Sulfur Dioxide

Sulfur dioxide is the pollutant behind acid rain in 0620. It is released whenever fuels containing sulfur impurities are burned, and because it is an acidic oxide it turns rainwater acidic — the chain of reasoning most exam questions want you to trace.

Key facts

Property	Detail
Formula	SO2
Relative molecular mass (Mr)	64 (S 32, O 16)
Bonding / structure	Simple covalent molecule
Appearance and smell	Colourless gas with a choking, pungent smell
Classification	Acidic oxide (non-metal oxide)
Main sources	Burning sulfur-containing fossil fuels; volcanoes

Where this compound appears in 0620

• Air quality and climate change (Topic 11): SO2 as an air pollutant — its source, its role in acid rain, and how it is controlled.
• Oxides (Topic 8): an acidic, non-metal oxide that reacts with alkalis.
• Sulfuric acid manufacture (Supplement): SO2 is the intermediate in the Contact process, S -> SO2 -> SO3 -> H2SO4. See sulfuric acid.

Formation

S(s) + O2(g) -> SO2(g)

Coal and oil contain sulfur impurities, so burning them in power stations and engines releases SO2.

Reactions you must know

Acid rain formation:

1. SO2(g) + H2O(l) -> H2SO3(aq) (sulfurous acid)
2. 2SO2(g) + O2(g) -> 2SO3(g) (further oxidation in the atmosphere)
3. SO3(g) + H2O(l) -> H2SO4(aq) (sulfuric acid)

As an acidic oxide (reacting with an alkali):

SO2(g) + 2NaOH(aq) -> Na2SO3(aq) + H2O(l) (sodium sulfite)

This reaction with an alkali confirms SO2 is acidic, and it is also the principle behind removing SO2 from waste gases.

Effects and prevention

Acid rain corrodes limestone buildings and metal structures, acidifies lakes and kills aquatic life, and damages trees and soil. It is reduced by:

• Flue gas desulfurisation — passing waste gases through calcium oxide or calcium carbonate, which neutralise the acidic SO2 before release.
• Using low-sulfur fuels — removing sulfur before combustion.
• Switching to renewable energy — avoiding sulfur-bearing fuels altogether.

Worked exam question

Coal contains sulfur as an impurity. When it is burned in a power station, sulfur dioxide is released.

(a) Write the equation for the formation of sulfur dioxide from sulfur. (1 mark)

(b) Explain how sulfur dioxide from the power station leads to acid rain. Include one equation. (2 marks)

(c) Describe one method of reducing the sulfur dioxide released and explain how it works. (2 marks)

Mark scheme

• (a) S + O2 -> SO2 [1]
• (b) SO2 dissolves in rainwater to form an acidic solution / sulfurous acid, e.g. SO2 + H2O -> H2SO3 [1]; the acidic rain then falls as acid rain (may be oxidised to sulfuric acid) [1]
• (c) Flue gas desulfurisation: the waste gases are reacted with calcium oxide or calcium carbonate [1]; the basic solid neutralises the acidic SO2 so it is not released (e.g. CaO + SO2 -> CaSO3) [1]. Accept: use low-sulfur fuel, removing sulfur before burning.

Examiner note: in (c) the method must be paired with a reason — “fit a filter” scores nothing, because SO2 is a soluble gas, not solid particulates, so a filter cannot trap it. In (b), naming the acid formed earns credit.

Common exam mistakes

• Confusing SO2 with CO2. Sulfur dioxide (with nitrogen oxides) causes acid rain; carbon dioxide is the main greenhouse gas. Mixing up which gas causes which problem loses marks.
• Writing sulfuric acid for the first step. SO2 + H2O gives sulfurous acid, H2SO3; sulfuric acid H2SO4 only forms after oxidation to SO3.
• Removing SO2 with a catalytic converter or filter. SO2 is a soluble acidic gas removed by neutralisation (scrubbing with CaO/CaCO3); catalytic converters deal with CO and NOx, not SO2.
• Forgetting SO2 is acidic. As a non-metal oxide it reacts with alkalis (SO2 + 2NaOH -> Na2SO3 + H2O) — it never reacts with acids.