Problem 129 - A Complete Resource Book in Chemistry for JEE Main

Question

Metal \(X\) on heating in nitrogen gas gives Y.Y on treatment with \(\mathrm{H}_{2} \mathrm{O}\) gives a colourless gas which when passed through \(\mathrm{CuSO}_{4}\) solution gives a blue colour. Y is (a) \(\mathrm{MgO}\) (b) \(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\) (c) \(\mathrm{Mg}_{3} \mathrm{~N}_{2}\) (d) \(\mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

Y is (c) \(\text{Mg}_3\text{N}_2\).

Step 1: Identify Reaction of X with Nitrogen

Metal X reacts with nitrogen gas (N₂) on heating. Generally, metals like magnesium form metal nitrides. The reaction can be expressed as: \[ \text{3Mg (s) + N}_2\text{ (g) } \rightarrow \text{Mg}_3\text{N}_2\text{ (s)} \]Thus, Y is likely a metal nitride, such as magnesium nitride \(\text{Mg}_3\text{N}_2\) (option c).

Step 2: Reaction of Y with Water

When metal nitride Y reacts with water, it forms ammonia (a colorless gas) and magnesium hydroxide. The reaction is as follows:\[ \text{Mg}_3\text{N}_2\text{ (s) } + 6\text{H}_2\text{O (l)} \rightarrow 3\text{Mg(OH)}_2\text{ (aq)} + 2\text{NH}_3\text{ (g)} \]This means that \(\text{Mg}_3\text{N}_2\) reacts with water to produce a colorless gas, ammonia \(\text{NH}_3\).

Step 3: Testing Gas with CuSO4

The colorless gas produced from the reaction with water is ammonia \(\text{NH}_3\). When this gas is passed through copper sulfate solution (\(\text{CuSO}_4\)), it forms a complex that has a blue color due to the formation of tetraaminecopper(II) complex \([\text{Cu(NH}_3)_4]^{2+}\). This confirms the presence of ammonia in solution.

Step 4: Conclusion and Verification

Based on the reactions and the test with \(\text{CuSO}_4\), Y must be \(\text{Mg}_3\text{N}_2\) since it reacts with water to produce \(\text{NH}_3\), which in turn gives a blue color with \(\text{CuSO}_4\). Therefore, the correct answer is (c) \(\text{Mg}_3\text{N}_2\).

Key Concepts

Metal Nitrides

Metal nitrides are formed when metals react with nitrogen gas. These are compounds composed of metal and nitrogen, and are symbolized with a chemical formula like \( \text{Mg}_3\text{N}_2 \), representing three magnesium atoms bonded with two nitrogen atoms. Typically, such reactions occur under heating, as metals generally need higher temperatures to react with nitrogen.

• It is important to note that not all metals form nitrides easily. Alkali and alkaline earth metals, like magnesium, are more prone to forming stable nitrides.
• In our exercise, metal \( X \) is magnesium (\( \text{Mg} \)), which reacts with nitrogen to form magnesium nitride \( \text{Mg}_3\text{N}_2 \).

This reaction can be summarized as:\[3\text{Mg (s)} + \text{N}_2\text{ (g)} \rightarrow \text{Mg}_3\text{N}_2\text{ (s)}\] This particular formation emphasizes the concept of direct metal-nonmetal reaction under specific conditions leading to a metal nitride.

Reactions with Water

When metal nitrides react with water, they can produce several products depending on their composition. One common outcome is the release of ammonia gas, which is colorless and has a distinct pungent smell.In the case of magnesium nitride \( \text{Mg}_3\text{N}_2 \), the interaction with water is characterized by the production of magnesium hydroxide and ammonia. The balanced chemical equation for this reaction is:\[\text{Mg}_3\text{N}_2\text{ (s)} + 6\text{H}_2\text{O (l)} \rightarrow 3\text{Mg(OH)}_2\text{ (aq)} + 2\text{NH}_3\text{ (g)}\]

• The ammonia gas is colorless and detectable through its strong odor.
• Magnesium hydroxide forms as a precipitate or in solution, depending on the conditions.

This reaction highlights the concept of metal nitrides as sources of ammonia gas upon reaction with water.

Complex Formation with CuSO4

The ammonia produced from the reaction of magnesium nitride with water is not only significant for its presence but also for its behavior in subsequent reactions. When this colorless gas is passed through a copper sulfate solution, a striking change occurs - the solution turns blue.This color transformation is due to the formation of a complex compound known as the tetraaminecopper(II) complex, denoted as \([\text{Cu(NH}_3)_4]^{2+}\). The steps leading to this phenomenon include:

• Ammonia molecules coordinate with the copper ions in \( \text{CuSO}_4 \) solution.
• The interaction causes the copper ions to form a complex with ammonia, yielding the characteristic blue color.

This is a fine example of coordination chemistry, where metal ions form complexes with various ligands, altering properties like color. Thus, the presence of \( \text{NH}_3 \) confirms it through reactions indicating formation of complexes.