C6.3 The Extraction of Aluminium

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: why aluminium is extracted by electrolysis, how the cell works, the electrode reactions and why the process is expensive. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain why metals more reactive than carbon, such as aluminium, are extracted by electrolysis
• Describe the electrolysis of aluminium oxide dissolved in molten cryolite
• Explain why cryolite is used and why the carbon anodes must be replaced
• Explain why extracting metals by electrolysis is expensive
• Write half equations for the reactions at each electrode (Higher tier)

Key words

• Ore: Rock that contains enough of a metal compound to make extraction worthwhile. The main aluminium ore is bauxite.
• Aluminium oxide: Al₂O₃, purified from bauxite. It is an ionic compound containing Al³⁺ and O²− ions.
• Cryolite: An aluminium compound. Molten cryolite dissolves aluminium oxide so the cell can work at a much lower temperature.
• Anode: The positive electrode. In this cell the anodes are large blocks of carbon (graphite).
• Cathode: The negative electrode. Here it is the carbon lining of the steel tank.

Why use electrolysis?

Metals less reactive than carbon, such as zinc, iron and lead, can be extracted by heating their oxides with carbon: carbon takes the oxygen away (reduction). Aluminium is more reactive than carbon, so carbon cannot remove the oxygen from aluminium oxide. Instead aluminium is extracted by electrolysis of a molten compound. Other very reactive metals, such as sodium, potassium and magnesium, are also extracted by electrolysis.

Aluminium is the most common metal in the Earth’s crust, but it is always found in compounds. Its main ore, bauxite, is mostly aluminium oxide mixed with impurities such as iron(III) oxide. The ore is purified to give pure aluminium oxide before electrolysis.

The electrolysis cell

Aluminium oxide melts at over 2000 °C. Keeping a cell that hot would need a huge amount of energy. So the aluminium oxide is dissolved in molten cryolite. The mixture is liquid at about 950 °C, and the ions are still free to move.

Watch out: Cryolite is not a catalyst. It is a solvent: it dissolves the aluminium oxide so that the electrolyte is liquid at a lower temperature.

What happens at the electrodes

• Ions attracted; Cathode (−) Al³⁺; Anode (+) O²−
• What happens; Cathode (−) aluminium ions gain electrons (reduction); molten aluminium sinks to the bottom and is tapped off; Anode (+) oxide ions lose electrons (oxidation); oxygen gas forms
• Half equation (Higher tier); Cathode (−) Al³⁺ + 3e− → Al; Anode (+) 2O²− → O₂ + 4e−

Overall: aluminium oxide → aluminium + oxygen; 2Al₂O₃(l) → 4Al(l) + 3O₂(g).

The anodes are made of carbon. At 950 °C the oxygen produced reacts with the carbon to form carbon dioxide: C(s) + O₂(g) → CO₂(g). The anodes slowly burn away, so they must be replaced regularly. This adds to the cost and releases a greenhouse gas.

Higher tier: To balance electrons over the whole cell: 4Al³⁺ + 12e− → 4Al and 6O²− → 3O₂ + 12e−. The 12 electrons lost at the anode are gained at the cathode.

Why is the process expensive?

• Energy to melt the electrolyte: even with cryolite, the cell must be kept at about 950 °C.
• Electricity: a very large current is needed to produce the aluminium.
• Replacing the anodes: the carbon anodes burn away and must be replaced.
• Plants are often built near cheap electricity, e.g. hydroelectric power. Recycling aluminium needs only about 5% of the energy used to extract it from its ore.

Worked example: mass of aluminium from aluminium oxide

Mr of Al₂O₃ = (2 × 27) + (3 × 16) = 102. Of this, 2 × 27 = 54 is aluminium.
Percentage by mass of aluminium = 54 ÷ 102 × 100 = 52.9%
Mass of aluminium in 68 tonnes of aluminium oxide = 68 × 54 ÷ 102 = 36 tonnes

Answer: 36 tonnes of aluminium

Uses of aluminium: it has a low density, resists corrosion (a thin oxide layer protects it) and conducts electricity well, so it is used for aircraft, drinks cans, foil, window frames and overhead power cables.

Common mistakes

• Saying cryolite is a catalyst. It is a solvent that lowers the operating temperature.
• Saying aluminium oxide can be heated with carbon. Aluminium is more reactive than carbon, so this does not work.
• Saying the cathode is replaced. It is the carbon anodes that react with oxygen and burn away.
• Writing the oxide ion as O−. It is O²−, and oxygen gas is O₂.

Revision questions

Try these, then check your answers below each one.

Question 1. Why is aluminium extracted by electrolysis instead of by heating aluminium oxide with carbon? (Choose from: Aluminium is more reactive than carbon, Carbon is more reactive than aluminium, Aluminium oxide is a covalent compound, Electrolysis is cheaper than heating with carbon.)

Answer: Aluminium is more reactive than carbon

Working: Aluminium is above carbon in the reactivity series, so carbon cannot reduce aluminium oxide. Electrolysis is used even though it is expensive.

Question 2. Name the compound in which aluminium oxide is dissolved in the electrolysis cell.

Answer: cryolite

Working: Aluminium oxide is dissolved in molten cryolite, so the cell works at about 950 °C.

Question 3. True or false? The carbon anodes must be replaced regularly because they react with the oxygen formed on them.

Answer: True

Working: True. Oxygen formed at the anodes reacts with the hot carbon: C + O₂ → CO₂, so the anodes burn away.

Related lessons

• Previous lesson: C6.2 Changes at the Electrodes
• Next lesson: C6.4 Electrolysis of Aqueous Solution
• In this chapter: C6.1 Introduction to Electrolysis
• In this chapter: C6 RP Required Practical: Electrolysis

Continue in the full course

This lesson is section C6.3 (AGCSC2303) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.