1. C 2. B 3. D 4. D 5. A 6. B

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1 SOLUCIONARIO REDOX PRUEBA09/10/11 1. C 2. B 3. D 4. D 5. A 6. B 7. labelled polarities of positive and negative electrodes; Electrodes can be labelled positive or negative or + and signs can be used. direction of electron flow; e does not have to be labelled but arrow essential. power source and molten electrolyte/ni 2+ (l) and Br (l)/nibr 2 (l); State symbol necessary for M3 unless molten electrolyte stated. Power source does not need to be labelled if correct symbol used (i.e. short line and long line). Cathode/negative electrode equation: Ni e Ni; Anode/positive electrode equation: 2Br Br 2 + 2e ; Accept balanced half-equation with one e.

2 Award [1 max] for M4 and M5 if electrodes are not identified or if equations are given wrong way round or incorrectly labelled. Penalize once only. Ignore state symbols for M4 and M5. 8. Dilute sodium chloride: 2H 2 O O 2 + 4H + + 4e / 4OH O 2 + 2H 2 O + 4e ; Concentrated sodium chloride: 2Cl Cl 2 + 2e ; Accept alternative balanced half-equations with correct number of electrons. Award [1 max] if equations are given the wrong way round. Award [2] if correct equations are written in order with dilute sodium chloride first and concentrated sodium chloride second but processes not stated explicitly. Penalize once only. 9. (i) Sn + Cu 2+ Sn 2+ + Cu; Penalize once only. (ii) ( ) = (+) 0.48 V; (iii) 1.0 mol dm 3 solutions and 25 C/298 K; 10. Cd 2+ is a stronger oxidizing agent than H 2 O and will be displaced to produce Cd / OWTTE; Cr 2+ is a weaker oxidizing agent than H 2 O and H 2 will displace in preference to Cr / OWTTE; Award [1 max] for stating Cd 2+ stronger oxidizing agent than H 2 O and Cr 2+ weaker oxidizing agent than H 2 O / OWTTE. 11. (i) Ni; only requires 2 mol of e for each mol of Ni / Sn requires 4 mol of e / Cr requires 3 mol of e / Ni 2+ needs least number of e to produce 1 mol of Ni metal; occurs. cathode / negative electrode;do not award M3 for metal deposited at cathode where oxidation 2

3 (ii) temperature of solution; [Sn 4+ ]; surface area/size of electrode; material of electrodes; Do not allow nature of electrodes. 12. (i) NH 3 : 3; NO: +2; N 2 : 0; 3 Penalize incorrect notation such as 3, III, 2+, 2, II once only. (ii) Oxidation: 2NH 3 N 2 + 6H + + 6e ; Reduction: 2NO + 4H + + 4e N 2 + 2H 2 O; Award [1 max] for M1 and M2 if redox processes are not identified or if equations are given wrong way round. Penalize once only. Oxidizing agent: NO; Allow either formula or name. (iii) NH 3 /ammonia (in excess by) and 10 (dm 3 ); 25.0 (dm 3 ); C 14. B 15. C 16. (i) 3

4 correct diagram including voltmeter/meter, 4 correct species (state symbols not required) and connecting wires; No credit if wires to electrodes immersed in the solutions. labelled salt bridge; Do not accept name of salt (e.g. potassium nitrate) in place of salt bridge. correctly labelled electrodes (+)/cathode and ( )/anode; flow of electrons from Fe to Cu in external circuit; (ii) positive electrode: Cu e Cu; negative electrode: Fe Fe e ; Award [1] if equations correct but at wrong electrodes or if electrodes are missing. Award [2] for correct equations if electrodes are missing but were correctly labelled in diagram. Accept e instead of e. Penalize once only in equations in (ii) and (iii). (iii) Fe + Cu 2+ Fe 2+ + Cu; Cu 2+ is the oxidizing agent and the species that is reduced; 17. D 18. B 19. (i) the potential difference/voltage obtained when a half-cell is connected to a standard hydrogen electrode; under standard conditions / 1.00 mol dm 3 solutions, 298 K; (ii) the electrons flow from the half-cell to the standard hydrogen electrode / the half-cell forms the negative electrode when connected to the standard half-cell / Fe is a better reducing agent than H 2 / Fe is above H 2 in electrochemical series;accept the half reaction is not spontaneous. 20. (i) bromine/br 2 ; (ii) hydrogen/h 2 ; (iii) iron/fe; Ignore coefficients for Br 2, H 2 or Fe. 21. (i) (the solution changes) from orange to green; 4

