AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to predict the products of the electrolysis of…
Full specification wording
Electrolysis of aqueous solutions: Students should be able to predict the products of the electrolysis of aqueous solutions containing a single ionic compound.
To predict products, first identify the ions present: the compound supplies a positive ion and a negative ion, while water supplies H⁺ and OH⁻.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 5 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
To predict products, first identify the ions present: the compound supplies a positive ion and a negative ion, while water supplies H⁺ and OH⁻. At the cathode, the less reactive positive ion is usually discharged; if the metal is more reactive than hydrogen, hydrogen is produced instead. At the anode, halide ions are discharged in preference to hydroxide ions, giving a halogen; otherwise oxygen is produced. For example, electrolysing copper chloride solution gives copper at the cathode and chlorine at the anode, while electrolysing sodium sulfate solution gives hydrogen and oxygen. The prediction must name the product at each electrode and can be supported by half-equations.
What earns the marks
- List all ions present, including H⁺ and OH⁻ from water.
- Apply the cathode rule: metal below hydrogen in reactivity gives the metal; otherwise hydrogen is produced.
- Apply the anode rule: halide present gives the halogen; otherwise oxygen is produced.
- Name the products at both electrodes for a given single ionic compound.
- Support predictions with correct half-equations where required.
- Explain the prediction using discharge of the appropriate ions.
Common mistakes and how to put them right
Ignoring water-derived ions and predicting only from the dissolved compound.
Put it right by including H⁺ and OH⁻ in the ion list.
Predicting a reactive metal such as sodium at the cathode.
Put it right by stating that hydrogen is produced when the metal is more reactive than hydrogen.
Predicting oxygen at the anode when a halide is present.
Put it right by applying the halide preference rule.
Exam tips
- Write the ion list before making any prediction.
- State the product at each electrode separately and name the electrode.
- Use reactivity series position for the cathode and halide presence for the anode as your two decision steps.
Try the lesson
The start of the Electrolysis of aqueous solutions: Students should be able to predict the products of the electrolysis of aqueous solutions containing a single ionic compound. lesson, made from checked teaching notes for this specification point.
Have a go first
Aqueous copper chloride is electrolysed with inert electrodes. Which product forms at the negative electrode?
Listen
To predict products, first identify the ions present: the compound supplies a positive ion and a negative ion, while water supplies H⁺ and OH⁻. At the cathode, the less reactive positive ion is usually discharged; if the metal is more reactive than hydrogen, hydrogen is produced instead. At the anode, halide ions are discharged in preference to hydroxide ions, giving a halogen; otherwise oxygen is produced. For example, electrolysing copper chloride solution gives copper at the cathode and chlorine at the anode, while electrolysing sodium sulfate solution gives hydrogen and oxygen. The prediction must name the product at each electrode and can be supported by half-equations. A strong answer earns marks for these points. List all ions present, including H⁺ and OH⁻ from water. Apply the cathode rule: metal below hydrogen in reactivity gives the metal; otherwise hydrogen is produced. Apply the anode rule: halide present gives the halogen; otherwise oxygen is produced. Name the products at both electrodes for a given single ionic compound. A common slip: Ignoring water-derived ions and predicting only from the dissolved compound. Instead, including H⁺ and OH⁻ in the ion list. Exam tip: Write the ion list before making any prediction.
See it
Key words
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Predicting electrolysis products
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Reactivity series and cathode discharge
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Halide preference at the anode
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Ion competition in aqueous solution
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See every definition
- Predicting electrolysis products
- Identifying the ions present, including H⁺ and OH⁻ from water, then applying the cathode and anode rules to name the product at each electrode.
- Reactivity series and cathode discharge
- The less reactive positive ion is usually discharged at the cathode; if the metal is more reactive than hydrogen, hydrogen is produced instead.
- Halide preference at the anode
- Halide ions are discharged at the anode in preference to hydroxide ions, giving a halogen; otherwise oxygen is produced.
- Ion competition in aqueous solution
- The compound supplies a positive ion and a negative ion, while water supplies H⁺ and OH⁻, and the appropriate ions are discharged at each electrode.
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