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  • MapPwner's Avatar
    138 posts since Aug '15
    • Originally posted by UltimaOnline:


      Thanks for making the effort and taking the trouble giving a detailed look at the entire P2, MapPwner, appreciate it :)

      So your top 10% friend and some sarcastic teachers in your school said it was easy, while you found it moderate in difficulty. The rest of your peers? And your friends in other schools?

      From your description, quite as expected, Cambridge balanced out the paper very nicely, with a fair mix of easy, standard and difficult questions. Let's see if this continues for P3 and P1.

      Let me know if you want me to comment on any question in particular. Good to see some interesting questions in there, so at least Cambridge didn't disappoint me in that regard ;)

      Regarding the friend who found it easy,yeah when he stepped out of the hall he said only lose a few marks but after discussing most of the paper that snowballed to 25 marks.

      My class on average the B graders(only 1 person) lost about same marks as me(15-17,he thought the paper was easy as he felt he only lost 2 marks but guess not),the rest probably might have done worse,another i asked lost 22 marks supposedly(he got E for prelims).Rest lost 25-30 or so.But i estimate the rest to lose on average 25-30 or so in all seriousness,as one couldnt even decipher the skeletal formula given in one of the question to deduce proper reagents and conditions.

      Of course i have friends in AJ losing same marks as me or even more,up to 30.My Hwa Chong friend who did much worse than me for prac aced this paper,lost about 5-6 marks

      I too,felt the paper was actually simple(except for qn 6 and 7,but qn 7 was like a repeat question from my secondary sch sec 2 chem paper for a few parts so it was actually familiar),but panic probably killed a lot of people.

      If i were to give an honest evaluation i would rate Qn 1 to 2 as easy(but i screwed up question 2 quite badly),qn 3 to 5 as moderate,qn 6 and 7 as moderate-challenging difficulty.

      Question 6 especially,as the question was unusual in the sense we need to state the purpose of benzoic acid that makes it useful for this calculation of heat capacity of calorimeter in a water,without considering about the water.

      Majority of the ppl were confused(>50% in my school) and other JCs,they didnt use -(q x(avg temp rise)/amt of benzoic acid in moles for enthalpy change of combustion,but other ways,which were wrong.

      Organic chemistry component was moderately difficult to me,yet a fun question to do even though it was a pain doing so as it was really interesting for me(only lost 1 mark for organic component part,which was Qn 3 and 4).

       

       

       

      Edited by MapPwner 07 Nov `17, 5:01PM
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • Originally posted by MapPwner:

      Regarding the friend who found it easy,yeah when he stepped out of the hall he said only lose a few marks but after discussing most of the paper that snowballed to 25 marks.

      My class on average the B graders(only 1 person) lost about same marks as me(15-17,he thought the paper was easy as he felt he only lost 2 marks but guess not),the rest probably might have done worse,another i asked lost 22 marks supposedly(he got E for prelims).Rest lost 25-30 or so.But i estimate the rest to lose on average 25-30 or so in all seriousness,as one couldnt even decipher the skeletal formula given in one of the question to deduce proper reagents and conditions.

      Of course i have friends in AJ losing same marks as me or even more,up to 30.My Hwa Chong friend who did much worse than me for prac aced this paper,lost about 5-6 marks

      I too,felt the paper was actually simple(except for qn 6 and 7,but qn 7 was like a repeat question from my secondary sch sec 2 chem paper so it was actually familiar),but panic probably killed a lot of people.

      If i were to give an honest evaluation i would rate Qn 1 to 2 as easy(but i screwed up question 2 quite badly),qn 3 to 5 as moderate,qn 6 and 7 as moderate-challenging difficulty.

      Question 6 especially,as the question was unusual in the sense we need to state the purpose of benzoic acid that makes it useful for this calculation of heat capacity of calorimeter in a water,without considering about the water.

      Majority of the ppl were confused(>50% in my school) and other JCs,they didnt use -(q x(avg temp rise)/amt of benzoic acid in moles for enthalpy change of combustion,but other ways,which were wrong.

      Organic chemistry component was moderately difficult to me,yet a fun question to do even though it was a pain doing so as it was really interesting for me(only lost 1 mark for organic component part).

       

       

       


      Ahh, so your top 10% friend didn't do as well as he initially thought, but prolly still well enough to get A grade. In contrast, your HCJC friend really only lost about 5 marks for P2, enough to make up for his P4 errors. So MapPwner, yourself, you're well on track for A grade (so far overall score > 80%, yes?).

