Problem 114 - A Complete Resource Book in Chemistry for JEE Main

Question

A substance 'A' is obtained by boiling an aqueous solution of \(\mathrm{NH}_{3}\) with an aqueous solution of sodium hypochlorite in the presence of a little glue. 'A' forms salts with \(\mathrm{HCl}\) and \(\mathrm{H}_{2} \mathrm{SO}_{4} \cdot \mathrm{A}^{\prime}\) is a powerful reducing agent and reduces \(\mathrm{FeCl}_{3}\) solution and acidified \(\mathrm{KMnO}_{4}\) solution. This reaction is being accompanied by evolution of inactive gas 'B'. Identify 'A' and 'B'. (a) \(\mathrm{A}=\mathrm{O}_{3} ; \mathrm{B}=\mathrm{H}_{2} \mathrm{O}_{2}\) (b) \(\mathrm{A}=\mathrm{NH}_{2} \cdot \mathrm{NH}_{2} ; \mathrm{B}=\mathrm{N}_{2}\) (c) \(\mathrm{A}=\mathrm{N}_{2} ; \mathrm{B}=\mathrm{NO}_{2}\) (d) \(\mathrm{A}=\mathrm{N}_{3} \mathrm{H} ; \mathrm{B}=\mathrm{N}_{2}\)

Step-by-Step Solution

Answer

(b) \(\mathrm{A} = \mathrm{NH}_2\mathrm{NH}_2\) ; \(\mathrm{B} = \mathrm{N}_2\)

Step 1: Identify the Reaction

The reaction described involves boiling ammonia (\(\mathrm{NH}_3\)) with sodium hypochlorite (\(\mathrm{NaOCl}\)) in the presence of glue. This is a classic preparation method for hydrazine (\(\mathrm{NH}_2\cdot\mathrm{NH}_2\)), known as the Raschig process. Hence, 'A' is likely \(\mathrm{NH}_2\cdot\mathrm{NH}_2\).

Step 2: Understand Formation of Salts

Substance 'A', which we've identified as \(\mathrm{NH}_2\cdot\mathrm{NH}_2\) (hydrazine), forms salts with \(\mathrm{HCl}\) and \(\mathrm{H}_2\mathrm{SO}_4\). Hydrazine does indeed react with acids to form salts, such as \(\mathrm{NH}_2\cdot\mathrm{NH}_2\cdot\mathrm{2HCl}\) and similar salts with sulfuric acid.

Step 3: Investigate Reduction Reactions

Hydrazine is a known powerful reducing agent. It can reduce \(\mathrm{FeCl}_3\) to \(\mathrm{FeCl}_2\) and acidified \(\mathrm{KMnO}_4\) to manganese compounds like \(\mathrm{Mn}^{2+}\), causing the purple color of \(\mathrm{KMnO}_4\) to disappear.

Step 4: Identify the Inactive Gas

The reaction of hydrazine with various agents often produces nitrogen gas \(\mathrm{N}_2\) as a byproduct. This aligns with the description of 'B' being an inactive gas (inert under normal conditions). Nitrogen, with its strong triple bond, is a very stable molecule.

Step 5: Conclusion

Combining all insights, \(\mathrm{A}\) is hydrazine (\(\mathrm{NH}_2\cdot\mathrm{NH}_2\)) and the gas \(\mathrm{B}\) is nitrogen (\(\mathrm{N}_2\)). Therefore, the correct answer is option (b).

Key Concepts

Hydrazine Preparation

The Raschig process is a standard method for preparing hydrazine (\(\mathrm{NH}_2\cdot\mathrm{NH}_2\)), a versatile compound in chemistry. The process begins with a reaction between ammonia (\(\mathrm{NH}_3\)) and sodium hypochlorite (\(\mathrm{NaOCl}\)). This reaction is carried out by boiling these substances together in the presence of a stabilizer like glue to prevent unwanted side reactions.

This process takes place in two key steps:

• Formation of chloramine \((\mathrm{NH}_2\cdot\mathrm{Cl})\) from sodium hypochlorite and ammonia.
• Reduction of chloramine by excess ammonia to yield hydrazine.

Throughout this reaction, hydrazine forms, but care must be exercised because it is both toxic and unstable in nature. Despite its challenges, the Raschig process remains essential for hydrazine preparation due to its relatively straightforward setup and effectiveness.

Reducing Agents in Chemistry

Reducing agents are substances that donate electrons to other molecules during chemical reactions. In essence, they "reduce" the other molecule by supplying electrons. Hydrazine is a well-known reducing agent and displays this behavior effectively in fertilizer and rocket fuel applications.

Hydrazine's reducing power is put to use in many reactions:

• It can convert ferric chloride \((\mathrm{FeCl}_3)\) to ferrous chloride \((\mathrm{FeCl}_2)\).
• It reduces potassium permanganate \((\mathrm{KMnO}_4)\), diminishing its characteristic purple color to manganese ions \((\mathrm{Mn}^{2+})\) or manganese dioxide \((\mathrm{MnO}_2)\).

These reduction reactions highlight hydrazine's ability to donate electrons effectively. In doing so, it alters the oxidation state of other substances, a principle crucial in various industrial and laboratory settings.

Nitrogen Gas

Nitrogen gas \((\mathrm{N}_2)\) is a prevalent substance in the Earth's atmosphere, comprising approximately 78% of it. It is an extremely stable molecule due to its robust triple bond, making it essentially inert under typical conditions.

In many chemical reactions, including the hydrazine reactions described, nitrogen gas is often a byproduct. When hydrazine is oxidized or decomposed, nitrogen gas is frequently formed. This production of nitrogen gas demonstrates its role as a safe and inert byproduct, as it does not readily react with other substances.

The non-reactive nature of \((\mathrm{N}_2)\) makes it an ideal blanketing gas for storing reactive materials, as it doesn't interact with them. Additionally, due to its abundance and properties, nitrogen gas is frequently utilized in industrial processes and scientific research as a standard inert atmosphere.