GCSE Chemistry Revision — Electrolysis of Aqueous Solutions
Revise Electrolysis of Aqueous Solutions for GCSE Chemistry with a topic explanation, worked example and common mistakes. Check the board notes for specification differences.
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What is Electrolysis of Aqueous Solutions?
When an ionic compound is dissolved in water, the water itself can also be broken down during electrolysis. This means there is a mixture of ions at each electrode. At the cathode, hydrogen gas will be produced if the metal is more reactive than hydrogen. At the anode, oxygen gas will be produced unless the solution contains halide ions.
Board notes: The electrolysis of aqueous solutions is a higher-tier topic for all boards. It requires a good understanding of the reactivity series and the ability to apply the rules for preferential discharge at the electrodes. Writing half-equations is also a key skill.
Step-by-step explanationWorked examples
Worked example
In the electrolysis of aqueous sodium chloride (NaCl), the ions present are Na⁺, Cl⁻, H⁺, and OH⁻. At the cathode, hydrogen is less reactive than sodium, so hydrogen gas is produced (2H⁺ + 2e⁻ → H₂). At the anode, chloride ions are present, so chlorine gas is produced (2Cl⁻ → Cl₂ + 2e⁻).
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Common mistakes
- 1Forgetting the presence of H⁺ and OH⁻ ions from the water, which compete with the ions from the solute.
- 2Applying the rules for predicting the products incorrectly. At the cathode, the least reactive positive ion is discharged. At the anode, a halide is produced if present; otherwise, oxygen is produced.
- 3Not being able to write the half-equations for the production of hydrogen and oxygen gas.
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Frequently asked questions
What is left behind in the solution after electrolysis of aqueous NaCl?
Since the Na⁺ and OH⁻ ions are not discharged, they are left in the solution, forming sodium hydroxide (NaOH).
When is the metal produced at the cathode in aqueous electrolysis?
The metal is produced at the cathode only if it is less reactive than hydrogen. This applies to metals like copper, silver, and gold.