Alkanes

Alkanes are a family of compounds made only of carbon and hydrogen. Compounds containing only these two elements are called hydrocarbons. Most hydrocarbons in crude oil are alkanes.

Alkanes form a homologous series: their formulae follow the same general pattern. For an alkane with $$n$$ carbon atoms, the general formula is $$\ce{C_nH_{2n+2}}$$. Each successive alkane has one more carbon atom and two more hydrogen atoms than the one before it.

The first four alkanes

• Methane: $$\ce{CH4}$$
• Ethane: $$\ce{C2H6}$$
• Propane: $$\ce{C3H8}$$
• Butane: $$\ce{C4H10}$$

For example, ethane has two carbon atoms, so $$n = 2$$. The general formula gives $$2(2) + 2 = 6$$ hydrogen atoms, giving $$\ce{C2H6}$$.

Recognising alkanes from formulae

A molecular formula gives the number of each type of atom in a molecule. To check whether it fits the alkane pattern, make sure it contains only carbon and hydrogen, then check whether the numbers fit $$\ce{C_nH_{2n+2}}$$. For example, $$\ce{C3H8}$$ fits the pattern, but $$\ce{C3H6}$$ does not.

A displayed formula shows the atoms and the bonds joining them. In an alkane, carbon atoms are joined in an open chain, which may be branched, and all the bonds are single bonds. Each carbon atom has four bonds in total, while each hydrogen atom has one. The displayed formula of ethane is:

The displayed formula shows two carbon atoms joined by a single bond. Each carbon also bonds to three hydrogen atoms, giving the molecular formula $$\ce{C2H6}$$.

Crude oil

Crude oil is a natural fossil resource found in rocks. It formed from the remains of ancient living material, mainly plankton, which became buried in mud. Over very long periods of time, the buried material changed to form crude oil.

Crude oil is finite: it forms much more slowly than humans use it, so the available supply will eventually run out.

What crude oil contains

Crude oil is not a single substance. It is a mixture containing a very large number of different compounds. Most of these compounds are hydrocarbons: substances made only from hydrogen and carbon atoms. Different hydrocarbons are different compounds, but they all contain only these two elements.