AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
During electrolysis, at the cathode (negative electrode), positively…
Full specification wording
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, which is the negative electrode, positively charged ions move towards it because opposite charges attract.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
At the cathode, which is the negative electrode, positively charged ions move towards it because opposite charges attract. Each positive ion gains one or more electrons from the electrode and becomes a neutral atom or molecule. Gaining electrons is reduction, so every cathode reaction in electrolysis is a reduction. For example, in copper chloride solution, Cu²⁺(aq) gains two electrons to form copper metal: Cu²⁺(aq) + 2e⁻ → Cu(s). In dilute sulfuric acid, 2H⁺(aq) + 2e⁻ → H₂(g). You show this by writing a balanced half equation with the correct charges and state symbols, and higher-tier questions assess whether you can construct and interpret such equations.
What earns the marks
- State that the cathode is the negative electrode and that positive ions move towards it.
- Explain that positive ions gain electrons at the cathode and that electron gain is reduction.
- Write a balanced cathode half equation showing the ion, electrons and product with correct formulae and state symbols.
- Balance the number of electrons gained with the charge on the ion, for example two electrons for a 2⁺ ion.
- Link the cathode half equation to the overall electrolysis reaction and to the product observed.
Common mistakes and how to put them right
Writing the cathode as the positive electrode.
Put it right by remembering that the cathode is negative and attracts positive ions.
Showing electrons lost at the cathode.
Put it right by writing electrons as gained, for example Cu²⁺(aq) + 2e⁻ → Cu(s).
Omitting state symbols or charges in the half equation.
Put it right by including (aq), (s), (g) or (l) and the ion charge where needed.
Exam tips
- Check the charge balance: the total charge on the left must equal the total charge on the right.
- Use the phrase gain electrons equals reduction to justify why a cathode reaction is a reduction.
- Include state symbols in half equations because they show the form of each substance.
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
Which half equation correctly represents the reduction of copper ions at the cathode during electrolysis?
Listen
At the cathode, which is the negative electrode, positively charged ions move towards it because opposite charges attract. Each positive ion gains one or more electrons from the electrode and becomes a neutral atom or molecule. Gaining electrons is reduction, so every cathode reaction in electrolysis is a reduction. For example, in copper chloride solution, Cu²⁺(aq) gains two electrons to form copper metal: Cu²⁺(aq) + 2e⁻ → Cu(s). In dilute sulfuric acid, 2H⁺(aq) + 2e⁻ → H₂(g). You show this by writing a balanced half equation with the correct charges and state symbols, and higher-tier questions assess whether you can construct and interpret such equations. A strong answer earns marks for these points. State that the cathode is the negative electrode and that positive ions move towards it. Explain that positive ions gain electrons at the cathode and that electron gain is reduction. Write a balanced cathode half equation showing the ion, electrons and product with correct formulae and state symbols. Balance the number of electrons gained with the charge on the ion, for example two electrons for a 2⁺ ion. A common slip: Writing the cathode as the positive electrode. Instead, remembering that the cathode is negative and attracts positive ions. Exam tip: Check the charge balance: the total charge on the left must equal the total charge on the right.
See it
Key words
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Cathode
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Reduction
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Balanced half equation
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State symbols
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See every definition
- Cathode
- The negative electrode, towards which positively charged ions move because opposite charges attract.
- Reduction
- Gaining electrons, so every cathode reaction in electrolysis is a reduction.
- Balanced half equation
- A balanced equation with the correct charges and state symbols showing the ion, electrons and product.
- State symbols
- Symbols such as (aq), (s), (g) or (l) included in a half equation to show the form of each substance.
Label the diagram
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1Box 1
2Box 2
3Box 3
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