Electrochemistry Answers

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1 Electrochemistry Answers

2 Chemistry - AQA GCE Mark Scheme 200 June series Q Part Sub Part Marking Guidance Mark Comments 7 (a) Hydrogen /H 2 gas/bubbles.0 mol dm 3 HCl / H + At 298K and 00kPa Pt (electrode) 7 (b) Li + + MnO 2 + e LiMnO 2 0.3(V) 7 (c) Fe 3+ ions reduced to Fe 2+ Because E(Fe 3+ (/Fe 2+ )) > E(H + /H 2 ) /E(hydrogen) Allow bar instead of 00 kpa Do not allow atm Ignore state symbols Can score from equation/scheme Allow emf/e cell +ve/0.77v Allow Fe 3+ better oxidising agent than H + Allow H 2 better reducing agent than Fe 2+ Only award this explanation mark if previous mark given 4

3 Chemistry - AQA GCE Mark Scheme 200 June series 7 (d) Moles Cr 2 O 7 2 =23.7 x 0.0/000 = 2.37 x 0 4 mol Cr 2 O 7 2 reacts with 6 mol Fe 2+ so moles Fe 2+ in 25 cm 3 = 6 x 2.37 x 0 4 =.422 x 0 3 Moles Fe 2+ in 250 cm 3 =.422 x 0 2 Original moles Fe 2+ = 0.00/277.9 = Moles Fe 2+ oxidised = = % oxidised = ( )/ = 60.5% M x 6 M2 x 0 or M4/0 Independent mark M4 M3 (M5 x 00)/M4 Allow 60 to 6 Note Max 3 if mol ratio for M2 wrong eg :5 gives 67.% : gives 93.4% Note also, 39.5% (39-40) scores M, M2, M3 and M4 (4 marks) 5

4 Chemistry - AQA GCE Mark Scheme 200 January series Question Part Sub Part Marking Guidance Mark Comments 2 (a).4 V Allow + or - 2 (b) 2NiO(OH) + 2H 2 O + Cd 2Ni(OH) 2 + Cd(OH) 2 Balanced 2 (c) NiO(OH) or Ni(III) or nickel Mark for species, Deduct a mark for additional species (eg OH - ) but allow balance mark If equation is reversed CE=0 +3 Allow conseq on wrong species 4

5 Chemistry - AQA GCE Mark Scheme 200 January series Question Part Sub Part Marking Guidance Mark Comments 3 (a) By definition allow 'set to this value' 3 (b).23 V Allow + or - 3 (c) Pt H 2 (g) OH (aq),h 2 O(l) O 2 (g) H 2 O(l),OH (aq) Pt H 2 O not essential, allow reverse order Correct but with Pt missing Includes Pt with correct representation 3 (d) Uses O 2 + 2H 2 O + 4e - 4OH - And (2x) 2OH - +H 2 2H 2 O + 2e - 2H 2 + O 2 2H 2 O 3 (e) Increases the surface area (so reaction faster) 3 (f) Overall reaction is the same (2H 2 + O 2 2H 2 O) Or shows e.m.f. is the same 3 (g) Hydrogen and oxygen supplied continuously OR Or can be refuelled quickly Allow any one mark Can be operated without stopping to recharge 3 (h) Hydrogen may need to be made using an energy source that is not carbon neutral 5

6 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 5: Energetics, Redox and Inorganic Chemistry June 20 Question Marking Guidance Mark Comments 5(a) loses electrons / donates electrons penalise donates electron pair 5(b) Zn (most) negative E o / lowest E o / least positive can only score M2 if M correct do not allow e.m.f instead of E o 5(c) E o F 2 (/F ) > E o O 2 (/H 2 O) or e.m.f is positive or e.m.f =.64 V Fluorine reacts to form oxygen (can score from equation in M3 even if equation unbalanced provided no contradiction) or fluorine oxidises water or fluorine is a more powerful oxidising agent than oxygen 2F 2 + 2H 2 O 4F + 4H + + O 2 allow 4HF in equation balanced equation scores M2 and M3 0

