Lecture on Alkanes

Introduction

• Alkanes are a homologous series of hydrocarbons.
• They contain only carbon and hydrogen atoms.
• No double bonds are present.

Names of Alkanes

• The first four alkanes were covered in the previous video.

Properties of Alkanes

1. Boiling Point:
   • Increases with chain length.
   • Short-chain alkanes (≤ 4 carbons) have low boiling points and are gases at room temperature.
   • Longer alkanes (> 4 carbons) can be liquids or solids.
2. Volatility:
   • Shorter alkanes are more volatile (evaporate easily).
3. Viscosity:
   • Longer alkanes are more viscous (thicker and sticky, like honey).
4. Flammability:
   • Shorter alkanes are more flammable.

Combustion Reactions

• Alkanes are used as fuels because they release a lot of energy when burned with oxygen.
• Complete Combustion:
  • Occurs with sufficient oxygen.
  • Produces carbon dioxide and water, releasing energy (exothermic reaction).
  • Carbon and hydrogen in hydrocarbons are oxidized.

Writing and Balancing Combustion Equations

• Example: Propane (C₃H₈):
  • Reacts with oxygen (O₂) to form CO₂ and H₂O.
  • Balanced equation:
    • 3 CO₂ from 3 carbons.
    • 4 H₂O from 8 hydrogens.
    • Requires 5 O₂ molecules for 10 oxygens.
• Example: Nonane (C₉H₂₀):
  • Reacts with oxygen.
  • Balanced equation:
    • 9 CO₂ from 9 carbons.
    • 10 H₂O from 20 hydrogens.
    • Requires 14 O₂ molecules for 28 oxygens.

Key Takeaways

• Properties of hydrocarbons depend on carbon chain length.
• Shortest hydrocarbons have the lowest boiling points, most volatile and flammable – great for fuels.
• Combustion requires oxygen to produce CO₂ and H₂O, releasing energy.

Conclusion

• Understanding these properties is crucial for using hydrocarbons effectively as fuels.