2. Alkanes

• Initiation

  • This step generates the free radicals
• Propagation

  • There are often multiple propagation steps.
  • These involve a free radical, and generate a new free radical.
• Termination

  • A termination step is one which involves two free radicals forming a covalent bond.
  • It is said to ‘consume’ free radicals
• UV light is needed for the initiation step in the chlorination of alkanes
• Alkanes are non-polar molecules as the shape is symmetrical and only experience van der Waals forces
• Carbon and hydrogen have similar electronegativities as such they form non-polar bonds
• Crude oil is a mixture containing mainly alkane hydrocarbons that can be separated by fractional distillation
• Fractional distillation is the continual evaporation and condensation of a mixture causing the components to separate because of a difference in their boiling points
• A fraction is a group of compounds that have similar boiling points and are removed at the same level of a fractionating column
• fill

  A) high

  B) high

  C) high

  D) low

  E) 1

  F) zeolite

  G) alkenes

  H) hydrocarbons

  
• Alkanes are insoluble in water because they are non polar
• Alkanes are soluble in organic solvents
• Economic reasons for cracking

  • The petroleum fractions with shorter C chains are in more demand than larger fractions.

  • To make use of excess larger hydrocarbons and to supply demand for shorter ones, longer hydrocarbons are cracked.

  • The products of cracking are more valuable than the starting materials
• Alkanes readily burn in the presence of oxygen

  • highly exothermic
• Complete combustion:

  • In excess O2
  • products are CO2 and H2O
• If there is a limited amount of oxygen then incomplete combustion occurs, producing CO and/or C
• Incomplete combustion produces less energy per mole than complete combustion
• Carbon (soot) can cause global dimming - reflection of the sun’s light
• Fuel : releases heat energy when burnt
• Incomplete combustion is more likely with longer chain alkanes, as they need more oxygen to combust completely
• The internal combustion engine

  • The combustion of alkanes powers cars via the internal combustion engine.

  • Common impurities from the internal combustion engine are:
  • Nitrous oxides
  • Unburned hydrocarbons
  • Carbon monoxide
• Nitrous oxides react to form ozone at ground level

  • Ozone irritates eyes and can cause lung damage
• Formation of oxides of nitrogen:

  Nitrogen and oxygen from air react at high temperatures
• Alkanes are good fuels as most burn readily to produce large amounts of energy
• Carbon monoxide is poisonous as it binds to haemoglobin and prevents oxygen binding. Carbon monoxide can be removed using a catalytic converter