CHEMICAL REACTIONS

A chemical reaction is a process in which one or more substances (reactants) are converted into new substances (products) with different physical and chemical properties.

Chemical reactions involve:
• Breaking of old chemical bonds
• Formation of new chemical bonds
• Rearrangement of atoms

Chemical reactions are represented using chemical equations.

Example:
Magnesium + Oxygen → Magnesium Oxide
2Mg + O₂ → 2MgO

2. CHEMICAL EQUATION
A chemical equation is a symbolic representation of a chemical reaction using symbols and formulas.

Components:
• Reactants – substances that take part in reaction
• Products – substances formed after reaction

Example:
Zn + H₂SO₄ → ZnSO₄ + H₂↑

3. BALANCED CHEMICAL EQUATION
According to the Law of Conservation of Mass, mass can neither be created nor destroyed.

Therefore:
Number of atoms of each element must be equal on both sides.

Example:
Unbalanced: H₂ + O₂ → H₂O
Balanced: 2H₂ + O₂ → 2H₂O

4. TYPES OF CHEMICAL REACTIONS

A. Combination Reaction
Two or more substances combine to form a single product.

Example:
CaO + H₂O → Ca(OH)₂
Applications:
• Slaking of lime
• Formation of water

B. Decomposition Reaction
A single compound breaks down into two or more simpler substances.

Types:
• Thermal decomposition
• Electrolytic decomposition
• Photochemical decomposition

Example:
CaCO₃ → CaO + CO₂

C. Displacement Reaction
A more reactive element displaces a less reactive element from its compound.

Example:
Zn + CuSO₄ → ZnSO₄ + Cu

D. Double Displacement Reaction
Exchange of ions between two compounds.

Example:
Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl

Includes:
• Precipitation reactions
• Neutralisation reactions

E. Redox Reaction
Reactions involving both oxidation and reduction simultaneously.

Oxidation – loss of electrons / gain of oxygen
Reduction – gain of electrons / loss of oxygen

Example:
CuO + H₂ → Cu + H₂O

5. EFFECTS OF OXIDATION REACTIONS

A. Corrosion
Slow destruction of metals due to reaction with air, moisture or chemicals.

Example:
Rusting of iron
4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O

Prevention:
• Painting
• Galvanisation
• Greasing
• Alloying

B. Rancidity
Oxidation of fats and oils in food leading to bad smell and taste.

Prevention:
• Refrigeration
• Adding antioxidants
• Vacuum packing

6. ENERGY CHANGES IN CHEMICAL REACTIONS

Exothermic Reaction:
Releases heat energy.

Example:
CH₄ + 2O₂ → CO₂ + 2H₂O + Heat

Endothermic Reaction:
Absorbs heat energy.

Example:
CaCO₃ → CaO + CO₂ (Heat absorbed)

7. RATE OF CHEMICAL REACTION

Factors affecting rate:
• Nature of reactants
• Concentration
• Temperature
• Surface area
• Presence of catalyst

Catalyst:
A substance that increases rate without being consumed.

Example:
MnO₂ in decomposition of H₂O₂

8. CORROSION AND ITS ECONOMIC IMPACT
• Loss of metals
• Damage to bridges, pipelines, ships
• Economic loss to industries

Use of corrosion-resistant alloys and coatings is essential.

9. CHEMICAL REACTIONS IN DAILY LIFE
• Respiration – oxidation of glucose
• Photosynthesis – reduction of CO₂
• Digestion of food
• Burning of fuels
• Battery operation
 

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Subject: Science

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