An alkene is a hydrocarbon that contains a carbon-carbon double bond.
Hexane can be cracked to form butane and ethene:
hexane → butane + ethene
C6H14 → C4H10 + C2H4
The longer alkanes are heated and their vapours are passed over a hot catalyst. This causes covalent bonds to break and reform. The slideshow describes this process.
Reasons for cracking
Cracking is important for two main reasons:
It helps to match the supply of fractions with the demand for them.
The supply is how much of a fraction an oil refinery produces. The demand is how much of a fraction customers want to buy. Fractional distillation of crude oil usually produces more of the larger hydrocarbons than can be sold, and less of the smaller hydrocarbons than customers want. Smaller hydrocarbons, such as petrol, are more useful as fuels than larger hydrocarbons. Since cracking converts larger hydrocarbons into smaller hydrocarbons, the supply of fuels is improved. This helps to match supply with demand.
It produces alkenes. Alkenes are more reactive than alkanes. They are used as feedstock for the petrochemical industry.