Mark Scheme (Results) January 2008

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1 Mark Scheme (Results) January 008 GCE GCE Chemistry (645) Paper 1 Edexcel Limited. Registered in England and Wales No Registered Office: One90 High Holborn, London WC1V 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 1 / means that the responses are alternatives and either answer should receive full credit. ( ) 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.

3 1.(a)(i) Cr [Ar] d 5 4s 1 and Cr [Ar] d 1 4s 1 d 5 and d D 5 4S 1 and D 4S 1 D 5 and D d 5 4s 1 and d 4s 1 d 5 and d D 5 4S 1 and D 4S 1 D 5 and D ALLOW 1s s etc for [Ar] provided it is complete and correct 1.(a)(ii) octahedral diagram for name 6 electron pairs around Cr (ion) these repel to a position of minimum repulsion / maximum separation 6 bonds, could be drawn on diagram bonds/atoms repelling 1.(a)(iii) (gelatinous) green ppt green solid any shade of green (dissolves) to green solution 1.(a)(iv) [Cr(H O) 6 ] OH - equations with NaOH Cr (aq) Cr(OH) (H O) H O eg NaOH on LHS Na on RHS [Cr(H O) 6 ] OH - Cr(OH) 6H O If H O is missing from Cr(OH) (H O) OH - RHS of both equations, allow for both [Cr(OH) 6 ] - H O correct Cr species on RHS Cr(OH) OH - [Cr(OH) 6 ] - Ignore state symbols

4 1.(b)(i) Reactant Product molecular formulae Formula of a: Formula of a: primary alcohol aldehyde names with no formulae primary alcohol carboxylic acid COH for aldehyde, secondary alcohol ketone unless structure shown as well aldehyde carboxylic acid 1.(b)(ii) θ - E cell for MnO 4 reacting with Cl - 4 = () 0.15 (V) θ E cell for Cr O - 7 reacting with Cl - = (V) E o cell for Cr reacting with Cl = ()0.0(V) MnO - 4 will oxidise Cl - /HCl so HCl cannot be used - MnO 4 16H 10Cl - Mn 8H O 5Cl so HCl cannot be used Cr O 7 - will not oxidise Cl - /HCl so HCl can be used 1.(b)(iii) oxidation number of Cr remains at 6 gain or loss of 1 electrons ALLOW this mark if the oxidation numbers are written under the species in the equation oxidation number does not change if it is not specified or is incorrect

5 . ACCEPT NAMES FMULAE F REAGENTS IF BOTH ARE GIVEN, BOTH MUST BE CRECT. CONDITION MARKS ARE ONLY AVAILABLE F CRECT REAGENTS.(a)(i) concentrated nitric acid concentrated formulae c for concentrated concentrated sulphuric acid [penalise lack of concentrated once] temperature o C stand alone any temperature or range of temperatures within this range more than 40 o C less than 60 o C.(a)(ii) HNO H SO 4 H O HSO 4 arrow to or from 4 NO charges Can be shown in two stages Kekule structures HNO H SO 4 H O HSO 4 NO NO NO H NO H (HSO - 4 ) NO H ( H SO 4 ) if HSO 4 is used in the last step, arrow must come from O Curly arrow from ring towards (space between C in ring and) N in NO Correct intermediate Curved arrow from C H bond back into ring curly arrow from within ring.(b) tin and conc hydrochloric acid IGNE heat or any stated temperature Fe or Zn and conc HCl H Pt/Ni/Pd LiAlH 4 reduction loss of oxygen and gain of hydrogen redox.(c) sodium nitrite/ NaNO and hydrochloric acid/hcl (aq) 0-10 o C benzene diazonium chloride sodium nitrate(iii) dilute or concentrated acid any temperature or range of temperatures within this range just HCl temperature value qualified by below / above

6 .(d) collect gas in gas syringe/over water in graduated apparatus or diagram measure volume of gas at regular time intervals If [BDC] measured only the following marks are available: Label [BDC] and time on axes 6 label volume and time on axes sketch including horizontal finish/final volume Sketch find at least half lives, first order if half lives are constant 1 st half life is time taken to half final volume, nd half life is time from half to ¾ these could be shown on graph Half lives constant (therefore 1 st order) STAND ALONE ALTERNATIVE F LAST 4 MARKS measure final volume and calculate (V final -V t ) Label (V final -V t ) and time on axes sketch For ph method only the following marks are available: use a ph probe measure ph at regular time intervals half lives constant If candidate mixes answers, mark them as if separate and award the highest mark find at least half lives, first order if half lives are constant collect gas in gas syringe/over water in graduated apparatus or diagram find volume of gas after fixed time and calculate rate = vol/time repeat for different values of [X] label rate and [X] on axes sketch straight line rate proportional to [X], so first order Mass loss method could be applied to any of above

