AQA Qualifications. A-LEVEL Chemistry. CHEM4 Kinetic, Equilibria and Organic Chemistry Mark scheme June Version: 1.

Transcription

1 AQA Qualifications A-LEVEL Chemistry CEM4 Kinetic, Equilibria and rganic Chemistry Mark scheme 2420 June 206 Version:.0 Final

2 Mark schemes are prepared by the Lead Assessment Writer and considered, together with the relevant questions, by a panel of subject teachers. This mark scheme includes any amendments made at the standardisation events which all associates participate in and is the scheme which was used by them in this examination. The standardisation process ensures that the mark scheme covers the students responses to questions and that every associate understands and applies it in the same correct way. As preparation for standardisation each associate analyses a number of students scripts: alternative answers not already covered by the mark scheme are discussed and legislated for. If, after the standardisation process, associates encounter unusual answers which have not been raised they are required to refer these to the Lead Assessment Writer. It must be stressed that a mark scheme is a working document, in many cases further developed and expanded on the basis of students reactions to a particular paper. Assumptions about future mark schemes on the basis of one year s document should be avoided; whilst the guiding principles of assessment remain constant, details will change, depending on the content of a particular examination paper. Further copies of this Mark Scheme are available from aqa.org.uk Copyright 205 AQA and its licensors. All rights reserved. AQA retains the copyright on all its publications. owever, registered schools/colleges for AQA are permitted to copy material from this booklet for their own internal use, with the following important exception: AQA cannot give permission to schools/colleges to photocopy any material that is acknowledged to a third party even for internal use within the centre.

3 MARK SCEME A-LEVEL CEMISTRY CEM JUNE 206 Question Answers Mark Additional Comments/Guidance ID detail a C 3 C + 2 C 3 C Must show allow C 3 C 2, C 3 C 2 R Ignore state symbols C 3 C C 3 C + + b C 3 C + N 3 C 3 C N 3 Ignore Allow C 3 C 2, C 3 C 2 2 +, C 3 C + () 2 3 of 20

4 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 c(i) marked with c(ii) 00 ( new [N 3 ] = [ + ] = ) 50 M [ + ] = 8.3(3) 0-3 (mol dm -3 ) R new [N 3 ] = mol N 3 = total vol M2 p = log M R 2.08 Must be 2 dp Allow correct p conseq to their [ + ] concentration c(ii) M mol Na (= ) = marked with c(i) M2 Subtraction of M from moles of N 3 ( or conseq from c(i)) M2 allow ecf for subtraction of mol Excess mol + = If no subtraction, no further marks M3 [ + ] = M R = M3 if no use of volume, no further marks (p=3.5) If incorrect volume used, can score M4 M4 p = log M3 R 2.32 M4 Allow 2.33 Must be 2 dp 4 of 20

5 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 d(i) M K a = [ ][C [C [ ] M2 K a = [C C] C C] - ] or with numbers or with A penalise ( ) once here Not [+][A-] / [A] if K a expression wrong Allow correct p conseq to their [ + ] concentration M4 only M3 [ + ] = [ ( )] = mark for answer M4 p = 3.33 Must be 2 dp Allow correct p conseq to their [ + ] concentration (p = 3.83 can score M, M2 and M4) G d(ii) sodium ethanoate Ignore formula allow sodium acetate d(iii) M [ + ] = M2 [salt] [C3C ] Ka (R = [acid] [C C] [ ] M3.2(0) )= e M (Electronegative) chlorine withdraws electrons M2 Stabilises/reduces charge on C- R weakens - bond R makes - more polar Accept or If M incorrect CE=0 Inclusion of in calculation can only score M ignore units.4 x 0-5 gives.24 Allow Cl has negative inductive effect Ignore chloroethanoic acid dissociates more readily Mark independently 5 of 20

