Problem 34 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following reactions does hydrogen peroxide acts as a reducing agent? (a) \(2 \mathrm{KI}+\mathrm{H}_{2} \mathrm{O}_{2}+\mathrm{H}_{2} \mathrm{SO}_{4} \Rightarrow \mathrm{K}_{2} \mathrm{SO}_{4}+1_{2}+\mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{Ag}_{2} \mathrm{O}\) (moist) \(+\mathrm{H}_{2} \mathrm{O}_{2} \Rightarrow 2 \mathrm{Ag}+\mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}\) (c) \(\mathrm{Na}_{2} \mathrm{SO}_{3}+\mathrm{H}_{2} \mathrm{O}_{2} \rightarrow \mathrm{Na}_{2} \mathrm{SO}_{4}+\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{PbS}+4 \mathrm{H}_{2} \mathrm{O}_{2}^{2} \rightarrow \mathrm{PbSO}_{4}+4 \mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

Hydrogen peroxide acts as a reducing agent in reaction (b).

Step 1: Understand the Role of a Reducing Agent

A reducing agent is a substance that donates electrons to another species in a chemical reaction. It gets oxidized in the process, meaning it loses electrons.

Step 2: Analyze Reaction (a)

In the reaction \(2 \mathrm{KI}+\mathrm{H}_{2} \mathrm{O}_{2}+\mathrm{H}_{2} \mathrm{SO}_{4} \Rightarrow \mathrm{K}_{2} \mathrm{SO}_{4}+\mathrm{I}_{2}+\mathrm{H}_{2} \mathrm{O}\), iodine (I_2) is produced, indicating that iodide ions are oxidized to iodine. This suggests that hydrogen peroxide is acting as an oxidizing agent, not a reducing agent.

Step 3: Analyze Reaction (b)

The reaction \(\mathrm{Ag}_{2} \mathrm{O} (moist) + \mathrm{H}_{2} \mathrm{O}_{2} \Rightarrow 2 \mathrm{Ag} + \mathrm{H}_{2} \mathrm{O} + \mathrm{O}_{2}\) involves the reduction of silver oxide (\u00bAg_2O\u00b) to silver (\u00bAg\u00b) and the decomposition of \(\mathrm{H}_{2} \mathrm{O}_{2}\) into oxygen gas. Here, hydrogen peroxide is reduced by losing oxygen atoms, indicating it acts as a reducing agent.

Step 4: Analyze Reaction (c)

In the reaction \(\mathrm{Na}_{2} \mathrm{SO}_{3}+\mathrm{H}_{2} \mathrm{O}_{2} \rightarrow \mathrm{Na}_{2} \mathrm{SO}_{4}+\mathrm{H}_{2} \mathrm{O}\), sulphite ions (\u00bNa_2SO_3\u00b) are oxidized to sulphate ions (\u00bNa_2SO_4\u00b). This means hydrogen peroxide is acting as an oxidizing agent, not a reducing agent.

Step 5: Analyze Reaction (d)

In the reaction \(\mathrm{PbS} + 4 \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow \mathrm{PbSO}_{4} + 4 \mathrm{H}_{2} \mathrm{O}\), lead sulfide (\u00bPbS\u00b) is oxidized to lead sulfate (\u00bPbSO_4\u00b). Hydrogen peroxide is again acting as an oxidizing agent by accepting electrons from \u00bPbS\u00b.

Step 6: Determine the Reaction with H2O2 as a Reducing Agent

Only in Reaction (b) does hydrogen peroxide act as a reducing agent by providing electrons to the silver ions, facilitating the reduction of \(\mathrm{Ag}_{2} \mathrm{O}\) to \(\mathrm{Ag}\).

Key Concepts

Reducing Agent

A reducing agent is a key player in chemical reactions. It donates electrons to other substances, effectively reducing them. During this electron donation process, the reducing agent itself becomes oxidized, meaning it loses electrons.

• This electron transfer is crucial in many chemical processes.
• Common reducing agents include hydrogen, carbon, and metals like zinc and iron.
• The effectiveness of a reducing agent depends on its ability to donate electrons easily.

Understanding this concept is essential to analyzing redox reactions, such as the behavior of hydrogen peroxide in different contexts.

Oxidizing Agent

In contrast to a reducing agent, an oxidizing agent gains electrons in a chemical reaction. By accepting electrons, it causes the other substance to be oxidized. This is why it is known as an oxidizing agent—because it facilitates oxidation.

• The oxidizing agent itself is reduced in the process, often resulting in a lower oxidation state.
• Typical oxidizing agents include oxygen, fluorine, and hydrogen peroxide under certain conditions.
• An oxidizing agent's strength is determined by its willingness to accept electrons.

In the case of hydrogen peroxide, it can serve as both an oxidizing and a reducing agent, depending on the reaction conditions.

Chemical Reactions Analysis

Analyzing chemical reactions involves understanding the roles of all reactants and products. In redox reactions, identifying which substances act as oxidizing and reducing agents is crucial.

• Careful observation of electron transfer between species helps deduce the roles played in the reaction.
• In the given exercises, we see hydrogen peroxide acting differently based on the reaction it participates in.
• In reaction (b), hydrogen peroxide acts as a reducing agent, donating electrons to reduce silver oxide to silver.

Determining the agents involved can be achieved by observing changes in oxidation states throughout the reactions. This understanding aids in comprehending the underlying principles of redox processes.