CIE IGCSE Chemistry

Revision Notes

14.1.2 Fuels

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
  • 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)

 

Crude-oil-under-the-sea, IGCSE & GCSE Chemistry revision notesDiagram 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 increases as the carbon chain gets longer

Process of fractional distillation

Fractional-Distillation, IGCSE & GCSE Chemistry revision notesDiagram showing the process of fractional distillation to separate crude oil in a fractionating column

 

Explanation

  • Fractional distillation is carried out in a fractionating column
  • The fractionating column is hot at the bottom and cool 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 the main fractions of crude oil

  • 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. So 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 the different fractions obtained from petroleum (crude oil)

  • 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

Trend in boiling point of the main fractions

Trend in boiling point of the main fractions table, IGCSE & GCSE Chemistry revision notes

Exam Tip

Some fractions may have different names in the UK and the USA. Names above in brackets are the UK while those underlined are the names used in the USA e.g. gasoline is the name used in the USA for petrol.

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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