6 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 f M Strong acids (almost) completely dissociated/ionised R not an equilibrium R equilibrium lies far to the right Cannot have K a value for a reaction not in equilibrium scores both marks M2 K a value for strong acids tends to infinity/is very large R can t divide by zero in K a Total 20 Question Answers Mark Additional Comments/Guidance ID details G 2a(i) Nucleophilic addition any extra loses the mark allow minor spelling errors e.g. nucleophyllic 2a(ii) C 3 C 2 3 C C 2 MM2b(i) Equal mixture of enantiomers/optical isomers WTTE M for arrow from lone pair on oxygen in ethanol to C of C= (or to space half way between and C) M2 for arrow from C= bond to oxygen in ethanal Do not allow M2 as first step without nucleophilic attack, but can allow M for attack on C+ produced + rather than + on C= loses M2 Ignore any further steps Mark independently 2b(ii) (non-superimposable) mirror images Ignore rotates light in opposite directions Ignore stereoisomers 6 of 20

7 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 2c(i) Ethanal 0.33 Ethanol 4.6 Allow 4.2 for ethanol 2c(ii) [ acetal][ 2] K c = 2 [ C C][ C C ] 3 (0.37 / 0.3)(0.65/ 0.3) 2 (0.58/ 0.3)(3.76 / 0.3) 3 2 or with names (0.37)(0.65) R 2 (0.58)(3.76) 0.3 M M2 Ignore slips in acetal structure or formula C If K c wrong, allow M4 only for units conseq to their K c If volume omitted (gives ) may only score M and M4 If volume used = 30 cm 3 allow M2 then award M3 for only and M4 for mol cm 3 only mol - dm 3 M3 M4 Treat error in converting 30 cm 3 to dm 3 as AE Allow range Not moles - dm 3 2d 2 C C 2 C 3 C Total 2 7 of 20

8 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 Question Answers Mark Additional Comments/Guidance 3a(i) (2-)chloroethan (--) oyl chloride 2 not required but penalise - or other numbers at start. Ignore in ethanoyl Ignore hyphens, commas, spaces 3a(ii) Cl C 2 C Cl RN 2 ClC 2 R C N Cl 4 M for arrow from lp on N to C (or to space half way between N and C) If full amine drawn, ignore slips except in N 2 M2 for arrow from C= bond to Not score M2 as an independent first step, but can allow M for attack on C+ produced If Cl lost at this stage, Max for M M3 for structure of ion including 2 charges M4 for 3 arrows and lp on - may be scored in two steps Ignore use of RN 2 to remove + in M4, but penalise use of Cl - 3b G nucleophilic substitution allow minor spelling errors e.g. nucleophyllic 3c A 9 8 of 20

9 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 3d M r = 234(.0) % = 9.4(0) 9.4 scores 2 marks 22 M2 = M 00 If M r = 234 not shown, can score M if their answer 234 = their no of 3e tertiary amine R 3 o amine R III o amine Ignore N- substituted 3f(i) If a given: CE=0, can only score if answer given is b NTE there is N mark for b alone M lp on N b or on b Alternatives: M2 alkyl groups donate electron density or positive inductive effect or electron donating groups attached M3 (lp on N b ) more available or protonated amine stabilised or better lp donor/ + acceptor Ignore reference to nucleophiles M lp on N a or on a M2 lp or electrons (on N a ) delocalised into ring /towards in C= M3 (lp on N a ) less available (to bond to + /accept proton) 3f(ii) Salt is ionic Independent marks (more) soluble (in blood/body fluids/water) Total 5 9 of 20

