A-Level Chemistry May/June 2024 Q4(a): Define standard electrode potential, Eº, including a description of standard conditions.

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  The standard electrode potential, $E^\circ$, is the potential difference (voltage) of a half-cell measured against the standard hydrogen electrode (SHE).

  This measurement is taken under standard conditions, which are:

  • A concentration of $1.0\ \text{mol dm}^{-3}$ for all aqueous ions.
  • A pressure of $101\ \text{kPa}$ for all gases.
  • A temperature of $298\ \text{K}$ ($25\ ^\circ\text{C}$).

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  How the marks are awarded

  • M1 — Defining the standard electrode potential as the voltage of a half-cell measured relative to the standard hydrogen electrode (SHE).
  • M2 — Correctly stating all three standard conditions: a concentration of 1 mol dm⁻³, a pressure of 101 kPa, and a temperature of 298 K.

  Common mistakes

  • Forgetting to mention that the potential is measured relative to the standard hydrogen electrode (SHE).
  • Stating 'room temperature' instead of the specific value of 298 K or 25 °C.
  • Omitting one of the three standard conditions, most commonly the pressure for gases.
  • Using incorrect values for standard conditions, such as 100 kPa for pressure or stating a concentration of 1 g dm⁻³.

  Examiner tip: For definition questions, memorise the full, precise wording and all associated numerical values and units for standard conditions to secure every mark.

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