7 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 5: Energetics, Redox and Inorganic Chemistry June 20 5(d)(i) order correct Zn Zn 2+ Ag 2 O Ag or reverse of this order ignore ss, H + and H 2 O, no. of moles all phase boundaries correct e.g. Zn Zn 2+ Ag 2 O Ag or Ag Ag 2 O Zn 2+ Zn scores 2 allow Zn Zn 2+ Ag 2 O,Ag or Zn Zn 2+ Ag 2 O H + Ag for M & M2 M2 cannot be gained unless M scored allow H + either side of Ag 2 O with comma or for M2 penalise wrong phase boundary (allow dashed lines for salt bridge) Pt use of + (from half equation) water/h + outside Ag in Ag electrode 5(d)(ii). (V) Allow no units, penalise wrong units allow correct answer even if no answer to (d)(i) or answer to (d)(i) incorrect allow. if silver electrode on Left in (d)(i) even if the species are in the wrong order. 5(d)(iii) Reaction(s) not reversible or H 2 O electrolyses do not allow hard to reverse mention of primary cell is not enough to show that reaction(s) are irreversible 5(e)(i) 0.46 (V) Allow no units, penalise wrong units

8 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 5: Energetics, Redox and Inorganic Chemistry June 20 5(e)(ii) 2PbSO 4 + 2H 2 O Pb +PbO 2 + 2HSO 4 + 2H + lead species correct on correct sides of equation equation balanced and includes H 2 O, HSO 4 and H + (or H 2 SO 4 ) allow ions / species must be fully cancelled out or combined allow /2 for balanced reverse equation 5(f)(i) reagents / PbO 2 / H 2 SO 4 /acid / ions used up (or concentration decreases) 5(f)(ii) fuel cell Ignore any other words 5(f)(iii) reagents / fuel supplied continuously concentrations (of reagents) remain constant 2

9 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 5: Energetics, Redox and Inorganic Chemistry January 20 Question Marking Guidance Mark Comments 5(a)(i) Co/Cobalt If Co or Cobalt not given CE = 0 ignore case in symbol for Co (+) 4 (+) 3 Allow 4 and 3 in either order 5(a)(ii) Li Li + + e Ignore state symbols Allow e without -ve sign Do not allow equilibrium sign 5(a)(iii) Platinum is a conductor Ignore mention of surface area or catalyst (Platinum is) unreactive/inert Allow 2 marks if two properties given on one answer line Apply list principle to contradictions/wrong answers Do not allow platinum resists corrosion 5(a)(iv) Li reacts with water/forms lithium hydroxide Allow water breaks down (or is electrolysed) on recharge

10 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 5: Energetics, Redox and Inorganic Chemistry January 20 5(b)(i) Pt SO 3 2 (aq), SO 4 2 (aq) ClO 3 (aq), Cl (aq) Pt 2 State symbols and, not necessary Allow in place of ',' NOT, in place of Ignore H + and H 2 O Deduct one mark for each mistake (e.g. Pt missed twice counts as two mistakes) Allow reverse order for whole cell Pt Cl -, ClO 3 - SO 4 2-, SO 3 2- Pt 5(b)(ii) ClO SO 3 Cl 2 + 3SO 4 Oxidising agent ClO 3 2 Reducing agent SO 3 2

11 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 5: Energetics, Redox and Inorganic Chemistry June 202 Question Marking Guidance Mark Comments 5(a) Electron acceptor / gains electrons / takes electrons away Do not allow electron pair acceptor / gain of electrons / definition of redox (QWC) 5(b) Cd(OH) 2 Species (on LHS) with the least positive/most negative electrode potential / lowest E / smallest E Do not allow Cd(OH) 2 /Cd Only allow this mark if M answer given correctly or blank Do not allow negative emf 5(c)(i).5 (V) /.50 5(c)(ii) 2MnO 2 + 2H 2 O + Zn 2MnO(OH) + 2OH - + Zn 2+ Ignore state symbols e - must be cancelled (take care that Zn 2+ is on RHS) 5(c)(iii) Allows ions to pass (through it) or words to that effect Penalise passage of electrons Allow mention of particular ions 5(c)(iv) Allows electrons to flow / makes electrical contact / conductor Allow acts as an (inert) electrode / anode / cathode 5(c)(v) Zn is 'used up' / has reacted / oxidised Allow idea that zinc reacts Do not allow just zinc corrodes 9

