AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
(HT only) Throughout Section 4.4.3 Higher Tier students should be able to…
Full specification wording
The process of electrolysis: (HT only) Throughout Section 4.4.3 Higher Tier students should be able to write half equations for the reactions occurring at the electrodes during electrolysis, and may be required to complete and balance supplied half equations.
A half equation shows the gain or loss of electrons by ions at an electrode.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 7 of 7
Study notes
From the checked teaching notes for this specification point.
The idea
A half equation shows the gain or loss of electrons by ions at an electrode. At the cathode (negative electrode), positive ions gain electrons (reduction). For example, Pb²⁺ + 2e⁻ → Pb. At the anode (positive electrode), negative ions lose electrons (oxidation). For example, 2Br⁻ → Br₂ + 2e⁻. Students may also need to complete and balance supplied half equations by adding electrons, ions, or adjusting coefficients to ensure conservation of charge and mass. Balancing requires that the number of atoms of each element and the total charge are the same on both sides. State symbols may be required.
What earns the marks
- Write half equations for reduction at the cathode, showing electrons gained by positive ions.
- Write half equations for oxidation at the anode, showing electrons lost by negative ions.
- Balance half equations for both atoms and charge.
- Include state symbols where appropriate (e.g., (l), (g), (aq)).
- Complete supplied half equations by adding missing electrons, ions, or coefficients.
Common mistakes and how to put them right
Forgetting to balance charge in half equations.
Put it right: Ensure the total charge on each side is equal by adding the appropriate number of electrons.
Placing electrons on the wrong side of the equation.
Put it right: Electrons are gained (reactants) at the cathode and lost (products) at the anode.
Omitting state symbols when they are required.
Put it right: Include (l), (g), (aq), or (s) as appropriate for the species involved.
Exam tips
- Always check that both atoms and charge are balanced in half equations.
- Remember that reduction occurs at the cathode (electrons on the left) and oxidation at the anode (electrons on the right).
- Practice completing half equations by identifying the missing species, often electrons or water molecules in aqueous solutions.
Try the lesson
The start of the The process of electrolysis: (HT only) Throughout Section 4.4.3 Higher Tier students should be able to write half equations for the reactions occurring at the electrodes during electrolysis, and may be required to complete and balance supplied half equations. lesson, made from checked teaching notes for this specification point.
Have a go first
Which half equation correctly represents the reaction at the cathode when molten sodium chloride is electrolysed?
Listen
A half equation shows the gain or loss of electrons by ions at an electrode. At the cathode (negative electrode), positive ions gain electrons (reduction). For example, Pb²⁺ + 2e⁻ → Pb. At the anode (positive electrode), negative ions lose electrons (oxidation). For example, 2Br⁻ → Br₂ + 2e⁻. Students may also need to complete and balance supplied half equations by adding electrons, ions, or adjusting coefficients to ensure conservation of charge and mass. Balancing requires that the number of atoms of each element and the total charge are the same on both sides. State symbols may be required. A strong answer earns marks for these points. Write half equations for reduction at the cathode, showing electrons gained by positive ions. Write half equations for oxidation at the anode, showing electrons lost by negative ions. Balance half equations for both atoms and charge. Include state symbols where appropriate (e.g., (l), (g), (aq)). A common slip: Forgetting to balance charge in half equations. Instead, ensure the total charge on each side is equal by adding the appropriate number of electrons. Exam tip: Always check that both atoms and charge are balanced in half equations.
See it
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of The process of electrolysis: (HT only) Throughout Section 4.4.3 Higher Tier students should be able to write half equations for the reactions occurring at the electrodes during electrolysis, and may be required to complete and balance supplied half equations.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge