Electrolysis of Water

        Pure 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

            E⁰_cell  = E⁰_cathode – E⁰_anode =  (-0.41) – (+0.82) 
                                                    = -1.23v

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.