# Explain why the standard electrode potential (E°) for the Fe³⁺/Fe²⁺ half-cell cannot be measured directly, and outline the procedure used to assign it a value.

> IB DP Chemistry Higher Level (2023 syllabus) — R3.2 Electron transfer reactions · Explain · 4 marks

> Standard electrode potentials are measured relative to the standard hydrogen electrode (SHE), which is assigned a value of 0.00 V. The Fe³⁺/Fe²⁺ half-cell has a standard electrode potential of +0.77 V.

## Mark scheme (4 marks)

1. Electrode potentials are relative (not absolute) values, so a reference electrode is required for measurement.
2. The Fe³⁺/Fe²⁺ half-cell uses a platinum (inert) electrode immersed in a solution containing equal concentrations (both 1 mol dm⁻³) of Fe³⁺ and Fe²⁺ ions.
3. This half-cell is connected to the standard hydrogen electrode (SHE) under standard conditions (298 K, 100 kPa) via a salt bridge and external circuit.
4. The measured cell voltage equals the E° of the Fe³⁺/Fe²⁺ half-cell (+0.77 V) because E°(SHE) = 0.00 V by definition; the positive sign indicates Fe³⁺/Fe²⁺ is the cathode (reduction occurs at this electrode).

## Key terms

- [standard electrode potential (E°)](https://www.gradenine.co.uk/glossary/standard-electrode-potential-e)

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