GCSE Chemistry revision

Fractional distillation and petrochemicals

Carbon compounds as fuels and feedstock

AQA 4.7.1.2 Foundation & Higher

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Crude oil is a fossil fuel extracted from deep underground and is a complex mixture of many different compounds.
Almost all compounds in crude oil are hydrocarbons, which contain only hydrogen and carbon atoms.
The most common type of hydrocarbon found in crude oil is alkanes.

Crude oil formed over millions of years from the remains of dead plants and animals, particularly plankton, buried in mud.
High pressures and temperatures underground converted this organic biomass into crude oil over millions of years.
The crude oil soaked into surrounding rocks and can be extracted by drilling down and pumping it to the surface.

2. How Crude Oil Forms AQA

Crude oil formed over millions of years from the remains of dead plants and animals, particularly plankton, buried in mud.
High pressures and temperatures underground converted this organic biomass into crude oil over millions of years.
The crude oil soaked into surrounding rocks and can be extracted by drilling down and pumping it to the surface.

Because crude oil takes millions of years to form, it is a finite resource that will eventually run out.
Fossil fuels such as coal, oil, and gas are described as non-renewable fuels because they cannot be replaced on a human timescale.

Fractional distillation is the process used to separate crude oil into its different components, called fractions.
The process works because different hydrocarbon compounds have different boiling points.
Crude oil is heated until most of it vaporises, and the gaseous mixture is passed into a fractionating column that is hot at the bottom and cooler at the top.
As gases rise up the column, each hydrocarbon condenses back into a liquid when it reaches a region cooler than its boiling point, and drains out of the column.

4. The Principle of Fractional Distillation AQA

Fractional distillation is the process used to separate crude oil into its different components, called fractions.
The process works because different hydrocarbon compounds have different boiling points.
Crude oil is heated until most of it vaporises, and the gaseous mixture is passed into a fractionating column that is hot at the bottom and cooler at the top.
As gases rise up the column, each hydrocarbon condenses back into a liquid when it reaches a region cooler than its boiling point, and drains out of the column.

Crude oil is heated in a chamber until most of it vaporises, and the gaseous mixture is then fed into a fractionating column.
The fractionating column is very hot at the bottom and gradually cooler towards the top.
As gases rise up the column, they condense into liquids when they reach a region cooler than their boiling point, and drain out of the column at that level.
Longer-chain hydrocarbons have higher boiling points, so they condense and are collected near the bottom of the column, whilst shorter-chain hydrocarbons condense higher up.

5. How Fractional Distillation Works AQA

Crude oil is heated in a chamber until most of it vaporises, and the gaseous mixture is then fed into a fractionating column.
The fractionating column is very hot at the bottom and gradually cooler towards the top.
As gases rise up the column, they condense into liquids when they reach a region cooler than their boiling point, and drain out of the column at that level.
Longer-chain hydrocarbons have higher boiling points, so they condense and are collected near the bottom of the column, whilst shorter-chain hydrocarbons condense higher up.

Long-chain fractions collected near the bottom include bitumen, used for surfacing roads, and heavy fuel oil, used for heating and lubrication.
Medium-chain fractions include diesel and petrol, used in road vehicles, and kerosene, used as jet fuel.
Very short-chain hydrocarbons, such as propane and butane, have such low boiling points that they remain as gases and are collected as LPG (liquefied petroleum gas).

6. Fractions and Their Uses AQA

Long-chain fractions collected near the bottom include bitumen, used for surfacing roads, and heavy fuel oil, used for heating and lubrication.
Medium-chain fractions include diesel and petrol, used in road vehicles, and kerosene, used as jet fuel.
Very short-chain hydrocarbons, such as propane and butane, have such low boiling points that they remain as gases and are collected as LPG (liquefied petroleum gas).

Shorter chain fractions near the top of the column are the most flammable and therefore make the best fuels.
Longer chain fractions at the bottom are often poor fuels and are used for other purposes or broken down further by a process called cracking.

All substances obtained from crude oil are collectively known as petrochemicals.
Petrochemicals can be used as feedstock, meaning raw materials, for the petrochemical industry.
Products made from petrochemical feedstock include solvents, lubricants, polymers, and detergents.