AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
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.
(HT only) Half equations represent the reactions occurring at individual electrodes during electrolysis, showing the transfer of electrons (e⁻).
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
(HT only) Half equations represent the reactions occurring at individual electrodes during electrolysis, showing the transfer of electrons (e⁻). At the cathode, positive ions gain electrons (reduction), so e⁻ are written on the left side of the equation. For example, Pb²⁺ + 2e⁻ → Pb. At the anode, negative ions lose electrons (oxidation), so e⁻ appear on the right. For example, 2Br⁻ → Br₂ + 2e⁻. To write a half equation, identify the ion and the element formed, balance the atoms, and then add electrons to balance the charge. When completing a supplied half equation, always balance atoms first, then charge. Combining half equations gives the full balanced equation.
What earns the marks
- (HT only) A half equation represents the reaction at one electrode and includes electrons as e⁻.
- (HT only) At the cathode, positive ions gain electrons (reduction), so electrons are written on the left of the arrow.
- (HT only) At the anode, negative ions lose electrons (oxidation), so electrons are written on the right of the arrow.
- (HT only) Half equations must balance both atoms and charge; the number of electrons added must match the charge change.
- (HT only) To complete a supplied half equation, balance the atoms of each element first, then balance the overall charge using electrons.
Common mistakes and how to put them right
Putting electrons on the wrong side.
Put it right: Reduction at the cathode gains electrons (e⁻ on the left), and oxidation at the anode loses electrons (e⁻ on the right).
Forgetting to balance charge as well as atoms.
Put it right: Check that the total charge is exactly the same on both sides of the arrow.
Writing e instead of e⁻ or omitting the charge.
Put it right: Show the electron as e⁻ with its negative charge.
Exam tips
- (HT only) Balance atoms before adding electrons, then check the total charge on both sides last.
- (HT only) For a supplied half equation, count the charge on each side and add the missing electrons to the side that needs them.
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
During the electrolysis of molten sodium chloride, sodium ions are discharged at the cathode. Which half equation correctly represents this reaction?
Listen
HT only) Half equations represent the reactions occurring at individual electrodes during electrolysis, showing the transfer of electrons (e⁻). At the cathode, positive ions gain electrons (reduction), so e⁻ are written on the left side of the equation. For example, Pb²⁺ + 2e⁻ → Pb. At the anode, negative ions lose electrons (oxidation), so e⁻ appear on the right. For example, 2Br⁻ → Br₂ + 2e⁻. To write a half equation, identify the ion and the element formed, balance the atoms, and then add electrons to balance the charge. When completing a supplied half equation, always balance atoms first, then charge. Combining half equations gives the full balanced equation. A strong answer earns marks for these points. HT only) A half equation represents the reaction at one electrode and includes electrons as e⁻. HT only) At the cathode, positive ions gain electrons (reduction), so electrons are written on the left of the arrow. HT only) At the anode, negative ions lose electrons (oxidation), so electrons are written on the right of the arrow. HT only) Half equations must balance both atoms and charge; the number of electrons added must match the charge change. A common slip: Putting electrons on the wrong side. Instead, reduction at the cathode gains electrons (e⁻ on the left), and oxidation at the anode loses electrons (e⁻ on the right). Exam tip: (HT only) Balance atoms before adding electrons, then check the total charge on both sides last.
Key words
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Half equation
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Reduction
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Oxidation
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Balancing a half equation
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See every definition
- Half equation
- Represents the reaction at one electrode and includes electrons as e⁻.
- Reduction
- At the cathode, positive ions gain electrons, so electrons are written on the left of the arrow.
- Oxidation
- At the anode, negative ions lose electrons, so electrons are written on the right of the arrow.
- Balancing a half equation
- Atoms and charge must both balance, with the number of electrons added matching the charge change.
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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.
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