Problem 24 - A Complete Resource Book in Chemistry for JEE Main

Question

The alkali metal that reacts with nitrogen directly to form nitride is (a) \(\mathrm{K}\) (b) \(\mathrm{Na}\) (c) Li (d) \(\mathrm{Rb}\)

Step-by-Step Solution

Answer

The alkali metal that reacts with nitrogen directly to form nitride is (c) Li.

Step 1: Understanding Direct Reaction with Nitrogen

Alkali metals normally do not react directly with nitrogen to form nitrides under standard conditions, except for lithium. This is due to lithium's unique properties compared to other alkali metals.

Step 2: Identifying the Unique Alkali Metal

Lithium is an outlier among alkali metals due to its small atomic size and higher ionization energy. These properties allow lithium to react directly with nitrogen, forming lithium nitride ({Li}_3{N}).

Step 3: Reasoning Through the Options

Review the given options: (a) {K} (potassium), (b) {Na} (sodium), (c) {Li} (lithium), (d) {Rb} (rubidium). Based on the understanding that lithium uniquely reacts with nitrogen, identify option (c) {Li} as the correct answer.

Step 4: Verifying the Reaction

To verify, consider the chemical reaction for lithium and nitrogen: {6Li + N_2 ightarrow 2Li_3N}. This reaction forms lithium nitride, proving that lithium reacts directly with nitrogen to form a nitride.

Key Concepts

Lithium Nitride Formation

Lithium uniquely participates in a direct chemical reaction with nitrogen to form lithium nitride, represented by the formula \( \mathrm{Li}_3\mathrm{N} \). This reaction is unusual for alkali metals since most do not naturally react with nitrogen. The formation of lithium nitride occurs under regular conditions without requiring extra reagents or special environments. The chemical equation illustrating this process is:

• \[ 6\,\mathrm{Li} + \mathrm{N}_2 \rightarrow 2\,\mathrm{Li}_3\mathrm{N} \]

Here, lithium atoms combine with nitrogen molecules, producing lithium nitride as a result. This compound is characterized by a dark-colored appearance and demonstrates lithium's distinctive behavior compared to other alkali metals. This reaction showcases lithium's strong affinity for nitrogen due to its small atomic size and high ionization energy.

Chemical Reactions of Alkali Metals

Alkali metals are known for their vigorous reactions, particularly with water and halogens, but their interaction with nitrogen is different. Except for lithium, alkali metals do not typically form nitrides by reacting directly with nitrogen under standard conditions. This is because larger alkali metals like sodium (Na), potassium (K), and rubidium (Rb) do not possess the necessary properties to bind directly with nitrogen effectively.
The unique reactivity of alkali metals is attributed to their single valence electron, which they readily lose to form positive ions.

• Lithium reacts directly with nitrogen due to its small size and high ionization energy, forming lithium nitride.
• Other alkali metals require specific conditions to form nitrides or do not form them directly at all.

Understanding these nuances helps grasp why lithium stands out in the periodic table for its ability to react with nitrogen unlike its group companions.

Properties of Lithium

Lithium is the lightest metal and exhibits unique characteristics that differentiate it from other alkali metals. With its small atomic radius, lithium is highly reactive but less so than its heavier group members with water. This small size also leads to a higher ionization energy.
Another distinctive property of lithium is its ability to form a nitride with nitrogen, as mentioned before. It achieves this through its small radius, which facilitates the formation of stable compounds with non-metals such as nitrogen.

• Lithium's ionization energy is higher than that of sodium or potassium, aiding in its unique reactivity.
• Its light weight and high reactivity make it valuable in a variety of applications, including batteries.

These properties not only explain its chemical behavior but also guide its use in industry and technology, distinguishing it from other metals in its group.