Chemical Reactions and Equations

Introduction

• Everyday examples of chemical changes:
  • Milk souring
  • Rusting of iron
  • Fermentation
  • Cooking and digestion
  • Respiration
• Chemical reactions involve a change in the substance's nature and identity.
• Indicators include change in state, colour, gas evolution, and temperature change.

Chemical Equations

Writing Equations

• Convert long sentence descriptions to word equations.
  • Example: Magnesium + Oxygen → Magnesium oxide
  • Reactants on LHS, products on RHS, arrow indicating reaction direction

Using Chemical Formulas

• Chemical formulas can be used for concise equations.
  • Example: Mg + O_2 → MgO
• Count atoms to ensure balance; if unequal, equation is unbalanced.

Balancing Equations

• Law of conservation of mass: Mass of reactants = Mass of products
• Step-by-step approach using hit-and-trial method:
  1. Draw boxes around formulas
  2. List number of atoms on both sides
  3. Adjust coefficients to balance atoms
  4. Indicate physical states (s, l, g, aq) and conditions like temperature.

Example

• Fe + H2O → Fe3O4 + H2
  • Balanced: 3Fe + 4H2O → Fe3O4 + 4H2

Types of Chemical Reactions

Combination Reaction

• Two or more substances form a single product.
• Example: Calcium oxide + Water → Calcium hydroxide (Exothermic)

Decomposition Reaction

• Single substance breaks into two or more substances.
• Types: Thermal, photolytic, electrolytic
• Example: 2AgCl → 2Ag + Cl2 (Photolytic)

Displacement Reaction

• One element displaces another in a compound.
• Example: Fe + CuSO4 → FeSO4 + Cu

Double Displacement Reaction

• Exchange of ions between two compounds.
• Example: Na2SO4 + BaCl2 → BaSO4 + 2NaCl (Precipitation)

Oxidation and Reduction

• Oxidation: Gain of oxygen / Loss of hydrogen
• Reduction: Loss of oxygen / Gain of hydrogen
• Example: CuO + H2 → Cu + H2O

Everyday Effects

Corrosion

• Metal reacts with environment, causing damage.
• Example: Rusting of iron

Rancidity

• Fats and oils oxidize, changing taste and smell.
• Prevention: Antioxidants, airtight storage, and nitrogen flushing.

Conclusion

• Chemical reactions are represented by balanced chemical equations.
• Reactions are classified into combination, decomposition, displacement, double displacement, and redox.
• Oxidation-reduction reactions (Redox) involve transfer of oxygen/hydrogen.