GCSE Chemistry Electrolysis Notes
Electrolysis uses electricity to break down ionic compounds. Positive ions go to the cathode; negative ions go to the anode.
Key concepts
- Electrolyte
- A molten or dissolved ionic compound whose ions can move and carry charge.
- Cathode
- Negative electrode. Positive ions (cations) are reduced here.
- Anode
- Positive electrode. Negative ions (anions) are oxidised here.
- Half equation
- Shows electron gain (reduction) or loss (oxidation) at one electrode.
- In aqueous solutions, hydrogen is often produced at the cathode instead of the metal, unless the metal is less reactive than hydrogen (e.g. copper).
- At the anode, halide ions produce the halogen; otherwise oxygen is produced from water.
- Aluminium is extracted by electrolysis of molten aluminium oxide dissolved in cryolite to lower the melting point and save energy.
Worked example
Write the half equations for the electrolysis of molten lead bromide.
- 1Cathode (reduction): Pb²⁺ ions gain electrons.
- 2Pb²⁺ + 2e⁻ → Pb.
- 3Anode (oxidation): Br⁻ ions lose electrons.
- 42Br⁻ → Br₂ + 2e⁻.
Answer: Cathode: Pb²⁺ + 2e⁻ → Pb; Anode: 2Br⁻ → Br₂ + 2e⁻
Common mistakes
✗ Balancing half equations without electrons.
✓ Electrons balance the charge — always include them.
✗ Predicting Na at the cathode in aqueous NaCl.
✓ Sodium is more reactive than hydrogen, so hydrogen is produced instead.
✗ Forgetting to melt or dissolve the compound.
✓ Ions must be free to move — solid ionic compounds do not conduct.
Quick check
Try these, then reveal the answers.
Why must an electrolyte be molten or dissolved?Reveal answer
Ions must be free to move and carry charge.
Product at cathode in molten NaCl?Reveal answer
Sodium metal (Na⁺ + e⁻ → Na).
Product at anode in aqueous copper sulfate (inert electrodes)?Reveal answer
Oxygen (from OH⁻/water).
Exam tips
- Label reduction at cathode and oxidation at anode using OIL RIG.
- For aqueous solutions, list H⁺, OH⁻, and the salt ions before deciding products.
- Use inert electrodes (usually graphite) unless the question says otherwise.
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Frequently asked questions
Why is aluminium extracted by electrolysis?+
Aluminium is more reactive than carbon, so it cannot be reduced by heating with carbon; electrolysis of molten Al₂O₃ (in cryolite) is used instead.
What is a half equation?+
An equation showing the gain (reduction) or loss (oxidation) of electrons at a single electrode.
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