Electrolysis of Aqueous Solutions
Revise Electrolysis of Aqueous Solutions for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. The required practical investigates electrolysis of solutions — hydrogen forms at the cathode unless the metal is less reactive than hydrogen.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Electrolysing an aqueous solution is more complicated than a molten compound, because water is present and provides additional ions. This is a required practical.
At the cathode, hydrogen is produced unless the metal is less reactive than hydrogen, in which case the metal is produced. So copper sulfate solution gives copper, but sodium chloride solution gives hydrogen.
At the anode, oxygen is produced unless a halide ion is present, in which case the halogen is produced. So sodium chloride solution gives chlorine, but copper sulfate solution gives oxygen. Applying these two rules in order answers almost every question on this topic.
Revision notes
The cathode rule
Hydrogen is produced unless the metal is less reactive than hydrogen.
If the metal is below hydrogen in the reactivity series — copper, silver, gold — the metal is produced instead. Otherwise the hydrogen ions from water are discharged.
The anode rule
Oxygen is produced unless a halide ion is present.
If chloride, bromide or iodide ions are present, the halogen is produced. Otherwise the hydroxide ions from water are discharged, giving oxygen.
Testing the products
Hydrogen: a lighted splint gives a squeaky pop. Oxygen: a glowing splint relights.
Chlorine: damp litmus paper is bleached white. These tests are frequently examined alongside the prediction.
Key points
- Water provides extra ions in aqueous solutions.
- Hydrogen forms at the cathode unless the metal is less reactive.
- Copper, silver and gold are produced at the cathode.
- Oxygen forms at the anode unless a halide is present.
- A halide gives the halogen at the anode.
- Chlorine bleaches damp litmus paper.
Worked examples
Example 1
Predict the products of electrolysing sodium chloride solution. [2 marks]
Model answer
Hydrogen at the cathode, because sodium is more reactive than hydrogen (1 mark). Chlorine at the anode, because chloride ions are present (1 mark).
Example 2
Predict the products of electrolysing copper sulfate solution. [2 marks]
Model answer
Copper at the cathode, because copper is less reactive than hydrogen (1 mark). Oxygen at the anode, because no halide ions are present (1 mark).
Example 3
Describe the test for chlorine gas. [2 marks]
Model answer
Hold damp litmus paper in the gas (1 mark). If chlorine is present the litmus paper is bleached white (1 mark).
Common mistakes
Applying the molten rules to solutions.
Water provides extra ions, so the two rules for aqueous solutions must be used.
Forgetting to check the metal's reactivity.
Only metals below hydrogen are produced at the cathode.
Predicting oxygen when a halide is present.
A halide always gives the halogen at the anode.
Confusing the gas tests.
Squeaky pop is hydrogen, relighting splint is oxygen, bleaching is chlorine.
Exam tips
- Apply the cathode rule and the anode rule separately.
- Check the metal against hydrogen in the reactivity series.
- Look for halide ions before predicting the anode product.
- Learn all three gas tests with their observations.
Key terms
- Aqueous
- Dissolved in water.
- Halide
- A chloride, bromide or iodide ion.
- Discharge
- The conversion of an ion into a neutral atom at an electrode.
- Reactivity series
- The order used to decide the cathode product.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.