Introduction:
Chemical reactions are part of our everyday life — from the digestion of food in our stomachs to the rusting of iron, the burning of fuels, to photosynthesis in plants. In this chapter, we’ll learn what chemical reactions and equations are, how to represent them using chemical equations, and explore different types of chemical reactions.
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What is a Chemical Reaction?
A chemical reaction is a process in which one or more substances (reactants) are transformed into one or more new substances (products) with new chemical properties.
Tip: Jab do ya zyada substances milkar naye substance banate hain jiska nature alag hota hai, use hi hum chemical reaction kehte hain.
Daily Life Examples of Chemical Reactions
- Cooking food
- Burning of wood or LPG
- Rusting of iron
- Digestion of food
- Photosynthesis in plants
Chemical Equations
Chemical reactions are written in symbolic form called chemical equations.
Example:
When magnesium burns in air:
Magnesium (Mg)+Oxygen (O₂)→Magnesium oxide (MgO)
Word Equation:
Magnesium + Oxygen → Magnesium oxide
Chemical Equation:
Mg+O2→MgO
But this equation is not balanced. Let’s balance it.
Balanced Chemical Equation:
2Mg+O2→2MgO
Note: According to the Law of Conservation of Mass, the number of atoms of each element must be the same on both sides of the equation.Balancing Chemical Equations
Balancing ensures mass is conserved during a chemical reaction. Here’s how:
Steps to Balance a Chemical Equation:
- Write the unbalanced equation.
- Count atoms of each element on both sides.
- Add coefficients to balance atoms.
- Ensure coefficients are in the simplest ratio.
Balancing ensures the Law of Conservation of Mass is followed — matter is neither created nor destroyed in a chemical reaction.
Unbalance Equation: Fe + H₂O → Fe₃O₄ + H₂
Balance Equation: 3Fe + 4H₂O → Fe₃O₄ + 4H₂
Types of Chemical Reactions
1. Combination Reaction
Two or more substances combine to form a single product.
Example:
CaO+H2O→Ca(OH)2
Jab do ya zyada reactants milkar ek hi product banate hain.2. Decomposition Reaction
One compound breaks into two or more simpler substances.
Example:
2Pb(NO3)2 → 2PbO+4NO2+O2
Heat, light, or electricity can cause decomposition.Types:
- Thermal decomposition (heat required)
- Photolytic decomposition (light required)
- Electrolytic decomposition (electricity required)
3. Displacement Reaction
A more reactive element displaces a less reactive element from a compound.
Example:
Zn + CuSO₄ → ZnSO₄ + Cu
4. Double Displacement Reaction
Exchange of ions between two compounds forming new compounds.
Example:
Na₂SO₄ + BaCl₂ → BaSO₄ ↓ + 2NaCl
Usually forms a precipitate (insoluble solid).5. Redox Reaction
Reaction involving both oxidation and reduction.
- Oxidation: Addition of oxygen/removal of hydrogen
- Reduction: Removal of oxygen/addition of hydrogen
Example:
MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂
Here, HCl is oxidized to Cl2 and MnO2 is reduced to MnCl2.
Exothermic and Endothermic Reactions
- Exothermic Reaction: Heat is released. Example: Combustion of fuels.
- Endothermic Reaction: Heat is absorbed. Example: Photosynthesis.
Effects of Chemical Reactions
- Change in state
- Change in color
- Evolution of gas
- Change in temperature
- Formation of precipitate
Corrosion
Corrosion is the gradual destruction of metals due to chemical reactions with environmental elements like air and moisture.
Example:
4Fe + 3O₂ + 6H₂O → Fe₂O₃·xH₂O (Rust)
Prevention:
- Painting
- Galvanization
- Oiling/Greasing
Rancidity
Rancidity is the oxidation of oils and fats in food that leads to bad taste and smell.
Prevention:
- Adding antioxidants (e.g., BHA, BHT)
- Storing food in airtight containers
- Refrigeration
- Vacuum packing
Interesting Facts
💡 The smallest unit of a chemical reaction is a molecule, not an atom.
💡 A balanced chemical equation follows the Law of Conservation of Mass – total mass of reactants = total mass of products.
💡 All chemical changes are irreversible, while physical changes may or may not be.
💡 Redox reactions are a combination of oxidation and reduction happening simultaneously.
💡 Not all reactions are visible to the naked eye—some happen at the molecular level.
💡 Heat, light, electricity, or catalysts can initiate a chemical reaction.
💡 Rusting is a slow oxidation reaction of iron with moisture and oxygen.
💡 Decomposition reactions are generally endothermic (require heat, light, or electricity).
💡 AgCl and AgBr decompose in sunlight, which is why they’re stored in dark bottles.
💡 Evolution of gas is a strong sign of a chemical reaction (e.g., reaction of acid with metal).
Activity (Exercise):
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Quiz:
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Supplementary Materials:
Provide downloadable materials for learners to review:
- – PDF Guide: “Coming Soon”
- – Cheat Sheet: “Coming Soon”
- – Video Source: “JNG ACADEMY“
- – Articles: “Blog Page“
FAQs:
1. What is a chemical reaction?
2. What are the main characteristics of a chemical reaction?
Change in state
Evolution of gas
Change in temperature
Formation of precipitate
3. What is the difference between a balanced and an unbalanced chemical equation?
a) A balanced equation has equal numbers of atoms for each element on both sides.
b) An unbalanced equation does not follow the Law of Conservation of Mass.
4. Why should chemical equations be balanced?
5. What are the types of chemical reactions?
Decomposition Reaction
Displacement Reaction
Double Displacement Reaction
Redox Reaction
6. What is an example of a redox reaction?
Here, CuO is reduced and H₂ is oxidized.
7. What is corrosion?
8. What is rancidity, and how can it be prevented?
9. What is the difference between exothermic and endothermic reactions?
Endothermic: Absorbs heat (e.g., photosynthesis)
10. What are decomposition reactions and their types?
Photolytic
Electrolytic