2015 A Level H2 Chemistry Paper 1, 2, 3

• chemistryiskool

  6 Nov 15, 20:55

  Hello!

  Thank you so much for doing this :)

  I would like to check with you:

  1) For the distinguishing test between HCl and HBr, I used acidified potassium dichromate solution. HBr would turn the solution from orange to green and there will be no decolourisation for HCl (I wrote down the E values).

  2) For the Sn2 reaction, my product contained a NH3+ and I neutralised it by adding OH- to generate RNH2 and a water molecule. Is this correct or do I get HI instead (the lone pair of electrons from I- coordinates with the H+)

  3) For the ionic equation for the redox half reaction (Ag2O), I wrote my reduction equation as Ag+ plus e- -> Ag as I thought that the ionic oxide will dissociate fully into its constitutent ions in aq soln. Is this correct?

  Thank you for your time!

• Weihaw08

  6 Nov 15, 20:57

  I believe that to distinguish HBr and HCl, we can make use of a displacement reaction since both are strong acids which dissociate fully to form Br- and Cl- ions. 

  Bubble Cl2(g) into both solutions.

  For HCl, the solution will remain colourless

  For HBr, the solution will turn from colourless to yellow-orange.

• poq9

  6 Nov 15, 21:06

  Shouldn't the answer to the last question be the cholorobenzene with the 3 cl attached to adjacent carbon atoms since Cl is 2,4 directing hence only one NO2 can be substituted at position 4?

   

  Oh and anyone know the answer to the first calculation question asking for energy released per dm3 or smth, is it 360?

• Weihaw08

  6 Nov 15, 21:19

  Originally posted by poq9:

  Shouldn't the answer to the last question be the cholorobenzene with the 3 cl attached to adjacent carbon atoms since Cl is 2,4 directing hence only one NO2 can be substituted at position 4?

   

  Oh and anyone know the answer to the first calculation question asking for energy released per dm3 or smth, is it 360?

  I got 3.87 for the energy released per dm^3. 
  
  My workings: 
  
  At 25 C, 101kPa, it releases 0.03xx MJ of energy per dm^3. Under these conditions, the amount of fuel in moles must be a particular value, say X.
  
  So, X mol produces 0.03xxMJ of energy.
  
  Calculate the volume of X mol of fuel at 26.8MPa. Since temperature and X remain the same, we know that p and V are inversely proportional.
  
  From here, we can deduce that at 26.8MPa, X mol of fuel occupy 0.00407 dm^3.

  Thus, energy released per volume = 0.03xx/0.00407 = 3.87.
  
  (Not sure if 3.87 is the answer I obtained in the exam, but the thought process which I did here is the same as what I did during the paper.)

• Sugarfortress

  6 Nov 15, 22:45

  Hi Ultima,

  Really appreciate all the advice and help you've rendered to us on this forum!

  I have some questions on today's paper:

  1. For the question on carbon monoxide and how it inhibits the transport of blood, will it suffice to just elaborate on the strength of the ligand, formation of dative bonds and ligand exchange? Or is further elaboration on "binding irreversibly" required?

  2. For the question on the reagents and conditions (for nucleophilic addition), Step 1, is it required to indicate 10-20 deg C for HCN and NaCN/KCN trace catalyst, or is room temperature sufficient? This is because I recalled some reference literature not specifying temperature conditions.

  3. For the question on why a brief exposure to light is sufficient, will it suffice to elaborate on how the presence of reactive free radicals due to initial homolytic fission of Cl-Cl propagates a chain reaction or must we elaborate on the regeneration of free radicals?

  4. For the question pertaining to the reactivity of Br2 with H2 compared to Cl2 and H2 (with reference to Data Booklet bond energies), will it suffice to discuss about the exothermicity of the formation of H-Br and H-Cl (based on Bond Energies) only or are candidates expected to discuss the overall enthalpy changes of the reactions? I felt that since the latter requires a consideration of the Bond Energies of Br-Br and Cl-Cl, and since the BE of Br-Br is less endothermic than that of Cl-Cl, does it really explain why Br2 is less reactive than Cl2 if its bonds are easily broken?

  Thank you for your time!

