Originally posted by captainkidzs:yYeah I at first thought oh hydrogen bonds but after I see that anomaly and the shape then I zoom in to their shape
the question asked why the triester had a lower melting point than the fatty acids. Therefore houdini's answer wouldnt make sense since if you say triester has more electrons then that means the ester's mp should be higher-which isnt conforming to question requirements.
also if you noticed, the next part also asked for number of electrons/size of electron cloud, so a levels wouldnt expect the same answer for 2 consequtive parts so of course the thought about hydrogen bonds should come in
just be more open-minded for paper 3 and dont always think that what you have learnt are the only things possible! Bye!
Originally posted by alliance88:This paper is quite challenging, the level of difficulty is almost on par with some of the Prelim Schools exam papers.
I would say Ultimaonline gauge of difficulty would not be accurate. Every single year after taking the A level H2 Chem paper, he would say the paper is pretty standard. He is a private tutor, who is also an Ex-MOE teacher, with a Bachelor Degree in Science. Of course, to him , he would find the paper simple.
But to us, A level students, our maximum knowledge for Chemistry is merely just from the lecture notes and lessons from lecturers, which only sufficiently prepare us for the papers. But he has more than knowledge in chemistry as he pursued a degree in chemistry, of course, he has a unfair advantage over this.
So, in conclusion, the best estimate of how hard or how easy the paper is, is we students. The 2016 cohort, who are taking the paper to rate what is the difficulty of paper, by comparing with out peers in our JC. Ultimately, we are competing with the cohort in the different JC and not a tuition teacher.
The benchmark of the A grade should not be determined by Pte tuition teachers, (I'm fine with school teacher, they know how their students fare in A levels). As he himself is doing the paper as a pte candidate, then he would find it easy, like a stroll in a park, and he will set the same benchmark every year.
Of course, you could ask him questions about the paper , as it serves as a platform for learning, but never ask him about the A grade of the paper, he would not know? These are moderations made by the examiners, performance of entire JC cohort (30000 students), just merely few students of him is highly inaccurate...
By the way, you just put the words right out of my mouth. You can tell me that the 'A' rate dangles around 70%, give and take 1-3%, but you can't give me a definite value. Unless Cambridge releases an official statement in any form, or there are insiders revealing any of these sensitive information, I doubt we would actually know. We can estimate ten marks gone for a paper but instead lose fifteen due to careless mistakes. We can estimate fifteen marks gone but end up only losing ten marks due to over-estimation. Students can report back to their tutors that they scored around 74% but didn't get an A when they prolly did slightly worse due to miscalculations. It's just as simple as that. I do feel a little (...) when I see statements like that right after a paper: "The A rate is now around 75% because the paper was of a rather decent standard, and this is solely based on paper two alone" and right after paper three, "the A rate has officially risen up to 77% because of the easy standard of paper three!" It just doesn't seem right or accurate at all. I would still choose to think that if you manage to score a 70 and the paper was of a decent/average standard, you still stand your chance of scoring an A. As simple as that.
Originally posted by Thefire521:the question asked why the triester had a lower melting point than the fatty acids. Therefore houdini's answer wouldnt make sense since if you say triester has more electrons then that means the ester's mp should be higher-which isnt conforming to question requirements.
also if you noticed, the next part also asked for number of electrons/size of electron cloud, so a levels wouldnt expect the same answer for 2 consequtive parts so of course the thought about hydrogen bonds should come in
just be more open-minded for paper 3 and dont always think that what you have learnt are the only things possible! Bye!
Originally posted by Thefire521:the question asked why the triester had a lower melting point than the fatty acids. Therefore houdini's answer wouldnt make sense since if you say triester has more electrons then that means the ester's mp should be higher-which isnt conforming to question requirements.
also if you noticed, the next part also asked for number of electrons/size of electron cloud, so a levels wouldnt expect the same answer for 2 consequtive parts so of course the thought about hydrogen bonds should come in
just be more open-minded for paper 3 and dont always think that what you have learnt are the only things possible! Bye!
I was proposing that the fatty acids are being bonded by hydrogen bonding which is stronger. Triester has more electrons and technically has stronger dispersion forces which outweighs the hydrogen bonding. But it is spherical and compact, hence steric hindrance and reduced surface for dispersion forces to act on. The dispersion forces on Triester is weaker (this dispersion force outweighs h bonding on triester because of steric hindrance) and thus lower melting point?
Originally posted by ArJoe:
are you from top 5 jc? You seem to be very good at chem
haha no i setted the paper was just interested in the discussions of students sitting for my little test hahaha
@houdini yeah i think so long as the examiner sees you write h bond stronger that should be more or less it. 3 lines they dont expect you to explain so much so even if you gave that extra d.f thinigs, it should be fine so long you say one has h bond one dont have
Thus, let's not give ourselves death sentences over one paper alone. I estimated a loss of 20/60 today for Paper 2 but I'm still optimistic that as long as you strive for Paper 3 and score decently to make up for your grades, you can still make it. A or B, that's Cambridge's job to assign our grades for us isn't it. Jiayou!
Originally posted by ArJoe:
are you from top 5 jc? You seem to be very good at chem
Fire521's concept sounds very solid.
Originally posted by Thefire521:haha no i setted the paper was just interested in the discussions of students sitting for my little test hahaha
@houdini yeah i think so long as the examiner sees you write h bond stronger that should be more or less it. 3 lines they dont expect you to explain so much so even if you gave that extra d.f thinigs, it should be fine so long you say one has h bond one dont have
Originally posted by Thomas00:Fire521's concept sounds very solid.
