HydrocarbonsOxford AQA IGCSE Chemistry: Revision notes

A hydrocarbon is a molecule made up of hydrogen and carbon atoms only. Crude oil is a mixture of a very large number of different hydrocarbon compounds.

• Most of the hydrocarbons in crude oil belong to a family called the alkanes
• Alkanes are saturated hydrocarbons: every carbon-carbon bond is a single covalent bond
• The general formula for an alkane is CnH2n+2, where n is the number of carbon atoms

Alkanes can be shown in molecular, displayed or structural form. The first three alkanes are:

• Methane, CH4 (1 carbon)
• Ethane, C2H6 (2 carbons)
• Propane, C3H8 (3 carbons)

In a displayed formula, every bond is drawn out as a line, and every carbon has four single bonds (to hydrogen or to other carbons).

As the number of carbon atoms in a hydrocarbon chain increases:

• Boiling point increases - larger molecules have stronger intermolecular forces to overcome, so more energy (a higher temperature) is needed to separate them
• Viscosity increases - longer molecules tangle together more, making the liquid thicker and flow less easily
• Flammability decreases - larger molecules are harder to vaporise and ignite, so they burn less easily

Crude oil itself is not very useful, but it can be separated into simpler, more useful mixtures called fractions by fractional distillation.

1. Crude oil is heated until most of it evaporates
2. The vapours rise up a fractionating column, which is hotter at the bottom and cooler at the top
3. Different fractions condense and are collected at different heights, according to their boiling points
4. Fractions containing shorter hydrocarbon chains (lower boiling points) are collected nearer the top; fractions with longer chains (higher boiling points) are collected nearer the bottom