Question

The $E^{\circ}_{M^{3+}/ M^{2+}}$ 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. $Cr$
• B. $Mn$
• C. $Fe$
• D. $Co$

Answer 1

Answer: (A)

$E_{ Cr ^{3+} / Cr ^{2+}}^{\circ}=-0.41\, V $

$E_{ Mn ^{3+} / Mn ^{2+}}^{\circ}=+1.57\, V $

$ E^{\circ} Fe ^{3+} / Fe ^{2+} =+0.77 \,V$

$ E_{ Co ^{3+} / C 0^{2}+}^{\circ} =+1.9$

More negative value of $E_{\text {red }}^{\circ}$ indicates better reducing agent, thus easily oxidised. Thus, oxidation of $Cr ^{2+}$ to $Cr ^{3+}$ is the easiest.