chemistry C1 (5)

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  • Created by: haidee
  • Created on: 26-04-12 17:32

Cracking hydrocarbons

  • the smaller hydrocarbons fractions obtained from crude oil are the most useful and are in heavy demand, particularly the petrol fractions
  • the larger fractions are less usefuk as fuels and are not in such high demand and the supply exceeds demand
  • it is possible to break down larger, less useful alkane molecules into smaller more useful ones
  • the process uses heat and a catalyst and is known as catalytic cracking
  • the large molecules are heated until they vapourise and passed over a hot catalyst
  • this splits the molecule in a reaction known as thermal decomposition
  • as well as producing a smaller more useul alkane molecule, other products called alkenes are formed
  • decane to octane + ethene
  • C10H22 to C8H18 + C2H4
  • alkenes are another homologous series of hydrocarbons
  • they have a different general formula to that of the alkanes


  • alkenes are unsaturated hydrocarbons- they do not contain as many hydrocarbons atoms as possible because they have a double bond between two of the carbon atoms
  • the name of every alkenes ends in -ene
  • methene doesn't exist as alkenes need a double bond so therefore they must have at least 2 carbon atoms
  • the general formula is CnH2n
  • bromine water can be used to test between alkenes and alkanes
  • when it is alkenes the bromine water turns from yellow to colourless as the double bonds breaks and bonds to the bromine
  • alkenes burn with a smoky flame, so do not make such good fuels

Key terms about crude oil

  • distillation: separation method used to get a pure liquid from a mixture
  • cracking: the use of heat and a catalyst to break down a long chain alkane to form an alkane and an alkene
  • refining: to remove impurities
  • saturated: a molecule with only single bonds
  • unsaturated: a molecule with a C=C bond
  • hydrocarbon: contains only carbon and hydrogen
  • propene: a molecule with 3 carbon atoms that would decolourise bromine water
  • propane: a molecule with 3 carbon atoms that would not decolorise bromine water

Making polymers from alkenes

  • Alkenes produced from the catalytic cracking of large alkane molecules have many uses
  • One of the most important uses is in manufacture of plastic
  • There are many different types of plastics, but most have one thing in common- they are polymers made from alkenes
  • A polymer is a huge, long chain molecule made by joining lots of small reactive alkene molecules together
  • The small molecules are called monomers
  • Different types of plastics have different properties because they are made from different monomers
  • A plastic bag is made from a polymer called poly(ethene)
  • Huge numbers of ethene monomers join together when they are heated, under pressure, in the presence of a catalyst
  • The double bond in each ethene monomer breaks, and they join together forming a long chain polymer
  • This is known as addition polymerisation because the reaction is an addition reaction
  • Poly(ethene) is used for bags, bottles, buckets, washing up bowls


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