Cracking and alkenes

Long-chain hydrocarbons are less useful as fuels, and crude oil contains more of them than we need. Cracking breaks them down into shorter, more useful molecules. It is a thermal decomposition reaction.

• Catalytic cracking: the hydrocarbon is heated to a vapour and passed over a hot catalyst.
• Steam cracking: the vapour is mixed with steam and heated to a very high temperature.

Cracking produces shorter alkanes and alkenes:

C₁₀H₂₂ → C₈H₁₈ + C₂H₄

decane → octane + ethene

The shorter alkanes are used as fuels such as petrol. The alkenes are used to make polymers and other chemicals.

Alkenes are hydrocarbons with a carbon–carbon double bond (C=C). They have two fewer hydrogen atoms than the alkane with the same number of carbons, so they are unsaturated.

General formula of alkenes: CₙH₂ₙ

The double bond makes alkenes more reactive than alkanes.

In each alkene, the double bond counts as two of a carbon atom's four bonds.

Ethene, C₂H₄:

H   H
|   |
C = C
|   |
H   H

Propene, C₃H₆:

H   H   H
|   |   |
C = C - C - H
|       |
H       H

Butene, C₄H₈:

H   H   H   H
|   |   |   |
C = C - C - C - H
|       |   |
H       H   H

Pentene, C₅H₁₀:

H   H   H   H   H
|   |   |   |   |
C = C - C - C - C - H
|       |   |   |
H       H   H   H

Bromine water is orange. Shake it with a hydrocarbon:

• With an alkene, it turns colourless (it is decolourised). The bromine adds across the double bond.
• With an alkane, it stays orange, because there is no double bond for the bromine to react with.

This is how you tell a saturated hydrocarbon from an unsaturated one.