Crude Oil, Hydrocarbons and Alkanes — AQA GCSE Chemistry
Crude oil is the source of most fuels and the raw material for the petrochemical industry.
Crude oil
Crude oil is a finite resource formed over millions of years from the remains of ancient organisms (mainly plankton). It is a mixture of a very large number of compounds, most of which are hydrocarbons.
A hydrocarbon is a compound made of hydrogen and carbon only.
Alkanes
Most of the hydrocarbons in crude oil are alkanes — a homologous series (a family of compounds with the same general formula and gradually changing properties).
- General formula: CₙH₂ₙ₊₂.
- The first four: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀).
- Alkanes are saturated — they contain only single carbon–carbon bonds.
Trends down the series
As the molecules get larger (more carbon atoms):
- boiling point increases,
- they become more viscous (thicker), and
- they are less flammable (harder to ignite).
These trends are due to stronger intermolecular forces in larger molecules.
Exam tips
- Define a hydrocarbon as hydrogen and carbon only.
- Learn the alkane general formula CₙH₂ₙ₊₂ and the first four names.
- Alkanes are saturated (single bonds only).
- Learn the trends in boiling point, viscosity and flammability with size.