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 (option c).

Step 1: Identify the Reaction Products

When iron sulphide ({FeS}) is heated in air, it reacts with oxygen to form iron(III) oxide ({Fe_2O_3}) and sulphur dioxide ({SO_2}). Hence, {A} is {SO_2}, an oxide of sulphur.

Step 2: Dissolve the Oxide in Water

Sulphur dioxide ({SO_2}) is dissolved in water to form sulphurous acid ({H_2SO_3}). The reaction is: {SO_2 + H_2O ightarrow H_2SO_3}.

Step 3: Determine the Basicity of the Acid

Basicity is defined as the number of hydrogen ions ({H^+}) that can be replaced by a base in an acid. {H_2SO_3} can donate two hydrogen ions, so its basicity is 2.

Key Concepts

Iron Sulphide Reaction

When we talk about the iron sulphide reaction, it is important to understand the process and products of this chemical reaction. Here, iron sulphide (FeS) is exposed to oxygen under heat, leading to the formation of new compounds.

• Iron(III) oxide is one of these products, having the formula \( Fe_2O_3 \).
• The main focus for our topic is the formation of sulphur dioxide, which is a gas and another product of this reaction.

Understanding how these substances interact is key to grasping how the oxide of sulphur is formed. It's this sulphur dioxide (\( SO_2 \)) that further evolves in our next steps.

Sulphur Dioxide

Sulphur dioxide, chemically represented as \( SO_2 \), plays a fundamental role in pollution as well as in various chemical processes. It is a colorless gas with a pungent odor, usually found as a byproduct from industrial activities such as burning fossil fuels.When considering its formation from the reaction of iron sulphide with oxygen, it's essential to note how it acts as an oxide of sulphur. This oxide's reactivity opens up further chemical possibilities, one of which is forming acids when dissolved in water. Hence, it serves as a simple but key player in illustrating how chemical reactions can transform substances.

Sulphurous Acid

Upon dissolving sulphur dioxide in water, we get sulphurous acid, denoted chemically as \( H_2SO_3 \). This transformation is crucial for understanding its property as an acid.

• Sulphurous acid is weak and diprotic, which means it can release two hydrogen ions in solutions.
• This behaviour is significant in determining the acidity and reactivity of the compound.

Though not as stable as other sulphur-based acids, understanding \( H_2SO_3 \) gives insights into natural processes like acid rain and its effect on the environment.

Chemical Reactions Involving Oxides

Chemical reactions involving oxides, like those forming from sulphur dioxide, demonstrate the dynamic nature of chemical bonds.

• Oxides are commonly seen products when elements react with oxygen, as in the case of iron sulphide heated in air.
• They can serve as intermediates in forming new, more complex substances, such as acids or bases when interacting with other substances like water.

The sulphur dioxide to sulphurous acid reaction, \( SO_2 + H_2O \rightarrow H_2SO_3 \), is a classic example of how oxides are more than mere byproducts—they are active participants in chemical cycles that matter in both environmental and industrial contexts.

Hydrogen Ion Donation

The concept of hydrogen ion donation is central to understanding the basicity of acids like sulphurous acid (\( H_2SO_3 \)). Basicity refers to the number of hydrogen ions \( (H^+) \) an acid can donate in a reaction.

• In the case of \( H_2SO_3 \), it can donate two hydrogen ions, hence it is known as diprotic.
• This ability impacts how the acid reacts with other substances, influencing its strength and behaviour as an acid.

Acquiring insight into the mechanics of hydrogen ion donation is fundamental for understanding the broader topics of acidity, acid-base reactions, and how these processes come into play in everyday chemical interactions.