0606REDOX EQMREDOX EQM Primary Cell, Secondary Cell & Fuel Cell

C. Y. Yeung (CHW, 2009)p.01

Primary Cell: Primary Cell: non-rechargable cellnon-rechargable celle.g. Zinc-carbon Cell

cathode

anode

absorb H2

electrolyte

p.02Anode: Zn(s) Zn2+(aq) + 2e-

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 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.

Cell equation:

Zn(s) + 2NH4+(aq) + 2MnO2(s) Zn2+(aq) + 2NH3(aq) + Mn2O3(s) + H2O(l)

Zn - C Cell

p.03

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

p.04

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

p.05

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

p.06Secondary Cell: Secondary Cell: rechargable cellrechargable celle.g. Lead-Acid Accumulator

Pb

PbSO4

PbO2

0

+2

+4

H2SO4(l) [electrolyte]

HH22SOSO44(l)(l)

coated with coated with PbSOPbSO44(s)(s) PbPb

PbPb

V

coated with coated with PbOPbO22(s)(s)

PbOPbO22(s)(s)

Pb(s)Pb(s)

PbSOPbSO44(s)(s)

reductionreduction

oxidationoxidation

dischargingdischarging

Pb + SO42- PbSO4 + 2e-

e- e-

PbO2 + SO42- + 4H+ + 2e- PbSO4 + 2H2O

ANODECATHODE

Cell Diagram:

[Pb(s)+ SO42-(aq)] | PbSO4(s) | H2SO4(l) |

[PbO2(s)+ SO42-(aq) + 4H+(aq)], [PbSO4(s) + 2H2O(l)] | Pb(s)

p.07

HH22SOSO44(l)(l)

PbSO4 + 2e- Pb + SO42-

e-e-

PbSO4 + 2H2O PbO2 + SO42- + 4H+ + 2e-

ANODECATHODE

PbOPbO22(s)(s)

Pb(s)Pb(s)

PbSOPbSO44(s)(s)

chargingcharging

reductionreduction

oxidationoxidation

coated with coated with PbSOPbSO44(s)(s)

coated with coated with PbSOPbSO44(s)(s)

Lead-Acid Accumulator is RECHARGABLE, because …~ All the products of the electrochemical rxns remain in the system.

The reverse rxn can occur upon charging.

i.e. PbO2(s) and Pb(s) are regenerated.

p.08

Lead-Acid Accumulator

p.09

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

2 H2(g) + O2(g) 2 H2O(l)

p.10

Fuel Cell : Fuel Cell : Combustion & Redox RxnCombustion & Redox Rxn

Give out HEAT through Combustion.

Give out ELECTRICITY through Redox Rxns.

Use FUEL to give out ELECTRICITY through Redox Rxns !!

ELECTROLYSIS

(Input = Electricity)

REDOXREDOX

(Output = Electricity????)(Output = Electricity????)

COMBUSTION

(Output = Heat)

Advantages: Less Energy Loss + Compact + Less Pollution

p.11

How does FUEL CELLFUEL CELL work? (1)Step 1:

Step 2:

(catalyst = Pt or Ni)

p.12

How does FUEL CELLFUEL CELL work? (2)Step 3:

Step 4:

p.13

How does FUEL CELLFUEL CELL work? (3)Step 5:

p.14

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

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

How does FUEL CELLFUEL CELL work? (4)

Cell Diagram:

Ni(s) | H2(g) | 2H+(aq) [O2(g) + 4H+(aq)], 2H2O(l)] | NiO(s)

Cell Diagram:

Ni(s) | [H2(g) + 2OH-(aq)], 2H2O(l) [O2(g) + 2H2O(l)], 4OH-(aq) | NiO(s)

p.15

Fuel Cells

Article Reading:

A hybrid between “Generator” & “Battery” -- the “Fuel Cell”

** Advantages of Fuel Cell?

p.16

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

p.17

AssignmentAssignment

Article Reading Article Reading [due date: 25/5 (Mon)] [due date: 25/5 (Mon)]

Next ….Next ….Book 3A p.2 – 26 (**p.8-9 **)Book 3A p.2 – 26 (**p.8-9 **)

p.229 Q.12, 23 p.229 Q.12, 23 [due date: 25/5 (Mon)] [due date: 25/5 (Mon)]

Eqm Completed! Book 2 finished!Eqm Completed! Book 2 finished!