Alkanes

• Alkanes are saturated hydrocarbons
• List in order from top to bottom the products of fractional distillation (KGNKDFB)

  .

  • Refinery gases
  • Gasoline/ petrol (naptha)
  • Kerosine
  • Lubricating oil
  • Diesel oil
  • Fuel oil
  • Bitumen
• Cracking involves breaking C-C bonds in alkanes
• Describe the differences between thermal and catalytic cracking


  Thermal cracking takes place at a high temperature and pressure and produces a high percentage of alkenes

  Catalytic cracking takes place at slight pressure and high temperature and with a zeolite catalyst. Used mainly to produce motor fuels and aromatic hydrocarbons
• Describe the economic reasons for cracking of alkanes


  Cracking produces shorter chain more useful alkanes and reactive alkenes that are used frequently as chemical feedstock
• Shorter chain alkanes burn completely in a plentiful supply of oxygen to produce carbon dioxide and water
• In a limited supply of oxygen carbon monoxide is produced
• What pollutants may be produced in the combustion of alkanes

  Carbon monoxide

  Carbon dioxide

  Carbon particulates

  Sulfur dioxide

  Nitrogen oxides

  Water vapour
• Describe the process of flue gas desulfurisation

  A slurry of calcium oxide (lime) and water is sprayed into the flue gas. These react to form calcium sulfite, which can be further oxidised to calcium sulfate (gypsum) for use in plasterboard.

  
  

  Calcium carbonate can also be used
• What is a catalytic converter?

  A honeycomb made of ceramic materials coated in rhodium and platinum metals to act as catalysts. The honeycomb shape provides a large surface area.
• How does a catalytic converter work?

  As the exhaust gases pass over the catalyst metal they react with each other to form less harmful products

  carbon monoxide + nitrogen oxide --> nitrogen + carbon dioxide

  hydrocarbons + nitrogen oxide --> nitrogen + carbon dioxide + water
• What are the 3 stages of free radical substitution?

  Initiation, Propagation, Termination
• Describe the initiation of FRS of methane

  Cl-Cl bonds are broken by UV light which creates free radicals

  Cl₂ ---> 2Cl·
• Describe the propagation of FRS of methane

  There are 2 stages:

  1. A chlorine free radical takes a hydrogen from methane to form hydrogen chloride. This leaves a methyl free radical

  Cl· + Ch4 ---> HCl + ·Ch3

  2. The methyl free radical reacts with the second chlorine free radical which produces another chlorine free radical and a molecule of chloromethane

  ·Ch3 + Cl ---> CH3Cl + Cl·
• Describe the termination of FRS of methane


  This is where the free radicals are removed and stable compounds are made. There are 3 combinations of products

  Cl· + Cl· ---> Cl2

  ·CH3 + Cl· ---> CH4Cl

  ·CH3 + ·CH3 ---> C2H6
• Why is homolytic free radical substitution considered a chain reaction?

  The chlorine radicals act like a catalyst and are not used up. This means they can continue to react with other molecules and continue the reaction until they are destroyed in the termination step after thousands of uses.