Chemical Reactions and Equations - Previous Year Questions Solved Examples (Class 10 Science)
Chemical reactions form the foundation of chemistry. These questions test your ability to identify reaction types, write balanced equations, and understand
TL;DR: Chemical reactions form the foundation of chemistry. These questions test your ability to identify reaction types, write balanced equations, and under…
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Chemical reactions form the foundation of chemistry. These questions test your ability to identify reaction types, write balanced equations, and understand
Chemical Reactions and Equations - Previous Year Questions — Solved Numerical Examples (Step by Step)
Example 1: Write a balanced chemical equation for the reaction between ferric oxide and aluminum powder. Name the type of reaction. [3 marks]
Solution: Fe2O3 + 2Al = 2Fe + Al2O3. This is a displacement reaction because aluminum displaces iron from its oxide. This is also a thermochemical reaction releasing a large amount of heat (thermite reaction), used in welding.
Example 2: A magnesium ribbon burns in air. Write the equation and identify the type of reaction. [2 marks]
Solution: 2Mg + O2 = 2MgO. This is a combination reaction where two or more reactants combine to form a single product. It is also an oxidation reaction as magnesium loses electrons.
Example 3: What happens when ferrous sulphate is heated? Write the balanced equation. [2 marks]
Solution: 2FeSO4 = Fe2O3 + SO2 + SO3 (when heated strongly). This is a decomposition reaction where ferrous sulphate breaks down into ferric oxide and gases.
Example 4: Write the equation for the reaction between copper and dilute nitric acid. Name the brown gas produced. [3 marks]
Solution: 3Cu + 8HNO3 (dilute) = 3Cu(NO3)2 + 2NO + 4H2O. The brown gas produced is nitrogen monoxide (NO), which turns brown when exposed to oxygen in air (forms NO2). This is a displacement and redox reaction.
Example 5: What is meant by an exothermic and endothermic reaction? Give one example of each. [3 marks]
Solution: Exothermic reactions release heat energy to surroundings, temperature increases, and ΔH is negative. Example: combustion of coal, burning of candle. Endothermic reactions absorb heat energy from surroundings, temperature decreases, and ΔH is positive. Example: melting of ice, evaporation of water, photosynthesis.
Example 6: Silver chloride decomposes in sunlight. Write the equation and explain why. [2 marks]
Solution: 2AgCl = 2Ag + Cl2 (in sunlight). Silver chloride is unstable and decomposes when exposed to sunlight. This is why photographic plates containing silver chloride must be stored away from light. This is a photochemical decomposition reaction.
Example 7: Distinguish between a physical change and a chemical change with examples. [3 marks]
Solution: Physical change: No new substance forms, reversible, affects only physical properties like shape, size, color. Examples: melting of ice, dissolving salt in water, tearing paper. Chemical change: New substances with different properties form, irreversible, involves breaking and making of bonds. Examples: burning of coal, rusting of iron, cooking of food.
Tips
- Always check if equation is balanced by counting atoms of each element on both sides.
- Identify the type of reaction: combination, decomposition, displacement, or double displacement.
- Use oxidation states to identify redox reactions and displacement reactions.
- For thermite and similar reactions, mention if reaction is exothermic and produces heat.
Frequently Asked Questions
What is the difference between a displacement and double displacement reaction?
Displacement: one element replaces another in a compound (A + BC = AC + B). Double displacement: ions exchange between two compounds (AB + CD = AD + CB).
Why must chemical equations be balanced?
To obey the law of conservation of mass: atoms cannot be created or destroyed in a chemical reaction.
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