Homologous Series: Alkanes – A Level Chemistry Revision

Beyond Science returns with this A Level blog on Alkanes from the Homologous Series. Get ahead with KS5 revision today. Here, we’ll cover the different chemical properties and reactions of alkanes. We will also look at how they are obtained through fractional distillation of petroleum and how they are used as fuels.

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Alkanes are saturated hydrocarbons. This means that they contain carbon and hydrogen atoms joined by single covalent bonds. Alkanes can be straight chains, branched or cyclic.

Functional GroupGeneral Formula
saturated hydrocarbons

Straight Chain Alkanes


Chemical Properties

Alkanes are relatively unreactive.

Reactions of alkanes include:

  • Complete Combustion
    Small alkane molecules are particularly useful as fuels because they combust readily, releasing large amounts of energy.
    alkane + oxygen → carbon dioxide + water
  • Incomplete Combustion
    This occurs when there is insufficient oxygen for complete combustion. This produces carbon particulates and carbon monoxide instead of, or as well as, carbon dioxide.
    alkane + limited oxygen → carbon monoxide + carbon dioxide + carbon + water
  • Chlorination
    Alkanes react with chlorine, Cl2, in the presence of UV light to produce chloroalkanes and a mixture of other products, in a free-radical substitution reaction.

Other information

  • Alkanes are often used as fuels.
  • Alkanes are mainly obtained through the fractional distillation of petroleum.
  • Large alkane molecules can undergo catalytic cracking or thermal cracking to produce more useful, smaller alkane molecules and alkene molecules.
  • When hydrocarbon fuels are burnt in car engines, they produce nitrogen oxides (NOx), carbon monoxide (CO), carbon particulates and unburned hydrocarbons. These must be removed from exhaust fumes by catalytic converters.
  • The combustion of hydrocarbon fuels containing sulfur releases the harmful pollutant sulfur dioxide (SO2), which causes acid rain.

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