Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{SO}_{2}\) acts as a temporary bleaching agent but \(\mathrm{Cl}_{2}\) acts as a permanent bleaching agent. Why? (a) \(\mathrm{Cl}_{2}\) bleaches due to oxidation but \(\mathrm{SO}_{2}\) bleaches due to reduction (b) \(\mathrm{Cl}_{2}\) bleaches due to reduction but \(\mathrm{SO}_{2}\) bleaches due to oxidation (c) both (a) and (b) (d) none of these

Step-by-Step Solution

Answer

(a) \( \mathrm{Cl}_{2} \) bleaches due to oxidation but \( \mathrm{SO}_{2} \) bleaches due to reduction.

Step 1: Understanding Bleaching by \( \mathrm{Cl}_{2} \)

Chlorine (\( \mathrm{Cl}_{2} \)) is known to act as a bleaching agent through the process of oxidation. When it comes into contact with moisture, it forms hypochlorous acid (\( \mathrm{HOCl} \)), which can release nascent oxygen. This nascent oxygen is a powerful oxidizing agent that effectively destroys the color-causing substances by converting them into colorless compounds.

Step 2: Understanding Bleaching by \( \mathrm{SO}_{2} \)

Sulfur dioxide (\( \mathrm{SO}_{2} \)) bleaches via a reduction reaction. It adds sulfur dioxide to the dye, thereby reducing it and making it colorless temporarily. This change is reversible, particularly when exposed to air, as the oxygen in the air can re-oxidize the colorless compound, restoring its original color.

Step 3: Comparing Permanent and Temporary Effects

The effect of \( \mathrm{Cl}_{2} \) is permanent because the bleaching occurs due to oxidation, which alters the chemical structure of the colored compounds permanently. On the other hand, the effect of \( \mathrm{SO}_{2} \) is temporary as it forms a colorless compound through reduction that can revert back to its original form when oxidized.

Step 4: Identify the Correct Choice

By analyzing both agents: \( \mathrm{Cl}_{2} \) indeed bleaches through oxidation and \( \mathrm{SO}_{2} \) bleaches by reduction. Therefore, option (a) is the correct choice. Option (b) is incorrect, because \( \mathrm{Cl}_{2} \) bleaches due to oxidation, not reduction.

Key Concepts

Oxidation

Oxidation is a chemical process that involves the loss of electrons by a molecule, atom, or ion. In the context of bleaching, oxidation refers to the way in which certain bleaching agents, like chlorine \( (\mathrm{Cl}_{2}) \), work to remove color. Here's how it happens:

• When bleach containing chlorine is applied, it comes into contact with water, forming hypochlorous acid \( (\mathrm{HOCl}) \).
• Hypochlorous acid can release nascent or active oxygen.
• This nascent oxygen is highly reactive and breaks down the chemical bonds in color-causing agents.
• By altering the chemical structure of the colored compounds, they become colorless permanently.

Bleaching through oxidation is a permanent change because it fundamentally alters the molecules responsible for color. Therefore, agents like chlorine are preferred for applications where permanent color removal is desired.

Reduction

Reduction is the opposite of oxidation. It is a chemical reaction that involves the gain of electrons by a molecule, atom, or ion. In the case of bleaching, some agents, like sulfur dioxide \( (\mathrm{SO}_{2}) \), operate through reduction.

• When \( \mathrm{SO}_{2} \) is used for bleaching, it adds sulfur dioxide to the dye molecules.
• This addition results in the gain of electrons by the dye molecules, thus reducing them.
• The compounds become colorless due to this reduction process.

The bleaching by sulfur dioxide is temporary because the dye's chemical structure isn't permanently altered. When exposed to air, the oxygen can re-oxidize the reduced compounds, restoring the original color. This makes \( \mathrm{SO}_{2} \) an ideal agent for applications where temporary color removal is acceptable or desired.

Bleaching Agents

Bleaching agents are substances used to whiten or remove the color from materials. They achieve this by chemical reactions that either oxidize or reduce the color-causing compounds. Let's look at the characteristics of the bleaching agents discussed:

• Chlorine \( (\mathrm{Cl}_{2}) \): Acts by oxidation, forming hypochlorous acid that releases nascent oxygen, permanently removing colors by altering the chemical structure of dyes.
• Sulfur Dioxide \( (\mathrm{SO}_{2}) \): Acts by reduction, temporarily altering the color by adding electrons to the dye, which can revert when exposed to air.

The choice of bleaching agent depends greatly on the desired outcome—permanent or temporary color removal. Chlorine provides a permanent solution beneficial for white fabrics and materials, whereas sulfur dioxide offers a temporary fix useful in certain situations where color recovery is needed.