Question

The ${}_{M^{3+}/M^{2+}}^o$ values for Cr, Mn, Fe and Co are $-0.41,+1.57,+0.77$ and $+1.97$V respectively. For which one of these metals the change in oxidation state from $+2$ to $+3$ is easiest?

• A. Fe
• B. Mn
• C. Cr
• D. Co

Answer 1

The correct option is C Cr

Given standard reduction potential as:

$C{r}^{3+}\left(aq\right)+e^{-}\rightleftharpoons C{r}^{2+},-0.41$ V

$M{n}^{3+}\left(aq\right)+e^{-}\rightleftharpoons M{n}^{2+},1.57$ V

$F{e}^{3+}\left(aq\right)+e^{-}\rightleftharpoons F{e}^{2+},+0.77$ V

$C{o}^{3+}\left(aq\right)+e^{-}\rightleftharpoons C{o}^{2+},1.97$ V

Reduction potential is a measure of the tendency of a chemical species to acquire electrons and thereby get reduced. Higher is the reduction potential more is the tendency to get reduced and change its oxidation from +3 to +2. Stronger is the oxidising agent or vice-versa.

Thus lower reduction potential specie will easily oxidised from +2 to +3 oxidation state.

$C{r}^{3+}\left(aq\right)+e^{-}\rightleftharpoons C{r}^{2+},-0.41$ V has lowest reduction potential is the strongest reducing agent and will be easiest to change its oxidation state from +2 to +3.