Chemicals of the Natural Environment.

C5 Lesson 8

Learning objective: To explain how aluminium is extracted by electrolysis.

• Must: Know where aluminium comes from.• Should: Know the main processes involved in

the electrolysis of aluminium oxide.• Could: Explain why the electrolysis of aluminium

oxide is a redox reaction.• Starter: Give the properties and uses of

aluminium.• Keywords: Bauxite, cryolite, molten, redox and

expensive.

Aluminium is a very common element. In fact it is the third most abundant element in the Earth's crust.

The Eros Statue in Piccadilly Circus, London was built in 1893. Now we would never build a statue out of a cheap metal like aluminium but at that time aluminium was a very expensive metal (more expensive than gold)

Why was aluminium so expensive??

Piccadilly Circus

The Eros Statue

Properties and uses of aluminium.

• Properties:• Conducts heat and electricity well. It has

a low density for a metal. It does not corrode.

• Uses:• Aeroplanes, window frames, foil, drinks

cans, electricity cables and pans.

Where does aluminium come from?

• Aluminium is extracted from its ore bauxite.

• Bauxite is mainly aluminium oxide.

• Formula Al2O3

A bauxite mine in western Australia

Why is electrolysis used?

• Where is aluminium in the reactivity series?

• Near the top. It is quite a reactive metal.

Why is electrolysis used?

• Why can’t it be extracted using carbon?

• It is more reactive than carbon and so carbon cannot remove the oxygen (reduction).

Key features of the electrolysis of aluminium oxide.

• Aluminium oxide is molten.• Oxygen is formed at the graphite anode.• The anodes are gradually worn away by

oxidation.• Aluminium is formed at the graphite cathode.• The process has a high electrical energy

requirement.

Electrolysis of aluminium oxide

Electrolysis of Aluminium OxideElectrolysis of Aluminium OxideElectrolysis of Aluminium OxideElectrolysis of Aluminium Oxide

Electrolysis of Aluminium Oxide

Electrode reactions.

• At the cathode:• Al3+ + 3e- Al

• Aluminium ions are reduced (gain electrons) to aluminium atoms .

• At the anode:

• 2O2- - 4e- O2

• Oxygen ions are oxidised (lose electrons) to oxygen molecules.

Understanding the process.

• Why must the aluminium oxide be molten?

• Why are the anodes gradually worn away?

• Why is cryolite used?• Why is aluminium expensive?

Answers

• The aluminium oxide must be molten for electrolysis to take place. When molten the ions are free to move, in the solid form they are fixed.

• The oxygen formed at the anode reacts with the carbon anode to from carbon dioxide.

Answers

• Cryolite lowers the melting point and so saves energy.

• The electrolytic process requires a lot of electrical energy. Most aluminium extraction plants are located near power stations, usually hydroelectric plants.