Fractional distillation separates crude oil into fractions of similar hydrocarbons
Definition
Hydrocarbon
A compound containing hydrogen and carbon atoms only.
- Crude oil is a mixture of many different hydrocarbons.
- Fractional distillation is a physical process that separates these hydrocarbons into groups called fractions.
- A fraction is a mixture of hydrocarbons whose molecules contain a similar number of carbon atoms.
- The separation is possible because hydrocarbons of different sizes have different boiling points.
Key Idea
- The hydrocarbons are separated using their different boiling points, so they evaporate and condense at different temperatures.
- The process makes no new substances, because it physically separates a mixture that is already there.
A hot-to-cool column makes hydrocarbons condense at different heights
Definition
Boiling point
The temperature at which a substance rapidly turns from a liquid into a gas throughout the liquid.
- Crude oil is heated so that most of it evaporates into a gas.
- The hot vapours enter the bottom of a tall fractionating column.
- The column has a temperature gradient: it is hot at the bottom and cooler towards the top.
- As the vapours rise through the column, they cool down.
- Each hydrocarbon condenses back to a liquid once the temperature drops below its boiling point.
- Hydrocarbons with high boiling points, which have long chains, condense low down where it is hot.
- Hydrocarbons with low boiling points, which have short chains, rise higher and condense near the cooler top.
- Each fraction is drawn off the column at the height where it condenses.
Exam technique
- To explain fractional distillation for full marks, link each step to a change of state: heating causes evaporation, and cooling in the column causes condensation at each hydrocarbon's boiling point.
- Say the column separates a mixture rather than burning it, and that each fraction is still a mixture, not a pure substance.
Crude-oil fractions become fuels and chemical feedstock
Definition
Liquefied petroleum gases
Hydrocarbon fuels obtained from crude oil that are stored as liquids under pressure and released as gases.
- Fuels from crude oil
- A fuel is a substance that releases energy when it burns.
- Fractions are processed into fuels such as petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases (LPG).
- You are not expected to name any other specific fractions or fuels.
- Feedstock for the petrochemical industry
- A feedstock is a raw material used to make other useful substances.
- The petrochemical industry processes substances from crude oil into fuels and other carbon-based materials.
- Useful materials made from petrochemicals
- Solvents dissolve other substances.
- Lubricants reduce friction between moving surfaces.
- Polymers are used to make plastics and synthetic fibres.
- Detergents help water lift away grease and dirt.
Common Mistake
- Do not say the crude oil burns in the column; heating makes the hydrocarbons evaporate, not burn.
- Do not call a fraction a pure substance; each fraction is still a mixture of hydrocarbons.
Carbon forms huge families of natural and synthetic compounds
Definition
Polymer
A substance made from very large molecules in which many atoms are linked by strong covalent bonds.
- Carbon atoms can bond to each other and to hydrogen in many different arrangements.
- This lets related carbon compounds be grouped into families that are built in similar ways.
- Natural carbon compounds occur in nature, such as the hydrocarbons in crude oil.
- Synthetic carbon compounds are made by chemical processes, such as many polymers and detergents.
- Carbon's ability to form families of similar compounds is why there is such a vast array of natural and synthetic carbon compounds.
Self review
- What two changes of state happen during fractional distillation?
- Why do long-chain hydrocarbons condense low down in the column?
- What is a feedstock?
- Name three fuels and two other useful materials made from crude oil.
- Why can carbon form such a large number of different compounds?
