Alkanes

• What is the first step in the process of refining crude oil?


  Oil is pre-heated
• What happens to the fractions during fractional distillation?


  The fractions condense at different heights
• How does the temperature change in the fractional distillation column?


  The temperature of the column decreases upwards
• What does the separation of fractions in fractional distillation depend on?


  It depends on boiling point
• What factor influences the boiling point of hydrocarbons?


  Boiling point depends on the size of molecules
• How do van der Waals forces relate to the size of molecules?


  The larger the molecule, the larger the van der Waals forces
• What happens to similar molecules during fractional distillation?


  Similar molecules condense together
• Where do small molecules condense in the fractional distillation column?


  Small molecules condense at the top at lower temperatures
• Where do big molecules condense in the fractional distillation column?


  Big molecules condense at the bottom at higher temperatures
• What type of process is fractional distillation?


  • It is a physical process
  • Involves the splitting of weak van der Waals forces between molecules
• What is the purpose of a vacuum distillation unit?


  It distills heavy residues from the fractionating column under a vacuum
• How does lowering the pressure affect boiling points in vacuum distillation?


  Lowering the pressure over a liquid will lower its boiling point
• What is the advantage of vacuum distillation?


  It allows heavier fractions to be further separated without high temperatures
• What is petroleum primarily composed of?

  Petroleum is a mixture consisting mainly of alkane hydrocarbons
• What is a petroleum fraction?


  A petroleum fraction is a mixture of hydrocarbons with a similar chain length and boiling point range
• What are some examples of petroleum fractions?

  • Naphtha (chemicals)
  • Fuel oil
  • Bitumen
  • Lubricating oils
  • Diesel oil
  • Kerosene (jet fuel)
  • Fuel gas (bottled)
  • Petrol/gasoline
• What is the temperature range for petrol/gasoline in fractional distillation?


  Petrol/gasoline condenses at around 20°C
• What is the temperature range for kerosene (jet fuel) in fractional distillation?


  Kerosene condenses at around 180°C
• What is the temperature range for diesel oil in fractional distillation?


  Diesel oil condenses at around 250°C
• What is the temperature range for lubricating oils in fractional distillation?


  Lubricating oils condense at around 300°C
• What is the temperature range for bitumen in fractional distillation?


  Bitumen condenses at around 340°C
• What is the process of cracking in petroleum refining?


  • Conversion of large hydrocarbons to smaller hydrocarbon molecules
  • Involves breakage of C-C bonds
  • Used to meet demand for shorter hydrocarbons
• Why is cracking economically important?


  It is used to make use of excess larger hydrocarbons and supply demand for shorter ones
• What are the products of cracking?


  The products of cracking are more valuable than the starting materials
• What is the general equation for cracking large hydrocarbons?


  High Mr alkanes → smaller Mr alkanes + alkenes + (hydrogen)
• What are the two main types of cracking?

  • Thermal cracking
  • Catalytic cracking
• What is the main product of thermal cracking?


  Thermal cracking produces mostly alkenes
• What are the conditions for thermal cracking?


  High pressure (7000 kPa) and high temperature (400°C to 900°C)
• What types of bonds can be broken during thermal cracking?


  Bonds can be broken anywhere in the molecule by C-C bond fission and C-H bond fission
• Write the equation for the thermal cracking of octane (<latex>C_8H_{18}<\/latex>).


  <latex>C_8H_{18} \rightarrow C_6H_{14} + C_2H_{4}<\/latex>
• Write the equation for the thermal cracking of dodecane (<latex>C_{12}H_{26}<\/latex>).


  <latex>C_{12}H_{26} \rightarrow C_{10}H_{22} + C_{2}H_{4}<\/latex>
• What is the advantage of catalytic cracking over thermal cracking?


  Catalytic cracking is cheaper because it saves energy with lower temperatures and pressures
• What are the conditions for catalytic cracking?


  Slight or moderate pressure and high temperature (450°C) with a zeolite catalyst
• What types of hydrocarbons does catalytic cracking produce?


  Catalytic cracking produces branched and cyclic alkanes and aromatic hydrocarbons
• Why are branched and cyclic hydrocarbons preferred in fuels?


  They burn more cleanly and provide a higher octane number
• What is the combustion of alkanes?


  • Alkanes readily burn in the presence of oxygen
  • Combustion is highly exothermic
  • Explains their use as fuels
• What is the equation for complete combustion of octane (<latex>C_8H_{18}<\/latex>)?


  $C_8H_{18}(g) +$$12.5 O_2(g) \rightarrow 8CO_2(g) +$$9 H_2O(l)$
• What are the products of complete combustion of alkanes?


  The products are CO2 and H2O
• What occurs during incomplete combustion of alkanes?


  It produces CO (toxic) and/or C (soot)
• What is the equation for incomplete combustion of methane (<latex>CH_4<\/latex>) producing carbon monoxide?


  $CH_4(g) +$$\frac{3}{2} O_2(g) \rightarrow CO(g) +$$2 H_2O(l)$