A-Level Chemistry May/June 2025 Q5(d)(i): Copper metal can be oxidised by acidified KMnO4. The relevant half-equations and their…

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  The Nernst equation at 298 K is: $E = E^\ominus + \frac{0.059}{z} \log \frac{[\text{oxidised species}]}{[\text{reduced species}]}$

  For the half-equation $\text{MnO}_4^- + 8\text{H}^+ + 5\text{e}^- \rightleftharpoons \text{Mn}^{2+} + 4\text{H}_2\text{O}$:

  • The number of electrons transferred, $z = 5$.
  • The oxidised species are $\text{MnO}_4^-$ and $\text{H}^+$.
  • The reduced species is $\text{Mn}^{2+}$.

  Substituting the given values into the Nernst equation: $E = 1.52 + \frac{0.059}{5} \log \frac{[\text{MnO}_4^-][\text{H}^+]^8}{[\text{Mn}^{2+}]}$ $E = 1.52 + \frac{0.059}{5} \log \frac{(0.0020)(1.0)^8}{(1.0)}$ $E = 1.52 + \frac{0.059}{5} \log(0.0020)$

  Calculating the final value: $E = 1.52 + (0.0118 \times -2.699)$ $E = 1.52 - 0.0318...$ $E = +1.488 \text{ V} \approx +1.49 \text{ V}$

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  How the marks are awarded

  • M1 — Correctly stating the Nernst equation in the form E = E° + (0.059/z) log([oxidised species]/[reduced species]).
  • M2 — Correctly substituting all values into the equation, specifically E°=+1.52V, z=5, and the concentrations which simplify the log term to log(0.0020) as shown in the working.

  Common mistakes

  • Using the wrong value for 'z' (the number of electrons transferred), for example using z=2 from the copper half-equation instead of z=5.
  • Incorrectly constructing the log term, for example by inverting the fraction to log([red]/[ox]) without changing the preceding '+' sign to a '-'.
  • Forgetting to raise the concentration of H+ ions to the power of its stoichiometric coefficient (8) in the log term. Although it makes no numerical difference here as [H+]=1.0, it is a crucial conceptual step.
  • Using the natural logarithm (ln) instead of the base-10 logarithm (log) with the 0.059 constant.

  Examiner tip: Always write out the full expression for the Nernst equation's log term, including all species and their stoichiometric powers, before substituting numerical values to avoid simplification errors.

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