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Question: "Explain why aluminium is extracted by electrolysis rather than carbon reduction." (4 marks)
Aluminium is more reactive than carbon, so it sits above carbon in the reactivity series. [1] This means carbon is not reactive enough to displace aluminium from its oxide — carbon cannot reduce aluminium oxide. [1] Electrolysis is used instead, which uses electrical energy to decompose the aluminium oxide: aluminium ions gain electrons at the cathode (Al³⁺ + 3e⁻ → Al). [1] To allow the ions to move, the aluminium oxide is dissolved in molten cryolite, which reduces the operating temperature from 2072°C to approximately 950°C, significantly reducing energy costs. [1]
Examiner note: The four marks correspond to: Al is more reactive than carbon, carbon cannot reduce Al₂O₃, electrolysis decomposes the oxide (with cathode equation or mechanism), and the role of cryolite in lowering the melting point. Simply stating "electrolysis is used because carbon doesn't work" earns only 1 mark. The cryolite point is often missed — it is a stand-alone mark that rewards students who understand the practical challenge of the high melting point.
→ See also: Electrolysis of Aqueous Solutions — what happens when water is present and ions compete for discharge.
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