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How does electrolysis split compounds, and what is formed at each electrode?

Electrolysis of molten ionic compounds and aqueous solutions; the products at each electrode; half-equations; and the extraction of aluminium.

A focused answer to AQA GCSE Chemistry 4.4.3, covering electrolysis of molten compounds and aqueous solutions, predicting the products at each electrode, writing half-equations, and the extraction of aluminium by electrolysis.

Generated by Claude Opus 4.89 min answer

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  1. What this dot point is asking
  2. What electrolysis is
  3. Electrolysis of molten compounds
  4. Electrolysis of solutions
  5. Half-equations
  6. Extraction of aluminium
  7. Try this

What this dot point is asking

AQA wants you to explain electrolysis of molten ionic compounds and aqueous solutions, predict the products at the cathode and anode using the reactivity series and the rules for solutions, write half-equations for the electrode reactions, and describe the extraction of aluminium. Electrolysis is the key industrial method for extracting and purifying reactive metals, so the rules for predicting products are tested on almost every Paper 1.

What electrolysis is

Positive ions (cations) move to the cathode and are reduced (gain electrons). Negative ions (anions) move to the anode and are oxidised (lose electrons). A solid ionic compound cannot be electrolysed because its ions are locked in the lattice and cannot move to carry charge.

Electrolysis of molten compounds

For a molten ionic compound, the metal forms at the cathode and the non-metal forms at the anode. For example, molten lead bromide gives lead at the cathode and bromine at the anode. Because there is no water present, the rules are simple: the metal cation is always reduced at the cathode and the non-metal anion is always oxidised at the anode.

Electrolysis of solutions

So electrolysing dilute sulfuric acid gives hydrogen and oxygen (effectively splitting water), while electrolysing sodium chloride solution gives hydrogen at the cathode (sodium is more reactive than hydrogen) and chlorine at the anode (a halide is present).

Half-equations

Half-equations show what happens at each electrode, with electrons gained at the cathode and lost at the anode:

  • Cathode (reduction): 2H++2e−→H22H^+ + 2e^- \rightarrow H_2.
  • Anode (oxidation): 2Cl−→Cl2+2e−2Cl^- \rightarrow Cl_2 + 2e^-.

The number of electrons must balance the charges so that the half-equation is balanced for both atoms and charge.

Extraction of aluminium

Aluminium is more reactive than carbon, so it is extracted by electrolysis of molten aluminium oxide. Cryolite is added to lower the melting point, so less energy is needed to keep the mixture molten. Aluminium forms at the cathode and oxygen at the anode, where the oxygen reacts with the hot carbon anodes to form carbon dioxide, so the anodes burn away and must be replaced. The large amount of electricity needed makes the process expensive, which is why recycling aluminium saves so much energy.

Try this

Q1. Name the products at each electrode when molten sodium chloride is electrolysed. [2 marks]

  • Cue. Sodium at the cathode, chlorine at the anode.

Q2. Explain why cryolite is added when extracting aluminium. [1 mark]

  • Cue. It lowers the melting point of the aluminium oxide, saving energy.

Q3. Write the half-equation for the formation of copper at the cathode from copper ions. [1 mark]

  • Cue. Cu2++2e−→CuCu^{2+} + 2e^- \rightarrow Cu.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20194 marksConcentrated copper chloride solution is electrolysed using inert electrodes. Name the product formed at each electrode, and write a half-equation for the reaction at the cathode and at the anode.
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A 4-mark Paper 1 question on aqueous electrolysis products and half-equations.

Cathode: copper is deposited, because copper is less reactive than hydrogen, so the metal forms rather than hydrogen (1 mark). Half-equation: Cu2++2e−→CuCu^{2+} + 2e^- \rightarrow Cu (1 mark). Anode: chlorine gas forms, because a halide is present (1 mark). Half-equation: 2Cl−→Cl2+2e−2Cl^- \rightarrow Cl_2 + 2e^- (1 mark).

Markers require balanced half-equations with electrons on the correct side (gained at the cathode, lost at the anode).

AQA 20215 marksAluminium is extracted from purified aluminium oxide by electrolysis. Explain why electrolysis is used rather than reduction with carbon, why cryolite is added, and why the positive electrodes must be replaced regularly. Write a half-equation for the reaction at the negative electrode.
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A 5-mark question combining extraction reasoning with a half-equation.

Electrolysis used (1 mark): aluminium is more reactive than carbon, so carbon cannot remove the oxygen from aluminium oxide. Cryolite (1 mark): it lowers the melting point of the aluminium oxide, so less energy is needed to keep it molten. Electrodes replaced (2 marks): oxygen is produced at the positive carbon electrodes; at the high temperature this oxygen reacts with the carbon to form carbon dioxide, so the electrodes gradually burn away and must be replaced. Half-equation at cathode (1 mark): Al3++3e−→AlAl^{3+} + 3e^- \rightarrow Al.

Markers reward the explicit reactivity comparison and the oxygen-burns-the-carbon point.

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