alkanes and alkenes

• Alkanes


  A homologous series of hydrocarbons that contain only carbons and hydrogen atoms with no double bonds
• Alkanes


  • As the length of the carbon chain increases:
  • Boiling point increases
  • Shorter alkanes are more volatile and evaporate more easily
  • Longer alkanes are more viscous and thick
  • Shorter alkanes are more flammable and easier to ignite or burn
• Combustion reactions


  1. Hydrocarbon reacts with oxygen

  2. Forms carbon dioxide and water

  3. Releases energy (exothermic)
• Complete combustion


  Hydrocarbon reacts with enough oxygen to form carbon dioxide and water
• Writing balanced combustion equations

  1. Write unbalanced equation

  2. Balance carbon atoms

  3. Balance hydrogen atoms

  4. Balance oxygen atoms
• Hydrocarbons like alkanes are used as fuels because they release energy when combusted
• Crude oil


  A mixture of lots of different hydrocarbons
• Fractional distillation


  Separates out 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 apart into two smaller hydrocarbons
• Steam cracking


  Vaporize hydrocarbons, mix with steam, heat to high temperature which causes long hydrocarbon chains to split apart into shorter ones
• 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 on each side of the equation must be the same
• Alkenes


  • 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
• Decane cracking


  • Decane (C10H22) cracks to form ethene (C2H4) and another hydrocarbon