GCSE Chemistry · AQA 8462 · Topic 4.4.3
Electrolysis
Key points
- Electrolysis passes a current through an electrolyte (a molten or dissolved ionic compound). Positive ions move to the cathode (−) and negative ions to the anode (+).
- Molten lead bromide gives lead at the cathode and bromine at the anode.
- Aluminium is extracted by electrolysis of aluminium oxide dissolved in molten cryolite (lowering the melting point). Oxygen formed at the carbon anodes reacts with them, so the anodes must be replaced.
- In aqueous solutions, hydrogen is produced at the cathode if the metal is more reactive than hydrogen. Oxygen is produced at the anode unless the solution contains halide ions, which give the halogen.
- Electrolysing sodium chloride solution gives hydrogen, chlorine and sodium hydroxide solution.
- (Higher) Half equations: at the cathode ions gain electrons (reduction), e.g. ; at the anode ions lose electrons (oxidation), e.g. .
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Electrolysis uses electricity to break down ionic compounds. It is how reactive metals such as aluminium are extracted, and how chlorine, hydrogen and sodium hydroxide are made from salt solution.
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