Primary Cells

1. Primary Cells A primary cell is a cell that can be used once only and cannot be recharged. The reactants cannot be regenerated

2. Primary cells non-rechargeable • These cells are not rechargeable. ․Zinc-carbon cells Recharging is dangerous as it produces H2 and heat which results in an explosion.

3. Primary cells • These cells are not rechargeable. ․Alkaline manganese cells

4. Primary cells • These cells are not rechargeable. ․Silver oxide cells (button cells)

5. Primary cells • These cells are not rechargeable. ․Lithium primary cells (button cells)

6. Zinc-carbon Cells

7. At anode: At cathode: 2MnO2(s) + 2NH4+(aq) + 2e– Mn2O3(s) + 2NH3(aq) + H2O(l) Zn(s) Zn2+(aq) + 2e– Zinc-carbon Cells Overall reaction: Zn(s) + 2MnO2(s) + 2NH4+(aq) Zn2+(aq) + Mn2O3(s) + 2NH3(aq) + H2O(l) Ecell = +1.50 V

8. Zn(s) | Zn2+(aq) [2MnO2(s) + 2NH4+(aq)], [Mn2O3(s) + 2NH3(aq) + H2O(l)] | C(graphite) Zinc-carbon Cells The cell diagram for the zinc-carbon cell is: Overall reaction: Zn(s) + 2MnO2(s) + 2NH4+(aq) Zn2+(aq) + Mn2O3(s) + 2NH3(aq) + H2O(l)

9. At anode, Zn(s) + 2OH(aq) ZnO(s) + H2O(l) + 2e At cathode, Ag2O(s) + H2O(l) + 2e 2Ag(s) + 2OH(aq) Overall reaction :Zn(s) + Ag2O(s)  ZnO(s) + 2Ag(s)

10. At anode, Zn(s) + 2OH(aq) ZnO(s) + H2O(l) + 2e At cathode, HgO(s) + H2O(l) + 2e Hg(l) + 2OH(aq) Q.20(a) HgO(s) Overall reaction : Zn(s) + HgO(s)  ZnO(s) + Hg(l)

11. Q.20(b) HgO(s) = +0.098V – (1.216V) = 1.314V

12. Secondary Cells Electrochemical cells that can be recharged. Examples : - Lead-acid accumulators Nickel-cadmium cells (NiCad) Nickel-Metal hydride(NiMH) cells Lithium-ion cells

13. Lead grids coated with PbSO4(s) Pb(s) + H2SO4(aq)  PbSO4(s) + H2(g)

14. Negative electrode : PbSO4(s) + 2e Pb(s) + SO42(aq) spongy lead Lead grids coated with PbSO4(s) During charging

15. Positive electrode : PbSO4(s) + 2H2O(l) PbO2(s) + 4H+(aq) + SO42(aq) + 2e spongy PbO2 Lead grids coated with PbSO4(s) During charging

16. Anode : Pb(s) + SO42(aq) PbSO4(s)+ 2e Lead grids coated with PbSO4(s) During discharging

17. Cathode : PbO2(s) + 4H+(aq) + SO42(aq) + 2e PbSO4(s) + 2H2O(l) Lead grids coated with PbSO4(s) During discharging

18. discharge Pb(s) + PbO2(s) + 2H2SO4(aq) 2PbSO4(s) + 2H2O(l) charge The cell diagram for the lead-acid accumulator is: Pb(s) | PbSO4(s) [PbO2(s) + 4H+(aq) + SO42–(aq)], [2PbSO4(s) + 2H2O(l)] | Pb(s) Overall reaction : -

19. discharge Pb(s) + PbO2(s) + 2H2SO4(aq) 2PbSO4(s) + 2H2O(l) charge Overall reaction : - PbSO4 is coated on the electrodes, The reversed processes are made possible.

20. discharge Pb(s) + PbO2(s) + 2H2SO4(aq) 2PbSO4(s) + 2H2O(l) charge Overall reaction : - The cell should be charged soon after complete discharge Otherwise, fine ppt of PbSO4 will become coarser and inactive, making the reversed process less efficient.

