Lecture on Cracking of Hydrocarbons
Overview
- Crude oil is a mixture of hydrocarbons.
- Separation by fractional distillation based on carbon chain length.
- Short hydrocarbons: flammable; good fuels.
- Long hydrocarbons: viscous; less useful.
Cracking Process
- Converts long hydrocarbons to shorter, more useful ones.
- Cracking: thermal decomposition reaction.
- Thermal: involves heating.
- Decomposition: breaking down molecules.
Methods of Cracking
- Catalytic Cracking
- Heat long hydrocarbons to vaporize.
- Pass vapor over hot powdered aluminum oxide (catalyst).
- Long hydrocarbons split into smaller ones.
- Steam Cracking
- Vaporize hydrocarbons.
- Mix with steam and heat to high temperature.
- Causes splitting of long hydrocarbon chains.
Chemical Equations for Cracking
- Write balanced equations for cracking long-chain alkanes.
- Example: Decane (C₁₀H₂₂) to Heptane and Propene.
- Balance carbons and hydrogens on both sides.
- One product must be an alkene (double bond).
Example Equation
- Decane: C₁₀H₂₂ cracked to Ethene: C₂H₄ + unknown.
- Calculate unknown alkane:
- Carbons: 10 - 2 = 8 (C₈H₁₈)
- Hydrogens: 22 - 4 = 18
- Balanced equation: C₁₀H₂₂ → C₂H₄ + C₈H₁₈
Alkenes vs. Alkanes
- Alkenes: unsaturated; have double bonds.
- Alkanes: saturated; only single bonds.
Properties of Alkenes
- More reactive than alkanes.
- React with bromine:
- Bromine water test: turns from orange to colorless.
- Can form polymers:
- Double bonds break to form additional bonds.
Conclusion
- Alkenes are similar but more reactive than alkanes.
- Cracking is essential for converting less useful long hydrocarbons into more useful short ones.
This concludes the lecture on cracking of hydrocarbons.