Lurkers, register with SgForums and ask ur H2 Chem qns here!

• Flying grenade

  19 Nov 15, 16:38

  If you want further clarification, join my BedokFunland JC tuition (assuming you're retaking your A levels) and discuss it during tuition. Alternatively, as is the case with 99% of Singapore JC students, just "memorize it and don't ask so much" as JC teachers would instruct.

   

  The other 1 % exceptional one are bedokjc's students *Two thumbs up!!!!!!*

• UltimaOnline

  19 Nov 15, 16:56

  Originally posted by Flying grenade:

  Huh?? But the picture of the intermediate, shows the U across the 3rd C too, relative to the C atom that is fully filled

  
  Just as O levels you were lied to that all compounds are either covalent or ionic, then at A levels you learnt the truth that all compounds lie within a spectrum between covalent and ionic; concordantly A levels is similarly simplified for the purpose of preparing you for University studies, which is when you start to properly learn the leading-edge relative truths, to the best of capacities of human science on this planet as of now.
• Flying grenade

  19 Nov 15, 17:07

  Hydroysis of amide

  Esterification of amine

  Nu sub of Carboxylic acid to ester

  Hydrolysis if esters

  Nu sub if alcohol into ester

  Oxdn of alcohol

  Oxdn of carbonyl cpds

  Hydrolysis of nitriles

  Side chain oxdn of methylbenzene

  Elimination , NuSub of halogenoalkanes into nitriles and alcohol

  Esub of NO2 into halogenoarene

   

  Ive listed down all the organic reactions in H2 syllabus that requires heat/ heat under reflux as (one of) its condition

  One perennial qns that puzzles many:

  When need use/write heat under reflux or heat??

  Pleaseee help!!!

   

  For my own notes i wrote all as heat only, refering to kim seng chan's book and the hwach's pam peng cheong summary book, none stated HUR

• Flying grenade

  19 Nov 15, 17:27

  Positive formal charge is not delocalised through meta C , is it because meta group is supposed to be electron withdrawing 

  But haha there's no group attached to the C there for that compound

• Flying grenade

  19 Nov 15, 17:30

  2012 p3 qn 2eii)

  Different answers from different publishers ultima please help!!!!

  I concur with this answer https://www.dropbox.com/s/dbdp64mdsbog5dr/20151119_172600.jpg?dl=0

   

  https://www.dropbox.com/s/fviekfa65r21zte/20151119_172611.jpg?dl=0

   

  Can help check, thank u ultima

• UltimaOnline

  19 Nov 15, 17:34

  Eh don't confuse yourself in the last few hours before each paper lah! Don't worry so much lah, if Chan Kim Seng or George Chong or CS Toh say "heat", then you write "heat" lor. In almost all cases for H2 syllabus, just "heating" without "reflux" is acceptable. But if Cambridge asks you questions about reflux, or why / when it is needed, I wrote about it a few weeks ago :
  
  You need to know how to draw, label and describe the use of the reflux apparatus (when it came out in the Singapore A levels a few years ago, most Singapore JC students couldn't do this question). There are 2 main types of reflux setups that can be tested at A levels : simple reflux, and reflux with distillation.
  
  Heating is required for most organic chemistry reactions (other than those whose reactants or products are vulnerable to thermal decomposition or undesired side reactions, then room temperature or warming is required instead of heating), as most organic chemistry reactions involve breaking of covalent bonds and hence involve high Ea.
  
  However, heating alone without reflux (ie. the use of a condenser) causes problems : organic reactants and solvents, being simple covalent molecular with weak intermolecular van der Waals are volatitle (ie. low boiling point). As such, either the solvent, or one or more of reactant(s), may vaporize and escape, instead of remaining in the reaction mixture, which sabotages the reaction by preventing it from continuing and/or reaching equilibrium yield for the product.
  
