Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following reactions: (1) \(\mathrm{H}_{2} \mathrm{O}_{2}+2 \mathrm{HX} \longrightarrow \mathrm{X}_{2}+2 \mathrm{H}_{2} \mathrm{O}\) (2) \(\mathrm{H}_{2} \mathrm{O}_{2}+\mathrm{O}_{3} \longrightarrow 2 \mathrm{O}_{2}+\mathrm{H}_{2} \mathrm{O}\) Here, hydrogen peroxide acts as (a) an oxidizing agent in both (1) and (2) (b) an oxidizing agent in (2) and reducing agent in (1)

Step-by-Step Solution

Answer

Hydrogen peroxide acts as (b): oxidizing agent in (2) and reducing agent in (1).

Step 1: Identify Hydrogen Peroxide's Role in Reaction (1)

In reaction (1), \( \mathrm{H}_{2} \mathrm{O}_{2} \) reacts with \( \mathrm{HX} \) to produce \( \mathrm{X}_{2} \). Here, \( \mathrm{H}_{2} \mathrm{O}_{2} \) causes the oxidation of \( \mathrm{X}^{-} \) ions to \( \mathrm{X}_{2} \), indicating that \( \mathrm{H}_{2} \mathrm{O}_{2} \) is acting as an oxidizing agent, since it accepts electrons.

Step 2: Identify Hydrogen Peroxide's Role in Reaction (2)

In reaction (2), \( \mathrm{H}_{2} \mathrm{O}_{2} \) reacts with \( \mathrm{O}_{3} \) to produce \( \mathrm{O}_{2} \). Here, \( \mathrm{H}_{2} \mathrm{O}_{2} \) is being oxidized to \( \mathrm{O}_2 \) itself while reducing \( \mathrm{O}_3 \), meaning that \( \mathrm{H}_{2} \mathrm{O}_{2} \) acts as a reducing agent.

Step 3: Conclusion from Steps 1 and 2

In reaction (1), \( \mathrm{H}_{2} \mathrm{O}_{2} \) acts as an oxidizing agent. In reaction (2), \( \mathrm{H}_{2} \mathrm{O}_{2} \) acts as a reducing agent. These observations match choice (b): hydrogen peroxide is an oxidizing agent in (2) and a reducing agent in (1).

Key Concepts

Oxidizing Agent

An oxidizing agent is a chemical species that causes the oxidation of another substance by accepting electrons from it. In a redox reaction, the oxidizing agent itself gets reduced as it gains electrons.
For example, consider reaction (1): \( \mathrm{H}_{2} \mathrm{O}_{2} + 2 \mathrm{HX} \rightarrow \mathrm{X}_{2} + 2 \mathrm{H}_{2} \mathrm{O} \). Here, hydrogen peroxide (\(\mathrm{H}_{2} \mathrm{O}_{2}\)) is the oxidizing agent. It oxidizes \(\mathrm{X}^-\) ions to form \(\mathrm{X}_{2}\) by accepting electrons from it.

• Oxidizing agents are essential in various chemical reactions, including combustion, respiration, and many industrial processes.
• The stronger the oxidizing agent, the more readily it accepts electrons.

Recognizing an oxidizing agent involves identifying what is gaining electrons during a reaction. If a compound is accepting electrons, it is likely acting as an oxidizing agent. In redox reactions, one cannot exist without the other—a reducing agent.

Reducing Agent

A reducing agent is a substance that donates electrons to another chemical species, causing the reduction of that species. In a redox reaction, the reducing agent itself gets oxidized because it loses electrons.
Look at reaction (2): \( \mathrm{H}_{2} \mathrm{O}_{2} + \mathrm{O}_{3} \rightarrow 2 \mathrm{O}_{2} + \mathrm{H}_{2} \mathrm{O} \). In this case, hydrogen peroxide (\(\mathrm{H}_{2} \mathrm{O}_{2}\)) is the reducing agent. It donates electrons to ozone (\(\mathrm{O}_{3}\)), thus reducing it to oxygen (\(\mathrm{O}_{2}\)).

• Reducing agents are crucial in multiple processes such as photosynthesis and various metallurgical processes.
• The power of a reducing agent is judged by its ability to donate electrons easily.

To identify a reducing agent, check which compound loses electrons during the reaction. If something is being used up to donate electrons, it is functioning as a reducing agent, completing the redox pair with an oxidizing agent.

Electrons Transfer

Electron transfer is the foundation of redox reactions. In such reactions, electrons move from one substance (the reducing agent) to another (the oxidizing agent).
This transfer process is exclusive to redox reactions, making them unique in chemistry. Understanding electron transfer is key to grasping how substances undergo oxidation or reduction.

• Oxidation involves the loss of electrons, leading to an increase in oxidation state.
• Reduction involves the gain of electrons, resulting in a decrease in oxidation state.

Consider that every redox reaction must include both oxidation and reduction events happening simultaneously. By focusing on the flow of electrons, you can determine which substance is undergoing oxidation, which is undergoing reduction, and thus, identify both the oxidizing and reducing agents.
Understanding electron transfer not only sharpens your skills in balancing redox equations but also enlightens the role of redox reactions in biological processes, energy production, and technological applications.