🔬Science (Class 10)
Chemical Reactions & Equations
Types of reactions, balancing equations, decomposition, displacement, and redox.
Class 10
What You Will Learn in Chemical Reactions & Equations
A chemical reaction involves the rearrangement of atoms in reactants to form new substances (products) with different properties.
- Types: combination (A+B→AB), decomposition (AB→A+B), displacement, double displacement, oxidation-reduction (redox).
- Balancing equations: law of conservation of mass — atoms of each element must be equal on both sides.
- Oxidation: loss of electrons / gain of oxygen / loss of hydrogen. Reduction: gain of electrons / loss of oxygen / gain of hydrogen.
- Exothermic reactions: release heat (combustion, respiration). Endothermic: absorb heat (photosynthesis, electrolysis).
- Corrosion: slow oxidation of metals (rusting of iron: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ → Fe₂O₃·xH₂O).
- Rancidity: oxidation of fats/oils (prevented by antioxidants, vacuum packaging, nitrogen flushing).
Key Formulas
Balanced equation example: 2H₂ + O₂ → 2H₂OFe₂O₃ + 2Al → Al₂O₃ + 2Fe (thermite)
Example
Burning magnesium (2Mg + O₂ → 2MgO) is a combination reaction releasing bright white light.
Expected Exam Questions — Chemical Reactions & Equations
Q1.What is a redox reaction? Give an example.
Answer: A redox reaction involves simultaneous oxidation (electron loss) and reduction (electron gain). Example: CuO + H₂ → Cu + H₂O. CuO is reduced (O removed, gains H) to Cu; H₂ is oxidised (gains O) to H₂O. OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
Q2.Differentiate between exothermic and endothermic reactions with examples.
Answer: Exothermic: heat is released (ΔH < 0). Examples: combustion (CH₄+O₂→CO₂+H₂O+heat), neutralisation (HCl+NaOH→NaCl+H₂O). Endothermic: heat is absorbed (ΔH > 0). Examples: photosynthesis (6CO₂+6H₂O+light→C₆H₁₂O₆+6O₂), decomposition of CaCO₃ on heating.
Q3.Why does iron corrode faster in salt water?
Answer: Salt water (NaCl solution) is a better electrolyte than pure water, increasing the rate of electrochemical reactions that cause rusting. Chloride ions (Cl⁻) remove the protective oxide layer and accelerate oxidation.
🔘 MCQ Practice — Chemical Reactions & Equations
MCQ 1.Which of the following is a decomposition reaction?
A. 2H₂+O₂→2H₂O
B. 2KClO₃→2KCl+3O₂ ✓
C. Fe+S→FeS
D. NaOH+HCl→NaCl+H₂O
✓ Correct Answer: 2KClO₃→2KCl+3O₂
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