  Hence heating under reflux (ie. with the use of a condenser) has 2 main purposes :
  1) Ensuring the solvent does not vaporize away and cause the reaction vessel to boil dry, stopping the reaction. In addition, as a particular solvent has its own fixed boiling point, hence by careful choice of solvent, you can control the temperature within a very narrow range, to allow the reaction will proceed at the temperature ideal for that particular reaction. The constant boiling action also serves to continuously mix the solution, ensuring the reactants can continue to effectively mix and collide and thus reach equilibrium yield for the product.
  2) Ensuring that the organic, volatile reactants (1 or more of which may have boiling points below the solvent and products) do not vaporize away and cause the reaction to slow down and stop, which prevents reaching the maximum equilibrium yield possible.

  Originally posted by Flying grenade:

  Hydroysis of amide

  Esterification of amine

  Nu sub of Carboxylic acid to ester

  Hydrolysis if esters

  Nu sub if alcohol into ester

  Oxdn of alcohol

  Oxdn of carbonyl cpds

  Hydrolysis of nitriles

  Side chain oxdn of methylbenzene

  Elimination , NuSub of halogenoalkanes into nitriles and alcohol

  Esub of NO2 into halogenoarene

   

  Ive listed down all the organic reactions in H2 syllabus that requires heat/ heat under reflux as (one of) its condition

  One perennial qns that puzzles many:

  When need use/write heat under reflux or heat??

  Pleaseee help!!!

   

  For my own notes i wrote all as heat only, refering to kim seng chan's book and the hwach's pam peng cheong summary book, none stated HUR

  
  
• UltimaOnline

  19 Nov 15, 17:37

  Originally posted by Flying grenade:

  2012 p3 qn 2eii)

  Different answers from different publishers ultima please help!!!!

  I concur with this answer https://www.dropbox.com/s/dbdp64mdsbog5dr/20151119_172600.jpg?dl=0

   

  https://www.dropbox.com/s/fviekfa65r21zte/20151119_172611.jpg?dl=0

   

  Can help check, thank u ultima

  
  Eh both are the same answers lah. If you wanna be even more precise than these publishers' answers, you can indicate the unipositive formal charge directly next to the N atom, and the uninegative formal charge directly next to the Cl atom.
• Flying grenade

  19 Nov 15, 17:45

  Originally posted by UltimaOnline:

  
  Eh both are the same answers lah. If you wanna be even more precise than these publishers' answers, you can indicate the unipositive formal charge directly next to the N atom, and the uninegative formal charge directly next to the Cl atom.

  OHHHH i understand this alr. Thank u god 😄

  If we use sulfuric acid instead, then there will be so42- attached, instead of cl-?

• UltimaOnline

  19 Nov 15, 17:49

  Originally posted by Flying grenade:

  OHHHH i understand this alr. Thank u god 

  If we use sulfuric acid instead, then there will be so42- attached, instead of cl-?

  
  Either HSO4- or SO4 2-, depending on context. Unless the question specifies "compound" (ie. meaning both the main cation / anion, together with its counter anion / cation, has to be specified), in most cases Cambridge will accept your answer even if you do not include the counter ion, as long as your main product is correct.
• Flying grenade

  19 Nov 15, 17:50

  Originally posted by UltimaOnline:

  Eh don't confuse yourself in the last few hours before each paper lah! Don't worry so much lah, if Chan Kim Seng or George Chong or CS Toh say "heat", then you write "heat" lor. In almost all cases for H2 syllabus, just "heating" without "reflux" is acceptable. But if Cambridge asks you questions about reflux, or why / when it is needed, I wrote about it a few weeks ago :
  
  You need to know how to draw, label and describe the use of the reflux apparatus (when it came out in the Singapore A levels a few years ago, most Singapore JC students couldn't do this question). There are 2 main types of reflux setups that can be tested at A levels : simple reflux, and reflux with distillation.
  
  Heating is required for most organic chemistry reactions (other than those whose reactants or products are vulnerable to thermal decomposition or undesired side reactions, then room temperature or warming is required instead of heating), as most organic chemistry reactions involve breaking of covalent bonds and hence involve high Ea.
  
