# A student constructs an electrochemical cell using a standard chromium half-cell (Cr³⁺(aq)/Cr(s), E° = −0.74 V) and a standard tin half-cell (Sn²⁺(aq)/Sn(s), E° = −0.14 V), connected by a salt bridge. Explain how the student can determine which electrode is the negative terminal, describe the direction of electron flow in the external circuit, and explain the role of the salt bridge in maintaining the cell's function.

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

> The standard electrode potentials for the two half-cells are: Cr³⁺(aq) + 3e⁻ → Cr(s), E° = −0.74 V and Sn²⁺(aq) + Sn(s), E° = −0.14 V.

## Mark scheme (5 marks)

1. The chromium half-cell has the more negative (lower) standard electrode potential, so it is the negative terminal (anode).
2. Chromium is oxidised at the negative electrode (anode), so it loses electrons.
3. Electrons flow through the external circuit from the chromium electrode to the tin electrode.
4. Tin(II) ions are reduced at the tin electrode (cathode), gaining electrons.
5. The salt bridge maintains electrical neutrality in both half-cells by allowing ions to move between them, completing the circuit.

## Key terms

- [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-28b74354) · Published by Druglandscape Ltd.