Electrolysis in Hall-Héroult process for aluminium extraction

Steps:

• Aluminium oxide dissolves in molten cryolite to form Al³⁺ and O²⁻ ions.
• At cathode, Al³⁺ ions gain electrons: Al³⁺ + 3e⁻ → Al (reaction 1, reduction).
• At anode, O²⁻ ions react with carbon: 2O²⁻ + C → CO₂ + 4e⁻ (reaction 2, oxidation).
• Reaction 3 likely involves direct O₂ evolution, which does not occur due to carbon anode reaction.

Why B is correct:

• Reactions 1 and 2 represent the actual electrode processes, as defined by the Hall-Héroult electrolysis where carbon anodes prevent O₂ formation and produce CO₂ instead.

Why the others are wrong:

• A includes 3, but O₂ evolution (3) is incorrect as carbon reacts to form CO₂.
• C omits 2, ignoring the essential anode oxidation with carbon.
• D excludes 1, missing the key cathodic reduction of Al³⁺ to aluminium metal.

Final answer: B