Hydrocarbons and crude oil

In a nutshell

Hydrocarbon compounds are extracted from crude oil in a process known as fractional distillation. The use of crude oil is controversial; although the hydrocarbon components are useful, it is an unsustainable fossil fuel.

Hydrocarbons

Definition

Hydrocarbons are compounds which contain hydrogen and carbon atoms only. There are two important types of hydrocarbons: alkenes and alkanes.

Alkanes and alkenes

Alkanes, such as ethane, are saturated hydrocarbons, which means they contain only single bonds between each carbon atom ( $C-C$ )
The general formula of an alkane is:

$C_nH_{2n+2}$

Alkenes, such as ethene, are unsaturated hydrocarbons, which means they contain one or more double bond(s) between the carbon atoms ( $C=C$ )
The general formula of an alkene is:

$C_nH_{2n}$

Below is the homologous series of alkanes:

Chemistry; Fuels and Earth science; KS4 Year 10; Hydrocarbons and crude oil

1.	Name
2.	Structural formula
3.	Displayed formula

Below is the homologous series of alkenes:

Chemistry; Fuels and Earth science; KS4 Year 10; Hydrocarbons and crude oil

1.	Name
2.	Structural formula
3.	Displayed formula

Properties of hydrocarbons

As the length of the hydrocarbon chain increases:

Viscosity increases. Viscosity is related to how easily a fluid flows; less viscous substances, such as short-chain hydrocarbons, flow more easily.
Boiling and melting points increase.
Flammability (how easily a substance ignites) decreases.
Volatility (how easily a substance vaporises) decreases.

Note: Longer-chained hydrocarbons have higher boiling/melting points, are more viscous and less volatile. This is because there are stronger forces between the molecules (intermolecular forces), compared to shorter-chained hydrocarbons.

Fractional distillation

Crude oil is a fossil fuel, formed from the remains of dead organisms, millions of years ago. Crude oil is a mixture of hydrocarbons. Fractional distillation can be used to separate hydrocarbons in crude oil. Hydrocarbons with a similar chain length are known as a fraction. Each fraction has a different use, such as fuel and feedstock. Feedstock refers to raw materials used in industrial processes.

Process of fractional distillation

The process of fractional distillation is described below.

1.	Crude oil is heated up at the bottom of the fractioning column.
2.	The crude oil evaporates.
3.	Temperature decreases as you go up the column - it is hottest at the bottom and coolest at the top.
4.	Shorter-chain hydrocarbons condense further up the column.
5.	Longer-chain hydrocarbons condense closer to the bottom.
6.	This separates the crude oil into different fractions by their boiling points.

Chemistry; Fuels and Earth science; KS4 Year 10; Hydrocarbons and crude oil

A.	Coolest region of the fractionating column
B.	Hottest region of the fractionating column
1.	Where the crude oil goes in.
2.	Refinery gas (bottled gas)
3.	Gasoline (car fuel)
4.	Kerosene (aircraft fuel)
5.	Diesel (car fuel)
6.	Fuel oil (fuel for ships)
7.	Bitumen (used in various forms of construction)

Sustainability

Sustainability refers to using resources in a way that minimises harm to the environment and leaves enough resources for future generations. Non-renewable fossil fuels, including crude oil, are unsustainable. This means they cannot be remade as quickly as we use them up, because they take millions of years to make. Fossil fuels are unsustainable as they release pollution when burned. Pollution refers to harmful substances in the environment which can harm humans, animals and plants.