Question

The standard redox potentials for the reactions
$M{n}^{2+}+2e^{-}\to Mn$ and $M{n}^{3+}+e^{-}\to M{n}^{2+}$
are $-1.18V$ and $1.51V$ respectively. What is the redox potential for the reaction $M{n}^{3+}+3e^{-}\to Mn$

• A. $0.33V$
• B. $1.69V$
• C. $-0.28V$
• D. $-0.85V$
• E. $0.85V$

Answer 1

Answer: (C)

Given: (i) $M{n}^{2+}+2e^{-}\to Mn;E{}^{\circ }=-1.18V;$
$\Delta G_1^o=-2\times F\times -1.18J$ (ii)
$M{n}^{3+}+e^{-}\to M{n}^{2+};E^o=1.51V;$
$\Delta G_2^o=-1\times F\times 1.51J$ Aim: $M{n}^{3+}+3e^{-}\to Mn;\Delta G_3^o=?$
(i) + (ii) gives the required results,
i.e., $\Delta G_3^o=\Delta G_1^o+\Delta G_2^o$
$=2.36F+(-1.51F)=0.85F$
$\therefore$ $-nF{E}_{M{n}^{3+}/Mn}^o=0.85F$
Or $E_{M{n}^{3+}/Mn}^o=-\frac{0.85}{3}V$
$(\because n=3)$ $=-0.28V$