Question

From the following data at $ 25^{\circ}C $ ,
$ Cr^{3+}_{(aq)} + e^{-} \to Cr^{ 2+}_{(aq)}, E^{\circ} = - 0.424\,V $
$ Cr^{2+}_{(aq)} + 2e^{-} \to Cr_{(s)}, E^{\circ} = - 0.900\,V $
Find $ E^{\circ} $ at $ 25^{\circ} C $ for the reaction,
$ Cr^{3+} +3e^- \to Cr_{(s)} $

• A. - 0.741 V
• B. - 1.324 V
• C. - 0.476 V
• D. + 0.741 V

Answer 1

Answer: (B)

$(i) \,Cr^{3+} + e^- \to Cr^{2+}, E_1 ^ {\circ}= - 0.424 \,V$
$(ii) \,Cr^{2+} + 2e^- \to Cr^-, E_2^{\circ} = - 9.00\,V$
$(iii)\, Cr^{3+} + 3e^- \to Cr, E^{\circ} =?$
Since, reaction $(i)$ and $(ii)$ are performed at cathode. Therefore, for reaction $(iii)$
$E^{\circ} =E_1^{\circ} + E_2^{\circ}$
$= (-0.424)+ (-0.900)$
$= - 1.324 \,V$