5 (ii) +6; Do not accept 6, 6+ or the use of Roman numerals. (iii) Cr 2 O H + + 6e 2Cr H 2 O; (iv) CH 3 CH 2 OH CH 3 CHO + 2H + + 2e ; Cr 2 O CH 3 CH 2 OH + 8H + 2Cr CH 3 CHO + 7H 2 O For second equation award [1] for correct reactants and products and [1] for correct balancing. (v) H + is a reactant / OWTTE; (vi) ethanoic acid / CH 3 COOH / acid; Accept acetic acid. 22. (i) sodium is a very powerful reducing agent/high in electrochemical series; any chemical reducing agent would need to be even higher in ECS to reduce Na + / OWTTE; (ii) H + ions gain electrons more readily than Na + / hydrogen is evolved instead; hydrogen is below Na in ECS; if sodium were to be formed it would react with the water in the solution / OWTTE; 23. B 24. C 25. B 26. C 27. (i) 5

6 correctly labelled electrodes and solutions; labelled salt bridge; voltmeter; Allow bulb or ammeter. direction of electron flow; (ii) Oxidation: Mg(s) Mg 2+ (aq) + 2e ; Reduction: Ag + (aq) + e Ag(s); Award [1 max] if equations not labelled reduction or oxidation or labelled the wrong way round. Penalize equilibrium sign or reversible arrows once only. 28. (i) Zn > Cd > Ni > Ag Zn most reactive; rest of order correct; (ii) Best oxidizing agent: Ag + ; Do not accept Ag. Best reducing agent: Zn; Do not accept Zn (i) sodium chloride crystals consist of ions in a (rigid) lattice / ions cannot move (to electrodes) / OWTTE; when melted ions free to move / ions move when potential difference/ voltage applied; positive sodium ions/na + move to negative electrode/cathode and negative chloride ions/cl move to positive electrode/anode; electrons released to positive electrode/anode by negative ions and accepted from negative electrode/cathode by positive ions / reduction occurs at the negative electrode/cathode and oxidation occurs at the positive electrode/ anode / Na + ions are reduced and Cl ions are oxidized; 6

7 (Positive electrode/anode): 2Cl Cl 2 + 2e / Cl 1 2 Cl 2 + e ; (Negative electrode/cathode): 2Na + + 2e 2Na / Na + + e Na; Award [1 max] if equations not labelled or labelled wrong way round. Penalize equilibrium sign or reversible (ii) arrows once only. 30. B 31. D 32. C 33. (a) (i) correctly labelled electrodes and solutions; labelled salt bridge; voltmeter; Allow bulb or ammeter. (ii) Oxidation: Mg(s) Mg 2+ (aq) + 2e ; Reduction: Ag + (aq) + e Ag(s); direction of electron flow; Award [1 max] if equations not labelled reduction or oxidation or labelled the wrong way round. Penalize equilibrium sign or reversible arrows once only. (iii) correct data; ( 2.37) = 3.17 V 7

