Cracking + Alkenes

• Cracking

  Hydrocarbons can be broken down (cracked) to produce smaller, more useful molecules.
• Products of cracking (long-chain hydrocarbons)

  • useful as fuels
  • polymers e.g plastics
• Cracking

  • Thermal decomposition
• Two types of cracking

  • Catalytic cracking
  • Steam cracking
• Catalytic cracking

  • heat the long-chain hydrocarbon to vaporise / evaporate them
  • passed over a hot, powdered catalyst
  • crack on the surface on the specs of the catalyst
• Steam cracking

  • heat the long-chain hydrocarbon to vaporise / evaporate them
  • mixed with steam and heated to a very high temperature
  • lead to the thermal decomposition of long-chain hydrocarbon molecules to form smaller one
• Give two conditions used to crack large alkane molecules.

  • High temperatures
  • Catalyst OR steam
• Similarity between Catalytic cracking and Steam cracking

  • heat long-chain hydrocarbons to vaporise / evaporate them
• Products of cracking 2

  • alkanes
  • alkenes
• Cracking of decane

  • decane = ethene + octane
• Economic reasons for cracking

  • There is a high demand for fuels with small molecules
• Alkenes

  • unsaturated hydrocarbons
  • homologous series
• General formula of alkenes

  • CnH2n
• Alkenes are hydrocarbons 


  • contain hydrogen and carbon atoms only
• Reactivity : alkenes

  • more reactive than alkanes
• Bonds in alkenes

  • form a single, double carbon-carbon bond
  • unsaturated
• Carbon bonds in alkenes

  • carbon atoms always 4 bonds by HYDROGEN atoms make only one
• Uses of alkenes

  • produce polymers e.g ethene = plastics
  • starting materials for the production of other chemicals
• Why are alkenes more reactive than alkanes?

  • double carbon-carbon bond can open up to become a single bond
  • this allows the two carbon atoms to bond to other atoms
• First four alkenes

  • ethene
  • propene
  • butene
  • pentene
• How do you know C5H10 is an alkene?

  • matches the general formula of an alkene : CnH2n
• Alkenes + bromine water 


  • When alkenes react with bromine water, the colour changes from orange to colourless
• Alkanes + bromine water

  • When alkanes react with bromine water, no reaction occurs and solution remains orange
• Pentene is useful because many pentene molecules can join together

  to form .................................................. .

  • poly (pentene)
• Complete Combustion of alkenes

  • plentiful supply of oxygen
  • combust completely
  • produce carbon dioxide + water (vapour)
• Incomplete combustion

  • limited supply of oxygen
  • also produces, carbon monoxide + carbon (soot)
• Complete combustion v Incomplete combustion

  • less energy is released by incomplete combustion
• long chain hydrocarbon -->

  • long chain hydrocarbon --> shorter chain molecule alkane + alkene
• Alkenes : functional group

  • contain a double carbon-carbon functional group
• Why are alkenes considered part of a homologous series?

  • all contain double carbon-carbon functional group
  • all react in similar ways
• Why are alkene molecules unsaturated?

  • contain two fewer hydrogen carbon atoms than the alkane with the same number of carbon atoms
• Flame in incomplete combustion of alkenes

  • burn with a smoky yellow flame
• Why are alkenes useful?


  • reactive
• Feedstock 


  • Solvents
  • Detergents
• Fuels 


  • petrol
  • kerosene
• Catalyst that can be used for catalytic cracking

  • aluminium oxide
• Suggest how the use of a suspension of limestone decreases one of the environmental impacts that the waste gases would cause

  • acid rain is caused by sulphur dioxide
  • global warming is caused by carbon dioxide
  • carbon dioxide dissolve in suspension by limestone