More about fractions

Properties of fractions

Each fraction contains a mixture of hydrocarbons. The hydrocarbons in a fraction are mostly hydrocarbons called alkanes. They have similar but not identical:

For example, the gases fraction contains hydrocarbons with between one and four carbon atoms. These have boiling points below room temperature. They are very flammable.

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.

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 structuresTable showing four alkanes, their molecule formulae and their structures

The general formula for the alkanes is CnH2n+2, with n standing for the number of carbon atoms.

As the number of carbon atoms increases:

Question

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.