Fossil Fuels (Cambridge (CIE) O Level Chemistry): Revision Note

Common Fossil Fuels

• 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 & Fractional Distillation

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