alkane

• petroleum is a mixture consisting of mainly alkane hydrocarbons
• petroleum fraction is a mixture of hydrocarbons with similar chain length and boiling point range
• fractional distillation is a physical process involving the splitting of weak van der waals forces between molecules
• fractional distillation

  oil is preheated and passed into column where temperature decreases upwards, fractions condense at different heights depending on boiling point due to size of molecule
• similar molecules (size, bp, mass) condense together
• small molecules condense at the top at lower temperatures

  big molecules condense at the bottom at higher temperatures
• vacuum distillation unit

  heavy residues from fractionating column are distilled again under a vacuum

  lowering the pressure over a liquid will lower its boiling point
• vacuum distillation allows heavier fractions to be further separated without high temperatures which could break them down
• fractional distillation in lab

  1. heat flask with Bunsen burner or electric mantle so vapours of all components in mixture are produce
  2. vapours pass up fractionating column
  3. vapour with lower boiling point reaches top of fractionating column first
  4. thermometer should be at or below boiling point of most volatile substance
  5. vapours with highest boiling point condense back into flask
  6. only most volatile vapour passes into condenser, liquid produced is collected
• cracking is conversion of large hydrocarbons to smaller hydrocarbons by breakage of C-C bonds

  chemical process that requires high temperatures

  can be thermal or catalytic
• high Mr alkanes --> smaller Mr alkanes + alkenes + (hydrogen)
• economic reasons for cracking

  1. petroleum fractions with shorter C chains (petrol) are more in demand than larger fractions
  2. make use of excess larger hydrocarbons and to supply demand for shorter ones, larger hydrocarbons cracked
  3. cracking products are more valuable than starting materials, branched alkanes for motor fuels
• thermal cracking produces mostly alkenes (ethene for ethanol) and sometimes hydrogen (used in haber process)

  Bonds can be broken anywhere in the molecule by C-C bond fission and C-H bond fission.
• thermal cracking conditions

  high pressure 7000kPa

  high temperature 400-900°C
• catalytic cracking produces branched/cyclic alkanes and aromatic hydrocarbons used for making motor fuels

  Branched and cyclic hydrocarbons burn more cleanly and are used to give fuels a higher octane number
• catalytic cracking conditions

  conditions: slight or moderate pressure

  high temperature: 450°C

  zeolite catalyst
• catalytic cracking uses lower temperatures and pressures so is cheaper than thermal cracking as less energy is required
• fuel releases heat energy when burnt
• combustion of alkanes is exothermic

  alkanes burn readily in oxygen
• in excess oxygen complete combustion occurs leading to the formation of CO2 and H2O
• in limited oxygen, incomplete combustion occurs producing CO which is toxic and C
• incomplete combustion produces less energy per mole than complete combustion
• carbon (soot) can cause global-dimming

  which is reflection of the sun's light

  can also cause respiratory problems
• sulfur containing impurities are found in petroleum fractions which produce SO2 when burnt
• SO2 will dissolve in atmospheric water to make acid rain
• SO2 can be removed from the waste gases from furnaces (e.g. coal fired power stations) by flue gas desulfurisation. The gases pass through a scrubber containing basic calcium oxide which reacts with sulfur dioxide to neutralise
• neutralisation of sulfur dioxide and calcium oxide

  SO2 + CaO --> CaSO3

  calcium sulfite formed can make calcium sulfate for plasterboard
• nitrogen oxides form when N2 and O2 in car engine react

  high temperature and spark in engine provides sufficient energy to break strong N2 bond
• nitrogen oxides equation

  N2 + O2 --> 2NO

  N2 + 2O2 --> 2NO2
• NO is toxic and can form acidic gas NO2

  NO2 is toxic and acidic and forms acid rain
• carbon monoxide is toxic
• carbon dioxide contributes to global warming
• unburnt hydrocarbons contributes towards formation of smog
• catalytic converters remove CO, NOx and unburned hydrocarbons from exhaust gases and converts them to N2, CO2 and H2O
• catalytic converters have a ceramic honeycomb structure coated with a thin layer of catalyst metals (platinum, palladium, rhodium) to give a large surface area
• catalytic converter equations

  2 CO + 2 NO --> 2 CO2 + N2

  C8H18 + 25 NO --> 8 CO2 + 12½ N2 +
• water vapour is the main greenhouse gas (natural), followed by carbon dioxide and methane
• UV wavelength radiation passes through atmosphere to Earth's surface and heats it up

  the earth radiates out infrared long wavelength radiation

  C=O bonds in CO2 absorb IR radiation so it does not escape from atmosphere

  energy transferred to other molecules in atmosphere by collisions so atmosphere is warmed
• carbon dioxide levels rising because more fossil fuels burnt

  largely responsible for global warming
• in the presence of uv light, alkanes react with chlorine to form a mixture of products with halogens substituting hydrogen atoms