c7 - organic chemistry

• bromine water is used to test for alkenes
• orange bromine water becomes colourless if alkene is present
• alkenes are more reactive than alkanes
• alkenes are used to produce polymers
• hydrocarbons include molecules of carbon and hydrogen atoms only
• viscosity and boiling point increase with chain length
• flammability decreases with chain length
• alkanes general formula = CnH2n+2
• fractional distillation separates the hydrocarbons from crude oil into fractions
• a fraction contains molecules with similar numbers of carbon atoms
• in complete combustion, hydrogen and water are produced
• in complete combustion water and hydrogen is produced
• in incomplete combustion carbon monoxide is produced (toxic)
• smaller molecules are more useful - high demand
• cracking produces short chain alkenes and alkanes
• catalytic cracking - vapours passed over hot powdered aluminium oxide catalyst
• steam cracking - hydrocarbon vapours mixed with steam and then heated to very high temperatures
• alkenes general formula = CnH2n
• incomplete combustion - burn with a smoky flame
• unsaturated - alkenes, double bonds
• saturated - alkanes, single bonds
• fractional distillation produces fuels and feedstock for the petrochemical industry
• how is steam cracking different to catalytic cracking?

  • steam cracking is different because there is no catalyst involved
  • the vaporised long chain alkane is mixed with steam at very high temperatures
• what is crude oil?

  • mixture of compounds : a fossil fuel consisting of the remains of ancient biomass
  • finite resource
• what is a homologous series?

  • series of compounds with the same general formula, same functional groups & similar chemical properties
• describe the combustion of hydrocarbons

  • exothermic reaction ; occurs when hydrocarbons reacted with oxygen
  • complete combustion = CO2 & H2O
  • incomplete combustion = carbon or carbon monoxide & water
• describe the physical properties of alkanes

  • first few are gases, then liquids & solids
  • boiling points & viscosity increases as molecules get bigger
  • volatility & flammability decrease as molecules get bigger
  • poor reactivity
• what is cracking?

  • when large hydrocarbons are thermally broken down into smaller & useful molecules
• how are the products of cracking used?

  • used as polymers & starting materials for synthesis
• describe the combustion of alkenes

  • burn with smoky flames due to incomplete combustion
• first 4 alkanes : methane, ethane, propane & butane