Combustion of hydrocarbons containing sulfur leads to sulfur dioxide that…

The idea

Crude oil contains sulfur compounds, and any sulfur left in a fuel is oxidised when that fuel burns: S + O₂ → SO₂. Sulfur dioxide is an acidic gas. Dissolved in atmospheric water it forms sulfurous acid, SO₂ + H₂O → H₂SO₃, and further oxidation in the air gives sulfuric acid, H₂SO₄. This is acid rain. Aqueous sulfur dioxide is acidic enough to register around pH 2 to 3, which is why it attacks carbonate stonework such as limestone and marble, corrodes metal structures, acidifies lakes so that fish die, and damages the leaves and roots of trees. The practical response is to remove sulfur compounds from fuels at the refinery and to scrub sulfur dioxide from flue gases at power stations.

What earns the marks

• The equation S + O₂ → SO₂ for the oxidation of sulfur during combustion.
• Sulfur dioxide dissolving in water to give an acidic solution, with an equation such as SO₂ + H₂O → H₂SO₃.
• Naming acid rain and a specific effect, such as dissolving limestone or acidifying lakes.
• A measure taken to reduce emissions, such as removing sulfur from the fuel beforehand.

Common mistakes and how to put them right

• Writing SO₃ as the direct product of burning sulfur in an engine, when SO₂ forms first.

• Saying sulfur dioxide causes the greenhouse effect rather than acid rain.

• Giving it is bad for the environment instead of a named effect such as damage to limestone.

• Forgetting that the sulfur comes from impurities in the fuel rather than from the hydrocarbon chain itself.

Exam tips

• Write equations for both stages: sulfur to sulfur dioxide, then sulfur dioxide to an acid.
• Name a specific consequence of acid rain, because general environmental harm scores nothing.
• Keep acid rain separate from global warming in your answer, since different gases cause them.