Brine is electrolysed by using inert electrodes. The reaction at the anode is:

1. Cl-(aq) → \(\frac{1}{2}\)Cl2(g) + e-\(E_{cell}^{o}\) = 1.36 V

2. 2H2O(l) → O2(g) + 4H+ +  4 e-;  \(E_{cell}^{o}\) = 1.23 V

3. Na+(aq) + e- → Na(s); \(E_{cell}^{o}\) = 2.71 V

4. H+(aq) + e-\(\frac{1}{2}\)H2(g); \(E_{cell}^{o}\) = 0.00 V


 
 


HINT: Cl2 produces at anode.
Explanation:
STEP 1:
Brine is electrolysed by using inert electrodes. The possible reactions occurring at anode are : 
Cl-aq12Cl2g+e-;                                                       ECell=1.36V
2H2OlO2g+ 4H+ + 4e- ;                                            ECell= 1.23 V

STEP 2:
The reaction at anode with lower value of E° is preferred and therefore water should get oxidised in preference to Cl-(aq). However, Cl2 is produced instead of O2.
This unexpected result is explained on the basis of the fact that water needs greater voltage for oxidation to O2 (as it is kinetically slow process) than that needed for oxidation of Cl-ions to Cl2