AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Representation of reactions at electrodes as half equations (HT only): During electrolysis, at the cathode (negative electrode), positively charged ions gain electrons and so the reactions are reductions.
At the cathode during electrolysis, positively charged ions (cations) gain electrons from the electrode.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
At the cathode during electrolysis, positively charged ions (cations) gain electrons from the electrode. This electron gain is defined as reduction. For example, in molten lead(II) bromide, Pb²⁺ ions gain two electrons to form lead metal: Pb²⁺ + 2e⁻ → Pb. In aqueous solutions, hydrogen ions (H⁺) may gain electrons to form hydrogen gas: 2H⁺ + 2e⁻ → H₂. You represent these changes as half equations, balancing both atoms and charges. The cathode is the negative electrode, so it attracts cations. Reduction always involves electron gain, which can be remembered using the OIL RIG mnemonic (Oxidation Is Loss, Reduction Is Gain of electrons).
What earns the marks
- Identify the cathode as the negative electrode where positively charged ions (cations) are attracted.
- State that positively charged ions gain electrons at the cathode, and that this electron gain is reduction.
- Write balanced half equations for cathode reactions, ensuring both atoms and charges are balanced.
- Apply the concept of reduction as electron gain to explain products formed at the cathode in different electrolytes.
Common mistakes and how to put them right
Confusing the cathode with the positive electrode.
Put it right: The cathode is the negative electrode; cations move towards it.
Writing half equations that are not charge-balanced.
Put it right: Add electrons to the left side for reduction so that total charge is equal on both sides.
Stating that reduction is loss of electrons.
Put it right: Reduction is gain of electrons; use OIL RIG to remember.
Exam tips
- Always check that half equations are balanced for both atoms and charge before finalising your answer.
- Use the phrase 'gain of electrons' when defining reduction, and link it directly to the cathode.
Try the lesson
The start of the Representation of reactions at electrodes as half equations (HT only): During electrolysis, at the cathode (negative electrode), positively charged ions gain electrons and so the reactions are reductions. lesson, made from checked teaching notes for this specification point.
Have a go first
During the electrolysis of molten lead bromide, Pb²⁺ ions move to the cathode. Which process happens to these ions at the cathode?
Listen
At the cathode during electrolysis, positively charged ions (cations) gain electrons from the electrode. This electron gain is defined as reduction. For example, in molten lead(II) bromide, Pb²⁺ ions gain two electrons to form lead metal: Pb²⁺ + 2e⁻ → Pb. In aqueous solutions, hydrogen ions (H⁺) may gain electrons to form hydrogen gas: 2H⁺ + 2e⁻ → H₂. You represent these changes as half equations, balancing both atoms and charges. The cathode is the negative electrode, so it attracts cations. Reduction always involves electron gain, which can be remembered using the OIL RIG mnemonic (Oxidation Is Loss, Reduction Is Gain of electrons). A strong answer earns marks for these points. Identify the cathode as the negative electrode where positively charged ions (cations) are attracted. State that positively charged ions gain electrons at the cathode, and that this electron gain is reduction. Write balanced half equations for cathode reactions, ensuring both atoms and charges are balanced. Apply the concept of reduction as electron gain to explain products formed at the cathode in different electrolytes. A common slip: Confusing the cathode with the positive electrode. Instead, the cathode is the negative electrode; cations move towards it. Exam tip: Always check that half equations are balanced for both atoms and charge before finalising your answer.
See it
Key words
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Reduction as electron gain
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Cathode reactions in electrolysis
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Half equations and balancing
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See every definition
- Reduction as electron gain
- Positively charged ions gain electrons at the cathode, and this electron gain is defined as reduction.
- Cathode reactions in electrolysis
- At the negative electrode, positively charged ions are attracted and gain electrons, so the reactions are reductions.
- Half equations and balancing
- These represent the changes at electrodes, balancing both atoms and charges, adding electrons to the left side for reduction.
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