Assertion: Electrolysis of NaCl solution gives Cl₂ gas at anode instead of O₂.

Reason: Formation of oxygen at anode requires overvoltage.

Assertion and Reason both are correct statements and reason is the correct explanation for assertion.

Formation of O₂ has lower value of E^o than formation of Cl₂ even than it is not formed because it requires overvoltage.

Overvoltage: When an e- transfers from electrode to solution or vice-versa then certain activation energy (due to transfer from solid to liquid state or vice-versa) is involved, which is termed as overvoltage. It can be as high as 1V for O₂.