29 Nov, 05:24AM in sunny Singapore!

Recent Posts by UltimaOnline

Subscribe to Recent Posts by UltimaOnline

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by oayihzeet:

      Hi, with regards to Q29, may I ask why is the answer B? 
      My own interpretation of the question is that aqueous bromine
      - Adds 1 bromine atom across the alkene
      - The carboxylic group is unaffected
      - The -OH group is 2,4 directing while the electron-rich (alkene/carboxylic acid) side chain either reinforce the 2,4 position(with respect to the -OH group) or enables the other 2 position to be attacked by Bromine.
      Thus, I arrive at the answer of 3 or 5 brome atoms being incorporated, with C being my answer as 3 is not part of the choices.

      Thank you for the help in advance. :)


      There are many possible products, but Cambridge specified maximum possible number of Br atoms.

      Although Br and OH is expected to be added across the alkene double bond when aqueous bromine is used, but as a minor product, it's also possible for 2 Br atoms to be added across instead. So that's 2.

      The remaining 2 Br atoms come from the electrophilic aromatic substitution of 2 H+ by 2 Br+ electrophile cations on both ortho positions with respect to the strongest activator group, the phenolic OH group which donates electrons by resonance.

      In contrast, the typical alkyl group only donates electrons by induction, which is a much weaker effect and insufficient for halogenation via electrophilic aromatic substitution.

      Interestingly for this particular question, the alkyl group actually withdraws electrons both by induction (due to higher % s character of the sp2 C atom of the alkene group), as well as by resonance delocalization of electrons over to the electron-withdrawing COOH group as there is conjugation throughout the entire molecule from one end to the other.

      So that's a total of 4 Br atoms, as the maximum possible number. (Cambridge knew that most Singapore students would automatically assume Br and OH added across the double bond, so Cambridge very kindly eliminated 3 Br atoms as a possible option).

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Runepout posted :
      Ahhhh ok... fingers crossed I guess >.<?

      In life, always hope for the best, but be prepared for the worst.

      JC grads should now be perusing over their options for the next phase of their lives, and begin seriously planning for their choice of University and degree course.

      Ok fine, take a well-deserved break and enjoy yourselves for now. But do give your future some serious and careful deliberation. Don't choose the wrong Uni course for the wrong reasons, or you may regret it for the rest of your life.

      That said, Happy Holidays! image

      Edited by UltimaOnline 24 Nov `14, 11:45PM
  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally Posted by Runepout :
      Yup... I feel that this year's paper is on the whole much easier than past few years paper because there are less out of syllabus questions/challenging questions???

      P2 & P3 are of similar difficulty as previous years (most students don't realize the real correct answers for a couple of distinction level P2 & P3 sub-questions, require going beyond the H2 syllabus, as it did in recent years 2010 to 2014; whether Cambridge accepts the simplified H2 version that JCs teach, is a separate point); it's P1 that is significantly easier.

      Hence my upward revision for the A grade cut-off overall (ie. P1 + P2 + P3 + Spa) % from (my previous estimate of) 75% to (my final estimate of) 77%. When self-marking or self-estimating your score, err on the side of caution, both because you never know when you might have missed some careless mistake you didn't even know you made, as well as because I personally feel the standard H2 answers taught by Singapore JCs and other tuition teachers to be lacking (when it comes to the couple of distinction level P2 & P3 sub-questions I mentioned previously).

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by rynal:

      Hi may i know why Q21 is A and not C :) and is 33/40 (mcq) and 63/80 (p3) and 60/72 (p2) plus practicals that went fine enuf for an A judging frm ur experience :) and thx for the effort in replying us :)


      Q21. C is the transition state for SN2, but the diagram indicates SN1.