• UltimaOnline

  6 Nov 15, 23:21

  Originally posted by hallopanda:

  Phenol? White ppt formed for Br?

  
  Nope.
• UltimaOnline

  6 Nov 15, 23:23

  Originally posted by bobasd:

  Hi, for the distinguishing test for Hcl and Hbr aq, i wrote Na2Co3 (aq). Pass the gaseous product(CO2) through calcium hydroxide and the one which gives a fixed mass of white ppt at a faster rate would be HCl << Careless. However, is this considered a physical test if let's say I wrote HBr that gives a faster rate of ppt formation?
  
  And is it possible to gauge the minimum marks required for an A grade? I found this paper to be quite tricky so I lost quite a lot of marks due to carelessness :(

  
  Nope cannot, calcium halides are soluble.
  
  A grade still depends on P3 and P1, as P2 wasn't particularly easy nor tough. A grade boundaries are usually between 70% to 75% for most years, after bell-curve moderation.
• bobasd

  6 Nov 15, 23:26

  Originally posted by UltimaOnline:

  
  Nope cannot, calcium halides are soluble.
  
  A grade still depends on P3 and P1, as P2 wasn't particularly easy nor tough. A grade boundaries are usually between 70% to 75% for most years, after bell-curve moderation.

  Thank you for the reply! But I think you might have read the test wrongly haha, as in I pass the gaseous co2 into calcium hydroxide (limewater) not the halide product :(

• UltimaOnline

  6 Nov 15, 23:26

  Originally posted by Soulfunk:

  Hi, for the hcl and hbr question, does na2co3 react with hbr?

  Also, how do u measure the amounts of the component present in the mixture for the planning qn?

  
  All the hydrohalic acids are acidic enough (even the weaker HF acid) to protonate CO3 2- ion to generate H2CO3, which decomposes to H2O(l) and CO2(g) (observed as effervescence).
  
  No comments for Planning, it's too tedious (and not sufficiently interesting) to discuss it here.
• UltimaOnline

  6 Nov 15, 23:31

  Originally posted by Vitalitylx:

  For the distinguishing test, I wrote adding H2SO4 and brown Br2 is formed?

  I just thought of another which is to add hexane and red brown Br2 layer formed?

  Also for the one that requires drawing Hydrogen bonding between serine side chain and the other compound( cant remember what it is ) , is it required to draw out the entire structure of serine? I drew the side chain with a -R

  
  Nope, can't. For the redox reaction to occur between HBr and H2SO4, both must be concentrated, but the question specified "dilute aqueous". Hexane wouldn't oxidize Br- to Br2 (you started with Br-, not Br2).
  
  The question is a bit ambiguous on this requirement, but it would have been better to play it safe and draw out the entire structure, as the question specified "displayed (structural) formula". Nonetheless, it's possible you could still get all of the marks, due to the ambiguous phrasing of the question which is open to interpretation.
• UltimaOnline

  6 Nov 15, 23:32

  Originally posted by bestrobber97:

  Can you explain why the answer to the last question is 1,3,5-trichlorobenzene? All my friends said that was the correct answer but I don't get it

  
  Because it is the only symmetrical isomer and hence it doesn't matter where the NO2+ electrophile was substituted on.
• UltimaOnline

  6 Nov 15, 23:33

  Originally posted by Soulfunk:

  is 1,3,5-trichlorobenzene the same as 2,4,6?

  
  Not acceptable, because you should always reduce your numbering by choosing the right C atom as carbon #1.
• UltimaOnline

  6 Nov 15, 23:34

  Originally posted by breadman15:

  To distinguish between HCl (g) and HBr (g),

  1) Dissolve both gases in separate solutions of aqueous NaOH

  2) Add concentrated H2SO4 to each solution

  3) Pungent SO2 gas formed for solution with NaBr

  
  That would indeed work, if it were HX(g). Unfortunately, the question specified "dilute aqueous solution".
• UltimaOnline

  6 Nov 15, 23:38

  Originally posted by chemistryiskool:

  Hello!

  Thank you so much for doing this :)

  I would like to check with you:

  1) For the distinguishing test between HCl and HBr, I used acidified potassium dichromate solution. HBr would turn the solution from orange to green and there will be no decolourisation for HCl (I wrote down the E values).

