hij Mark Scheme Chemistry 6421 General Certificate of Education Thermodynamics and Further Inorganic Chemistry 2009 examination - June series

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1 Version.0: 24/06/09 hij General Certificate of Education Chemistry 642 CHM5 Thermodynamics and Further Inorganic Chemistry Mark Scheme 2009 examination - June series

2 Mark 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 Mark Scheme are available to download from the AQA Website: Copyright 2009 AQA and its licensors. All rights reserved. COPYRIGHT AQA retains the copyright on all its publications. However, 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 ) and a registered charity (registered charity number ). Registered address: AQA, Devas Street, Manchester M5 6EX Dr Michael Cresswell Director General

3 (a) Enthalpy change when mol of a substance/compound/ionic lattice/product; is formed from its elements; Allow heat change/heat energy change/heat absorbed Not energy change, not products All substances in their standard states; (b) O - repels electrons/both O - and e - are negative/idea of repulsion; (c) Arrows on cycle and correct ΔH values/correct labels; -ΔH = Or normal states under standard conditions If confused with ionisation energy or combustion or bond enthalpy etc CE=0 Ignore reference to standard conditions only Allow (O - ion) forced to accept an electron Allow lattice enthalpy arrow going up ΔH(lattice formation) = (kj mol - ); (d) Ca 2+ ions are smaller (than Ba 2+ ions)/ca 2+ ions higher charge to size ratio/greater charge density; More attraction for O 2- /stronger attraction; Correct answer scores scores scores Ignore incorrect or missing units Must mention ions or M 2+ Must imply between ions Mark independently CE if mention molecules 3

4 2 (a) (i) BaO + H 2 O Ba OH - ; Accept Ba(OH) 2 2 (a) (ii) Moles of BaO = 2.00/53.3 = Allow 0.03(0) 0.03 [Ba 2+ ] = mol dm -3 ; [OH - ] = mol dm -3 ; [H + ] = K w / [OH - ] = ph = log 0 [H + ] = 3.42 One mark for x 0 method mark One mark for 2 x [Ba 2+ ] method mark Method mark K w / [OH - ] or K w / previous answer Allow 3.40 to for correct answer. Method mark for -log 0 [answer 4] provided answer to 2dp 2 (b) Mg(OH) 2 (much) less soluble (than Ba(OH) 2 )/Ba(OH) 2 more soluble (than Mg(OH) 2 ); 3. scores 4 (3.0 to 3.3) (omits factor 2) 2.4 scores 4 (2.40 to 2.43) (omits factor 0) 2. scores 3 (2.0 to 2.3) (omits x 2 and x 0) Allow Mg(OH) 2 sparingly soluble 4

5 3 (a) ΔG = ΔH TΔS 3 (b) (i) Positive/+ More moles of gas products (2 mol gas gives 3 mol gas); More disorder/increase in disorder/very disordered; 3 (b) (ii) ΔH is negative and ΔS is positive/ TΔS +ve/ -TΔS -ve; QWC Allow molecules instead of moles Mark independently even if + not given So ΔG will be negative (at all temperatures); Mark independently and indep. of (b) (i) 3 (b) (iii) Too slow/activation energy too high/speeds up reaction; higher yield is contradiction, scores 0 3 (c) (i) ΔH = ΣΔH(formation products) - ΣΔH(formation reactants); Allow correct cycle = 4-4 ( ); = -936 (kj mol - ); ΔS = ΣS(products) - ΣS(reactants); = (329 + (4 58)) = -243 (J K - mol - ); 3 (c) (ii) Limiting condition ΔG = 0/ 0 = ΔH TΔS/ T = ΔH/ΔS ; = ( )/-243 = 3852 (K) (allow range 3850 to 3852); 3 marks for correct value +936 scores (-924 scores ie assumed ΔH f (Na(l) = 0) 2 marks for correct value Penalise wrong units If answer must show units kj K - mol - Using values given (allow use of these with own value for ΔH or ΔS) ( )/-32; 2603 (2600 to 2603) Allow consequential on answer to (c)(i) provided not ve T. Penalise wrong units T = 3.85 scores M only 5

