For example, hexane can be cracked to form butane and ethene:
hexane → butane + ethene
C6H14 → C4H10 + C2H4
The larger alkanes are heated to around 650°C and their vapours are passed over a hot catalyst containing aluminium oxide. This causes covalent bonds to break and reform. The slideshow describes this process.
Cracking is important for two main reasons:
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 often produces more of the larger hydrocarbons than can be sold, and less of the smaller hydrocarbons than customers want.
Smaller hydrocarbons 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.
Alkanes and alkenes both form homologous series of hydrocarbons, but: