Topic 8

• Hydrocarbons


  Compounds that contain hydrogen and carbon atoms only
• Crude oil


  • A complex mixture of hydrocarbons
  • Contains molecules with rings or chains of carbon atoms
  • An important source of useful substances like fuels and feedstocks for the petrochemical industry
  • A finite resource
• Crude oil can be found under the sea and ground
• Finite


  Will run out
• Fractional distillation


  1. Crude oil is vaporised before it enters a fractionating column

  2. The fractionating column is hotter at the bottom than at the top. The vapours rise up and condense at different fractions depending on their boiling points

  3. Hydrocarbons with low boiling points will be tapped off the top of the column and hydrocarbons with high boiling points will be tapped off the bottom of the column
• Reason for separating crude oil

  Unseparated crude oil isn't very useful but the separated products (such as petrol) are very useful
• Uses of refinery gas


  • Heating and cooking
• Fuels used in cars

  • Petrol (gasoline)
  • Diesel
• Use of kerosene


  • Aircraft fuel
• Use of diesel oil


  • Fuel for some cars and trains
• Use of bitumen


  • Road surfacing and roofs
• Use of fuel oil


  • Fuel for large ships and in some power stations
• Differences between hydrocarbons in each fraction

  • Boiling points
  • Ease of ignition
  • Viscosity
  • The number of hydrogen and carbon atoms their molecules have
• Viscosity


  How thick and sticky a substance is
• Properties of hydrocarbons tapped from the top of the fractionating column

  • Low boiling point
  • Highly volatile
  • Easily ignited
  • Shorter carbon chains (small molecules)
• Homologous series


  Series of compounds which have the same general formula, have similar chemical properties, differ by CH2 in molecular formula form neighbouring molecules, and have slight variation in physical properties
• Products from crude oil mostly belong to the alkane homologous series
• The only products when a hydrocarbon fuel undergoes complete combustion are water (H2O) and carbon dioxide (CO2)
• Energy is given out when a fuel undergoes complete combustion
• Complete combustion of ethane

  2C2H6 + 7O2 → 4CO2 + 6H2O
• Incomplete combustion


  Occurs when there is an insufficient supply of oxygen
• To ensure complete combustion occurs, the reaction should be carried out with excess oxygen
• Products of incomplete combustion

  • Carbon particulates - soot (C)
  • Carbon monoxide (CO)
  • Water (H2O)
• Incomplete combustion of methane to form carbon monoxide

  2CH4 + 3O2 → 2CO + 4H2O
• Incomplete combustion of methane to form carbon particulates

  CH4 + O2 → C + 2H2O
• Problems with carbon monoxide

  • Carbon monoxide is a toxic gas
  • It is colourless and odourless and if breathed in can cause death by preventing the red blood cells from carrying oxygen around the body
• Problems with incomplete combustion

  • Produces carbon monoxide which is toxic and can be fatal if breathed in
  • Produces carbon particulates (soot) which cause global dimming and respiratory problems
• Some hydrocarbon fuels contain sulfur impurities. When the fuel is burned, the sulfur reacts with oxygen to form sulfur dioxide
• How acid rain is produced

  Sulfur dioxide (produced when combusting impure hydrocarbon fuels) evaporates into the air. It reacts with water in the clouds to form sulfuric acid. This is 'acid rain'
• Problems caused by acid rain

  • Corrodes buildings and statues made of limestone
  • Kills/damages the vegetation
  • Lowers the pH of large bodies of water, killing the wildlife
• How acid rain is produced

  1. Sulfur dioxide (produced when combusting impure hydrocarbon fuels) evaporates into the air

  2. It reacts with water in the clouds to form sulfuric acid

  3. This is 'acid rain'
• Fuel is burned, the sulfur reacts with oxygen to form sulfur dioxide
• Problems caused by acid rain

  • Corrodes buildings and statues made of limestone
  • Kills/damages the vegetation
  • Lowers the pH of large bodies of water, killing the wildlife
• How oxides of nitrogen are produced from car engines


  The high temperature and pressure of a car engine causes nitrogen and oxygen from the air to react together
• Problems associated with oxides of nitrogen

  • Pollutants
  • Produce acid rain with similar effects as SO2
  • Cause respiratory problems
• Advantages of using hydrogen as a fuel in cars

  • It releases more energy per kg compared to most other fuels
  • Water is the only product so no pollutants
  • Renewable source as hydrogen can be extracted from water
• Disadvantages of using hydrogen as a fuel in cars

  • It is expensive to produce and a lot of energy is required for the electrolysis of water to acquire the hydrogen
  • Difficult and dangerous to store hydrogen because it is very volatile and easily ignites
• Methane


  Non-renewable fossil fuel found in natural gas
• Petrol, kerosene and diesel are non-renewable fuels
• Cracking


  • Breaking down large hydrocarbons into smaller more useful ones
  • Saturated alkanes are cracked into shorter chain alkanes and short chain unsaturated alkenes