Problem 35 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following reactions does \(\mathrm{H}_{2} \mathrm{O}_{2}\) acts as a reducing agent? (a) \(\mathrm{PbO}_{2}(\mathrm{~s})+\mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq}) \longrightarrow \mathrm{PbO}(\mathrm{s})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})\) \(+\mathrm{O}_{2}(\mathrm{~g})\) (b) \(\mathrm{KNO}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq}) \longrightarrow \mathrm{KNO}_{3}(\mathrm{aq})+\) \(\mathrm{H}_{2} \mathrm{O}(1)\) (c) \(2 \mathrm{KI}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq}) \longrightarrow 2 \mathrm{KOH}(\mathrm{aq})+\) \(\mathrm{I}_{2}(\mathrm{~s})\) (d) \(\mathrm{Na}_{2} \mathrm{SO}_{3}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq}) \longrightarrow \mathrm{Na}_{2} \mathrm{SO}_{4}(\mathrm{aq})\) \(+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})\)

Step-by-Step Solution

Answer

In reaction (a), \(\mathrm{H}_{2} \mathrm{O}_{2}\) acts as a reducing agent.

Step 1: Understand What is a Reducing Agent

A reducing agent is a substance that donates electrons to another species, undergoing oxidation itself. It causes the reduction of another species.

Step 2: Analyze Reaction (a)

In reaction (a), \(\mathrm{PbO}_{2}\) is reduced to \(\mathrm{PbO}\) and \(\mathrm{H}_{2}\) is oxidized to form \(\mathrm{O}_{2}\). Here, \(\mathrm{H}_{2} \mathrm{O}_{2}\) donates electrons and thus acts as a reducing agent.

Step 3: Analyze Reaction (b)

In reaction (b), \(\mathrm{NO}_{2}^{-}\) is oxidized to \(\mathrm{NO}_{3}^{-}\) and \(\mathrm{H}_{2} \mathrm{O}_{2}\) is reduced to \(\mathrm{H}_2\mathrm{O}\). \(\mathrm{H}_{2} \mathrm{O}_{2}\) here acts as an oxidizing agent, not a reducing agent.

Step 4: Analyze Reaction (c)

In reaction (c), \(\mathrm{I}^{-}\) ions are oxidized to \(\mathrm{I}_2\), while \(\mathrm{H}_{2} \mathrm{O}_{2}\) is reduced to produce \(\mathrm{OH}^-\). Here, \(\mathrm{H}_{2} \mathrm{O}_{2}\) is acting as an oxidizing agent.

Step 5: Analyze Reaction (d)

In reaction (d), \(\mathrm{SO}_3^{2-}\) is oxidized to \(\mathrm{SO}_4^{2-}\) and \(\mathrm{H}_{2} \mathrm{O}_{2}\) is reduced to \(\mathrm{H}_2\mathrm{O}\). \(\mathrm{H}_{2} \mathrm{O}_{2}\) serves as an oxidizing agent in this scenario.

Key Concepts

Redox Reactions

Redox reactions are fundamental chemical processes where oxidation and reduction occur simultaneously. These reactions are characterized by the transfer of electrons between chemical species. Redox is short for reduction-oxidation. In a redox reaction, one substance donates electrons (is oxidized) and another accepts electrons (is reduced).
Understanding how electron transfer works is essential to grasp redox reactions. It's like a dance of electrons where one partner loses and another gains. Consider

• Oxidation: loss of electrons
• Reduction: gain of electrons

An important identifier of a redox reaction is a change in the oxidation states of the involved atoms. This implies that the number of electrons assigned to atoms changes during the process, helping in the identification of which atoms are oxidized and which are reduced.
Redox reactions are ubiquitous in chemistry. From the burning of fuels to the cellular respiration process in our bodies, these reactions are essential to multiple disciplines.

Oxidation and Reduction

Understanding oxidation and reduction, often summed up as "redox", helps students identify the roles substances play in reactions. In these contexts, achieving a clear insight into how electrons move and change oxidation states is vital.
Oxidation is when a substance loses electrons, thus becoming more positively charged. For example, when \[ \mathrm{H}_{2} \mathrm{O}_{2} \] in reaction \((a)\) turns into oxygen, it means electrons have been transferred away from it.
Reduction is the gain of electrons, meaning the substance becomes more negatively charged. In an oxidizing environment, another substance accepts these electrons. For instance, when \( \mathrm{PbO}_{2} \) is reduced to \( \mathrm{PbO} \), it accepts extra electrons.
A helpful hint for remembering these processes is OIL RIG: "Oxidation Is Loss, Reduction Is Gain". This formula simplifies the understanding of what really happens behind the scenes whenever substances interact through electron exchanges.

Chemical Reaction Analysis

Analyzing chemical reactions involves looking at changes in structure, composition, and electron configurations in a chemical process. This is a critical skill for students wanting to understand the intricate details of reactions involving substances like hydrogen peroxide.
When evaluating a chemical equation, a student should:

• Identify the reactants and products
• Check if there is an observable change in oxidation states
• Determine which substance is the reducing agent and which is the oxidizing agent based on electron transfer

For example, in reaction (a) with hydrogen peroxide \( ( \mathrm{H}_{2} \mathrm{O}_{2} ) \), it acts as a reducing agent because it donates electrons to lead dioxide \( ( \mathrm{PbO}_{2} ) \), transforming it to lead oxide \( ( \mathrm{PbO} ) \).
To correctly identify agents in these reactions:

• The reducing agent loses electrons, supporting another substance's reduction
• The oxidizing agent gains electrons, supporting another substance's oxidation

With practice, students can master the analysis of chemical reactions by consistently applying these observations.