Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2019 / Paper 1 / Section A(iii) / 6 / ii

With $n=2$, the standard reaction Gibbs free energy and dimensionless equilibrium constant areFor the reaction as written, $Q=a_{\mathrm{F}{\mathrm{e}}^{2+}}/a_{\mathrm{S}{\mathrm{n}}^{2+}}$, and the Nernst equation gives $E=E^{\circ }-(RT/2F)\ln Q$. Consequently zero cell potential requires $[\mathrm{F}{\mathrm{e}}^{2+}]/[\mathrm{S}{\mathrm{n}}^{2+}]=K\simeq 1.93\times 10^{10}$ under the specified unit-activity-coefficient assumption. Reversing that ratio reverses the conclusion. This mathematical ratio does not by itself guarantee experimentally attainable concentrations without precipitation or other chemistry.