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.

$M{n}_4^{-}\left(aq\right)+8H^{+}\left(aq\right)+5e^{-}\to M{n}^{2+}\left(aq\right)+4H_{2}O\left(l\right);E^{\circ }=1.51V$
$C{r}_2{O}_7^{2-}\left(aq\right)+14H^{+}+6e^{-}\to 2C{r}^3+\left(aq\right)+7H_{2}O\left(l\right);E^{\circ }=138V$
$F{e}^{3+}\left(aq\right)+e^{-}\to F{e}^{2+}\left(aq\right);E^{\circ }=0.77V$
$C{l}_2\left(g\right)+2e^{-}\to 2C{l}^{-}\left(aq\right);E^{\circ }=1.40V$

Identify the incorrect statement regarding the quantitative estimation of gaseous $Fe(N{O}_3{)}_2$.

• A. $Mn{O}_4^{-}$ can be used in aqueous $HCl$
• B. $C{r}_2{O}_7^{2-}$ can be used in aqueous $HCl$
• C. $Mn{O}_4^{-}$ can be used in aqueous $H_{2}S{O}_4$
• D. $C{r}_2{O}_7^{2-}$ can be used in aqueous $H_{2}S{O}_4$

Answer 1

The correct option is A $Mn{O}_4^{-}$ can be used in aqueous $HCl$

In aqueous HCl medium,

$Mn{O}_4^{-}$ can also oxidise $C{l}^{-}$ to $C{l}_2$

$Mn{O}_4^{-}\left(aq\right)+8H^{+}\left(aq\right)+5e^{-}\to M{n}^{2+}\left(aq\right)+4H_{2}O\left(l\right)$

$2C{l}^{-}\left(aq\right)\to C{l}_2\left(g\right)+2e^{-}$

The standard emf of the cell made from the above two half-reactions is:

$E_{cell}^0$ $=E_R^0-E_L^0$

$=E_{Mn}^0-E_{Cl}^0$

$=1.51-1.40$

$=0.11V$

Since $E_{cell}^0>0$, the reaction is spontaneous.

In all other cases, the redox process between the oxidising agent and medium ($HCl$ or $H_{2}S{O}_4$) are non-spontaneous, would not interfere with oxidation of $F{e}^{2+}$.

Hence, option A is correct.