  However, heating alone without reflux (ie. the use of a condenser) causes problems : organic reactants and solvents, being simple covalent molecular with weak intermolecular van der Waals are volatitle (ie. low boiling point). As such, either the solvent, or one or more of reactant(s), may vaporize and escape, instead of remaining in the reaction mixture, which sabotages the reaction by preventing it from continuing and/or reaching equilibrium yield for the product.
  
  Hence heating under reflux (ie. with the use of a condenser) has 2 main purposes :
  1) Ensuring the solvent does not vaporize away and cause the reaction vessel to boil dry, stopping the reaction. In addition, as a particular solvent has its own fixed boiling point, hence by careful choice of solvent, you can control the temperature within a very narrow range, to allow the reaction will proceed at the temperature ideal for that particular reaction. The constant boiling action also serves to continuously mix the solution, ensuring the reactants can continue to effectively mix and collide and thus reach equilibrium yield for the product.
  2) Ensuring that the organic, volatile reactants (1 or more of which may have boiling points below the solvent and products) do not vaporize away and cause the reaction to slow down and stop, which prevents reaching the maximum equilibrium yield possible.

   

  Thk u ultima 

   

  Dun get 2)

  So if it does not vaporise its bad right? Then why still want to ensure that it doesnt vaporise? 

   

  Update : understood it alr. Thank you!

   

  'Eh don't confuse yourself' 

  Wont, asking u is gaining knowledge, not diluting knowledge, and your explanations are always top notch and clear cut. Thanks ! !!

  https://www.dropbox.com/s/kpwhn7if4vfi249/8715de6a97ecefb9e71c87cd684afc32.jpg?dl=0

• Flying grenade

  19 Nov 15, 17:51

  Originally posted by UltimaOnline:

  
  Either HSO4- or H2SO4, depending on context. Unless the question specifies "compound" (ie. meaning both the main cation / anion, together with its counter anion / cation, has to be specified), in most cases Cambridge will accept your answer even if you do not include the counter ion, as long as your main product is correct.

  Is it so42- u mean?

  How can h2so4 attach to it?

   

  Update : saw the edit alr. Thanks! (:

• UltimaOnline

  19 Nov 15, 18:00

  Originally posted by Flying grenade:

  Is it so42- u mean?

  How can h2so4 attach to it?

  
  Yah typo. Don't lose marks with typos like these : they can cost you your A grade if your score is around the grade boundary (which we'll have a better idea of after tmrw's P3).
• UltimaOnline

  19 Nov 15, 18:00

  Originally posted by Flying grenade:

  Thk u ultima 

   

  Dun get 2)

  So if it does not vaporise its bad right? Then why still want to ensure that it doesnt vaporise? 

  
  If the reactants vaporize away, then they don't get to react, then you don't get your product, then you've wasted your money.
• Flying grenade

  20 Nov 15, 11:44

  https://www.dropbox.com/s/zlp6sofy7wm49fe/20151120_114103.jpg?dl=0

   

  Why will heating several hours in inert solvent result in that compound? 

  Is it because strong oxidation result in bond cleavage? But i thought it only applies for double bond?

  And how is CO2 gas formed?

• UltimaOnline

  20 Nov 15, 11:48

  Originally posted by Flying grenade:

  https://www.dropbox.com/s/zlp6sofy7wm49fe/20151120_114103.jpg?dl=0

   

  Why will heating several hours in inert solvent result in that compound? 

  Is it because strong oxidation result in bond cleavage? But i thought it only applies for double bond?

  And how is CO2 gas formed?

  
  Because this is both an alpha and a beta keto-carboxylic acid, and thus being a beta-keto-carboxy acid, decarboxylation is possible. You can try drawing the mechanism for this, which I teach my BedokFunland JC students.
  
  Not gonna answer any more qns here liao. Will come back to the forum again only after Paper 3.
• Flying grenade

  20 Nov 15, 12:47

  https://www.dropbox.com/s/eql687yhg4w0znh/20151120_124356.jpg?dl=0

   

  What information are we lacking such that we cannot deduce its a 2nd order rxn?

