Crude oil and hydrocarbons

Hydrocarbons are compounds that contain hydrogen and carbon atoms only.

Crude oil

Crude oil is a finite resource that is found in the Earth’s crust. It is the remains of organisms that lived and died millions of years ago - mainly plankton which was buried in mud.

Crude oil is a complex mixture of hydrocarbons. The carbon atoms in these molecules are joined together in chains and rings. In the ball and stick models below, carbon atoms are black and hydrogen atoms are white.

Crude oil is an important source of:

• fuels such as petrol, diesel, kerosene, heavy fuel oil and liquefied petroleum gases
• feedstock for the petrochemical industry

A feedstock is a raw material used to provide reactants for an industrial reaction. A petrochemical is a substance made from crude oil using chemical reactions. For example, ethene is produced from crude oil. It is used as feedstock to make poly(ethene), a polymer.

Other useful substances made from compounds found in crude oil include:

• solvents
• lubricants
• detergents

The alkanes form a homologous series. Like all homologous series, the alkanes:

• have the same general formula
• differ by CH₂ in molecular formulae from neighbouring compounds
• show a gradual variation in physical properties, such as their boiling points
• have similar chemical properties

General formula

The general formula for the alkanes is C_nH_2n+2, where n is the number of carbon atoms in the molecule.

Example

Decane is an alkane. Its molecules contain 10 carbon atoms. Predict the molecular formula of decane and explain your answer.

The formula will be C₁₀H₂₂. This is because n = 10. So, 2n + 2 = (2 × 10) + 2 = 20 + 2 = 22.

The ball and stick models on the right show that the covalent bonds are not really at angles of 90°, as shown in the structures.

The alkanes are saturated hydrocarbons:

• hydrocarbons, because they are compounds containing hydrogen and carbon only
• saturated, because their carbon atoms are joined by C-C single bonds