Crude Oil and Fuels

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Crude oil contains many different compounds that boil at different temperatures.  They burn under different conditions and so crude oil needs to be separated first to make useful fuels.  We can separate a mixture of liquids by a process called distillation.  Simple distillation of crude oil can produce liquids that boil within different temperatures.  These liquids are called fractions.

Most of the compounds in crude oil are hydrocarbons.  This means that their molecules only contain hydrogen and carbon.  Many of the hydrocarbons are alkanes, with the general formula CnH2n+2.  Alkanes contain as many hydrogen atoms as possible in each molecule and so we call them saturated hydrocarbons.

We can represent molecules in different ways.  A molecular formula shows the number of each type of atom in each molecule.  eg C2H6 shows a molecule of ethane.  We can represent molecules by a displayed formula that shows how the atoms are bonded together.


Crude oil is separated into fractions at refineries using fractional distillation.   This can be done because the boiling point of a hydrocarbon depends on the size of its molecule.  The larger the molecule, the higher the boiling point of the hydrocarbon.  The crude oil is vaporised and fed into a fractionating column.  This is a tall tower that is hot at the bottom and cooler at the top.  Inside the column there are many trays with holes to allow the gases through.  The vapours move up the column, getting cooler as they go up.  The hydrocarbons condense to liquids when they reach the level that is at their boiling point.  Different liquids collect on the trays at different levels and there are outlets to collect the fractions.  Hydrocarbons with the smallest molecules have the lowest boiling points and so are collected at the top of the column.  The fractions collected at the…


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