Hydrocarbons are substances that contain
Hydrogen and Carbon only.
Hydrocarbons are divided into a series of
families or groups called Homologous
A Homologous Series is a group of
compound which have a General Formula
and are chemically similar.
Examples of Homologous Series are the:
Alkanes, Alkenes and Cycloalkanes.
The Alkanes have a general formula of CnH2n+2
They run from C1 –C60 approx.
C1-C4 are gases
C5-C17 are liquids
C17-C60 are Waxy Solids
As we progress between successive members of the
series we find that there is a difference of CH2 in
their formulae. As a direct result there is a regular
increase in their Boiling Points since more
energy is required to break up the longer chains.
All Alkanes are flammable, have a pH of 7 and are
insoluble in water.
The Alkane members
Name Molecular Formula Common Uses
Nat.Gas.Household heating &
Portable cooking and Heating
Lighter fuel/Calor Gas
Octane * C8H18
The Alkenes are another Homologues
Series with General Formula CnH2n
All Alkenes must have a C=C double bond.
As a result the first member of the series is
Ethene ie. Methene does not exist.
The Cycloalkanes are another
Homologous series with General Formula
The Cycloalkanes are Ring compounds
which means that the first member of the
series is cyclopropane.
It should be noted that the Cycloalkanes
have the same general formula as the
alkenes i.e. CnH2n but that the Cyloalkanes
are Saturated whereas the Alkenes are
Isomers are molecules with the same
Molecular formula but different structural
Note: There must be the same number of
carbon and hydrogen atoms but there also
must be a completely different arrangement
of Carbon atoms.
Cracking of Crude Oil
Crude Oil is a mixture of Hydrocarbons called
To Separate the oil into its component fractions
we first boil it. This process is called Fractional
After Fractional Distillation there are a large
number of Long-Chain Hydrocarbons left.
To meet market demand these long-chain
Hydrocarbons are broken up into smaller more
desirable Hydrocarbon Chains ie they are said to
Cracking of Crude Oil (contd).representation
Steel Wool acts as a Catalyst
is passed over
the heated steel
The gas collected
is tested with
When an Alkane is cracked one of the products is
C16H34 C13H28 + C3H6
The Test for Saturation/Unsaturation
An Unsaturated compound is one that contains a C=C
A Saturated compound is one that contains C-C single
Alkanes are Saturated and Alkenes Unsaturated.
The Test: Add Br2(aq) to both an alkane and alkene.
Alkenes quickly decolourise the Br2(from brown to
clear) whereas alkanes only slowly decolourise the Br2
in the presence of UV light.