12 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 5: Energetics, Redox and Inorganic Chemistry June 202 5(d)(i) 3 / +3 / III 2Ni(OH) 2 + Cd(OH) 2 2NiO(OH) + Cd + 2H 2 O For correct nickel and cadmium species in correct order (allow H 2 O missing and OH - not cancelled) For balanced equation (also scores M2) Allow max for M2 and M3 if correct balanced equation but reversed. Ignore state symbols 5(d)(ii) Metal / metal compounds are re-used / supplies are not depleted / It (the cell) can be re-used Allow does not leak / no landfill problems / less mining / less energy to extract metals / less waste Do not allow less CO 2 unless explained 5(e)(i) C 2 H 5 OH + 3O 2 2CO 2 + 3H 2 O Allow C 2 H 6 O 5(e)(ii) C 2 H 5 OH + 3H 2 O 2CO 2 + 2H + + 2e - Allow C 2 H 6 O 5(e)(iii) (+)0.23 (V) 5(e)(iv) CO 2 released by combustion / fermentation / fuel cell / reaction with water Can be answered with the aid of equations (atmospheric) CO 2 taken up in photosynthesis 0

13 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 5: Energetics, Redox and Inorganic Chemistry January 202 Question Marking Guidance Mark Comments 4(a) HCl.0 mol dm -3 (Hydrogen at) 00kPa / bar 298 K Allow H 2 SO mol dm -3 Allow HNO 3.0 mol dm -3 Allow name or formula Concentration can be given after conditions 4(b) Pt / Platinum Mark on if no answer for M If wrong answer for M, only mark on if electrode is Au, Ag, Pb or Ti Inert / unreactive / does not create a potential difference Conducts electricity / allows electron flow / conducts / conductor 4(c) KCl Allow NaCl, KNO 3, Na 2 SO 4 etc NOT NH 4 Cl Does not react with either electrode / solution in electrode Allow unreactive / inert Ions can move Allow conducts electricity / electrical connection / carries charge Do not allow just connects / completes the circuit Do not allow conducts / carries electrons Mark these independently 8

14 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 5: Energetics, Redox and Inorganic Chemistry January 202 4(d) Pt H 2 H + Fe 3+,Fe 2+ Pt Ignore state symbols Order must be correct 4(e)(i) 2Fe 3+ + H 2 2Fe H + Allow multiples must be correct but allow instead of, separating Fe 3+ from Fe 2+ Allow, instead of separating H 2 and H + 4(e)(ii) The Fe 3+ ions would be used up / reaction completed Answer must relate to reactants in 4(e)(i) equation if given Allow reactant / reactants used up Do not allow concentration of Fe 3+ decreases Allow concentration of Fe 3+ falls to zero 9

15 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 5: Energetics, Redox and Inorganic Chemistry January 202 Question Marking Guidance Mark Comments 5(a) H 2 O 2 Ignore state symbols 5(b) E o Cl 2 /Cl - > E o O 2 /H 2 O Allow potential for chlorine/cl 2 greater than for oxygen/o 2 Allow.36 >.23 / E cell = 0.3 Cl 2 + H 2 O 2Cl - + /2O 2 + 2H + Allow multiples Allow + HCl 5(c) Activation energy is high / light/uv provides the activation energy / light breaks chlorine molecule / Cl Cl bond If light used to break Cl Cl bond award mark and ignore product e.g. Cl 5(d) O (-) (in H 2 O 2 ) Changes to O(-2) (in water) Must give oxidation state of O in H 2 O 2 = - Must give oxidation state of O in water = -2 CE = 0/2 if refers to oxidation state of H changing 5(e) E o H 2 O 2 /H 2 O > E o O 2 /H 2 O 2 Allow stated in words Allow.77 > 0.68 / E cell =.09 2H 2 O 2 O 2 + 2H 2 O Allow multiples H + and e - must be cancelled 0

