Atmospheric pollutants from fuels: predict the products of combustion of a fuel given appropriate information about the composition of the fuel and the conditions in which it is used.

The idea

Combustion products depend on what the fuel contains and on how much oxygen is available. Fuels made mainly of carbon and hydrogen burn in plentiful air to give carbon dioxide and water: methane + oxygen → carbon dioxide + water, CH₄ + 2O₂ → CO₂ + 2H₂O. If oxygen is limited, carbon monoxide and/or carbon particles (soot) also form, because carbon is only partly oxidised. A fuel containing sulfur produces sulfur dioxide, which contributes to acid rain. Nitrogen in the air can react with oxygen at the high temperatures of a flame to give nitrogen oxides. To predict products, identify the elements in the fuel, decide whether oxygen supply is plentiful or limited, then combine each element with oxygen: carbon → CO₂ or CO, hydrogen → H₂O, sulfur → SO₂.

What earns the marks

• Identify the elements present in the fuel from the information given, for example carbon, hydrogen and possibly sulfur.
• State that complete combustion in a plentiful supply of oxygen produces carbon dioxide and water.
• State that incomplete combustion in a limited supply of oxygen produces carbon monoxide and/or carbon (soot) as well as carbon dioxide and water.
• Deduce that a sulfur-containing fuel produces sulfur dioxide, which can cause acid rain.
• Recognise that nitrogen oxides can form when nitrogen and oxygen from the air react at the high temperature of a flame.
• Write word equations, and balanced symbol equations where required, for the combustion reactions predicted.

Common mistakes and how to put them right

• Assuming every fuel gives only carbon dioxide and water.

  Put it right by checking the fuel's composition and the oxygen supply, since sulfur gives sulfur dioxide and limited oxygen gives carbon monoxide or soot.

• Writing carbon monoxide as the product of complete combustion.

  Put it right by linking carbon monoxide to incomplete combustion with insufficient oxygen.

• Forgetting that nitrogen oxides come from the air rather than the fuel.

  Put it right by noting that nitrogen and oxygen react together at flame temperatures.

Exam tips

• Underline the composition of the fuel and the words describing the oxygen supply before you start predicting products.
• Use the phrase 'plentiful supply of oxygen' for complete combustion and 'limited supply of oxygen' for incomplete combustion to make your reasoning clear.
• Check that every element in the fuel appears in a product, and balance any symbol equation by counting atoms on each side.