Alkanes

• Properties of alkanes:

  Non-polar

  There is not much difference in electronegativity between C and H, so VDW forces between molecules. Larger molecules= stronger VDW
• Properties of alkanes:

  Boiling point low

  As molecules get larger, boiling point increases. More branching, vdw forces are weaker because less surfaces of contact, so lower bpt
• Properties of alkanes:

  Insoluble

  Water is polar

  Alkanes are non polar

  Therefore non polar does not dissolve in polar
• Petroleum fraction

  Mixture of hydrocarbons with similar chain length and boiling point range
• Crude oil is preheated then passed into column
• The fractions condense at different heights
• The temperature of the column decreases upwards
• Process of fractional distillation

  Vapours pass up the fractionating column.

  • The vapour of the substance with the lower boiling point reaches the top of the fractionating column first.

  • The thermometer should be at or below the boiling point of the most volatile substance.

  • The vapours with higher boiling points condense back into the flask.

  • Only the most volatile vapour passes into the condenser.

  • The condenser cools the vapours and condenses to a liquid and is collected.
• Cracking is the conversion of large hydrocarbons to smaller hydrocarbon molcules by brakeage of C-C bonds
• Thermal cracking

  High pressure (7000kPa)

  High temperature (400-900°C)

  Produces a high percentage of alkenes
• Catalytic cracking

  • Moderate pressure
  • High temperature (450°C)
  • Zeolite catalyst
  • Mainly used to produce motor fuels and aromatic hydrpcarbons

  Cheaper than thermal cracking because it saves energy as lower temperatures and pressures are used
• Economic reasons for cracking

  • The petroleum fractions with shorter C chains (e.g. petrol and naphtha) are in more demand than larger fractions.
  • To make use of excess larger hydrocarbons and to supply demand for shorter ones, longer hydrocarbons are cracked
  • Products of cracking are more valuable than starting materials
• Complete combustion

  The products of complete combustion are CO2 and H2O.

  C8H18(g) + 12.5 O2 (g) ->8CO2 (g) + 9 H2O(l)
• Incomplete combustion

  Producing CO (which is very toxic) and/or C (producing a sooty flame) and H2o
• Incomplete combustion produces less energy per mole than complete combustion
• Alkanes readily burn in the presence of oxygen. This combustion of alkanes is highly exothermic, explaining their use as fuels.
• SO2 can be removed from the waste gases from furnaces by flue gas desulfurisation. The gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction.
• The calcium sulfite which is formed from flue gas desulfurisation can be used to make calcium sulfate for plasterboard.
• Nitrogen oxides (formed when N2 in the air reacts at the high temperatures and spark in the engine) are toxic and can form acidic gas NO2
• These remove CO, NOx and unburned hydrocarbons (e.g. octane, C8H18) from the exhaust gases, turning them into ‘harmless’ CO2 , N2 and H2O.
• SO2 + CaO ->CaSO3 (calcium sulfate)
• CH4 (methane) + Cl2 ->CH3Cl (chloromethane)+ HCl
• In general, alkanes do not react with many reagents. This is because the C-C bond and the C-H bond are relatively strong
• Free radical substiution:

  Initiation

  Propagation

  Termination
• 1. Initiation- making radicals

  UV light required

  Cl2 -> 2Cl'

  A free radical is a reactive species which possess an unpaired electron

  A) Homolytic fission- each atom takes one electron

  
• 2. Propagation- using and making radicals

  CH4 + Cl'-> HCl + 'CH3

  • The chlorine free radicals are very reactive and take a H from the methane leaving a methyl free radical

  'CH3 + Cl2 ->CH3Cl + Cl'

  • The methyl free radical reacts with a Cl2 molecule to produce the main product and another Cl free radical
• In the presence of UV ligh alkanes react with chlorine


  The UV light supplies the energy to break the Cl-Cl bond. It is broken in preference to the others because it is the weakest.
• 3. Termination- using up radicals

  'CH3 + Cl' ->CH3Cl

  • Collision of two free radicals does not generate further free radicals: the chain is terminated.

  'CH3 + 'CH3 ->CH3CH3

  • Product must be written using structural formula
• thermal cracking conditions


  • high pressure, 7000kPa
  • high temp, 400-900 Celsius
• catalytic cracking conditions


  • slight or moderate pressure
  • high temp, 450 c
  • zeolite catalyst
• flue gas desulfurisation


  SO2 removed

  • gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction.
  • SO2 + CaO --> CaSO3

  Calcium sulfite formed can be used to make calcium sulfate for plasterboard
• catalytic converters


  • 2 CO + 2 NO --> 2 CO2 +N2
  • C8H18 + 25 NO --> 8 CO2 + 12.5 N2 + 9 H2O
• greenhouse effect

  • UV radiation passes through the atmosphere to Earth's surface and heats it up
  • Earth radiates out infrared long wavelength radiation
  • C double bond O in CO2 absorbs infrared radiation so IR does not escape from atmosphere
  • energy transferred to other molecules in atmosphere by collisions
  • so atmosphere is warmed