrow! ;Þ -
So did you find P3 easy or tough? How about your classmates / schoolmates? Which question did you skip?
Overall, I'd say this P3, like P2, is neither particularly easy nor tough. So the A grade boundary thus far (excluding P1) should be approximately 72.5%, give or take 1%. If P1 is surprisingly easy or tough, then the A grade boundary may be increased or decreased, up to 2% or so. -
Did 1,2,3,4 only didn't know how to derive the mechanism for q4 lol
Other than that everything else was good!
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It was quite easy :P
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Not really sure of the mechanism for Q4d (i).
Question says first order with respect to H+ and HMF itself, so i guess the slow step has to involve both H+ and HMF. all i can think of slow step is the subsequent loss of H2O as a leaving group as an Sn1, which i think is reasonable because the carbocation generated is stabilised by resonance away. this means hmf is part of the slow step (the rate determining step) but i couldn't incooperate the H+ in the slow step. not really sure how it goes
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A common groan I hear from students is, "Q5 looks tough at first glance but only after the paper then I realized it's actually the easiest question! So wasted!". True, but one little risk with Q5, is that there are parts d & e, hidden on the last page (ie. the qn looks like it's 2 pages long, but it's actually 3 pages long), and some students didn't realize this, skipped parts d & e, and lost 7 marks.
Some students found P3 easier than P2, while others found P2 easier than P3 (mainly because for many students the time constraints pose more of a problem for this year's P3 compared to P2). I'd say overall P2 and P3 are both of average difficulty.
So weaker students are now hoping P1 will be easier, so it'll be easier to pass. Stronger students are now hoping P1 will be tougher, otherwise the A grade boundary may increase from 72.5% to closer to 75%, and it'll be more difficult to get an A grade.
What do *you* wish for? We'll find out next Wed whether your wish will come true. ;Þ -
Originally posted by nelexus:
Not really sure of the mechanism for Q4d (i).
Question says first order with respect to H+ and HMF itself, so i guess the slow step has to involve both H+ and HMF. all i can think of slow step is the subsequent loss of H2O as a leaving group as an Sn1, which i think is reasonable because the carbocation generated is stabilised by resonance away. this means hmf is part of the slow step (the rate determining step) but i couldn't incooperate the H+ in the slow step. not really sure how it goes
This is indeed the trickiest question out of the entire P3 (and arguably P2+P3 too). Well done, your mechanism is correct. The thing you now need to figure out is, do you know why your mechanism obeys 1st order kinetics for both H+ and HMF? ;Þ -
Its really something like that? I did what nelexus did and just kinda left it cuz I couldnt figur it out. When you say its correct, is it something simple like the protonated alcohol was considered an intermediate and thus in the rate equation it is represented by H+ and HMF?
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Originally posted by Malachitear:
Its really something like that? I did what nelexus did and just kinda left it cuz I couldnt figur it out. When you say its correct, is it something simple like the protonated alcohol was considered an intermediate and thus in the rate equation it is represented by H+ and HMF?
I was kinda running out of time by question 4 so I was really thankful for that little string of short answer organic questions :D