Q. Given, standard electrode potentials ${Fe}^{2 +} + 2e^{-} \rightarrow Fe,\ E{^\circ} = - 0.440\ V$ ${Fe}^{3 +} + 3e^{-} \rightarrow Fe,\ E{^\circ} = - 0.036\ V$ The standard electrode potential $\left( E^{{^\circ}} \right)$ for ${Fe}^{3 +} + e^{-} \rightarrow {Fe}^{2 +}$ is
A) $+ \ 0.771\ V$
B) $- \ 0.771\ V$
C) $+ \ 0.417\ V$
D) $- \ 0.417\ V$
Explanation
(a)
$\mathrm{\Delta}G{^\circ} = - nFE{^\circ}$
$Fe^{2 +} + 2e^{-} \rightarrow Fe\ \ \ \ \ \ \$…(i)
$\mathrm{\Delta}G{^\circ} = - 2 \times F \times ( - 0.440\ V) = 0.880\ F$
${Fe}^{3 +} + 3e^{-} \rightarrow Fe\ \ \ \ \ \ \ \$…(ii)
$\mathrm{\Delta}G{^\circ} = - 3 \times F \times ( - 0.036) = 0.108\ F$
On subtracting Eqs. (i) from (ii), we get
${Fe}^{3 +} + e^{-} \rightarrow {Fe}^{2 +}$
$\mathrm{\Delta}G{^\circ} = 0.108\ F - 0.880\ F = - 0.772\ F$
$E{^\circ}\ for\ the\ reaction = - \frac{\mathrm{\Delta}G{^\circ}}{nF} = - \frac{( - 0.772\ F)}{1 \times F} = + 0.772\ V$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.