In the electrolysis of aqueous sodium chloride with sufficient chloride ion concentration, what are the primary products at the cathode and anode, respectively?

• A. Hydrogen gas at the cathode and chlorine gas at the anode
• B. Hydrogen gas at the cathode and oxygen gas at the anode
• C. Sodium metal at the cathode and chlorine gas at the anode
• D. Chlorine gas at the cathode and hydrogen gas at the anode

Answer: (A)

In this setup, the cathode drives reduction and the anode drives oxidation. The solution contains water and a lot of chloride, so the easier process at the cathode is the reduction of water to hydrogen gas (water takes electrons to form H2 and OH−). Sodium ions don’t get reduced to metal in aqueous solution because water is reduced first, so hydrogen gas is produced at the cathode.

At the anode, oxidation happens. Chloride ions are readily oxidized to chlorine gas, and this competes successfully with water oxidation to oxygen when chloride concentration is high. So chlorine gas evolves at the anode. If chloride were scarce, oxygen from water oxidation could appear instead.