The smallest of the hydrocarbons, methane (CH4)
through butane (C4H10),
are gases, which are familiar as industrial, cooking, and heating
Methane also is known as marsh gas because some bacteria in swamps
can oxidize hydrogen (produced from decomposing matter by other
bacteria), using C02 rather than
02, to yield methane and water -
4H2 + CO2
---> CH4 + 2H20
The flickering fire of ignited marsh gas in a swamp has been the
origin of any number of good tales of ghosts and the supernatural.
The larger the hydrocarbon molecule, the stronger are the van der
Waals forces between molecules, and the higher the temperature needed
to melt the solid or vaporize the liquid. Pentane (C5H12)
through heptadecane (C17H36)
are liquids at room temperature, and octadecane (C18H38)
and larger molecules are waxy solids (facing page.)
Polyethylene plastic, which is familiar as a tough, inert but flexible
material for laboratory and kitchenware, is a straight-chain hydrocarbon
with 5000 to 50,000 carbon atoms per chain.
Polyethylene is tough because the long molecules are entwined around
one another and are difficult to unwind and separate.
We use vast quantities of simple hydrocarbons as fuels and lubricants.
Natural gas is 85% methane.
Crude petroleum is a mixture of hydrocarbons, which usually are
separated by distillation. The fraction with 5 to 10 carbons is
sold as gasoline, and kerosene has 10 to 18 carbons.
Fuel oils typically have 18 to 22 carbons per molecule, and paraffin
waxes have 20 or more. Crude oil contains all of these compounds,
plus various ring hydrocarbons.
Much of the natural petroleum originally came from the decomposition
of organic matter from once-living organisms, under conditions of
high temperature and pressure beneath the surface of the Earth.
Petroleum thus is a "fossil fuel" like coal is, and the
prospect of soon running out of the supply of fossil fuels is currently
a distressing one for our energy-devouring economy. What has taken
hundreds of millions of years to deposit, we are in danger of depleting
in less than two centuries.