AQA GCSE Chemistry

Revision Notes

7.1.1 Crude Oil, Hydrocarbons & Alkanes

Organic Chemistry

  • Organic chemistry is the chemistry of carbon compounds
  • Carbon forms a vast number of compounds because it can form strong covalent bonds with itself
  • This enables it to form long chains of carbon atoms, and hence an almost infinite variety of carbon compounds are known
  • Carbon always forms four covalent bonds which can be single, double or triple bonds
  • A functional group is a specific atom or group of atoms which confer certain physical and chemical properties onto the molecule
  • Organic molecules are classified by the dominant functional group on the molecule
    • Organic compounds with the same functional group, but a different number of carbon atoms, are said to belong to the same homologous series
    • Every time a carbon atom is added to the chain, two hydrogen atoms are also added
  • Organic compounds can be represented in a number of ways

Types of Formulae:

  • General Formulae
    • This type of formula tells you the composition of any member of a whole homologous series of organic compound
    • For example, all of the alkanes have the general formula CnH2n+2
    • This tells you that however many carbon atoms there are in the alkane, doubling this number and adding two will give you the number of hydrogen atoms present in the alkane
  • Displayed Formulae
    • This shows the spatial arrangement of all the atoms and bonds in a molecule
  • Molecular Formulae
    • This shows the actual number of each atom in a molecule, one molecule at a time

The displayed formulae and molecular formulae of butane and butene

  • Structural Formulae
    • This gives enough information to make the structure clear, but most of the actual covalent bonds are omitted
    • Only important bonds are shown, such as double and triple bonds
    • Identical groups can be bracketed together

The displayed formula followed by the structural formula of pentane 

Representing structural formulae of two alkenes

Homologous Series

  • Things we can say about a homologous series:
    • each member has the same functional group
    • each member has the same general formula
    • each member has similar chemical properties
    • each subsequent member differs by  -CH2
    • members have gradually changing physical properties, for example, boiling point, melting point and density
  • As a homologous series is ascended, the size of the molecule increases
  • This has an effect on the physical properties, such as boiling point and density

Homologous Series of Alkanes Table

Homologous Series of Alkanes, downloadable IGCSE & GCSE Chemistry revision notes

Composition of Crude Oil

  • Crude oil is a finite resource which we find in the Earth’s crust
  • It is also called petroleum and is a complex mixture of hydrocarbons which also contains natural gas
    • Hydrocarbons are compounds that are made of carbon and hydrogen atoms only
  • The hydrocarbon molecules in crude oil consist of a carbon backbone which can be in a ring or chain, with hydrogen atoms attached to the carbon atoms
  • The mixture contains molecules with many different ring sizes and chain lengths
  • It is a thick, sticky, black liquid that is found in porous rock (under the ground and under the sea)
  • Crude oil formed over millions of years from the effects of high pressures and temperatures on the remains of plants and animals
  • It is being used up much faster than it is being formed, which is why we say crude oil is a finite resource

Crude-oil-under-the-sea, IGCSE & GCSE Chemistry revision notes

Crude oil found under the sea

Exam Tip

Crude oil is also sometimes referred to as petroleum. Some fractions may have different names in the UK and the USA., e.g. gasoline is the name used in the USA for petrol.

You may be asked to give a definition of the term hydrocarbon – be careful! You must say a compound which contains carbon and hydrogen atoms only. If you do not say only, then you will not get the mark.


  • Alkanes are a group of saturated hydrocarbons
    • The term saturated means that they only have single carbon-carbon bonds, there are no double bonds
  • The general formula of the alkanes is CnH2n+2
  • They are colourless compounds which have a gradual change in their physical properties as the number of carbon atoms in the chain increases
  • Alkanes are generally unreactive compounds but they do undergo combustion reactions, can be cracked into smaller molecules and can react with halogens in the presence of light
  • Methane is an alkane and is the major component of natural gas

Alkanes - First 4 members 1, downloadable IGCSE & GCSE Chemistry revision notesAlkanes - First 4 members 1 2, downloadable IGCSE & GCSE Chemistry revision notes

This table shows the first four members of the alkane homologous series

Exam Tip

For your exam, you need to be able to name, draw and give the appropriate formula for the first four alkanes.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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