Problem 123 - A Complete Resource Book in Chemistry for JEE Main

Question

A liquid \(\mathrm{A}\) is treated with \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) solution. A mixture of two salts \(\mathrm{B}\) and \(\mathrm{C}\) are produced in the solution. The mixture on acidification with sulphuric acid and distillation produces the liquid \(\mathrm{A}\) again. Identify A. (a) \(\mathrm{Cl}_{2}\) (b) \(\mathrm{O}_{2}\) (c) \(\mathrm{Br}_{2}\) (d) \(\mathrm{N}_{2}^{2}\)

Step-by-Step Solution

Answer

The liquid \( A \) is \( \text{Br}_2 \), option (c).

Step 1: Understand the Reaction Scheme

Liquid \( A \) reacts with \( \text{Na}_2\text{CO}_3 \) to form a mixture of salts \( B \) and \( C \). When these salts are treated with sulfuric acid \((\text{H}_2\text{SO}_4)\) and distilled, they regenerate liquid \( A \). This suggests a reversible reaction.

Step 2: Identify the Nature of Liquid A

To ascertain what \( A \) could be, consider the reactions involved. \( \text{Na}_2\text{CO}_3 \) reacts with acidic gases to form salts and \( A \) is regenerated when these salts are treated with \( \text{H}_2\text{SO}_4 \). This property is typical of halogens (like \( \text{Cl}_2 \) and \( \text{Br}_2 \)) that can form salts with sodium carbonate.

Step 3: Analyze the Regeneration Process

Salts like sodium chloride or sodium bromide when treated with \( \text{H}_2\text{SO}_4 \) can release \( \text{Cl}_2 \) or \( \text{Br}_2 \) gas, respectively, which aligns with the regeneration of liquid \( A \). Considering the boiling points, \( \text{Br}_2 \) fits the description of 'liquid' between the options given.

Step 4: Eliminate Options

\( \text{Cl}_2 \) and \( \text{Br}_2 \) are possible candidates. However, \( \text{Br}_2 \) is a liquid at room temperature, whereas \( \text{Cl}_2 \) is a gas. Therefore, \( A \) is likely \( \text{Br}_2 \). \( \text{O}_2 \) and \( \text{N}_2 \) do not fit the product nor regeneration description in presence of \( \text{Na}_2\text{CO}_3 \) and \( \text{H}_2\text{SO}_4 \).

Step 5: Conclusion

The choice of \( A \) based on the described reactions and physical state should be \( \text{Br}_2 \), as \( \text{Br}_2 \) is the only halogen among the options that is liquid at room temperature and follows the described reaction pathway.

Key Concepts

Reversible Reactions

Reversible reactions are chemical reactions where products can revert back to reactants under certain conditions. This concept is crucial in understanding how chemical equilibria are established. When a reaction is reversible, the products formed can decompose back into the original reactants either partially or completely. This dynamic balance is a hallmark of reversible reactions.

In the exercise, the reaction between liquid A and sodium carbonate results in salts B and C. These salts, when treated with sulfuric acid, regenerate liquid A. This beautifully highlights the reversible nature of the chemical process. Reversible reactions often occur under closed conditions, allowing the system to achieve equilibrium where both reactants and products coexist.

• In a reversible reaction: reactants can form products and products can revert to reactants.
• Often involves physical changes like phase changes (solid to liquid and vice versa).
• Temperature and pressure can affect the direction of reversible reactions.

When analyzing such reactions, it's important to consider both forward (reactants to products) and backward (products back to reactants) pathways.

Halogen Chemistry

The halogens are a group of elements in Group 17 of the periodic table. They include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements are highly reactive, especially with alkali metals and alkaline earth metals, forming various salts. Their high reactivity derives from their ability to gain an electron to achieve a stable electronic configuration.

Halogen chemistry is central to the problem described in the exercise. Liquid A, identified as bromine (Br2), is a halogen. Halogens can easily form salts with metals. For instance, bromine reacts with sodium carbonate to form sodium bromide, showcasing typical halogen behavior.

• Halogens exist as diatomic molecules (e.g., Br2).
• They have varied states at room temperature: fluorine and chlorine are gases, bromine is a liquid, iodine, and astatine are solids.
• These elements can form compounds through simple electron gain, resulting in ions with a negative charge.

The transformation of bromine between its elemental and ionic forms is a classic example of halogen chemistry in action.

Inorganic Salts

Inorganic salts are ionic compounds typically formed from the reaction of an acid with a base, where the acid's hydrogen ions are replaced by metal ions from the base. These salts are usually crystalline solids that dissolve in water, providing the ions essential for many biological and chemical processes.

In the context of the exercise, the formation of salts B and C from the reaction of liquid A with sodium carbonate illustrates the creation of inorganic salts. Here, bromine (Br2) reacts with sodium to yield sodium bromide, showcasing the classic formation of such salts.

• Inorganic salts are characterized by their strong ionic bonds.
• These compounds often have high melting and boiling points.
• They are soluble in water and can conduct electricity in the molten or dissolved state.

In addition to their role in laboratory reactions, inorganic salts are crucial components in everything from minerals and ores to biological systems.

Acid-Base Reactions

Acid-base reactions are chemical processes where an acid reacts with a base to produce salt and water. These reactions constitute a significant portion of chemical processes and are foundational for understanding pH changes in various environments.

In the given exercise, sulfuric acid (H2SO4) is used to treat salts B and C, leading to the reformation of liquid A, bromine. This process underscores the acid's role in liberating bromine from the sodium bromide salt. The interaction between hydrochloric acid and such salts is a classic example of acid-base neutralization reactions.

• Acids donate protons (H^+) to bases, which accept them.
• These reactions result in the formation of water and an ionic salt.
• The pH of the solution shifts depending on the strength and concentration of the acids and bases involved.

Such reactions are not only vital in chemical synthesis but also in maintaining balance and functionality in living organisms.