# 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

> 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.

## 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

- [standard electrode potential](https://www.gradenine.co.uk/glossary/standard-electrode-potential)
- [negative electrode](https://www.gradenine.co.uk/glossary/negative-electrode)
- [salt bridge](https://www.gradenine.co.uk/glossary/salt-bridge)
- [external circuit](https://www.gradenine.co.uk/glossary/external-circuit)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-constructs-an-electrochemical-cell-e4cd2bcb) · Published by Druglandscape Ltd.