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Half Equation

Key Stage 4

Meaning

A half equation is a type of symbol equation which shows how individual ions gain or lose electrons at an electrode during electrolysis.

About Half Equations

In half equations only one ion is considered at a time, or the changes at one electrode.
Half equations use the letter 'e' to represent electrons.
Half equations can be taken from balanced symbol equations.
The total charge on each side of a half equation must add to zero.
The number of each element must be the same on both sides of a half equation.

Examples

Balanced Symbol Equation 2Li2O(l) → 4Li(l) + O2(g) CuCl2(aq) → Cu(s) + Cl2(g) 2H2O(l) → 2H2(g) + O2(g)
Half Equation at cathode Li+ + e- → Li Cu+2(aq) + 2e- → Cu(s) 2H+(aq) + 2e- → H2(g)
Half Equation at anode 2O-2 → O2 + 4e- 2Cl-(aq) → Cl2(g) + 2e- 4OH- → 2H2O(l) + O2(g) + 4e-

References

AQA

Half equations, pages 104, 289-91, GCSE Chemistry, Hodder, AQA
Half equations, pages 142-144, GCSE Combined Science Trilogy; Chemistry, CGP, AQA
Half equations, pages 167-169, GCSE Chemistry, CGP, AQA
Half equations, pages 206, 255-7, GCSE Combined Science Trilogy 1, Hodder, AQA
Half equations, pages 59, 65, GCSE Chemistry; The Revision Guide, CGP, AQA
Half-equations, pages 87, 104-105, 107, 108, 123, GCSE Chemistry; Third Edition, Oxford University Press, AQA

Edexcel

Half equations, pages 110-112, GCSE Combined Science; The Revision Guide, CGP, Edexcel
Half equations, pages 134, 135, GCSE Chemistry, CGP, Edexcel
Half equations, pages 210, 225, 231, GCSE Combined Science, Pearson Edexcel
Half equations, pages 48-50, GCSE Chemistry; The Revision Guide, CGP, Edexcel
Half equations, pages 66, 81, 87, GCSE Chemistry, Pearson, Edexcel

OCR

Half equations, pages 37, 47, 48, Gateway GCSE Chemistry; The Revision Guide, CGP, OCR
Half equations, pages 94, 111, 124, Gateway GCSE Chemistry, Oxford, OCR