Question #44147

This problem tests the student's ability to correctly calculate standard cell potentials from a Latimer diagram using Gibbs free energy additivity, rather than simply adding the potentials (a common trap leading to Option C).

Furthermore, it challenges the intuitive assumption that comproportionation to an intermediate oxidation state is always spontaneous.

By setting the reduction potential of the intermediate ($\ce{An^{3+}} \rightarrow \ce{An^{2+}}$) higher than the potential forming it ($\ce{AnO2^+} \rightarrow \ce{An^{3+}}$), the net $E^\circ_{\text{cell}}$ becomes negative, resulting in a positive $\Delta G^\circ$.

Students must also correctly identify the number of electrons transferred ($n=2$) in the balanced overall reaction to compute the correct extensive property $\Delta G^\circ$ (failing to do so leads to Option B).