C6.2k To be able
to explain the separation of crude oil by fractional distillation by size of
molecules and intermolecular forces
Crude oil is a
mixture of hydrocarbons. These
molecules of hydrogen and carbon are mainly linear structures where carbon
atoms are bonded to each other in a “chain”.
These hydrocarbons
contain only single bonds joining the carbon atoms together and are called
alkanes. Alkanes all fit the general
formula CnH2n+2
Here is a list of the first
few members of this series of hydrocarbons:
These hydrocarbons are more
useful to us if they are separated into different groups or fractions. In
a previous post I described this separation.
Why do these
hydrocarbons have different boiling points that allow us to separate the larger
molecules from the smaller molecules?
Here is a
list of some hydrocarbons with their boiling points:
I hope you
can see a pattern to the formulae and boiling points in the above diagram
Larger
hydrocarbon molecules have higher boiling points.
Smaller
hydrocarbon molecules have lower boiling points and you can see that best on this chart of boiling point and number of carbon atoms in the hydrocarbon alkane.
The
explanation for this gradation in boiling points is to do with the weak forces
that exist between the molecules.
Alkane
molecules attract each other by forces that are called Van der Waals forces.
Heating the
hydrocarbon gives the molecules sufficient energy to move from the liquid to
the gaseous state and to do so the molecules separate from each other
overcoming the Van der Waals forces holding them together.
Van der
Waals forces are very weak hence the low boiling points of the simpler
hydrocarbons.
Only when
the molecules start to increase in chain length do the forces become so large
that at room temperature the molecules are in the solid state.
See the
table below for the state of some hydrocarbons.
Finally,
here is a high level explanation and summary giving you a little more than is required from
the GCSE specification:
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