6 4 (a) Gains electrons; 4 (b) Zero; By definition/by convention; 4 (c) Salt bridge transfers charge/ions/ to allow (electrical) connection between the two electrodes/ complete the circuit; Not reference electrode Allow conduct electricity Not transfer electrons Made from KCl etc; conducts electrons/ collects electrons/transfers electrons/gains electrons/loses electrons; Any group or ammonium chloride/nitrate/sulphate NOT salt alone Must have idea of electron transfer Allow electron source/sink Pt 4 (d) (+)6/six/VI; 4 (e) Cr 2 O H + + 6e 2Cr H 2 O Allow spectator ions, K 2 Cr 2 O 7 etc Zn Zn e Allow reverse equation Cr 2 O H + + 3Zn 2Cr H 2 O + 3Zn 2+ Allow spectators 4 (f) Zn(s) Zn 2+ (aq) Cr 2 O 2 7 (aq),(h + (aq)),cr 3+ (aq)( Pt) H +, Pt, state symbols not essential Ignore water Ignore multiple ions 2- Ignore SO 4 Allow I instead of, (comma) Allow extra (phase boundary) at either end Allow X instead of Pt 6

7 General comments for Q5 Reagent must be a compound not an ion but mark on/mark on with wrong formula For no reaction do not allow nothing/no observation, do allow no change CE if no reagent given 5 (a) Reagent: (dil) sulphuric acid/identified soluble sulphate; Obs with MgCl 2 : no reaction/ no ppt; Obs with BaCl 2 : white ppt; Reagent: NaOH/NH 3 Obs with MgCl 2 : white ppt Obs with BaCl 2 : no reaction 5 (b) Reagent: NaHCO 3 or allow Na 2 CO 3 Obs with CH 3 COOH: bubbles of gas; Obs with CH 3 COCH 3 : no reaction; Equation: CH 3 COOH + HCO 3 - CH 3 COO - + CO 2 + H 2 O etc; Allow correct acid/base indicator Correct acid indication No change CH 3 COOH CH 3 COO - + H + Allow Reagent: Mg/Zn Obs with CH 3 COOH: bubbles of gas Obs with CH 3 COCH 3 : no reaction Equation: 2CH 3 COOH + Mg/Zn Mg 2+ /Zn CH 3 COO - + H 2 Allow (2,4)-dnph/Brady s reagent Obs with CH 3 COOH: no change Obs with CH 3 COCH 3 : yellow/orange ppt Equation:CH 3 COCH 3 + (NO 2 ) 2 C 6 H 3 NHNH 2 (NO 2 ) 2 C 6 H 3 NHN=C(CH 3 ) 2 + H 2 O 7

8 5 (c) Reagent: AgNO 3 Obs with KF: no change Obs with KCl: white ppt 5 (d) Reagent: excess NaOH Obs with CrCl 3 : green solution (or ppt dissolves) Obs with FeCl 2 : green ppt Ignore NH 3 if AgNO 3 observation correct Reagent: NaOH Obs with CrCl 3 : green ppt Obs with FeCl 2 : green ppt goes brown on standing Reagent: NH 3 Obs with CrCl 3 : green ppt Obs with FeCl 2 : green ppt goes brown on standing Reagent: excess NH 3 (or conc) Obs with CrCl 3 : purple solution Obs with FeCl 2 : green ppt Reagent: Na 2 CO 3 (or NaHCO 3 ) Obs with CrCl 3 : green ppt gas evolved Obs with FeCl 2 : green ppt or white ppt Reagent: H 2 O 2 /NaOH Obs with CrCl 3 : yellow solution Obs with FeCl 2 : brown ppt Reagent: Zn/HCl Obs with CrCl 3 : blue solution Obs with FeCl 2 : no change Note where two reagents necessary, only one must be fully specified e.g. Zn/H + scores the reagent mark 8