      Agree with you that the calorimetry question is a good challenging question for Singapore JC H2 students. I can actually think of 5 different reasons why benzoic acid is used as a standard for calorimetry (just as I can think of 3 different reasons why LiAlH4 is a stronger reducing agent than NaBH4), but Cambridge as expected, is already very lenient : any 1 reason (for both the benzoic acid and LiAlH4 questions) will suffice to score the 1 mark allocated. But the majority of Singapore JC H2 students are unlikely to have prior exposure to such questions, which is why these are, all things considered, good and fair distinction questions (not too unreasonable as to require 3 reasons for each question, but challenging enough to differentiate A graders from the rest).

      Similarly for the Naphthalene aromaticity question (from your description, the entire question is about Naphthalene being aromatic with resonance delocalization of pi electrons, while in the basic H2 syllabus only the simplest aromatic species, benzene, is taught), another excellent question that (from your description), didn't disappoint me, in the sense it is interesting and A grade differentiating.

      But as you (and your sarcastic school teachers) pointed out, Cambridge has already been very fair, since these are only a sprinkling of challenging questions amidst mostly easy or standard questions. So (unless P3 and P1 are hellish... hell yeah! I should set the A level papers Lol!) I doubt the bell-curve will deviate too much from past years.

      Keep up your excellent A grade performance, MapPwner! :)

  • do12453's Avatar
    13 posts since Nov '17
    • Originally posted by UltimaOnline:


      The rest of your peers in your school also found it easy? Or just you? How about your friends from other schools?

      Generally more complaints about single mark losses here and there due to carelessness than about the toughness of the paper. Overall, managed a -13 to -15 in the worst case (Assuming zero marks for questions that I may have gotten a few points) and assuming they are very strict on the marking (eg penalising for lack of equation generation of nucleophile for question on reduction of aldehyde with NaBH4, even though the question already told you to assume H- is the nucleophile) etc. 

       

       

  • MapPwner's Avatar
    138 posts since Aug '15
    • Originally posted by UltimaOnline:


      Ahh, so your top 10% friend didn't do as well as he initially thought, but prolly still well enough to get A grade. In contrast, your HCJC friend really only lost about 5 marks for P2, enough to make up for his P4 errors. So MapPwner, yourself, you're well on track for A grade (so far overall score > 80%, yes?).

      Agree with you that the calorimetry question is a good challenging question for Singapore JC H2 students. I can actually think of 5 different reasons why benzoic acid is used as a standard for calorimetry (just as I can think of 3 different reasons why LiAlH4 is a stronger reducing agent than NaBH4), but Cambridge as expected, is already very lenient : any 1 reason (for both the benzoic acid and LiAlH4 questions) will suffice to score the 1 mark allocated. But the majority of Singapore JC H2 students are unlikely to have prior exposure to such questions, which is why these are, all things considered, good and fair distinction questions (not too unreasonable as to require 3 reasons for each question, but challenging enough to differentiate A graders from the rest).

      Similarly for the Naphthalene aromaticity question (from your description, the entire question is about Naphthalene being aromatic with resonance delocalization of pi electrons, while in the basic H2 syllabus only the simplest aromatic species, benzene, is taught), another excellent question that (from your description), didn't disappoint me, in the sense it is interesting and A grade differentiating.

      But as you (and your sarcastic school teachers) pointed out, Cambridge has already been very fair, since these are only a sprinkling of challenging questions amidst mostly easy or standard questions. So (unless P3 and P1 are hellish... hell yeah! I should set the A level papers Lol!) I doubt the bell-curve will deviate too much from past years.

      Keep up your excellent A grade performance, MapPwner! :)

      Overall score no lah how over 80% lol,my prac was rather bad.Prob for prac alone is 56%,this paper is 77-80%(pretty sure my estimates are very accurate,in line with my estimates for past chem papers for my exams this year,even if they deviate the score is higher than the estimated score).

      My prelim raw % scores for individual components were p1:80%,p2:72%,p3:76%,p4:56%,so more or less similar to a levels.

      As with regards about my HC friend,he probably will lose way more than 6 marks,i estimate eventually same marks lost as me as he only immediately excluded those he know is definitely wrong.[Edit:Yep just asked her more about it,he lost more marks(3 marks) due to the wrong calculation of enthalpy change of combustion as he used mc delta T,where m is mass of water/substance instead of only c delta T.So i'm pretty sure he will end up losing abt 15-17 marks too as we only discussed a few parts and already 9 marks lost].

      Edit:I miscalculated my prac %,shld be 56%,same as my prelim score instead of 60%.But thats somewhat decent as opposed to most of my friends who said they would be glad if they passed.