      Based on the overall weightage (see pg 4 below), if your overall score is 77% or greater, you'll most likely score A grade. But take your school teacher's or private tutor's answers with a pinch of salt, and self-mark on the side of caution (so if you think you scored > 77% but turns out you didn't get an A grade, your actual score is most likely < 77%).

      https://www.seab.gov.sg/content/syllabus/alevel/2015Syllabus/9647_2015.pdf

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by enorh12:

      Hi,

      For qn33. Since SO4 is much bigger than M, shouldnt Hsoln LE be counted as insignificant compated to hydration energy. Since the increasing size of M will only affect LE minimally as SO4 is alr so big. My answer was D...

       

      Qn37. My answer was C because the conditions given didny specify excess conc NH3. So polyalkylation likely to take place and primary amines formed will undergo further nucleo sub

       

      Thanks! 


      Q33. You're already implicitly considering both lattice dissociation enthalpy as well as hydration enthalpy, in determining solution enthalpy.

      Q34. Even if polyalkylation takes place, this is still one of the possible (and immediate) products.

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by Richard22:

      For the past years tys I'd done, I realise cambridge dont really care about ambiguity. For example, H2 and nickel can happen at rtp without heating to 200degrees, phenol can form ester without forming phenoxide first etc, unlike what we are told in sch. So I dont think the conditions of 'excess' NH3 'heat in a sealed tube' is 'required' for it to be the answer 


      Cambridge isn't always 100% consistent (afterall, it's not always the same exact Cambridge dude writing all the mark schemes for all the questions for all the papers for all the decades... if it was, he might have already gone on strike for being severely overworked and underpaid). Some years Cambridge may not require "excess", some years they may require it.. So when calculating your own marks, it's safer to err on the side of caution.

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by Darkcygnet:

      Hi

      For question 15, doesn't black phosphorus conduct electricity in solid state ?

      Thanks!


      That's only 1 out of many different allotropes of P.

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by Immasheep:

      Sorry i meant why is qn 3 not D, thanks!


      You have to write out the balanced combustion equation for compound G, given as C7H12O3. To balance O atoms, 8.5 molecules required.

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by Immasheep:

      Hi, why option 3 for qn 37 is wrong and why is qn2 not D?


      Option 3 for Qn 37 is right (not wrong, as you wrongly said).

      For Q2, you have to write the half-equations and balanced disproportionation redox equation, and the question specifies equal moles of ClO2 and OH- ions in the final balanced disproportionation redox equation.

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by Adelineeeee (::

      why is q36 option 1 correct? 


      Because 3 is obviously wrong and 2 is obviously correct, and there is no option which says "only 2 is correct".

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by Momento:

      Are the worked solutions for p2 and p3 out yet?


      Yes, many schools and private tutors have released P2 and P3 full solutions for their own students, but (since you're not their student) you have to search for these solutions on your own. I will not be releasing P2 and P3 full solutions, since I've already answered many P2 and P3 queries here on the forum.

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by chemrock:

      For question 33 why do the lattice energies have to  be take into account? Since the lattice energies of the group II sulfates decrease down the group,  taking the  LEs might contradict the trend of decreasing solubilities  So should the answer be D instead 


      By Hess Law, you have to consider *both* the lattice dissociation enthalpies as well as the hydration enthalpies. See my BedokFunland JC question (posted on my BedokFunland JC website) here :

      image

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by stusentxxx:

      Sorry, I don't understand why qns15 is Al?

      and why is qns37 A, option 3 I rotate 360degrees but is still unable to get the product structure. 




      Q15
      A says it's a metal, eg. Al
      B says its P
      C says it's P or Al
      D says it's Al

      Q37
      Look at the 1st product of the E1 (ie. unimolecular elimination mechanism) reaction on the left (the products on the extreme right are the result of a carbocation rearrangement due to hydride shift, which is beyond H2 syllabus).

      image

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally Posted by Gaga_ View :
      Hello may I ask if the 77% is a lower range estimate or higher range estimate? Higher range estimate being that it almost confirm won't be higher than 77%?

