More about fractions

Properties of fractions

Each crude oil fraction contains a mixture of hydrocarbons. The hydrocarbons in a fraction are mostly hydrocarbons called alkanes. The alkanes in each fraction have similar (but not identical):

For example, the gases fraction contains hydrocarbons with one to four carbon atoms. These have boiling points below room temperature. They are very flammable (easily set on fire) and have a low viscosity (because they are gases).

The hydrocarbons in different fractions differ in these properties. For example, the bitumen fraction contains hydrocarbons with more than 35 carbon atoms. These have boiling points well above room temperature. They are very difficult to ignite and have a high viscosity (they flow with difficulty).

Alkanes as a homologous series

A homologous series is series of compounds which:

The table shows four alkanes, their molecular formulae and their structures.

Table showing four alkanes, their molecule formulae and their structures

The general formula for the alkanes is CnH2n+2 (where n stands for the number of carbon atoms). As the number of carbon atoms increases:

  • the intermolecular forces between alkane molecules become stronger
  • more energy is needed to overcome these forces
  • the boiling point increases
The molecules C2H6, C8H18 and C15H32 are all alkanes because they fit the general formula CnH2n+2.

Describe how you can tell from their molecular formulae that ethane, propane and butane are neighbouring compounds in the same homologous series.

Their molecular formulae differ by CH2 from one compound to the next. For example, ethane is C2H6 but propane is C3H8 (and the next one, butane, is C4H10).