Crude Oil and Fuels » Crude Oil, Hydrocarbons and Alkanes

Where does crude oil come from?

Crude oil is a thick, dark liquid found in rocks underground. It is the remains of an ancient biomass. Biomass means material that comes from living things. The living things were mainly plankton, tiny creatures that lived in ancient seas. When they died, they sank and were buried in mud. Over a very long time, the buried remains changed into crude oil.

This took millions of years. We use crude oil much faster than it forms. So we say crude oil is a finite resource. One day it will run out.

Key terms:

• Crude oil: a mixture of a very large number of compounds, formed from the remains of ancient plankton buried in mud.
• Finite resource: a resource that is used faster than it is made, so it will run out.
• Biomass: material that comes from living things.

A mixture of compounds

Crude oil is not one substance. It is a mixture of a very large number of different compounds. Most of these compounds are hydrocarbons.

Key terms:

• Hydrocarbon: a molecule made up of hydrogen and carbon atoms only.
• Alkane: the most common type of hydrocarbon in crude oil.

The alkanes

Alkanes belong to a homologous series. This is a family of compounds that follow the same pattern. Each member is a little bigger than the one before it. The pattern is given by a general formula.

The general formula for the alkanes is:

C_nH_2n+2

Here, n is the number of carbon atoms. The number of hydrogen atoms is always twice the number of carbon atoms, plus two.

Key terms:

• Homologous series: a family of compounds with the same general formula.
• General formula: a formula that works for every member of a family, using n for a number.

The first four alkanes

You need to know the names and formulae of the first four alkanes. Use the general formula to check each one.

A tip to remember the order: Monkeys Eat Peeled Bananas. The first letters match methane, ethane, propane and butane.

You do not need to know the names of any other alkanes. Just learn these four.

Ways to show an alkane

Alkane molecules can be shown in different ways. You can write the molecular formula, such as C₂H₆. You can also draw the molecule showing every atom and bond. In these drawings, each carbon atom has four single bonds and each hydrogen atom has one. The carbon atoms are joined in a chain, and hydrogen atoms fill the remaining bonds.

You should be able to recognise an alkane from its formula or its drawing. Two checks are enough:

• Does it contain only carbon and hydrogen?
• Does it follow C_nH_2n+2?

Models help too. A molecular modelling kit lets you build alkane molecules. You can see how each carbon atom makes four bonds and each hydrogen atom makes one.

Exam-style question

Crude oil is a mixture of hydrocarbons. (a) Explain how crude oil was formed. (b) What is a hydrocarbon? (c) Name the alkane with 3 carbon atoms and give its formula. (d) Explain why crude oil is described as finite.