Fossil Fuels (Cambridge O Level Chemistry)

• A fuel is a substance which when burned, releases heat energy
• This heat can be transferred into electricity, which we use in our daily lives
• Most common fossil fuels include coal, natural gas and hydrocarbons such as methane and propane which are obtained from crude oil
• Hydrocarbons are made from hydrogen and carbon atoms only
• The main constituent of natural gas is methane, CH₄

Petroleum

• Petroleum is also called crude oil and is a complex mixture of hydrocarbons which also contains natural gas
• It is a thick, sticky, black liquid that is found under porous rock (under the ground and under the sea) 

Diagram showing crude oil under the sea

 

• Petroleum itself as a mixture isn't very useful but each component part of the mixture, called a fraction, is useful and each fraction has different applications
• The fractions in petroleum are separated from each other in a process called fractional distillation
• The molecules in each fraction have similar properties and boiling points, which depend on the number of carbon atoms in the chain
• The boiling point and viscosity of each fraction increase as the carbon chain gets longer

Fractional Distillation

Diagram showing the process of fractional distillation

• Fractional distillation is carried out in a fractionating column
• The fractionating column is hot at the bottom and cools at the top
• Crude oil enters the fractionating column and is heated so vapours rise
• Vapours of hydrocarbons with very high boiling points will immediately turn into liquid and are tapped off at the bottom of the column
• Vapours of hydrocarbons with low boiling points will rise up the column and condense at the top to be tapped off
• The different fractions condense at different heights according to their boiling points and are tapped off as liquids.
• The fractions containing smaller hydrocarbons are collected at the top of the fractionating column as gases
• The fractions containing bigger hydrocarbons are collected at the lower sections of the fractionating column

Properties of Fractions

• Viscosity

  • This refers to the ease of flow of a liquid.
  • High viscosity liquids are thick and flow less easily.
  • If the number of carbon atoms increases, the attraction between the hydrocarbon molecules also increases which results in the liquid becoming more viscous with the increasing length of the hydrocarbon chain.
  • The liquid flows less easily with increasing molecular mass

• Colour

  • As carbon chain length increases the colour of the liquid gets darker as it gets thicker and more viscous

• Melting point/boiling point

  • As the molecules get larger, the intermolecular attraction becomes greater.
  • More heat is needed to separate the molecules.
  • With increasing molecular size there is an increase in boiling point

• Volatility

  • Volatility refers to the tendency of a substance to vaporise.
  • With increasing molecular size hydrocarbon liquids become less volatile.
  • This is because the attraction between the molecules increases with increasing molecular size

Uses of Fractions

• Refinery gas: heating and cooking
• Gasoline: fuel for cars (petrol)
• Naphtha: raw product for producing chemicals
• Kerosene: for making jet fuel (paraffin)
• Diesel: fuel for diesel engines (gas oil)
• Fuel oil: fuel for ships and for home heating
• Lubricating oil: for lubricants, polishes, waxes
• Bitumen: for surfacing roads

Trends in Properties