2015 A Level H2 Chemistry Paper 1, 2, 3

• UltimaOnline

  27 Oct 15, 04:01

  After each paper, you may post your discussions about specific questions from each paper, here in this thread.
  
  Be sure to get at least 8 hours of sleep the night before each paper (it's actually better to sleep more than mug more the night before), and of course a healthy, nutritious, light and safe meal (nothing too spicy, fatty, raw or dangerous enough to cause vomiting and/or diarrhoea) about an hour before each paper so you've sufficient glucose in your brain.
  
  Alternatively, if you're too rushed to have a meal, at least have some energy snacks (eg. Kit Kat, Kinder Bueno, Snickers, Mars, etc) and a can of isotonic drink (eg. Aquarius, Hundred Plus, H2O, Pokka Sports Water, Pocari Sweat, etc).
  
  And if you've a headache or at risk of getting one during the paper, swallow a pill / tablet / caplet of Panadol, the locally available brand of Paracetamol, ie. N-(4-hydroxyphenyl)ethanamide just before the paper, *provided* you've already safely taken Paracetamol many times in your life, and so you're 100% sure you're not allergic to Paracetamol, which will be deathly fatal for those allergic to it.
  
  Enjoy your papers! ;Þ
• UltimaOnline

  5 Nov 15, 21:54

  Final Tip : print out and use Owl Cove summaries for your final revision in the last few hours before each A level Chem paper.
• UltimaOnline

  6 Nov 15, 10:28

  How did *you* find P2? Easy / average / tough?
  
  How did your classmates find P2? Easy / average / tough?
  
  You guys go ahead and discuss first. I'll post my comments or reply to any queries later tonight.
• bestrobber97

  6 Nov 15, 11:17

  What's the minimum % you need overall to get an A? Just wanna see if I'm on track

• hallopanda

  6 Nov 15, 13:12

  Hi, just finished the paper. Quite worred as I didn't do well for the planning question. Basically left half blank. Anyway, can you help me with the following questions:

  Q1. Is the mechanism for HCl Free Radical Substitution? Also, is it because there is an increasing rate of reaction because the H• radical will react with Cl2 to reform Cl• radical which can further react with H2? I forgot to mention chain reaction. Will lose marks?

  Q2. Is it because Br atom radius is larger that's why there is less degree of overlap between Br and H to form HBr. As such, the reaction is less exothermic and hence the formation of HBr is less reactive?

  Thank you.

• Mankey123

  6 Nov 15, 14:36

  Hi, how is this paper compared to precious years? some say is manageable but some also say harder than prelims

  1. For the question on hydrogen and methanol fuel cell, why is there no value for combustion of petrol? Can I write cos combustion of petrol cannot be done under standard conditions?

  2. For FRS question, why is there increasing rate of reaction after bright light removed? Can I say radicals are very reactive and the subsequent termination steps are faster cos it only involve radicals?

  3. Why is there a reaction only after there's bright light? Isit cos to lower activation energy? 

  4. I carelessly drew sn1 instead of sn2. Is that zero marks?

  5.For planning, I only write out the steps and no calculations and explanations on how to find the concentrations, how many marks is that?

• M4r

  6 Nov 15, 16:03

  Hi can i know what is the minium % to get an A over the years. Is losing over 10 marks equvient to no more A? ( marks are Excluding planning)

• bestrobber97

  6 Nov 15, 17:03

   I lost around 10 marks excluding planning too

• UltimaOnline

  6 Nov 15, 17:27

  Originally posted by bestrobber97:

  What's the minimum % you need overall to get an A? Just wanna see if I'm on track

  
  

  Originally posted by M4r:

  Hi can i know what is the minium % to get an A over the years. Is losing over 10 marks equvient to no more A? ( marks are Excluding planning)

  
  bestrobber97 and M4r
  
  The exact grade boundaries differ every year, as a different bell-curve is produced every year, based on varying difficulty of papers, and varying performance of cohort. Nonetheless, to play it safe, you should consider the A grade boundary to be 75%, rather than 70%. It's usually somewhere in between, depending on the bell-curve.
• UltimaOnline

  6 Nov 15, 17:31

  Originally posted by hallopanda:

  Hi, just finished the paper. Quite worred as I didn't do well for the planning question. Basically left half blank. Anyway, can you help me with the following questions:

  Q1. Is the mechanism for HCl Free Radical Substitution? Also, is it because there is an increasing rate of reaction because the H• radical will react with Cl2 to reform Cl• radical which can further react with H2? I forgot to mention chain reaction. Will lose marks?

  Q2. Is it because Br atom radius is larger that's why there is less degree of overlap between Br and H to form HBr. As such, the reaction is less exothermic and hence the formation of HBr is less reactive?

