The electrolysis of molten compounds
4 marks · NormalAluminium oxide can be electrolysed when molten.
State the names and formulas of the ions present in molten aluminium oxide.
Write the half-equation for the reaction at the cathode.
Write the half-equation for the reaction at the anode.
Identify the product formed at each electrode.
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(a) Aluminium ions (Al^{3+}) and oxide ions (O^{2-})
(b) Al^{3+} + 3e^- → Al
(c) 2O^{2-} → O_2 + 4e^-
(d) Aluminium at the cathode; oxygen at the anode
Common mistakes: Incorrect charge or formula for oxide ion (e.g., writing O⁻ instead of O²⁻); Wrong balance of electrons in half-equations; Swapping cathode and anode products
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Marking points
B1 (a) · 1 mark · States both ions present as aluminium (Al^{3+}) and oxide (O^{2-})
B2 (b) · 1 mark · Correct half-equation at the cathode: Al^{3+} + 3e^- → Al
B3 (c) · 1 mark · Correct half-equation at the anode: 2O^{2-} → O_2 + 4e^-
B4 (d) · 1 mark · Identifies aluminium forms at the cathode and oxygen forms at the anode
Worked solution
(a) The ions present in molten aluminium oxide are aluminium ions, Al^{3+}, and oxide ions, O^{2-}.
(b) The cathode half-equation is:
(c) The anode half-equation is:
(d) Aluminium is produced at the cathode, and oxygen is produced at the anode.