52.Alkanes: Properties & Combustion

Alkanes: Properties & Combustion

Based on the video, here are the notes on the physical properties of alkanes and their behavior during combustion.

1. Trends in Physical Properties

The properties of alkanes change predictably as the length of the carbon chain increases:

  • Boiling Point: Increases as the chain length increases.

    • Short chains (1-4 carbons): Have low boiling points and exist as gases at room temperature.

    • Longer chains: Have higher boiling points and exist as liquids or solids.

  • Volatility: Shorter alkanes are more volatile, meaning they evaporate much more easily due to their lower boiling points.

  • Viscosity: Longer alkanes are more viscous, meaning they are thick and sticky (like honey). Shorter alkanes are much more fluid.

  • Flammability: Shorter alkanes are more flammable, making them easier to ignite and burn, which is why they are frequently used as fuels.

2. Complete Combustion

Complete combustion occurs when a hydrocarbon is burned in a plentiful supply of oxygen.

  • The Reaction: Hydrocarbon + Oxygen → Carbon Dioxide + Water

  • Energy: The reaction is exothermic, meaning it releases a large amount of energy.

  • Oxidation: During the process, both the carbon and hydrogen in the fuel are oxidized as they combine with oxygen to form CO₂ and H₂O.

3. Balancing Combustion Equations

To write a balanced equation for complete combustion, you must ensure the number of atoms for each element is the same on both sides.

Example: The combustion of Propane (C₃H₈)

  1. Identify the reactant: Propane is C₃H₈.

  2. Balance Carbons: 3 carbons in propane requires 3 CO₂ molecules on the product side.

  3. Balance Hydrogens: 8 hydrogens in propane requires 4 H₂O molecules on the product side (since each water has 2 hydrogens).

  4. Balance Oxygens: Count the oxygens on the right (3 × 2 = 6 from CO₂ plus 4 from H₂O = 10 total). To get 10 on the left, you need 5 O₂ molecules.

  5. Final Equation: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

4. Summary Checklist

  • Short chains: Low boiling point, high volatility, high flammability, low viscosity.

  • Long chains: High boiling point, low volatility, low flammability, high viscosity.

  • Complete combustion products: Always Carbon Dioxide (CO₂) and Water (H₂O).