Question

Why electrolysis of NaBr and NaI gives Br2 and I2 respectively while that of NaF gives O2 instead of F2?

Answer 1

Any product that is formed at the cathode and the anode on electrolysis of an aqueous solution depends on the redox potential of the reactions that can occur at the respective electrodes. 

The reactions at anode involve oxidation. 

The e0 value for the oxidation of both Br- ion and I- ion are more positive than that of water.  

We will compare the redox potentials of the possible reactions that can take place. Because the redox potential of Na+ is lower than that of water which means that Na+ has a lower tendency to get reduced than water. So the reaction with a higher reduction potential will occur at the cathode and as water has a high reduction potential, so the reaction will be formed at cathode forming water.