A reaction in which oxygen combines with another substance is called an oxidation reaction.
Fuels react rapidly with oxygen, transferring energy to the surroundings by heating and by light. This type of oxidation reaction is known as burning or combustion. The products of combustion then enter the atmosphere.
Complete combustion of a hydrocarbon fuel happens when there is a good supply of air or oxygen. Carbon and hydrogen atoms in the fuel react with oxygen:
hydrocarbon + oxygen → carbon dioxide + water
Write a balanced chemical equation for the complete combustion of propane (C3H8) with oxygen (O2). Explain why the equation is balanced.
C3H8 + 5O2 → 3CO2 + 4H2O
Reactants: 3 × C, 8 × H, (5 × 2 × O) = 10 × O
Products: 3 × C, (4 × 2 × H) = 8 × H, (3 × 2 × O) + (4 × O) = 10 × O
Incomplete combustion happens when the supply of air or oxygen is poor. Water is still produced, but carbon monoxide and carbon are the other products. Less energy is released than during complete combustion.
Here is one possible equation for the incomplete combustion of propane:
propane + oxygen → carbon monoxide + carbon + water
C3H8 + 3O2 → 2CO + C + 4H2O
Notice that fewer oxygen molecules are needed to balance the equation than are needed for the complete combustion of propane.
The carbon is released as fine black particles, which we can see in smoky flames, and is deposited as soot. Soot can cause breathing problems and it blackens buildings. It may block boilers and other appliances, or cause a fire.
Carbon monoxide is a toxic gas. It is absorbed in the lungs and binds with the haemoglobin in the red blood cells. This reduces the capacity of the blood to carry oxygen. Carbon monoxide causes drowsiness, and affected people may fall unconscious or even die.