Alkanes

• C and H have very similar electronegativity, so the bonds are non-polar. This means all alkane molecules will also be non-polar. As a result, neighbouring molecules will be attracted to each other by weak induced dipole-dipole forces.
• Water molecules are held together by hydrogen bonds. These are much stronger than the weak induced dipole-dipole forces between alkane molecules. As a result, alkanes are not soluble in water.
• Why is the boiling point of octane greater than of butane?

  As the length of the carbon chain increases the boiling point of the alkane increases. This is beacuase the molecules are getting larger and there are more electrons in the molecule. As a result the increased number of induced dipole-dipole forces between the molecules get stronger and require more energy to be broken.
• As the number of branches in a molecule increases the boiling point decreases. This is as there are fewer points of contact between the molecules/ the surface area of the molecules decreases so the induced dipole-dipole forces between the molecules get weaker thus require less energy to be broken.
• Fractional Distillation

  • Crude oil is vapourised and introduce near the bottom of the fractionating column.
  • Vapours rise up the column.
  • there is a temperature gradient in the column, so i is cooler at the top of the column than at the bottom.
  • Hydrocarbons have different boiling points as they condense at different levels and are collected.
  • The boiling point depends on the size of the hydrocarbon.
  • Hydrocarbons with low boiling points do not condense and are collected as gases at the top.
  • Hydrocarbons with higher boiling points are collected as thick residue at the bottom of the column.
• Thermal Cracking

  • Temperature: 400C to 900C
  • Pressure: High
  • Catalyst: None
  • Mechanism: Free Radical
  • Products: High proportion of alkenes
• Catalytic Cracking

  • Temperature: 450C
  • Pressure: Slightly above 100kPa
  • Catalyst: Zeolites
  • Mechanism: Carbocation Intermediate
  • Products: Motor fuels and aromatic hydrocarbons.
• The complete combustion of alkanes produces carbon dioxide and water.
• The incomplete combustion of alkanes occurs when there is a limited supply of oxygen and the products are carbon monoxide and water.
• When there is a very limited supply of oxygen, the products of combustion are carbon ( soot ) and water.
• The carbon dioxide formed contributes to global warming as carbon dioxide is a greenhouse gas.
• Oxides of nitrogen are formed when the nitrogen and oxygen in the air react. These contribute to acid rain and photochemical smog.
• Sulphur dioxide is produced whilst burning coal and can be neutralised by coating the chimneys by calcium oxide or calcium carbonate.
• What is the condition needed for Free Radical Substitution?

  UV Light.
• Why is UV Light needed in Free Radical Substitution?

  The UV Light provides energy to break the bond between the halogens, this energy is greater than the activation energy.
• During the conversion of Ethane to Bromoethane, what is a termination step, that forms a product with the empirical formula of C2H5?

  .CH2CH3 + .CH2CH3 -> C4H10