      It's a safe estimate. There's a very small possibility it might be higher than 77%, but it's highly unlikely.

      If you thought you got > 77% and yet you didn't get A grade, then it's almost definitely your self-marking and self-estimate that is incorrect. For instance, many JC students self-mark their P2 and P3 answers based on their school teachers or private tutors' answers, which has to be taken with a pinch of salt (since it's common knowledge that answers commonly differ between school teachers, both across schools and even within a school).

      Originally Posted by EightyOne
      Hey Ultima im confused 77% includes SPA?

      Yes. Since your school does not release Spa grades, you have to estimate that for yourself. I recommend you play safe and assume you only did so-so for Spa.

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by joel tan 123:

      Hi sir could you please post h1 answers as well. thanks alor


      Sorry, I don't have the H1 qn papers. You can look for your H1 school teacher for that.

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by alycx:

      Hi could you explain the answer for q7, 21 and 39(why cant there be hbonding in pr structure)


      Q7. Both cation and anion have lowest charge densities, hence weakest ionic bonds.

      Q21. Diagram indicates SN1, hence X is the transition state of the leaving group being eliminated.

      Q39. Primary structure refers to the sequence of amino acids, which are held together by peptide covalent bonds only.

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by Dylan96:

      I got 31/40 for P1, have chance of getting A mah for chem? 


      Depends on your overall % (P1 + 2 + 3 + Spa) combined.

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by Momento:

      Sir, What will be the cutoff for A and B this year based on the difficulties of the papers?


      Approximately 77% for A grade, and 63% for B grade.

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by Alohamadda:

      Hmm, counting that in i got 27/40... Haiz b alr

      Could u draw them structures out? I triied drawing it agn but still got 6 lol

      Thx sir


      The 5 isomers are :
      1-chloro-2-methylpentane
      2-chloro-2-methylpentane
      3-chloro-2-methylpentane
      4-chloro-2-methylpentane
      5-chloro-2-methylpentane

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by Alohamadda:

      When u use dilute nitric acid, i think there might be possible multiple substituitions resulting in white ppt. So i dunnoe whether d is correct orwrong.

      And qn 20, why not D?

       


      Updated : Q28 is C, not D (since nitrophenols are yellow coloured).

      Check that your drawing of your skeletal structure for P has the correct no. of C atoms. Then notice that the 2 terminal methyl groups are identical. Hence only 5 possible isomers.

      Edited by UltimaOnline 24 Nov `14, 4:00PM
  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • Originally posted by Kingweixian:

      For qn 28 what is the identity of the white ppt formed in Option C


      Phenol is white ppt. When deprotonated by NaOH, white ppt dissolves because phenoxide ion is a lot more soluble in water (due to thermodynamically favourable ion - permanent dipole interactions).

  • Moderator
    A Levels H1 & H2 Chemistry Tuition @ BedokFunland JC
    UltimaOnline's Avatar
    4,624 posts since May '05
    • 2014 H2 Chemistry Paper 1 MCQ Answers (by BedokFunland JC)

      If you disagree with a particular answer, post your explanation here.

      Q1. B
      Q2. C
      Q3. A
      Q4. A
      Q5. A
      Q6. B
      Q7. A
      Q8. C
      Q9. C
      Q10. B
      Q11. C
      Q12. C
      Q13. B
      Q14. B
      Q15. B
      Q16. D
      Q17. C
      Q18. A
      Q19. C
      Q20. B
      Q21. A
      Q22. D
      Q23. D
      Q24. D
      Q25. D
      Q26. C
      Q27. D
      Q28. C (D generates nitrophenols, which are yellow coloured)
      Q29. B
      Q30. B
      Q31. B
      Q32. B
      Q33. B
      Q34. B
      Q35. D
      Q36. A
      Q37. A
      Q38. B
      Q39. C
      Q40. A

      Edited by UltimaOnline 24 Nov `14, 3:57PM