Half Equations in Electrolysis
Master Half Equations in Electrolysis for GCSE Chemistry with this free worksheet and full mark scheme — Higher-tier exam-style questions with worked answers for AQA, Edexcel and OCR. A Higher topic: write half equations showing electrons gained at the cathode and lost at the anode, e.g. 2H⁺ + 2e⁻ → H₂.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
This is a Higher tier only topic, so there's no Foundation paper — only the Higher worksheet and mark scheme below.
Topic overview
Half equations show what happens to electrons at each electrode during electrolysis. This is a Higher-only topic.
At the cathode, positive ions gain electrons, so electrons appear on the left-hand side. For example: Pb²⁺ + 2e⁻ → Pb. This is reduction.
At the anode, negative ions lose electrons, so electrons appear on the right-hand side. For example: 2Br⁻ → Br₂ + 2e⁻. This is oxidation. The number of electrons must balance the charge, and for halogens and oxygen the product is a molecule, so two ions are needed.
Revision notes
Cathode half equations
Positive ions gain electrons, so electrons are written on the left.
Pb²⁺ + 2e⁻ → Pb. Na⁺ + e⁻ → Na. Al³⁺ + 3e⁻ → Al. The number of electrons equals the charge on the ion.
Anode half equations
Negative ions lose electrons, so electrons are written on the right.
2Br⁻ → Br₂ + 2e⁻. 2Cl⁻ → Cl₂ + 2e⁻. Two ions are needed because the halogen product is a diatomic molecule.
Balancing charge
The total charge must be the same on both sides.
For oxygen at the anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻. Counting the charges on each side is the reliable way to check a half equation is correct.
Key points
- Half equations show electron transfer at each electrode.
- At the cathode electrons are on the left.
- At the anode electrons are on the right.
- The number of electrons matches the ion charge.
- Halogens need two ions to form a molecule.
- Charge must balance on both sides.
Worked examples
Example 1
Write the half equation for the formation of lead at the cathode from Pb²⁺ ions. [2 marks]
Model answer
Pb²⁺ + 2e⁻ → Pb. Correct electrons on the left (1 mark), with the number matching the 2+ charge (1 mark).
Example 2
Write the half equation for the formation of chlorine at the anode from Cl⁻ ions. [2 marks]
Model answer
2Cl⁻ → Cl₂ + 2e⁻. Two chloride ions used because chlorine is diatomic (1 mark), with two electrons on the right (1 mark).
Example 3
Explain why electrons appear on different sides in cathode and anode half equations. [2 marks]
Model answer
At the cathode positive ions gain electrons, so the electrons are a reactant and appear on the left (1 mark). At the anode negative ions lose electrons, so the electrons are a product and appear on the right (1 mark).
Common mistakes
Putting electrons on the wrong side.
Cathode: electrons gained, on the left. Anode: electrons lost, on the right.
Forgetting halogens are diatomic.
Two chloride ions are needed to make one Cl₂ molecule.
Using the wrong number of electrons.
It must match the total charge being neutralised.
Not checking the charge balance.
Count charges on both sides to confirm the equation is correct.
Exam tips
- Match the number of electrons to the ion charge.
- Remember halogens and oxygen form molecules, so two or more ions are needed.
- Check the total charge balances on both sides.
- Remember this is a Higher-only topic.
Key terms
- Half equation
- An equation showing electron transfer at one electrode.
- Electron
- The particle transferred during electrolysis, written e⁻.
- Diatomic
- Existing as a molecule of two atoms.
- Charge balance
- Having equal total charge on both sides of an equation.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.