Alkanes

• Cracking is a physical process involving the splitting of weak van de waals forces between molecules
• Fractional distillatiton

  • oil is preheated and then passed on into column
  • Fraction condense at different boiling points
  • temperature of the column decrease upwards
  • Boiling point depends on the size of the molecule
  • Larger the molecule the larger the van de waals forces
  • Similar molecules condense together
  • Small molecules condense at the top at lower temperature, and big molecule condenses at the bottom at higher temperature
• Vacuum distillation unit

  • heavy residues from the fractionating column are distilled again under a vacuum
  • lowering the pressure over a liquid will lower boiling point
• Vacuum distillation allows heavier fractions to be further separated without high temperatures which could break them down
• Fractional distillation in the lab

  • Heat the flask, with a Bunsen burner or electric mantle
  • causes the vapours of all the components in the mixture to be produced
  • vapours with lower temp reaches the top of the fractionating column first
  • Thermometer should be at or below the BP of the most volatile su
  • Vapours with higher BP condense back into the flask
  • only the most volatile vapour passes into the condenser
  • Condenser cools the vapour and condenses to a liquid and collected
• Cracking: Conversion of large hydrocarbons into smaller hydrocarbons molecules by breaking C-C bonds
• economic reasons for cracking

  • Petroleum fraction with shorter C chains are more in demand than larger
  • To make use of excess larger hydrocarbons and to supply demand for shorter ones, longer ones are cracked
  • Product of cracking are more valuable than the starting materials
• Cracking is a chemical process involving the splitting of strong covalent bonds so require high temperature
• Thermal cracking conditions 


  High pressure (7000kPa)

  High temperature (400-900C)
• Thermal cracking


  high pressure and temp, produce mostly alkenes, H2 gas
• Catalytic cracking


  high temp, slight pressure zeolite catalysts

  produce branched cyclic alkanes and aromatic hydrocarbons
• why do free radicals cause depletion of ozone?

  Cl' is regenerated, causes chain reaction to continue