  Thank you.

  
  Q2. Yes. It's about the different strengths of the H-X bond (in turn due to the different atomic radii), and accordingly the exothermicity and thermodynamic favorability of the reactions. You need to quote Data Booklet bond energy values as indicated in the question, to get full marks.
• UltimaOnline

  6 Nov 15, 17:40

  Originally posted by Mankey123:

  Hi, how is this paper compared to precious years? some say is manageable but some also say harder than prelims

  1. For the question on hydrogen and methanol fuel cell, why is there no value for combustion of petrol? Can I write cos combustion of petrol cannot be done under standard conditions?

  2. For FRS question, why is there increasing rate of reaction after bright light removed? Can I say radicals are very reactive and the subsequent termination steps are faster cos it only involve radicals?

  3. Why is there a reaction only after there's bright light? Isit cos to lower activation energy? 

  4. I carelessly drew sn1 instead of sn2. Is that zero marks?

  5.For planning, I only write out the steps and no calculations and explanations on how to find the concentrations, how many marks is that?

  
  Q1. Petrol is a mixture of alkanes, cycloalkanes, and alkenes, with between 4 and 12 carbon atoms per molecule. Hence it doesn't have a fixed combustion enthalpy value.
  
  Q2. 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.
  
  Q3. To *provide* the activation energy, not lower it.
  
  Q4. Yes, zero marks.
  
  Q5. Depends on exactly what you wrote. It's impossible to determine exactly how many marks you can get from Planning, and you shouldn't bother so much with it. Just do your best with the remaining Papers, and if you do well enough for them (since the overall score is an aggregate of all papers), you can still get your A grade.
• Weihaw08

  6 Nov 15, 18:51

  This paper was quite manageable. Apart from the planning question, I think the other structured questions are doable but require some thinking.

  The planning question involves the determination of Kc of an esterification reaction. It requires us to briefly explain how we are suppose to obtain the equilibrium amount of each compound for the calculation of Kc. I forget to do the necessary scaling and I cocked up quite badly there.

• efghijklm

  6 Nov 15, 19:27

  Hi! Will you be posting answers up? 

• Vitalitylx

  6 Nov 15, 19:36

  For the last question where they asked to suggest formulas of Y, I drew the structures but carelessly named all 3 wrongly. Is that 0 marks?

  Also I drew mechanisms and structures etc with pencil, is that accepted?

• UltimaOnline

  6 Nov 15, 19:44

  Originally posted by efghijklm:

  Hi! Will you be posting answers up? 

  
  Nope. I'm not gonna do the work of the TYS authors for them (they're reading this thread, and in the past, have copied my answers to use in their TYS books published months later).
  
  I'm here to comment and advise on specific queries you might have regarding the questions that came out, and whether your own answers are acceptable.
• UltimaOnline

  6 Nov 15, 19:45

  Originally posted by Vitalitylx:

  For the last question where they asked to suggest formulas of Y, I drew the structures but carelessly named all 3 wrongly. Is that 0 marks?

  Also I drew mechanisms and structures etc with pencil, is that accepted?

  
  No, you get some marks as long as your structures are drawn correctly. Yes, answers in pencil are fine as long as they're clear.
• efghijklm

  6 Nov 15, 19:51

  hmm for the part that asks us to distinguish between hbr and hcl, is by adding h2so4? i wrote that cause i couldn't think of anything else but it seems a little weird cause we already wrote that for the parts above.

  thank you in advance!

• UltimaOnline

  6 Nov 15, 20:16

  Originally posted by efghijklm:

  hmm for the part that asks us to distinguish between hbr and hcl, is by adding h2so4? i wrote that cause i couldn't think of anything else but it seems a little weird cause we already wrote that for the parts above.

  thank you in advance!

  
  Unfortunately that doesn't work with "dilute aqueous solutions" as in this question. You'll require gaseous HBr and concentrated H2SO4 (which is a serious safety hazard and thus shouldn't be used for distinguishing tests). But you may still get 1 out of 2 marks, provided you correctly described the smell of SO2(g) evolved as a "strong, choking odor".
  
  I won't reveal the correct answer just yet, as I'll like to see what the rest of you wrote. Rest of you, go ahead and share your answers to this question (which is one of the tougher questions for today's P2), and see what you came up with.
• hallopanda

  6 Nov 15, 20:22

  Phenol? White ppt formed for Br?

• bobasd

  6 Nov 15, 20:23

  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 :(

• Soulfunk

  6 Nov 15, 20:34

  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?

• Vitalitylx

  6 Nov 15, 20:35

  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

• bestrobber97

  6 Nov 15, 20:36

  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

• Soulfunk

  6 Nov 15, 20:39

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

• breadman15

  6 Nov 15, 20:43

  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