7.1.2Organic Chemistry
Fractional Distillation and Petrochemicals — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 8 parts Lesson 3 of 5
Part 1 of 6Crude oil as a starting material
What crude oil is
- What crude oil is Crude oil is a mixture of many hydrocarbons, which are compounds containing only hydrogen and carbon atoms.
- Most of the hydrocarbons are alkanes, with chains that range from one carbon atom to more than fifty.
Where it comes from
- Where it comes from It is a finite resource formed over millions of years from the remains of ancient marine organisms buried under sediment and exposed to heat and pressure.
- Because it forms so slowly, it is described as non-renewable.
Why it must be separated
- Why it must be separated Crude oil is not much use as it comes out of the ground,
- because the different hydrocarbons in it have different properties and uses.
- Separating the mixture into groups of similar molecules gives products that suit particular jobs.
Part 2 of 6How fractional distillation works
1
Crude oil is heated until it vaporises and the hot vapour passes into the bottom of a fractionating column.
2
The column is hottest at the bottom and cooler towards the top, so there is a temperature gradient along its height.
3
As vapours rise, each hydrocarbon cools until it reaches its boiling point, then condenses to a liquid.
4
Fractions with short molecules and low boiling points rise highest; those with long molecules condense near the bottom.
5
Each fraction, a mixture of hydrocarbons with similar chain lengths, is drawn off at its own level.
6
Any gases that never condense leave from the top, while the thick residue that stays liquid is removed from the base.
Part 3 of 6Fractions and their uses
| Fraction | Main use | Position in column |
|---|---|---|
| Refinery gases | Heating and cooking gas (LPG) | Top |
| Petrol | Fuel for cars | Upper |
| Kerosene | Aircraft fuel | Middle |
| Diesel | Fuel for lorries, buses and trains | Middle |
| Fuel oil | Ships and power stations | Lower |
| Bitumen | Surfacing roads and roofs | Bottom |
Part 4 of 6Trends down the column
As chains get longer
Boiling point
- Boiling point Longer molecules have stronger forces between them, so more energy is needed to separate them and boiling points rise.
Viscosity
- Viscosity The liquids become thicker and flow less easily because long chains tangle and attract one another.
Flammability
- Flammability Short-chain fractions ignite easily;
- long-chain ones are harder to set alight and burn with a smoky flame.
Colour
- Colour Fractions become darker, from colourless gases to black bitumen.
Part 5 of 6Why fractions are useful
Fuels
- Fuels Hydrocarbons release a lot of energy when they burn, so they power transport, heating and electricity generation.
- Different engines need different fractions, for example petrol for cars and kerosene for jet aircraft.
Petrochemicals
- Petrochemicals Some fractions are feedstocks, the starting materials for the petrochemical industry.
- Chemical reactions turn them into new substances that do not occur naturally.
Made from fractions
- Made from fractions Solvents, lubricants, polymers, detergents and many other useful materials are made from hydrocarbons.
- Using oil as a feedstock means it is not available for burning, which is one reason to conserve it.
Part 6 of 6Matching the fraction to the job
1Short chain
- Short chain Low boiling point and easy ignition make refinery gases and petrol suitable as clean, quick-burning fuels.
2Medium chain
- Medium chain Kerosene and diesel are runny enough to be pumped through engines yet store a large amount of energy.
3Long chain
- Long chain Fuel oil is thicker and used where heat is needed in bulk, such as in ships and power stations.
4Very long chain
- Very long chain Bitumen is so viscous that it is not used as a fuel at all and instead makes a waterproof surface for roads and roofs.
Key terms
| Term | Meaning |
|---|---|
| Fractional distillation | Separating a mixture of liquids into fractions using differences in boiling point. |
| Fraction | A group of hydrocarbons with similar boiling points and chain lengths. |
| Fractionating column | A tall tower that is hot at the bottom and cooler at the top, in which vapours condense at different heights. |
| Viscosity | How thick a liquid is and how reluctantly it flows. |
| Feedstock | A raw material used to make other chemicals. |
| Petrochemical | A chemical made from crude oil or natural gas. |
| Hydrocarbon | A compound made of hydrogen and carbon atoms only. |
Avoid these mistakes
1
Saying the compounds are separated by chemical reactions; the process is physical, using boiling points.
2
Putting long-chain hydrocarbons at the top of the column, when they condense at the hot bottom.
3
Describing a fraction as a pure compound, when it is a mixture of similar-sized molecules.
4
Writing that molecules 'burn off' in the column; they only evaporate and condense.
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