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  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,570 posts since May '05
    • Originally posted by MapPwner:

      I daren't say my OC mastery is the top,theres 4.5% people in my school taking chemistry who are more talented than me in chemistry.And yeah,the VJC OC deductive quesiton was the hardest in the set of 10 questions.The other 7 questions were integrated organic chemistry questions,the bulk coming from MJC Prelim 2016 P2/P3.I felt that the questions they took from MJC were the hardest,which was the purification synthesis question from 2016 prelim.The other MJC  questions such as fentanyl,morphine is rather standard and questions taken from other schools such as AJC Prelim on relative rate of substituiton of pri/sec/ter H atom of hydrocarbon were simple.My friends in Band 2 found the fentanyl,morphine question from MJC 2016 Prelim hard though,as their set of questions also have that question included.

      My school provides revision packages for H2 Chem covering every single topic compiled from past year prelim papers from other schools from 2011 to 2016.So considering I just started doing a few questions on the Ksp/Transition elements revision package my school has given only,I guess for now I can just get by with it as I also have other subjects to juggle too.


      Fair enough. Though I'd still recommend you take the initiative to self-compile all the Ksp and TM questions from all the Prelim papers you can get your hands on (eg. last 5 years), since these topics are your (and all JC students' too, just that the average JC student isn't even as prepared as you are in all the other topics as well) Achilles heel. Remember to share any TYS or Prelim paper Ksp and TM questions that you find difficult.

  • Jasolee99's Avatar
    5 posts since Jul '17
    • hey guys my friend created https://tick.ninja where they pooled tgt lots of O Levels and A Levels notes for their juniors. hope they help! cheers :)

      Edited by Jasolee99 01 Aug `17, 11:31PM
  • MapPwner's Avatar
    106 posts since Aug '15
    • For me,comparing to all prelim electrolysis questions and TYS,the hardest(and only) electrolysis question I have come across so far is the 2012 A level H2 Chem P3 electrolysis question about drawing of the diagram of an electrochemical cell to measure the relative oxidising powers of chlorine/iodine.The question itself felt ambiguous as I only merely thought of putting SHE at 1 end and pumping the halogen at 1 mol dm^-3,1 atm,25 degree into the solution of the halide ions at another end only,never expecting iodine to be one end,chlorine the other end as i cant imagine I2(s) being in solution.

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,570 posts since May '05
    • Originally posted by MapPwner:

      For me,comparing to all prelim electrolysis questions and TYS,the hardest(and only) electrolysis question I have come across so far is the 2012 A level H2 Chem P3 electrolysis question about drawing of the diagram of an electrochemical cell to measure the relative oxidising powers of chlorine/iodine.The question itself felt ambiguous as I only merely thought of putting SHE at 1 end and pumping the halogen at 1 mol dm^-3,1 atm,25 degree into the solution of the halide ions at another end only,never expecting iodine to be one end,chlorine the other end as i cant imagine I2(s) being in solution.


      No, your method of answering the question is not acceptable. The clue in the question is that you're tasked to measure the *relative* oxidizing powers of the *two* different halogens in a single setup (implied by the question).

      If you used a SHE setup, you need to carry out two separate setups instead of one, in which case you'll be heavily penalized and Cambridge will at most give you half the number of marks available for this question.

      Also, the new syllabus requires 1 bar, not 1 atm for standard pressure.

      Lastly, I2(s) is not part of the required setup. What is required is 1M of I-(aq) and 1M of I3-(aq), ie. the triiodide anion.

      In this setup, the Cl2 will be reduced to Cl- at the cathode, and the I- anion is oxidized to the I3- anion at the anode. The reading from the (high resistance) voltmeter gives the required answer.

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


      No, your method of answering the question is not acceptable. The clue in the question is that you're tasked to measure the *relative* oxidizing powers of the *two* different halogens in a single setup (implied by the question).

