Q44 / 180 · COMEDK 2025 (10 May Shift 2) · Chemistry
The Standard Reduction potential at 25°C for $(\text{MnO}_4)^{-1}/\text{H}^+$ is +1.49 V. The $E^0$ values for four redox couples:
(a). $\text{Co}^{3+}/\text{Co}^{2+}$, (b). $\text{Cr}^{3+}/\text{Cr}$, (c). $\text{Au}^{3+}/\text{Au}$ and (d). $\text{Ag}^+/\text{Ag}$ are +1.81 V, –0.74 V, +1.50 V and +0.8 V respectively.
Identify the two reduced species which cannot be oxidised by $(\text{MnO}_4)^{-1}/\text{H}^+$
- Correct answerAa & c
- Bb & d
- Cb & c
- Da & d
Solution
$(MnO_4)^-/H^+$ has $E° = +1.49$ V. It can only oxidise species whose $E°$ is less than $+1.49$ V.
Species with $E° >$ +1.49 V cannot be oxidised by permanganate:
$Co^{3+}/Co^{2+}$: $E° = +1.81$ V $>$ +1.49 V $\Rightarrow$ cannot be oxidised
$Au^{3+}/Au$: $E° = +1.50$ V $>$ +1.49 V $\Rightarrow$ cannot be oxidised
$Cr^{3+}/Cr$: $E° = -0.74$ V $<$ +1.49 V $\Rightarrow$ can be oxidised
$Ag^+/Ag$: $E° = +0.8$ V $<$ +1.49 V $\Rightarrow$ can be oxidised
The two species that cannot be oxidised are (a) and (c).
Species with $E° >$ +1.49 V cannot be oxidised by permanganate:
$Co^{3+}/Co^{2+}$: $E° = +1.81$ V $>$ +1.49 V $\Rightarrow$ cannot be oxidised
$Au^{3+}/Au$: $E° = +1.50$ V $>$ +1.49 V $\Rightarrow$ cannot be oxidised
$Cr^{3+}/Cr$: $E° = -0.74$ V $<$ +1.49 V $\Rightarrow$ can be oxidised
$Ag^+/Ag$: $E° = +0.8$ V $<$ +1.49 V $\Rightarrow$ can be oxidised
The two species that cannot be oxidised are (a) and (c).
Hence, the correct answer is option: 'A'.