9 5 (e) Reagent: excess NaOH Obs with AlCl 3 : colourless solution with excess; Obs with MgCl 2 : white ppt; Equation: AlCl 3 + 6NaOH 3Na + + [Al(OH) 6 ] NaCl (or two equations) excess can also be gained from M2 Or white ppt dissolves in excess Allow + 4NaOH Na + + [Al(OH) 4 ] - + 3NaCl Reagent: Na 2 CO 3 /NaHCO 3 Obs with AlCl 3 : bubbles (white ppt) Obs with MgCl 2 : white ppt Equation: Any equation showing release of CO 2 Reagent: Acid base indicator Obs with AlCl 3 : Correct acid indication Obs with MgCl 2 : No change/neutral indication Equation: [Al(H 2 0) 6 ] 3+ [Al(H 2 0) 5 (OH)] 2+ + H + Reagent: Mg/Zn Obs with AlCl 3 : bubbles Obs with MgCl 2 : no reaction Equation: 2[Al(H 2 0) 6 ] 3+ +Mg/Zn 2[Al(H 2 0) 5 (OH)] 2+ + H 2 + Mg 2+ /Zn 2+ 9

10 6 (a) Power/ability of an element/atom to attract/pull/withdraw electrons/electron density; Not an electron In a covalent bond/shared pair; 6 (b) Na ion/na + has low charge to size ratio/charge density/charge 6 (c) (i) Ionic (lattice); Aluminium ion/al 3+ has a high charge to size ratio/charge density/charge; Al 3+ has higher charge density than Na + scores M and M2 Al 3+ polarises Cl /chloride ion; Only allow if idea of attraction given in M If bond breaking or ionisation implied, CE=0 Or Big difference in electronegativity between Na and Cl; Or small difference in electronegativity between Al and Cl Electronegativity difference between Na and Cl > that between Al and Cl scores M and M2 Allow explanation or description of polarisation even if polarised not stated Strong forces of attraction between ions; Or strong electrostatic attraction Mention of ions also scores first mark Any reference to molecules or atoms CE=0 0

11 6 (c) (ii) SiCl 4 molecular/molecules/simple covalent; van der Waals'/vdW forces between molecules /VdW intermolecular forces/vdw IMF PCl 5 ionic; PCl 4 + PCl 6 Ionic forces stronger than van der Waals ; 6 (d) MgCl 2 Mg Cl SiCl 4 + 2H 2 O SiO 2 + 4HCl PCl 5 + 4H 2 O H 3 PO 4 + 5HCl Or London or temp dipole-dipole etc Mention of molecules here also scores M Lose this mark if confused with PCl 5 molecules Also scores M3 Allow van der Waals forces weak, ionic bonds/forces strong Allow without state symbols Allow [Mg(H 2 O) 6 ] 2+ Ignore inclusion of aq in equation Equations must be balanced or + 4H 2 O Si(OH) 4 + 4HCl HCl can be H + + Cl - Or ionic Allow PCl 5 + H 2 O POCl 3 + 2HCl

12 7 (a) Electron pair donor/lone pair donor/lewis base; Number of electron pairs donated from ligands; 7 (b) d electrons can be excited/move to a higher energy level; By absorbing visible light/energy; Allow number of co-ordinate bonds Not number of donor atoms Not number of ligands Allow there is an energy gap between d orbitals/can have d to d transitions Remaining visible/coloured light reflected or transmitted; Or in M2, if some visible light/energy is absorbed, this scores M2 and M3 If visible not mentioned in M3 but is mentioned in M2, can score this M3 mark 2

13 7 (c) (i) Add NH 3 /ammonia; [Co(NH 3 ) 6 ] 2+ ions; Yellow/straw/pale brown; Can be scored from an equation even if not balanced Colour must be correct for species given If no species do not allow colour mark Or Na 4 EDTA/EDTA [CoEDTA] 2- blue/purple (allow any colour not pink) Or H 2 NCH 2 CH 2 NH 2 [Co(H 2 NCH 2 CH 2 NH 2 ) 3 ] 2+ yellow (allow any colour not pink) 7 (c) (ii) Add HCl/NaCl/KCl [CoCl 4 ] 2 Blue (solution); 7 (c) (iii) Add NH 3 and H 2 O 2 / O 2 /air; [Co(NH 3 ) 6 ] 3+ ions; (Dark) brown (solution); Or (COO) 2 Na 2 [Co(C 2 O 4 ) 3 ] 4- allow any colour not pink Not Cl - but mark on, ignore concentration of solutions Not blue ppt Mark M and M2 independently 3

