Electrochemistry 2 Question 31
30. While $\mathrm{Fe}^{3+}$ is stable, $\mathrm{Mn}^{3+}$ is not stable in acid solution because
(a) $\mathrm{O}_{2}$ oxidises $\mathrm{Mn}^{2+}$ to $\mathrm{Mn}^{3+}$
(b) $\mathrm{O}_{2}$ oxidises both $\mathrm{Mn}^{2+}$ to $\mathrm{Mn}^{3+}$ and $\mathrm{Fe}^{2+}$ to $\mathrm{Fe}^{3+}$
(c) $\mathrm{Fe}^{3+}$ oxidises $\mathrm{H}{2} \mathrm{O}$ to $\mathrm{O}{2}$
(d) $\mathrm{Mn}^{3+}$ oxidises $\mathrm{H}{2} \mathrm{O}$ to $\mathrm{O}{2}$
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Solution:
- For the reaction :
(i) $4 \mathrm{Fe}^{3+}+2 \mathrm{H}{2} \mathrm{O} \longrightarrow 4 \mathrm{Fe}^{2+}+4 \mathrm{H}^{+}+\mathrm{O}{2} ; E^{\circ}=-0.46 \mathrm{~V}$
(ii) $4 \mathrm{Mn}^{3+}+2 \mathrm{H}{2} \mathrm{O} \rightarrow 4 \mathrm{Mn}^{2+}+4 \mathrm{H}^{+}+\mathrm{O}{2} ; E^{\circ}=+0.27 \mathrm{~V}$
As evidenced above, reaction (i) is non-spontaneous, therefore, $\mathrm{Fe}^{3+}$ is stable in acid solution. However, reaction (ii) is spontaneous $\mathrm{Mn}^{3+}$ oxidises $\mathrm{H}{2} \mathrm{O}$ to $\mathrm{O}{2}$ and itself reduced to $\mathrm{Mn}^{2+}$ in acidic medium.
