Cracking Crude Oil & Alkanes

• Crude oil

  A mixture of lots of different hydrocarbons
• Fractional distillation


  Separates different hydrocarbons by the length of their carbon chains
• Shortest hydrocarbons


  • Most flammable, make the best fuels
• Longer chain hydrocarbons

  • Thick viscous liquids, comparatively less useful
• Cracking

  Breaks down longer less useful hydrocarbons into shorter more flammable hydrocarbons
• Thermal decomposition

  Breaking down molecules by heating them
• Cracking methods

  • Catalytic cracking
  • Steam cracking
• Catalytic cracking

  Heat long chain hydrocarbons and vaporize, pass over hot powdered aluminium oxide catalyst which splits them into smaller hydrocarbons
• Steam cracking

  Vaporize hydrocarbons, mix with steam, heat to high temperature to split into smaller hydrocarbons
• Balanced chemical equation for cracking long chain alkanes


  Long chain alkane (e.g. decane) goes to form shorter alkane (e.g. heptane) and alkene (e.g. propene)
• Number of carbons and hydrogens must be the same on both sides of the equation
• Alkenes


  • Unsaturated, more reactive than alkanes, can be added together to make polymers
• Adding bromine water to a solution of alkenes

  Decolorizes the bromine water from orange to colorless
• Cracking reaction

  • Decane (C10H22) can be cracked into ethene (C2H4) and another hydrocarbon
• Alkenes are another example of a homologous series, similar to alkanes but with a double bond instead of single bonds