Which gas is produced at the anode when chloride ions are present in the electrolyte during electrolysis?

• A. Oxygen
• B. Chlorine
• C. Hydrogen
• D. Nitrogen

Answer: (B)

At the anode, oxidation occurs, so the species that loses electrons there determines what gas is released. When chloride ions are present, they are oxidized to chlorine gas: two Cl– ions lose two electrons to form Cl2. So the gas evolved at the anode is chlorine.

Water would have to be oxidized to produce oxygen, but in a chloride-containing electrolyte the oxidation of chloride ions to Cl2 is the dominant process at the anode, giving chlorine gas as the product.