16 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 5: Energetics, Redox and Inorganic Chemistry June 203 Question Marking Guidance Mark Comments 5(a) It has mobile ions / ions can move through it / free ions Do not allow movement of electrons. Allow specific ions provided they are moving but do not react. 5(b) Chloride ions react with copper ions / Cu 2+ OR [CuCl 4 ] 2- formed If incorrect chemistry, mark = 0 5(c) The Cu 2+ ions / CuSO 4 in the left-hand electrode more concentrated Allow converse. So the reaction of Cu 2+ with 2e - will occur (in preference at) left-hand electrode / Cu Cu 2+ + electrons at right-hand electrode Allow left-hand electrode positive / right-hand electrode negative. Also reduction at left-hand electrode / oxidation at right-hand electrode. Also left-hand electrode has oxidising agent / right-hand electrode has reducing agent. Allow E left-hand side > E right-hand side 5(d) (Eventually) the copper ions / CuSO 4 in each electrode will be at the same concentration 5(e)(i) (V) Must have minus sign only.

17 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 5: Energetics, Redox and Inorganic Chemistry June 203 5(e)(ii) LiMnO 2 Li + MnO 2 correct equation Allow for reverse equation. Allow multiples. Correct direction If Li+ not cancelled but otherwise correct, max = If electrons not cancelled, CE = 0 LiMnO 2 Li + MnO 2 scores 2 Li + + LiMnO 2 Li + + Li + MnO 2 scores Li + MnO 2 LiMnO 2 scores 5(e)(iii) Electricity for recharging the cell may come from power stations burning (fossil) fuel Allow any reference to burning (of carboncontaining) fuels. Note combustion = burning. 2

18 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 5: Energetics, Redox and Inorganic Chemistry January 203 Question Marking Guidance Mark Comments 7(a) Diagram of an Fe 3+ /Fe 2+ electrode that includes the following parts labelled: Solution containing Fe 2+ and Fe 3+ ions Platinum electrode connected to one terminal of a voltmeter Salt bridge 298 K and 00 kpa / bar all solutions unit / mol dm 3 concentration Must be in the solution of iron ions (one type will suffice) Do not allow incorrect material for salt bridge and salt bridge must be in the solution (ie it must be shown crossing a meniscus) Allow zero current / high resistanve voltmeter as alternative to M4 or M5 Ignore hydrogen electrode even if incorrect 7(b) Cu 2+ + Fe Cu + Fe 2+ Fe Fe 2+ Cu 2+ Cu correct order Phase boundaries and salt bridge correct, no Pt Copper electrode Ignore state symbols Allow Cu Cu 2+ Fe 2+ Fe Allow single/double dashed line for salt bridge Penalise phase boundary at either electrode end Can only score M3 if M2 correct Allow any reference to copper 2

19 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 5: Energetics, Redox and Inorganic Chemistry January 203 7(c) E o Au + (/Au) > E o O 2 (/H 2 O) So Au + ions will oxidise water / water reduces Au + 2Au + + H 2 O 2Au + /2O 2 + 2H + Allow E cell/e.m.f. = 0.45 V Allow.68 >.23 QoL Allow multiples 7(d) E o Ag + (/Ag) > E o Fe 2+ (/Fe) Allow E cell/e.m.f. =.24 Allow 0.80 > And E o Ag + (/Ag) > E o Fe 3+ (/Fe 2+ ) Allow E cell/e.m.f. = 0.03 Allow 0.80 > 0.77 So silver ions will oxidise iron (to iron(ii) ions) and then oxidise Fe(II) ions (further to Fe(III) ions producing silver metal) Allow Ag + ions will oxidise iron to iron(iii) 3

