Question

Bleaching powder is effective with cold water, but ineffective with hot water. This is because of (a) oxidation of chloride radical (b) reduction of chloride radical (c) reduction of hypochlorite radical (d) hydrolysis of hypochlorite radical

Short answer

Answer: Bleaching powder is more effective with cold water than with hot water due to the hydrolysis of the hypochlorite radical. In hot water, the rate of hydrolysis of the hypochlorite radical increases, decreasing its concentration and making it less effective as a bleaching agent.

Step-by-step solution

Determine the chemical formula of bleaching powder

Bleaching powder, also known as calcium hypochlorite, has a chemical formula of Ca(OCl)_2.

Write the reaction of bleaching powder with water

When bleaching powder reacts with water, it forms calcium hydroxide and hypochlorous acid. Ca(OCl)_2 + 2H_2O → Ca(OH)_2 + 2HOCl

Analyze the effect of temperature on the reaction

As the temperature increases, the rate of hydrolysis of hypochlorite radical increases. Hypochlorite radical is responsible for the bleaching action. With an increase in the rate of hydrolysis, as in hot water, the concentration of hypochlorite radical decreases and becomes less effective as a bleaching agent.

Choose the correct option

Based on our analysis, we can conclude that the reason why bleaching powder is effective with cold water but ineffective with hot water is due to the hydrolysis of the hypochlorite radical. So the correct option is: (d) hydrolysis of hypochlorite radical.

Key concepts

Calcium Hypochlorite

Calcium hypochlorite, known chemically as Ca(OCl)₂, is a white solid compound with a strong chlorine odor. It serves as an active ingredient in many commercial bleaching powders and is used as a disinfectant in water treatment, especially in swimming pools.

Its strong oxidizing properties allow it to break down various organic compounds, making it an effective sanitizer. When it comes into contact with water, it releases hypochlorous acid (HOCl), which is the active species responsible for the bleaching and disinfecting action. It's important to handle calcium hypochlorite with care, as it reacts vigorously with organic materials and can cause ignition upon contact with oils or other easily oxidizable substances.

Hydrolysis of Hypochlorite

Hydrolysis of hypochlorite refers to the chemical process where hypochlorite ions (OCl^-) in water react to form hypochlorous acid (HOCl) and hydroxide ions (OH^-). The reaction can be represented as follows:

OCl^- + H₂O → HOCl + OH^-

The produced hypochlorous acid is a powerful bleaching agent, which is why substances releasing hypochlorite ions are commonly used in bleaches. However, hypochlorous acid is more stable in colder water, which contributes to the effectiveness of bleaching agents in cold conditions as opposed to hot ones.

Chemical Reaction of Bleaching Powder

The chemical reaction involving bleaching powder, when mixed with water, is a classic example of hydrolysis. Bleaching powder, consisting mainly of calcium hypochlorite, reacts with water to release calcium hydroxide and hypochlorous acid, as per the equation:

Ca(OCl)₂ + 2H₂O → Ca(OH)₂ + 2HOCl

This reaction is crucial for the bleaching action as the hypochlorous acid generated acts on dyes and stains, altering their chemical structures and resulting in a 'bleached' appearance. Moreover, this process showcases the dual functionality of bleaching powder as it not only acts as a bleaching agent but also as a disinfectant due to the antimicrobial properties of the resultant hypochlorous acid.

Effect of Temperature on Bleaching Powder

Temperature plays a significant role in the effectiveness of bleaching powder. Specifically, the hydrolysis of hypochlorite, which is a critical function of bleaching agents, is temperature-dependent. With an increase in temperature, the rate at which hypochlorite ions are converted into hypochlorous acid increases. While this might suggest a more potent bleaching action, the reality is that higher temperatures lead to a faster decomposition of hypochlorous acid into less effective species. Consequently, at higher temperatures, the bleaching powder's ability to whiten and disinfect diminishes significantly. Thus, for optimal use of bleaching powder as a bleaching agent and disinfectant, it is recommended to use it with cold water.