Problem 15 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A metal ' \(\mathrm{M}\) ' reacts with nitrogen gas to afford ' \(\mathrm{M}_{3} \mathrm{~N}^{\prime}\). ' \(\mathrm{M}_{3} \mathrm{~N}\) ' on heating at high temperature gives back ' \(\mathrm{M}\) ' and on reaction with water produces a gas ' \(\mathrm{B}\) '. Gas ' \(\mathrm{B}\) ' reacts with aqueous solution of \(\mathrm{CuSO}_{4}\) to form a deep blue compound. 'M' and 'B' respectively are : [Main Online April 8, 2017] (a) \(\mathrm{Li}\) and \(\mathrm{NH}_{3}\) (b) \(\mathrm{Ba}\) and \(\mathrm{N}_{2}\) (c) \(\mathrm{Na}\) and \(\mathrm{NH}_{3}\) (d) \(\mathrm{Al}\) and \(\mathrm{N}_{2}\)

Step-by-Step Solution

Answer

M is Li and B is NH3.

Step 1: Understanding the Reaction with Nitrogen

The metal \( M \) reacts with nitrogen gas to form \( M_3N \), indicating that \( M \) forms a nitride with nitrogen. A common reaction pattern involves metals from group 1 (alkali metals) or group 2 (alkaline earth metals) that form nitrides. Here, \( M \) should be reactive enough with nitrogen.

Step 2: Analyzing Reaction with Water

When \( M_3N \) reacts with water, it produces gas \( B \). A typical gas formed by nitrides reacting with water is ammonia (\( NH_3 \)). This suggests that \( M_3N \) is likely to be a nitride that can release ammonia.

Step 3: Examining the Reaction with \( \mathrm{CuSO}_4 \)

The problem states that gas \( B \) reacts with an aqueous solution of \( \mathrm{CuSO}_4 \) to form a deep blue compound. This matches the reaction of ammonia with \( \mathrm{CuSO}_4 \), where it forms a deep blue complex such as \([Cu(NH_3)_4]^{2+}\). Thus, \( B \) is \( NH_3 \).

Step 4: Identifying Metal \( M \)

From the previous analysis, \( B \) being \( NH_3 \) suggests that \( M \) could be a metal like lithium (\( Li \)), which forms \( Li_3N \), known to release ammonia upon reaction with water. This matches option (a), which lists \( Li \) and \( NH_3 \).

Step 5: Confirming the Option

Given the reactions and known elements, option (a) with \( \mathrm{Li} \) (metal \( M \)) and \( \mathrm{NH}_3 \) (gas \( B \)) conforms to the reactions described: \( \mathrm{Li} \) reacts with nitrogen to form \( \mathrm{Li}_3\mathrm{N} \), producing \( \mathrm{NH}_3 \) when reacting with water, which indeed forms a deep blue compound with \( \mathrm{CuSO}_4 \).

Key Concepts

Metal Nitrides

Metal nitrides are compounds formed between metals and nitrogen. These are typically created when a reactive metal directly combines with nitrogen gas. Certain metals, like lithium (\( \mathrm{Li} \)), form well-known nitrides like lithium nitride (\( \mathrm{Li}_3\mathrm{N} \)).

• Nitride formation is common among alkali and alkaline earth metals due to their reactivity.
• They possess interesting properties, such as stability and high melting points.
• Metal nitrides are utilized in various applications, including electronics and ceramics.

Understanding the formation of metal nitrides helps in predicting the behavior of metals under specific conditions, such as their reaction with atmospheric nitrogen.

Reaction with Water

The reaction of metal nitrides with water is particularly significant because it often results in the release of gases. Particularly, when lithium nitride (\( \mathrm{Li}_3\mathrm{N} \)) reacts with water, it produces ammonia gas (\( \mathrm{NH}_3 \)).

• This process can be described by the chemical reaction: \[ \mathrm{Li}_3\mathrm{N} + 3\mathrm{H}_2\mathrm{O} \rightarrow 3\mathrm{LiOH} + \mathrm{NH}_3 \]
• Nitrides typically react vigorously with water, making them significant in chemical synthesis.
• The resulting products, like ammonia, are used in various industrial applications.

Grasping how metal nitrides interact with water is crucial for comprehending broader chemical patterns and predicting the behavior of these compounds in aqueous environments.

Complex Formation with Copper Sulfate

When ammonia (\( \mathrm{NH}_3 \)) is introduced to an aqueous solution of copper(II) sulfate (\( \mathrm{CuSO}_4 \)), a characteristic deep blue complex is formed. This deep blue color is due to the formation of the complex ion \([\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+}\).

• Ammonia acts as a ligand, surrounding the central copper ion to form a coordination complex.
• The deep blue complex is a hallmark of such reactions and is utilized in tests to confirm the presence of ammonia.
• This type of reaction is also significant in analytical chemistry.

Comprehending the formation of these complexes involves understanding coordination chemistry, where ligands form bonds to central metal atoms or ions, influencing the compound's physical properties.