hydrocarbons

• crude oil is a finite resource found in rocks

  it is the remains of an ancient biomass consisting mainly of plankton that was buried in mud
• crude oil is a mixture of hydrocarbons, which are molecules made up of hydrogen and carbon atoms only
• the hydrocarbons found in crude oil are called alkanes

  general formula = CnH2n+2

  they only have single covalent bonds between the carbon atoms
• first 4 alkanes:

  • methane (CH4)
  • ethane (C2H6)
  • propane (C3H8)
  • butane (C4H10)
• each carbon atom has 4 bonds

  in alkanes, there are only single bonds
• alkanes are saturated molecules, as the carbon atoms are fully bonded to hydrogen atoms
• as the size of hydrocarbons increase:

  • viscosity increases
  • boiling point increases
  • volatility decreases
  • flammability decreases, hard to burn
• short chain hydrocarbons are highly flammable and have low bp vs longer which are less flammable and higher bp and higher viscosity
• what run on hydrocarbon fuels:

  • ships
  • planes
  • cars
• the combustion of hydrocarbon fuels releases energy
• during combustion on hydrocarbon fuels, the carbon and hydrogen in the fuels are oxidised
• the complete combustion (when oxygen is unlimited) of hydrocarbon produces carbon dioxide and water
• in order for the hydrocarbons in crude oils to be useful, we have to separate them - fractional distillation which separate them into fractions: these contain hydrocarbons with a similar number of carbon atoms
• 1. crude oil is heated to a very high temp boil, evaporation (to gas)
  2. the vapour is put into a fractional distillation column: hotter at bottom, cooler at top
  3. vapour rises up the column and condense when they reach their bp (liquid) --> removed
  4. remaining hydrocarbons move up the column and again condense when they reach their bp
• points for fractional distillation of crude oil:

  • very long chain hydrocarbons have very high bp
  • so are removed from the BOTTOM of the column
  • very short chain hydrocarbons have very low bp
  • don't condense, so remove at TOP of column, as gases
• fractions contain hydrocarbons with a similar number of carbon atoms
• uses of fractions:

  • fuels (petrol, diesel, kerosene)
  • feedstock for petrochemical industry (solvents, lubricants, detergents, polymers)