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  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • If no one can give the correct answer by this evening, anyone who’s really worried can PM me for the answer. Or you can just google it out yourself, seriously.

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

      May I know where to download chemistry specimen paper 2017?

       


      It's not available online. Only some JCs gave their students a hardcopy (ie. no softcopy to share online) to practice on. No worries lah, just practice the 2017 Prelim Papers.

  • do12453's Avatar
    13 posts since Nov '17
    • Hi, the amide hybridisation question, does it have to do with the fact that the lone pair

      delocalises into the Pi electron cloud of the C=O bond? Hence, compared to amine which has 3 bonds as well (but also a lone pair), it is sp2 hybridised

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

      Hi, the amide hybridisation question, does it have to do with the fact that the lone pair

      delocalises into the Pi electron cloud of the C=O bond? Hence, compared to amine which has 3 bonds as well (but also a lone pair), it is sp2 hybridised


      Yes, you're almost there. So which orbital does the lone pair reside in?

  • ofji4o's Avatar
    3 posts since Feb '17
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • Originally posted by ofji4o:

      any specific topics to emphasize on for tmr paper? 


      No. Spotting won't work for Singapore H2 Chemistry papers.

      Even after tomorrow's P2, you still can't reliably spot topics for P3. Let alone before P2.

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

      Last 24 hours left before Paper 2. Any final questions from the 2017 Prelim Papers or TYS, better faster ask now!

      After P2 is done tomorrow (and P3 on Fri), I'll leave it to you students (eg. MapPwner) to lead the discussions on the questions.

      If anyone wants my comments on any P2 or P3 questions, pls type out the qn from memory (for P2), or upload a photo of the qn (for P3).

      Sure I will try to lead the chem discussions.

      Chem revision a bit unproductive after GP exam,no mood to do chem so even some simple questions i'm getting wrong/careless.

      Hopefully everything clears up when i wake up tomorrow,as usual.

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

      Sure I will try to lead the chem discussions.

      Chem revision a bit unproductive after GP exam,no mood to do chem so even some simple questions i'm getting wrong/careless.

      Hopefully everything clears up when i wake up tomorrow,as usual.


      Well this year, if P2, 3, 1 are as tough as P4, the bell-curve will be 'steeper' than in recent years. Whether that works for or against you (ie. each student), depends partly on yourself (you'll have to be significantly above average no matter what) and also partly on how SEAB chooses to apply the bell-curve (it's up to SEAB to shift the bell-curve to the left or right, the % of A graders are different every year, it's not a 'hard' or absolute bell-curve grading, but a 'soft' one).

  • MapPwner's Avatar
    138 posts since Aug '15
    • I see,well I'm gonna aim for at least 80% of the total score possible for each of the remaining components,to compensate for practical.

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

      I see,well I'm gonna aim for at least 80% of the total score possible for each of the remaining components,to compensate for practical.


      That's safe. The A grade boundary has almost never exceeded 75% (but is always above 70%).

      The problem is students, when counting their own scores, usually miss out on errors they didn't realize they made, or they think their answer phrasing (eg. copied from school notes) would get full marks from Cambridge, when it wouldn't (especially if the question has a twist in it, so regurgitating school notes won't be enough).

      So if you (ie. all students) counted you got > 75% but still didn't get A grade, it's far more likely that you overestimated your own score, rather than the actual grade boundary being > 75%. So always play safe when counting your own score, and just assume you get zero marks for iffy questions.

  • MapPwner's Avatar
    138 posts since Aug '15
    • Based on my estimations using my internal examination papers as an example,I think my estimates are relatively accurate or deviate slightly from the actual score,in which the actual score is higher than the predicted score.

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

      Based on my estimations using my internal examination papers as an example,I think my estimates are relatively accurate or deviate slightly from the actual score,in which the actual score is higher than the predicted score.


      Was there ever any doubt in your mind that you'll get an A grade? ;Þ

      So what are you applying for, Medicine?

      Edited by UltimaOnline 06 Nov `17, 8:17PM
  • MapPwner's Avatar
    138 posts since Aug '15
    • Originally posted by UltimaOnline:


      Was there ever any doubt in your mind that you'll get an A grade? ;Þ

      So what are you applying for, Medicine?

