Sunday, May 29, 2011

A6. Cracking

6. Describe how long-chain hydrocarbons are cracked to give more short-chain hydrocarbons using silica or alumina as a catalyst at a temperature in the range of 600-700°C


Because fractional distillation forms more long-chain molecules than the more in-demand short-chain molecules, a process called Cracking is used to break down the longer molecules.


The longer molecules are heated to a temperature in the range of 600-700°C and passed over a catalyst like alumina or silica, this breaks down the molecules into smaller ones.

A5. Too many long chains...

5. Recall that fractional distillation of crude oil produces more long-chain and fewer short-chain hydrocarbons than required.

Fractional distillation produces many more of the long chain molecules e.g. bitumen than the useful short-chain molecules, and so the molecules need to be shortened in length to produce the more in-demand short-chain molecules.

A4. Properties of fractions

4. Describe the trend in physical properties and main uses of the main fractions (focus on boiling point and viscosity)


Small molecules (i.e. Petroleum to Kerosene)
  • Low boiling point
  • Light in colour
  • Easy to light and burn well (Flammable)
  • Runny
Large molecules (i.e. Diesel to Bitumen)
  • High boiling point
  • Dark in colour
  • Hard to burn/light (reasonably inflammable)
  • Viscous or thick (Bitumen is a solid at room temperature)

Basically, the longer the molecule, the higher the boiling point, the darker the colour, the viscosity increases and they become increasingly harder to set alight.

A3. Hydrocarbons of crude oil

3. Recall the names and the uses of fractions obtained from crude oil


  • Petroleum gases: (such as propane and butane): Cooking and Heating
  • Gasoline (petrol): Car fuel
  • Kerosene (paraffin): Aeroplane/Jet fuel
  • Diesel: Fuel for larger vehicles and cars
  • Lubricating oil: surprisingly... used as a lubricant
  • Heavy fuel oil: used as fuel for large ships
  • Bitumen: used for road surfaces

A2. Fractional distillation of crude oil

2. Describe how the process of fractional distillation can be used to separate the hydrocarbons in crude oil.


The oil is heated up until it vaporises.
It then passes through a fractionating column with a high temperature at the base, and a much lower temperature at the top
As the vapours pass up the tower they cool, and the larger molecules condense to liquid and are collected
And the smaller ones rise until they condense
Separating the crude oil into many useful oils.