8 answer with unit; Award [1] for 3.17 V or correct working of wrong values. (b) (i) Cd/Cd(s); Do not allow Cd 2+. (ii) 5Cd(s) + 2MnO 4 (aq) + 16H + (aq) 5Cd 2+ (aq) + 2Mn 2+ (aq) + 8H 2 O(l) correct reactants and products; correct balancing of this equation; (c) Accept suitable diagram with the following indicated: Pt electrode; [H + (aq)] = 1 mol dm 3 /0.5 mol dm 3 H 2 SO 4 ; H 2 gas; at 1 atm / Pa; Do not award mark for pressure if no hydrogen gas given. 298 K / 25 C; 34. (i) sodium chloride crystals consist of ions in a (rigid) lattice / ions cannot move (to electrodes) / OWTTE; when melted ions free to move / ions move when potential difference/voltage applied; (ii) positive sodium ions/na + move to the negative electrode/cathode and negative chloride ions/cl move to the positive electrode/anode; electrons are released to positive electrode/anode by negative ions and accepted from negative electrode/cathode by positive ions / reduction occurs at the negative electrode/cathode and oxidation occurs at the positive electrode/anode / Na + ions are reduced and Cl ions are oxidized; (Positive electrode/anode): 2Cl Cl 2 + 2e / Cl 1 Cl 2 + e ; (Negative electrode/cathode) 2Na + + 2e 2Na / Na + + e Na; 2 Award [1 max] if equations not labelled or labelled wrong way round. Penalize equilibrium sign or reversible arrows once only. (iii) Products: oxygen at positive electrode and hydrogen at negative electrode; moles of Mg = 0.5 / mole ratio of O 2 :H 2 is 1:2; Can be implied by calculation mass oxygen = = 8.00 g; mass hydrogen = = 1.01 g; Do not apply SD rule here. 8

9 35. B 36. A 37. A 38. (i) Pb: 0, PbO 2 : +4, PbSO 4 : +2; Need sign for mark. Do not accept notations such as 4+, 2+ or IV, II. (ii) Negative/ /anode Pb(s) + SO 2 4 (aq) PbSO 4 (s) + 2e / Pb(s) Pb 2+ (aq) + 2e ; Positive/+/cathode PbO 2 (s) + 4H + (aq) + SO 4 2 (aq) + 2e PbSO 4 (s) + 2H 2 O(l) / PbO 2 (s) + 4H + (aq) + 2e Pb 2+ (aq) + 2H 2 O(l) / PbO 2 (s) + H 2 SO 4 (aq) + 2H + (aq) + 2e PbSO 4 (s) + 2H 2 O(l); Accept Pb e Pb 2+. oxidizing agent is PbO 2 / lead(iv) oxide/lead dioxide and reducing agent is Pb/lead; from negative/ /anode/pb to positive/+/cathode/pbo 2 (through the external circuit/wire); (iii) Pb(s) + Cu 2+ (aq) Pb 2+ (aq) + Cu(s) Pb(s) + 2Ag + (aq) Pb 2+ (aq) + 2Ag(s) Cu(s) + 2Ag + (aq) Cu 2+ (aq) + 2Ag(s) Award [2] for three correct, award [1] for any two correct, one correct scores no mark. Penalize unbalanced equations once only. Pb is a stronger reducing agent than Cu and/or Ag / Pb most reactive as it can reduce/displace both Cu 2+ and Ag + ; Cu is a stronger reducing agent than Ag but not Pb / Cu in the middle (of the three) as it can reduce/displace Ag + but not Pb 2+ ; Accept converse argument. Decreasing order: Pb, Cu, Ag / Pb > Cu > Ag; Do not accept: Pb 2+, Cu 2+, Ag D 40. (i) Positive/+/anode 2Cl (l) Cl 2 (g) + 2e ; Negative/ /cathode Na + (l) + e Na(l); Penalize missing or incorrect states such as (aq) or (s) once only. Award only [1] if electrodes not specified or if equations switched. 9

