Problem 83 - A Complete Resource Book in Chemistry for JEE Main

Question

Bleaching powder is used as a disinfectant for purification of water. When water-borne germs are killed, the disinfectant activity is destroyed. This is due to its disproportionation into (a) \(\mathrm{CaCl}_{2}\) and \(\mathrm{Ca}\left(\mathrm{ClO}_{3}\right)_{2}\) (b) \(\mathrm{CaO}, \mathrm{Cl}_{2}\) and \(\mathrm{CaCl}_{2}\) (c) \(\mathrm{CaO}\) and \(\mathrm{Cl}_{2}\) (d) \(\mathrm{CaCl}_{2}\) and \(\mathrm{Cl}_{2}\)

Step-by-Step Solution

Answer

The answer is (d) CaCl₂ and Cl₂.

Step 1: Understand the Concept of Disproportionation

Disproportionation reactions involve a single substance being simultaneously reduced and oxidized to form two different products. In this case, bleaching powder, a common disinfectant in water treatment, goes through this kind of reaction when it loses its disinfectant activity.

Step 2: Identify the Reaction of Bleaching Powder

Bleaching powder is chemically represented as Ca(OCl)₂. When it decomposes or undergoes a disproportionation reaction, it forms several possible compounds, including calcium compounds and chlorine-related species.

Step 3: Analyze the Options for Disproportionation

We need to determine which option describes the products formed from the disproportionation of bleaching powder: (a) CaCl₂ and Ca(ClO₃)₂ (b) CaO, Cl₂, and CaCl₂ (c) CaO and Cl₂ (d) CaCl₂ and Cl₂. Bleaching powder is known to produce calcium chloride (CaCl₂) and chlorine gas (Cl₂), aligning with option (d).

Step 4: Conclusion on the Correct Products

After analyzing, the products CaCl₂ and Cl₂ in option (d) accurately represent the conversion of bleaching powder when it loses disinfectant activity. This aligns with the known chemistry of bleaching powder's disproportionation in water treatment.

Key Concepts

Bleaching Powder

Bleaching powder, scientifically known as calcium hypochlorite, is a powerful chemical disinfectant widely used for the purification of water. This compound has a chemical formula of \( ext{Ca(OCl)}_2 \), and it works effectively to kill pathogens present in water. Its ability to disinfect comes from the release of chlorine. Once introduced into water, a chemical reaction takes place, where the bleaching powder disintegrates and releases chlorine.

Here's how it affects water:

• It can eliminate microorganisms by breaking down into bleach and releasing chlorine gas.
• This chlorine is what leads to the chemical's disinfectant properties.
• The introduction of bleaching powder to water also results in a decrease in the content of harmful bacteria and pathogens, ensuring water is safe for use.

Disinfectant Activity

The disinfectant activity of bleaching powder is primarily due to the presence of chlorine. Once released, the chlorine works effectively to disinfect water by deactivating microorganisms. This is achieved through oxidizing their cell structures, leading to their death.

However, the activity does not last indefinitely. Over time, or during certain conditions, the bleaching powder undergoes disproportionation, which reduces its effectiveness:

• This process involves the decomposition of the compound where it both accepts and donates electrons.
• As a result, bleaching powder is broken down into simpler compounds such as calcium chloride (\( ext{CaCl}_2 \)) and chlorine gas (\( ext{Cl}_2 \)).
• This breakdown concludes with its disinfectant properties nullified, as the chlorine which provides these properties is consumed in the reaction.

Calcium Compounds

Calcium compounds produced from the breakdown of bleaching powder play an essential role in completing the chemical reaction. During disproportionation, calcium hypochlorite decomposes, forming calcium chloride and releasing chlorine gas:

• Calcium chloride (\( ext{CaCl}_2 \)) is one of the products. It is a salt that remains dissolved in the water post-reaction.
• This process highlights calcium compounds' role as intermediates in the reaction.
• Understanding the intermediate formation of calcium compounds helps in grasping the entire cycle of a disproportionation reaction, as it ensures water does not become re-contaminated afterward.

Water Treatment

In water treatment, bleaching powder is a vital agent for ensuring the safety and hygiene of water supplies. Its use is critical in treating drinking water and maintaining public health standards. Here's how it fits into the process:

• The addition of bleaching powder clarifies water by removing microbes that cause diseases.
• By breaking down into its chemical constituents, it undergoes a process that includes both oxidation of impurities and disinfection.
• It also requires careful handling to ensure that the right amounts are used; excess can lead to harmful side effects.

Water treatment systems control the precise amounts and ensure that the resultant by-products of the powder are safe and not harmful to humans or animals. Monitoring these water treatments is essential to balance disinfection efficiency and safety.