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 \(\mathrm{H}_{2} \mathrm{SO}_{4} \cdot^{\prime} \mathrm{A}\) ' 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 ' \(\mathrm{B}\) '. Identify 'A' and ' \(\mathrm{B}\) '. (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) A=NH_2NH_2; B=N_2.

Step 1: Understand the Reaction

When aqueous H_3 is boiled with aqueous NaOCl in the presence of a little glue, it leads to a reaction that typically results in the formation of hydrazine, NH_2NH_2, as 'A'. This is known as the Raschig synthesis of hydrazine.

Step 2: Evaluating 'A'

Hydrazine ( NH_2NH_2) is known to form salts with HCl and H_2SO_4, which matches the description given in the problem statement.

Step 3: Check Reducing Properties

'A' is described as a powerful reducing agent. Hydrazine is indeed a powerful reducing agent that can reduce both FeCl_3 and acidified KMnO_4 solutions, which is consistent with the reaction characteristics described.

Step 4: Identify the Evolution of 'B'

During the reduction process involving NH_2NH_2, nitrogen gas ( N_2) is often evolved as a byproduct in these reactions. This would be the inactive gas 'B' mentioned.

Step 5: Select the Correct Option

Based on the analysis above, the compound 'A' is hydrazine ( NH_2NH_2) and 'B' is nitrogen gas ( N_2). Therefore, the correct answer is option (b) A=NH_2NH_2; B=N_2.

Key Concepts

Raschig process

The Raschig process is a fascinating chemical reaction named after Friedrich Raschig. This process is primarily used for the industrial production of hydrazine (\(\text{NH}_2\text{NH}_2\)) by reacting ammonia (\(\text{NH}_3\)) with sodium hypochlorite (\(\text{NaOCl}\)). What makes it intriguing is the way simple household chemicals are transformed into a compound with wide-ranging industrial applications.
The reaction occurs under mild conditions, which is a significant advantage in an industrial setting. In this process, ammonia acts with sodium hypochlorite, and with the assistance of a catalyst such as glue, produces hydrazine along with water and other minor byproducts. The role of glue here is to stabilize and speed up the course of the reaction, ensuring the successful synthesis of hydrazine. Without it, the reaction could be less efficient. Hydrazine, which is the reaction's primary product, is a vital component in rocket fuel and also serves as a precursor to various pharmaceuticals and agrichemical products.
It's crucial to control the reaction environment since both temperature and concentration of reactants can influence the yield of \(\text{NH}_2\text{NH}_2\). The Raschig process remains one of the foundational industrial methods demonstrating the intersection of basic chemistry principles with large-scale manufacturing needs.

Reducing agent

A reducing agent is a substance that donates electrons to another species in a chemical reaction, thus reducing that species. In the realm of chemistry, hydrazine (\(\text{NH}_2\text{NH}_2\)) is renowned for its powerful reducing properties. This means that it can effectively donate electrons to other compounds, facilitating their reduction. When hydrazine acts as a reducing agent, it becomes oxidized itself.
This property is particularly important in chemical industries where hydrazine finds its application in processes such as water treatment, where it reduces dissolved oxygen, preventing corrosion. Moreover, its reducing capability makes hydrazine a prime choice in the synthesis of numerous organic compounds. Its ability to participate in versatile reactions also underlines its utility in laboratories.
What truly sets hydrazine apart is its capacity to reduce a range of compounds, including metal salts and oxides. It makes hydrazine indispensable in analytical chemistry, where its reductive prowess is employed to assess the concentration of various components in samples. By understanding the concept of reduction, students can appreciate how broadly hydrazine's reducing power is applied across different chemical contexts and industries.

Chemical reactions involving FeCl3 and KMnO4

The chemical reactions involving hydrazine interacting with \(\text{FeCl}_3\) (ferric chloride) and acidified \(\text{KMnO}_4\) (potassium permanganate) showcase its reducing power. In these reactions, hydrazine acts as the reducing agent.
Let's consider the reaction with \(\text{FeCl}_3\): when hydrazine is added, it reduces \(\text{Fe}^{3+}\) ions to \(\text{Fe}^{2+}\) ions. The overall reaction also causes the dissolution of brown iron \(\text{III}\) chloride solution into a lighter iron \(\text{II}\) chloride solution.
Similarly, in the case of acidified \(\text{KMnO}_4\), hydrazine reduces \(\text{MnO}_4^-\) ions to \(\text{Mn}^{2+}\) ions, leading to a color change in the reaction mixture from purple to colorless or pale pink. This change is visually indicative of the reaction's progress. These reactions underline the ability of hydrazine to efficiently act as a reducing agent. It demonstrates how students can practically apply knowledge to predict products of reactions and understand the mechanics behind complex chemical processes.

Evolution of nitrogen gas

The evolution of nitrogen gas (\(\text{N}_2\)) is a common occurrence when hydrazine is used in reactions as a reducing agent. During the reduction of metal oxides or other substances, nitrogen gas is often released as a byproduct. This evolutionary process is key to understanding the reactions' completion and can often be observed as bubbles forming in the reaction mixture.
Nitrogen gas is considered "inactive" due to its inert characteristics at normal temperatures and pressures. It's colorless, odorless, and is not involved in many chemical reactions, providing a non-reactive backdrop for the reactions at hand.
This characteristic of being "inactive" makes nitrogen an excellent marker for the end of specific reactions, especially in educational demonstrations. Observing the liberation of \(\text{N}_2\) can also serve as a practical indicator for students to understand when the reducing process is complete, thus deepening their grasp of chemical behavior. By connecting theory to observable phenomena, students learn not just about specific compounds like hydrazine, but also about the dynamics of chemical reactions in general.