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Cracking

is a reaction in which larger are broken down into smaller, more useful hydrocarbon molecules, some of which are unsaturated:

  • the original starting hydrocarbons are
  • the products of cracking include alkanes and , members of a different

For example, hexane can be cracked to form butane and ethene:

Hexane 鈫 butane + ethene

C6H14 鈫 C4H10 + C2H4

The larger alkanes are heated to around 650掳C and their vapours are passed over a hot containing aluminium oxide. This causes to break and reform. The slideshow describes this process.

Image gallerySkip image gallerySlide 1 of 3, Structure of hexane showing covalent bonds.,

Reasons for cracking

Cracking is important for two main reasons:

  1. it helps to match the supply of with the demand for them
  2. it produces alkenes, which are useful as for the industry

Supply and demand

The supply is how much of a fraction an oil refinery produces. The demand is how much of a fraction customers want to buy. of 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 than larger hydrocarbons. Since cracking converts larger hydrocarbons into smaller hydrocarbons, the supply of fuels is improved. This helps to match supply with demand.

Alkenes

Alkanes and alkenes both form homologous series of hydrocarbons, but:

  • alkanes are (their carbon atoms are only joined by C-C single bonds)
  • alkenes are (they contain at least one C=C double bond)

As a result, alkenes are more than alkanes. Alkenes can take part in reactions that alkanes cannot. For example, ethene molecules can react together to form poly(ethene), a .