Types of Chemical Reactions — Class 10 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 10 Science, Chemistry — Chapter 10. Writing equations and classifying how substances react.
1. About this chapter
This chapter covers chemical equations and their balancing, the main types of reactions (combination, decomposition, displacement, double displacement), oxidation–reduction, and exothermic/endothermic changes.
2. Chemical equations and balancing
- A chemical equation represents a reaction with formulae of reactants → products.
- By the law of conservation of mass, an equation must be balanced — equal atoms of each element on both sides. e.g. 2H₂ + O₂ → 2H₂O.
3. The four main types
| Type | General form | Example |
|---|---|---|
| Combination | A + B → AB | C + O₂ → CO₂ |
| Decomposition | AB → A + B | CaCO₃ → CaO + CO₂ (thermal) |
| Displacement | A + BC → AC + B | Zn + CuSO₄ → ZnSO₄ + Cu |
| Double displacement | AB + CD → AD + CB | AgNO₃ + NaCl → AgCl↓ + NaNO₃ |
- Decomposition may be thermal (heat), electrolytic (electricity), or photochemical (light).
- Double displacement includes precipitation (insoluble solid forms) and neutralisation (acid + base → salt + water).
4. Redox and energy changes
- Oxidation: gain of oxygen / loss of hydrogen / loss of electrons.
- Reduction: loss of oxygen / gain of hydrogen / gain of electrons.
- Oxidation and reduction occur together — a redox reaction. e.g. CuO + H₂ → Cu + H₂O (CuO reduced, H₂ oxidised).
- Exothermic reactions release heat (e.g., burning); endothermic reactions absorb heat (e.g., decomposition of CaCO₃).
5. Worked examples
Example 1. Balance: H₂ + O₂ → H₂O. 2H₂ + O₂ → 2H₂O (4 H and 2 O on each side).
Example 2. Classify: Zn + 2HCl → ZnCl₂ + H₂. Zinc displaces hydrogen → displacement reaction.
Example 3. In CuO + H₂ → Cu + H₂O, what is oxidised and reduced? CuO loses oxygen → reduced; H₂ gains oxygen → oxidised.
6. Common mistakes
- Mistake: Changing formulae to balance an equation. Fix: Only adjust coefficients, never the subscripts in a formula.
- Mistake: Calling a double displacement a displacement. Fix: Double displacement exchanges ions between two compounds (AB + CD → AD + CB).
- Mistake: Treating oxidation as only "addition of oxygen." Fix: More generally, oxidation is loss of electrons (reduction is gain).
7. Practice (book-back style)
- State the law of conservation of mass.
- Balance: Fe + O₂ → Fe₂O₃.
- Give one example each of combination and decomposition reactions.
- Define oxidation and reduction in terms of electrons.
- Differentiate exothermic and endothermic reactions with an example.
8. Answer key
- Mass is neither created nor destroyed in a chemical reaction.
- 4Fe + 3O₂ → 2Fe₂O₃.
- Combination: C + O₂ → CO₂; Decomposition: CaCO₃ → CaO + CO₂.
- Oxidation = loss of electrons; reduction = gain of electrons.
- Exothermic releases heat (e.g., combustion); endothermic absorbs heat (e.g., thermal decomposition of limestone).
9. Quick revision
- Chemistry Ch 10 · equations, balancing, reaction types, redox.
- Balance by changing coefficients only (conservation of mass).
- Types: combination, decomposition, displacement, double displacement.
- Redox: oxidation = loss of electrons; reduction = gain (occur together).
- Exothermic releases heat; endothermic absorbs heat.