      Ever since chem prac kinda rekted me yeah the thought of not getting an A is somewhat worrying.Same thing happened to me for O levels,i flunked all my 3 pracs(didnt cared after that for rest of papers)+only scored 29/40 for chem p1 after cross checking answers online,in the end i got an A2.So hopefully a miracle will happen lol?

      Med ah,don't intend to think so far,currently only got early invite for NUS scholarship.If my results enable me to go there I will definitely try.At least GP for me so far was fine(my classmates and sec sch classmates from other JCs found it rather difficult).

      I will probably somewhat flood this post right after the end of Chem P2 tomorrow to discuss.

      Edited by MapPwner 06 Nov `17, 8:28PM
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • Originally posted by MapPwner:

      Ever since chem prac kinda rekted me yeah the thought of not getting an A is somewhat worrying.Same thing happened to me for O levels,i flunked all my 3 pracs(didnt cared after that for rest of papers)+only scored 29/40 for chem p1 after cross checking answers online,in the end i got an A2.So hopefully a miracle will happen lol?

      Med ah,don't intend to think so far,currently only got early invite for NUS scholarship.If my results enable me to go there I will definitely try.At least GP for me so far was fine(my classmates and sec sch classmates from other JCs found it rather difficult).

      I will probably somewhat flood this post right after the end of Chem P2 tomorrow to discuss.


      Yeah, every year there are a couple of tragic cases that actually do worse for A levels than Prelims, so you're prudent not to be complacent.

      Yes, pls do try to reproduce from memory some of the trickier questions from tmrw's P2 to post here, so that I can give my comments on them and on your submitted answers for these qns.

      Edited by UltimaOnline 06 Nov `17, 8:50PM
  • do12453's Avatar
    13 posts since Nov '17
    • I will als try to rmb some qns frm the paper

      Btw, the lone pair is in the p orbital right

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

      I will als try to rmb some qns frm the paper

      Btw, the lone pair is in the p orbital right


      Yes, and u must specify *unhybridized*. Hence the hybridization of the remaining orbitals to form the sigma bonds.

      Yes, rmbr as many qns as u can, particularly those u want to ask me on, then post em here together with ur submitted answers, for my comments.

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


      Yeah, every year there are a couple of tragic cases that actually do worse for A levels than Prelims, so you're prudent not to be complacent.

      Yes, pls do try to reproduce from memory some of the trickier questions from tmrw's P2 to post here, so that I can give my comments on them and on your submitted answers for these qns.

      Yep sure will try to remember the questions(especially the tougher ones) to the best of my abilities tomorrow,if after doing the paper i still have time left,which should have based on past papers as on average i take 75mins-90mins to finish P2.

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

      Yep sure will try to remember the questions(especially the tougher ones) to the best of my abilities tomorrow,if after doing the paper i still have time left,which should have based on past papers as on average i take 75mins-90mins to finish P2.


      Paper begins in 1 hour's time. Have fun! I know you will! ;Þ

  • MapPwner's Avatar
    138 posts since Aug '15
    • Paper was considered of moderate difficulty but required much prudence to score.

       

      Weird questions were why Lialh4 stronger reducing agent than nabh4(orbital overlap between al and H less effective easier break to release more molecules to reduce molecules) and suggesting why c6h5cooh is useful to measure specific heat capacity of calorimeter.Determining specific heat capacity of calorimeter for this particular energetics question is somewhat unusual,using only -(ct)/amt of benzoic acid) for enthalpy change of combustion of c6h5cooh,whereby c is specific heat capacity of calorimeter and t is average temperature rise.

       

      More standard questions were defining homogenous catalyst,deducing another structure for c2h4o other than etheneol and ethanal(which is a triangular structure with c-o-c bond),calculation of ph of soln and concentration of a salt in a buffer solution.