10 1Cl 2 to 2Na; (ii) (choice of Cl or H 2 O/OH to be oxidized), Cl oxidized because of concentrated solution/higher concentration / OWTTE; (choice of Na + or H 2 O/H + to be reduced), H 2 O/H + reduced because Na + is a (much) weaker oxidizing agent / Na + not reduced to Na in water / H + easier to reduce than Na + / OWTTE; positive/+/anode 2Cl (aq) Cl 2 (g) + 2e ; negative/ /cathode 2H 2 O(l) + 2e H 2 (g) + 2OH (aq) / 2H + (aq) + 2e H 2 (g); Penalize missing or incorrect states once only. Award only [1] out of the last two marks if electrodes not specified or if equations switched. 41. Positive/+/anode 2H 2 O(l) O 2 (g) + 4H + (aq) + 4e 1 /H 2 O(l) O 2 (g) + 2H + (aq) + 2e / 4OH (aq) 2H 2 O(l) + O 2 (g) + 4e ; Negative/ /cathode Cu 2+ (aq) + 2e Cu(s); Award only [1] if electrodes not specified or if equations switched. Observations: [2 max] blue colour of Cu 2+ (aq) fades; Cu/metal deposited on negative/ /cathode/tin (jewelry); gas produced/bubbles formed (at positive/+/anode); ph of solution decreases/acidity increases (observed with indicator/ph paper); 42. C A 44. C 45. (i) increase in the oxidation number; (ii) (NO 3 )+ 5 and (NO 2 ) + 3; Accept V and III. Do not penalize missing charges on numbers. 46. change in oxidation numbers: Ag from 0 to +1 and N from +5 to +2; Do not penalize missing charges on numbers. balanced equation: 3Ag + NO 3 + 4H + 3Ag + + NO + 2H 2 O Award [1] for correct reactants and product; 10

11 Award [3] for correct balanced equation. 47. B 48. A 49. C 50. (i) contains ions which are free to move (only) in molten state; Mg 2+ move to cathode/negative electrode and Cl move to anode/positive electrode / OWTTE; (ii) anode/positive electrode; 2Cl Cl 2 + 2e / Cl 1 Cl 2 + e ; Accept e instead of e. Do not accept Cl Cl + e. 2 (iii) magnesium has large negative electrode potential / E O ; reduction of H 2 O/H + to H 2 has less negative electrode potential; Mg 2+ not readily reduced (in comparison to H 2 O); if formed, magnesium would (immediately) react with water to form Mg 2+ ; magnesium more reactive than hydrogen; Do not accept Mg too reactive. 51. A 52. C 53. (a) Loss of (one or more) electrons; (b) (i) (KMnO 4 ) + 7; (MnCl 2 ) + 2; Must have + sign for mark. [1 max] if roman numerals or 7+ or 2+ used or if + signs are missing. 2 (ii) Cl / chloride / chlorine / Cl (has been oxidized) / HCl; oxidation number from 1 to 0 / has increased by one; If HCl is given for first mark, it must be clear that it is the Cl that has the change of oxidation number. 54. B 11

12 55. B 56. (i) the voltage obtained when the half-cell is connected to the standard hydrogen electrode; under standard conditions of 298 K and 1 mol dm 3 solutions; electrons flow (in the external circuit) from the half-cell to the hydrogen electrode / the metal in the half-cell is above hydrogen in the ECS / Fe is a better reducing agent than H 2 / Fe is oxidized more readily than H 2 ; (ii) 0.28 V; (iii) Co 2+ /cobalt(ii) ion; (iv) 2Al + 3Fe 2+ 3Fe + 2Al 3+ ; Award [1] for correct reactants and products and [1] for correctly balanced, ignore states. Do not accept (v) to complete the electrical circuit / OWTTE; by allowing the movement of ions; 57. (a) (i) diagram to show: battery/source of electricity connected to two electrodes in the solution with positive and negative electrodes correctly labelled; electrons/current flowing from the cell to the negative electrode; labelled solution of sodium chloride; If the connecting wires to electrodes are immersed in the solution [1 max]. 3 (ii) Na +, H + /H 3 O +, Cl, OH All four correct [2], any three correct [1]. (iii) hydrogen at ( )/cathode and oxygen at (+)/anode; 2H + + 2e H 2 / 2H 2 O + 2e H 2 + 2OH ; 4OH O 2 + 2H 2 O + 4e / 2H 2 O O 2 + 4H + + 4e ; Accept e instead of e If electrodes omitted or wrong way round [2 max]. (iv) Ratio of H 2 :O 2 is 2:1; 12

