Dilute sulfuric acid is electrolysed using inert electrodes. Explain what happens at each electrode during this electrolysis, including the type of reaction occurring and the product formed at each electrode.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.9 Electrolysis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Dilute sulfuric acid contains hydrogen ions (H⁺) and sulfate ions (SO₄²⁻) as well as water molecules. Inert electrodes are used so that the electrode material does not affect the reactions.

Model answer (4 marks)

At the cathode (negative electrode) the hydrogen ions in the solution accept electrons (are reduced) to form hydrogen gas:

2H⁺ + 2e⁻ → H₂(g)

At the anode (positive electrode) water molecules (or hydroxide ions) lose electrons (are oxidised) to give oxygen gas:

2H₂O → O₂(g) + 4H⁺ + 4e⁻

Thus H₂ is produced at the cathode and O₂ at the anode.

Examiner tips

  • Use the correct half‑reactions and show electron transfer; include the gas produced at each electrode; mention that inert electrodes mean no electrode material is involved.
  • Remember to state the type of reaction (reduction at cathode, oxidation at anode).

Common mistakes

  • Confusing the products (e.g., writing H₂O instead of H₂ at the cathode); not recognising that water is oxidised at the anode; omitting the electron transfer in the equations.

Mark scheme (4 marks)

  1. At the cathode (negative electrode), hydrogen ions gain electrons / are reduced
  2. Hydrogen gas is produced at the cathode
  3. At the anode (positive electrode), hydroxide ions / water molecules lose electrons / are oxidised
  4. Oxygen gas is produced at the anode

Key terms in this question

inert electrode · electrolysis

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