Alkanes
Crude oil is a mixture of compounds called hydrocarbons. Many useful materials can be
produced from crude oil. It can be separated into different fractions using fractional
distillation, and some of these can be used as fuels.
Crude oil forms naturally over millions of years from the remains of living things. Most of the
compounds in crude oil are hydrocarbons. These are compounds that contain hydrogen and carbon
atoms only, joined together by chemical bonds called covalent bonds. There are different types of
hydrocarbon, but most of the ones in crude oil are alkanes.
The alkanes are a family of hydrocarbons that share the same general formula. This is:
CnH2n+2
The general formula means that the number of hydrogen atoms in an alkane is double the number of
carbon atoms, plus two. For example, methane is CH4 and ethane is C2H6.
Alkane molecules can be represented by displayed formulas. In a displayed formula, each atom is
shown as its symbol (C or H) and each covalent bond by a straight line. This table shows four different
alkanes.
Structure of alkanes
Notice that the molecular models show that the bonds are not really at 90°C, but this makes displayed
formulas easier to draw.
Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to
each other by single bonds. This makes them relatively unreactive, apart from burning
or combustion, which is their reaction with oxygen in the air.
Distillation is a process that can be used to separate a pure liquid from a mixture of liquids. It works
when the liquids have different boiling points. Distillation is commonly used to separate ethanol (the
alcohol in alcoholic drinks) from water.
Distillation process to separate ethanol from water
