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

The answer is (c) 1, 3, and 4.

Step 1: Understanding the Role of an Oxidizing Agent

An oxidizing agent is a substance that gains electrons during a chemical reaction. It is reduced as it allows another substance to be oxidized. In these reactions, we will see if ozone (\(\mathrm{O}_3\)) leads to oxidation of other substances, by looking at how the oxidation numbers change.

Step 2: Analyze Reaction 1

For reaction (1): \(2 \mathrm{HCl}+\mathrm{O}_{3} \longrightarrow \mathrm{Cl}_{2}+\mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}\), chlorine goes from -1 in HCl to 0 in \(\mathrm{Cl}_2\), indicating loss of electrons by chlorine and gain by \(\mathrm{O}_3\). Thus, ozone is an oxidizing agent here.

Step 3: Analyze Reaction 2

For reaction (2): \(\mathrm{BaO}_{2}+\mathrm{O}_{3} \longrightarrow \mathrm{BaO}+2 \mathrm{O}_{2}\), oxygen remains in the same oxidation state in both reactant and product. Ozone is not acting as an oxidizing agent here as there's no element that changes oxidation states.

Step 4: Analyze Reaction 3

For reaction (3): \(2 \mathrm{KI}+\mathrm{O}_{3}+\mathrm{H}_{2} \mathrm{O} \longrightarrow 2 \mathrm{KOH}+\mathrm{I}_{2}+\mathrm{O}_{2}\), iodine goes from -1 in \(\mathrm{KI}\) to 0 in \(\mathrm{I}_2\), indicating loss of electrons. Ozone acts as an oxidizing agent here by inducing the oxidation of iodine.

Step 5: Analyze Reaction 4

For reaction (4): \(\mathrm{PbS}+4 \mathrm{O}_{3} \longrightarrow \mathrm{PbSO}_{4}+4 \mathrm{O}_{2}\), sulfur changes from -2 in \(\mathrm{PbS}\) to +6 in \(\mathrm{PbSO}_{4}\), indicating it has been oxidized, with ozone acting as the oxidizing agent.

Step 6: Conclusion from all Reactions

After analysis, we see that ozone acts as the oxidizing agent in reactions 1, 3, and 4, but not in reaction 2.

Key Concepts

Ozone Reactions

Ozone, represented as \( \mathrm{O}_3 \), is a molecule made up of three oxygen atoms. It's well known for its role in absorbing the sun's harmful ultraviolet rays high in the Earth's atmosphere. However, in the context of chemistry, ozone is a potent oxidizing agent. This means it has the ability to accept electrons from other substances during chemical reactions.
When ozone enters a reaction, it often changes the state of other compounds by taking electrons away from them. This process is known as oxidation. Ozone is particularly powerful because it can react with a wide variety of substances, leading to significant changes in their chemical properties. Its use in chemical reactions is widespread because of its strong oxidizing abilities, making it crucial for processes that require the conversion of one substance into another.

Oxidation States

An oxidation state, often called an oxidation number, is a theoretical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. In simpler terms, it represents the number of electrons an atom can gain, lose, or share when it forms compounds.
Each element in a chemical reaction will have an oxidation state, and the focus in determining oxidizing or reducing agents is on these states. The changes in oxidation state help us identify which substances are oxidized and which are reduced.

• An increase in oxidation state means a substance is oxidized and loses electrons.
• A decrease in oxidation state means a substance is reduced and gains electrons.

By observing these changes, scientists and students can determine the role of different elements in a reaction. For instance, in our exercise, ozone helps other elements lose electrons, hence serving as an oxidizing agent.

Chemical Reactions Analysis

Analyzing chemical reactions involves understanding the transformation of reactants to products. This process is critical to deciphering how substances interact and change during a reaction. To analyze a chemical reaction involving oxidizing agents like ozone, several steps are essential.
First, identify the reactants and products in the chemical equation. Next, determine the oxidation states of the key elements involved in the reaction. These states are essential to pinpoint which elements are oxidized and reduced.

• Write down the changes in oxidation states for each element involved.
• Check which element lost electrons (oxidized) and which gained electrons (reduced).
• Confirm whether the oxidizing agent, such as ozone, facilitated the electron transfer.

Through this systematic approach, you can determine the roles of various substances in a reaction. In the provided exercise, this analysis helped conclude where ozone acts as the oxidizing agent, distinguishing between reactions where it facilitates electron transfer and those where it does not.