CBSE Class 10 Science — Chemical Reactions and Equations: Complete Notes and MCQs
Chemical Reactions and Equations is the first and most fundamental chapter of CBSE Class 10 Science. Carrying 15-20% weightage in board exams, this chapter lays the foundation for all chemistry topics in Classes 11 and 12. This comprehensive guide covers balancing equations, types of chemical reactions, oxidation-reduction, and 10 MCQs for board exam preparation.
What is a Chemical Reaction?
A chemical reaction involves breaking of bonds in reactants and formation of new bonds in products. Signs that a chemical reaction has occurred: (1) Change in colour, (2) Change in temperature (exothermic/endothermic), (3) Evolution of gas, (4) Formation of precipitate, (5) Change in state. Chemical equations use symbols and formulae. A balanced equation has equal number of atoms of each element on both sides (Law of Conservation of Mass).
Balancing Chemical Equations
Example: Fe + H2O to Fe3O4 + H2 (unbalanced). Balanced: 3Fe + 4H2O → Fe3O4 + 4H2. Steps: (1) Write skeleton equation. (2) Count atoms on each side. (3) Balance by adding coefficients (never change subscripts). (4) Verify balance. Burning of magnesium: 2Mg + O2 → 2MgO.
Types of Chemical Reactions
| Type | Definition | Example |
|---|---|---|
| Combination | Two reactants form one product | 2H2 + O2 → 2H2O |
| Decomposition | One reactant breaks into products | 2PbO → 2Pb + O2 (thermal) |
| Displacement | More reactive element replaces less reactive | Zn + CuSO4 → ZnSO4 + Cu |
| Double Displacement | Exchange of ions between two compounds | Na2SO4 + BaCl2 → BaSO4 + 2NaCl |
| Redox (Oxidation-Reduction) | Simultaneous oxidation and reduction | CuO + H2 → Cu + H2O |
Oxidation, Reduction and Redox Reactions
- Oxidation: gain of oxygen OR loss of hydrogen OR loss of electrons (OIL)
- Reduction: loss of oxygen OR gain of hydrogen OR gain of electrons (RIG)
- Oxidising agent: substance that causes oxidation (gets reduced itself)
- Reducing agent: substance that causes reduction (gets oxidised itself)
- In CuO + H2 → Cu + H2O: H2 is oxidised (reducing agent), CuO is reduced (oxidising agent)
- Corrosion: iron rusts (Fe + O2 + H2O → Fe2O3), slow oxidation
- Rancidity: oxidation of oils and fats causing bad smell; prevented by antioxidants, vacuum packing, refrigeration
Exothermic and Endothermic Reactions
Exothermic reactions release heat energy: combustion (CH4 + 2O2 → CO2 + 2H2O + heat), respiration, neutralisation. Endothermic reactions absorb heat energy: decomposition of calcium carbonate (CaCO3 → CaO + CO2), photosynthesis, dissolution of ammonium chloride in water. Test: if temperature of mixture rises, exothermic; if temperature falls, endothermic.
Frequently Asked Questions — CBSE Class 10 Chemical Reactions
What is the Law of Conservation of Mass?
Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products. This is why chemical equations must be balanced — same number of atoms of each element on both sides.
What is the difference between precipitation reaction and double displacement?
Double displacement is a broader term where two compounds exchange their ions. Precipitation reaction is a special type of double displacement where one of the products formed is insoluble in water and settles as a precipitate (solid). Example: Na2SO4 + BaCl2 → BaSO4 (white precipitate) + 2NaCl.
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