In the industrial Hall-Héroult process, aluminium is extracted by the electrolysis of alumina (Al2O3\text{Al}_2\text{O}_3Al2O3) dissolved in molten cryolite (Na3AlF6\text{Na}_3\text{AlF}_6Na3AlF6).
Which statement correctly explains why this molten mixture can act as an electrolyte and describes the chemical change occurring at the cathode?
The mixture contains delocalised electrons that carry the current through the liquid, allowing Al3+\text{Al}^{3+}Al3+ ions to undergo reduction to Al\text{Al}Al at the cathode.
The mixture contains mobile ions (such as Na+\text{Na}^+Na+, Al3+\text{Al}^{3+}Al3+, F−\text{F}^-F−, and O2−\text{O}^{2-}O2−) that carry the current, and Al3+\text{Al}^{3+}Al3+ ions undergo reduction to Al\text{Al}Al at the cathode.
The mixture contains mobile ions (such as Na+\text{Na}^+Na+, Al3+\text{Al}^{3+}Al3+, F−\text{F}^-F−, and O2−\text{O}^{2-}O2−) that carry the current, and O2−\text{O}^{2-}O2− ions undergo oxidation to O2\text{O}_2O2 at the cathode.
The mixture contains neutral Al2O3\text{Al}_2\text{O}_3Al2O3 molecules that migrate to carry the current, and Al3+\text{Al}^{3+}Al3+ ions undergo oxidation to Al\text{Al}Al at the cathode.
83 exam-style questions on Edexcel GCSE Chemistry 4.2 Electrolytic processes, covering 4.2.1 Electrolytes and the process of electrolysis, 4.2.2 Products of electrolysis using inert electrodes, 4.2.3 Half equations, oxidation and reduction in electrolysis, and 4.2.4 Electrolysis of copper sulfate and purifying copper. Each one has a worked solution and a mark scheme showing where the marks go.