NCERT Solutions
Class 12 Chemistry
Electrochemistry

Q.18
Predict the products of electrolysis in each of the following:
(i) An aqueous solution of AgNO 3 with silver electrodes.
(ii) An aqueous solution of AgNO 3with platinum electrodes.
(iii) A dilute solution of H 2SO4 with platinum electrodes.
(iv) An aqueous solution of CuCl2 with platinum electrodes.
- At cathode:
The following reduction reactions compete to take place at the cathode.
Ag+ (aq) + e- à Ag(s); E0 = +0.80V
H+ (aq) + e- à (1/2) H2 (g); E0= 0.00V
The reaction with a higher value of E0 takes place at the cathode. Therefore, deposition of silver will take place at the cathode.
At anode:
The Ag anode is attacked by NO3- ions. Therefore, the silver electrode at the anode dissolves in the solution to form Ag+.
- At cathode:
The following reduction reactions compete to take place at the cathode.
Ag+ (aq) + e- à Ag(s); E0 = +0.80V
H+ (aq) + e- à (1/2) H2 (g); E0= 0.00V
The reaction with a higher value of E 0 takes place at the cathode. Therefore, deposition of silver will take place at the cathode.
At anode:
Since Pt electrodes are inert, the anode is not attacked by NO 3- ions. Therefore, OH− or NO3- ions can be oxidized at the anode. But OH − ions having a lower discharge potential and get preference and decompose to liberate O2.
OH- OH + e- 4OH- H2O + O2
(iii) At the cathode, the following reduction reaction occurs to produce H 2
gas.
H+ (aq) + e- à (1/2) H2 (g)
At the anode, the following processes are possible.
For dilute sulphuric acid, reaction (i) is preferred to produce O2 gas. But for concentrated sulphuric acid, reaction (ii) occurs.
(iv) At cathode:
The following reduction reactions compete to take place at the cathode.
Cu2+ (aq) + 2e- à Cu(s); E0 = +0.34V
H+ (aq) + e- à (1/2) H2 (g); E0= 0.00V
The reaction with a higher value of E 0 takes place at the cathode. Therefore, deposition of copper will take place at the cathode.
At anode:
The following oxidation reactions are possible at the anode.
Cl-(aq) à (1/2) Cl2 (g) +e-1; E0 = 1.36 V
2H2O (l) à O2 (g) + 4H+ (aq) + 4e- ; E0 =+1.23 V
At the anode, the reaction with a lower value of E 0 is preferred. But due to the over-potential of oxygen, Cl− gets oxidized at the anode to produce Cl 2 gas.