b
\(F e^{3+}+3 e^{-} \rightarrow F e, E^{o}_{F e^{3} 4 / F e}\)

\(=-0.036 V \ldots(i)\)

\(F e^{2+}+2 e^{-} \rightarrow F e, E^{\circ}_{F e^{2} / / F e}\)

\(=-0.439 V \ldots(i i)\)

we have to calculate

\(F e^{3+}+e^{-} \rightarrow F e^{2+}, \Delta G=?\)

To obtain this equation subtract equ \(( ii )\) from \(( i )\) we get

\(F e^{3+}+e^{-} \rightarrow F e^{2+} \ldots(i i i)\)

As we know that \(\Delta G=-n F E\)

Thus for reaction ( iii)

\(\Delta G=\Delta G_{1}-\Delta G\)

\(-n F E^{\circ}=-n F E_{1}-\left(-n F E_{2}\right)\)

\(-n F E^{\circ}=n F E_{2}-n F E_{1}\)

\(-1 F E^{\circ}=2 \times 0.439 F-3 \times 0.036 F\)

\(- F E^{\circ}=0.770 F\)

\(\therefore \quad E^{\circ}=-0.770 V\)

\(O^{-}>F^{-}>N a^{+}>M g^{++}>A l^{3+}\)