Topic 8: Fuels and Earth science

• Hydrocarbons


  Compounds that contain carbon and hydrogen only
• Crude oil


  • A complex mixture of hydrocarbons
  • Containing molecules in which carbon atoms are in chains or rings
  • An important source of useful substances (fuels and feedstock for the petrochemical industry)
  • A finite resource
• Fractional distillation of crude oil

  1. Crude oil is a mixture of different hydrocarbons with different boiling points (longer chain hydrocarbons have higher boiling points)

  2. The crude oil is heated in the fractionating column and the oil evaporates and condenses at a number of different temperatures

  3. The fractionating column works continuously, heated crude oil is piped in at the bottom, the vaporised oil rises up the column and the various fractions are constantly tapped off at the different levels where they condense

  4. The fractions can be processed to produce fuels and feedstock for the petrochemical industry
• Fractions from crude oil

  • Gases (domestic heating and cooking)
  • Petrol (fuel for cars)
  • Kerosene (fuel for aircraft)
  • Diesel oil (fuel for some cars and trains)
  • Fuel oil (fuel for large ships and in some power stations)
  • Bitumen (surface roads and roofs)
• Hydrocarbons in different fractions

  • Differ in the number of carbon and hydrogen atoms their molecules contain
  • Differ in boiling points
  • Differ in ease of ignition
  • Differ in viscosity
  • Mostly members of the alkane homologous series
• Homologous series


  A series of compounds which have the same general formula, differ by CH2 in molecular formulae from neighbouring compounds, show a gradual variation in physical properties, and have similar chemical properties
• Complete combustion of hydrocarbon fuels

  1. CO2 and H2O are produced

  2. Energy is given out (exothermic)
• Incomplete combustion of hydrocarbons

  1. If there's not enough oxygen, some of the fuel doesn't burn - this is partial combustion, solid particles of soot (carbons) and unburnt fuel are released

  2. Carbon monoxide (CO) is also released when there isn't enough oxygen to produce CO2 instead
• Carbon monoxide


  A colourless and odourless gas which, if breathed in, prevents red blood cells carrying oxygen around your body, which leads to death
• Problems caused by incomplete combustion producing carbon monoxide and soot in appliances that use carbon compounds as fuels
• Production of sulfur dioxide from impurities in hydrocarbon fuels

  1. Most fuels, including coal, contain carbon and/or hydrogen and may also contain some sulfur

  2. When the fuels are burnt in oxygen, this sulfur can react to form sulfur dioxide
• Problems associated with acid rain caused when sulfur dioxide dissolves in rain water
• Production of oxides of nitrogen when fuels are burned in engines

  1. Nitrogen and oxygen from the air combine to produce nitrogen monoxide

  2. When this nitrogen monoxide is released from vehicle exhaust systems, it combines with oxygen in the air to form nitrogen dioxide

  3. Nitrogen monoxide and nitrogen dioxide are pollutants
• Advantages of using hydrogen rather than petrol as a fuel in cars

  • Use of hydrogen - petrol is from crude oil, a finite resource
  • Only produces water - no CO2 produced which contributes to global warming
• Disadvantages of using hydrogen rather than petrol as a fuel in cars

  • Expensive
  • Difficult to transport and store hydrogen
  • Dangerous - hydrogen can be explosive
• Petrol, kerosene and diesel oil are non-renewable fossil fuels obtained from crude oil, and methane is a non-renewable fossil fuel found in natural gas
• Cracking of hydrocarbons


  1. Involves the breaking down of larger, saturated hydrocarbon molecules (alkanes) into smaller, more useful ones, some of which are unsaturated (alkenes)

  2. Hydrocarbons can be cracked by heating them to vaporise them, then either passing the vapours over a hot catalyst or mixing them with steam and heating to a very high temperature so that thermal decomposition reactions can occur

  3. The products of cracking include alkanes and unsaturated hydrocarbons called alkenes

  4. Alkenes have the general formula CnH2n and the first 2 alkenes are ethene and propene, they are unsaturated because they have a double bond
• Cracking is necessary because the demand for smaller chained alkanes is much greater than that for longer chained alkanes, and shorter chained hydrocarbons ignite more easily and so are more useful as fuels