The $E_{{\mathrm{M}}^{3+}\mid {\mathrm{M}}^{2+}}^{\circ }$ values for$\mathrm{Cr}$ $\mathrm{Mn}$, $\mathrm{Fe}$ and $\mathrm{Co}$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 easiest?

We are given that ${\mathrm{Cr}}^{3+}+{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{Cr}}^{2+}{E}^{\circ }=-0.41\mathrm{V}{\mathrm{Fe}}^{3+}+{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{Fe}}^{2+}{E}^{\circ }=0.77\mathrm{V}$${\mathrm{Mn}}^{3+}+{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{Mn}}^{2+}{E}^{\circ }=1.57\mathrm{V}{\mathrm{Co}}^{3+}+{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{Co}}^{2+}{E}^{\circ }=1.97\mathrm{V}$${\mathrm{Cr}}^{3+}$ will show least tendency to convert to ${\mathrm{Cr}}^{2+}$. On the other hand ${\mathrm{Cr}}^{2+}$ will show maximum tendency to convert to ${\mathrm{Cr}}^{3-}$.