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 correct answer is (a) \( \text{CaCl}_2 \) and \( \text{Ca(ClO}_3)_2 \).

Step 1: Understand the Disproportionation Reaction

Disproportionation reactions occur when a substance is simultaneously oxidized and reduced to form two different products. In the case of bleaching powder, which is chemically known as calcium hypochlorite, \( \text{Ca(OCl)}_2 \), it undergoes disproportionation to form other compounds.

Step 2: Write the Disproportionation Reaction

The disproportionation of calcium hypochlorite typically results in the formation of calcium chloride \( \text{CaCl}_2 \) and calcium chlorate \( \text{Ca(ClO}_3)_2 \). The reaction can be expressed as follows:\[3\text{Ca(OCl)}_2 \rightarrow 2\text{CaCl}_2 + \text{Ca(ClO}_3)_2\]

Step 3: Choose the Correct Options

Based on the disproportionation reaction written in Step 2, we identify the products as \( \text{CaCl}_2 \) and \( \text{Ca(ClO}_3)_2 \). Among the provided options, option (a) matches exactly with these products.

Step 4: Conclusion: Select the Correct Answer

Therefore, the correct answer is option (a), as it matches the products of the disproportionation of bleaching powder, which are \( \text{CaCl}_2 \) and \( \text{Ca(ClO}_3)_2 \).

Key Concepts

Calcium Hypochlorite

Commonly known as bleaching powder, calcium hypochlorite is a chemical compound with the formula \( \text{Ca(OCl)}_2 \). It is a white solid, often providing the active ingredient in commercial products such as pool cleaners and certain disinfectants. Its primary utility comes from its ability to release chlorine upon contact with water, lending it strong oxidizing and microbicidal properties. These qualities make it suitable for disinfecting swimming pools, drinking water, and for bleaching paper and textiles. In its solid state, it is highly stable, but in the presence of moisture, a disproportionation reaction occurs, yielding calcium chloride \( \text{CaCl}_2 \) and calcium chlorate \( \text{Ca(ClO}_3)_2 \). This diverse chemical behavior is what makes it a critical component in various disinfecting practices.

Bleaching Powder

While calcium hypochlorite is the chemical name, it is more popularly known as bleaching powder. It has wide applications due to its disinfectant properties. The powder, known for its strong chlorine odor, is typically a mixture of calcium hydroxide, calcium chloride, and calcium hypochlorite. Upon exposure to air and moisture, it releases chlorine, which can eliminate microorganisms by breaking down their cellular walls. The bleaching action isn't limited to just disinfection; it is also crucial in the paper and textile industries where bleaching powder plays a key role in whitening materials. When used properly, it can help maintain public health standards by disinfecting water supplies.

Disinfection of Water

Disinfecting water with bleaching powder is a staple practice in ensuring water safety. When this chemical is added to water, it introduces chlorine into the system. Chlorine acts as a powerful oxidizing agent, effectively neutralizing harmful bacteria, viruses, and other pathogens. The typical reaction results in the breakdown of organic molecules within the microorganisms, leading to their death. After the disinfection, it is crucial to manage the chemical's concentrations carefully, to ensure that the water remains safe to drink, without any residual chlorine taste or smell. This balancing act makes bleaching powder a widely adopted solution for water treatment facilities worldwide.

Calcium Chloride

One of the by-products of calcium hypochlorite's disproportionation reaction is calcium chloride \( \text{CaCl}_2 \). This compound is colorless and highly soluble in water. Beyond its role in chemical reactions, calcium chloride is versatile. It finds applications as a de-icing agent on roads, thanks to its ability to lower freezing points. It also retains moisture and is thus used in food products as a firming agent, and in concrete mixtures to speed curing. In the context of disinfection, calcium chloride doesn’t contribute directly to killing bacteria but forms part of the balanced chemical equation, indicating efficient usage of calcium hypochlorite.

Calcium Chlorate

Calcium chlorate \( \text{Ca(ClO}_3)_2 \) is another product of the disproportionation of calcium hypochlorite. Though not commonly used in disinfection, it highlights the complexity of reactions bleaching powder undergoes. Calcium chlorate is a powerful oxidizing agent, finding its place more in pyrotechnics and as an herbicide than in water treatment. Its presence in the reaction, however, points to the robust chemical transformations taking place. Although it’s less frequently discussed than its companions produced in the reaction, understanding its properties can provide further insight into creating balanced chemical environments during the disinfection process.