Question

Hydrogen peroxide acts as an oxidising and as a reducing agent, depending upon the nature of the reacting species. In which of the following cases $H_2{O}_2$ acts as a reducing agent in acid medium?
(A) ${MnO}_4^{-}$
(B) ${Cr}_2{O}_7^{2-}$
(C) ${SO}_3^{2-}$
(D) $KI$

Answer 1

Hint: In redox reactions, one species is oxidised while the other gets reduced. Not all chemical species can oxidise or reduce each other. Whether it will be oxidised or reduced by another species depends upon their redox potential which is a measure of the tendency of a species to accept or donate electrons to an electrode (generally the Standard hydrogen electrode) and thus be reduced or oxidised respectively.

Complete step by step solution:
Before solving this question, we need to understand the meaning of a redox reaction. In redox reactions, one species is oxidised while the other gets reduced. Not all chemical species can oxidise or reduce each other. Whether it will be oxidised or reduced by another species depends upon their redox potential which is a measure of the tendency of a species to accept or donate electrons to an electrode (generally the Standard hydrogen electrode) and thus be reduced or oxidised respectively.
An oxidising agent will itself get reduced by accepting electrons thereby oxidising the other species. A reducing agent gets itself oxidised by donating electrons thereby reducing the other species. If we have two species that can undergo redox reaction, then the species with greater redox potential will get reduced since it will have a greater tendency to get reduced as is indicated by its greater redox potential. It will accept electrons from the other species; itself getting reduced while oxidising the other species. The redox potential for a particular species also depends upon the medium of the reaction i.e. whether the reaction is carried out in acidic medium, basic medium or in neutral medium. Hence the value of the redox potential for a particular species will be different depending on whether the reaction is carried out in acidic, basic or neutral medium. Also, if for a redox reaction, its total redox potential comes out to be positive then the reaction is feasible.
Now, the standard reduction potential for $H_2{O}_2$ in acidic medium is 1.763 V while its oxidation potential is -0.682 V. The standard reduction potential for ${MnO}_4^{-}$ in acidic medium is 1.49 V. The standard reduction potential for ${Cr}_2{O}_7^{2-}$ in acidic medium is 1.33 V. ${SO}_3^{2-}$ ion is already reduced, hence it will not get reduced by $H_2{O}_2$. Similarly in KI, the iodide ion is already reduced, hence it will not get reduced by $H_2{O}_2$. The reduction potential of $K^{+}$ ion in acidic medium is -2.925 V.
From all these values we come to the conclusion that the redox potential for the reaction involving the oxidation of $H_2{O}_2$ and reduction of ${MnO}_4^{-}$ is 0.808 V. Similarly the redox potential for the reaction involving the oxidation of $H_2{O}_2$ and reduction of ${Cr}_2{O}_7^{2-}$ is 0.648 V. Hence $H_2{O}_2$ will act as a stronger reducing agent with ${MnO}_4^{-}$ than with ${Cr}_2{O}_7^{2-}$ (although both reactions are feasible).

Hence the correct answer is (A) ${MnO}_4^{-}$.

Note: The redox potentials are actually the reduction potentials of the species and not the oxidation potentials so if the redox potential of a species is given, it is actually its reduction potential. Also if the redox potential of a species is more negative than the other, then the former has a less tendency to get reduced and the latter has a more tendency to get reduced.