Explain how the sign and magnitude of a standard cell potential (E°cell) can be used to predict whether a redox reaction is spontaneous under standard conditions.

IB DP Chemistry Standard Level (2023 syllabus) — R3.2 Electron transfer reactions · Explain · 4 marks · View as Markdown

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 and used to determine the feasibility of redox reactions.

Model answer (4 marks)

E°cell is calculated as E°(cathode) – E°(anode) or, equivalently, as the potential of the more positive half‑cell minus that of the less positive half‑cell.
A positive E°cell means the reaction is spontaneous under standard conditions; a negative E°cell means it is non‑spontaneous and the reverse reaction would be spontaneous.
The magnitude of E°cell shows the strength of the driving force – the larger the positive value, the greater the tendency for the redox reaction to proceed spontaneously.

Examiner tips

  • State the formula for E°cell and explain the sign of each half‑cell potential.
  • Link a positive E°cell to spontaneity and a negative one to non‑spontaneity.
  • Mention that a larger positive value gives a stronger driving force.
  • Use the exact wording ‘standard hydrogen electrode’ and ‘standard conditions’.

Common mistakes

  • Confusing the sign of the half‑cell potentials (writing E°cell = E°(anode) – E°(cathode)).
  • Forgetting to specify that the potentials are measured relative to the SHE.
  • Claiming that any positive E°cell guarantees a reaction will occur without noting it is under standard conditions.

Mark scheme (4 marks)

  1. E°cell is calculated as E°(cathode/reduction) minus E°(anode/oxidation), or equivalently E°(more positive half-cell) minus E°(less positive half-cell).
  2. A positive E°cell indicates the reaction is spontaneous (feasible) under standard conditions.
  3. A negative E°cell indicates the reaction is non-spontaneous under standard conditions (the reverse reaction would be spontaneous).
  4. The magnitude of E°cell indicates the relative driving force (tendency) of the reaction — a larger positive value corresponds to a greater tendency for the redox reaction to proceed spontaneously.

Key terms in this question

standard cell potential (E°cell)

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