fuels

• hydrocarbons are compounds that contain carbon and hydrogen only
• crude oil is:

  • a complex mixture of hydrocarbons with chains of differing lengths
  • 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
• crude oil is separated using fractional distillation, because different hydrocarbons have different boiling points depending on the length of their chains.
• describing separating crude oil:

  • crude oil is heated + evaporated in a fractionating column
  • heated crude oil enters near the bottom of a tall fractionating column, which is hot at the bottom and gets cooler towards the top
  • vapours from the oil rise through the column
  • vapours condense when the temperature of the column is cool enough
  • liquids are led out of the column at different heights, depending on their boiling point and hydrocarbon chain length
• diagram of separating crude oil:

  
• hydrocarbons with longer chains stay at the bottom of the fractionating tower as liquids because:

  • they have higher boiling points
  • due to the stronger intermolecular forces between them
  • because in longer chains there are more molecules
  • these forces require more energy to overcome
• hydrocarbons with shorter chains rise to the top of the fractionating column as gases because:

  • they have lower boiling points
  • due to the weaker intermolecular forces between them overall
  • because in shorter chains there are not as many molecules
  • those forces require less energy to break
  • lower temperatures required for them to condense into liquids than the fractionating column has
• • crude oil is separated into different fractions.
  • the fractions are then separated into different products.
  • fractions are the different parts of the original mixture
• uses of the following fractions:

  • gases = used in domestic heating and cooking
  • petrol = used as fuel for cars
  • kerosene = used as fuel for aircraft
  • diesel oil = used as fuel for some cars and trains
  • fuel oil = used as fuel for large ships and in some power stations
  • bitumen = used to surface roads and roofs
• hydrocarbons with more hydrogen and carbon atoms in each molecule have:

  • larger chains
  • higher boiling points
  • more viscous
  • less flammable
  • are mostly members of the alkane homologous series
• hydrocarbons with less hydrogen and carbon atoms in each molecule have:

  • smaller chains
  • lower boiling points
  • less viscous
  • more flammable
  • are mostly members of the alkane homologous series
• a homologous series is 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, as exemplified by their boiling points
  • have similar chemical properties
• the complete combustion of hydrocarbon fuels:

  • produces carbon dioxide and water
  • releases energy
• the incomplete combustion of hydrocarbon fuels:

  • produces carbon (e.g soot)/ carbon-monoxide + water

  because:

  • incomplete combustion means there is limited oxygen so
  • not all the fuel burns (partial combustion)
  • or not enough oxygen to produce carbon-dioxide
  • so carbon-monoxide is produced instead
• carbon monoxide behaves as a toxic gas because:

  • it binds to haemoglobin, replacing oxygen
  • reduces blood’s capacity to carry oxygen
  • can result in death
  • it is colourless and odourless
• problems caused by incomplete combustion:

  • soot is a carbon particulate which can cause breathing problems, blackens buildings, blocked waste-gas pipes
  • carbon-monoxide is a toxic gas that is detrimental to health (reduces body’s supply of oxygen)
• • some hydrocarbon fuels contain sulfur as an impurity
  • when these hydrocarbons are burnt in oxygen, the sulfur oxidises to become sulfur dioxide gas
  • sulfur dioxide + water make sulfurous acid
  • sulfurous acid + oxygen in the air makes sulfuric acid
  • which is acid rain
• problems with acid rain:

  • reacts with metals and rocks such as limestone, weakening and damaging buildings and statues
  • makes rivers and lakes too acidic for some aquatic life to survive
  • damages plants and trees, leaches minerals from soil, making them lose their leaves and die
• High temperatures are reached when fuels are burned in engines. At these high temperatures, nitrogen and oxygen from the air can react together to produce nitrogen monoxide. When nitrogen monoxide is released from vehicle exhaust systems it combines with oxygen in the air to form nitrogen dioxide.
• oxides of nitrogen are atmospheric pollutants:

  • they react in sunlight with other substances to produce a hazy, harmful smog
  • nitrogen monoxide + dioxide forms dilute nitric acid when it dissolves in water in clouds so can cause acid rain
  • nitrogen dioxide can cause respiratory diseases like bronchitis
• advantages of using hydrogen as fuel in cars instead of petrol:

  • use​ ​of​ ​hydrogen​ ​–​ ​petrol​ ​is​ ​from​ ​crude​ ​oil,​ ​a​ ​finite​ ​resource but hydrogen is renewable
  • combustion of hydrogen only​ ​produces​ ​water vapour​ ​–​ ​no​ ​carbon-dioxide​​ ​produced​ ​which​ ​contributes​ ​to​ ​global warming
  • combustion of hydrogen releases more energy than petrol
  • very flammable
• disadvantages of using hydrogen as fuel in cars instead of petrol:

  • expensive
  • difficult​ ​to​ ​transport​ ​and​ ​store​ ​hydrogen since it is a gas at room temperature unlike petrol which is a liquid
  • dangerous​ ​–​ ​hydrogen​ ​can​ ​be​ ​explosive
  • requires a lot of energy to produce
• • petrol, kerosene and diesel oil are non-renewable fossil fuels obtained from crude oil
  • methane is a non-renewable fossil fuel found in natural gas