Useful substances from crude oil

Alkenes and Alkanes 

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Intro- Keywords


Cracking- a chemical reaction which produces smaller hydrocarbons, including alkanes and alkenes.

Hydrocarbons- a group of compounds containing hydrogen and carbon. 

Polymers- large molecules formed from many identical smaller molecules (monomers) 

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Alkenes (1)

  • Feuls made from oil mixtures containing large hydrocarbons are often not efficient because they are difficult to ignite or do not flow easily. 
  • Often, crude oil contains too many large hydrocarbon molecules and not enough small ones to meet demand.
  • Cracking allows large molecules to be broken down into smaller and more useful hydrocarbon molecules. 
  • Fractions large hydrocarbon molecules are heated to vapourise them, then the vapour is either passed over a hot catalyst or mixed with steam and heated to a very high temperature. 
  • This breaks chemical bonds in the molecules (thermal decomposition). Cracking produces smaller alkanes and alkenes. 
  • These smaller hydrocarbons can now be used as feuls and the alkenes are used to make polymers in plastics manufacture. 
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Alkenes (2)

  • The products of cracking include alkenes (eg ethene and propene). 
  • The alkenes are a family of hydrocarbons that share the same general formula:CnH2n
  • The number of hydrogen atoms in an alkene is double the number of carbon atoms. For example, ethene is C2H4 and propene is C3H6.
  • Alkenes are unsaturated hydrocarbons. They contain a double covalent bond, which is shown as two lines between two of the carbon atoms.
  • The presence of this double bond allows alkenes to react in ways that alkanes cannot. 
  • They can react with oxygen in the air, so they could be used as fuels. But they are more useful than that: they can be used to make ethanol and polymers (plastics)
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