Combustion of hydrocarbon fuels

Most fuels contain carbon and/or hydrogen. When hydrogen atoms are present in a fuel, they are oxidised to water. Water is not a pollutant.

When there are lots of oxygen atoms present during combustion, the carbon atoms are completely oxidised to carbon dioxide. When there are fewer oxygen atoms present, carbon monoxide or carbon may be produced during combustion.

Complete combustion

Complete combustion of a hydrocarbon fuel occurs when there is a good supply of oxygen. It releases the maximum amount of energy and produces carbon dioxide and water.

To construct a symbol equation for the complete combustion of a hydrocarbon fuel, remember that the fuel reacts with O2 and the only products are CO2 and H2O. For example, using methane (CH4) as the fuel:

methane + oxygen → carbon dioxide + water

CH4 + O2 → CO2 + H2O

When this equation is balanced, you get:

CH4 + 2O2 → CO2 + 2H2O

Incomplete combustion

Incomplete combustion of a hydrocarbon fuel occurs when there is a poor supply of oxygen. Less energy is released. Water is still produced from the hydrogen atoms. Instead of carbon dioxide, you might get carbon monoxide or particulate carbon, known commonly as soot, or a mixture of both.

To construct a symbol equation for the incomplete combustion of a fuel, you need to read the question to see whether carbon monoxide (CO) or carbon (C) is produced. For example, using ethane (C2H6) as a fuel, and producing CO as one of the products:

ethane + oxygen → carbon monoxide + water

C2H6 + O2 → CO + H2O

When this equation is balanced, you get:

2C2H6 + 5O2 → 4CO + 6H2O

Problems with incomplete combustion

Carbon monoxide is a poisonous gas which binds to haemoglobin in your red blood cells, preventing them from carrying oxygen to the cells in your body. Carbon monoxide is colourless and has no smell, so it is very difficult to tell if you are breathing it in. It can be detected by electronic detectors which are often fitted near to boilers.

Particulate carbon can cause health problems for humans because it irritates the lining of the lungs, can make asthma worse, and perhaps even cause cancer. Particulate carbon can also cause global dimming, which may reduce rainfall.