Chemical Changes and Reactions
Introduction
Chemical reactions involve the transformation of one or more substances into new substances. For ICSE Class 9, understanding the different types of reactions and the energy changes involved is essential.
Types of Chemical Reactions
1. Combination Reactions
Two or more substances combine to form a single product.
A + B → AB
Examples:
- 2Mg + O₂ → 2MgO (magnesium burns in oxygen)
- CaO + H₂O → Ca(OH)₂ (quicklime with water)
- 2H₂ + O₂ → 2H₂O (formation of water)
2. Decomposition Reactions
A single compound breaks down into two or more simpler substances.
AB → A + B
Examples:
- CaCO₃ (heat) → CaO + CO₂ (thermal decomposition)
- 2H₂O (electricity) → 2H₂ + O₂ (electrolysis of water)
- 2AgCl (sunlight) → 2Ag + Cl₂ (photochemical decomposition)
3. Displacement Reactions
A more reactive element displaces a less reactive element from its compound.
A + BC → AC + B
Examples:
- Zn + CuSO₄ → ZnSO₄ + Cu (zinc displaces copper)
- Fe + CuSO₄ → FeSO₄ + Cu (iron displaces copper)
- Cl₂ + 2KI → 2KCl + I₂ (chlorine displaces iodine)
Reactivity Series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au
4. Double Displacement Reactions
Two compounds exchange their ions to form two new compounds.
AB + CD → AD + CB
Examples:
- AgNO₃ + NaCl → AgCl + NaNO₃ (white precipitate of AgCl)
- Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl (white precipitate of BaSO₄)
5. Redox (Oxidation-Reduction) Reactions
Oxidation: Gain of oxygen or loss of hydrogen (or loss of electrons) Reduction: Loss of oxygen or gain of hydrogen (or gain of electrons)
Example: CuO + H₂ → Cu + H₂O
- CuO is reduced to Cu (loss of oxygen)
- H₂ is oxidised to H₂O (gain of oxygen)
Oxidising Agent: Substance that provides oxygen (CuO in the example) Reducing Agent: Substance that removes oxygen (H₂ in the example)
Energy Changes in Reactions
Exothermic Reactions
Reactions that release heat energy.
- Heat is written on the product side
- Examples: Combustion, respiration, neutralisation
- C + O₂ → CO₂ + Heat
Endothermic Reactions
Reactions that absorb heat energy.
- Heat is written on the reactant side
- Examples: Photosynthesis, electrolysis, thermal decomposition
- CaCO₃ + Heat → CaO + CO₂
| Feature | Exothermic | Endothermic |
|---|---|---|
| Energy change | Releases heat | Absorbs heat |
| Temperature | Surroundings get hotter | Surroundings get colder |
| Examples | Burning, neutralisation | Photosynthesis, electrolysis |
Applications in Daily Life
- Rancidity: Oxidation of fats in food (prevented by antioxidants and airtight packing)
- Corrosion: Oxidation of metals (rusting of iron)
- Bleaching: Oxidation of coloured substances
- Ripening of fruits: Chemical changes involving enzymes
Common Mistakes With Fixes
| Mistake | Correction |
|---|---|
| Confusing displacement and double displacement | Displacement: one element displaces another. Double displacement: exchange of ions |
| Thinking all reactions are either exothermic or endothermic | Most reactions have energy changes; some are spontaneous |
| Oxidation always involves oxygen | Oxidation also includes loss of electrons or gain of hydrogen |
| Catalysts change the products | Catalysts only change the RATE, not the products |
ICSE Exam Focus
| Topic | Marks (approx.) | Frequency |
|---|---|---|
| Identifying types of reactions | 4-5 marks | Very common |
| Redox reactions (oxidising/reducing agents) | 4 marks | Common |
| Exothermic vs endothermic | 3 marks | Common |
| Reactivity series applications | 3-4 marks | Frequently asked |
Self-Test
Q1: Classify as combination, decomposition, displacement, or double displacement: (i) CaCO₃ → CaO + CO₂ (ii) Zn + CuSO₄ → ZnSO₄ + Cu
Q2: In the reaction CuO + H₂ → Cu + H₂O, identify the oxidising and reducing agents.
Q3: Distinguish between exothermic and endothermic reactions with examples.
Q4: Why does iron displace copper from copper sulphate solution?
Q5: What is rancidity? How can it be prevented?
