6.3 The Extraction of Aluminium

A free GCSE Chemistry lesson for AQA (8462) from OBA School: why aluminium is extracted by electrolysis, the electrolysis cell, the electrode reactions and the uses of aluminium. 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 aluminium cannot be extracted from its ore by heating with carbon, using the reactivity series
• Describe the stages from bauxite ore to aluminium metal
• Describe the electrolysis of aluminium oxide dissolved in molten cryolite
• Explain why cryolite is used and why the anodes must be replaced
• (Higher tier) write half equations for the reactions at each electrode
• Calculate masses of aluminium produced, and explain why the process is expensive

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. Also called alumina.
• Cryolite: An aluminium compound. Molten cryolite dissolves aluminium oxide and lowers the operating 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 iron and zinc, can be extracted by heating their oxides with carbon. Aluminium is more reactive than carbon, so carbon cannot remove the oxygen from aluminium oxide. Instead, aluminium is extracted by electrolysis. Electrolysis uses large amounts of energy, so it is only used when a cheaper method will not work.

Key fact: Metals above carbon in the reactivity series (potassium to aluminium) are extracted by electrolysis of a molten compound. Aluminium is less reactive than magnesium but more reactive than zinc and iron.

Aluminium is the most common metal in the Earth’s crust, but it is always found combined in compounds. Its main ore, bauxite, is dug from the surface by open-cast mining. Bauxite is mostly aluminium oxide mixed with rocky impurities, especially iron(III) oxide. The ore is purified first: the impurities are removed as a brown waste slurry, which has to be stored safely in large lagoons. Heating the purified product gives pure aluminium oxide. Every stage needs energy.

The electrolysis cell

Aluminium oxide melts at over 2000 °C. Keeping that temperature would cost a huge amount of energy. So the aluminium oxide is dissolved in molten cryolite, which lets the cell run at about 950 °C. The ions are still free to move.

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⁺(l) + 3e− → Al(l); Anode (+) 2O²−(l) → O₂(g) + 4e−

Overall: aluminium oxide → aluminium + oxygen; 2Al₂O₃(l) → 4Al(l) + 3O₂(g). The aluminium is molten at the temperature of the cell, so it is shown as (l).

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

Exam tip: To balance the electrons in the Higher half equations, use 4Al³⁺ + 12e− → 4Al and 6O²− → 3O₂ + 12e−. The 12 electrons lost at the anode are gained at the cathode.

Cost, environment and uses

• Cost: a large electric current is needed for a long time, and energy is needed to keep the electrolyte molten. Plants are often built near cheap electricity, e.g. hydroelectric power.
• Environment: carbon dioxide from the anodes; mining bauxite; electricity may come from fossil fuels. Recycling aluminium uses only about 5% of the energy needed to extract it from its ore.
• Uses: low density, does not corrode (a thin oxide layer protects it) and a good conductor. Used for aircraft and car bodies, drinks cans, foil, window frames and overhead power cables.

Worked example: mass of aluminium from aluminium oxide

Mr of Al₂O₃ = (2 × 27) + (3 × 16) = 102
Percentage by mass of aluminium = (54 ÷ 102) × 100 = 52.9%
Mass of aluminium in 51 tonnes of aluminium oxide = 51 × 54 ÷ 102 = 27 tonnes

Answer: 27 tonnes of aluminium

Common mistakes

• Saying cryolite is a catalyst. It is a solvent that lowers the operating temperature.
• Saying aluminium oxide is heated with carbon. Aluminium is more reactive than carbon, so this does not work.
• Forgetting why the anodes are replaced: oxygen reacts with the carbon to make carbon dioxide.
• Writing the oxide ion as O−. It is O²−, and oxygen gas is O₂.
• Saying the cathode is replaced. It is the carbon anodes (positive electrodes) that burn away.

Revision questions

Try these, then check your answers below each one.

Question 1. Why is aluminium extracted by electrolysis instead of by heating its oxide with carbon? (Choose from: Aluminium is more reactive than carbon, Aluminium is less reactive than carbon, Aluminium oxide is a gas, Carbon is too expensive.)

Answer: Aluminium is more reactive than carbon

Working: Aluminium is above carbon in the reactivity series, so carbon cannot reduce aluminium oxide.

Question 2. Name the ore from which aluminium oxide is obtained.

Answer: bauxite

Working: Bauxite. It is purified to remove impurities such as iron(III) oxide, giving pure aluminium oxide.

Question 3. Why is aluminium oxide dissolved in molten cryolite? (Choose from: To lower the temperature needed, which saves energy, Cryolite is a catalyst for the reaction, Cryolite provides the aluminium ions, To stop the anodes burning.)

Answer: To lower the temperature needed, which saves energy

Working: The mixture is molten at about 950 °C, far below the melting point of aluminium oxide, so less energy is needed. Cryolite is not a catalyst.

Related lessons

• Previous lesson: 6.2 Changes at the Electrodes
• Next lesson: 6.4 Electrolysis of Aqueous Solutions
• In this chapter: 6.1 Introduction to Electrolysis
• In this chapter: 6.5 Required Practicals: Electrolysis

Continue in the full course

This lesson is section 6.3 (AGCCH0603) of our free GCSE Chemistry 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 Chemistry course.