Alkanes

• Fractional Distillation: Industrially


  1. Oil is pre-heated

  2. Then passed into column

  3. The fractions condense at different heights

  4. The temperature of column decreases upwards

  5. The separation depends on boiling point

  6. Boiling point depends on size of molecules

  7. The larger the molecule the larger the van der waals forces

  8. Similar molecules (size, bp, mass) condense together

  9. Small molecules condense at the top at lower temperatures

  10. Big molecules condense at the bottom at higher temperatures
• Fractional Distillation


  • It is a physical process involving the splitting of weak van der waals forces between molecules
• Vacuum Distillation Unit


  1. Heavy residues from the fractionating column are distilled again under a vacuum

  2. 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
• Petroleum


  A mixture consisting mainly of alkane hydrocarbons
• Petroleum Fraction


  A mixture of hydrocarbons with a similar chain length and boiling point range
• Fractional Distillation: In the Laboratory

  1. Heat the flask

  2. Vapours pass up the fractionating column

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

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

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

  6. Only the most volatile vapour passes into the condenser

  7. The condenser cools the vapours and condenses to a liquid and is collected
• Fractional Distillation


  Used to separate liquids with similar boiling points
• 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
• The products of cracking are more valuable than the starting materials (e.g. ethene used to make poly(ethene), branched alkanes for motor fuels, etc.)
• Cracking


  Conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds
• Cracking


  • It is a chemical process involving the splitting of strong covalent bonds so requires high temperatures
• Types of Cracking


  • Thermal Cracking
  • Catalytic Cracking
• Thermal Cracking


  1. High pressure (7000 kPa)

  2. High temperature (400°C to 900°C)

  3. Bonds can be broken anywhere in the molecule by C-C bond fission and C-H bond fission
• Thermal Cracking Equations


  • C8H18 C6H14 + C2H4
  • C12H26 C10H22 + C2H4
• Catalytic Cracking


  Cheaper than thermal cracking because it saves energy as lower temperatures and pressures are used
• Catalytic Cracking


  1. Slight or moderate pressure

  2. High temperature (450°C)

  3. Zeolite catalyst
• Catalytic Cracking Products


  • Produces branched and 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
• Complete Combustion of Alkanes

  C8H18(g) + 12.5 O2(g) 8CO2(g) + 9 H2O(l)
• Alkanes


  • Readily burn in the presence of oxygen
  • Combustion of alkanes is highly exothermic, explaining their use as fuels
• Incomplete Combustion of Alkanes

  1. CH4(g) + 3/2 O2(g) CO(g) + 2 H2O(l)

  2. CH4(g) + O2(g) C(s) + 2 H2O(l)
• Incomplete Combustion


  • Produces less energy per mole than complete combustion
  • Can produce CO (which is very toxic) and/or C (producing a sooty flame)
• The products of complete combustion are CO2 and H2O
• In excess oxygen alkanes will burn with complete combustion
• Carbon (soot)


  Can cause global dimming- reflection of the sun's light
• Flue Gas Desulfurisation


  1. The gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction

  2. The calcium sulfite which is formed can be used to make calcium sulfate for plasterboard
• Sulfur containing impurities are found in petroleum fractions which produce SO2 when they are burned
• Coal is high in sulfur content, and large amounts of sulfur dioxide are emitted from power stations
• SO2 dissolves in atmospheric water

  Can produce acid rain
• Pollutants from Combustion


  • Nitrogen oxides
  • Carbon monoxide
  • Carbon dioxide
  • Unburnt hydrocarbons
  • Soot
• Nitrogen Oxides


  • Formed when N2 in the air reacts at the high temperatures and spark in the engine
  • NO is toxic and can form acidic gas NO2
  • NO2 is toxic and acidic and forms acid rain
• Carbon Monoxide


  Toxic
• Carbon Dioxide


  Contributes towards global warming
• Unburnt Hydrocarbons


  Contributes towards formation of smog
• Soot


  Global dimming and respiratory problems
• Catalytic Converters


  Remove CO, NOx and unburned hydrocarbons (e.g. octane, C8H18) from the exhaust gases, turning them into 'harmless' CO2, N2 and H2O
• Water is the main greenhouse gas (but is natural), followed by carbon dioxide and methane
• Carbon dioxide levels have risen significantly in recent years due to increasing burning of fossil fuels
• Carbon dioxide is a particularly effective greenhouse gas and its increase is thought to be largely responsible for global warming