Aluminium Oxide

Aluminium oxide earns exam marks in three separate topics: as the feedstock for extracting aluminium by electrolysis, as an amphoteric oxide, and as the invisible layer that explains why a reactive metal resists corrosion.

Key facts

Property	Detail
Formula	Al2O3
Relative formula mass (Mr)	102 (Al 27, O 16)
Bonding / structure	Giant ionic lattice (Al3+ and O2- ions)
Melting point	about 2072 C (very high — high ionic charges)
Classification	Amphoteric oxide (Supplement)
Main exam roles	Ore for aluminium extraction, amphoteric oxide, protective layer

Where this compound appears in 0620

• Electrolysis (Topic 4): the molten Al2O3 / cryolite cell, with half-equations at each electrode.
• Extraction of metals (Topic 10): why aluminium, being above carbon in the reactivity series, must be extracted electrolytically rather than by reduction with carbon.
• Oxides (Topic 8): amphoteric classification.
• Corrosion: the protective oxide layer, contrasted with flaky rust.

Reactions you must know

Extraction by electrolysis (Al2O3 dissolved in molten cryolite):

Aluminium oxide melts at about 2072 C because the small, highly charged Al3+ and O2- ions attract each other strongly. Melting it directly would cost too much energy, so it is dissolved in molten cryolite to bring the operating temperature down to roughly 950 C.

• Cathode (negative): Al3+ + 3e- -> Al
• Anode (positive): 2O2- -> O2 + 4e-

The oxygen produced attacks the hot carbon anodes, so they burn away and must be replaced: C(s) + O2(g) -> CO2(g). See extracting aluminium by electrolysis for the full cell.

Amphoteric behaviour — with acids:

Al2O3(s) + 6HCl(aq) -> 2AlCl3(aq) + 3H2O(l) Al2O3(s) + 3H2SO4(aq) -> Al2(SO4)3(aq) + 3H2O(l)

with alkalis:

Al2O3(s) + 2NaOH(aq) -> 2NaAlO2(aq) + H2O(l) (sodium aluminate)

Why aluminium resists corrosion

Exposed to air, aluminium instantly forms a thin, coherent layer of Al2O3 across its surface. Unlike rust, this layer does not flake off — it seals the metal underneath from oxygen and water, so the reaction stops. This is why aluminium is used for window frames, drink cans and aircraft bodies despite sitting high in the reactivity series.

Worked exam question

Aluminium is extracted by the electrolysis of aluminium oxide dissolved in molten cryolite.

(a) Write the ionic half-equations for the reactions at the cathode and at the anode. (2 marks)

(b) Explain why the carbon anodes must be replaced regularly. (2 marks)

(c) State why the aluminium oxide is dissolved in cryolite. (1 mark)

Mark scheme

• (a) Cathode: Al3+ + 3e- -> Al [1]; Anode: 2O2- -> O2 + 4e- [1]
• (b) Oxygen is produced at the anode and reacts with the hot carbon [1]; forming carbon dioxide (C + O2 -> CO2), so the anode gradually burns away [1]
• (c) It dissolves the aluminium oxide and lowers the melting point / operating temperature, saving energy [1]

Examiner note: in (a) the electrons must balance the charge — a very common error is writing 2O2- -> O2 + 2e- (should be 4e-). In (b), “the anode gets used up” alone scores nothing; you must name oxygen reacting with carbon to form CO2.

Common exam mistakes

• Unbalanced anode half-equation. 2O2- -> O2 + 4e- has four electrons; losing count and writing 2e- loses the mark.
• Saying cryolite “reacts with” or “reduces” the ore. Cryolite is a solvent: it dissolves Al2O3 and lowers the melting point. It is not consumed and does not reduce anything.
• Claiming Al2O3 is basic or inert. It is amphoteric — it reacts with sodium hydroxide (Al2O3 + 2NaOH -> 2NaAlO2 + H2O) as well as with acids.
• Confusing the oxide layer with rust. The Al2O3 layer is coherent and protective, so it stops corrosion; rust is flaky and lets corrosion continue.