       

       

      Edited by MapPwner 08 Nov `17, 8:41AM
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • Nice nice. So that means you're on par with, or better than, both your JC's top 10% friend, and your HCJC friend.

      image:)

      image

      Good good, as long as you don't cockup your P3 and P1 (ie. about same performance as your Prelims), your overall score as a safe estimate will be between 75% to 80%, so that's a guaranteed A grade (as I said before, when students are shocked they didn't get A grade, it's not that the A grade boundary > 75%, it's that they overestimated their own scores, but this shouldn't be an issue for you MapPwner).

      So P2 was mostly easy to average, with only a couple of tough questions. But since you're the top 20% of the national cohort, MapPwner, perhaps the average Singapore H2 student found this paper mostly average in difficulty, with quite a few tough questions. But that's still totally within expectations of a Cambridge paper, so the bell-curve shouldn't have too many surprises this year.

      Originally posted by MapPwner:

      Overall score no lah how over 80% lol,my prac was rather bad.Prob for prac alone is 60%,this paper is 77-80%(pretty sure my estimates are very accurate,in line with my estimates for past chem papers for my exams this year,even if they deviate the score is higher than the estimated score).

      My prelim raw % scores for individual components were p1:80%,p2:72%,p3:76%,p4:56%,so more or less similar to a levels.

      As with regards about my HC friend,she probably will lose way more than 6 marks,i estimate eventually same marks lost as me as she only immediately excluded those she know is definitely wrong.

       


      Edited by UltimaOnline 10 Nov `17, 3:55AM
  • MapPwner's Avatar
    138 posts since Aug '15
    • Originally posted by UltimaOnline:

      Nice nice. So that means you're on par with, or better than, both your JC's top 10% friend, and your HCJC friend.

      image:)

      image:)

      Good good, as long as you don't cockup your P3 and P1 (ie. about same performance as your Prelims), your overall score as a safe estimate will be between 75% to 80%, so that's a guaranteed A grade (as I said before, when students are shocked they didn't get A grade, it's not that the A grade boundary > 75%, it's that they overestimated their own scores, but this shouldn't be an issue for you MapPwner).

      So P2 was mostly easy to average, with only a couple of tough questions. But since you're the top 20% of the national cohort, MapPwner, perhaps the average Singapore H2 student found this paper mostly average in difficulty, with quite a few tough questions. But that's still totally within expectations of a Cambridge paper, so the bell-curve shouldn't have too many surprises this year.

       

      Well hopefully can possibly get A haha.Ultimately my aim is to just enter a decently good course in University.

      My AJ friend who replied me told me he lost at least 20 after checking.

      Nevertheless,the amount of careless mistakes i made in my opinion,were avoidable,such as definition of homogenous catalyst,saying that 'it is in the same state as the reactants,which are aqueous',whereby the word 'phase' is used in place of 'state',so carelessly threw 1 mark away.

      Edited by MapPwner 07 Nov `17, 5:57PM
  • Henry999's Avatar
    2 posts since Nov '17
    • Hi, can I ask , for today's A level Chemistry paper 2

      for questions 1) there's a part asking about why Hydrogen cannot be placed under group 1 as well as for a physical property

      can the reason be because hydrogen cannot delocalise its electrons to form metallic binding unlike a group 1 element. And hence the physical property that it lacks is electrical conductivity?

      another question is would an A grade for Chemistry still be as high as 70-75%? Given that this year's practical was generally tough across all schools and it's the first year that Cambridge reintroduced the SPA exam in the standardised format 

       

      Thank you!

  • do12453's Avatar
    13 posts since Nov '17
    • Originally posted by Henry999:

      Hi, can I ask , for today's A level Chemistry paper 2

      for questions 1) there's a part asking about why Hydrogen cannot be placed under group 1 as well as for a physical property

      can the reason be because hydrogen cannot delocalise its electrons to form metallic binding unlike a group 1 element. And hence the physical property that it lacks is electrical conductivity?

      another question is would an A grade for Chemistry still be as high as 70-75%? Given that this year's practical was generally tough across all schools and it's the first year that Cambridge reintroduced the SPA exam in the standardised format 

       

      Thank you!

      yes, state that hydrogen has SMS, weak dispersion forces, hence low elec conductivity (or mp/bp), and briefly compare that with metallic bonds in grp 1 metals

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • Originally posted by Henry999:

      Hi, can I ask , for today's A level Chemistry paper 2

      for questions 1) there's a part asking about why Hydrogen cannot be placed under group 1 as well as for a physical property

      can the reason be because hydrogen cannot delocalise its electrons to form metallic binding unlike a group 1 element. And hence the physical property that it lacks is electrical conductivity?

      another question is would an A grade for Chemistry still be as high as 70-75%? Given that this year's practical was generally tough across all schools and it's the first year that Cambridge reintroduced the SPA exam in the standardised format 

       

      Thank you!


