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Hydrocarbons

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Hydrocarbons

  1. 1. 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.
  2. 2. 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.
  3. 3. The Alkane members Name 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
  4. 4. 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 C2H4 Propene C3H4 Butene C4H8 Pentene C5H10 Hexene C6H12 Only the first three are gases Ethene & propene are important starting materials in making other chemicals
  5. 5. 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
  6. 6. 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.
  7. 7. 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
  8. 8. Cracking of Crude Oil (contd).representation Steel Wool acts as a Catalyst The hot Paraffin vapour is passed over the heated steel wool The gas collected is tested with Bromine water Safety:Beware of Suck-back When an Alkane is cracked one of the products is always Unsaturated C16H34 C13H28 + C3H6
  9. 9. 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. Alkene and Bromine water shaken together Alkane and Bromine water shaken together 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.

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