Question

For $M^{2+}/M$ and $M^{3+}/M^{2+}$ systems the $E^{\ominus }$ values for some metals are as follows:

$C{r}^{2+}/Cr$	$-0.9V$$	$C{r}^3/C{r}^{2+}$	$-0.4V$
$M{n}^{2+}/Mn$	$-1.2V$	$M{n}^{3+}/M{n}^{2+}$	$+1.5V$
$F{e}^{2+}/Fe$	$-0.4V$	$F{e}^{3+}/F{e}^{2+}$	$+0.8V$

Use this data to comment upon:
(i) the stability of $F{e}^{3+}$ in acid solution as compared to that of $C{r}^{3+}$ or $M{n}^{3+}$ and
(ii) the ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal

Answer 1

(a) The reduction potential of $C{r}^{3+}/C{r}^{2+}$ is negative and hence, $Cr\left(III\right)$ is most stable as it cannot be reduced to $Cr\left(II\right)$.
$M{n}^{3+}/M{n}^{2+}$ has large positive value and $Mn\left(III\right)$ is least stable as it can be easily reduced to $Mn\left(II\right)$.
$F{e}^{3+}/F{e}^{2+}$ has small positive value and $Fe\left(III\right)$ is more stable than $Mn\left(III\right)$ but less stable than $Cr\left(III\right)$.
(b) $M{n}^{2+}/Mn$ has most negative reduction potential and it is most easily oxidised. The ease of oxidation is $Mn>Cr>Fe$.