Explain why a standard hydrogen electrode is assigned a standard electrode potential of exactly 0.00 V, and outline how this value is used to determine the standard electrode potential of another half-cell.

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).

Model answer (4 marks)

The standard hydrogen electrode (SHE) is defined by international convention to have a potential of 0.00 V, not because it is inert but because it provides a convenient reference point. It is therefore used as a universal reference electrode. To find the standard potential of another half‑cell, the unknown electrode is paired with the SHE in a galvanic cell under standard conditions (298 K, 1 atm, 1 mol dm⁻³). The cell voltage (e.m.f.) is measured. Since the SHE contributes 0.00 V, the measured e.m.f. equals the standard electrode potential of the unknown half‑cell; the sign of the voltage indicates whether the unknown electrode is the cathode (positive) or anode (negative) relative to the SHE.

Examiner tips

  • Define SHE as 0.00 V by convention; use this to explain its role as a reference.
  • Describe the standard conditions and the cell set‑up clearly.
  • State that the measured e.m.f. equals the unknown potential because SHE is 0.00 V.
  • Mention the sign of the voltage shows cathode/anode relationship.

Common mistakes

  • Confusing the SHE’s 0.00 V with it having no reaction tendency.
  • Failing to specify standard conditions (298 K, 1 atm, 1 mol dm⁻³).
  • Misstating that the measured voltage is the SHE potential rather than the unknown half‑cell potential.

Mark scheme (4 marks)

  1. The standard hydrogen electrode (SHE) is assigned 0.00 V by international convention / definition, not because it has no tendency to react.
  2. It acts as a universal reference electrode against which all other half-cell potentials are measured.
  3. The unknown half-cell is connected to the SHE under standard conditions (298 K, 100 kPa, 1 mol dm⁻³ solutions), and the voltage (e.m.f.) of the resulting electrochemical cell is measured.
  4. Because the SHE contributes 0.00 V, the measured cell voltage equals the standard electrode potential of the unknown half-cell; its sign indicates whether the half-cell acts as the positive (cathode) or negative (anode) terminal relative to the SHE.

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