Problem 95 - A Complete Resource Book in Chemistry for JEE Main

Question

Chlorine acts as a bleaching agent only in presence of (a) sunlight (b) pure oxygen (c) dry air (d) moisture

Step-by-Step Solution

Answer

Chlorine acts as a bleaching agent in the presence of moisture (option d).

Step 1: Understanding the Problem

The problem asks under what conditions chlorine can act as a bleaching agent. The options provided are sunlight, pure oxygen, dry air, and moisture. We need to consider the reaction mechanism by which chlorine bleaches to identify the correct option.

Step 2: Review Chlorine's Bleaching Mechanism

Chlorine acts as a bleaching agent through a chemical reaction whereby chlorine forms a bleaching compound. It usually involves reaction with water (moisture) to form hypochlorous acid, which manifests the bleaching effect.

Step 3: Identify Key Elements in the Reaction

For chlorine to effectively act as a bleaching agent, it needs to interact with moisture. Thus, the presence of water or moisture is crucial to its bleaching mechanism.

Step 4: Evaluating Options

Given the importance of moisture for chlorine's acting as a bleaching agent, options such as sunlight, pure oxygen, and dry air lack the necessary water component involved in forming the bleaching compound.

Step 5: Conclusion

Based on the analysis, the presence of moisture enables chlorine to act as a bleaching agent effectively. Therefore, the correct answer is option (d) moisture.

Key Concepts

Reaction Mechanism

When we talk about the reaction mechanism of chlorine as a bleaching agent, we are referring to the series of steps that chlorine undergoes to effect a bleaching action. Bleaching through chlorine involves a specific sequence of reactions that fundamentally require the presence of moisture. Upon contact with water, chlorine participates in a reaction that produces hypochlorous acid (HOCl).

The reaction can be represented as follows:

• Cl₂ + H₂O → HOCl + HCl

This equation describes chlorine reacting with water to form hypochlorous acid and hydrochloric acid. Hypochlorous acid is the key component in this reaction sequence, as it is the active agent responsible for the bleaching. This acid further participates in oxidation, which is a critical part of the overall bleaching mechanism.

It's important to understand that the shift in chemical structure as chlorine interacts with water is what leads to its bleaching capability. The generation of hypochlorous acid allows the chlorine to oxidize and thereby decolorize or "bleach" organic materials.

Chemical Reactions

Chemical reactions are integral to understanding how chlorine operates as a bleaching agent. Chlorine does not bleach on its own; it requires a chemical environment that promotes the generation of reactive substances such as hypochlorous acid. In discussing chemical reactions, it is crucial to appreciate that the transformation involves changes in the chemical composition and structure.

The interaction of chlorine with moisture is a classic example of hydrolysis, where water acts as a reactant. Here, the breakage of water molecules facilitates the conversion of chlorine into useful bleaching compounds. As mentioned:

• Cl₂ interacts with H₂O, leading to HOCl and HCl.

These reactions are not instantaneous and require the precise conditions of temperature and pH to proceed effectively. Additionally, as the reactions unfold, they yield particular products that can further interact and aid in the bleaching process.

The bleaching effect largely depends on the ability of the resulting compounds from these chemical reactions to engage in oxidation. This oxidation alters the molecular structure of dyes and pigments, effectively neutralizing the color.

Hypochlorous Acid

Hypochlorous acid (HOCl) is an essential intermediate product in the bleaching process involving chlorine. Its role is critical because it acts as the primary bleaching agent through oxidation. Once chlorine reacts with water to form hypochlorous acid, this compound behaves as a potent oxidizer. This property is what imparts chlorine its bleaching capabilities.

The effectiveness of hypochlorous acid comes from its ability to disrupt organic molecules. It does so by breaking the bonds within the molecules of pigments and stains, leading to a loss of color. This oxidative action is potent against a wide range of substances, making it a reliable choice for bleaching.

Moreover, hypochlorous acid's effectiveness as a disinfectant complements its role as a bleaching agent. Not only does it fade colors, but it also helps in reducing microbial presence. As such, its presence as part of the chlorine bleaching mechanism highlights its versatility and importance in both cleaning and sanitizing processes.

Remember, while hypochlorous acid is effective, it's also crucial to handle it with care due to its reactive nature.