       

       

  • do12453's Avatar
    13 posts since Nov '17
    • Paper was q easy. Weirdest qn was last question on Naphthalene but even then it was still doable after abit of thinking

      They asked for the third of c2h4o excluding ethenol and ethanal. Answer was an epoxide (which isnt in syllabus)? But enuf info to derive i guess

       

      Fyi the qn on Lialh4 and nabh4, i brought in the stronger polarising power of Li+ and larger, more polarisable electron cloud of alh4- hence Al-H bonds weakened to grwater exrent, easily releasing H. Is it right? Anw the qn was only 1m but i wrote a ton of lines. What were thry actually lookintgfor

      Edited by do12453 07 Nov `17, 10:49AM
  • MapPwner's Avatar
    138 posts since Aug '15
    • Originally posted by do12453:

      Paper was q easy. Weirdest qn was last question on Naphthalene but even then it was still doable after abit of thinking

      They asked for the third of c2h4o excluding ethenol and ethanal. Answer was an epoxide (which isnt in syllabus)? But enuf info to derive i guess

       

      Fyi the qn on Lialh4 and nabh4, i brought in the stronger polarising power of Li+ and larger, more polarisable electron cloud of alh4- hence Al-H bonds weakened to grwater exrent, easily releasing H. Is it right? Anw the qn was only 1m but i wrote a ton of lines. What were thry actually lookintgfor

      For lialh4 explanation i explained above thats the answer,orbital overlap so easier to release more hydride molecules(if u did other sch papers orbital overlap was the main reason why)Not forgetting one needs to show the generation of hydride molecule from BH4- in step 1 of nucleophilic addition.

      I wont dare say its easy,i have a friend,hes top 10% in level for chem,said the paper was easy and lost only 5 marks.We discussed only 70% of the paper and that increased to 25 marks loss total so far.However of course there were chem teachers who said it was simple and beared a tinge of sarcasm by saying qn 6 and 7 is 'only slightly harder'.

      As for me after stringent calculation of my score after discussion with many people,i probably lost 15-17 marks.

      Edited by MapPwner 07 Nov `17, 3:45PM
    •  

       Incompletely Compiled chemistry suggested answers so far(Answers may not be fully correct so take it with a pinch of salt):

      Qn 1(Ionisation energies of element A(chlorine) and hydrogen properties)

      Increase in I.E as successive electrons are removed from element A(1m),as it is increasingly difficult to remove a negatively charged electron from a positively charged element A ion(1m).Large increase from 7th to 8th i.e as the 8th electron is located in an inner electron shell,which is closer to the nucleus and thus more energy required to overcome the strong electrostatic forces of attraction,resulting in large rise in I.E.(2m)

       

      Element A is chlorine(1m).1s^22s^22^p63^s23p^5.(electronic configuration-1m).

       

      Hydrogen can be classified as a grp 1 element as it has 1 valence electron in its outermost shell(1m).

      Cannot be classified:

      Grp 1 elements-metal lattice structure vs simple covalent molecule structure for H+describe(1m)

      Contrasting properites=grp 1 higher melting/boiling point than H/H2+accounting for the reason why(1m)

       

      Qn 2(Acids and buffers of HCO2- and blood)

       

      part i)ph=2.60+showing calculation(3m)

      (ii)forgot the answer

      (iii)ph will fall if no buffer system in a human while exercising

       Qn 3,4,5(C2H4O/cinammealdehyde and transition elements)

       

      qn3)

      (i)showing molecular formula is c2h4o(relatively straightforward)

      (ii) structure is a triangular molecule with c-o-c bond and c-c bond between both c for c2h4o,with remaining h bonded to the 2 c ensuring octet rule fufilled.

      Reaction with Br2(aq)=orange br2 decolourises rapidly to colourless

      Na2co3:No vigorous effervescence,production of co2 observed.

      Kc is small which thus show most of the product is the (LHS one-I forgot the name) and thus position of equilibrium lies more to the left(1m).Kc shows the extent of favourability of product formed in the equilibrium and can be used to determine the amount of product formed based on Kc value.''If Kc small=poe left while if Kc large=poe right.''(Stating something like this is 1m I think)

       

      (iv) memorise balanced eqn of triiodomethane with ch3cho

      cant really remember much for qn 3...

