Alkanes are used as fuels because their combustion is very exothermic and they are easy to transport and store.Complete combustion happens in a plentiful supply of oxygen. All the carbon is converted into carbon dioxide and all the hydrogen into water.
C3H8 + 5O2 → 3CO2 + 4H2O
Worked example: balance the complete combustion of octane, C8H18.
8 C gives 8CO2; 18 H gives 9H2O. Oxygen atoms needed: 16 + 9 = 25, which is 12½ O2.
C8H18 + 12½O2 → 8CO2 + 9H2O
(or double every number: 2C8H18 + 25O2 → 16CO2 + 18H2O)Incomplete combustion happens when the oxygen supply is limited, for example in a car engine or a badly ventilated heater. Carbon monoxide and carbon (soot) are formed as well as water, and less energy is released.
C3H8 + 3½O2 → 3CO + 4H2O
C3H8 + 2O2 → 3C + 4H2O
Problems:
Carbon monoxide is colourless, odourless and toxic. It combines with haemoglobin in the blood more strongly than oxygen does, reducing the oxygen carried around the body.
Carbon particulates (soot) can cause breathing problems and blacken buildings.
Unburnt hydrocarbons are also released.
Carbon dioxide from complete combustion is a greenhouse gas that contributes to climate change.
Worked example: balance the complete combustion of octane, C8H18.
8 C gives 8CO2; 18 H gives 9H2O. Oxygen atoms needed: 16 + 9 = 25, which is 12½ O2.
(or double every number: 2C8H18 + 25O2 → 16CO2 + 18H2O)Incomplete combustion happens when the oxygen supply is limited, for example in a car engine or a badly ventilated heater. Carbon monoxide and carbon (soot) are formed as well as water, and less energy is released.
Problems:
Carbon monoxide is colourless, odourless and toxic. It combines with haemoglobin in the blood more strongly than oxygen does, reducing the oxygen carried around the body.
Carbon particulates (soot) can cause breathing problems and blacken buildings.
Unburnt hydrocarbons are also released.
Carbon dioxide from complete combustion is a greenhouse gas that contributes to climate change.