AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Representation of reactions at electrodes as half equations (HT only): Reactions at electrodes can be represented by half equations, for example:
A half equation shows what happens at one electrode only.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A half equation shows what happens at one electrode only. It includes the reactant ion or molecule, the product, and the electrons gained or lost. At the cathode (negative electrode), positive ions gain electrons and are reduced, for example Cu²⁺ + 2e⁻ → Cu. At the anode (positive electrode), negative ions lose electrons and are oxidised, for example 2Cl⁻ → Cl₂ + 2e⁻. To write a correct half equation, balance atoms first, then balance charge by adding electrons to the more positive side. State symbols may be included. The two half equations can be combined to give the overall electrolysis equation. Representing electrode reactions as half equations is assessed at Higher Tier only.
What earns the marks
- A half equation represents the reaction at one electrode only.
- Cathode half equations show reduction: positive ions gain electrons, for example Cu²⁺ + 2e⁻ → Cu.
- Anode half equations show oxidation: negative ions lose electrons, for example 2Cl⁻ → Cl₂ + 2e⁻.
- Atoms and charge must balance in a half equation.
- Electrons are shown as reactants in reduction and as products in oxidation.
- Combining two half equations gives the overall electrolysis equation.
Common mistakes and how to put them right
Putting electrons on the wrong side.
Put it right: Reduction gains electrons as reactants, oxidation releases electrons as products.
Forgetting to balance atoms before charge.
Put it right: Balance atoms first, then add electrons to balance charge.
Writing a half equation for both electrodes in one equation.
Put it right: Each half equation describes only one electrode.
Exam tips
- Balance atoms first, then charge, then check the final equation.
- Label each half equation as oxidation or reduction to show understanding.
- Use state symbols where they help clarify the product, such as Br₂(l) or O₂(g).
- Be prepared to write half equations if you are taking the Higher Tier paper.
Try the lesson
The start of the Representation of reactions at electrodes as half equations (HT only): Reactions at electrodes can be represented by half equations, for example: lesson, made from checked teaching notes for this specification point.
Have a go first
In the electrolysis of molten lead bromide, bromide ions lose electrons at the anode. Which half equation correctly represents this reaction at the anode?
Listen
A half equation shows what happens at one electrode only. It includes the reactant ion or molecule, the product, and the electrons gained or lost. At the cathode (negative electrode), positive ions gain electrons and are reduced, for example Cu²⁺ + 2e⁻ → Cu. At the anode (positive electrode), negative ions lose electrons and are oxidised, for example 2Cl⁻ → Cl₂ + 2e⁻. To write a correct half equation, balance atoms first, then balance charge by adding electrons to the more positive side. State symbols may be included. The two half equations can be combined to give the overall electrolysis equation. Representing electrode reactions as half equations is assessed at Higher Tier only. A strong answer earns marks for these points. A half equation represents the reaction at one electrode only. Cathode half equations show reduction: positive ions gain electrons, for example Cu²⁺ + 2e⁻ → Cu. Anode half equations show oxidation: negative ions lose electrons, for example 2Cl⁻ → Cl₂ + 2e⁻. Atoms and charge must balance in a half equation. A common slip: Putting electrons on the wrong side. Instead, reduction gains electrons as reactants, oxidation releases electrons as products. Exam tip: Balance atoms first, then charge, then check the final equation.
See it
Key words
Say what each word means before you turn the card.
Half equations
Try answering aloud or in your head before turning the card.
Reduction and oxidation
Try answering aloud or in your head before turning the card.
Charge balance
Try answering aloud or in your head before turning the card.
See every definition
- Half equations
- A representation of the reaction at one electrode only, including the reactant ion or molecule, the product, and the electrons gained or lost.
- Reduction and oxidation
- At the cathode positive ions gain electrons and are reduced, while at the anode negative ions lose electrons and are oxidised.
- Charge balance
- Atoms are balanced first, then charge is balanced by adding electrons to the more positive side.
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 Representation of reactions at electrodes as half equations (HT only): Reactions at electrodes can be represented by half equations, for example:
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