Problem 80 - A Complete Resource Book in Chemistry for JEE Main

Question

Bleaching powder reacts with a few drops of concentrated HCl to give (a) oxygen (b) calcium oxide (c) chlorine (d) hypochlorous acid

Step-by-Step Solution

Answer

The product is chlorine (c).

Step 1: Understanding the Chemical Reaction

The problem involves the reaction of bleaching powder with concentrated hydrochloric acid (HCl). Bleaching powder is chemically known as calcium hypochlorite, which has the formula Ca(OCl)_2. When concentrated HCl is added to it, a chemical reaction occurs. Our goal is to identify the product that is formed when concentrated HCl is added to bleaching powder.

Step 2: Rewriting the Chemical Equation

To solve the problem, we need to write the balanced chemical equation for the reaction between bleaching powder (calcium hypochlorite) and concentrated HCl. The chemical reaction can be represented as: Ca(OCl)_2 + 4HCl ightarrow CaCl_2 + 2HOCl + 2Cl_2 + 2H_2O

Step 3: Analyzing the Products

According to the balanced chemical equation, the reaction produces calcium chloride (CaCl_2), hypochlorous acid (HOCl), chlorine gas (Cl_2), and water (H_2O). The question asks us which of the given options is a product of the reaction.

Step 4: Identifying the Correct Answer

From the products of the reaction listed ( CaCl_2, 2HOCl, 2Cl_2, 2H_2O), it is clear that among the options given (a) oxygen, (b) calcium oxide, (c) chlorine, and (d) hypochlorous acid, both chlorine (c) and hypochlorous acid (d) are produced. However, only one option can be chosen based on typical multiple-choice format. Since chlorine (Cl_2) is a gas and often a key identifiable product in similar context problems, option (c) chlorine is more explicitly emphasized.

Key Concepts

Calcium Hypochlorite

Calcium hypochlorite is a white, powdered chemical compound with the formula \(\text{Ca(OCl)}_2\). It is commonly used in households and industries for disinfecting water and sanitizing surfaces. Bleaching powder, as it is widely known, plays a pivotal role in this process due to its ability to release chlorine when reacted with acids. This property makes it particularly useful for water treatment applications, such as in swimming pool maintenance.

When calcium hypochlorite is mixed with water, it partially dissociates to form hypochlorous acid and calcium hydroxide, contributing to its bleaching power. However, when it reacts with stronger acids like hydrochloric acid \((\text{HCl})\), it releases a significant amount of chlorine gas \((\text{Cl}_2)\). This process is leveraged in chemical reactions to yield the desired product, particularly chlorine, which is a vital component in industrial and household cleaning processes.

Chemical Reaction Equation

In chemistry, writing and balancing chemical equations is crucial for understanding the changes that occur during chemical reactions. The equation for the reaction between calcium hypochlorite \((\text{Ca(OCl)}_2)\) and hydrochloric acid \((\text{HCl})\) is:

• \(\text{Ca(OCl)}_2 + 4\text{HCl} \rightarrow \text{CaCl}_2 + 2\text{HOCl} + 2\text{Cl}_2 + 2\text{H}_2\text{O}\)

This balanced equation shows that one molecule of calcium hypochlorite reacts with four molecules of hydrochloric acid to produce several products, including:

• Calcium chloride \((\text{CaCl}_2)\)
• Hypochlorous acid \((2\text{HOCl})\)
• Chlorine gas \((2\text{Cl}_2})\)
• Water \((2\text{H}_2\text{O})\)

Balancing chemical equations ensures that the number of atoms of each element is conserved during the reaction, highlighting the importance of stoichiometry in chemical processes.

Chlorine Production

Chlorine gas \((\text{Cl}_2)\) is one of the products formed from the reaction of calcium hypochlorite with hydrochloric acid. It is a greenish-yellow gas with a pungent odor, widely used as a disinfectant and bleaching agent.

The gas is produced during the reaction when the bonds in calcium hypochlorite break and reform as hydrochloric acid reacts with it. As chlorine is a primary product identified in this reaction, it serves as a characteristic indicator for chemical reactions that involve bleaching agents. Chlorine plays crucial roles beyond disinfecting—to manufacture polyvinyl chloride (PVC) for plastics, making solvents, and in the food and paper industry.

Hypochlorous Acid Formation

Hypochlorous acid \((\text{HOCl})\) is another significant product of the reaction between calcium hypochlorite and hydrochloric acid. It forms a weak, unstable acid that can act as an effective oxidizing agent and disinfectant.

This chemical is crucial for hygiene and sanitation due to its bacteria-killing properties. The formation of hypochlorous acid arises when the chlorite ion \((\text{ClO}^-)\) reacts with hydrogen ions, resulting in \(\text{HOCl}\).
Hypochlorous acid is a safer alternative to free chlorine in many cleaning products because it has a milder effect on surfaces while still maintaining its disinfecting capability. Often, its formation is associated with the delivery of clean and safe drinking water and in-lab synthesis where it serves various chemical purposes.