1. TOPIC 5
Hydrocarbons

2. Hydrocarbons
Hydrocarbons are substances that contain Hydrogen and Carbon only.
Hydrocarbons are divided into a series of families or groups called Homologous Series.
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.

3. The Alkanes 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.

4. The Alkane members Name Methane CH4 Ethane C2H4 Propane C3H6 Butane
Molecular Formula
Common Uses
Methane
CH4
Nat.Gas.Household heating & cooking
Ethane
C2H4
Propane
C3H6
Portable cooking and Heating gas
Butane
C4H10
Lighter fuel/Calor Gas
Pentane
C5H12
Hexane
C6H14
Octane
* C8H18
Petrol derivative
Diesel
*C17H36
Transport
Bitumen
*>C60
Road/Playgrounds

5. Only the first three are gases
The Alkenes
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.
Ethene & propene are important starting materials in making other chemicals
Ethene
C2H4
Propene
C3H4
Butene
C4H8
Pentene
C5H10
Hexene
C6H12
Only the first three are gases

6. The Cycloalkanes
The Cycloalkanes are another Homologous series with General Formula CnH2n
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 Unsaturated

7. Isomers
Isomers are molecules with the same Molecular formula but different structural formula.
Note: There must be the same number of carbon and hydrogen atoms but there also must be a completely different arrangement of Carbon atoms.

8. Cracking of Crude Oil
Crude Oil is a mixture of Hydrocarbons called Fractions
To Separate the oil into its component fractions we first boil it. This process is called Fractional Distillation.
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 be Cracked

9. Cracking of Crude Oil
Steel Wool acts as a Catalyst
The gas collected is tested with Bromine water
The hot Paraffin vapour is passed over the heated steel wool
Safety:Beware of Suck-back
When an Alkane is cracked one of the products is always Unsaturated
C16H34
C13H28 + C3H6

10. The Test for Saturation/Unsaturation
An Unsaturated compound is one that contains a C=C double bond.
A Saturated compound is one that contains C-C single bonds only.
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.
Alkene and Bromine water shaken together
Alkane and Bromine water shaken together