1. Chemsheets AS006 (Electron arrangement)
22/09/2018
ELECTROCHEMISTRY

2. Galvanic Cell or Voltaic Animation (2 min)

3. Electrochemical Cells
spontaneous
redox reaction

4. Zn2+(aq) e–  Zn(s)

6. Electrochemical Cells
Cell Diagram
Zn (s) + Cu2+ (aq) Cu (s) + Zn2+ (aq)
[Cu2+] = 1 M & [Zn2+] = 1 M
Zn (s) | Zn2+ (1 M) || Cu2+ (1 M) | Cu (s)
anode
cathode

7. Zn  Zn2+ + 2 e- oxidation Cu2+ + 2 e-  Cu reduction
- electrode
anode
oxidation
+ electrode
cathode
reduction
electron flow
At this electrode the metal loses electrons and so is oxidised to metal ions.
These electrons make the electrode negative.
At this electrode the metal ions gain electrons and so is reduced to metal atoms.
As electrons are used up, this makes the electrode positive. Zn Cu
Zn  Zn e-
oxidation
Cu e-  Cu
reduction

9. ½ Equations and e- flow University Berkley (6 min)

11. Standard Conditions Concentration
1.0 mol dm-3 (ions involved in ½ equation)
Temperature
298 K
Pressure
100 kPa (if gases involved in ½ equation)
Current
Zero (use high resistance voltmeter)

12. S tandard
H ydrogen
E lectrode

13. University of Berkley S.H.E. 3 (min)

14. Emf = E = Eright - Eleft

17. Pt(s) | H2(g) | H+(aq) || Cu2+(aq) | Cu(s)

19. Non-rechargeable (primary) cells – Zinc-carbon
-0.80 V Zn(NH3) e-  Zn NH3
+0.70 V 2 MnO H e-  Mn2O3 + H2O
Standard cell
Short life
Determine: a) cell emf
b) overall reaction during discharge

20. Batteries Dry cell Leclanché cell Anode: Zn (s) Zn2+ (aq) + 2e-
Cathode:
2NH4 (aq) + 2MnO2 (s) + 2e Mn2O3 (s) + 2NH3 (aq) + H2O (l) +
Zn (s) + 2NH4 (aq) + 2MnO2 (s) Zn2+ (aq) + 2NH3 (aq) + H2O (l) + Mn2O3 (s)

21. Non-rechargeable (primary) cells – alkaline
-0.76 V Zn e-  Zn
+0.84 V MnO2 + H2O + e-  MnO(OH) + OH-
Determine: a) cell emf
b) overall reaction during discharge
Longer life

22. Non-rechargeable (primary) cells – lithium
Very long life
High voltage
Determine: a) cell emf
b) overall reaction during discharge

23. Batteries Mercury Battery Anode:
Zn(Hg) + 2OH- (aq) ZnO (s) + H2O (l) + 2e-
Cathode:
HgO (s) + H2O (l) + 2e Hg (l) + 2OH- (aq)
Zn(Hg) + HgO (s) ZnO (s) + Hg (l)

24. Rechargeable (secondary) cells
In non-rechargeable (primary) cells, the chemicals are used up so the voltage drops
In rechargeable (secondary) cells the reactions are reversible – they are reversed by applying an external current.
It is important that the products from the forward reaction stick to the electrodes and are not dispersed into the electrolyte.

25. Rechargeable (secondary) cells – Li ion
+0.60 V Li+ + CoO2 + e-  LiCoO2
-3.00 V Li+ + e-  Li
Rechargeable
Most common rechargeable cell
Determine: a) cell emf
b) overall reaction during discharge
c) overall reaction during re-charge

26. Rechargeable (secondary) cells – lead-acid
+1.68 V PbO2 + 3 H+ + HSO e-  PbSO4 + 2 H2O
-0.36 V PbSO4 + H e-  Pb + HSO4-
Determine: a) cell emf
b) overall reaction during discharge
c) overall reaction during re-charge
Used in sealed car batteries (6 cells giving about 12 V overall)

27. Rechargeable (secondary) cells – nickel-cadmium
+0.52 V NiO(OH) + 2 H2O + 2 e-  Ni(OH)2 + 2 OH-
-0.88 V Cd(OH) e-  Cd OH-
Determine: a) cell emf
b) overall reaction during discharge
c) overall reaction during re-charge

28. FUEL CELLS High efficiency (more efficient than burning hydrogen)
How is H2 made?
Input of H2/O2 to replenish so no need to recharge
+0.40 V O2 + 2 H2O + 4 e-  4 OH-
-0.83 V 2 H2O e-  H OH-
Determine: a) cell emf b) overall reaction

29. From wikipedia (public domain)

30. Batteries
A FUEL CELLis an electrochemical cell that requires a continuous supply of reactants to keep functioning
Anode:
2H2 (g) + 4OH- (aq) H2O (l) + 4e-
Cathode:
O2 (g) + 2H2O (l) + 4e OH- (aq)
2H2 (g) + O2 (g) H2O (l)

31. Pros & cons of cells
+ portable source of electricity
Pros & cons of non-rechargeable cells
+ cheap, small
– waste issues
Pros & cons of rechargeable cells
+ less waste, cheaper in long run
– still some waste issues
Pros & cons of fuel cells
+ water is only product
– most H2 is made using fossil fuels, fuels cells expensive