A student constructs an electrochemical cell by connecting a standard iron(II)/iron(III) half-cell (Fe³⁺(aq)/Fe²⁺(aq)) to a standard manganese half-cell (MnO₄⁻(aq)/Mn²⁺(aq)) using a salt bridge and a high-resistance voltmeter. The standard electrode potential for Fe³⁺/Fe²⁺ is +0.77 V and for MnO₄⁻/Mn²⁺ is +1.51 V. Explain which half-cell acts as the negative electrode, identify the overall direction of electron flow in the external circuit, and explain why a salt bridge is essential for the cell to function.

Edexcel A-Level Chemistry (9CH0) — 14.1 Electrode potentials and electrochemical cells · Explain · 5 marks · View as Markdown

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

Standard electrode potentials (measured against a standard hydrogen electrode): Fe³⁺(aq) + e⁻ → Fe²⁺(aq), E° = +0.77 V. MnO₄⁻(aq) + 8H⁺(aq) + 5e⁻ → Mn²⁺(aq) + 4H₂O(l), E° = +1.51 V.

Model answer (5 marks)

The iron(II)/iron(III) half‑cell is the negative electrode because its E° ( +0.77 V ) is less positive than that of the manganese half‑cell (+1.51 V). At the iron half‑cell oxidation occurs: Fe²⁺ → Fe³⁺ + e⁻. Electrons therefore leave the iron half‑cell, travel through the external circuit to the manganese half‑cell, where they are consumed in the reduction MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. The salt bridge is essential because it allows ions to move between the two half‑cells, maintaining charge neutrality. Without it, charge would accumulate in each half‑cell, stopping further electron flow and the cell would not operate.

Examiner tips

  • State which half‑cell is negative by comparing E° values. Explain the oxidation/reduction reactions at each electrode. Describe electron flow direction. Justify the role of the salt bridge in maintaining charge balance.

Common mistakes

  • Confusing the more positive electrode as the negative one. Forgetting to mention that the iron half‑cell undergoes oxidation. Claiming the salt bridge is optional or not explaining ion flow.

Mark scheme (5 marks)

  1. The iron(II)/iron(III) half-cell acts as the negative electrode because it has the less positive (lower) standard electrode potential.
  2. At the negative electrode (iron half-cell), oxidation occurs: Fe²⁺ is oxidised to Fe³⁺.
  3. Electrons flow in the external circuit from the iron(II)/iron(III) half-cell (negative electrode) to the manganese half-cell (positive electrode).
  4. The salt bridge completes the electrical circuit by allowing ions to flow between the two half-cells.
  5. Without the salt bridge, charge would build up in each half-cell, preventing further electron flow and stopping the cell from functioning.

Key terms in this question

standard electrode potential · negative electrode · salt bridge · external circuit

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