Electrolysis of WaterPure water is an electrical insulator. Addition of a little bit of acid, base or salt (such as Na2SO4) in water, the soln. behaves as a good conductor. Electrolysis of such aq. solution results in the decomposition of water at both electrodes.
At pH=7 (neutral condition), the redox reactions are :
Reduction at Cathode :
4 H2O(l) + 4e = 2 H2(g) + 4OH-(aq) E0 = - 0.41v
Oxidation at Anode :
2 H2O(l) = 4 H+(aq) + O2(g) + 4e E0 = +0.82v
E0cell = E0cathode – E0anode = (-0.41) – (+0.82)
The calculation shows that the electrolytic cell potential of water is -ve. Therefore, the redox reactions are not possible without any external energy. To derive a spontaneous redox reaction, the cell potential should be +ve. So unlike voltaic cell, the electrolytic cell needs potential input.The minimum voltage necessary to bring about electrolysis should be greater than +1.23v, as the electrolytic cell potential of water at pH 7 is -1.23v.