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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (4 marks)
Electrode potentials are relative values; a reference electrode is required for measurement.
A platinum electrode is immersed in a solution containing equal concentrations (1 mol dm⁻³) of Fe³⁺ and Fe²⁺ ions.
The half‑cell is connected to the standard hydrogen electrode (SHE) under standard conditions (298 K, 100 kPa) via a salt bridge and external circuit.
The measured cell voltage equals the standard potential 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).
A platinum electrode is immersed in a solution containing equal concentrations (1 mol dm⁻³) of Fe³⁺ and Fe²⁺ ions.
The half‑cell is connected to the standard hydrogen electrode (SHE) under standard conditions (298 K, 100 kPa) via a salt bridge and external circuit.
The measured cell voltage equals the standard potential 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).
Examiner tips
- State that potentials are relative and a reference electrode is needed.
- Describe the platinum electrode with 1 M Fe³⁺/Fe²⁺ solution.
- Mention connection to SHE via salt bridge at 298 K, 100 kPa.
- Explain that the measured voltage equals +0.77 V because SHE is 0.00 V.
Common mistakes
- Claiming the Fe³⁺/Fe²⁺ potential can be measured alone without a reference.
- Using a non‑inert electrode or unequal ion concentrations.
- Ignoring the role of the salt bridge and standard conditions.
Mark scheme (4 marks)
- Electrode potentials are relative (not absolute) values, so a reference electrode is required for measurement.
- 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.
- 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.
- 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 in this question
standard electrode potential (E°)
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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