  2) For the Sn2 reaction, my product contained a NH3+ and I neutralised it by adding OH- to generate RNH2 and a water molecule. Is this correct or do I get HI instead (the lone pair of electrons from I- coordinates with the H+)

  3) For the ionic equation for the redox half reaction (Ag2O), I wrote my reduction equation as Ag+ plus e- -> Ag as I thought that the ionic oxide will dissociate fully into its constitutent ions in aq soln. Is this correct?

  Thank you for your time!

  
  No prob :)
  
  Q1. Very good. Correct! ;)
  
  Q2. Unfortunately, the question asked for any by-products of the original reaction; you're not supposed to add NaOH. But you'll still get most of the marks.
  
  Q3. No cannot, you need to write it as Ag2O --> Ag and balance it accordingly.
• Vitalitylx

  6 Nov 15, 23:39

  Another question ( haha sorry for asking so many ),

  for the SN2 mechanism when HI is formed, is it acceptable to write HI or must it be H+ + I- since HI decomposes easily?

   

• UltimaOnline

  6 Nov 15, 23:39

  Originally posted by Weihaw08:

  I believe that to distinguish HBr and HCl, we can make use of a displacement reaction since both are strong acids which dissociate fully to form Br- and Cl- ions. 

  Bubble Cl2(g) into both solutions.

  For HCl, the solution will remain colourless

  For HBr, the solution will turn from colourless to yellow-orange.

  
  Very good. Correct! ;)
• LemonRay

  6 Nov 15, 23:41

  Overall, the paper is easy as compared to past year papers for H2 chem paper 2.

   

  For the distinguishing test. it should be added with Hexane and KMnO4.

   

  SN2 mechanism is the generation of an cation of RNH2CH3+. 

• UltimaOnline

  6 Nov 15, 23:42

  Originally posted by poq9:

  Shouldn't the answer to the last question be the cholorobenzene with the 3 cl attached to adjacent carbon atoms since Cl is 2,4 directing hence only one NO2 can be substituted at position 4?

   

  Oh and anyone know the answer to the first calculation question asking for energy released per dm3 or smth, is it 360?

  
  If the 3 Cl atoms are adjacent, and all 3 are ortho-para directing, it's possible to get 2 different positional isomeric mono-nitrated products.
  
  I forget the calculation question's numerical answer, so check this with someone else.
• UltimaOnline

  6 Nov 15, 23:44

  Originally posted by Weihaw08:

  I got 3.87 for the energy released per dm^3. 
  
  My workings: 
  
  At 25 C, 101kPa, it releases 0.03xx MJ of energy per dm^3. Under these conditions, the amount of fuel in moles must be a particular value, say X.
  
  So, X mol produces 0.03xxMJ of energy.
  
  Calculate the volume of X mol of fuel at 26.8MPa. Since temperature and X remain the same, we know that p and V are inversely proportional.
  
  From here, we can deduce that at 26.8MPa, X mol of fuel occupy 0.00407 dm^3.

  Thus, energy released per volume = 0.03xx/0.00407 = 3.87.
  
  (Not sure if 3.87 is the answer I obtained in the exam, but the thought process which I did here is the same as what I did during the paper.)

  
  I forget the specifics of the question and answer, but your thought process is sound. Well done. ;)
• UltimaOnline

  6 Nov 15, 23:58

  Originally posted by Sugarfortress:

  Hi Ultima,

  Really appreciate all the advice and help you've rendered to us on this forum!

  I have some questions on today's paper:

  1. For the question on carbon monoxide and how it inhibits the transport of blood, will it suffice to just elaborate on the strength of the ligand, formation of dative bonds and ligand exchange? Or is further elaboration on "binding irreversibly" required?

  2. For the question on the reagents and conditions (for nucleophilic addition), Step 1, is it required to indicate 10-20 deg C for HCN and NaCN/KCN trace catalyst, or is room temperature sufficient? This is because I recalled some reference literature not specifying temperature conditions.

  3. For the question on why a brief exposure to light is sufficient, will it suffice to elaborate on how the presence of reactive free radicals due to initial homolytic fission of Cl-Cl propagates a chain reaction or must we elaborate on the regeneration of free radicals?