21. discharge Pb(s) + PbO2(s) + 2H2SO4(aq) 2PbSO4(s) + 2H2O(l) charge Overall reaction : - charge Pb(s) and PbO(s) are on different electrodes Direct reaction is not possible Porous partition is not needed

22. discharge Pb(s) + PbO2(s) + 2H2SO4(aq) 2PbSO4(s) + 2H2O(l) charge Overall reaction : - During discharging, H2SO4 is being used up The density of electrolyte solution  The charging/discharging status can be monitored by a hydrometer.

23. Q.21 Ecell = Eocathode – Eoanode = (1.69V) – (0.35V)=2.04V

24. At anode, Cd(s) + 2OH(aq) Cd(OH)2(s) + 2e At cathode, NiO(OH)(s) + H2O(l) + e Ni(OH)2(s) + OH(aq) Nickel-cadmium cells – Nicad cells Q.22(a)

25. Nickel-cadmium cells – Nicad cells Q.22(b) Overall reaction : - 2NiO(OH)(s) + Cd(s) + 2H2O(l)  2Ni(OH)2(s) + Cd(OH)2(s)

26. Nickel metal hydride cell (NiMH) Cathode : NiO(OH) Anode : MH(s) where M is a hydrogen-absorbing alloy. More environmentally friendly than NiCad cell due to the absence of Cd.

27. On discharging, Anode : - MH(s) + OH(aq) M(s) + H2O(l) + e +1 0 At cathode, NiO(OH)(s) + H2O(l) + e Ni(OH)2(s) + OH(aq) +3 +2 Nickel metal hydride cell (NiMH)

28. Nickel metal hydride cell (NiMH) Voltage : 1.2 V Electrolyte : KOH 2 to 3 times the capacity of an equivalent NiCad cell From 1100 mAh up to 8000 mAh.

29. Anode is graphite into which Li+ are inserted Lithium ion cell

30. Cathode is metal oxide into which Li+ are inserted. Lithium ion cell

31. During charging, Li+ moves from cathode to anode Lithium ion cell

33. During discharge, Li+ moves from anode to cathode

35. Voltage is 3.6/3.7V Three times that of NiCad or NiMH Much higher

36. Q.23 The anode of lithium cell is made of reactive lithium metal. If the lithium anode is exposed to moisture and air, vigorous reactions will occur. Thus, lithium ion cell is safer to use.

37. Continuous supply of fuel, H2

38. Continuous supply of oxygen No need for recharging

39. Other fuels such as hydrocarbon, alcohol, or glucose are possible

40. Ni(s) and NiO(s) are catalysts for the half-cell reactions

41. Fuel Cells • At anode: • H2(g) + 2OH–(aq)  2H2O(l) + 2e– • At cathode: • O2(g) + 2H2O(l) + 4e– 4OH–(aq) • Overall reaction: • 2H2(g) + O2(g)  2H2O(l)

42. Q.24 Maximum energy that can be used to do useful work = (2)(96485)(1.22) = 235 kJ mol1

43. Q.25 • To increase the mobility of OH/K+ to balance the extra charges built up in half-cells. • [OH(aq)]  quickly at anode • [OH(aq)]  quickly at cathode • 2. To increase the solubility of KOH

44. Anode : - CH4 + 2H2O CO2 + 8H+ + 8e Cathode : - 2O2 + 8H+ + 8e 4H2O Q.26 Overall reaction : - CH4 + 2O2 CO2 + 2H2O

45. Supplementary notes from HKDSE Chemistry

46. What is a fuel cell?

47. Fuel cell • It is a primary cell. • It converts the chemical energy of a continuous supply of reactants (a fuel and an oxidant) into electrical energy. • The products are removed continuously.

48. anode (−) cathode (+) porous Ni electrodes How a fuel cell works

49. hot KOH electrolyte ( 200°C) Fuel : H2 steam (exhaust) H2 hydrogen How a fuel cell works e− e− Anode : H2(g) + 2OH(aq)  2H2O(g) + 2e

50. Fuel : H2 steam (exhaust) H2 O2 hydrogen oxygen How a fuel cell works Oxidant : O2 e− e− H2 hydrogen Cathode : O2(g) + 2H2O(g) + 4e  4OH(aq)