  Is it information of rate? What else?

• UltimaOnline

  20 Nov 15, 20:00

  Originally posted by Flying grenade:

  https://www.dropbox.com/s/eql687yhg4w0znh/20151120_124356.jpg?dl=0

   

  What information are we lacking such that we cannot deduce its a 2nd order rxn?

  Is it information of rate? What else?

  
  Before you can be 100% sure, you'll need experimental data (such as the table given for OH radicals) for CH4 as well.
• Ibenedictkoh

  22 Nov 15, 22:55

  hello there! i know it's not very wise to ask about the h2 chem bell curve since most teachers said to carry on and focus on other papers.

  could you give an estimated marks that one should get for each paper for 2015 to achieve an A, assuming that p1 is an easy paper? i heard from peers that p2 is manageable/weird, while p3 is generally ok and much easier that p2.

  Eg.

  Paper 1: /40

  Paper 2: /60

  Planning: /12

  Paper 3: /80

  it would help a lot thanks :)

• UltimaOnline

  22 Nov 15, 23:48

  Originally posted by Ibenedictkoh:

  hello there! i know it's not very wise to ask about the h2 chem bell curve since most teachers said to carry on and focus on other papers.

  could you give an estimated marks that one should get for each paper for 2015 to achieve an A, assuming that p1 is an easy paper? i heard from peers that p2 is manageable/weird, while p3 is generally ok and much easier that p2.

  Eg.

  Paper 1: /40

  Paper 2: /60

  Planning: /12

  Paper 3: /80

  it would help a lot thanks :)

  
  Look at overall % score, not individual papers. Of course, if all your individual papers score > 75%, then obviously your overall score is >75%, then your A grade is safe.
  
  (Disclaimer : if you thought your overall score > 75% but you didn't get an A grade, it means your overall score was < 75%, and some of your answers you thought were acceptable by Cambridge, weren't. So always play safe and calculate your marks conservatively, if you're not sure if you got the mark for a certain qn, just assume you didn't).
  
  The actual A grade boundary for almost every year lies somewhere between 70% and 75%, depending on the bell-curve for the cohort. P2 and P3 were of average difficulty, hence A grade boundary as it stands should be approx 72.5%, give or take 1-2% (depending on this cohort's performance). If P1 this coming Thurs turns out easier than average, A grade boundary will be shifted up closer to 75% (possibly exceeding slightly). If tougher, may be shifted downwards closer to 70% (but it will never be lower than 70%). So should you be hoping for an easy or tough Paper 1? Pros and Cons.
• Sugarfortress

  23 Nov 15, 00:23

  Originally posted by UltimaOnline:

  
  Excellent point, Sugarfortress (are you a H2 or H3 student? or not even a student?).
  
  This *appears* to be irreconcilable at A levels, given the stoichiometry and magnitudes of the relevant oxidation and reduction potentials. A easy workaround would be to suggest that excess I- be required (again, non-standard molarities). However, as it turns out, there's actually a more important reason that enables the redox reaction to proceed to generate CuI(s) even without excess I-. Which is, the reduction potential of Cu2+(aq) to Cu+(s) precipitate is actually far more positive than given in the Data Booklet, which is for aqueous species, ie. Cu2+(aq) to Cu+(aq). So much so that as CuI(s) precipitates out, the magnitude of the decrease in oxidation potential of I- to I2, is still far outweighed by the magnitude of increase in reduction potential of Cu2+(aq) to Cu+(aq) to Cu+(s). As such, admittedly my earlier explanation is simplified and incomplete, but would still suffice for A levels, should Cambridge ever ask this question.

  Hi Ultima,

  Really sorry for the late reply! I'm a H2 student. Also, thanks for that clarification pertaining to CuI!

