Alkanes

• Substitution reaction is a reaction where one atom or group of atoms s replaced by another atom or group of atoms
• Free radical is a species with an unpaired electron
• Homolytic fission is the breaking of a bond with 2 identical species formed ( 2 radicals )
• Heterolytic fission is the breaking of a bond with 2 different species formed (2 oppositely charged ions)
• Fractional distillation is used to separate compounds according to their boiling points
• Fractions are a mixture of compounds that have similar boiling points and are removed at the same level in a fractionating column
• Hydrocarbons have different boiling points

  The boiling points depend on size / Mr / chain length

  There is a negative temperature gradient in the column ( cooler at the top)

  Higher boiling point molecules at the bottom
• 1-4 = refinery gas

  5-12 = petrol

  7-14 = Naphtha

  11-15 = Kerosine

  15-19 = Gas oil

  20-30 = Mineral oils

  30+ = Bitumen
• Cracking is where long chains are broken down into shorter chains, more useful, molecules by breaking C-C bonds
• Thermal cracking:

  • High temperature
  • High pressure

  produces shorter chain alkanes and high percentage of alkenes
• Catalytic cracking:

  • High temperature
  • Sight pressure
  • Zeolite catalyst

  Produces cyclic alkanes, branched alkanes and aromatic hydrocarbons
• Catalytic cracking is better than thermal as it produces a high % of branched alkanes which burn more easily, it is cheaper as it uses less heat and pressure, and produces a higher proportion of arenas which feedstock chemicals. However, it cannot be used on all fractions whereas thermal cracking can
• Complete combustion occurs when the hydorcarbon is burned in plentiful supply of oxygen

  X + O2 -> CO2 + H2O
• Complete combustion is highly exothermic
• During complete combustion there is a clean blue flame
• Incomplete combustion is when the hydrocarbons are burned in a limited supply of oxygen

  X + O2 -> CO + H2O
• During incomplete combustion there is a cooler, dirty yellow flame
• Further incomplete combustion is when the hydrocarbons are burned in very limited supply of oxygen

  X + O2 -> C + H2O
• Under high temperatures and pressure in a car engine, nitrogen from the air can react with oxygen producing nitrogen oxides.

  They cause photochemical smog and acid rain
• Sulfur in the fuel reacts with oxygen to form sulphur dioxide which causes acid rain and photochemical smog (pollutant)
• Unburnt hydrocarbons can react with nitrogen oxides in the air to form low level ozone which contribute to photochemical smog
• Catalytic converters reduce the amount of carbon monoxide, nitrogen oxides and unburnt hydrocarbons by converting them into less polluting products
• The catalytic converter has a honeycomb structure to provide a larger surface area to increase the rate of reaction
• The removal of NO and CO:

  2NO + 2CO -> N2 + 2CO2
• Flue gas desulfurisation uses CaO or CaCO3 to neutralise the sulfur dioxide produced

  CaO + SO2 -> CaSO3

  CaCO3 + SO2 -> CaSO3 + CO2
• Initiation is the formation of radicals
• Propagation is the formation of products
• Termination is the removal of radicals