AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
At the positive electrode (anode), oxygen is produced unless the solution…
Full specification wording
Electrolysis of aqueous solutions: At the positive electrode (anode), oxygen is produced unless the solution contains halide ions when the halogen is produced.
In aqueous electrolysis, the anode attracts negative ions.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 3 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In aqueous electrolysis, the anode attracts negative ions. Hydroxide ions, OH⁻, are present from water and are discharged, producing oxygen gas. However, if the solution contains halide ions such as Cl⁻, Br⁻ or I⁻, these are discharged in preference, so the halogen forms instead. For example, electrolysing sodium chloride solution gives chlorine at the anode, while electrolysing sodium sulfate solution gives oxygen. The rule is therefore: halide present means halogen at the anode; otherwise oxygen. This links ion discharge to the relative ease of oxidation of the ions present.
What earns the marks
- State that the anode is the positive electrode and attracts negative ions.
- Identify hydroxide ions from water as the species discharged when no halide is present, giving oxygen.
- Explain that halide ions are discharged in preference to hydroxide ions, giving the halogen.
- (HT only) Write or recognise the anode half-equation for oxygen production: 4OH⁻ → O₂ + 2H₂O + 4e⁻.
- (HT only) Write or recognise a halogen half-equation, for example 2Cl⁻ → Cl₂ + 2e⁻.
- Apply the rule to a named solution, such as predicting chlorine from sodium chloride solution and oxygen from sodium sulfate solution.
Common mistakes and how to put them right
Thinking oxygen is always produced at the anode regardless of the ions present.
Put it right by checking for halide ions first and naming the halogen if they are present.
Confusing the anode with the cathode.
Put it right by remembering that the anode is positive and attracts negative ions.
(HT only) Writing the oxygen half-equation without balancing electrons or water.
Put it right by balancing atoms and charge so the equation reads 4OH⁻ → O₂ + 2H₂O + 4e⁻.
Exam tips
- Name the solution and identify all ions present before predicting the anode product.
- Use the phrase 'halide present, halogen produced; otherwise oxygen' to structure your answer.
- (HT only) When asked to write half-equations, ensure both atoms and charges are balanced.
Try the lesson
The start of the Electrolysis of aqueous solutions: At the positive electrode (anode), oxygen is produced unless the solution contains halide ions when the halogen is produced. lesson, made from checked teaching notes for this specification point.
Have a go first
Concentrated aqueous sodium chloride is electrolysed using inert electrodes. Which product forms at the positive electrode?
Listen
In aqueous electrolysis, the anode attracts negative ions. Hydroxide ions, OH⁻, are present from water and are discharged, producing oxygen gas. However, if the solution contains halide ions such as Cl⁻, Br⁻ or I⁻, these are discharged in preference, so the halogen forms instead. For example, electrolysing sodium chloride solution gives chlorine at the anode, while electrolysing sodium sulfate solution gives oxygen. The rule is therefore: halide present means halogen at the anode; otherwise oxygen. This links ion discharge to the relative ease of oxidation of the ions present. A strong answer earns marks for these points. State that the anode is the positive electrode and attracts negative ions. Identify hydroxide ions from water as the species discharged when no halide is present, giving oxygen. Explain that halide ions are discharged in preference to hydroxide ions, giving the halogen. HT only) Write or recognise the anode half-equation for oxygen production: 4OH⁻ → O₂ + 2H₂O + 4e⁻. A common slip: Thinking oxygen is always produced at the anode regardless of the ions present. Instead, checking for halide ions first and naming the halogen if they are present. Exam tip: Name the solution and identify all ions present before predicting the anode product.
See it
Key words
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anode
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hydroxide ions
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halide ions
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ion discharge
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See every definition
- anode
- The positive electrode, which attracts negative ions.
- hydroxide ions
- Ions, OH⁻, present from water, which are discharged to produce oxygen gas.
- halide ions
- Ions such as Cl⁻, Br⁻ or I⁻, which are discharged in preference to hydroxide ions, so the halogen forms instead.
- ion discharge
- The process linked to the relative ease of oxidation of the ions present, giving oxygen or a halogen at the anode.
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