Problem 101 - A Complete Resource Book in Chemistry for JEE Main

Question

A metal (A) heating in nitrogen gas gives (B). (B) on treatment with \(\mathrm{H}_{2} \mathrm{O}\) gives a colourless gas which on passing through \(\mathrm{CuSO}_{4}\) solution turns it deep blue. Here (B) is (a) \(\mathrm{NH}_{3}\) (b) \(\mathrm{MgO}\) (c) \(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\) (d) \(\mathrm{Mg}_{3} \mathrm{~N}_{2}\)

Step-by-Step Solution

Answer

Compound (B) is (d) \( \mathrm{Mg}_{3} \mathrm{~N}_{2} \).

Step 1: Understand the Reaction Context

We are given that a metal (A) reacts with nitrogen gas to form a compound (B). This compound (B) then reacts with water to form a colorless gas, which changes the color of a CuSO_{4} solution to deep blue.

Step 2: Identify Chemical Reactions Involved

First, we need to determine what metal-nitrogen compound could be formed. Typically, when a metal reacts with nitrogen gas, a metal nitride is formed. In this scenario, (B) is a candidate metal-nitride.

Step 3: Reaction of Metal (A) with Nitrogen

Let's consider magnesium (a common metal) as metal (A). When magnesium reacts with nitrogen, it forms magnesium nitride, Mg_{3}N_{2}:\[3Mg + N_{2} \rightarrow Mg_{3}N_{2}\]

Step 4: Reaction of Compound (B) with Water

This magnesium nitride (B) can react with water to produce ammonia (NH_{3}, a colorless gas):\[Mg_{3}N_{2} + 6H_{2}O \rightarrow 3Mg(OH)_{2} + 2NH_{3}\]

Step 5: Interaction with Copper Sulfate Solution

Ammonia is known to react with CuSO_{4} to form a deep blue complex [Cu(NH_{3})_{4}]SO_{4}, confirming the presence of ammonia:\[Cu^{2+} + 4NH_{3} \rightarrow [Cu(NH_{3})_{4}]^{2+}\]

Step 6: Identify Compound (B)

Since magnesium nitride ( Mg_{3}N_{2}) is the compound that leads to the formation of ammonia, which then reacts to turn the CuSO_{4} solution deep blue, compound (B) is Mg_{3}N_{2}.

Key Concepts

Metal-Nitrogen Reactions

When metals react with nitrogen, they typically form metal nitrides, which are compounds consisting of metal cations and nitride anions \((\text{N}^{3-})\). The reaction involves the direct combination of a metal with nitrogen gas \(\text{N}_2\), resulting in a crystalline solid. This type of reaction is essential for forming various metal nitrides that have important applications in industry and technology. Not every metal can easily form a nitride, as it depends significantly on the reactivity of the metal. For example, metals like magnesium, lithium, and aluminum react more easily with nitrogen to form nitrides. The general reaction can be simplified as follows:\

• Metal + Nitrogen gas \(\rightarrow\) Metal Nitride.

Metal Nitrides

Metal nitrides are compounds where metal atoms are bonded to nitrogen. They are represented generally as \(\text{M}_x\text{N}_y\), where \(\text{M}\) stands for the metal and \(\text{N}\) is for nitrogen. These compounds usually consist of a lattice structure, giving them some remarkable properties such as hardness, high melting points, and conductivity. \

• Example: For magnesium, when it reacts with nitrogen, it forms magnesium nitride \(\text{Mg}_3\text{N}_2\).
• In practical applications, nitrides are used in ceramics, cutting tools, and as semiconductor materials due to their high-temperature stability and wear resistance.

Emphasizing magnesium nitride, its formation can be explained by the reaction:\[3 \text{Mg} + \text{N}_2 \rightarrow \text{Mg}_3\text{N}_2\] This specific type of compound readily undergoes a chemical change with certain reagents, demonstrating significant chemical behavior.

Ammonia Formation

Ammonia \(\text{(NH}_3\text{)}\) is a simple molecule, consisting of nitrogen and hydrogen. It is produced when magnesium nitride \(\text{Mg}_3\text{N}_2\) reacts with water. This reaction is of significant industrial and chemical importance as ammonia serves as a crucial nitrogen source for fertilizers. \When magnesium nitride is treated with water, it decomposes to form magnesium hydroxide and ammonia. The balanced chemical equation for this reaction is:\[\text{Mg}_3\text{N}_2 + 6 \text{H}_2\text{O} \rightarrow 3 \text{Mg(OH)}_2 + 2 \text{NH}_3\] \

• Ammonia forms as a colorless gas, notable for its pungent smell.
• Its capability to act as a ligand allows it to participate in a variety of complexation reactions, such as with copper sulfate solution.

Chemical Reactions with Water

Reactions of metal nitrides with water are interesting processes that produce other compounds, such as hydroxides and gases. When magnesium nitride \(\text{Mg}_3\text{N}_2\) is added to water, it reacts vigorously. Water acts as a reactant that breaks the bond between the metal and nitrogen, ultimately forming a hydroxide and a gas. \

Key insights:

\

• In the reaction, water provides oxygen for the hydroxide and hydrogen, which combines with nitrogen to form ammonia:
• The general reaction expression for these transformations can be expressed simply as:

\From the equation discussed in earlier sections, understand how the water molecules not only help release the colorless ammonia gas \((\text{NH}_3)\) but also form magnesium hydroxide \((\text{Mg(OH)}_2)\), a solid precipitate. This precipitate is less soluble in water and appears as a milky white suspension, completing the transformation process from solid nitride to useful compounds.