1. 06 REDOX EQM Primary Cell: non-rechargable cell
C. Y. Yeung (CHW, 2009) 06
REDOX EQM
Primary Cell, Secondary Cell & Fuel Cell
Primary Cell: non-rechargable cell
e.g. Zinc-carbon Cell
cathode
anode
absorb H2
electrolyte

2. Cell Diagram : p.02 Anode: Zn(s)  Zn2+(aq) + 2e-
Zn - C Cell
Cathode: 2NH4+(aq) + 2e-  2NH3(aq) + H2(g)
2MnO2(s) + H2(g)  Mn2O3(s) + H2O(l)
Cathode: 2NH4+(aq) + 2MnO2(s) + 2e-  2NH3(aq) + Mn2O3(s) + H2O(l)
Cell equation:
Zn(s) + 2NH4+(aq) + 2MnO2(s)  Zn2+(aq) + 2NH3(aq) + Mn2O3(s) + H2O(l)
Cell Diagram :
Zn(s) | Zn2+(aq) 
[2MnO2(s) + 2NH4+(aq)] , [2NH3(aq) + Mn2O3(s) + H2O(l)] | C(graphite)
~ Primary cells are not rechargable, because some of the products of the electrochemical rxns are no longer available in the system (i.e. H2). The reverse rxn cannot occur upon charging.

3. p.03
p. 235 Q.23 HKALE 2001 Paper 1 Q.4(b)

4. p.04
p. 230 Q.5 HKALE 1995 Paper 2 Q.3(c)

5. p. 230 Q.5 HKALE 1995 Paper 2 Q.3(c) --- continue

6. Secondary Cell: rechargable cell
p.06
Secondary Cell: rechargable cell V
e.g. Lead-Acid Accumulator e- e- Pb
PbSO4
PbO2
H2SO4(l) [electrolyte] +2 +4 Pb
coated with PbSO4(s)
coated with PbO2(s)
PbO2(s)
Pb(s)
PbSO4(s)
discharging
CATHODE
ANODE
reduction
H2SO4(l)
Pb + SO42-  PbSO4 + 2e-
PbO2 + SO H+ + 2e-  PbSO4 + 2H2O
oxidation
Cell Diagram:
[Pb(s)+ SO42-(aq)] | PbSO4(s) | H2SO4(l) |
[PbO2(s)+ SO42-(aq) + 4H+(aq)], [PbSO4(s) + 2H2O(l)] | Pb(s)

7. e- e- p.07 Lead-Acid Accumulator is RECHARGABLE, because …
~ All the products of the electrochemical rxns remain in the system. The reverse rxn can occur upon charging.
PbO2(s)
Pb(s)
PbSO4(s)
H2SO4(l)
oxidation e- e-
reduction
coated with PbSO4(s)
charging
i.e. PbO2(s) and Pb(s) are regenerated.
ANODE
CATHODE
PbSO4 + 2e- Pb + SO42-
PbSO4 + 2H2O  PbO2 + SO H+ + 2e-

8. Lead-Acid Accumulator
p.08
Lead-Acid Accumulator

9. p.09
p. 232 Q.12 HKALE 1998 Paper 2 Q.4(a)

10. (Output = Electricity????)
Fuel Cell : Combustion & Redox Rxn
Give out HEAT through Combustion.
Give out ELECTRICITY through Redox Rxns.
Use FUEL to give out ELECTRICITY through Redox Rxns !!
2 H2(g) + O2(g) H2O(l)
ELECTROLYSIS
(Input = Electricity)
COMBUSTION
(Output = Heat)
REDOX
(Output = Electricity????)
Advantages: Less Energy Loss + Compact + Less Pollution

11. How does FUEL CELL work? (1)
p.11
How does FUEL CELL work? (1)
Step 1:
Step 2:
(catalyst = Pt or Ni)

12. How does FUEL CELL work? (2)
p.12
How does FUEL CELL work? (2)
Step 3:
Step 4:

13. How does FUEL CELL work? (3)
p.13
How does FUEL CELL work? (3)
Step 5:

14. How does FUEL CELL work? (4)
p.14
How does FUEL CELL work? (4)
Anode: H2(g)  2H+(aq) + 2e-
Cathode: O2(g) + 4H+(aq) + 4e-  2H2O(l)
(catalysed by Ni)
(catalysed by NiO)
When Acid is used as electrolyte
Cell Diagram:
Ni(s) | H2(g) | 2H+(aq)  [O2(g) + 4H+(aq)], 2H2O(l)] | NiO(s)
Anode: H2(g) +2OH-(aq)  2H2O(l) + 2e-
Cathode: O2(g) + 2H2O(l) + 4e-  4OH-(aq)
(catalysed by Ni)
(catalysed by NiO)
When Alkali is used as electrolyte
Cell Diagram:
Ni(s) | [H2(g) + 2OH-(aq)], 2H2O(l)  [O2(g) + 2H2O(l)], 4OH-(aq) | NiO(s)

15. ** Advantages of Fuel Cell?
p.15
Fuel Cells
Article Reading:
A hybrid between “Generator” & “Battery” -- the “Fuel Cell”
** Advantages of Fuel Cell?

16. p.16
p. 236 Q.27(a) HKALE 2002 Paper 2 Q.3(b)

17. Assignment Next …. Eqm Completed! Book 2 finished!
Article Reading [due date: 25/5 (Mon)]
p.229 Q.12, 23 [due date: 25/5 (Mon)]
Eqm Completed! Book 2 finished!
Next ….
Book 3A p.2 – 26 (**p.8-9 **)