# Explain why the standard electrode potential of a half-cell is measured against the standard hydrogen electrode (SHE), and state the conditions required for a measurement to be considered 'standard'.

> AQA A-Level Chemistry (7405) — 3.1.11 Electrode potentials and electrochemical cells (A-Level only) · Explain · 5 marks

## Mark scheme (5 marks)

1. The SHE is used as a universal reference point / zero reference so that all half-cell potentials can be compared on the same scale
2. It is not possible to measure the potential of a single half-cell in isolation; a complete circuit (two half-cells) is required
3. Temperature must be 298 K (25 °C)
4. All ionic / aqueous species must be at a concentration of 1 mol dm⁻³
5. All gases must be at a pressure of 100 kPa (1 bar) / 101 kPa (1 atm)

## Key terms

- [standard electrode potential](https://www.gradenine.co.uk/glossary/standard-electrode-potential)
- [standard hydrogen electrode (SHE)](https://www.gradenine.co.uk/glossary/standard-hydrogen-electrode-she)
- [half-cell](https://www.gradenine.co.uk/glossary/half-cell)

## Related

- [Revision notes for AQA A-Level Chemistry (7405)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-standard-electrode-potential-d5ee4456) · Published by Druglandscape Ltd.