Overview
This lecture explains how long-chain hydrocarbons from crude oil can be converted into shorter, more useful molecules using the process of cracking, and discusses the properties and uses of alkanes and alkenes.
Fractional Distillation Review
- Crude oil is a mixture of hydrocarbons separated by chain length using fractional distillation.
- Short-chain hydrocarbons are more flammable and better fuels.
- Long-chain hydrocarbons are thick and less useful.
Cracking: Purpose and Methods
- Cracking breaks down long-chain hydrocarbons into shorter, more useful ones.
- It is a thermal decomposition reaction, meaning molecules are broken down by heating.
- Two main types: catalytic cracking (uses hot powdered aluminum oxide as a catalyst) and steam cracking (uses steam and high temperature).
Cracking Process Details
- Hydrocarbons are first vaporized (turned into gas).
- In catalytic cracking, vaporized hydrocarbons pass over a hot catalyst and split.
- In steam cracking, vaporized hydrocarbons are mixed with steam and heated, causing them to split.
Chemical Equations in Cracking
- Cracking produces a shorter alkane and at least one alkene.
- The equation must be balanced: the number of carbons and hydrogens on each side must be equal.
- Example: Decane (C10H22) can be cracked into octane (C8H18) and ethene (C2H4).
Alkanes vs Alkenes
- Alkanes: saturated hydrocarbons with single bonds.
- Alkenes: unsaturated hydrocarbons with at least one double bond.
- Alkenes are more reactive and can be tested using bromine water (alkenes turn bromine water from orange to colorless).
- Alkenes are used to make polymers due to their double bonds.
Key Terms & Definitions
- Crude Oil — a mixture of hydrocarbons found in the earth.
- Fractional Distillation — process separating hydrocarbons by boiling point.
- Cracking — breaking long-chain hydrocarbons into shorter hydrocarbons by heating.
- Thermal Decomposition — chemical breakdown by heat.
- Catalytic Cracking — cracking using a catalyst (e.g., aluminum oxide).
- Steam Cracking — cracking using steam and high temperatures.
- Alkanes — saturated hydrocarbons with only single bonds.
- Alkenes — unsaturated hydrocarbons with at least one double bond.
- Homologous Series — group of compounds with similar properties and structures.
Action Items / Next Steps
- Practice writing balanced chemical equations for cracking reactions.
- Memorize the test for alkenes using bromine water.