🧪 Chapter 1: Chemical Reactions and Equations
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1. Introduction to Chemical Reactions
Changes around us can be physical (reversible, no new substance) or chemical.
🔬 Scientific Definition
A chemical reaction is a process where the original substances (reactants) lose their nature and identity to form new chemical substances (products) with different properties.
How do we know a reaction has taken place?
- Change in state
- Change in color
- Evolution of a gas
- Change in temperature
🔥 The Magnesium Ribbon Experiment
When a magnesium ribbon is burnt in air, it burns with a dazzling white flame and changes into a white powder.
2Mg + O₂ → 2MgO + Heat
Board Tip: Magnesium ribbon must be cleaned with sandpaper before burning to remove the protective layer of basic Magnesium Carbonate (or Magnesium Oxide) so it can react easily.
2. Chemical Equations & Balancing
A chemical equation is the symbolic representation of a chemical reaction using symbols and formulae.
⚖️ The Law of Conservation of Mass
Mass can neither be created nor destroyed in a chemical reaction. The total mass of the elements present in the products must be equal to the total mass of the elements present in the reactants.
⚡ How to Balance an Equation (Shortcut Trick)
Step-by-step trick: Balance Metals first → then Non-Metals → then Oxygen → then Hydrogen.
3. Types of Chemical Reactions
A. Combination Reaction
Two or more reactants combine to form a single product. Usually Exothermic (releases heat).
B. Decomposition Reaction
A single reactant breaks down into simpler products. Usually Endothermic (requires energy).
| Type | Energy Source | Important Reaction (VVIP) |
|---|---|---|
| Thermal | Heat (Δ) | CaCO₃ → CaO + CO₂ |
| Electrolytic | Electricity | 2H₂O → 2H₂ + O₂ |
| Photolytic | Sunlight | 2AgCl → 2Ag + Cl₂ (Used in B&W Photography) |
C. Displacement Reaction
More reactive element displaces a less reactive element from its compound.
Fe(s) + CuSO₄(aq) [Blue] → FeSO₄(aq) [Green] + Cu(s)
⚠️ Common Mistake
Cu cannot displace Fe. Cu + FeSO₄ → No Reaction! (Because Cu is less reactive than Fe in the reactivity series).
D. Double Displacement Reaction
Exchange of ions between two reactants. Often forms an insoluble solid called a precipitate.
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) [White ppt] + 2NaCl(aq)
4. Redox Reactions & Everyday Life
📈 Oxidation
- Addition of Oxygen (O)
- Removal of Hydrogen (H)
📉 Reduction
- Addition of Hydrogen (H)
- Removal of Oxygen (O)
CuO + H₂ → Cu + H₂O
Here, CuO is reduced to Cu, and H₂ is oxidized to H₂O.
Effects of Oxidation in Everyday Life
A. Corrosion
Gradual destruction of metals by moisture, acids, etc.
Example: Rusting of Iron (Hydrated Iron III Oxide - Fe₂O₃·xH₂O). It forms a brown flaky substance.
Prevention: Painting, oiling, greasing, and galvanization (coating iron with zinc).
B. Rancidity
The condition produced by aerial oxidation of fats and oils in foods marked by an unpleasant smell and taste.
Real Life Example: When butter or chips are left open for a long time, they smell and taste foul.
Prevention:
- Adding antioxidants (BHA, BHT).
- Flushing packaging with Nitrogen gas (this is why potato chips packets are filled with air; it's actually N₂ gas to prevent oxidation!).
🎁 BONUS: ExamSpark Revision Hub
⭐ Top 5 Most Repeated Board Questions
- Why should a magnesium ribbon be cleaned before burning in air?
- Translate the following into a balanced chemical equation: Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
- Why is respiration considered an exothermic reaction?
- What happens when silver chloride is exposed to sunlight? Write its application.
- Why are food items flushed with nitrogen?
🧠 Mind Map Summary
- Chemical Equation: Balanced vs Skeletal.
- Combination: A + B → AB (Exothermic).
- Decomposition: AB → A + B (Endothermic - Thermal, Light, Electric).
- Displacement: Fast metal kicks out slow metal.
- Double Displacement: Ion exchange (Precipitate forms).
- Redox: Oxidation (Gain O) & Reduction (Lose O).