  4. For the question pertaining to the reactivity of Br2 with H2 compared to Cl2 and H2 (with reference to Data Booklet bond energies), will it suffice to discuss about the exothermicity of the formation of H-Br and H-Cl (based on Bond Energies) only or are candidates expected to discuss the overall enthalpy changes of the reactions? I felt that since the latter requires a consideration of the Bond Energies of Br-Br and Cl-Cl, and since the BE of Br-Br is less endothermic than that of Cl-Cl, does it really explain why Br2 is less reactive than Cl2 if its bonds are easily broken?

  Thank you for your time!

  
  U're welcome ;)
  
  Q1. For such questions, Cambridge provides a bit of leeway, in that out of say, 4 acceptable key marking points for a 3 mark question, you can score the full 3 marks if you wrote any 3 of the 4 marking points correctly. So it's still possible to score full marks without mention of "binds permanently / irreversibly". No worries.
  
  Q2. When in doubt, it's always safer to write it in. But in this particular case, unless Cambridge specifically asked for "conditions" or "temperature", otherwise just for "reagents", Cambridge will accept either "HCN" or "HCN and CN-". So no worries, but for P3, do yourself a favor and just play it safe when in doubt :)
  
  Q3. Because the question specifically mentioned, "The rate of reaction increases, explain why", hence to get full marks for this question, you need to specify "Due to the large no. of different possible propagation reactions, free-radical concentrations will continue to increase, hence increasing rate of reaction, until the rate of termination reactions catch up" OWTTE (Or Words To That Effect). But you should still get the marks for the rest of the question.
  
  Q4. To obtain full marks, other than quoting Data Booklet bond enthalpies (or energies, for A level Chem purposes they're interchangeable), you need to mention that the decrease in magnitude of exothermic H-X bond formation enthalpy outweighs the decrease in magnitude of endothermic X-X bond dissociation enthalpy, going from Cl to Br. So yes, best answer is, as you say, take into consideration both the bond dissociation and bond formation enthalpies, leading to the conclusion that the overall enthalpy change of reaction to generate H-Cl is more exothermic (hence more thermodynamically favorable) compared to H-Br.
• UltimaOnline

  7 Nov 15, 00:00

  Originally posted by LemonRay:

  Overall, the paper is easy as compared to past year papers for H2 chem paper 2.

   

  For the distinguishing test. it should be added with Hexane and KMnO4.

   

  SN2 mechanism is the generation of an cation of RNH2CH3+. 

  
  You're a private tutor, yes?
• efghijklm

  7 Nov 15, 00:04

  What do you think would be a reasonable range of score for an A for this paper?

• LemonRay

  7 Nov 15, 00:13

  Originally posted by UltimaOnline:

  
  You're a private tutor, yes?

  Nope! I'm a student. I have benefitted a lot from your past year threads on the after-paper discussions for H2 Chemistry. Your threads explained many concepts which I was unclear about (as well as teach and expose me to questions even more so than what my teacher has given us).

   

   

• UltimaOnline

  7 Nov 15, 00:14

  Originally posted by efghijklm:

  What do you think would be a reasonable range of score for an A for this paper?

  
  I've no comments on the individual scores for individual papers. I'm only interested in looking at the overall grade thresholds after considering all 3 Papers as a whole. Today's P2 is neither particularly easy nor tough. So it wouldn't affect the bell-curve that much (which will probably remain somewhere close to the middle between 70% and 75%). So the difficulties of P3 and P1 will determine whether the overall A grade threshold for this year will be closer to the low end of 70%, or closer to the high end of 75%. Unless the remaining papers are terribly easy or terribly tough, for *most* years the A grade thresholds do not go below 70% or above 75%.
• UltimaOnline

  7 Nov 15, 00:16

  Originally posted by LemonRay:

  Nope! I'm a student. I have benefitted a lot from your past year threads on the after-paper discussions for H2 Chemistry. Your threads explained many concepts which I was unclear about (as well as teach and expose me to questions even more so than what my teacher has given us).

   

   

  
  You've followed my past year threads (plural) on the after-paper discussions for H2 Chemistry, even when you were in Sec 4 / IP Yr 4, and JC 1? Or you dug up these old threads to read them? Either way, good to know you really love Chemistry ;)