Alkanes

• Fractional distillation:

  • crude oil is vaporised at 350 degrees
  • vaporised crude oil rises up the fractionating column
  • a temperature gradient is created
  • because boiling points of alkanes increase as the molecules get bigger, each fraction condenses at a different temperature.
• Cracking- breaking long-chain alkanes into smaller hydrocarbons, involves breaking the C-C bonds. Done as stuff that's in high demand is much more valuable than the stuff that isn't.
• Thermal cracking- high temperature (up to 1000 degrees), high pressure (up to 70atm). Produces high proportion of alkenes.
• Catalytic cracking- zeolite catalyst (hydrated aluminosilicate), slight pressure, high temperature (500 degrees), mostly produces aromatic hydrocarbons and the alkanes needed to produce motor fuels.
• Carbon monoxide- made during incomplete combustion. Poisonous as bind to the same sites on haemoglobin molecules in red blood cells as oxygen molecules. Can be removed from exhaust gases by catalytic converters on cars.
• Nitrogen oxides- toxic and poisonous. Produced when high pressure and temperature in a car engine cause the nitrogen and oxygen atoms from the air to react together. Can be removed by catalytic converters in cars.

  N2 (g) + O2 (g) -> 2NO (g)

  2NO (g) +O2 (g) -> 2NO2 (g)
• Unburnt hydrocarbons- formed from incomplete combustion. React with nitrogen oxides in the presence of sunlight to form ground-level ozone (O3) which is a major component of smog. Can be removed by catalytic converters.
• Sulphur dioxide- formed from sulphur containing impurities in fuel oxidise when burnt. It dissolves in the moisture in the atmosphere and in converted to sulphuric acid which produces acid rain. Can be removed from power station flue gases by desulphurisation.

  O2 + 2SO2 + 2H2O -> 2H2SO4
• Desulphurisation- powdered calcium carbonate (limestone) is mixed with water to make an alkaline slurry. When the flue gas mix with the alkaline slurry, the acidic sulphur dioxide gas reacts with the calcium compounds to form salt as SO2 is acidic and CaO is basic.

  Ca + SO2 -> CaSO3

  CaCO3 + SO2 + 1/2 O2 -> CaSO4 +CO2
• Catalytic converters:

  • ceramic honeycomb to give a large surface area
  • catalysts: palladium, platinum and rhodium

  2CO + O2 -> 2CO2

  2NO + 2CO -> N2 + CO2