7 .(a) If diagram slopes up to left, could still score other two marks Straight liquid or vapour line If 109 o C labelled at lower temp than 8 o C, can only score liquid and vapour mark vapour and liquid lines reasonably drawn with no maximum or minimum Sloping up to the right areas labelled.(b) draws more than 1 tie line, starting at 0.75, connected by verticals (and heading correctly towards the lower bp component) 4 states that (equilibrium) vapour is richer in the more volatile component / propan-1-ol STAND ALONE describes repeated distillations (with correct reference to tie lines) give rise to (first) distillate of pure propan-1-ol / -methylpropan-1-ol left in the flask

8 4.(a) Ester(s) triester(s) triglyceride(s) Ether(s) lipid(s) 1 4.(b)(i) Any example e.g. H R C C H R [R can be any group/atom other than hydrogen, R can be the same or different] both hydrogen atoms on the same side both larger groups on the same side 4.(b)(ii) saturates pack more closely together than unsaturates (due to cis isomers) breaking single / double / σ / π bonds saturates have higher/stronger dispersion/van der Waals forces than unsaturates (so more energy is required to melt) 4.(c)(i) RCOONa / RCOO - Na CH OHCH(OH)CH OH RCO Na Full structural formulae Covalent bond shown between O and Na. RCOOH C H 8 O 4.(c)(ii) Making/manufacture of: soap/soapy detergents or soap production saponification 1

9 4.(d)(i) Any one from: Reagent - methylpropan- -ol obs propanoic acid obs NaHCO no change effervescence Na CO no change effervescence Observation marks conditional on correct reagent Answer involving formation of an ester, identified by smell, for either acid or alcohol Description of test for CO instead of effervescence IGNE references to heat 4.(d)(ii) Any one from: Reagent Propanal obs Propanone obs Fehlings blue to red no change ppt Tollens silver no change mirror/ppt Cr O - 7 /H orange to no change green/blue/ brown I NaOH no change yellow ppt Benedicts Ammoniacal AgNO MnO 4 - /H with correct colour changes Observation marks conditional on correct reagent IGNE references to heat 4.(e)(i) H C H C C O (:)CN H C Each arrow H C O C CN H CN O C CN CH OH H C C CN CN - CN - or CN arrows start from negative charge on O or C arrow to H or to HCN in nd step 4 CH both arrows CH 4.(e)(ii) higher [H ] (so) lower [CN - ] and rate slower

10 5.(a)(i) electrode platinum/pt Fe and Fe 1 mol dm - conditional on both ions being present 5.(a)(ii) 5.(a)(iii) to bring the solutions to the same potential/connect solutions without setting up a p.d. to allow the movement of ions to complete the circuit to allow flow of electrons (saturated) potassium chloride (saturated) potassium nitrate Formulae Sodium nitrate or chloride (a)(iv) x 0.4 = 0.4 x = 0.77 V Correct answer with some working () 5.(a)(v) Fe e - Fe e for electron Fe e - Fe Cu Cu e - Cu Cu e - Cu e - Cu Cu e - Cu [not cq on (iv)] 5.(a)(vi) Cu Fe Fe Cu Fe Cu Cu 1 Fe if both half equations in opposite direction in (v)

11 5.(b) ½ O e - H O / OH - species balance ignore state symbols multiples 5.(c) Moles S O - = (16.5 )x0.1 = 1.65x (Moles I = 1.65x10 - = 8.5x10-4 ) Moles Cu = 1.65x10 - Conc CuSO 4 = 1.65x10 - x(1000) = 0.066(mol dm - ) 5 Penalise incorrect unit

Mark Scheme (Results) January 2008

Mark Scheme (Results) January 2008 GCE GCE Chemistry (6246) Paper 2 Edexcel Limited. Registered in England and Wales No. 4496750 Registered Office: One90 High Holborn, London WCV 7BH General Marking Guidance