Alkanes and Alkenes

• Hydrocarbon molecules vary in size, affecting their properties and how they can be used as fuels
• The larger the hydrocarbon:

  • the more viscous it is (flows less easily)
  • the higher its boiling point
  • the less volatile it is and the less easily it ignites
• Each fraction contains hydrocarbon molecules with a similar number of carbon atoms and similar boiling points
• Most hydrocarbons obtained from fractional distillation are alkanes
• During fractional distillation, the crude oil is heated until it evaporates
• The vapour moves up the fractionating column, with the top being much colder than the bottom
• Shorter hydrocarbon molecules reach the top of the column before condensing and are collected
• Longer hydrocarbon molecules condense at higher temperatures and are collected lower down the column
• Hydrogen can be added to alkenes to produce alkanes, using a nickel catalyst
• Example: propene + hydrogen -> propane
• This is an addition reaction
• Ethanol can be produced by reacting ethene with steam in the presence of phosphoric acid catalyst
• Example: ethene + steam -> ethanol
• Bromine Water test
• Alkenes react with bromine water, turning it from orange to colourless
• This reaction can be used to differentiate between alkanes and alkenes
• Example: ethene (colourless) + bromine water (orange brown) -> colourless solution
• Example: ethane (colourless) + bromine water (orange brown) -> remains orange brown solution
• Cracking is the process of breaking longer-chain hydrocarbons into shorter, more useful hydrocarbons
• During catalytic cracking:

  • Hydrocarbons are heated until they vaporise
  • The vapour is passed over a hot catalyst
  • A thermal decomposition reaction takes place
  • The products include alkanes and alkenes
• During steam cracking, hydrocarbons are mixed with steam and heated to a high temperature
• Products of cracking are useful as fuels
• There is a high demand for fuels with small chains of carbon atoms because they are easy to ignite and have low boiling points