13 (b) (i) ( )/(cathode)2h + + 2e H 2 / 2H 2 O + 2e H 2 + 2OH ; (+)/(anode) 2Cl Cl 2 + 2e ; Accept e instead of e. If electrodes omitted or wrong way round [1 max] (ii) ( )/(cathode) Na + + e Na; (+)/(anode) 2Br Br 2 + 2e ; Accept e instead of e. If electrodes omitted or wrong way round [1 max]. 58. B 59. B 60. (i) increase in the oxidation number; (ii) Annotated diagram of cell showing: power supply/battery; electrolyte; cathode/negative electrode and anode/positive electrode; (iii) (solid) ions in a lattice / ions cannot move; (molten) ions mobile / ions free to move; (iv) reduction occurs at the cathode/negative electrode and oxidation occurs at the anode/positive electrode; Cathode/negative electrode: Na + + e Na; Anode/positive electrode: 2Cl Cl 2 + 2e / Cl 1 Cl 2 + e ; Award [1 max] if the two electrodes are not labelled/labelled incorrectly for the two half-equations. Overall cell reaction: Na + (l) + Cl (l) Na(l) + Award [1] for correct equation and [1] for correct state symbols. Allow NaCl(l) instead of Na + (l) and Cl (l). (v) Cl 2 (g) Al does not corrode/rust / Al is less dense/better conductor/more

14 malleable; Accept Al is a lighter (metal compared to Fe). Accept converse argument. (vi) electrolytic cell converts electrical energy to chemical energy and voltaic cell converts chemical energy to electrical energy / electrolytic cell uses electricity to carry out a (redox) chemical reaction and voltaic cell uses a (redox) chemical reaction to produce electricity / electrolytic cell requires a power supply and voltaic cell does not; electrolytic cell involves a non-spontaneous (redox) reaction and voltaic cell involves a spontaneous (redox) reaction; in an electrolytic cell, cathode is negative and anode is positive and vice-versa for a voltaic cell / electrolytic cell, anode is positive and voltaic cell, anode is negative / electrolytic cell, cathode is negative and voltaic cell, cathode is positive; voltaic cell has two separate solutions and electrolytic cell has one solution / voltaic cell has salt bridge and electrolytic cell has no salt bridge; electrolytic cell, oxidation occurs at the positive electrode/anode and voltaic cell, oxidation occurs at the negative electrode/anode and vice-versa; 61. C 62. A 63. D 64. D 65. D 66. A 67. B 68. electrolytic cell converts electrical energy to chemical energy and voltaic cell converts chemical energy to electrical energy / electrolytic cell uses electricity to carry out a (redox) chemical reaction and voltaic cell uses a (redox) chemical reaction to produce electricity / electrolytic cell requires a power supply and voltaic cell does not; electrolytic cell involves a non-spontaneous (redox) reaction and voltaic cell involves a spontaneous (redox) reaction; in an electrolytic cell, cathode is negative and anode is positive and vice-versa for a voltaic cell / electrolytic cell, anode is positive and voltaic cell, anode is negative / electrolytic cell, cathode is negative and voltaic cell, cathode is positive; 14

15 voltaic cell has two separate solutions and electrolytic cell has one solution / voltaic cell has salt bridge and electrolytic cell has no salt bridge; electrolytic cell, oxidation occurs at the positive electrode/anode and voltaic cell, oxidation occurs at the negative electrode/anode/vice-versa; If descriptions are reversed for electrolytic and voltaic cell, penalize first marking point but award second marking point as ECF. 69. (i) 2Al(s) + 3Ni 2+ (aq) 2Al 3+ (aq) + 3Ni(s); Correct reactants and products, award [1] Balancing award [1]. Ignore state symbols and equilibrium sign. (ii) (+) 1.40 (V) ; (iii) aluminium anode/negative electrode; nickel cathode/positive electrode; electron movement from Al to Ni; correct movement of cations and anions through salt bridge; If electron movement shown correctly but not labelled, award the mark 70. A 71. A 72. C 73. B 74. D 15

16 75. C 76. A 77. (a) Pt electrode; 1 mol dm 3 [H + (aq)]; H 2 gas; at 1 atm/ Pa; 298 K/25 C; 5 Accept suitable labelled diagram with the above. (b) electron acceptor; F 2 /fluorine; 2 (c) (i) (+)0.48 (V); (ii) Cu 2+ (aq) + Sn(s) Sn 2+ (aq) + Cu(s) Award [1] for correct reactants and products from (c)(i), and [1] for state symbols. 16