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. and big molecules condense at the bottom at higher temperatures
• Fractional Distillation

  • This 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, with a Bunsen burner or electric mantle

  2. This causes vapours of all the components in the mixture to be produced

  3. Vapours pass up the fractionating column

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

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

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

  7. Only the most volatile vapour passes into the condenser

  8. 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
• 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


  1. Slight or moderate pressure

  2. High temperature (450°C)

  3. Zeolite catalyst
• Catalytic Cracking


  • 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
• The products of complete combustion are CO2 and H2O
• In excess oxygen alkanes will burn with complete combustion
• Incomplete Combustion


  • Produces less energy per mole than complete combustion
  • If there is a limited amount of oxygen then incomplete combustion occurs, producing CO (which is very toxic) and/or C (producing a sooty flame)
• 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 will dissolve in atmospheric water and 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 dioxide


  Contributes towards global warming
• Unburnt hydrocarbons


  Not all fuel burns in the engine, 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
• Nitrogen oxides form from the reaction between N2 and O2 inside the car engine
• Mechanism of Greenhouse Effect


  1. UV wavelength radiation passes through the atmosphere to the Earth's surface and heats up Earth's surface

  2. The Earth radiates out infrared long wavelength radiation

  3. The C=O Bonds in CO2 absorb infrared radiation so the IR radiation does not escape from the atmosphere

  4. This energy is transferred to other molecules in the atmosphere by collisions so the atmosphere is warmed
• 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
• Reaction of Alkanes with Bromine/Chlorine in UV Light

  In the presence of UV light alkanes react with chlorine to form a mixture of products with the halogens substituting hydrogen atoms

  
• Mechanism of Halogenation Reaction


  • Free radical substitution
  • Initiation step: Cl2 splits into Cl. radicals
  • Propagation step: Cl. removes H from alkane to form alkyl radical, which then reacts with Cl2
  • Termination step: Collision of two radicals
• Mechanism of Halogenation Reaction: Initiation

  Cl2 splits into Cl. radicals in the presence of UV light

  
• Mechanism of Halogenation Reaction: Propagation


  Cl. removes H from alkane to form alkyl radical, which then reacts with Cl2
• Mechanism of Halogenation Reaction: Termination


  Collision of two radicals