      If you used a SHE setup, you need to carry out two separate setups instead of one, in which case you'll be heavily penalized and Cambridge will at most give you half the number of marks available for this question.

      Also, the new syllabus requires 1 bar, not 1 atm for standard pressure.

      Lastly, I2(s) is not part of the required setup. What is required is 1M of I-(aq) and 1M of I3-(aq), ie. the triiodide anion.

      In this setup, the Cl2 will be reduced to Cl- at the cathode, and the I- anion is oxidized to the I3- anion at the anode. The reading from the (high resistance) voltmeter gives the required answer.

      Ah i see ok,understood.

    • Looked through several Transition Element questions,followed by the answers and there are 2 particular questions that left me puzzled(no matter what I definitely wont think of that answer at all in exams),which are HCI Prelim 2011/P2/Q2(c),regarding the sketching of the graph absorbance and also HCI Prelim 2013/P2/Q2(a) and (b) for deducing formula of complex ion in B.For that question,free chloride ions means that the chlorine ions are not part of the complex ion right?

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,570 posts since May '05
    • Originally posted by MapPwner:

      Looked through several Transition Element questions,followed by the answers and there are 2 particular questions that left me puzzled(no matter what I definitely wont think of that answer at all in exams),which are HCI Prelim 2011/P2/Q2(c),regarding the sketching of the graph absorbance and also HCI Prelim 2013/P2/Q2(a) and (b) for deducing formula of complex ion in B.For that question,free chloride ions means that the chlorine ions are not part of the complex ion right?


      Both questions are within H2 syllabus and can be asked by Cambridge, although the 2011 question is slightly less commonly encountered for Singapore JC students and hence might be a tad more confusing and accordingly a tad more challenging.

      For the 2011 question, as you said, you wouldn't be able to think of the answer the first time you come across the question. But now that you've seen the solution and understand it, if a similar question came out for the A levels, you can do it liao, right?

      For the 2013 question, yes 'free' ions means counter ions, not ligands.

  • MapPwner's Avatar
    106 posts since Aug '15
    • Actually yeah for 2011 question more or less everything can understand except for the graph sketching need more time to internalize the reasons why.

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,570 posts since May '05
    • _

      Edited by UltimaOnline 13 Oct `17, 2:49AM
    • image

      A BedokFunland JC Original H2 Chemistry Challenge

      Draw the full curved-arrow electron-flow reaction mechanism for every step of the full methamphetamine synthesis (starting from benzene!) via the Leuckart reaction pathway as illustrated below.

      image

    • [Chemistry / Medicine] - Trisha the Chemist's explanation missed out explicitly specifying the redox aspect of the 'copper dissolving' reaction (don't be misled thinking the redox reaction isn't thermodynamically feasible based on the standard redox potentials on the data booklet, because you'll be poisoned to death way before the [Cu2+] ever reaches 1M or standard molarity, tis a cunning trick question I hope Cambridge unleashes on H2 Chem students). In addition, there are several copper metabolic genetic medical conditions that make even a low [Cu2+] deadly for some people.

      image

      Chemist Debunks That Nasty Rumor About Moscow Mule Mugs Being Poisonous

      https://sg.news.yahoo.com/chemist-debunks-nasty-rumor-moscow-100024945.html

    • A 2017 BedokFunland JC Original H2 Chemistry Challenge, adapted from the Singapore-Cambridge 2008 H2 Chemistry A Level Exam

      When C6H5COCHO is reacted with OH-, it is converted into C6H5CH(OH)COO-.

      i) Draw out the full curved-arrow, electron-flow mechanism for the reaction.

      ii) Identify (with reasons) the rate determining step, and concordantly state the rate law for the reaction.

      iii) State the specific type of reaction (ie. not the type of mechanism) that has occurred, and explain why this reaction is considered unusual in organic chemistry.

      iv) Justify your answer using either OSes or FGLs.

