Question

An iron wire is immersed in a solution containing $ {\text{ZnS}}{{\text{O}}_{\text{4}}} $ and $ {\text{NiS}}{{\text{O}}_{\text{4}}} $ . Predict by giving reasons which of the following reactions is likely to proceed?
(A) Iron reduces $ {\text{Z}}{{\text{n}}^{{\text{2 + }}}} $ ions.
(B) Iron reduces $ {\text{N}}{{\text{i}}^{{\text{2 + }}}} $ ions.
Given: $ {\text{E}}_{{\text{Z}}{{\text{n}}^{{\text{2 + }}}}{\text{/Zn}}}^{\text{o}}{\text{ = }} - 0.76 $ volt; $ {\text{E}}_{{\text{F}}{{\text{e}}^{{\text{2 + }}}}{\text{/Fe}}}^{\text{o}}{\text{ = }} - 0.44 $ volt and $ {\text{E}}_{{\text{N}}{{\text{i}}^{{\text{2 + }}}}{\text{/Ni}}}^{\text{o}}{\text{ = }} - 0.25 $ volt.

Answer 1

Hint: In the above question, an iron wire is immersed in a solution containing $ {\text{ZnS}}{{\text{O}}_{\text{4}}} $ and $ {\text{NiS}}{{\text{O}}_{\text{4}}} $ . And we are provided with the reduction potential of different metals. We can answer the question on the basis of reduction potential. If the reduction potential of iron is more than the reduction potential of the metal in which it is dissolved then only the reaction will take place.

Complete step by step solution:
Since, we are given that an iron wire is immersed in a solution containing $ {\text{ZnS}}{{\text{O}}_{\text{4}}} $ and $ {\text{NiS}}{{\text{O}}_{\text{4}}} $ and reduction potential of zinc, iron and nickel are given and hence, we can directly answer the question on reduction potential basis.
Iron reduces $ {\text{Z}}{{\text{n}}^{{\text{2 + }}}} $ ions:
The reduction potential of iron, i.e. $ {\text{E}}_{{\text{F}}{{\text{e}}^{{\text{2 + }}}}{\text{/Fe}}}^{\text{o}}{\text{ = }} - 0.44 $ volt and the reduction potential of zinc, i.e., $ {\text{E}}_{{\text{Z}}{{\text{n}}^{{\text{2 + }}}}{\text{/Zn}}}^{\text{o}}{\text{ = }} - 0.76 $ volt. We can notice that reduction potential of iron is less than reduction potential of zinc and hence, no reaction takes place and iron does not reduce $ {\text{Z}}{{\text{n}}^{{\text{2 + }}}} $ ions.
Iron reduces $ {\text{N}}{{\text{i}}^{{\text{2 + }}}} $ ions:
The reduction potential of iron, i.e. $ {\text{E}}_{{\text{F}}{{\text{e}}^{{\text{2 + }}}}{\text{/Fe}}}^{\text{o}}{\text{ = }} - 0.44 $ volt and the reduction potential of nickel , i.e., $ {\text{E}}_{{\text{N}}{{\text{i}}^{{\text{2 + }}}}{\text{/Ni}}}^{\text{o}}{\text{ = }} - 0.25 $ volt. We can see that reduction potential of iron is more than reduction potential of nickel and hence, reaction takes place and iron gets oxidised and reduces the $ {\text{N}}{{\text{i}}^{{\text{2 + }}}} $ ions.

Note:
In these types of questions which involve the reduction potential, we should not consider the sign. Only the magnitude part should be considered. If the magnitude is greater, then its reduction potential is more.