Fractions in Crude Oil (WJEC GCSE Science (Double Award)): Revision Note

Fractions in Crude Oil

• The petrochemical industry is hugely important for modern society and development

• The fuels that are used in most modern methods of transport (cars, trains, airplanes etc.) are all based on oil products

• Polymers, lubricants, solvents, detergents and adhesives are all products that are obtained from crude oil

• The array of fractions in crude oil and the huge range of compounds we can produce from them all stem from carbon’s ability to form multiple strong covalent bonds with itself leading a huge number of organic compounds

• The main fractions and their uses are described in the following table:

Table to show the uses of fractions from crude oil 

Trends in Properties of Fractions

• Some properties of hydrocarbons depend on the size of their molecules

• These properties include:

  • Viscosity (the ability of the substance to flow)

  • Flammability (how easily the substance burns)

  • Colour

  • Cleanliness of burn 

• These properties influence how hydrocarbons are used as fuels

Boiling Point

• The bigger the hydrocarbon, the higher it's boiling point

• This is because the size of the intermolecular forces increases requiring more energy to overcome them

Graph to show the trend in boiling points of hydrocarbons

As the size of the hydrocarbon increases, the boiling point also increases

Viscosity

• Viscosity refers to the ease of flow of a liquid

  • The opposite of high viscosity is to say a liquid is runny

  • High viscosity liquids are thick and flow less easily

• Viscosity also increases with increasing chain length

  • This is also due to the increased intermolecular forces of attraction as molecular size increases

• Longer chain hydrocarbons would not be as useful as fuels for cars as they would be too thick and clog the engine

• Increased viscosity means that longer chained alkanes are useful as lubricants in machinery as they are less likely to burn and function to reduce friction between moving parts

Flammability

• Molecular size again influences the ease of ignition or flammability of hydrocarbons

• Smaller hydrocarbon molecules are more flammable and are easier to ignite than larger molecules

• This makes them very useful as fuels, releasing large amounts of energy when they burn

Colour

• The colour of the hydrocarbons within each fraction change as the size increases:

  • The smaller fractions formed at the top of the column are colourless

  • The middle fractions range from brown to yellow in colour 

  • The bottom fraction is black 

Cleanliness of Burn 

• Shorter hydrocarbons burn with a cleaner bluer flame

• Larger hydrocarbons burn with a more yellow, smoky flame 

• This is due to there not being enough oxygen to react with all of the atoms in the longer hydrocarbon