      Edited by UltimaOnline 13 Aug `17, 5:16PM
    • A 2017 BedokFunland JC Original H2 Chemistry Challenge, adapted from a Singapore-Cambridge A Level Exam Qn.

      Explain why, despite the Br-Br bond being weaker than the Cl-Cl bond, the rate of reaction between chlorine and phosphorus is faster than the rate of reaction between bromine and phosphorus (assuming the same allotrope of phosphorus is used, and assuming the same state or phase for both halogens).

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

      A 2017 BedokFunland JC Original H2 Chemistry Challenge, adapted from a Singapore-Cambridge A Level Exam Qn.

      Explain why, despite the Br-Br bond being weaker than the Cl-Cl bond, the rate of reaction between chlorine and phosphorus is faster than the rate of reaction between bromine and phosphorus (assuming the same allotrope of phosphorus is used, and assuming the same state or phase for both halogens).

      Something to do with p-orbital overlap?Since Cl has a smaller electron cloud it is able to have a larger effective orbital overlap with phosphrous thus has higher rate of rxn.Br with a larger electron cloud is more diffused and hence forms a less effective orbital overlap with phosphrous,thus resulting in a slower rate of rxn.

      Typed this out based on what i think,havent officially revised for chemistry yet.

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,570 posts since May '05
    • Originally posted by MapPwner:

      Something to do with p-orbital overlap?Since Cl has a smaller electron cloud it is able to have a larger effective orbital overlap with phosphrous thus has higher rate of rxn.Br with a larger electron cloud is more diffused and hence forms a less effective orbital overlap with phosphrous,thus resulting in a slower rate of rxn.

      Typed this out based on what i think,havent officially revised for chemistry yet.


      Nice try, but effectiveness of orbital overlap refers to the strength of bonds formed (while you conveniently ignored the conflicting corresponding strength of bonds broken), without directly addressing the different rates of reaction.

      This is a BedokFunland JC Challenge Qn, so JC students are not expected to be able to come up with the correct answer. Those who are curious can try asking your school teacher or private tutor, and see if they know the answer (some will, some won't).

      Edited by UltimaOnline 13 Aug `17, 4:06PM
  • MapPwner's Avatar
    106 posts since Aug '15
    • Hmm then probably cos Br has a larger electron cloud,thus more electrons to be polarised,leads to stronger id-id as compared to Cl with a smaller electron cloud thus weaker id-id.Thus Cl2 bond is more easily broken than Br2 bond in terms of the strength of instantaneous dipole-induced dipole forces of attraction,and thus can react faster with Phosphorus?

    • Originally posted by UltimaOnline:

      A 2017 BedokFunland JC Original H2 Chemistry Challenge, adapted from the Singapore-Cambridge 2008 H2 Chemistry A Level Exam

      When C6H5COCHO is reacted with OH-, it is converted into C6H5CH(OH)COO-.

      i) Draw out the full curved-arrow, electron-flow mechanism for the reaction.

      ii) Identify (with reasons) the rate determining step, and concordantly state the rate law for the reaction.

      iii) State the specific type of reaction (ie. not the type of mechanism) that has occurred.

      iv) Justify your answer using either OSes or FGLs.

      Then for this question,part (iii) is oxidation?Then part (iv) to justify just say the functional group level in C in COO- is +3 as compared to +2 CHO,showing oxidation?

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,570 posts since May '05
    • Originally posted by MapPwner:

      Hmm then probably cos Br has a larger electron cloud,thus more electrons to be polarised,leads to stronger id-id as compared to Cl with a smaller electron cloud thus weaker id-id.Thus Cl2 bond is more easily broken than Br2 bond in terms of the strength of instantaneous dipole-induced dipole forces of attraction,and thus can react faster with Phosphorus?


      Nice try again, but again no.

      You cannot use strength of intermolecular interactions to justify ease of breaking intramolecular bonds.

