Electrochemistry - Result 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 :
$$ \begin{aligned} & MnO _4^{-}(a q)+8 H^{+}(a q)+5 e^{-} \longrightarrow Mn^{2+}(a q)+4 H _2 O(l) \ & E^{\circ}=1.51 V \ & Cr _2 O _7^{2-}(a q)+14 H^{+}(a q)+6 e^{-} \longrightarrow 2 Cr^{3+}(a q)+7 H _2 O(l) \ & E^{\circ}=1.38 V \ & E^{\circ}=0.77 V \ & Fe^{3+}(a q)+e^{-} \longrightarrow Fe^{2+}(a q) \quad E^{\circ}=1.40 V \end{aligned} $$
Identify the incorrect statement regarding the quantitative estimation of aqueous $Fe\left(NO _3\right) _2$
$(2002,3 M)$
(a) $MnO _4^{-}$can be used in aqueous $HCl$
(b) $Cr _2 O _7^{2-}$ can be used in aqueous $HCl$
(c) $MnO _4^{-}$can be used in aqueous $H _2 SO _4$
(d) $Cr _2 O _7^{2-}$ can be used in aqueous $H _2 SO _4$
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Answer:
Correct Answer: 14. (d)
Solution:
- $MnO _4^{-}$cannot be used for oxidation of $Fe^{2+}$ in $HCl$ medium because the following reaction is spontaneous :
$MnO _4^{-}+Cl^{-} \longrightarrow Mn^{2+}+Cl _2 ; \quad E^{\circ}=1.51-1.40=0.11 V$
In all other cases, the redox process between oxidising agent and medium $\left(HCl\right.$ or $\left.H _2 SO _4\right)$ are non-spontaneous, would not interfere oxidation of $Fe^{2+}$.
