Question

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 :
$MnO^- _4(aq)+8H^+ (aq)+5e^- \longrightarrow Mn^{2+} (aq)+4H_2 O(l), E^\circ = 1.51\, V$
$Cr_2O^{2-} _7(aq)+14H^+ (aq)+6e^- \longrightarrow 2Cr^{3+} (aq)+7H_2 O(l), E^\circ = 1.38\, V$
$Fe^{3+} (aq)+e^- \longrightarrow Fer^{2+} (aq) E^\circ=0.77\, V$
$Cl_2 (g)+2e^- \longrightarrow 2Cl^{-} (aq) E^\circ=1.40\, V$
Identify the incorrect statement regarding the quantitative estimation of aqueous $Fe(NO_3)_2$

• A. $MnO^- _4$ can be used in aqueous $HCl$
• B. $Cr_2O^{2-} _7$ can be used in aqueous $HCl$
• C. $MnO^- _4$ can be used in aqueous $H_2 SO_4$
• D. $Cr_2O^{2-} _7$ can be used in aqueous $H_2 SO_4$

Answer 1

Answer: (A)

$MnO^- _4$ cannot be used for oxidation of $Fe^{2+}$ in $HCl$ medium because the following reaction is spontaneous :
$MnO^- _4 +Cl^- \longrightarrow Mn^{2+} \, +Cl_2; E^\circ=1.51-1.40=0.11\, V$
In all other cases, the redox process between oxidising agent and medium $(HCl$ or $ H_2 SO_4)$ are non-spontaneous, would not interfere oxidation of $Fe^{2+}$.