Problem 241 - A Complete Resource Book in Chemistry for JEE Main

Question

Iron (II) sulphide is heated in air to form compound \(' A^{\prime}\), an oxide of sulphur. Compound 'A' is dissolved in water to give an acid. The basicity of this acid is

Step-by-Step Solution

Answer

The basicity of sulphurous acid \( \text{H}_2\text{SO}_3 \) is 2.

Step 1: Identify the chemical reaction

Start by identifying the chemical reaction. When Iron (II) sulphide, which is FeS, is heated in air, it reacts with oxygen to form iron(III) oxide \( \text{Fe}_2\text{O}_3 \) and sulphur dioxide \( \text{SO}_2 \). This can be represented by the chemical equation: \( 4\text{FeS} + 7\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3 + 4\text{SO}_2 \). Here, the gaseous compound A is sulphur dioxide, \( \text{SO}_2 \).

Step 2: Dissolve sulphur dioxide in water

Next, identify the compound formed when \( \text{SO}_2 \) is dissolved in water. Sulphur dioxide, \( \text{SO}_2 \), reacts with water to form sulphurous acid, \( \text{H}_2\text{SO}_3 \). This reaction is: \( \text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 \).

Step 3: Determine the basicity of sulphurous acid

Basicity refers to the number of hydrogen ions \( \text{H}^+ \) that an acid can donate. Sulphurous acid \( \text{H}_2\text{SO}_3 \) contains two hydrogen atoms that can dissociate in solution, forming two \( \text{H}^+ \) ions. Therefore, the basicity of sulphurous acid is 2.

Key Concepts

Chemical Reactions

Chemical reactions are processes where substances, often called reactants, are transformed into different substances known as products. In the case of Iron (II) sulphide formed with sulphur and iron, these reactions can lead to the formation of entirely new compounds. For example, when Iron (II) sulphide is heated, it reacts with oxygen from the air.

• The Iron (II) sulphide (FeS) undergoes oxidation reactions, meaning it combines with oxygen.
• This results in the creation of iron(III) oxide (\( \text{Fe}_2\text{O}_3 \) ) and sulphur dioxide (\( \text{SO}_2 \) ).

These reactions are crucial in understanding how elements can recombine when energy (like heat) is applied to break and form bonds. Chemical reactions are not just transformations in labs, they are the forces behind many natural processes in the world. The specific reaction for Iron (II) sulphide is given by the equation: \[ 4\text{FeS} + 7\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3 + 4\text{SO}_2 \] Understanding these reactions provides insights into the basic processes that govern nature and materials science.

Sulphur Dioxide

Sulphur dioxide (\( \text{SO}_2 \) ) is a colourless gas with a pungent smell, usually produced from the combustion of sulphur-containing fuels. It plays an important role in the chemical processes as it is formed as one of the products when Iron (II) sulphide is heated in the air.

• This compound is represented by the formula \( \text{SO}_2 \) .
• In the air, it reacts with other chemicals, forming pollutants like sulphuric acid, contributing to acid rain.
• It can dissolve in water, which leads to the formation of acids.

While it's a crucial industrial chemical for processes like the manufacturing of sulphuric acid, it's also a significant atmospheric pollutant, affecting human health and the environment. Understanding its chemical formation and reactions helps in environmental conservation efforts and pollution control strategies.

Sulphurous Acid

Sulphurous acid (\( \text{H}_2\text{SO}_3 \) ) is formed when sulphur dioxide (\( \text{SO}_2 \) ) dissolves in water. This is a weak acid, meaning it partially dissociates in solution.
The equation representing this reaction is:
\[ \text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 \]

• This acid plays an important role in many chemical processes.
• It can donate hydrogen ions, making it capable of lowering the pH of solutions.
• Despite its instability, the acid is significant in certain industrial processes and acts as an intermediate in the conversion of sulphur dioxide to sulphuric acid.

In nature, it occasionally forms and contributes to the acidity of rainwater and other natural waters. The characteristic reactivity and donations of hydrogen ions make sulphurous acid an interesting study in both synthetic and environmental chemistry.

Basicity of Acids

The basicity of an acid refers to the number of \( \text{H}^+ \) ions that it can potentiall donate to a solution. This property determines the acid's strength and reactivity. Sulphurous acid (\( \text{H}_2\text{SO}_3 \) ) has a basicity of 2, meaning it can release two \( \text{H}^+ \) ions.

• Acids with basicity 1, like hydrochloric acid (\( \text{HCl} \) ), release one \( \text{H}^+ \) ion, making them monoprotic.
• Acids that can donate two \( \text{H}^+ \) ions, like sulphurous acid, are called diprotic.
• There are also triprotic acids, such as phosphoric acid (\( \text{H}_3\text{PO}_4 \) ), which can donate three \( \text{H}^+ \) ions.

Knowing the basicity of an acid helps chemists predict how the acid will behave in reactions, particularly those involving neutralization with bases. It aids in calculating the strength, reaction efficiency, and the capacity of the acid to participate in chemical processes essential for industrial and laboratory practices.