1.1 Crude oil is a mixture of a very
large number of compounds.
1.1.1 A mixture consists of two or more elements or compounds not
chemically combined together. The chemical properties of each
substance in the mixture are unchanged. It is possible to
separate the substances in a mixture by physical methods
184.108.40.206 Most of the compounds in crude oil consist of molecules made up of hydrogen and
carbon atoms only (hydrocarbons). Most of these are saturated hydrocarbons called
alkanes, which have the general formula CnH2n+2.
220.127.116.11.1 Saturated hydrocarbons have only single bonds. This
makes them relatively unreactive, apart from burning or
combustion, which is their reaction with oxygen in the air.
2 C1.4.2 Hydrocarbons
2.1 The many hydrocarbons in crude oil
may be separated into fractions, each
of which contains molecules with a
similar number of carbon atoms.
2.1.1 The crude oil is evaporated and its vapours condense at different temperatures in the
fractionating column. The column is hot at the bottom and cool at the top; substances
with high boiling points condense at the bottom and substances with lower boiling
points condense on the way to the top. This is known as fractional distillation.
18.104.22.168 Properties of
depend on the size
of their molecules;
used as fuels.
22.214.171.124.1 Smaller molecules have lower
boiling points, lower viscosity (flow
more easily) and higher flammability
(ignite more easily).
3 C1.4.3 Hydrocarbon Fuels
3.1 Most fuels, including coal, contain carbon
and/or hydrogen and may also contain some
sulfur. Fuels burn when they react with oxygen in
the air. If there is plenty of air, complete
combustion happens. Coal is mostly carbon.
During complete combustion, carbon is
oxidised. The gases released into the
atmosphere when a fuel burns may include
carbon dioxide and water vapour.
3.1.1 If there is insufficient air, incomplete combustion happens. Hydrogen is still oxidised to
water, but carbon monoxide forms instead of carbon dioxide; a toxic gas, so adequate
ventilation is important when burning fuels. Solid particles (particulates) are also
released. These contain carbon and are seen as soot or smoke. Particulates cause
global dimming; reducing the amount of sunlight reaching the Earth’s surface.
126.96.36.199 The combustion of hydrocarbon fuels releases
energy. During combustion the carbon and
hydrogen in the fuels are oxidised.
188.8.131.52.1 Sulfur dioxide and oxides of nitrogen cause acid
rain, carbon dioxide causes global warming, and
solid particles cause global dimming.
184.108.40.206.1.1 Sulfur can be removed from fuels
before they are burned; in vehicles.
Sulfur dioxide can be removed from
the waste gases after combustion;
in power stations.
220.127.116.11.1.1.1 Biofuels, including biodiesel and ethanol, are
produced from plant material. There are economic,
ethical and environmental issues surrounding their
use: agricultural land is used which may cause food
shortages or increases in the price of food; more
people are needed to produce biofuels than petrol
and diesel; increased income for farmers; lower fuel
prices; biodiesel naturally contains little sulfur and
they may be said to be carbon neutral however,
while biofuels produce less carbon dioxide, overall,
they are not carbon neutral - because fossil fuels
are used in their production, for example in making
fertilisers for the growing plants.