AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Representation of reactions at electrodes as half equations (HT only): and
The specification uses 'and' to join two key examples: the reduction of lead ions (Pb²⁺ + 2e⁻ → Pb) and the oxidation of bromide ions (2Br⁻ → Br₂ + 2e⁻).
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The specification uses 'and' to join two key examples: the reduction of lead ions (Pb²⁺ + 2e⁻ → Pb) and the oxidation of bromide ions (2Br⁻ → Br₂ + 2e⁻). Students may be asked to write these from scratch or complete and balance supplied half equations. Always verify that the total charge on the left equals the total charge on the right.
What earns the marks
- Identifies that reduction occurs at the cathode, involving the gain of electrons (e.g., Pb²⁺ + 2e⁻ → Pb).
- Identifies that oxidation occurs at the anode, involving the loss of electrons (e.g., 2Br⁻ → Br₂ + 2e⁻).
- Completes a supplied half equation by adding the correct number of electrons to the appropriate side.
- Balances a supplied half equation by ensuring the number of atoms and total electrical charge are equal on both sides.
Common mistakes and how to put them right
Adding electrons to the wrong side of a supplied equation.
Put it right: Reduction (cathode) gains electrons on the left, while oxidation (anode) loses electrons on the right.
Failing to balance the charges correctly, such as writing Br⁻ → Br₂ + e⁻ instead of 2Br⁻ → Br₂ + 2e⁻.
Put it right: Check that total charge is equal on both sides.
Forgetting to balance the atoms before balancing the charges, leading to an incorrect number of electrons being added to the half equation.
Exam tips
- When completing a half equation, first balance the atoms, then add electrons to balance the total charge.
- Use the OIL RIG acronym (Oxidation Is Loss, Reduction Is Gain) to check if electrons should be reactants or products.
Try the lesson
The start of the Representation of reactions at electrodes as half equations (HT only): and lesson, made from checked teaching notes for this specification point.
Have a go first
In the electrolysis of molten sodium chloride, sodium ions (Na⁺) are attracted to the cathode. Which half equation correctly represents the reaction at the cathode?
Listen
The specification uses 'and' to join two key examples: the reduction of lead ions (Pb²⁺ + 2e⁻ → Pb) and the oxidation of bromide ions (2Br⁻ → Br₂ + 2e⁻). Students may be asked to write these from scratch or complete and balance supplied half equations. Always verify that the total charge on the left equals the total charge on the right. A strong answer earns marks for these points. Identifies that reduction occurs at the cathode, involving the gain of electrons (e.g., Pb²⁺ + 2e⁻ → Pb). Identifies that oxidation occurs at the anode, involving the loss of electrons (e.g., 2Br⁻ → Br₂ + 2e⁻). Completes a supplied half equation by adding the correct number of electrons to the appropriate side. Balances a supplied half equation by ensuring the number of atoms and total electrical charge are equal on both sides. Exam tip: When completing a half equation, first balance the atoms, then add electrons to balance the total charge.
See it
Key words
Say what each word means before you turn the card.
Oxidation and reduction
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Balancing half equations
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Conservation of charge and mass
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See every definition
- Oxidation and reduction
- Oxidation involves the loss of electrons at the anode, while reduction involves the gain of electrons at the cathode.
- Balancing half equations
- Ensuring the number of atoms and total electrical charge are equal on both sides of a half equation.
- Conservation of charge and mass
- Checking that the total charge on the left equals the total charge on the right, and that atoms are balanced on both sides.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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