Problem 91 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following reactions ozone acts as an oxidizing agent? (1) \(2 \mathrm{HCl}+\mathrm{O}_{3} \longrightarrow \mathrm{Cl}_{2}+\mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}\) (2) \(\mathrm{BaO}_{2}+\mathrm{O}_{3} \longrightarrow \mathrm{BaO}+2 \mathrm{O}_{2}\) (3) \(2 \mathrm{KI}+\mathrm{O}_{3}+\mathrm{H}_{2} \mathrm{O} \longrightarrow 2 \mathrm{KOH}+\mathrm{I}_{2}+\mathrm{O}_{2}\) (4) \(\mathrm{PbS}+4 \mathrm{O}_{3} \longrightarrow \mathrm{PbSO}_{4}+4 \mathrm{O}_{2}\) (a) 1,2 and 4 (b) 2 and 3 (c) 1,3 and 4 (d) 1 and 2

Step-by-Step Solution

Answer

Option (c) 1, 3, and 4.

Step 1: Understand the Role of an Oxidizing Agent

An oxidizing agent is a substance that gains electrons in a redox chemical reaction. It oxidizes another substance and itself gets reduced.

Step 2: Analyze Reaction (1)

Consider the reaction \( 2 \mathrm{HCl}+\mathrm{O}_{3} \longrightarrow \mathrm{Cl}_{2}+\mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2} \). Here, ozone (\( \mathrm{O}_{3} \)) is reduced to \( \mathrm{O}_{2} \) by gaining electrons from \( \mathrm{HCl} \), which is oxidized to \( \mathrm{Cl}_2 \). Thus, \( \mathrm{O}_{3} \) acts as an oxidizing agent.

Step 3: Analyze Reaction (2)

For the reaction \( \mathrm{BaO}_{2}+\mathrm{O}_{3} \longrightarrow \mathrm{BaO}+2\mathrm{O}_{2} \), ozone (\( \mathrm{O}_{3} \)) is reduced to \( \mathrm{O}_{2} \), which suggests a role as an oxidizing agent as \( \mathrm{BaO}_{2} \)'s oxidation state remains unchanged here.

Step 4: Analyze Reaction (3)

In the reaction \( 2 \mathrm{KI}+\mathrm{O}_{3}+\mathrm{H}_{2} \mathrm{O} \longrightarrow 2 \mathrm{KOH}+\mathrm{I}_{2}+\mathrm{O}_{2} \), ozone is reduced to \( \mathrm{O}_{2} \) and iodide ions (\( \mathrm{I^-} \)) are oxidized to iodine (\( \mathrm{I}_{2} \)), suggesting ozone is an oxidizing agent.

Step 5: Analyze Reaction (4)

In the reaction \( \mathrm{PbS}+4\mathrm{O}_{3} \longrightarrow \mathrm{PbSO}_{4}+4\mathrm{O}_{2} \), \( \mathrm{O}_{3} \) is reduced to \( \mathrm{O}_{2} \), indicating it acts as an oxidizing agent since the sulfide \( \mathrm{S}^{2-} \) is oxidized to sulfate \( \mathrm{SO}_{4}^{2-} \).

Step 6: Determine the Correct Answer

Based on the analysis in steps 2 to 5, ozone acts as an oxidizing agent in reactions 1, 3, and 4. Option (c) 1, 3, and 4 is correct.

Key Concepts

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, are essential chemical processes where there is a transfer of electrons between two substances. In a redox reaction, one substance gains electrons (is reduced), while another substance loses electrons (is oxidized). Understanding which substance is oxidized and which is reduced is key to determining the oxidizing and reducing agents in a reaction.

For example, in Reaction (1) of the exercise, ozone (3 eq3) acts as the oxidizing agent because it accepts electrons from HCl. HCl is thereby oxidized to Cl4 while ozone is reduced to O2. Similarly, in Reaction (4), ozone is again reduced, facilitating the oxidation of PbS to PbSO4. Such reactions highlight the dual role of substances as both electron donors and acceptors. Always remember, a substance that gains electrons (gets reduced) is the oxidizing agent, as they help another substance to lose electrons.

Electron Transfer

Electron transfer is a fundamental concept in redox reactions and is essential to understand how these reactions occur. During these reactions, electrons are transferred from the reducing agent to the oxidizing agent.

• Reducing Agent: The substance that donates electrons. It gets oxidized.
• Oxidizing Agent: The substance that accepts electrons. It gets reduced.

In Reaction (3), ozone (3 eq3) acts as the oxidizing agent. It gains electrons from iodide ions, which are part of the compound KI. This electron transfer results in the formation of iodine (I2) from iodide ions (I5). Such reactions demonstrate the shift of electrons from one reactant to another, showing the interdependent relationship between oxidation and reduction. This process is critical for understanding how chemical compounds interact and transform.

Ozone Chemistry

Ozone (3 eq3) is a molecule composed of three oxygen atoms. It plays an interesting role in chemistry, especially in redox reactions due to its strong oxidizing properties. In chemical terms, ozone can easily accept electrons, which allows it to act as an oxidizing agent in various reactions, as seen in the exercise provided.

Ozone's structure gives it the capability to reduce to diatomic oxygen (O2), contributing to its ability to drive redox reactions. For instance, in all four reactions given in the exercise, ozone is involved in an electron transfer where it is invariably reduced from O^3- to O^2-. This reduction accompanies the oxidation of other substances like sulfides to sulfates or iodides to iodine.

The chemistry of ozone is not just limited to laboratory reactions; it has significant implications for atmospheric chemistry too. High up in the atmosphere, ozone absorbs ultraviolet radiation, protecting life on Earth. Meanwhile, as an oxidizing agent, it also helps in breaking down pollutants closer to the ground. As learners explore ozone's dual role, they appreciate its importance both in theory and practical applications.