1. Chemistry

2. Session
Electrochemistry - 3

3. Session Objective Nernst equation
Equilibrium constant and Nernst equation
Primary cell(Batteries)
Secondary cell(Batteries)
Fuel Cell
Corrosion and its prevention.

4. Nernst Equation For a general reduction reaction,
The Nernst equation can be written as
(At 298K)
Where n = Number of electrons involved
[Mn+] = molar concentrations at 298K

5. Illustrative Example
Calculate the electrode potential at a copper electrode dipped in a 0.1M solution of copper sulphate at 250C . The standard potential of Cu2+/Cu system is 0.34 volt at 298 K.
Solution:
Cu2+ + 2e-  Cu

6. Equilibrium constant from Nernst equation
Consider the following cell reaction in equilibrium then
Then Nernst equation is:
Electrode potential
Reduction half reaction
Equilibrium point
Oxidation half reaction
Progress of reaction

7. Illustrative Example
Calculate the equilibrium constant of the reaction:
Cu(s)+2Ag+(aq.)Cu2+(aq.)+Cu(s) E0=0.46 V
Solution :

8. Electrochemical cell and Gibbs Energy of the reaction

9. Illustrative Example
Calculate DG° for Zn-Cu cell at standard state conditions
[Given = –0.76 V, = V ]
Solution
= V – (–0.76 V) = 1.10 V
DG° = –nFE°
= 2×96500×1.10
= –212.3 kJ mol–1

10. Commercial Cells
Primary Cell
Dry Cell

11. Commercial Cells
The oxidation taking place at the negative zinc electrode.
Anode:
The reduction takes place at positive electrode
Cathode:
The net cell reaction is
The emf of the cell is about 1.45 V.

12. Secondary Cells Lead storage battery
Net cell reaction is reversible. Hence, it can be recharged.

13. Lead storage battery
At anode:
At cathode:
Overall reaction:

14. Secondary Cell
In the above equation H2SO4 is used up during the discharge.During recharging the reactions are the reverse of those that occurs during discharge.
At cathode:
At anode:
Overall reaction:

15. Fuel Cells
Galvanic cells which converts energy of combustion of fuel like hydrogen, methane and methanol etc. directly into electrical energy are called fuel cells.

16. Example
One of the most successful fuel cell uses hydrogen and oxygen reaction to form water.
At cathode:
O2(g)+2H2O(l)+4e-  4OH-(aq.)
At anode:
4H2O(l)+4e-  2H2 + 4OH-(aq.)
Overall cell reaction is:
2H2(g)+O2(g)  2H2O(l)
Efficiency of fuel cell is 70% much more as compared thermal plants(40%).

17. Corrosion
Process of slowly eating away of the metal due to attack of atmospheric gases on the surface of the metal.
Examples of corrosion
Rusting of iron
Tarnishing of silver
Development of green coating on copper and bronze, etc.

18. Corrosion

19. Methods of preventing corrosion
Barrier protection
Using anti rust solutions
Sacrificial protection
For example iron surface is covered with a metal which has higher tendency to get oxidized (larger negative value of standard reduction potential) than iron.
Zinc is used for covering iron and the process is called galvanization.

20. Illustrative Example
The standard reduction potentials of Sn+2/Sn and Zn+2/Zn are respectively –0.14V, -0.76V. Predict whether the corrosion of tin can be prevented by coating with zinc or not.
Solution :
Zinc lies above tin in the electrochemical series, therefore it has a lower reduction potential than tin.This property is employed to prevent corrosion of tin by coating it with zinc as zinc acts as a sacrificial electrode.

21. Thank you