20 MARK SCHEME A-LEVEL CHEMISTRY CHEM5 JUNE 204 Question Marking Guidance Mark Comments 5(a) Pt H 2 H + Fe 2+ Fe Note, allow one mark only for correct symbol in reverse: Fe Fe 2+ H + H 2 Pt 2 Allow for correct order of symbols but lose second mark for a wrong phase boundary(s) / Pt missing / extra Pt on RHS, additional phase boundary Allow dashed lines for salt bridge Ignore state symbols Ignore 2 if used before H + 5(b) Electron donor Allow (species that) loses electrons Do not allow reference to electron pairs 5(c) Cl 2 / chlorine If M blank or incorrect cannot score M2 (Species on RHS / electron donor) has most positive / largest E o / has highest potential Do not allow reference to e.m.f. or E(cell) 5(d)(i) Cl / chlorine 5(d)(ii) Chlorine + to chlorine 0 CE if chlorine not identified in 5(d)(i) Allow chlorine + to chlorine (in Cl ) Allow oxidation state decreases by one OR two Allow oxidation state changes by OR 2 4 of 2

21 MARK SCHEME A-LEVEL CHEMISTRY CHEM5 JUNE 204 5(e) 4HOCl + 4H + + 4OH 2Cl 2 + O 2 + 6H 2 O OR 4HOCl 2Cl 2 + O 2 + 2H 2 O 2 Allow one mark for any incorrect equation that shows HOCl Cl 2 + O 2 Allow multiples Ignore state symbols Penalise one mark for uncancelled or uncombined species (eg H 2 O + H 2 O instead of 2H 2 O) 5(f)(i) e.m.f. = 0.40 (.25) =.65 (V) / +.65 (V) Allow.65 (V) 5(f)(ii) 2Zn + O 2 2ZnO Allow multiples Ignore state symbols Do not allow uncancelled species If more than one equation given, choose the best 5(f)(iii) A / stainless lid If M incorrect or blank CE=0 O 2 (electrode) has a more positive E o / oxygen (electrode) requires / gains electrons from external circuit Or reference to the overall equation and a link to electrons going into A OR Zinc (electrode) has more negative E o Allow oxygen is reduced and reduction occurs at the positive electrode Do not allow reference to e.m.f. or E(cell) 5(f)(iv) (Cell) reaction(s) cannot be reversed / zinc oxide cannot be reduced to zinc by passing a current through it / zinc cannot be regenerated Allow danger from production of gas / oxygen produced / hydrogen produced 5 of 2

22 MARK SCHEME A-LEVEL CHEMISTRY CHEM5 JUNE 204 Question Marking Guidance Mark Comments 6(a)(i) H 2 + 2OH 2H 2 O + 2e / H 2 2H + + 2e Any order O 2 + 4e + 2H 2 O 4OH / O 2 + 4H + + 4e 2H 2 O 6(a)(ii) Hydrogen (electrode) produces electrons Ignore reference to salt bridge Do not allow at negative / positive electrode must identify hydrogen and oxygen Oxygen (electrode) accepts electrons Allow electrons flow to the oxygen electrode 6(b) Hydrogen / the fuel / reactants supplied continuously / fed in Do not accept oxygen supplied as the only statement 6(c) In the fuel cell, a greater proportion of the energy available from the hydrogen oxygen reaction is converted into useful energy Allow less energy wasted / more efficient Do not allow reference to safety 6(d) Hydrogen is flammable / H + corrosive / OH corrosive / hydrogen explosive 6 of 2

23 MARK SCHEME A-LEVEL CHEMISTRY CHEM5 JUNE 205 Question Marking Guidance Mark Comments 6(a) Electron acceptor / gains electrons do not allow electron pair acceptor 6(b) Fe 2+ ions Fe 2+ /Fe or Fe 2+ or it has smallest / most negative electrode potential / E o Do not allow Fe/Fe 2+ Cannot score M2 if M incorrect 6(c) Pt H 2 H + Ag + Ag M for H 2 H + Ag + Ag in correct order allow dashed phase boundaries M2 for Pt correct and correct phase boundaries 2H + loses one mark (M2) Ignore state symbols. M must be correct to score M2 Any two correct conditions If answer correct but all in reverse order allow mark out of two 298 K /25 o C 00 kpa both solutions of unit concentration zero current 2 Allow bar Do not apply list principle, mark correct answers. 4 of 22

