Students should be able to: describe how carbon monoxide, soot (carbon…

The idea

Burning fuels releases pollutants depending on the fuel's composition and the conditions. Incomplete combustion, caused by limited oxygen, produces carbon monoxide and soot. Carbon monoxide forms when carbon in the fuel is only partly oxidised; soot forms as unburnt carbon particles. Sulfur dioxide forms when sulfur impurities in the fuel burn and combine with oxygen. Oxides of nitrogen form when the high temperature inside an engine or furnace allows nitrogen and oxygen from the air to react. To predict products, identify the elements present in the fuel and the oxygen supply: complete combustion of a hydrocarbon gives carbon dioxide and water, while limited oxygen adds carbon monoxide and soot. Sulfur impurities add sulfur dioxide, and high temperatures add oxides of nitrogen.

What earns the marks

• Incomplete combustion occurs when oxygen supply is limited, producing carbon monoxide and soot instead of only carbon dioxide and water.
• Carbon monoxide is produced by partial oxidation of carbon in the fuel; soot is unburnt carbon released as particles.
• Sulfur dioxide is produced when sulfur impurities in a fuel burn and react with oxygen.
• Oxides of nitrogen are produced when nitrogen and oxygen from the air react at the high temperatures found in engines and furnaces.
• Predicting products requires identifying the elements in the fuel and the conditions, especially whether oxygen is plentiful or limited and whether temperatures are high.
• Complete combustion of a hydrocarbon fuel produces carbon dioxide and water; additional pollutants depend on impurities and conditions.

Common mistakes and how to put them right

• Thinking carbon monoxide is produced only from carbon dioxide.

  Put it right: It forms when carbon is partly oxidised because oxygen is limited.

• Assuming sulfur dioxide comes from atmospheric nitrogen.

  Put it right: It comes from sulfur impurities in the fuel.

• Believing oxides of nitrogen form from fuel nitrogen only.

  Put it right: They mainly form when nitrogen and oxygen in the air react at high temperature.

• Writing that soot is carbon dioxide.

  Put it right: Soot is unburnt carbon particles.

Exam tips

• Link each pollutant to its source: carbon monoxide and soot to incomplete combustion, sulfur dioxide to sulfur impurities, oxides of nitrogen to high-temperature reaction of air.
• When predicting products, first list the elements in the fuel, then decide whether oxygen is plentiful or limited and whether conditions are hot enough for nitrogen oxides.
• Use the phrase 'limited oxygen supply' rather than vague wording such as 'not enough air' to show precise understanding.
• Check that every product you name can be formed from the elements present; do not invent products containing elements absent from the fuel and air.