Mark Scheme (Results) January 2008

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1 Mark Scheme (Results) January 2008 GCE GCE Chemistry (6246) Paper 2 Edexcel Limited. Registered in England and Wales No Registered Office: One90 High Holborn, London WCV 7BH

2 General Marking Guidance All candidates must receive the same treatment. Examiners must mark the first candidate in exactly the same way as they mark the last. Mark schemes should be applied positively. Candidates must be rewarded for what they have shown they can do rather than penalised for omissions. Examiners should mark according to the mark scheme not according to their perception of where the grade boundaries may lie. There is no ceiling on achievement. All marks on the mark scheme should be used appropriately. All the marks on the mark scheme are designed to be awarded. Examiners should always award full marks if deserved, i.e. if the answer matches the mark scheme. Examiners should also be prepared to award zero marks if the candidate s response is not worthy of credit according to the mark scheme. Where some judgement is required, mark schemes will provide the principles by which marks will be awarded and exemplification may be limited. When examiners are in doubt regarding the application of the mark scheme to a candidate s response, the team leader must be consulted. Crossed out work should be marked UNLESS the candidate has replaced it with an alternative response. Using the mark scheme / means that the responses are alternatives and either answer should receive full credit. 2 ( ) means that a phrase/word is not essential for the award of the mark, but helps the examiner to get the sense of the expected answer. [ ] words inside square brackets are instructions or guidance for examiners. 4 Phrases/words in bold indicate that the meaning of the phrase or the actual word is essential to the answer. 5 ecf/te/cq (error carried forward) means that a wrong answer given in an earlier part of a question is used correctly in answer to a later part of the same question. Quality of Written Communication s which involve the writing of continuous prose will expect candidates to: show clarity of expression construct and present coherent arguments demonstrate an effective use of grammar, punctuation and spelling. Full marks will be awarded if the candidate has demonstrated the above abilities. s where QWC is likely to be particularly important are indicated QWC in the mark scheme BUT this does not preclude others.

3 .(a) CH COOH + C 2 H 5 OH CH COOC 2 H CH CO 2 H CH OCOC 2 H H 2 O () CH CH 2 for C 2 H 5.(b) catalyst /speed up reaction () dehydrating agent.(c) flask with still head () condenser and a receiver () thermometer at correct place () penalty of () if apparatus sealed or open at the wrong place or doesn t work for some other reason.

4 .(d) mol ethanoic acid = 2.6(0) = 0.2 () 60 (mol ethyl ethanoate = 0.2) theoretical mass ethyl ethanoate = 0.2 x 88 = 8.48 g or 8.5g () % yield = 0.60 x 00 = 57 () 8.48 Allow or 57.6 or 57.4 Theoretical mol ethanoic acid = = 0.2() (mol ethyl ethanoate = 0.2) actual moles of ethyl ethanoate = = 0.2 () % yield = 0.2 x00 = 57 () 0.2 Allow 57. or 57.4 CQ ON FMULAE IN (a) but these must be possible compounds. IGNE S.F..(e)(i).(e)(ii) C 2 H 5 OH + CH COCl CH COOC 2 H 5 + HCl () CH CH 2 for C 2 H 5 CH OCOC 2 H 5 Reaction with ethanoic acid reaches equilibrium/is reversible Reaction with ethanoic acid is incomplete Reaction with ethanoyl chloride is not reversible/goes to completion ()

5 .(f)(i).(f)(ii).(f)(iii).(f)(iv) (Phenyl benzoate) must be soluble in the hot solvent and less/almost insoluble in cold solvent () to remove insoluble/un-dissolved impurities () to remove solid from soluble impurities () Just collect the product. to wash away remaining solution/soluble impurities /remove surface impurity. ().(f)(v) measure melting temperature () check value same as data book/sharp melting point () Use gas-liquid chromatography () Showing only one peak () Mix with known sample and measure melting temperature. Any other instrumental method. 2

6 2.(a) ionic lattice () Na + ions have 6 nearest neighbours of Cl - ions and vice-versa / 6:6 coordination () Labelled sketch can score both marks but must have some D extension. 2 2.(b) electrostatic attractions (in solid NaCl) overcome () by the attractions between the ions and dipoles in water () ; this can be shown in a diagram. Attractions overcome by solvation of ions scores () only 2 Water has a high dielectric constant/relative permittivity () which reduces the forces of attraction between ions in the solution () 2.(c) NaCl(s) (+aq) Na + (aq) + Cl - (aq) Cycle () Na + (g) + Cl - (g) (+aq) Arrows labelled with names or values () Check arrow direction agrees with label/sign of the value H soln = (-77) = + (kj mol - ) () + sign not essential Negative value 2.(d) sodium hydroxide/naoh () hydrogen /H 2 () H anode 2Br - Br 2 + 2e (-) Br 2Br - - 2e (-) Br 2 () or halved.

