Standard electrode potential data are useful for understanding the suitability of an oxidant in a redox titration. Some half cell reaction and their standard potentials are given below:
`MnO_(4)^(-)(aq) +8H^(+)(aq) +5e^(-) rarr Mn^(2+)(aq) +4H_(2)O(l) E^(@) = 1.51V`
`Cr_(2)O_(7)^(2-)(aq) +14H^(+) (aq) +6e^(-) rarr 2Cr^(3+)(aq) +7H_(2)O(l), E^(@) = 1.38V`
`Fe^(3+) (aq) +e^(-) rarr Fe^(2+) (aq), E^(@) = 0.77V`
`CI_(2)(g) +2e^(-) rarr 2CI^(-)(aq), E^(@) = 1.40V`
Identify the only correct statement regarding quantitative estimation of aqueous `Fe(NO_(3))_(2)`

Options

A. `MnO_(4)^(-)` can be used in aqeuous 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)`.

Answer

A

Explanation

When `MnO_(4)^(-)` is used in a queous HCl, `MnO_(2)` reacts wit `Fe^(2+)` as well as with HCl as explained below.
`MnO_(4)^(-)+5Fe^(2+)+8H^(+)rarr5Fe^(3+)+ 4H_(2)O+Mn^(2+)`
`E_("Cell")^(@)=1.51- 0.77=0.74Vgt0`
`MnO_(4)^(-)+16H^(+)+10Cl^(-)rarrMn^(2+)+8H_(2)O+5Cl_(2)`
`E_("cell"^(@)=1.51-0.77=0.74Vgt0`
As such `MnO_(4)^(-)` cannot be used for the quantitative estimation of aqueous `Fe(NO_(3))_(2)` . Thus option A is not correct. Similarly we can show that `Cr_(2)O_(7)^(2-)` will react with `Fe^(2+)` but not with HCl.
`Cr_(2)O_(7)^(2-)+6Fe^(2+)+14H^(+)rarr2Cr^(3+)+7H_(2)O+6 Fe^(3+)`
`E_("cell")^(@)=1.38-0.77=0.61Vgt0`
`Cr_(2)O_(7)^(2-)+ 6Cl^(-)+14H^(+)rarr2Cr^(3+)+7H_(2)O^(+)3Cl_(2)`
`E_("cell")^(@)=1.38-1.40=-0.02Vlt0`
Thus, the normal reactions is not possible Thus `Cr_(2)O^(7)^(-)` in aqueous HCl can be used for the quantitative estimation of aqueous `Fe(NO_(3))_(2)`.
Option C and D are correct.