Question

Which of the following order of $E^0$ $M^{+3}$/$M^{+2}$ is not correct?

• A. Sc<V
• B. Mn < Zn
• C. Mn < Fe
• D. Sc < Zn

Answer 1

Answer: (C)

(3)

Answer 2

The standard electrode potential $E^0 (M^{+3}/M^{+2})$ represents the tendency of the $M^{+3}$ ion to be reduced to the $M^{+2}$ ion:

$M^{+3}(aq) + e^- \rightarrow M^{+2}(aq)$

A higher (more positive) $E^0$ value indicates a greater tendency for $M^{+3}$ to be reduced to $M^{+2}$, implying that $M^{+2}$ is more stable than $M^{+3}$. Conversely, a lower (more negative) $E^0$ value indicates that $M^{+3}$ is more stable than $M^{+2}$.

• $E^0 (Sc^{+3}/Sc^{+2})$ is very negative because $Sc^{+3}$ (noble gas configuration) is extremely stable, making $Sc^{+2}$ very unstable.
• $E^0 (V^{+3}/V^{+2})$ is -0.26 V.
• $E^0 (Mn^{+3}/Mn^{+2})$ is +1.57 V because $Mn^{+2}$ ($3d^5$) is exceptionally stable, so $Mn^{+3}$ readily reduces.
• $E^0 (Fe^{+3}/Fe^{+2})$ is +0.77 V because $Fe^{+3}$ ($3d^5$) is stable, making its reduction less favorable than $Mn^{+3}$.
• $E^0 (Zn^{+3}/Zn^{+2})$ is very positive because $Zn^{+2}$ ($3d^{10}$) is extremely stable, making $Zn^{+3}$ highly unstable and its reduction very favorable.

Comparing the values, the incorrect order is Mn < Fe (+1.57 V < +0.77 V) because +1.57 V is greater than +0.77 V.