 Solution

ID: past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2021/paper-1/6/i/solution

The species that oxidizes nickel is gold(III) ion, rather than metallic gold. The half-reactions are

Ni A u^{3 +} + 3 e^{-} ⟶ N i^{2 +} + 2 e^{-}, ⟶ Au .

Multiplying the first by three and the second by two gives

3Ni + 2A u^{3 +} ⟶ 3N i^{2 +} + 2Au .

Standard electrode potentials are not multiplied by the balancing factors. Thus

E_{cell}^{\circ} = 1.50 - (- 0.25) = 1.75 V

and six moles of electrons are transferred per mole of this balanced reaction:

Δ G^{\circ} = - 6 F E_{cell}^{\circ} ≃ - 1.01 \times 1 0^{6} J mo l^{- 1} .

The negative sign predicts a thermodynamically favorable galvanic replacement under standard conditions.

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