Alkanes

• Alkanes are a homologous series of saturated hydrocarbons. A saturated hydrocarbon is when the compound only contains single bonds between carbon atoms.
• General formula:

  CnH2n+2
• Physical properties:

  • Non-polar - small difference in electronegativity between C and H
  • Insoluble in water
  • Will dissolve in other non-polar liquids
• Fractional distillation:

  Crude oil is heated in a furnace until a portion changes state into vapour, which is then passed up a fractionating tower. The fractionating tower cools towards the top, when a fraction reaches a layer where it is cool enough, it codences and is piped off.

  The shorter (carbon) chain molecules are collected at the top of the tower as they have lower boiling points.
• Petroleum is a mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation.
• Industrial cracking: Thermal cracking

  Long, useless chain alkanes are heated, between 1000K to 1200K, at 70 atm. Produce a high percentage of shorter, useful chain alkanes, or carbon and hydrogen individually.

  Example - C15H32→ C7H16 + 4C2H4
• Industrial cracking: Catalytic cracking

  Long chain alkanes are heated to a high temperature, 800K to 1000K, under slight pressure, 1-2 atm, in the presence of a zeolite catalyst.

  Zeolite has a honeycomb structure, giving it a larger surface area, which increases the rate of reaction. Produces mainly branched alkanes, cycloalkanes and aromatic compounds, used for motor fuels and aromatic hydrocarbons.
• Combustion of alkanes:

  • Alkanes are used as fuels.
  • Combustion of alkanes and other organic compounds can be complete, producing CO2 or incomplete, producing CO, water is produced by both.
  • Combustion requires an excess supply of oxygen, incomplete combustion occurs when the supply is limited.
• Combustion in an internal combustion engine:

  Small amount of fuel mixed with a large excess of air is compressed and ignited. The products of combustion are released through the exhaust.
• Combustion in an internal combustion engine: Products

  • The internal combustion engine produces a number of pollutants including NOx , CO, carbon and unburned hydrocarbons. Hydrocarbons containing sulfur produce sulfur dioxide, another pollutant.
  • NOx = Acid rain and photochemical smog.
  • CO = Toxic gas.
  • C = Particles exacerbate asthma.
  • Unburned hydrocarbons = Reacts with NOx, forming ground level ozone, causing respiratory issues.
  • SO2= Acid rain and air pollution.
• Combustion in an internal combustion engine: Solution

  These gaseous pollutants (CO, NOx and unburnt hydrocarbons) from internal combustion engines can be removed using catalytic converters.

  At the centre, there is a honeycomb of ceramic material covered in catalysts, platinum and rhodium. The honeycomb structure provides a large surface area, increasing the rate of reaction. As the gasses pass over the catalysts, they react with each other, producing less polluting products.
• Removing sulfur:

  • Sulfur containing compounds are removed from petrol and diesel.
  • Flue gas desulfurisation - Gasses are passed through a slurry, containing calcium oxide or calcium carbonate. These bases neutralise the acidic sulfur dioxide to form calcium sulphate, which is dumped into waste pits or oxidised into calcium sulfate, which is commercially useful as a construction material.
• Chlorination of alkanes:

  A) Free radical substitution

  B) UV light

  C) Initiation

  D) Propagation

  E) Termination