4.1.3Guiding Spaceship Earth Towards a Sustainable Future
Fractional Distillation Of Crude Oil — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 9 parts Lesson 4 of 15
Section 1 of 9Crude oil
A mixture
- A mixture Crude oil is a finite resource formed over millions of years from the remains of ancient marine organisms.
- It is a mixture of many hydrocarbons, compounds of hydrogen and carbon only.
- Because there are no bonds between the different molecules, their properties stay the same after they have been separated.
Mostly alkanes
- Mostly alkanes Most are alkanes with the general formula CnH2n+2, for example methane CH4 and octane C8H18.
- The hydrocarbons are not chemically bonded to each other, so they can be separated physically.
Section 2 of 9The fractionating column
1
Crude oil is heated until most of it is vapour and is fed in near the bottom of the column.
2
The column is hot at the bottom and cooler towards the top.
3
Vapours rise up the column. Each hydrocarbon condenses where the temperature falls below its boiling point.
4
Large molecules with high boiling points condense low down; small molecules with low boiling points rise to the top.
5
Each fraction, a mixture of hydrocarbons with similar boiling points, is drawn off at a different height.
Section 3 of 9Fractions from top to bottom
| Fraction | Carbon atoms (approx.) | Typical use |
|---|---|---|
| Refinery gases | 1–4 | Heating and cooking gas |
| Petrol | 5–10 | Fuel for cars |
| Kerosene | 10–16 | Aircraft fuel |
| Diesel oil | 14–20 | Fuel for lorries and trains |
| Fuel oil | 20–30 | Ships and power stations |
| Bitumen | 50 or more | Surfacing roads and roofs |
Section 4 of 9Trends down the column
1Longer chains
- Longer chains The hydrocarbon molecules get larger.
2Higher boiling points
- Higher boiling points Stronger intermolecular forces need more energy to overcome.
3Higher viscosity
- Higher viscosity Fractions become thicker and flow less easily.
4Less flammable
- Less flammable They ignite less easily and burn with a smokier flame.
Section 5 of 9Uses and consequences
Crude oil in society
Fuels
- Fuels Most fractions are burned as fuels, releasing energy and carbon dioxide, which contributes to climate change.
Feedstock
- Feedstock Some fractions are raw materials for the petrochemical industry, which makes plastics, solvents, lubricants and detergents.
Finite resource
- Finite resource Crude oil takes millions of years to form and is being used faster than it forms, so it will eventually run out.
Section 6 of 9Comparing the fractions
| Property | Small molecules (top) | Large molecules (bottom) |
|---|---|---|
| Boiling point | Low | High |
| Viscosity | Low, flows easily | High, thick and sticky |
| Flammability | High, ignites easily | Low, hard to ignite |
| Colour | Colourless | Dark and tarry |
Section 7 of 9Linking to other topics
Cracking
- Cracking Large, less useful hydrocarbons are broken into smaller, more useful alkanes and alkenes,
- because demand for petrol is greater than for bitumen.
Environmental impact
- Environmental impact Burning fuels makes carbon dioxide, and incomplete combustion makes carbon monoxide and soot, so oil use has to be managed.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Hydrocarbon | A compound made of hydrogen and carbon atoms only. |
| Fraction | A mixture of hydrocarbons with similar boiling points collected together. |
| Fractional distillation | Separating a mixture into fractions according to their boiling points. |
| Alkane | A saturated hydrocarbon with the general formula CnH2n+2. |
| Viscosity | How thick a liquid is and how slowly it flows. |
| Finite resource | A resource that is used up faster than it is formed and so will run out. |
| Feedstock | A raw material used by industry to make other substances. |
Section 9 of 9Exam technique
1
Saying the molecules are broken apart during fractional distillation. This is a physical process, so bonds are not broken.
2
Writing that each fraction is a single pure compound; it is still a mixture.
3
Thinking the gases condense at the bottom. The smallest molecules condense highest up the column.
4
Describing crude oil as a compound rather than a mixture.
5
Forgetting that boiling point rises as the carbon chain gets longer.
6
Mixing up the trendslarge molecules have higher boiling points and are more viscous, not less.
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