CBSE Class 12-science Answered

 

The half-cell is represented as Hg/ Hg₂Cl₂ (salt)/ KCl (sat)
Calomel electrode can act either as anode or cathode depending on the other electrode used 
For Anode:- 2 Hg + 2Cl - Hg₂Cl₂ + 2e-

For Cathode:- Hg₂Cl₂ + 2e^-  → 2 Hg + 2Cl^-

Net reaction is:- Hg₂Cl₂ (s) + 2e- → 2 Hg + 2Cl^-

The electrode potential may be represented by the Nernst equation as
E = E^o – 0.059/n log [Cl -]² 
E = E^o – 0.0591/2 log [Cl-]²
E = E^o – 0.0591 log [Cl ] at 298 K

The potential of the calomel electrode depends on the concentration of KCl used.
The electrode potential decreases with increase in the concentration of chloride ions.
For 0.1N KCl E = 0.33V
1N KCl E = 0.28V
Saturated KCl E = 0.24V