55. Cracking & Alkenes

1. What is Cracking?

Cracking is a thermal decomposition reaction used to break down long-chain, less useful hydrocarbons (from fractional distillation) into shorter, more useful, and more flammable hydrocarbons.

  • Thermal: Involves heating.

  • Decomposition: Breaking down molecules.

2. Methods of Cracking

Regardless of the method, the first step is to heat the long-chain hydrocarbons to vaporize them into a gas.

  • Catalytic Cracking: The hydrocarbon vapor is passed over a hot, powdered aluminum oxide catalyst. Contact with the catalyst splits the long chains apart.

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

3. Cracking Equations

In a cracking reaction, a long-chain alkane is broken down into a shorter alkane and an alkene.

  • Key Rule: The total number of carbon and hydrogen atoms must be the same on both sides of the equation.

  • Example: Decane (C₁₀H₂₂) → Octane (C₈H₁₈) + Ethene (C₂H₄).

4. Alkenes vs. Alkanes

Cracking produces alkenes because there aren't enough hydrogen atoms left to keep all the new molecules "saturated."

Feature

Alkanes

Alkenes

Bonding

Single bonds only

At least one double bond (C=C)

Saturation

Saturated

Unsaturated

Reactivity

Less reactive

More reactive

General Use

Fuels

Making polymers and other chemicals

5. The Chemical Test for Alkenes

Because alkenes are more reactive, they can be identified using bromine water:

  • Procedure: Add orange bromine water to the sample.

  • Result for Alkanes: Bromine water stays orange.

  • Result for Alkenes: The bromine water is decolorized (turns from orange to colorless).

6. Uses of Alkenes

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

  • Feedstock: They serve as starting materials for many other industrial chemicals.