Cracking

• Cracking:

  • Saturated molecules contain single bonds only whereas unsaturated molecules contain double bonds between their carbon atoms
  • Alkanes are saturated compounds and alkenes are unsaturated compounds
  • Long chain alkane molecules are further processed to produce other products consisting of smaller chain molecules
  • A process called cracking is used to convert them into short chain molecules which are more useful
  • Small alkenes and hydrogen are produced using this process
  • Kerosene and diesel oil are often cracked to produce petrol, other alkenes and hydrogen
• Decane is cracked to produce octane for petrol and ethene for ethanol synthesis:

  
• Decane is cracked to produce octane for petrol and ethene for ethanol synthesis:

  
• • Cracking involves heating the hydrocarbon molecules to around 600 – 700°C to vaporise them
  • The vapours then pass over a hot powdered catalyst of alumina or silica
  • This process breaks covalent bonds in the molecules as they come into contact with the surface of the catalyst, causing thermal decomposition reactions
  • The molecules are broken up in a random way which produces a mixture of smaller alkanes and alkenes
  • Hydrogen and a higher proportion of alkenes are formed at higher temperatures and higher pressure
• Writing Equations for Cracking:

  • We can use the general formulae for alkanes and alkenes to check that we have correctly balanced equations for cracking
  • Hexane for example, can be cracked to form butane and ethene, both of which are very useful molecules
  • Ethene as the starting material for the production of alcohol and butane is used as a fuel
  • The equation for this cracking reaction is:

  C6H14 ⟶ C4H10 + C2H4
• • Note that the starting compound for this reaction is an alkane and thus the general formula CnH2n+2 applies
  • Butane is also an alkane and so the same rule applies
  • Ethene is an alkene and so its formula will follow the CnH2n rule