Alkanes and Alkenes

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Alkanes are Hydrocarbons

  • Hydrocarbon are molecules that are made up of Hydrogen and Carbon atoms ONLY.
  • Need to know the names and the displayed formulas of the first five alkanes.

Methane-CH4, Ethane-C2H6, Propane-C3H8, Butane-C4H10, Pentane-C5H12

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Alkanes are a Homologous Series

  • It is a group of compunds that can all be represented by the same general formula.
  • Alkanes= CnH2n+2
  • No more atoms can join onto the carbon atoms so Alkanes are saturated.

Complete Combustion

  • Where there is alot of Oxygen about Alkane burns to produce Carbon Dioxide and Water
  • Alkane+Oxygen=Carbon Dioxide+Water
  • The gas burns with a clean blue flame.
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Alkanes (Continued)

Incomplete Combustion

  • When there is not enough oxygen, alkane combustion will be incomplete. Although Carbon and Water are still produced, you can still get Carbon and Carbon Monoxide which is a toxic gas.
  • Alkane+Oxygen= Carbon+Carbon Monoxide+Carbon Dioxide+Water
  • Incomplete combustion means a smoky yellow flame, and less energy.

Halogens React with Alkanes to make Haloalkanes

  • Halogens react with alkanes with the present of UV light.
  • A Hydrogen atom from the alkane (e.g.Methane) is substituted by Bromine for example.
  • Bromine and Methane react together to form bromomethane.
  • E.g. CH4+Br2 = CH3Br + HBr
  • Here is the Structural Formula:
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Alkenes have C=C Double Bond

  • They are hydrocarbons which have a double bond between two of the Carbon atoms.
  • They are unsaturated molecules because they can make more bonds.
  • The first three alkenes are: Ethene-C2H4, Propene-C3H6 and Butene-C4H8.
  • When two molecules have identical molecular formulas but different structures they are called Isomers.

Halogens React with Alkenes, forming Haloalkenes

  • Halogens can react with alkenes to make haloalkenes.
  • E.g. when Bromine and Ethene react, they form dibromoethane.
  • It has a "-ane" at the end as C=C double bond are split and Halogen atom is added to each of the Carbons.

Test for Carbon-Carbon double bonds

  • When you shake an alkene with Orange Bromine Water, the solution becomes colourless- this is because the bromine molecules, which are orange are reacting with the Alkene to make a diamoalkane, which is colourless.
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Fractional Distillation

Fractional Distillation

  • Fractional distillation differs from distillation only in that it separates a mixture into a number of different parts, called fractions.
  • A tall column is fitted above the mixture, with several condensers coming off at different heights
  • The column is hot at the bottom and cool at the top. Substances with high boiling points condense at the bottom and substances with low boiling points condense at the top
  • Like distillation, fractional distillation works because the different substances in the mixture have different boiling points.

Fractional Distillation in Crude Oil

  • Because they have different boiling points, the substances in crude oil can be separated using fractional distillation. The crude oil is evaporated and its vapours allowed to condense at different temperatures in the fractionating column
  • Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.
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