10 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 Question Answers Mark Additional Comments/Guidance 4a(i) 2(.0) Not 2.5 ignore working 4a(ii) Cl C C R C C Cl Must be displayed and show + on relevant carbon Penalise extra dots or [ ] + penalise 37 Cl 4a(iii) [ClC 2 CCl] +. [ 35 ClC 2 ]. + [C 37 Cl] + Allow without brackets M for molecular ion M2 for both fragments with mass numbers Allow dot or + anywhere on formula 4b(i) Cl C 2 C Allow [ClC 2 C] + 4b(ii) CC 2 ClM M3 M2 CC 2 Cl M Arrow from inside hexagon to C or + on C on correct electrophile M2 Structure of intermediate horseshoe centred on C; + in intermediate not too close to C (allow on or below a line from C2 to C6) M3 Arrow from bond to into ring Allow M3 arrow independent of M2 structure + on in intermediate loses M2 not M3 Ignore Cl - removing + (different from Qu3a(ii)) 0 of 20

11 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 4c reag ent water (Aqueous) silver nitrate Na followed by acidified silver nitrate (Water +) named indicator Named alcohol Na 2 C 3 or NaC 3 ammonia P no reaction no reaction (or slow formation of ppt) no reaction (or slow formation of ppt) No colour change NVC NVC no reaction Do NT award No observation Q Steamy /misty/ white fumes white precipitate (immediately formed) white precipitate (immediately formed) Indicator turns to correct acid colour Fruity or sweet smell or misty fumes Fizzing or effervescence (not just gas produced) white smoke 4d(i) C 2 C 4d(ii) 2 C R C 2 C R C C 2 ignore n and brackets ne unit only must have trailing bonds allow C 2 C C C Allow C for C= 4e(i) C 2 C C 2 ne unit only must have trailing bonds C 3 ignore n and brackets of 20

12 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 4e(ii) PGA sutures react/dissolve/break down/are biodegradable/ are hydrolysed / attacked by water or nucleophiles /no need to remove R Polypropene not biodegradeable/ not hydrolysed / not attacked by water/nucleophiles (Ester links have) polar bonds polypropene contains non-polar bonds ignore intermolecular forces Total 6 Question Answers Mark Additional Comments/Guidance 5a(i) C 3 C C N C C N R C C N C C N C 3 C C N C C N C 3 C C N C C N nly one molecule of each used 2 M for 2 amide links M2 C 2 and C(C 3 ) Allow mark after one error C 3 Dipeptide max Treat both trailing bonds missing as one error Ignore n ID details 2 of 20

13 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 5a(ii) C N R C N No need to show lp The covalent bond and the hydrogen bond either side of the must be linear. Allow N C N C N N 5b(i) 2-amino-4-methylpentan(--)oic acid Ignore hyphens, commas, spaces 5b(ii) C 2 Allow -N 3 + C C N 3 5b(iii) C 2 C C N 2 C 2 C C N 2 5b(iv) C(C 2 ) 2 Allow -N 3 + C C N 3 Total 7 3 of 20

14 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 Question Answers Mark Additional Comments/Guidance G 6a rder wrt D = R first R [D] R [D] rder wrt E = 2 R second R [E] 2 Ignore working ID details 6b (At time zero/start) the concentrations are known 6c M (Calculate) gradient (of tangent/curve/graph) M2 at t=0 or at start of graph/curve Total 5 Allow description of gradient calculation: Change in conc / time M2 scored only if M gained Ignore the word initial 4 of 20

15 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 Question Answers Mark Additional Comments/Guidance ID details 7a Iodine is not involved in (or before) the rate determining / slow(est) / limiting step (in the mechanism) Ignore, iodine does not appear in the rate equation or iodine concentration does not affect the rate 7b k = ( 2 ( ) ( ) = 3.(2) 0-5 ) Mark for answer mol dm +3 s Mark units separately, i.e. only these units but can be in any order 7c rate = k [ + ] (large excess of propanone) so [C 3 CC 3 ] is (effectively) constant If wrong or missing CE = 0 Total 5 5 of 20

