You will find out:

> about complete combustion and incomplete combustion

> about pollution caused by burning fossil fuels

> how to write a balanced symbol equation to describe a fuel burning

Incomplete combustion

If a fuel burns in a shortage of oxygen the combustion is incomplete.

The burning fuel gives off soot and unwanted gases. These gases are toxic.

One of the toxic fumes is a gas called carbon monoxide.

Carbon monoxide is a poisonous gas and is very dangerous if it is breathed in.

A heater should be checked regularly to make sure it is burning properly.

Bunsen burner flame

A Bunsen burner flame produces energy from burning gas.

> If the air hole is open the flame burns in plenty of oxygen. Combustion is complete and a blue flame is seen. This means there is more energy released.

> If the air hole is closed there is a shortage of oxygen. Combustion is incomplete and less energy is transferred.

A blue flame from a Bunsen burner transfers more energy than a yellow Bunsen flame as complete combustion gives a blue flame. Incomplete combustion gives a yellow flame and so less energy is released.

When combustion is incomplete, a yellow flame is seen. This is because a yellow flame produces a lot of soot. Carbon monoxide, soot and water vapour are produced as well as carbon dioxide.

FIGURE 5: Why should heaters in people’s homes be checked regularly?

FIGURE 6: What colour is the flame from a Bunsen burner when the air hole is open?


7What is given off when fuels burn in a shortage of oxygen?

8Why is this a problem?

9Which Bunsen burner flame transfers the most energy?

10What are the products of incomplete combustion?

Complete and incomplete combustion

Complete combustion occurs when a fuel burns completely in air.

A fuel, such as methane, uses oxygen in the air to produce products.

When there is a plentiful supply of oxygen the products are carbon dioxide and water.

methane + oxygen → carbon dioxide + water

The advantage of complete combustion is that more energy is released and no toxic gases or soot are produced. More energy is released during complete combustion than during incomplete combustion.

FIGURE 7: What has caused this stone gargoyle to erode?

Incomplete combustion

Incomplete combustion occurs when a fuel burns in limited oxygen. The products are carbon monoxide and water. Complete combustion is better than incomplete combustion because:

> less soot is made
> more heat is released more heat is released
> toxic carbon monoxide gas is not produced.

Pollution problems

A fossil fuel contributes to the greenhouse effect when it burns because it produces carbon dioxide. This is believed to contribute to global warming.

Fuels also cause other problems. Coal has sulfur in it. Burning coal gives off sulfur dioxide. Sulfur dioxide dissolves in rainwater to make acid rain, which damages stone buildings and statues and kills fish and trees.

FIGURE 8: Trees killed by acid rain in Scandinavia. Suggest why forests are particularly badly hit in these countries.


11If carbon monoxide was the product made during incomplete combustion of methane, how would you construct the word equation for the reaction?

12What are the products of complete combustion?

13Which type of combustion releases more useful energy?

14What effect does carbon dioxide contribute to?

15What problems do fuels containing sulfur cause?

More on combustion

Complete combustion releases useful energy.

The formulae for the products of complete combustion are:
carbon dioxide CO2
water H2O

Methane is a common hydrocarbon fuel. The formula for methane is CH4.

Complete combustion can be shown by the equation:
CH4 + O2 → CO2 + H2O

The equation must be made to balance.

There are two oxygen atoms in the reactants and three oxygen atoms in the products.

The balanced equation is:
CH4 + 2O2 → CO2 + 2H2O

There are now four oxygen atoms in both reactants and products.

Incomplete combustion happens when less oxygen is available. Carbon monoxide, CO, and carbon, C, can be made.

CH4 + O2 → C + 2H2O Equations need to be balanced.

First, count up the numbers of atoms in each molecule (shown by the subscript numbers).

Don’t change these numbers.

Then, if necessary, add to the molecule number (large number in front of formula) in order to balance the numbers on either side of the arrow.

Complete combustion gives carbon dioxide and water. Incomplete combustion gives carbon monoxide, carbon and water.

You need to remember the formulae of carbon dioxide, CO2, carbon monoxide, CO, water, H2O, and oxygen, O2.


16Write a balanced equation for the complete combustion of propane, C3H8.

... carbon monoxide ... incomplete combustion
... greenhouse effect ... acid rain ... complete combustion ... sulfur dioxide