Electrolysis of Aqueous Solutions

This lesson covers the electrolysis of aqueous solutions as required by the AQA GCSE Combined Science Trilogy specification (8464). When an ionic compound is dissolved in water, there are additional ions from the water. You must be able to predict the products using simple rules and write half equations (higher tier).

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Why Are Aqueous Solutions Different?

When an ionic compound is dissolved in water, the water itself provides additional ions:

$\text{H}_2\text{O}(l) \rightleftharpoons \text{H}^+(aq) + \text{OH}^-(aq)$H2​O(l)⇌H+(aq)+OH−(aq)

So an aqueous solution contains:

• Cations from the compound (e.g. Na⁺, Cu²⁺)
• Anions from the compound (e.g. Cl⁻, SO₄²⁻)
• H⁺ ions from water
• OH⁻ ions from water

At each electrode, there is competition between the ions, and rules determine which ion is discharged.

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Rules for Predicting Products

At the Cathode (Reduction — Positive Ions)

flowchart TD
    A["Which cation is<br/>present?"] --> B{"Is the metal LESS<br/>reactive than hydrogen?"}
    B -- "Yes<br/>(e.g. Cu²⁺, Ag⁺)" --> C["The METAL is<br/>deposited"]
    B -- "No<br/>(e.g. Na⁺, K⁺, Ca²⁺, Al³⁺)" --> D["HYDROGEN gas<br/>is produced<br/>(H⁺ ions are reduced<br/>instead)"]

    style C fill:#2e7d32,color:#fff
    style D fill:#1565c0,color:#fff