Crude oil as a finite resource; hydrocarbons and the alkane homologous series; the general formula; and how properties change with chain length.

What this dot point is asking

AQA wants you to describe crude oil as a finite resource made from ancient organisms, define hydrocarbons and the alkane homologous series, give the general formula of alkanes, and describe how properties change as the carbon chain gets longer. These trends underpin the next dot point on fractional distillation, because it is the boiling-point trend that lets the fractions be separated.

Crude oil

Because it forms over millions of years but is being used far faster than it can be replaced, crude oil is non-renewable: it is finite and will eventually run out.

Hydrocarbons and alkanes

The first four alkanes are methane ($CH_4$), ethane ($C_2H_6$), propane ($C_3H_8$) and butane ($C_4H_{10}$). Alkanes are saturated, meaning they contain only single carbon to carbon bonds. A homologous series is a family of compounds that share the same general formula, differ by a $CH_2$ unit from one member to the next, and have similar chemical properties but gradually changing physical properties.

Trends with chain length

These trends arise because longer molecules have stronger intermolecular forces, so more energy is needed to separate them (higher boiling point) and they flow less easily (higher viscosity). They mean shorter hydrocarbons make better fuels because they are more flammable and easily ignited, while longer ones are more viscous and used for things like lubricants and bitumen.

Complete combustion

When a hydrocarbon burns completely in plenty of oxygen, it produces carbon dioxide and water, releasing energy. This is why hydrocarbons are used as fuels. For example, methane burns as $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$. During combustion the carbon and hydrogen in the fuel are oxidised (they gain oxygen), which is why complete combustion is an oxidation reaction.

Why the trends matter for use

The trends with chain length explain how each fraction of crude oil is used. Short-chain hydrocarbons have low boiling points and are very flammable, so they make good fuels that ignite easily, such as the gases used for camping stoves and the petrol used in cars. Long-chain hydrocarbons have high boiling points and high viscosity, so they are used where a thick, hard-to-ignite material is wanted, such as lubricating oils and bitumen for road surfacing. These same property differences (especially the spread of boiling points) are what allow the mixture to be separated by fractional distillation, the subject of the next dot point.

Try this

Q1. State what a hydrocarbon is. [1 mark]

• Cue. A compound made of hydrogen and carbon only.

Q2. Describe how the boiling point of hydrocarbons changes as the carbon chain gets longer. [1 mark]

• Cue. The boiling point increases.

Q3. Write the molecular formula of the alkane with five carbon atoms. [1 mark]

• Cue. $C_5H_{12}$ (using $C_nH_{2n+2}$).