4.3.5 Required Practical: Electrolysis of Aqueous Solutions

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The electrolysis of aqueous solutions introduces additional complexity compared to the electrolysis of molten ionic compounds. This complexity arises because water itself can undergo electrolysis, producing hydrogen ions (H⁺) and hydroxide ions (OH⁻), which participate in the reactions at the electrodes.

Electrolysis of Water

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When water is present in an aqueous solution, it partially dissociates into H⁺ and OH⁻ ions through the following reversible reaction:

\ce{H 2O(l)⇌H+(aq)+OH−(aq)}

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This dissociation means that during electrolysis, not only are the ions from the dissolved compound involved, but the H⁺ and OH⁻ ions from water can also react at the electrodes.

Reactions at the Electrodes

The specific reactions that occur at the electrodes during the electrolysis of an aqueous solution depend on the reactivity of the ions present. The general rule is:

At the Cathode (Negative Electrode): The ion that is less reactive will be reduced.
At the Anode (Positive Electrode): The ion that can be oxidised more easily will be oxidised.

lightbulbExample

Example 1: Electrolysis of Copper (II) Sulfate (CuSO₄) Solution

At the Cathode:

The solution contains Cu²⁺ ions and H⁺ ions (from water).
Copper (Cu) is less reactive than hydrogen, so Cu²⁺ ions are reduced at the cathode:

\ce{Cu^2+(aq) +2e^−→Cu(s)}

This produces solid copper, which deposits on the cathode.

At the Anode:

The solution contains SO₄²⁻ (sulfate) ions and OH⁻ ions (from water).
Since there are no halide ions present, OH⁻ ions are oxidised at the anode:

\ce4OH^−(aq)→2H2O(l)+O2(g)+4e^−

This produces oxygen gas and water.

Example 2: Electrolysis of Sodium Chloride (NaCl) Solution

At the Cathode:

The solution contains Na⁺ ions and H⁺ ions (from water).
Sodium (Na) is more reactive than hydrogen, so H⁺ ions are reduced at the cathode:

\ce2H^+(aq)+2e^−→H2(g)

This produces hydrogen gas.

At the Anode:

The solution contains Cl⁻ (chloride) ions and OH⁻ ions (from water).
Chloride (Cl⁻) ions are oxidised at the anode because they are halide ions:

\ce2Cl^−(aq)→Cl2(g)+2e^−

This produces chlorine gas.

Summary

In the electrolysis of aqueous solutions, the presence of water adds H⁺ and OH⁻ ions to the system, complicating the reactions at the electrodes. The actual products formed at each electrode depend on the reactivity of the ions present:

At the cathode, less reactive cations (such as Cu²⁺) are reduced, while more reactive ones (such as Na⁺) leave the H⁺ ions to be reduced instead.
At the anode, halide ions (such as Cl⁻) are usually oxidised if present, otherwise, OH⁻ ions are oxidised, leading to the production of oxygen gas.

Required Practical: Electrolysis of Aqueous Solutions Simplified Revision Notes for GCSE AQA Chemistry Combined Science

4.3.5 Required Practical: Electrolysis of Aqueous Solutions

Electrolysis of Water

Reactions at the Electrodes

Example 2: Electrolysis of Sodium Chloride (NaCl) Solution

Summary

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