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 \(\overrightarrow{\mathrm{Cl}}_{2}\)

Step-by-Step Solution

Answer

The correct answer is (a) \(\mathrm{CaCl}_{2}\) and \(\mathrm{Ca(ClO}_{3})_{2}\).

Step 1: Understand Disproportionation Reaction

In a disproportionation reaction, a substance is simultaneously oxidized and reduced, producing two different products. For bleaching powder, this reaction is essential for destroying its disinfectant properties.

Step 2: Analyze the Reaction of Bleaching Powder

Bleaching powder, chemically known as calcium hypochlorite \ (\(\text{Ca(OCl)}_{2}\)), decomposes during disproportionation to yield calcium chloride (\(\text{CaCl}_{2}\)) and calcium chlorate (\(\text{Ca(ClO}_{3})_{2}\)). It effectively releases chlorine gas which participates in the germ-killing process.

Step 3: Determine the Products of the Disproportionation

Upon disproportionation, the primary products are \(\text{CaCl}_{2}\) and \(\text{Ca(ClO}_{3})_{2}\). These are formed when each chlorine atom in \(\text{Ca(OCl)}_{2}\) undergoes a change in its oxidation state, leading to these compounds.

Key Concepts

Bleaching Powder

Bleaching powder is a common name for a chemical compound that is used frequently for disinfection and bleaching purposes. This compound is predominantly calcium hypochlorite \((\text{Ca(OCl)}_{2}\)). It is a loose, white powder with a strong smell of chlorine. Bleaching powder is widely used for treating water to make it safe for drinking.

• It sanitizes water by killing harmful bacteria and other pathogens, thereby preventing waterborne diseases.
• Due to its effectiveness in killing germs, it's a popular choice for maintaining swimming pool hygiene.
• It is also used as a bleaching agent in the textile and paper industries.

Bleaching powder works by releasing chlorine when it comes in contact with water. This chlorine is responsible for its disinfectant and bleaching actions.

Calcium Hypochlorite

Calcium hypochlorite is a chemical compound represented by the formula \(\text{Ca(OCl)}_{2}\). It is the active ingredient in bleaching powder and is known for its strong oxidizing properties. Let's unpack what makes calcium hypochlorite effective.
Firstly, it has excellent microbial properties, which make it effective in killing germs found in contaminated water. When dissolved in water, calcium hypochlorite produces chlorine, which is a powerful disinfectant.

• The compound is stable in dry form, making it easy to store and use over an extended period.
• It's often used in granule or tablet form for commercial and residential water treatment.
It plays a key role in public health by ensuring safe drinking water and reducing incidences of waterborne epidemics.
• This compound degrades into less harmful by-products, making it an environmentally friendly choice for water disinfection.

Disinfectant Properties

The disinfectant properties of bleaching powder arise mainly from its ability to release chlorine. Chlorine is a recognized germicide used to kill a variety of pathogenic microorganisms. Here's how it works:
When chlorine enters the water, it reacts to form hypochlorous acid (HOCl), which is capable of penetrating cell walls of bacteria and viruses. This penetration disrupts their internal structures, effectively killing them.

• This destructiveness is why bleaching powder is effective in keeping swimming pools and drinking water free from contaminants.
• It's also utilized for sanitizing household surfaces, preventing the spread of bacteria and viruses.
• The by-products of this reaction are typically harmless, ensuring that the treated water remains safe for human consumption.

Although stable, dispensing calcium hypochlorite requires careful handling because of its strong oxidizing nature.

Oxidation-Reduction Reactions

An oxidation-reduction (redox) reaction is a crucial concept to understand the behavior of bleaching powder. These reactions involve the transfer of electrons between chemical species.
In the context of bleaching powder, a disproportionation reaction occurs, wherein one substance acts as both the oxidizing agent and the reducing agent.

• The chlorine in calcium hypochlorite undergoes both oxidation and reduction processes.
• During the disproportionation of calcium hypochlorite, chlorine is converted to calcium chloride \(\text{CaCl}_{2}\) and calcium chlorate \(\text{Ca(ClO}_{3})_{2}\).
• This alteration in the oxidation state helps us understand how bleaching powder's disinfecting action eventually degrades or is destroyed.

Knowing about redox reactions provides clarity on how substances like bleaching powder are utilized and how they function chemically during disinfection tasks.