Why is an external emf of more than 2.2V

2Cl– (aq) + 2H2O(l) → 2OH– (aq) + H2(g) + Cl2(g)

The ΔGᶱ for this reaction is +422kJmol-1.

Using the formula, ΔGᶱ = -nFE°

E° = 422 x 103/2×96500

we get = -2.2V

For the reaction to take place the external voltage should be greater than 2.2V otherwise the reaction will not happen.