  On another note, could I seek your advice pertaining to the Mcq question below? The answer provided is C, but we couldn't figure out why. We understand that they are asking for fractions instead of just concentration of X so it's less clear cut, yet we know that there is more than 1 variable in the fraction changing at any point in time (ie LHS, RHS, fraction numerator, fraction denominator). How should we approach such questions? 

  https://drive.google.com/file/d/0B0qaWFuyyNByRjBCSFJoRW1wQlE/view?usp=docslist_api

  Thanks so much! :) 

• UltimaOnline

  23 Nov 15, 04:39

  Originally posted by Sugarfortress:

  Hi Ultima,

  Really sorry for the late reply! I'm a H2 student. Also, thanks for that clarification pertaining to CuI!

  On another note, could I seek your advice pertaining to the Mcq question below? The answer provided is C, but we couldn't figure out why. We understand that they are asking for fractions instead of just concentration of X so it's less clear cut, yet we know that there is more than 1 variable in the fraction changing at any point in time (ie LHS, RHS, fraction numerator, fraction denominator). How should we approach such questions? 

  https://drive.google.com/file/d/0B0qaWFuyyNByRjBCSFJoRW1wQlE/view?usp=docslist_api

  Thanks so much! :) 

  
  Your past posts revealed a deeper insight than possessed by most Singapore JC students, Sugarfortress. If indeed as you say you're currently only a JC student, you'd do well to consider taking Chemistry related disciplines in the University, eg. Medicine.
  
  This MCQ is a classic killer A level Chemistry MCQ that is designed to sieve out only the most able students for an A grade, and has been recycled over the years across different JC prelim papers. There are 2 parts to this MCQ.
  
  Firstly, the shape of the curve (at either pressure). The shape is consistent with either a product of an endothermic reaction, or the reactant of an exothermic reaction. Concordantly, 1 and 2 are possible options.
  
  Secondly, at higher pressures, there is less of the species, which is consistent with the species being gaseous, and belonging to the side (ie. LHS vs RHS) with greater moles of gas. Concordantly, 1, 2 and 3 are possible options.
  
  As such, the correct answer to this MCQ is B (1 & 2 only), instead of C (2 & 3 only).
  
  Assuming that's your paper and handwriting, you and your friends actually correctly worked it out, but were sabotaged by your teacher's wrong marking. Don't let the term "fraction" confuse you. Just replace it with the word "moles" or "molarity", then you're good to go.
  
  You were probably confused with the use of the term "fraction" only after you got back your script, and only because of your teacher's wrong marking, misleading you into thinking the use of the term "fraction" adds some deeper, more insidious, tricky element on the question. Btw, why didn't you and your friends go ask your school teacher or private tutor (whoever wrongly marked you wrong) to explain why he/she marked you wrong? Or if you did, what did he/she retort? ;Þ
  
  You're welcome ;)
• Laxmiglobal12

  23 Nov 15, 20:44

  discussing very useful information

• Light5

  26 Nov 15, 07:38

  Q: http://papers.xtremepapers.com/CIE/Cambridge%20International%20A%20and%20AS%20Level/Chemistry%20(9701)/9701_w09_qp_42.pdf

  ^Q5 part (a) of the above paper.

  I simply dont know how to solve this part....i do understand that in benzene, all carbons are arranged in trigonal planar compared to cyclohexane but i cant understand how methylpropane is not planar while butane can be planar

  A detailed explanation of how to solve this question would be very helpful. Thank You

  Mark Scheme : http://papers.xtremepapers.com/CIE/Cambridge%20International%20A%20and%20AS%20Level/Chemistry%20(9701)/9701_w09_ms_42.pdf

• Flying grenade

  26 Nov 15, 11:19

  Is the cj acid and cj base of H2O, H3O+ and OH- respectively?

  Is the cj base of H3O+, H2O? What would it be its cj acid then?

• Flying grenade

  26 Nov 15, 11:32

  Why higher pressure means less of the species, and

  on the side with greater number of moles of gas?

  Is it because greater number of moles of gas in a fixed vessel volume , there is higher pressure, and since there are greater no. moles of gas, the mole fraction of the gas we're interested in, is smaller, and hence less of the species??