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'.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The standard hydrogen electrode (SHE) is used as a universal reference point because it provides a zero potential against which all other half‑cell potentials can be compared.
A single half‑cell cannot be measured in isolation – a complete electrochemical cell with two half‑cells is required to obtain a potential.
For a measurement to be considered standard the following conditions must be met:
1. Temperature = 298 K (25 °C).
2. All aqueous species are at a concentration of 1 mol dm⁻³.
3. All gases are at a pressure of 100 kPa (1 bar) or 101 kPa (1 atm).
A single half‑cell cannot be measured in isolation – a complete electrochemical cell with two half‑cells is required to obtain a potential.
For a measurement to be considered standard the following conditions must be met:
1. Temperature = 298 K (25 °C).
2. All aqueous species are at a concentration of 1 mol dm⁻³.
3. All gases are at a pressure of 100 kPa (1 bar) or 101 kPa (1 atm).
Examiner tips
- Use the phrase ‘universal reference point’ to show understanding of SHE’s role. Explain that a complete cell is needed – this shows knowledge of measurement. List the three standard conditions explicitly.
- common_mistakes
- :
- Forgetting to mention the need for a complete cell. Confusing 1 bar with 1 atm – both are acceptable but state one clearly. Omitting the temperature requirement.
Mark scheme (5 marks)
- The SHE is used as a universal reference point / zero reference so that all half-cell potentials can be compared on the same scale
- It is not possible to measure the potential of a single half-cell in isolation; a complete circuit (two half-cells) is required
- Temperature must be 298 K (25 °C)
- All ionic / aqueous species must be at a concentration of 1 mol dm⁻³
- All gases must be at a pressure of 100 kPa (1 bar) / 101 kPa (1 atm)
Key terms in this question
standard electrode potential · standard hydrogen electrode (SHE) · half-cell
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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