During the industrial extraction of aluminium by the electrolysis of molten aluminium oxide (Al2O3\text{Al}_2\text{O}_3Al2O3), oxide ions are discharged at the carbon anodes to produce oxygen gas. Which half-equation correctly represents the formation of oxygen gas at the anode?
2O2−→O2+2e−2\text{O}^{2-} \rightarrow \text{O}_2 + 2\text{e}^-2O2−→O2+2e−
O2+4e−→2O2−\text{O}_2 + 4\text{e}^- \rightarrow 2\text{O}^{2-}O2+4e−→2O2−
2O2−→O2+4e−2\text{O}^{2-} \rightarrow \text{O}_2 + 4\text{e}^-2O2−→O2+4e−
O2−→O+2e−\text{O}^{2-} \rightarrow \text{O} + 2\text{e}^-O2−→O+2e−
34 exam-style questions on OCR GCSE Chemistry Electrolysis. Each one has a worked solution and a mark scheme showing where the marks go.