7 2.(e)(i) as intermediate lone pair not essential, arrow can start at on Br - and go to + on C 2.(e)(ii) initial attack (on ethene) is by an electrophile/br δ+ () 4 no Cl + / Cl δ+ available as the electrophile (so no dichloroethane formed) () then (nucleophilic) attack by Br - () Cl - can replace Br - (as nucleophile, so -bromo-2-chloroethane is formed) ()

8 .(a)(i) [(CH ) CCl] increases by.5 while [OH - ] remains constant, rate increases by.5 In expts A and B, [(CH ) CCl] increases by.5 and rate increases by.5 () so first order () [OH - ] zero order, with some explanation ().(a)(ii) (CH ) C Cl (CH ) C + + Cl - S N mechanism if [OH - ] first order () arrow () both ions (CH ) C + (:)OH - (CH ) C OH () arrow Must be S N 2 mechanism if st order wrt OH - in (i): Cl C H C CH CH (:)OH - Cl H C C CH H C OH - CH CH CH C OH + Cl - each arrow () x 2 intermediate ().(b)(i) CH CH C H CH O C CH 2 CH 2 C * CH 2 C H H ()

9 .(b)(ii) alkene 4 (aqueous) bromine () orange to colourless() (aqueous) potassium manganate(vii) (ignore alkaline/acid) () purple to colourless/brown () Purple to green. aldehyde any one matching pair from: reagent () observation (): Fehling s solution Tollens reagent blue (soln) to red/brown ppt silver mirror or black ppt Benedict s, same observation. Ammoniacal AgNO, same obs. 2,4 DNP.(b)(iii) (with NaBH 4 ) 2 H C H C C H C (with HBr) CH H CH 2 CH 2 C CH 2 C H H OH () Br CH O CH C CH 2 CH 2 CH 2 C CH 2 C CH H H ()

10 .(b)(iv) C 0 H 8 O + 4O 2 0CO 2 + 9H 2 O () Ignore any state symbols 4 Moles citronellal =.0/54 () = 6.49x0 - Moles CO 2 = 0x 6.49x0 - () = 6.49x0-2 Volume CO 2 = 24 x 6.49 x0-2 =.56 dm () allow.6 Allow cq from incorrectly balanced equation. Ignore sf 54g citronellal gives 240dm CO 2 () Vol CO 2 from g = 240/54 () =.56 dm ()

11 4.(a) silicon - giant atomic/ giant covalent /giant molecular/macromolecular () 5 phosphorus and chlorine (simple) molecular () covalent bonds broken in Si are stronger than intermolecular/dispersion/ Van der Waals / London/ induced dipole forces () phosphorus is P 4 and chlorine is Cl 2 () P 4 has more electrons (per molecule) so stronger dispersion (etc) forces () 4.(b) PCl + 4 tetrahedral () correct -D diagrams - PCl 6 octahedral () 4 or 6 pairs of electrons as far apart as possible to minimise repulsion () 4.(c) name of any specific alcohol () ROH + PCl 5 RCl + HCl + POCl () [R must apply to the specific alcohol] equation with R if mark lost for not giving a specific example Just alcohol 2 name of any specific carboxylic acid () RCOOH + PCl 5 RCOCl + HCl + POCl () [R must apply to the specific acid] Just acid

12 4.(d) PCl 5 PCl + Cl If eqm moles PCl 5 = and PCl =Cl 2 =0. eqm moles: answer = 0.5 and can () score last marks mole fraction: () partial pressures: 0.x4 0.67x4 0.67x4 () = 0.79 =.605 =.605 K p = ppcl x pcl 2 () ppcl 5 If.6 used here then final answer is.24 =.26 and atm() PCl 5 PCl + Cl 2 / 2/ 2/ eqm moles () mole fraction () partial press () K p = ppcl x pcl 2 () ppcl 5 =.2 atm () 4.(e)(i) H PO 4 + 2NaOH Na 2 HPO 4 + 2H 2 O () H PO 4 + 2OH - HPO H 2 O ()

13 4.(e)(ii) any point between

Mark Scheme (Results) January 2008

Mark Scheme (Results) January 008 GCE GCE Chemistry (645) Paper 1 Edexcel Limited. Registered in England and Wales No. 4496750 Registered Office: One90 High Holborn, London WC1V 7BH General Marking Guidance