Aluminium

Aluminium (Al) is the most abundant metal in the Earth’s crust. Its extraction by electrolysis and its useful properties make it a frequent 0620 exam topic.

Where it appears in 0620

Aluminium earns its place in the syllabus mainly because it is the standard example of a reactive metal that must be won by electrolysis:

• Extraction of metals: aluminium sits above carbon in the reactivity series, so it cannot be reduced by carbon and is extracted electrolytically.
• Electrolysis: the molten Al2O3 / cryolite cell is examined for the electrolyte, both half-equations, and why the anodes burn away. See the full walk-through in extracting aluminium by electrolysis.
• Oxides: aluminium oxide is amphoteric, reacting with both acids and alkalis.
• Alloys and uses: low density plus corrosion resistance explain aircraft, cables, and foil.

Position in the periodic table

• Symbol: Al
• Atomic number: 13
• Group 13 (III), Period 3
• Electron configuration: 2, 8, 3
• Forms Al3+ ions by losing 3 electrons

Physical properties

Property	Detail
Appearance	Silver-white, shiny
Density	Low (2.7 g/cm3) — about one-third that of steel
Melting point	660 C
Conductivity	Good conductor of heat and electricity
Malleability	Highly malleable and ductile

The protective oxide layer

Aluminium is high in the reactivity series (above zinc and iron), yet it resists corrosion in practice. A thin, tough layer of aluminium oxide (Al2O3) forms the moment the surface meets air, sealing the metal underneath and preventing further reaction. This is the reason aluminium does not corrode the way iron rusts, even though it is the more reactive metal.

Extraction by electrolysis

Aluminium is extracted from its purified ore, bauxite (Al2O3), by electrolysis.

Setup

Component	Detail
Electrolyte	Molten Al2O3 dissolved in cryolite (Na3AlF6)
Cathode	Carbon lining of the cell
Anode	Carbon (graphite) rods
Temperature	~950 C

Electrode reactions

At the cathode (reduction): Al3+ + 3e- -> Al

Molten aluminium collects at the bottom of the cell and is tapped off.

At the anode (oxidation): 2O2- -> O2 + 4e-

Oxygen is produced. At the operating temperature it reacts with the hot carbon anodes: C(s) + O2(g) -> CO2(g). The anodes slowly burn away and must be replaced.

Why extraction is expensive

• Electrolysis needs a continuous, large supply of electricity
• The carbon anodes are burnt away and replaced regularly
• The high temperature must be maintained
• Purifying the bauxite ore adds cost

Reactivity

Once the oxide layer is removed, aluminium reacts readily:

• With dilute hydrochloric acid: 2Al(s) + 6HCl(aq) -> 2AlCl3(aq) + 3H2(g)
• With sodium hydroxide solution: 2Al(s) + 2NaOH(aq) + 6H2O(l) -> 2NaAl(OH)4(aq) + 3H2(g) (aluminium is amphoteric)
• Thermite reaction: 2Al(s) + Fe2O3(s) -> Al2O3(s) + 2Fe(l) (a highly exothermic displacement)

Uses

Use	Property
Aircraft and vehicles	Low density, strong when alloyed
Overhead power cables	Good conductor, low density
Food packaging (foil)	Malleable, non-toxic, corrosion resistant
Cooking pans	Good thermal conductor
Window frames	Resistant to corrosion (oxide layer)

Key facts at a glance

Fact	Detail
Symbol	Al
Proton number	13
Group / Period	13 (III) / 3
Ion formed	Al3+
Extraction	Electrolysis of molten Al2O3 in cryolite
Key property	Low density with a self-repairing oxide layer

Common exam mistakes

• Confusing low reactivity with corrosion resistance. Aluminium is reactive (above zinc and iron); it survives because of the Al2O3 layer, not because it is unreactive.
• Balancing the cathode half-equation wrongly. Al3+ gains three electrons: Al3+ + 3e- -> Al. Writing “2e-” is a common slip.
• Saying carbon is not used “because it is too expensive”. Carbon cannot reduce aluminium oxide at all — aluminium is above carbon in the reactivity series. It is a chemistry limit, not a cost choice.
• Mis-stating cryolite’s job. Cryolite lowers the melting point of the aluminium oxide so the cell runs near 950 C instead of ~2000 C, saving energy. “Cryolite conducts electricity” scores nothing.

Exam-style questions

Aluminium is extracted by electrolysis of aluminium oxide dissolved in molten cryolite. Name the product formed at the negative electrode, write the ionic half-equation for its formation, and explain why cryolite is added. (4 marks)

Mark scheme

• aluminium (metal) is formed at the negative electrode (cathode) [1]
• Al3+ + 3e- -> Al [1]
• cryolite lowers the melting point of the aluminium oxide [1]
• so less energy / heat is needed / the cell runs at a lower temperature, reducing cost [1]

Examiner note: the most-lost mark is the half-equation — the electrons must balance the 3+ charge on Al3+.

Aluminium is more reactive than iron, yet aluminium objects corrode far less than iron objects. Explain why. (2 marks)

Mark scheme

• aluminium reacts with oxygen in the air to form a layer of aluminium oxide on its surface [1]
• this layer is tough and unreactive, stopping oxygen and water reaching the metal underneath, so corrosion stops [1]

Examiner note: candidates who write “aluminium is unreactive” cannot score — the mark is for the oxide layer, not low reactivity.