16 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 Question Answers Mark Additional Comments/Guidance 8a M Na M2 M3 Aqueous/(warm) (Fractional) distillation or described nly score M2 if M gained, but mark on from hydroxide. Mention of acid loses M & M2 ignore alcoholic / conc / dil. Not just evaporation; not reflux Allow chromatography ID details 8b M S is C 3 C(CN)C 2 C 3 Step 3 M2 KCN (mark on from CN - ) M3 Alcoholic (/aqueous) Allow without brackets Not CN, not KCN with acid Allow ethanolic can only score M3 if M2 gained Step 4 M4 2 LiAl 4 Na can only score M5 if M4 gained M5 Ni or Pt or Pd Ethoxyethane or ether LiAl 4 with acid loses both M4 and M5 Ignore followed by acid ethanol NT NaB 4 R Sn/Cl penalise other extras as list ignore pressure or temperature Total 8 6 of 20

17 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 Question Answers Mark Additional Comments/Guidance 9a Method M % = = 5.08 =.9 =.69 M2 3 7 M3 C 3 7 = 59 which is half of M r so MF = 2EF R Method 2 M 6% of 8 = 72.0 and.9 % of 8 = 4.0 M = 86 so oxygen = 32 out of 8 R 27.% of 8 = 32.0 M = 6 = 4 = 2 3 Method 3 Alternative using given molecular formula 2 6 M C = 00 = 6.0% 8 4 M2 = 00 =.9% 8 M3 = = 27.% 8 ID details 7 of 20

18 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 Question Additional Comments/Guidance 9b For this question, marks can be awarded either for a description of how the structure is derived or from the given structure itself. The maximum mark to be awarded is nine from the ten marks listed. Marks fall into three sections: Infrared evidence : two marks are available for use of the infrared evidence, (M and M0) Chemical evidence: one mark is available for use of the chemical evidence (M2) N.m.r. evidence: six marks are available for use of the n.m.r. evidence (M3 M8 inclusive) plus one mark (M9) for a completely correct structure. Suggested procedure for marking First look at the infrared spectrum: marks M and M0 may be scored there or in the written answer. Then look for use of the acidified potassium dichromate(vi) evidence, (M2). Then look at the final structure: this may lead to the award of marks M3 to M9 as shown on the structures below. Beware contradictions, e.g. using the chemical evidence they may state that R is a primary or secondary alcohol but then draw a tertiary alcohol. This will lose M2 but may score M3. The written evidence frequently simply contains extracts from the Table B on the Data Sheet and, if only this is given, is unlikely to score many marks. 8 of 20

19 MARK SCEME A-LEVEL CEMISTRY CEM4 JUNE 206 Q9b Described M infrared peak/absorbance at 3400 (cm - ) = - alcohol (reference to ir spectrum needed) M0 Either No peak between (cm - ) so no C= or not aldehyde/acid R peak at (cm - ) so C present M2 (Acidified potassium dichromate(vi) turns green) so primary alcohol or secondary alcohol or not tertiary alcohol r drawn Note: please check the spectrum If peak at 3000 (cm - ) is identified as acid then cannot score M (contradiction) Apply list principle to IR analysis for M0 Ignore aldehyde here Lose M2 if just tertiary alcohol in structure M3 = 3. singlet or integration = is - Award M3 if structure has - group only (can be primary, secondary or tertiary). Lose M3 if more than one group shown M4 two triplets at.4 & 3.8 = C 2 C 2 Allow C 2 C 2 C 2 M5 = 3.8 means C 2 attached to (in ether NT ester) =.4 means C 2 attached to C (but not to C=) M6 =. (singlet) integration 6 = 2 equivalent C 3 on same C C(C 3 ) 2 Allow C 2 C 2 C 2 C M7 =. singlet so no attached to C(C 3 ) 2 - R C(C 3 ) 2 R M8 = 3.2 singlet integration 3 = -C 3 C 3 M9 For completely correct If no structure given then Max 8 R is M4 C 3 C 2 C 2 C C3 M3 M5 C 3 M6 M7 M8 plus M9 This close alternative M4 C 3 C 3 C 2 C 2 C C3 M8 M5 M3 M6 M7 would not score M9, but could score up to 8 marks 9 of 20

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