For the lower melting point in question 1 part 1, why cant i say that the molecules contains more cis double bond thus less effective packing and thus lower degree of molecular symettry which leads to lower melting point? It you look at the table on fatty acids, the number of cis for each molecule went up thus shouldnt there havingg more cis in the triester outweigh that of the hydrogen bond formed?
Originally posted by houdini_00:Thus, let's not give ourselves death sentences over one paper alone. I estimated a loss of 20/60 today for Paper 2 but I'm still optimistic that as long as you strive for Paper 3 and score decently to make up for your grades, you can still make it. A or B, that's Cambridge's job to assign our grades for us isn't it. Jiayou!
Anybody found or have the suggested answers for P2 yet? Just curious and want to see :)
How many marks did you all lose approximately? All the best for remaining papers!
Originally posted by ArJoe:
for the cu dissolve in acid qn, i quote the eqn cu arrow cu2+ +2e- but irreversible arrow. Isit ok? Then the 2h+ +2e- arrow h2 also irreversible arrow. Isit ok? and for the no2 and so2 qn, i just wrote the eqn no2+so2arrow no+so3. Isit possible to get 1 mark lol.. i sound so pathetic
Since the question asked you to quote data from data booklet, you should have just copied wholesale and wrote down the e value since they only wanted you to test feasibility of the reaction, but not write down the reaction occuring (there is no reaction anyway) If this was an electrochem cell and they asked you write down the half equations of the reactions occuring at anode and cathode, then yes you use irreversible arrows.
though i think they wouldn't penalise you if they are lenient on this
no2+so2arrow no+so3 should be worth 1 mark and the regeneration another mark. writing that it is a homogeneous catalyst probably gives you the last mark and explaining
@thomas there wouldnt be suggested answers for paper 2 since nobody will guarantee (even ultimate tutor ultima) since its collected up
Originally posted by ArJoe:
You meant 20/72?
Planning's a separate component. I estimate 8/12 for planning because of calculation errors at the end and 38/60 for the other segment. Not a decent score. I need to use P3 and P1 to pull me up, therefore.
Originally posted by Sir3:For the lower melting point in question 1 part 1, why cant i say that the molecules contains more cis double bond thus less effective packing and thus lower degree of molecular symettry which leads to lower melting point? It you look at the table on fatty acids, the number of cis for each molecule went up thus shouldnt there havingg more cis in the triester outweigh that of the hydrogen bond formed?
This is bio-
A tri-ester is simply the compound formed after an esterification reaction between glycerol and fatty acids.
Then the question asked you: Why triester melting point lower than the fatty acid monomer?
First assumption you made is that the double bonds are cis. Do you know that they can be trans double bond and trans double bond can give MORE effective packing and therefore higher melting point? - so this is already a huge assumption you made
Indeed, while triester may be larger in terms of having more electrons/dispersion forces, but using this explanation does not answer the question because that should mean triester will have higher melting point!
But for you, your major assumption was that the double bonds was cis. And in fact the part about the packing and cis double bond forming kinks is not under chemistry syllabus
For the first question, i compared to strength of pdpd interaction vs hydrogen bond. Is that correct?
As for the c=c double bond qn, i rounded off my answer to 3? Will i be penalized?
For the reaction between NO2 and SO2 I used the data booklet so i ended up having So42- and N2 as the product instead? Is that wrong?
What's the equation for CCl3-CHO + OH-?
Originally posted by Ccii:What's the equation for CCl3-CHO + OH-?
Gives u methanoate hco2- and chcl3 liquid that insolubles
Yes pdpd vs h bond
No so4 and n2 is wrong.
C=c double bond they asked for 3s.f. so anything that is not 3.s.f will minus 1
I think i screwed up planning cause i only had like 15mins to write anD I WAS SO SLEEPY I COULDNT EVEN concentrate so i can barely read the qn wow!!! Oh but for planning is it right to use the calculated value which was like 4.6+g smth for the reaction? Btw what do the planning qn want i cant even remember cause i wasnt rlly awake.
Originally posted by AwkwardPotato:I think i screwed up planning cause i only had like 15mins to write anD I WAS SO SLEEPY I COULDNT EVEN concentrate so i can barely read the qn wow!!! Oh but for planning is it right to use the calculated value which was like 4.6+g smth for the reaction? Btw what do the planning qn want i cant even remember cause i wasnt rlly awake.
Please catch up on your sleep now so that you dont fall asleep for paper 3 as well
As to the concerns raised by alliance88 and houdini_00, I never claimed to be (in fact, I can assure you I'm *not*) speaking on behalf of Cambridge or MOE-SEAB regarding official grade boundaries. Afterall, the bell-curve is subject to the performance of the entire Singapore JC cohort, which I've no way of knowing. I'm only giving my opinion (for whatever it's worth) based on the relative difficulty of the paper, without considering the performance of the entire Singapore JC cohort.
I simply assumed (mistakenly, perhaps) it would be understood by everyone reading this thread, that I was just giving my own opinion, whatever it's worth, (similarly to how Bilahari Kausikan is not, and never claimed to be, speaking on behalf of the PAP government whenever he writes an opinion article) on how the A grade boundary might be swayed by the difficulty of the Paper, just as how many other private tutors and JC teachers would also give their own opinion on this matter, after each paper.
You're (not just alliance88 and houdini_00, but everyone else as well who feels likewise) welcome to ignore my future opinion posts on possible shifts on the A grade boundary, eg. after the upcoming P3 and P1, if you feel they're not useful or worse, discouraging.
if I forgot to x2 the Mr of Iodine, how many marks will I lose? my answer was 5.70 which was twice of 2.85. question was 4m.
Hi may i know why is so42- and n2 wrong? I wrote both eqns meaning one eqn produced so3 and n2o and the other produced so42- and n2... Will i get any marks