Fractional Distillation of Crude Oil (Oxford AQA International A Level Chemistry)

Revision Note

Richard Boole

Written by: Richard Boole

Reviewed by: Stewart Hird

Fractional Distillation of Crude Oil

  • Crude oil, or petroleum, is currently the world's main source of organic chemicals

  • It is a mixture of mostly alkanes

    • Alkanes are saturated hydrocarbons, which means that:

      • They contain only single covalent carbon-carbon bonds

      • They are compounds made from only hydrogen and carbon

    • They can be straight chain or branched chain

  • As a mixture, it isn’t a very useful substance

    • However, the different hydrocarbons that make up the mixture are useful

  • The mixture of alkanes from crude oil is separated into fractions, based on boiling points

    • Each fraction has different uses and consists of groups of hydrocarbons of similar chain lengths

    • Hydrocarbons of similar chain length have similar boiling points

  • The fractions are separated from each other in a process called fractional distillation

  • The size and length of each hydrocarbon molecule determines the fraction an alkane will be separated into

  • The size of each molecule is directly related to how many carbon and hydrogen atoms the molecule contains

The fractional distillation of crude oil

Diagram of a fractionating column
Fractional distillation separates crude oil into fractions by 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

  • Crude oil contains small amounts of other compounds

    • Some of these compounds may contain sulfur, and when burned, sulfur dioxide can be produced

      • This is one of the causes of acid rain

Examiner Tips and Tricks

Fractional distillation is a physical process, not a chemical one, so no covalent bonds are broken. The intermolecular forces between molecules are broken as the crude oil is heated. You do not need to memorise the fractions, you just need to understand the principle of what is happening during the process, and why it is used.

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Richard Boole

Author: Richard Boole

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.