Problem 4 - A Complete Resource Book in Chemistry for JEE Main

Question

A metal is burnt in air and the ash on moistening smells of ammonia. The metal is (a) \(\mathrm{Mg}\) (b) \(\mathrm{Al}\) (c) \(\mathrm{Fe}\) (d) \(\mathrm{Na}\)

Step-by-Step Solution

Answer

The metal is (d) Na.

Step 1: Analyze the Reaction

When a metal burns in air, it often forms metal oxides as a result of reacting with oxygen. To identify the metal, consider any unique chemical properties present in the by-products, such as the smell of ammonia in this case.

Step 2: Understand the Ammonia Connection

The smell of ammonia suggests the presence of an ammonium compound, which indicates a reaction involving nitrogen. Among commonly oxidized metals, sodium is known to form a compound leading to ammonia.

Step 3: Evaluate Metal Options

Evaluate each metal option based on its ability to form compounds that could release ammonia when moistened. Particularly, consider if burning each metal could theoretically lead to the formation of compounds that smell of ammonia.

Step 4: Deduce the Correct Metal

If sodium ( 7Na 7) burns in air, it can form sodium nitride under certain conditions. When sodium nitride reacts with water, it produces ammonia ( 7NH 7). Thus, the ash that resembles ammonium compounds when moistened supports sodium as the likely choice.

Key Concepts

Metal oxides formation

When metals burn in air, a common reaction is the formation of metal oxides. This occurs because metals react with oxygen from the air. For example, when magnesium burns, it forms magnesium oxide. This process is common and is supported by the fact that oxygen is a major component of air.

Metal oxides are usually basic in nature, and their formation is an exothermic process, meaning it releases energy. The general reaction can be written as:

• Metal + Oxygen -> Metal Oxide

For instance, sodium, when it reacts with oxygen, forms sodium oxide. Understanding these reactions is crucial in predicting the behavior of metals when exposed to air.

Ammonium compounds

Ammonium compounds are characterized by the presence of the ammonium ion, \( ext{NH}_4^+ \). These compounds often release a distinct smell of ammonia, especially when they come in contact with water. This can happen when certain nitrides react with water, producing ammonia in the process.

Ammonium compounds play a significant role in various chemical reactions and are prevalent in fertilizers due to their high nitrogen content, which is essential for plant growth. In our context, the presence of ammonia upon moistening suggests a nitrogen reaction, often related to ammonium production.

Sodium nitride

Sodium nitride is a compound that forms when sodium reacts with nitrogen under specific conditions. This compound is somewhat unstable and rare since sodium naturally prefers to react with oxygen. However, when sodium nitride is formed and subsequently reacts with water, it results in the release of ammonia. This reaction can be summarized as follows:

• Sodium nitride + Water -> Sodium hydroxide + Ammonia

This characteristic release of ammonia is what leads to the distinct smell when the ash of burnt sodium is moistened. Understanding sodium nitride's formation and behavior is key in chemical identification processes involving sodium.

Chemical properties of metals

The chemical properties of metals define their reactions with other elements. Metals like sodium have a strong tendency to lose electrons, making them reactive. This is why sodium burns easily in air to form compounds like sodium oxide or sodium nitride. They also conduct electricity, are malleable, and can form alloys with other metals.

Metals can form various types of compounds such as oxides, chlorides, and hydrides based on the reaction environment and reactants. Their reactivity series further explains their ability to displace other metals in reactions. Understanding these properties allows us to predict and explain their behavior in chemical processes.