    • Originally posted by MapPwner:

      Then for this question,part (iii) is oxidation?Then part (iv) to justify just say the functional group level in C in COO- is +3 as compared to +2 CHO,showing oxidation?


      You confuse FGL with OS, but that's a minor issue. More importantly, "oxidation" doesn't cut it. The require keyword to score the mark is more precise, less ambiguous. I'll edit my question to give a further clue.

  • MapPwner's Avatar
    106 posts since Aug '15
    • If i'm not wrong functional group level refers to the number of bonds a negatively charged element forms with carbon?So since COO- has 3 bonds,its FGL is 3,so i naturally assume C to be 3+.

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,570 posts since May '05
    • Originally posted by MapPwner:

      If i'm not wrong functional group level refers to the number of bonds a negatively charged element forms with carbon?So since COO- has 3 bonds,its FGL is 3,so i naturally assume C to be 3+.


      Not quite. In order not to spoil the fun for others, as well as to encourage self-learning and discourage spoon-feeding, anyone interested can check this out further online for yourself.

      Unlike Singapore JC teachers whom are more anal, Cambridge is more reasonable, and will accept the use of either OS or FGL in the Singapore A levels, and generally will accept (for all topics) alternative answers using either within-syllabus or beyond-syllabus concepts and terminology, as long as the answers are indeed correct.

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


      Not quite. In order not to spoil the fun for others, as well as to encourage self-learning and discourage spoon-feeding, anyone interested can check this out further online for yourself.

      Unlike Singapore JC teachers whom are more anal, Cambridge is more reasonable, and will accept the use of either OS or FGL in the Singapore A levels, and generally will accept (for all topics) alternative answers using either within-syllabus or beyond-syllabus concepts and terminology, as long as the answers are indeed correct.

      I came across FGL once during chem lecture where we did practice questions,and the explanation of FGL mentioned was something like what i said or so,from the HCI 2016 p3 prelim question i think.

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,570 posts since May '05
    • Originally posted by MapPwner:

      I came across FGL once during chem lecture where we did practice questions,and the explanation of FGL mentioned was something like what i said or so,from the HCI 2016 p3 prelim question i think.


      I know where Hwa Chong stole that question from.

      Anyways, small matter, but technically, FGL has "level" as a prefix, while OS has "+" or "-" as a prefix. Cambridge will accept either, no worries.

  • jyjyjy's Avatar
    3 posts since Jul '17
    • JJC P2 Q1(e) why is it that even though reaction is independant of [I2], when [I2] is halved, time is halved? wouldn't that be first order?

      Q5(c)(iii) How does this mechanism look like? Is it possible for the Br- from step 1 to attack instead?

      P3 Q1(b)(ii) Why does the answer state that when the reaction just becomes feasible G=0? Isn't the reaction feasible when G<0? When G=0 isn't it just at equilibrium?

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,570 posts since May '05
    • Originally posted by jyjyjy:

      JJC P2 Q1(e) why is it that even though reaction is independant of [I2], when [I2] is halved, time is halved? wouldn't that be first order?

      Q5(c)(iii) How does this mechanism look like? Is it possible for the Br- from step 1 to attack instead?

      P3 Q1(b)(ii) Why does the answer state that when the reaction just becomes feasible G=0? Isn't the reaction feasible when G<0? When G=0 isn't it just at equilibrium?


      P2 Q1e) If I2 was 1st order, then the half-life would be constant, ie. no change. Since I2 is a 0th order reactant, half-life is halved when molarity is halved.

      P2 Q5ciii) SN1. If the leaving group Br- attacked back, then you would end up with the original reactant, ie. no net reaction occurred.

      P3 Q1(b)(ii) Yes you're right, but what exact value are you going to substitute in for delta G? -1? -0.1? -0.01? -0.001? So just use delta G = 0 first, then state that for the reaction to be thermodynamically feasible, the temperature needs to be higher than 958K (ie. take initiative to improve on the given mark scheme answer, when answering such questions in the A levels).

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