Problem 19 - A Complete Resource Book in Chemistry for JEE Main

Question

Iron sulphide is heated in air to form \(\mathrm{A}\), an oxide of sulphur. A is dissolved in water to give an acid. The basicity of this acid is (a) zero (b) 1 (c) 2 (d) 3

Step-by-Step Solution

Answer

The basicity of the acid is 2.

Step 1: Understanding Iron Sulphide Reaction

When iron sulphide ( ext{FeS}) is heated in air, sulphur reacts with oxygen to form sulphur dioxide ( ext{SO}_2). The reaction is: \[ ext{4FeS} + ext{7O}_2 ightarrow ext{4FeO} + ext{4SO}_2 \] identify ext{SO}_2 (sulphur dioxide) as substance A, the oxide of sulphur formed.

Step 2: Dissolution of Sulphur Dioxide in Water

When sulphur dioxide ( ext{SO}_2) is dissolved in water, it forms sulfurous acid ( ext{H}_2 ext{SO}_3): \[ ext{SO}_2 (g) + ext{H}_2 ext{O} (l) ightarrow ext{H}_2 ext{SO}_3 (aq) \] Here, the compound formed is sulfurous acid.

Step 3: Determine the Basicity of Sulfurous Acid

Basicity of an acid refers to the number of ionizable hydrogen ions (protons) it can donate in a reaction. Sulfurous acid ( ext{H}_2 ext{SO}_3) has two ionizable hydrogen atoms. Thus, its basicity is 2.

Key Concepts

Iron Sulphide Reaction

When we think about chemical reactions involving iron sulphide, we need to start with what happens when it is heated in air. Iron sulphide, or \( \text{FeS} \), contains both iron and sulphur. When the substance is heated, the sulphur component reacts with the oxygen present in the air. This interaction leads to the formation of sulphur dioxide, \( \text{SO}_2 \).

The chemical equation for this reaction is given by:

• \( 4\text{FeS} + 7\text{O}_2 \rightarrow 4\text{FeO} + 4\text{SO}_2 \)

This equation tells us that four molecules of iron sulphide react with seven molecules of oxygen to produce four molecules of iron oxide and four molecules of sulphur dioxide. Here, sulphur dioxide is the compound identified as substance A in the exercise.

By understanding this reaction, it's clear that heating iron sulphide sets the stage for forming compounds that have implications in subsequent chemical transformations.

Sulphur Dioxide Formation

Once sulphur dioxide \( (\text{SO}_2) \) is formed from the reaction between iron sulphide and oxygen, it takes on significant chemical roles. The formation itself is important because \( \text{SO}_2 \) is a critical intermediary in producing other sulfur compounds.

Sulphur dioxide is a colorless gas that's easily soluble in water. When \( \text{SO}_2 \) dissolves in water, it undergoes a further chemical reaction to form sulfurous acid \( (\text{H}_2\text{SO}_3) \). The equation for this reaction is written as:

• \( \text{SO}_2 (g) + \text{H}_2\text{O} (l) \rightarrow \text{H}_2\text{SO}_3 (aq) \)

This reaction shows that a gas (sulphur dioxide) combines with liquid water to create an aqueous solution of sulfurous acid.

Solutions such as these are useful in many industrial processes and provide a basis for further chemical investigations involving acids and their properties.

Chemical Reactions

Chemical reactions are processes that involve the transformation of substances through breaking and forming chemical bonds. In the example with iron sulphide, we've seen how these reactions can lead to significant changes and the production of new compounds.

This example highlights two reactions:

• The reaction of iron sulphide with oxygen to form sulphur dioxide and iron oxide.
• The dissolution of sulphur dioxide in water to form sulfurous acid.

Both reactions involve important concepts in chemistry: reactivity with oxygen, formation of gases, and solubility in water.Moreover, the concept of basicity is introduced when studying sulfurous acid, \( \text{H}_2\text{SO}_3 \). Basicity refers to how many hydrogen ions an acid can donate. Sulfurous acid can donate two hydrogen ions, thus it has a basicity of 2.

Understanding chemical reactions helps chemists to predict the behavior of substances in various environments and applications, emphasizing the practical nature of these studies.