Question

Which of the following can function as an oxidizing as well as a reducing agent ?

• A. $H_{2}S$
• B. $H_{2}S{O}_4$
• C. $H_2{O}_2$
• D. $S{O}_3$

Answer 1

The correct option is C $H_2{O}_2$

A species can work both as an oxidizing agent and a reducing agent if the central atom is present in the intermediate oxidation state so that it can be oxidized (an increase of oxidation number), playing the role of a reducing agent, as well as reduced (decrease of oxidation number ), playing the role of an oxidizing agent.

For $S,$ the lowest oxidation number is $-2$ and the highest oxidation number is $+6$.
a)
In $H_{2}S$ , the oxidation number of $S$ is $-2$ which is the lowest O.N. of sulfur. Therefore it can acts as a reducing agent by oxidizing itself.
b)
In $H_{2}S{O}_4$ , the oxidation number of $S$ is $+6$ which is the highest O.N. of sulfur. Therefore it can acts as a oxidizing agent by reducing itself.
c)
Hydrogen peroxide acts as both a reducing and an oxidizing agent.
When $H_2{O}_2$ serves as an oxidizing agent, the oxygen of hydrogen peroxide (that is present in -1 oxidation state) is reduced to $H_{2}O$ (-2 oxidation state).
When $H_2{O}_2$ (-1 oxidation state) serves as a reducing agent, the oxygen of $H_2{O}_2$ is oxidized to $O_2$ ( 0 oxidation state) and bubbles are noticed. As a reducing agent:
$\stackrel{}{H_2}\stackrel{-1}{O_2}\to 2H_2\stackrel{-2}{O}+\stackrel{0}{O_2}$.

In $H_2{O}_2$ , the oxidation number of central metal atom $O$ is $-1$ as peroxide linkage is present. The oxygen here present in the intermediate oxidation state.Therefore it can acts as both oxidizing as well as reducing agent.

d)
In $S{O}_3$ , the oxidation number of $S$ is $+6$ which is the highest O.N. of sulfur. Therefore it can acts as a oxidizing agent by reducing itself.