Classify Reactions: Decomposition MCQs Quiz | Class 10

Understanding the Thermal Decomposition of Ferrous Sulphate

Decomposition reactions are fundamental chemical processes where a single compound breaks down into two or more simpler substances. Thermal decomposition specifically refers to decomposition caused by heating the compound. One classic example studied in Class 10 Science is the thermal decomposition of ferrous sulphate crystals.

The Action of Heat on Ferrous Sulphate Crystals (FeSO4.7H2O)

Ferrous sulphate crystals (FeSO4.7H2O) are typically light green in color. When these crystals are heated, the reaction proceeds in two main stages:

1. Loss of Water of Crystallization: Initially, when ferrous sulphate heptahydrate (FeSO4.7H2O) is heated gently, it loses its seven molecules of water of crystallization. This causes the light green crystals to turn into a white, anhydrous ferrous sulphate (FeSO4).

FeSO4.7H2O(s) → FeSO4(s) + 7H2O(g)

2. Thermal Decomposition: Upon strong heating, the anhydrous ferrous sulphate further decomposes. This is a true decomposition reaction where a single reactant breaks down into multiple products. The products include a solid residue, ferric oxide, and two pungent gases, sulfur dioxide and sulfur trioxide.

2FeSO4(s) → Fe2O3(s) + SO2(g) + SO3(g)

Observations During the Experiment:

• Color Change: The light green ferrous sulphate crystals first turn white (due to loss of water), and then upon strong heating, a reddish-brown solid residue (ferric oxide) is formed.
• Gas Evolution: Pungent smelling gases are evolved. These gases are sulfur dioxide (SO2) and sulfur trioxide (SO3), which have the characteristic smell of burning sulfur.

Products and Their Properties:

• Ferric Oxide (Fe2O3): This is a reddish-brown solid. It is also known as iron(III) oxide.
• Sulfur Dioxide (SO2): A colorless gas with a pungent, suffocating smell. It is an acidic oxide, meaning it reacts with water to form sulfurous acid.
• Sulfur Trioxide (SO3): Also a colorless gas with a pungent smell. It is a highly acidic oxide, reacting with water to form sulfuric acid.

Key Concepts to Remember:

• Thermal Decomposition: A reaction requiring heat energy to break down reactants. This makes it an endothermic process.
• Redox Reaction: Although primarily a decomposition, the oxidation states of sulfur and iron change during the process, indicating it involves both oxidation and reduction.
• Acidic Oxides: The gaseous products, SO2 and SO3, are acidic oxides, which can be confirmed by testing with moist blue litmus paper (it turns red).

Summary Table of Ferrous Sulphate Decomposition

Quick Revision Points:

• Ferrous sulphate crystals are green due to water of crystallization.
• Heating first removes water, turning it white (anhydrous FeSO4).
• Strong heating causes decomposition to reddish-brown ferric oxide and sulfur gases (SO2, SO3).
• The evolved gases have a characteristic burning sulfur smell.
• This is an endothermic thermal decomposition reaction.

Extra Practice Questions:

1. Why is anhydrous ferrous sulphate white, while hydrated ferrous sulphate is green?
2. What test could be performed to confirm the presence of sulphur dioxide gas?
3. Name another metal sulphate that undergoes thermal decomposition to produce a metal oxide and sulphur oxides.
4. How does the smell of the gases produced differ from that of hydrogen sulphide (H2S)?
5. Explain why this reaction is considered an example of a redox reaction.