      Practical P4 was tougher than average, but today's P2 (and likely P3 and P1) is mostly average, perhaps slightly on the tough side. So at most, the A grade boundary may shift slightly closer towards 70% than 75%, but for Singapore A level Chemistry (even across syllabus changes), the A grade boundary has almost always been between 70% and 75%. And considering today's mostly average paper, there is no reason to believe it will shift to less than 70% (it'll never happen Lol) or go beyond 75% (unless P3 and P1 are significantly easier than P4 and P2).

  • Henry999's Avatar
    2 posts since Nov '17
    • Hi does anyone remember what are the buffer equations for today's A level Chemistry Paper 2? 

      What was the chemicals in the buffer system and what are their equations?

  • MapPwner's Avatar
    138 posts since Aug '15
    • Originally posted by Henry999:

      Hi does anyone remember what are the buffer equations for today's A level Chemistry Paper 2? 

      What was the chemicals in the buffer system and what are their equations?

      Contrary to popular belief about the need for equation(s)[If you are referring to the blood and lactic acid buffer question for question 2],it is actually only 1 equation,which is the buffer buffering against H+(i forgot the exact eqn).

      Considering that question was 2 marks,and i just merely breezed through wrote 2 eqn,1 for H+ presence,the other for OH- without explaining how buffer works+ H+ source i will probably only get 1mark,since equation is only worth 1 mark.

      Edited by MapPwner 07 Nov `17, 7:39PM
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • For the purpose of H2 Chemistry A Level Paper discussions, it's ok to give the Reddit link. Discussions are a lot more efficient when cross-referencing is available. In other words, before you ask "What is the answer for this Qn huh?", go check the Reddit thread first.

      https://www.reddit.com/r/SGExams/comments/7ba5g0/a_level_h2_chemistry_paper_2_2017

      Fyi, MapPwner (SgForums) = Mappy39 (Reddit)

    • Last 2 days left for you to ask me any questions from the Singapore JCs 2017 Prelim Paper 3s, before your Judgment Day this Fri.

      Your performance this Fri could determine the path you take for the rest of your life*. So for the sake of the rest of your life, ASK AWAY (while you still can)!!!

      (Of course, this can be argued for every single paper across all subjects, but it sounds more pithy if we don't go there.)

  • do12453's Avatar
    13 posts since Nov '17
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • Originally posted by do12453:

      What are the 3 reasons for lialh4 being stronger than nabh4?


      Uhhh, I gotta leave some BedokFunland JC trade advantages for my own BedokFunland JC students (in future cohorts), too many private tutors silently lurking on the forums.

      Besides (more relevantly for you), since it was already asked in P2, no way Cambridge will ask again in P3 or P1. And as you saw for yourself, while I can think of 3 reasons, Cambridge will accept any 1 reason to get the full 1 mark, so no worries (u prolly already got that mark).

      U can go google out for yourself if interested. If u don't understand what you google out, you can link to it and I'll comment on it. This applies to any other Chemistry topic.

      Gambatte for P3! :D

  • MapPwner's Avatar
    138 posts since Aug '15
    • Originally posted by do12453:

      What are the 3 reasons for lialh4 being stronger than nabh4?

      Pretty sure 2 of the 3 reasons are Al-H vs B-H bond overlap due to diff in atomic size of Al and B,Al-H bond less effective orbital overlap,bond of Al-H thus longer and easier to break to release hydride.Another is diff polarising power of Li+ and Na+ and since Li+ caitionic radius is smaller it polarises,weakens the Al-H bond in Lialh4 to a greater extent to release hydride ions.

       

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • A few hours left, and still nobody has any questions from the 2017 Prelim Papers to consult me on??? Fine, I'll give you guys a challenging P3 question.

      An alkane with Mr 114.0 exists as three stereoisomers - two of which are chiral and the third is non-chiral. Draw the stereochemical formula of all 3 stereoisomers, and explain why 2 of them are chiral while the 3rd is not.

      Anyone who's game enough to attempt my BedokFunland JC question, upload your answer drawings and link from your post, and I'll tell you if you got it correct or not. As usual, I won't reveal the answer, I'll only say if any submitted answer is wrong, partly correct, or fully correct.

      Edited by UltimaOnline 09 Nov `17, 6:02PM
  • delurach's Avatar
    13 posts since Nov '17
    • Edited by delurach 09 Nov `17, 6:39PM
    • Originally posted by UltimaOnline:

      A few hours left, and still nobody has any questions from the 2017 Prelim Papers to consult me on??? Fine, I'll give you guys a challenging P3 question.

