Problem 84 - A Complete Resource Book in Chemistry for JEE Main

Question

A 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 \(\dot{\mathrm{CuSO}_{4}}\) solution gives a blue colour. Y is (a) \(\mathrm{Mg}_{3} \mathrm{~N}_{2}\) (b) \(\mathrm{MgO}\) (c) \(\mathrm{NH}_{3}\) (d) \(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\)

Step-by-Step Solution

Answer

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

Step 1: Identify the reaction of metal with nitrogen

When the metal \( X \) is heated in nitrogen gas, a metal nitride \( Y \) is formed. The compounds given in the options that can be considered metal nitrides are \( \mathrm{Mg}_{3} \mathrm{N}_{2} \) and possibly \( \mathrm{Mg( NO }_{3} \mathrm{ )}_{2} \), though legitimately \( \mathrm{Mg( NO }_{3} \mathrm{ )}_{2} \) is a nitrate and not a nitride.

Step 2: Determine the outcome when \(Y\) reacts with water

The problem states that upon treatment with water, \( Y \) releases a colorless gas. If \( \mathrm{Mg}_{3} \mathrm{N}_{2} \) reacts with water, it forms \( \mathrm{NH}_{3} \) (ammonia) and \( \mathrm{MgO} \). \( \mathrm{NH}_{3} \) is a colorless gas. \( \mathrm{Mg( NO }_{3} \mathrm{ )}_{2} \) would dissolve, but not produce \( \mathrm{NH}_{3} \).

Step 3: Analyze the interaction with \(\mathrm{CuSO}_4\)

When \( \mathrm{NH}_3 \) (produced from \( \mathrm{Mg}_{3} \mathrm{N}_{2} + \mathrm{H}_2 \mathrm{O} \)) interacts with \( \cdot \mathrm{CuSO}_{4} \), it forms \([\text{Cu} (\text{NH}_3)_4]^{2+}\), which has a blue color, confirming the passage of ammonia gas.

Step 4: Identify \( Y \) based on reactions and properties

Based on the process, \( Y \) that reacts with water to release \( \mathrm{NH}_3 \) that turns blue with \( \mathrm{CuSO}_4 \) must be \( \mathrm{Mg}_3 \mathrm{N}_2 \), which is magnesium nitride. Therefore, the compound \( Y \) is \( \mathrm{Mg}_3 \mathrm{N}_2 \).

Key Concepts

Chemical Reactions

Chemical reactions are processes in which substances, known as reactants, are transformed into different substances called products. These reactions can involve changes in energy, bonds, and the identities of the substances involved.
In the given exercise, the metal \( X \) reacts with nitrogen gas to form a metal nitride \( Y \). This type of reaction is known as a synthesis reaction, where two or more simple substances combine to form a more complex compound. Here are some important aspects of chemical reactions:

• Reactants and Products: Reactants are the starting materials, while products are the substances formed as a result of the reaction.
• Conservation of Mass: In a chemical reaction, the total mass of the reactants equals the total mass of the products.
• Indicators of a Reaction: Indicators can include color changes, gas formation, temperature changes, or the formation of a precipitate.

This exercise involves the formation of \( Y \), which upon further reactions reveals its properties as a metal nitride.

Inorganic Chemistry

Inorganic chemistry is a branch of chemistry that deals with inorganic compounds, which typically lack carbon-hydrogen bonds. It covers a wide range of substances, including metals, minerals, and organometallic compounds.
Metal nitrides, such as \( \mathrm{Mg}_3 \mathrm{N}_2 \), are a classic topic in inorganic chemistry. Magnesium nitride is formed when magnesium metal reacts with nitrogen gas. This compound illustrates key concepts of inorganic chemistry:

• Formation: Metal nitrides are formed when metals combine with nitrogen. This reaction often requires heating.
• Properties: Many metal nitrides, including \( \mathrm{Mg}_3 \mathrm{N}_2 \), are stable compounds that demonstrate characteristics such as high melting points.
• Reactivity with Water: Inorganic compounds like \( \mathrm{Mg}_3 \mathrm{N}_2 \) react with water to form ammonia, a key concept in understanding metal nitrogen reactions.

Understanding these properties and reactions is essential for mastering inorganic chemistry concepts.

Ammonia Formation

Ammonia, \( \mathrm{NH}_3 \), is a colorless gas with a characteristic pungent odor. It is a significant compound in both laboratory and industrial chemistry, often used in fertilizers and cleaning products.
The formation of ammonia in metal nitride reactions, such as the reaction of \( \mathrm{Mg}_3 \mathrm{N}_2 \) with water, is an important concept. Here's how this process occurs:

• Reaction with Water: When metal nitrides react with water, ammonia is released. For \( \mathrm{Mg}_3 \mathrm{N}_2 \), the reaction can be written as:
  \[ \mathrm{Mg}_3 \mathrm{N}_2 + 6 \mathrm{H}_2\mathrm{O} \rightarrow 3 \mathrm{MgO} + 2 \mathrm{NH}_3 \]
• Detecting Ammonia: Ammonia is commonly detected by its reaction with \( \mathrm{CuSO}_4 \), turning it blue. This property allows us to confirm ammonia formation in chemical reactions.

Ammonia plays a key role in this exercise by indicating the formation of specific compounds and helping to identify \( \mathrm{Mg}_3 \mathrm{N}_2 \) as the correct answer.