You might know that the petrol in a car and the tarmac on a road both come from crude oil, but how do we get such different substances from the same black liquid? Crude oil is a complex mixture of different hydrocarbons, which are compounds containing only hydrogen and carbon atoms.
To make crude oil useful, it is separated into simpler mixtures called fractions. A fraction is a mixture of hydrocarbons with similar boiling points and similar numbers of carbon atoms.
This separation is done using fractional distillation. First, the crude oil is heated until it evaporates. The resulting vapour enters a fractionating column that has a temperature gradient: it is hotter at the bottom and cooler at the top.
As the hot vapours rise up the column, they cool down. When a hydrocarbon reaches a height where the temperature is below its boiling point, it undergoes condensation, changing from a gas back into a liquid. The liquid fractions are then tapped off at different levels.
The fractions are tapped off the column in a specific order based on their boiling points. The lightest fractions with the lowest boiling points exit at the very top, while the heaviest form a thick residue at the bottom.
You must know the exact names and primary uses for the six main fractions required by Edexcel, listed here in order from top to bottom:
| Fraction | Specific Use |
|---|---|
| Gases | Used for domestic heating and cooking |
| Petrol | Used as fuel for cars |
| Kerosene | Used as fuel for aircraft |
| Diesel oil | Used as fuel for cars and trains |
| Fuel oil | Used as fuel for large ships and in some power stations |
| Bitumen | Used for surfacing roads and for roofing |
Note: "Gases" (often called refinery gases) have such low boiling points that they do not condense in the column and exit as a gas at the top. Bitumen does not evaporate easily and is collected as a thick liquid at the very bottom.
As you move down the fractionating column from gases to bitumen, the physical properties of the fractions change in a predictable way. Most of these hydrocarbons are alkanes, which follow the general formula . As you move down the column, the number of carbon atoms in the chain increases.
Explain why refinery gases are collected at the top of the fractionating column while bitumen is collected at the bottom.
Step 1: State the relationship between chain length and boiling point.
Step 2: Relate this to the temperature gradient in the column.
Step 3: Contrast this with the bottom fraction.
Students often swap the uses of kerosene and fuel oil. Remember that kerosene is lighter and used for aircraft, while fuel oil is heavier and used for large ships.
When asked for a use of a fraction, always use the exact phrase 'fuel for...' (e.g., 'fuel for cars' instead of just 'for engines') to ensure you get the mark.
In 3 or 4-mark description questions about fractional distillation, you must explicitly state that the column is 'hotter at the bottom and cooler at the top'.
For questions explaining trends in boiling points, examiners award marks for explicitly linking the higher boiling point to the greater energy required to break stronger intermolecular forces between longer hydrocarbon chains.
Hydrocarbon
A compound containing only hydrogen and carbon atoms.
Fraction
A mixture of hydrocarbons with similar boiling points and similar numbers of carbon atoms.
Fractional distillation
A process used to separate crude oil into simpler, more useful mixtures based on their different boiling points.
Condensation
The process where a gas cools down and changes into a liquid.
Viscosity
A measure of a fluid's resistance to flow; fluids with high viscosity are thick and sticky.
Put your knowledge into practice — try past paper questions for Chemistry
Hydrocarbon
A compound containing only hydrogen and carbon atoms.
Fraction
A mixture of hydrocarbons with similar boiling points and similar numbers of carbon atoms.
Fractional distillation
A process used to separate crude oil into simpler, more useful mixtures based on their different boiling points.
Condensation
The process where a gas cools down and changes into a liquid.
Viscosity
A measure of a fluid's resistance to flow; fluids with high viscosity are thick and sticky.