fuels

• hydrocarbons are compounds containing carbon hydrogen
• crude oil 


  • a complete mixture of hydrocarbons
  • contains rings/chains of carbon atoms
  • used in the industry
  • a finite resource
• crude oil is a mixture of different hydrocarbons, different hydrocarbons have different boiling points (longer chain hydrocarbons have higher boiling points)
• • the column is hotter at the bottom and coldest at the top
  • the vapour rise through the column and cools down
  • vapour condenses when they reach a part of column cool enough (below their melting point)
  • liquid falls into a tray and is piped away
  • vapours with the lowest boiling points do not condense at all and leave at the top as a mixture of gases
  • bitumen has the highest boiling point and leaves at the bottom as a hot liquid
• fractions and their uses

  refinery 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 some power stations

  bitumen - surfacing roads and roofs
• hydrocarbons from different fractions differ due to :

  some properties of hydrocarbons depend on the size of their molecules, these properties influence their use as fuels

  • shorter the molecules, the less viscous it is (more runny) and the longer the molecules, the more viscous it is
  • the shorter the molecules, the lower the temperature at which that fraction evaporates or condenses – and the lower its boiling point
  • the shorter the molecules – the more flammable it is, so the easier it is to ignite
• gases -> bitumen


  • the number of atoms in molecules increases down the fractions, 1-4 carbon atoms in gases compared to >35 in bitumen
  • the boiling point increases down the fractions (lowest in gases, highest in bitumen)
  • ease of ignition decreases down the group, gas is the easiest to ignite compared to bitumen
  • viscosity is lowest at the bottom whereas it flows with difficulty in bitumen
• homologous series 


  • a series of compounds
  • have the same general formula
  • differ by $CH_2$ in molecular formulae from neighbouring compounds
  • show a gradual variation in physical properties, as exemplified by their boiling points
  • have similar chemical properties
• complete combustion of hydrocarbon fuels

  • carbon dioxide and water are produced
  • energy is given out (exothermic)
• incomplete combustion


  • limited supply of oxygen
  • water is produced
  • less energy produces
  • carbon monoxide and carbon are produced
• If there’s not enough oxygen, some of the fuel doesn’t burn – this is partial combustion, here, solid particles of soot (carbons) and unburnt fuel are released, carbon monoxide ($CO$) is also released when there isn’t enough oxygen to produce $CO_2$ instead
• carbon monoxide is a toxic gas, it is colourless and odourless, it combines with haemoglobin in red blood cells, preventing oxygen from combining (decreases amount of oxygen carried round the body) meaning it could eventually lead to death
• carbon monoxide causes health problems and potentially death

  soot causes global warming and it can block pipes carrying waste gases from appliances
• impurities in some hydrocarbons fuels may contain sulphur compounds, most are removed at oil refineries to reduce the environmental problems they cause
• • when hydrocarbon fuel is burnt, sulphur reacts with oxygen to form sulphur dioxide which can fall as acid rain when the gas binds with water
• problems associated with acid rain

  • toxic/really deadly for living organisms
  • increases rate of corrosion of metals
  • soil becomes acidic so crops do not grow well
  • lowers the pH of water in lakes therefore killing fish
• when fuel is burned in engines, fuel is mixed with air and ignited inside the engine which provides a high enough temperature for nitrogen to react with oxygen, they produce oxides of nitrogen (atmospheric pollutants)
• hydrogen as fuel for cars instead of petrol

  advantages

  • environmental benefit as carbon dioxide is a greenhouse gas

  disadvantages

  • expensive
  • difficult to transport and store
  • dangerous as it can be explosive
• petrol, kerosene and diesel oil are non-renewable fossil fuels from crude oil

  methane is a non-renewable fossil fuel found in natural gas
• cracking


  heating of hydrocarbons to vaporise them

  • breaks covalent bonds in hydrocarbon molecules
  • crude oil fractions are heated to evaporate them, the vapours are passed through a catalyst (containing aluminium oxide) and heated to about 650 degrees speeding up the reactions that breaks down larger hydrocarbon molecules into smaller more useful molecules (alkenes)
• cracking is necessary as:

  • demand for smaller chained alkanes is much greater than that for longer chained alkanes
  • shorter chained hydrocarbons ignite more easily and so are more useful as fuels