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

Question

Metal \(\mathrm{X}\) 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 \( \mathrm{Mg}_{3}\mathrm{N}_{2} \).

Step 1: Identify Reaction Between Metal X and Nitrogen

Metal \( \mathrm{X} \) is heated in nitrogen gas to produce \( Y \). From the problem description, \( Y \) must involve nitrogen, suggesting the formation of a nitride. Metals generally form nitrides when heated with nitrogen, so the probable compound \( Y \) is \( \mathrm{Mg}_{3}\mathrm{N}_{2} \) (magnesium nitride), as \( \mathrm{Mg} \) is a metal capable of forming such a compound.

Step 2: Reaction of Y with Water

React \( Y \), which is \( \mathrm{Mg}_{3}\mathrm{N}_{2} \), with water. This reaction produces a colorless gas \( \mathrm{NH}_{3} \) (ammonia) and \( \mathrm{Mg(OH)}_{2} \). The reaction can be written as:\[\mathrm{Mg}_{3}\mathrm{N}_{2} + 6\,\mathrm{H}_{2}\mathrm{O} \rightarrow 3\,\mathrm{Mg(OH)}_{2} + 2\,\mathrm{NH}_{3} \]

Step 3: Identify the Reaction with \( \mathrm{CuSO}_{4} \)

According to the problem, the colorless gas produced (ammonia \( \mathrm{NH}_{3} \)) turns \( \mathrm{CuSO}_{4} \) solution blue when passed through it. This is due to the formation of \( \mathrm{Cu(NH}_{3})_{4}\mathrm{SO}_{4} \cdot \mathrm{H}_{2} \) which is a typical deep blue complex.

Step 4: Determination of Compound Y

From the above analysis, compound \( Y \) in the option list is identified as \( \mathrm{Mg}_{3}\mathrm{N}_{2} \). None of the other options fit all the reactions described in the problem.

Key Concepts

Metal Nitrides

Metal nitrides are formed when certain metals react with nitrogen gas at high temperatures. These compounds typically consist of metal cations and nitride anions. Nitride ions contain nitrogen with an oxidation state of -3.

One of the classic examples is magnesium nitride (\( \mathrm{Mg}_{3}\mathrm{N}_{2} \)). When magnesium, a reactive metal, is heated in a nitrogen atmosphere, it bonds with nitrogen to produce magnesium nitride. The reaction can be represented as:

• \( 3 \mathrm{Mg} + \mathrm{N}_2 \rightarrow \mathrm{Mg}_3 \mathrm{N}_2 \)

Metal nitrides are known for their high melting points and hardness. They are used in various applications, including electronics, ceramics, and even in cutting tools. Understanding the properties of metal nitrides is crucial for exploiting their applications effectively.

Ammonia Production

Magnesium nitride reacts vigorously with water to produce ammonia, a key industrial chemical. When \( \mathrm{Mg}_3\mathrm{N}_2 \) is exposed to water, it breaks down to form magnesium hydroxide and ammonia gas. This process can be described by the chemical equation:

• \( \mathrm{Mg}_3\mathrm{N}_2 + 6\,\mathrm{H}_2\mathrm{O} \rightarrow 3\,\mathrm{Mg(OH)}_2 + 2\,\mathrm{NH}_3 \)

Ammonia is a colorless gas with a distinctive odor. It is highly soluble in water, which makes it an essential component in fertilizers. The synthesis of ammonia on an industrial scale, known as the Haber-Bosch process, is vital to agriculture. Even in smaller-scale reactions, the formation of ammonia is crucial for chemical synthesis and various laboratory applications.

Complex Ion Formation

Complex ions are ions with a central metal atom/ion bonded to surrounding molecules or ions. These surrounding entities, called ligands, usually have lone electron pairs that they donate to the metal.

Ammonia, when passed through copper sulfate solution, forms a complex ion that showcases this behavior. Copper reacts with ammonia to form \( \mathrm{Cu(NH}_3)_{4}\mathrm{SO}_4 \cdot \mathrm{H}_2 \), which imparts a deep blue color to the solution. This is a result of the coordination of ammonia molecules to the copper ions. The reaction is a great example of how complex ions can alter a solution's properties, such as its color. By forming complexes, metal ions often exhibit unique chemical behaviors in solution.