Burning fuels is a major source of air pollution
Hydrocarbon
A compound containing hydrogen and carbon atoms only.
- Most fuels contain carbon and hydrogen, and some also contain sulfur as an impurity.
- Many fuels are hydrocarbons, which are made of hydrogen and carbon only.
- When a fuel burns, it reacts with oxygen from the air in a reaction called combustion.
- The pollutants released depend on how completely the fuel burns and on any impurities it contains.
Complete combustion makes carbon dioxide and water
Complete combustion
Combustion in a plentiful supply of oxygen, producing carbon dioxide from carbon and water from hydrogen.
- Complete combustion happens when there is plenty of oxygen.
- The carbon in the fuel is oxidised to carbon dioxide: C+O2→CO2\text{C} + \text{O}_2 \rightarrow \text{CO}_2C+O2→CO2.
- The hydrogen is oxidised to water: 2H2+O2→2H2O2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}2H2+O2→2H2O.
- For methane the overall reaction is CH4+2O2→CO2+2H2O\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}CH4+2O2→CO2+2H2O.
- Complete combustion also releases the most energy from the fuel.
Complete combustion oxidises carbon to carbon dioxide and hydrogen to water, releasing the maximum energy.
Incomplete combustion makes carbon monoxide and soot
Incomplete combustion
Combustion in a limited supply of oxygen, which may produce carbon monoxide, soot or unburned fuel.
Carbon monoxide
A toxic, colourless and odourless gas that reduces the amount of oxygen transported by the blood.
Soot
Fine solid carbon particles produced by the incomplete combustion of a carbon-containing fuel.
- Incomplete combustion happens when there is not enough oxygen to burn the fuel fully.
- Some carbon is only partly oxidised to carbon monoxide, CO\text{CO}CO: 2C+O2→2CO2\text{C} + \text{O}_2 \rightarrow 2\text{CO}2C+O2→2CO.
- Some carbon is not oxidised at all and escapes as soot, which is solid carbon particles.
- Some fuel may also escape unburned as hydrocarbon particles.
- Incomplete combustion releases less energy than complete combustion.
- Carbon monoxide, CO\text{CO}CO, has one oxygen atom, while carbon dioxide, CO2\text{CO}_2CO2, has two.
- Incomplete combustion happens when oxygen is limited, for example in a poorly ventilated boiler.
Sulfur and hot engines add more pollutants
Sulfur dioxide
A gaseous atmospheric pollutant that irritates the respiratory system and contributes to acid rain.
Oxides of nitrogen
Gaseous compounds containing nitrogen and oxygen that irritate the respiratory system and contribute to acid rain.
- If a fuel contains sulfur, the sulfur burns to form sulfur dioxide, SO2\text{SO}_2SO2: S+O2→SO2\text{S} + \text{O}_2 \rightarrow \text{SO}_2S+O2→SO2.
- Inside a hot engine, the high temperature makes nitrogen and oxygen from the air react together.
- This forms oxides of nitrogen, written as NOx\text{NO}_xNOx.
- So a car exhaust can contain carbon dioxide, water, carbon monoxide, soot, sulfur dioxide and oxides of nitrogen.
- Match each pollutant to its cause: carbon monoxide and soot from incomplete combustion; sulfur dioxide from sulfur in the fuel; oxides of nitrogen from the heat of the engine.
- When you write a combustion equation, balance it, and add state symbols if the question asks for them.
- Which two elements are found in most fuels, and which impurity may also be present?
- Write the equation for the complete combustion of methane.
- Which two products form when carbon burns incompletely?
- How is sulfur dioxide produced when a fuel burns?
- Why do oxides of nitrogen form inside a car engine?
