Electrochemistry 1 Question 14
14. Standard electrode potential data are useful for understanding the suitability of an oxidant in a redox titration. Some half-cell reactions and their standard potentials are given below :
$$ \begin{aligned} & \mathrm{MnO}{4}^{-}(a q)+8 \mathrm{H}^{+}(a q)+5 e^{-} \longrightarrow \mathrm{Mn}^{2+}(a q)+4 \mathrm{H}{2} \mathrm{O}(l) \ & E^{\circ}=1.51 \mathrm{~V} \ & \mathrm{Cr}{2} \mathrm{O}{7}^{2-}(a q)+14 \mathrm{H}^{+}(a q)+6 e^{-} \longrightarrow 2 \mathrm{Cr}^{3+}(a q)+7 \mathrm{H}_{2} \mathrm{O}(l) \ & E^{\circ}=1.38 \mathrm{~V} \ & E^{\circ}=0.77 \mathrm{~V} \ & \mathrm{Fe}^{3+}(a q)+e^{-} \longrightarrow \mathrm{Fe}^{2+}(a q) \quad E^{\circ}=1.40 \mathrm{~V} \end{aligned} $$
Identify the incorrect statement regarding the quantitative estimation of aqueous $\mathrm{Fe}\left(\mathrm{NO}{3}\right){2}$
$(2002,3 M)$
(a) $\mathrm{MnO}_{4}^{-}$can be used in aqueous $\mathrm{HCl}$
(b) $\mathrm{Cr}{2} \mathrm{O}{7}^{2-}$ can be used in aqueous $\mathrm{HCl}$
(c) $\mathrm{MnO}{4}^{-}$can be used in aqueous $\mathrm{H}{2} \mathrm{SO}_{4}$
(d) $\mathrm{Cr}{2} \mathrm{O}{7}^{2-}$ can be used in aqueous $\mathrm{H}{2} \mathrm{SO}{4}$
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Answer:
Correct Answer: 14. (d)
Solution:
- $\mathrm{MnO}_{4}^{-}$cannot be used for oxidation of $\mathrm{Fe}^{2+}$ in $\mathrm{HCl}$ medium because the following reaction is spontaneous :
$\mathrm{MnO}{4}^{-}+\mathrm{Cl}^{-} \longrightarrow \mathrm{Mn}^{2+}+\mathrm{Cl}{2} ; \quad E^{\circ}=1.51-1.40=0.11 \mathrm{~V}$
In all other cases, the redox process between oxidising agent and medium $\left(\mathrm{HCl}\right.$ or $\left.\mathrm{H}{2} \mathrm{SO}{4}\right)$ are non-spontaneous, would not interfere oxidation of $\mathrm{Fe}^{2+}$.
