Alkanes

Alkanes

1. Fractional Distillation of Crude Oil

   • Process
     • Crude oil is vaporised at 350 degrees Celcius.
     • Passed into the bottom of the fractionating column where it rises up.
     • Large hydrocarbons do not vaporise and run to the bottom to form a "gooey" residue.
     • As the vapor rises, it cools and forms a temperature gradient.
     • Boiling points increase as molecules increase so they condense at different temperatures and are drawn off at different levels in the column.
     • The smallest hydrocarbons do not condense and are drawn off as gas at the top of the column.
   • Petroleum is a fancy word for crude oil - the stuff in the ground. It is a mixture mostly made up of alkanes.

2. Modification of Alkanes by Cracking

   • Cracking is breaking long-chain hydrocarbons in to smaller ones. It involves breaking C-C bonds.
   • Types of Cracking
     • Thermal Cracking
       • Takes place at high temperatures (1000 C) and high pressures (70 atm).
       • Produces lots of alkenes.
     • Catalytic Cracking
       • Uses a zeolite catalyst (hydrated aluminosilicate).
       • Slight pressure used and a high temperature (500 C).
       • Mostly produces aromatic hydrocarbons and the alkanes used in motor fuels.
       • The catalyst cuts cost due to reducing pressure and temperature requirements and speeds up the reaction saving time and money.

3. Combustion of Alkanes

   • Alkanes are used as fuels because they release a large amount of energy when burnt.
   • Same as normal combustion - you get complete and incomplete.
   • Pollution Produced by Combustion
     • Nitrogen Oxides - toxic and poisonous molecules with the general formula NOx.
     • Not all hydrocarbons are burnt which react with nitrogen oxides in the presence of oxygen for form ground-level ozone (a component of smog)
     • Carbon Monoxide
     • Sulfur Dioxide is formed if the hydrocarbon contains sulfur. This dissolves in moisture and forms sulfuric acid which causes acid rain. Nitrogen Dioxide also does this.
     • Catalytic converters remove these gaseous pollutants.
   • General Equation: Hydrocarbon + Oxygen -> Carbon Dioxide + Water

4. Chlorination of Alkanes

5. What are they?

   • Saturated hydrocarbons.