56. Cracking Crude Oil & Alkenes

1. What is Cracking?

Cracking is a thermal decomposition reaction. This means breaking down large molecules into smaller ones using heat. It is used to turn long-chain hydrocarbons, which are thick and less useful, into shorter-chain hydrocarbons that are more flammable and make better fuels.

2. Methods of Cracking

The first step for both methods is to heat the long-chain hydrocarbons until they vaporize into a gas.

  • Catalytic Cracking: The hydrocarbon vapor is passed over a hot, powdered aluminum oxide catalyst. The long chains split apart when they come into contact with the catalyst.

  • Steam Cracking: The hydrocarbon vapor is mixed with steam and heated to a very high temperature, which causes the chains to split.

3. The Products of Cracking

Cracking a long-chain alkane always produces at least two products:

  1. A shorter alkane (useful as fuel).

  2. An alkene (used to make plastics and other chemicals).

4. Balancing Cracking Equations

When writing equations for cracking, the total number of carbon and hydrogen atoms must be the same on both sides of the equation.

  • Example: Decane (C10H22) can be cracked into Ethene (C2H4) and Octane (C8H18).

  • Calculation: 10 - 2 = 8 Carbons; 22 - 4 = 18 Hydrogens.

5. Alkanes vs. Alkenes

Alkenes are a separate homologous series from alkanes. The main structural difference is that alkenes have a double carbon-carbon bond (C=C).

  • Alkanes are saturated (contain only single bonds).

  • Alkenes are unsaturated (contain at least one double bond).

6. Testing for Alkenes

Alkenes are more reactive than alkanes. You can test for them using bromine water:

  • Alkanes: When added to orange bromine water, no reaction occurs, and the solution stays orange.

  • Alkenes: When added to orange bromine water, the alkene reacts with the bromine and decolorizes the solution (it turns from orange to colorless).

7. Uses of Alkenes

  • Polymers: Alkenes can be joined together to make polymers (plastics) because their double bonds can break to form new single bonds with neighboring molecules.

  • Starting Materials: They are used as raw materials for manufacturing many other industrial chemicals.