AlkanesAQA GCSE Chemistry: Revision notes

Alkanes are the main type of hydrocarbon found in crude oil. They are saturated hydrocarbons (all carbon-carbon bonds are single bonds), with the general formula:

CₙH₂ₙ₊₂

The first four members of the alkane family are:

Crude oil (a mixture of mostly alkanes) is separated into fractions by fractional distillation, based on differences in boiling point between the different-sized alkane molecules.

Small alkane fractions (like those found in petrol) have low boiling points and flow easily, while large alkane fractions (like heavy fuel oil) have high boiling points and are thick and viscous.

As the number of carbon atoms in an alkane molecule increases:

• Boiling point increases — larger molecules have stronger intermolecular forces to overcome
• Viscosity increases — the liquid becomes thicker and flows less easily
• Flammability decreases — larger molecules are harder to ignite

This is why short-chain alkanes (like propane and butane) make good portable gas fuels, while long-chain alkanes are used in thick, high-boiling-point products.

Alkanes undergo complete combustion in a plentiful supply of oxygen, releasing energy and producing carbon dioxide and water as the only products:

alkane + oxygen → carbon dioxide + water

For example:

CH₄ + 2O₂ → CO₂ + 2H₂O (methane)

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O (propane)

This combustion reaction is why alkane fractions from crude oil are widely used as fuels.

Long-chain alkane molecules produced from crude oil are often less useful as fuels than shorter, more flammable ones. Cracking is used to break large alkane molecules down into smaller, more useful molecules, including smaller alkanes and reactive alkenes — this is covered in more detail in the cracking topic, but it is one reason why alkanes from crude oil are chemically processed rather than simply used as extracted.