klm Mark Scheme Chemistry 2421 General Certificate of Education Kinetics, Equilibria and Organic Chemistry 2010 examination - January series

Transcription

1 Version.0: 02/200 klm General ertificate of Education hemistry 242 EM4 Kinetics, Equilibria and rganic hemistry Scheme 200 examination - January series

2 schemes are prepared by the Principal Examiner and considered, together with the relevant questions, by a panel of subject teachers. This mark scheme includes any amendments made at the standardisation meeting attended by all examiners and is the scheme which was used by them in this examination. The standardisation meeting ensures that the mark scheme covers the candidates responses to questions and that every examiner understands and applies it in the same correct way. As preparation for the standardisation meeting each examiner analyses a number of candidates scripts: alternative answers not already covered by the mark scheme are discussed at the meeting and legislated for. If, after this meeting, examiners encounter unusual answers which have not been discussed at the meeting they are required to refer these to the Principal Examiner. It must be stressed that a mark scheme is a working document, in many cases further developed and expanded on the basis of candidates 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 Scheme are available to download from the AQA Website: opyright 200 AQA and its licensors. All rights reserved. PYRIGT AQA retains the copyright on all its publications. owever, registered centres 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 centres to photocopy any material that is acknowledged to a third party even for internal use within the centre. Set and published by the Assessment and Qualifications Alliance. The Assessment and Qualifications Alliance (AQA) is a company limited by guarantee registered in England and Wales (company number 64472) and a registered charity (registered charity number 074). Registered address: AQA, Devas Street, Manchester M5 6EX Dr Michael resswell Director General

3 EM4 - AQA GE hemistry Scheme 200 January series part (a) (i) acid 0.46 alcohol.46 omments (a) (ii) (a) (iii) water 5.54 K c = [ [ (0.54/V)(5.54/V) (0.46/V)(.46/V) 4.45 or ][ ][ 2 2 ] ] = Allow without V [ ester][ water] penalise ( ) [ acid][ alcohol] cancel (as equal no of moles on each side of equation) allow molecular formulae or minor slip in formulae onseq on values in (a)(i) If values used wrongly or wrong values inserted or wrong Kc no marks for calc Part (a)(iii) for info = Possible wrong answers acid 0.46 gives alcohol.46 Kc =.59 water 4.46 X (b) (i) decrease or be reduced or fewer (b) (ii) decrease or be reduced or less time or faster or quicker (b) (iii) decrease or be reduced acid 0.46 gives alcohol.46 Kc = 0.44 water 0.54 X

4 EM4 - AQA GE hemistry Scheme 200 January series part omments 2 (a) (i) -log[ + ] or log/[ + ] penalise ( ) 2 (a) (ii) [ + ] = 0.56 mark for the answer; allow 2dp or more [ 2 S 4 ] = ½ 0.56 = (b) (i) + Na Na + 2 Allow 2 etc R (b) (ii) mol acid = ( ) 0.4 = or [Na] = / = 0.45(4) R mark for answer if not look back for error [Na] = / = or (b) (iii) cresol purple 2 (b) (iv) Na reacts with carbon dioxide (in the air) 2 (c) (i) K a = [ + ][ - ] allow molecular formulae or minor slip in formulae [] penalise ( ) allow + not allow A etc 4

5 EM4 - AQA GE hemistry Scheme 200 January series 2 (c) (ii) K a = [ + ] 2 [] or with numbers allow A etc here This can be scored in part(c)(i) but doesn t score there. [ + ] = ( ( ) = ( ) ) = p = 2.57 can give three ticks here for (c)(ii) penalise decimal places < 2 > 2 (c) (iii) M mol = ( ) 0.0 = M2 orig mol A = ( ) 0.4 = or or M mol A in buffer = orig mol A mol = or M4 mol A in buffer = mol = M5 [ + Ka x [ ] ] = ( [ - ] -5 (.74 x 0 ) ( ) or = ) -5 (.74 x 0 ) (0.0090) ( = or ) M6 p =.79 can give six ticks for.79 NB Unlike Qu 2(c)(ii), this p mark is NT awarded conseq to their [ + ] unless following AE mark for or either gives 2.57 p mark conseq on their [ + ] so 5.5 gets 2 marks where square root not taken If no subtraction or other wrong chemistry the max score is for M, M2 and M4 If A is wrong, max for M, M2 and M or use of p = pka log [A]/ [A ] is for insertion of correct numbers in correct expression for [ + ] if [A]/[ A ] upside down lose M5 & M6 If wrong method e.g. [ + ] 2 /[A] max for M, M2 and M Some may calculate concentrations [A] = and [A ] = and rounding this to gives p =.80 (which is K) BEWARE: using wrongly instead of gives p =.75 (this gets for M, M2 & M4) 5

6 EM4 - AQA GE hemistry Scheme 200 January series Part omment (a) 2 or two or second (b) k = (4.40)(0.82) = (min sfs) mark is for insertion of numbers into a correctly rearranged rate equ, k = etc if upside down, (or use of I 2 data) score only units mark mol - dm s - any order (c) no change or no effect or stays the same or (d) or 2 or and 2 rate equ doesn t involve I 2 or only step which includes 2 species in rate equ if wrong no further mark but mark on from no answer (e) + + any second arrow loses the mark 6