      An alkane with Mr 114.0 exists as three stereoisomers - two of which are chiral and the third is non-chiral. Draw the stereochemical formula of all 3 stereoisomers, and explain why 2 of them are chiral while the 3rd is not.

      Anyone who's game enough to attempt my BedokFunland JC question, upload your answer drawings and link from your post, and I'll tell you if you got it correct or not. As usual, I won't reveal the answer, I'll only say if any submitted answer is wrong, partly correct, or fully correct.

      https://i.imgur.com/a16fSCh.jpg Pls do check my answer! Thank u

    • Additionally i’d like to seek help for the following questions ...

      1) https://i.imgur.com/LTrzkWI.jpg

      how do you know step 2 is a reduction process?

      2) https://m.imgur.com/Ekfa5YK

      what are the answering techniques like for such question? Can I not just say equilibrium position shifting results in ppt dissolving or remaining without mentioning Ksp exceeds or is less than IP? And how do you know when do you need to include the calculation for e cell values? 

      3) for nucleophilic addition of CN- , is it necessary to include cold? Or will trace amount of NAoh, HcN , 10-20deg just suffice? 

      Thank you!!

  • CB2883J's Avatar
    352 posts since Sep '15
    • Originally posted by UltimaOnline:

      A few hours left, and still nobody has any questions from the 2017 Prelim Papers to consult me on??? Fine, I'll give you guys a challenging P3 question.

      An alkane with Mr 114.0 exists as three stereoisomers - two of which are chiral and the third is non-chiral. Draw the stereochemical formula of all 3 stereoisomers, and explain why 2 of them are chiral while the 3rd is not.

      Anyone who's game enough to attempt my BedokFunland JC question, upload your answer drawings and link from your post, and I'll tell you if you got it correct or not. As usual, I won't reveal the answer, I'll only say if any submitted answer is wrong, partly correct, or fully correct.

      Straight chain octane non chiral

      2-methylheptane non chiral

      3-methylheptane chiral

      2,4-dimethylhexane chiral

      3,4-dimethylhexane chiral

    • Originally posted by UltimaOnline:

      A few hours left, and still nobody has any questions from the 2017 Prelim Papers to consult me on??? Fine, I'll give you guys a challenging P3 question.

      An alkane with Mr 114.0 exists as three stereoisomers - two of which are chiral and the third is non-chiral. Draw the stereochemical formula of all 3 stereoisomers, and explain why 2 of them are chiral while the 3rd is not.

      Anyone who's game enough to attempt my BedokFunland JC question, upload your answer drawings and link from your post, and I'll tell you if you got it correct or not. As usual, I won't reveal the answer, I'll only say if any submitted answer is wrong, partly correct, or fully correct.

      Btw what is meant by stereoisomer here? The kind of isomerism I’m getting is positional/constitutional/structural isomerism.

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • Originally posted by CB2883J:

      Straight chain octane non chiral

      2-methylheptane non chiral

      3-methylheptane chiral

      2,4-dimethylhexane chiral

      3,4-dimethylhexane chiral


      Unfortunately, both CB2883J and delurach gave the wrong answers, because both of you offered constitutional isomers, instead of stereoisomers of the same alkane.

      Now that you guys mention it, I've another related BedokFunland JC Challenge Qn that allows for constitutional isomers, you guys can give this a try.

      How many monochlorinated isomers (including both constitutional isomers and stereoisomers) can be generated when 3-methylpentane is reacted with Cl2 under UV light? Name all of them.

    • Originally posted by delurach:

      Additionally i’d like to seek help for the following questions ...

      1) https://i.imgur.com/LTrzkWI.jpg

      how do you know step 2 is a reduction process?

      2) https://m.imgur.com/Ekfa5YK

      what are the answering techniques like for such question? Can I not just say equilibrium position shifting results in ppt dissolving or remaining without mentioning Ksp exceeds or is less than IP? And how do you know when do you need to include the calculation for e cell values? 

      3) for nucleophilic addition of CN- , is it necessary to include cold? Or will trace amount of NAoh, HcN , 10-20deg just suffice? 

      Thank you!!


      Because this sneaky question specifies, "Steps 2 and 3 in both pathways each refers to the same set of reagents and conditions.", so it's machiam solving simultaneous equations.

      Based on the number of marks allocated.

      "10-20deg" or "cold" both acceptable.

    • Originally posted by CB2883J:

      Btw what is meant by stereoisomer here? The kind of isomerism I’m getting is positional/constitutional/structural isomerism.


      image

      https://en.wikipedia.org/wiki/Stereoisomerism

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