14 8 (a) Rate: change of concentration with time; QWC Catalyst: speeds up a reaction/alters rate; Without being chemically changed/used(up); 8 (b) (TM ions have) variable oxidation states; (Fe 2+ ions) involved in alternative route/as intermediate 2Fe 2+ +S 2 O 8 2-2Fe SO 4 2-2Fe 3+ +2I - 2Fe 2+ + I 2 Activation energy lower; Fe 2+ (involved in reaction but) regenerated 8 (c) (i) Zeolite/alumino silicate/porous pot; 8 (c) (ii) Pt; 8 (c) (iii) AlCl 3 /FeCl 3 C 8 H 8 (CH 3 ) 3 CCH 2 CH 3 (or C 6 H 4 ) + C 2 H 4 CO + NO CO 2 + /2N 2 (or 2NO O 2 + N 2 ) Ignore reference to E a and alternative route Mark first feature only in any list Allow any unambiguous explanation that Fe 2+ is regenerated after involvement And/or Ir, Rh, Pd C 6 H 6 + CH 3 COCl C 6 H 5 COCH 3 + HCl (can use any acyl chloride) C 6 H 6 + RCOCl C 6 H 5 C + HCl 2 Organic product (), balanced equation () C 6 H 5 CH 3 CO could get equation mark but loses product mark 4

9 (a) Curly arrow from C=C double bond to Br; Curly arrow from Br Br bond to Br; Correct carbocation intermediate; Curly arrow from lone pair on :Br to C + can score this mark (M4) if primary

[Br] 2 /[Br 2 ]; 9 (c) (ii) Initial moles of Br 2 = 6.30/59.8 = 0.0394 Allow 0.039 to 0.040 (A) Eqbm moles of Br 2 = 0.8 0.0394 = 0.035 Moles of Br = 0.0576 [Br 2 ] = 0.035/2 = 0.

15 9 (a) Curly arrow from C=C double bond to Br; Curly arrow from Br Br bond to Br; Correct carbocation intermediate; Curly arrow from lone pair on :Br to C + can score this mark (M4) if primary carbocation is shown; 9 (b) Br + CH 4 CH 3 + HBr Ignore δ+ and δ-, penalise wrong + or If HBr used instead of Br 2, max 2 Allow free radical dot after CH 3 CH 3 + Br 2 CH 3 Br + Br 9 (c) (i) K c = [Br] 2 /[Br 2 ]; 9 (c) (ii) Initial moles of Br 2 = 6.30/59.8 = Allow to (A) Eqbm moles of Br 2 = = Moles of Br = [Br 2 ] = 0.035/2 = mol dm 3 and [Br] = /2 = Allow 0.03 to (0.8 x A) (A2) method mark Allow 0.5 to 0.6 (2 x (A A2) or 0.4 x A) method mark Method mark if both concentrations/2 (if not /2 CE do not mark on) K c = /0.058 = (mol dm 3 ) Allow 3.5 to 4.3 x 0-3 Allow consequential on incorrect [Br] and [Br 2 ] only if M4 gained 5

16 9 (c) (iii) The value of K c increases; The reaction is endothermic; The temperature increase is opposed by favouring the endothermic reaction; CE if K c does not increase Or the temperature increase is opposed by moving the equilibrium to lower the temperature/absorb heat/oppose the change 6

abc Mark Scheme Chemistry 6421 General Certificate of Education Thermodynamics and Further Inorganic Chemistry 2008 examination - June series

abc Mark Scheme Chemistry 6421 General Certificate of Education Thermodynamics and Further Inorganic Chemistry 2008 examination - June series Version 1.0 abc General Certificate of Education Chemistry 6421 CHM5 Thermodynamics and Further Inorganic Chemistry Mark Scheme 2008 examination - June series Mark schemes are prepared by the Principal