       

      qn(4)

      for cinammealdehyde,1 structure still has c=c and aldehyde just that it is arranged at different positions(swapped)=positional isomerism,while the other only has an aldehyde group and no alkene group=structural isomerism(Or show cis-trans version for one of them).

      After that is Hcn(aq),trace amount of nacn(aq),in the cold,10 to 20 degree.

      when the nitrile compound is reduced as shown in question by h2,alkene and nitrile is reduced,but for lialh4 alkene is not reduced.

      behind the mechanism is nucleophilic addition(relatively straightforward)

      Generation of Hydride ion:BH4- ->(reversible arrow) H^- +BH3.

      Then the slow and fast step relatively standard.

       

      Why lialh4 a stronger red. agent than nabh4=orbital overlap of Al and H in AL-H bond or different polarising power of Li+ and Na+ of anion electron cloud to release hydride ions.

       

      the naoh thingy just dilute the thing(1m) and state how to deduce order+brief procedure of expt such as recording time taken for ppt formation at diff concentrations via dilution and comparing it with diff expt to determine order(2m)[I made naoh more concentrated but as my back part is right i probably lost 1 mark as only dilution is accepted to concentration lower than 2mol dm^-3].

       

      Graph shows a first order downward sloping curve with constant half life(reaction time) halves for a doubled increase in successive concentrations of naoh up to 2moldm^-3.[I labelled beyond 2 moldm3 so if lenient i lost 1 mark at most,if strict 2 marks lost].[I forgot whether this was part of qn 4 or 5]

       

      qn 5

       

      for the electrolysis part,delta g is -284.0kj mol-1(to 3 sf)=reaction thus spontaneous at r.t.p

      Define homogenous catalyst(2m),show how Fe2+ catalyses using appropriate equations in the reaction of S2O8^2- + 2I^- -> I2+ 2SO4^2- (2m) and state the property,which is able to form one or more stable ions with variable oxidation states(1m).

       

      Contrast between colour for the 3 mark question of iron compounds[solid hydrated fe3+,aqueous fe3+ and solid hydrated fe2+] is relatively ok

       

      Purpose of using c6h5cooh:It is very reactive/combustible.

      Water jacket purpose:Ensures temperature of surrounding water at same temperature as calorimeter thus no heat loss.

      Presence of black solid:Incomplete combustion of c6h5cooh forming carbon,thus suggest excess use of o2.

       

      to find heat capacity of calorimeter is -(q x average temp rise for sample 2 and 3)/amt of benzoic acid in moles=-3230kj mol^-1,where q is the heat capacity of calorimeter.One should end up with something like -(qx30)/0.05= -3230kj mol^-1 before eventually arriving at 5.38kj bla bla for q.

       

      the below 3 marks shld be fine.

       

      Napthalene(Moth Balls) qn:

      -590kj mol -1

      ai value higher thus molecule very stable due to p orbital overlap between singly bonded and doubly bonded carbon atoms results in extensive delocalisation of electrons(or words to the effect).

       

      sp2 hybridisation,draw out the hybridisation between two c atoms.c=c is 1 sigma 1 pi,c-c technically is 1 sigma only but cos of delcoalisation of electrons is somewhat 1 sigma 1 pi too for bonds.Thus only 1 diagram to show hybridisation is sufficient between sp2-sp2.

       

      Showing is c10h12 is fine.

      drawing of structures too and reason.

      Reagent and condition:H2(g),Nickel catalyst,200 degrees.

       

       

      Edited by MapPwner 07 Nov `17, 2:58PM
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • Originally posted by MapPwner:

      I wont dare say its easy,i have a friend,hes top 10% in level for chem,said the paper was easy and lost only 5 marks.We discussed only 70% of the paper and that increased to 25 marks loss total so far.However of course there were chem teachers who said it was simple and beared a tinge of sarcasm by saying qn 6 and 7 is 'only slightly harder'.

      As for me after stringent calculation of my score after discussion with many people,i probably lost 15-17 marks.


      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 ;)

    • Originally posted by do12453:

      Paper was q easy. Weirdest qn was last question on Naphthalene but even then it was still doable after abit of thinking


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

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