Fossil Fuels (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

• 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 petroleum
• Methane, CH₄, is the main constituent of natural gas and is a hydrocarbon 
• Hydrocarbons are made from hydrogen and carbon atoms only

Petroleum

• Petroleum is also called crude oil and is a 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) 

The location of crude oil 

Crude oil is located 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

Fractional distillation

• Each fraction consists of groups of hydrocarbons of similar chain lengths
• 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 size of each molecule is directly related to how many carbon and hydrogen atoms the molecule contains
• Most fractions contain mainly alkanes, which are compounds of carbon and hydrogen with only single bonds between them

Diagram to show fractional distillation

The process separates the fractions according to their boiling point

• Fractional distillation is carried out in a fractionating column which has a temperature gradient 
  • It is very hot at the bottom of the column and cooler at the top
• During the process of fractional distillation:
  • The crude oil is heated and vapourises
  • The vapours of hydrocarbons enter the column which has a temperature gradient
  • The vapours of hydrocarbons with high boiling points condense at the bottom of the column
  • The vapours of hydrocarbons with lower boiling points rise up the column and condense at the top 

Properties of fractions

Extended tier only

• Melting point/boiling point
  • As the molecules get larger, the intermolecular attraction becomes greater.
  • More heat is needed to separate the molecules.
  • Going down the column, the boiling points of the fractions increases
• Chain length
  
  • As you go down the column, the chain length of the hydrocarbons increases 

Uses of fractions

• Refinery gas: heating and cooking
• Gasoline: fuel for cars (petrol)
• Kerosene: for making jet fuel (paraffin)
• Diesel: fuel for diesel engines (gas oil)
• Bitumen: for surfacing roads

Table to show the trends in properties

Fraction	Number of carbon atoms	Boiling point range / oC
Refinery gas	1-4	Below 25
Gasoline / petrol	4-12	40-100
Naptha	7-14	90-150
Kerosene / paraffin	12-16	150-240
Diesel / gas oil	14-18	220-300
Fuel oil	19-25	250-320
Lubricating oil	20-40	300-350
Bitumen	More than 70	More than 350