Combustion of fuels

Complete combustion

Fuels are substances that react with oxygen to release useful energy. Most of the energy is released as heat, but light energy is also released.
About 21 per cent of air is oxygen. When a fuel burns in plenty of air, it receives enough oxygen for complete combustion.
Complete combustion needs a plentiful supply of air so that the elements in the fuel react fully with oxygen.
Fuels such as natural gas and petrol contain hydrocarbons. These are compounds of hydrogen and carbon only. When they burn completely:
the carbon oxidises to carbon dioxide
the hydrogen oxidises to water (remember that water, H2O, is an oxide of hydrogen)

Incomplete combustion

Incomplete combustion occurs when the supply of air or oxygen is poor. Water is still produced, but carbon monoxide and carbon are produced instead of carbon dioxide.

Nitrogen oxides

When fuels are burned in vehicle engines, high temperatures are reached. At these high temperatures, nitrogen and oxygen from the air combine to produce nitrogen monoxide.
One nitrogen molecule (N2) reacts with one oxygen molecule (O2) to make two nitrogen monoxide molecules (NO).
nitrogen + oxygen → nitrogen monoxide
N2(g) + O2(g) → 2NO(g)

When this nitrogen monoxide is released from vehicle exhaust systems, it combines with oxygen in the air to form nitrogen dioxide.
One nitrogen molecule (N2) reacts with two oxygen molecules (O2) to make two nitrogen dioxide molecules (NO2)
nitrogen monoxide + oxygen → nitrogen dioxide
2NO(g) + O2(g) → 2NO2(g)
Nitrogen dioxide is a cause of acid rain.
Nitrogen monoxide and nitrogen dioxide are jointly referred to as NOx.

Sulfur dioxide and acid rain

Many fossil fuels contain sulfur impurities. When these fuels are burned, the sulfur is oxidised to form sulfur dioxide.
S(s) + O2(g) → SO2(g)
This sulfur dioxide then dissolves in droplets of rainwater to form sulfurous acid.
SO2(g) + H2O(l) → H2SO3(aq)

Effects of acid rain

Acid rain reacts with metals and rocks such as limestone. Buildings and statues are damaged as a result.
Acid rain damages the waxy layer on the leaves of trees and makes it more difficult for trees to absorb the minerals they need for healthy growth. They may die as a result.
Acid rain also makes rivers and lakes too acidic for some aquatic life to survive.

The carbon cycle

Most of the chemicals that make up living tissue contain carbon. When organisms die, the carbon is recycled so that it can be used by other organisms. The model that describes the processes involved is called the carbon cycle.

Stages in the carbon cycle

Methane

Methane, CH4, is a gas that can be produced by:
decomposition of vegetation
waste gases from digestion in animals
Methane is a powerful greenhouse gas and therefore contributes to global warming.