Lecture Notes: Basic Concepts of Chemistry

Introduction

• Topic: Basic concepts of chemistry (Class 11 Chemistry, Chapter 1)
• Importance: Understanding the relevance of chemistry and the chemicals we use
• Focus Areas:
  • Significance of chemistry in daily life
  • Definition and understanding of significant figures, elements, and mixtures
  • Introduction to chemical combinations
  • Laws of chemical combination

Laws of Chemical Combination

• Total Laws: 5 major laws governing chemical combinations
• Purpose: Determine how elements combine to form compounds
• Detailed Study: Mass ratios, mole ratios, gas division, etc.
• Primary Law: Law of Conservation of Mass

Law of Conservation of Mass

• Concept:
  • When two compounds combine, they form a new compound.
  • The mass of reactants is equal to the mass of the products.
  • Statement: Mass can neither be created nor destroyed.
  • Formula: Sum of mass of reactants = Sum of mass of products.
• Historical Context: Discovered in the 1740s-1780s.

Example: Carbon and Oxygen

• Reaction: Carbon (C) + Oxygen (O₂) → Carbon Dioxide (CO₂)
• Balanced Reaction: 1 C + 2 O → CO₂
• Atomic Masses:
  • Carbon (C) = 12
  • Oxygen (O) = 16
• Mass Calculation:
  • Carbon: 1 × 12 = 12
  • Oxygen: 2 × 16 = 32
  • CO₂: 12 + 2 × 16 = 44
• Verification:
  • Reactants Mass: 12 + 32 = 44
  • Products Mass: 44
  • Conclusion: Mass of reactants equals mass of products (44 = 44).

Special Note

• Modern Application: E=MC²
  • Some reactions release mass as energy.
  • Original laws did not account for this (as they are old).

Practice Question

• Question: Prove that the given reaction obeys the law of conservation of mass.
  • Given:
    • 1.5 grams of ethane completely combusts to give
    • 44 grams of CO₂
    • 2.7 grams of H₂O
• Task: Write out the reaction and show the mass balance.

Conclusion

• Wrap Up: Revise the Law of Conservation of Mass
• Homework: Prove the practice question and review the concepts discussed.