7 EM4 - AQA GE hemistry Scheme 200 January series Part omments 4 (a) nucleophilic addition M2 M M N N N M M for completely correct structure not including lp 4 Attack by N loses M and M2 M2 not allowed independent of M, but allow M for correct attack on + += loses M2 M2 only allowed if correct carbon attacked allow minus charge on N i.e. :N allow 7 in M M4 for lp and arrow 2-hydroxy-2-methylpentan(e)nitrile 4 (b) Product from Q is a racemic mixture/ equal amounts of enantiomers racemic mixture is inactive or inactive explained Product from R is inactive (molecule) or has no chiral centre allow without allow 2-hydroxy-2- methylpentanonitrile if no reference to products then no marks; not Q is optically active or has a chiral centre etc 4 (c) (i) mark the three sections of Qu 4(c) separately R or (c) (ii) [ 2 2 ] +. R [ 5 0 ] +. [ 2 ] R [ 5 ] allow molecular formulae allow without brackets if brackets not shown, allow dot anywhere on radical or + anywhere on ion 4 (c) (iii) m/z = 4 or 7 7

8 EM4 - AQA GE hemistry Scheme 200 January series Part Question 5 (a) (i) propan(e)-,2,-triol or,2,- propan(e)triol not propyl ignore hyphen, commas 5 (a) (ii) soaps allow anionic surfactant not cationic surfactant not detergents, not shampoos 5 (b) (i) (bio)diesel Allow fuel for diesel engines not biofuel, not oils 5 (b) (ii) ignore anything else attached except any more atoms. 5 (b) (iii) ( 2 ) 2 + 2½ not allow equation doubled R or 4/2 8

9 EM4 - AQA GE hemistry Scheme 200 January series Part omments 6 (a) (i) N allow 2 allow + N don t penalize position of + on N 6 (a) (ii) 2 N allow 2 allow N 2 allow 7 ( ) 2 6 (a) (iii) N allow 2 allow + N don t penalize position of + on N ( 2 ) 4 N 6 (b) 2 N 2 N N N ( ) 2 ( ) 2 allow 2 allow N 2 allow 7 allow as zwitterions if error in peptide link e.g. N if twice, penalise both times not polymers if wrong amino acid in both can score Max 9

10 EM4 - AQA GE hemistry Scheme 200 January series 6 (c) chromatography or electrophoresis ignore qualification to chromatography 0

11 EM4 - AQA GE hemistry Scheme 200 January series Part omments 7 (a) A allow B 2 2 or 2 must show = Penalise sticks once per pair 7 (b) D NT cyclopentane which is only 5 0 Penalise sticks once per pair 7 (c) E 2 Allow F 2 Allow 2 2 or Penalise sticks once per pair 7 (d) G 2 R R not 5 nor 4 9 Penalise sticks once per pair 2 2 ( ) 2 2 allow 7 allow 7 allow allow 2 5

12 EM4 - AQA GE hemistry Scheme 200 January series 7 (e) I allow N 2 J N( ) 2 NT 7 Penalise sticks once per pair 2

13 EM4 - AQA GE hemistry Scheme 200 January series Part (8) (a) (i) W X 4 omments Y 2 (8) (a) (ii) Si (8) (b) (i) + N 2 N S 4 N S 4 R + + displayed formula shows ALL bonds allow + anywhere can score in equation or use two equations via 2 N + N + 2 S 4 N S 4 (8) (b) (ii) electrophilic substitution + 2 Not Friedel rafts 2 N M N 2 2 N M2 M N 2 Allow Kekule structures + must be on N of + N 2 (which must be correct) both N 2 must be correctly positioned and bonded to gain M2 M arrow from circle or within it to N or to + on N horseshoe must not extend beyond 2 to 6 but can be smaller + not too close to M arrow into hexagon unless Kekule allow M arrow independent of M2 structure ignore base removing in M

14 EM4 - AQA GE hemistry Scheme 200 January series 8 (c) (i) 2 /Ni or 2 /Pt or Sn/l or Fe/l (conc or dil or neither) allow dil 2 S 4 ignore mention of Na Not NaB 4 Not LiAl 4 Not Na/ 2 5 not conc 2 S 4 or any N 2 N N 2 + 2[] 2 N N r 6 2 allow 6 4 (N 2 ) 2 etc, allow N 2 N 2 i.e. be lenient on structures, the mark is for balancing equ 8 (c) (ii) N N allow N- ignore [ ] n as in polymer st mark for correct peptide link 2 nd mark for the rest correct including trailing bonds 8 (c) (iii) M Kevlar is biodegradeable but polyalkenes not M2 Kevlar has polar bonds / is a (poly) amide / has peptide link M can be hydrolysed/attacked by nucleophiles/acids/bases/enzymes M4 polyalkenes non polar /has non-polar bonds 2 allow Kevlar is more biodegradeable comment on structure of Kevlar comment on structure of polyalkenes but not just strong bonds 4

15 EM4 - AQA GE hemistry Scheme 200 January series Part omments 9 (a) (nucleophilic) addition-elimination 2 M2 ( 2 5 )-N 2 M l N-ethylpropanamide M ( N l M4 for arrows and lp 2 N minus on N 2 loses M M2 not allowed independent of M, but allow M for correct attack on + += loses M2 only allow M4 after correct or very close M lose M4 for l removing + in mechanism, but ignore l as a product Not N-ethylpropaneamide 9 (b) N or ethan(e)nitrile or ethanonitrile for each step wrong or no reagent loses condition mark not ethanitrile but allow correct formula with ethanitrile contradiction loses mark Step l 2 uv or above 00 o Step 2 KN wrong or no reagent loses condition mark aq and alcoholic (both needed) Step 2 /Ni or LiAl 4 or Na/ 2 5 allow uv light / (sun)light / uv radiation not N but mark on NT N or KN + acid, and this loses condition mark NT NaB 4 Sn/l (forms aldehyde!) ignore conditions 5