24 MARK SCHEME A-LEVEL CHEMISTRY CHEM5 JUNE 205 6(d) E Au + (/Au) > E O 2 (/ H 2 O) OR e.m.f. / Ecell = 0.45 V If both species in electrode given, must be in correct order i.e. Au + /Au Au + (ions) oxidise water OR water reduces Au + (ions) Allow water donates electrons to Au+ Gold metal/solid/precipitate OR bubbles / effervescence of (oxygen gas) / gas produced Penalise incorrect observations 2Au + + H 2 O 2Au + 2H O 2 OR 4Au + + 2H 2 O 4Au + 4H + + O 2 Allow multiples 6(e)(i).24 (V) Do not allow (e)(ii) Chloride ions / Cl - react with / form a precipitate with silver ions/ Ag+ / form AgCl Penalise reaction of chloride ions with iron ions or iron 6(f) E O 2 (/H 2 O) > E Fe 3+ (/Fe 2+ ) (or e.m.f / Ecell = 0.46 V) Therefore the iron(ii) ions are oxidised (or converted) into iron(iii) ions (by oxygen) Species in electrode if all given must be in correct order If chloride ions oxidised to chlorine, lose M2 M2 can be obtained or lost from equation. Ignore observations. 5 of 22

25 MARK SCHEME A-LEVEL CHEMISTRY CHEM5 JUNE 206 Question Marking guidance Mark Comments 4ai Zn 2+ Zn 2+ (aq) Apply List 4aii 298 K /25 o C (solutions at) unit concentration / mol dm -3 (of Zn 2+ ) Ignore pressure Ignore standard conditions Ignore state symbols Ignore references to S.H.E 4b Identifying it is the Zn/Zn 2+ and Co 2+ /Co half cells Zn Zn 2+ Co 2+ Co correct order with phase boundaries and salt bridge correct, no Pt If this is correct it scores M and M2 Allow double dashed line for salt bridge Extra phase boundaries loses M2 Ignore state symbols Zn Zn e (-) M3 independent Allow 2e - on LHS 9 of 22

26 MARK SCHEME A-LEVEL CHEMISTRY CHEM5 JUNE 206 4c Co 3+ 2Co 3+ (aq) + H 2 O(l) 2Co 2+ (aq)+ ½ O 2 (g) + 2H + (aq) Oxygen /O 2 Mark independently. Ignore state symbols allow multiples Allow ½ O 2 4d E ϴ (O 2 ( H 2 O)) electrode < E ϴ (Au + ( Au)) OR E ϴ (Au + ( Au)) > E ϴ (O 2 ( H 2 O)) OR the E ϴ (Au + Au) electrode potential is more positive than the E ϴ (O 2 H 2 O) electrode OR The emf (for the reaction of Au and oxygen) is 0.45 V (and therefore not spontaneous) Mark independently so oxygen is unable to oxidise gold Ignore references to water Allow gold cannot reduce oxygen 0 of 22

27 MARK SCHEME A-LEVEL CHEMISTRY CHEM5 JUNE 206 Question Marking guidance Mark Comments 5ai M Positive electrode O 2 + 2H 2 O +4e (-) 4OH - M2 Negative electrode H 2 + 2OH - 2H 2 O + 2e (-) 2H 2 + O 2 2H 2 O Allow multiples, ignore state symbols If equations both correct but at the wrong electrodes allow mark Mark independently Must be this way round 5aii Increase (emf ) If decrease/no change then CE=0/2; if blank then mark on 2H 2 + O 2 2H 2 O will move to the right or overall equation moves to the right Allow O 2 + 2H 2 O +4e - 4OH - will move to the right / oxygen half equation moves to the RHS / E ϴ O 2 OH - half cell moves to the right 5b Must start at y-axis e.m.f /V time /min 5ci Unchanged of 22

28 MARK SCHEME A-LEVEL CHEMISTRY CHEM5 JUNE 206 5cii Water is the only product / fuel cell does not give out pollutants such as NO x or CO 2 or SO 2 or C or CO or C x H y or unburnt hydrocarbons Not fuel cell does not give out pollutants unless pollutant stated 5d CO 2 is released because fossil fuels are burned to produce electricity to generate hydrogen OR CO 2 is released when methane reacts with steam to produce hydrogen Allow CO 2 is released to produce the hydrogen 2 of 22