Coal and oil contain traces of sulfur. Burn them and sulfur dioxide pours out, turning rain acidic and eating away at limestone. The same gas, in tiny controlled amounts, keeps food fresh.
Fossil fuels contain sulfur, which burns to sulfur dioxide (S +4). In the air SO2 is oxidised to SO3 (S +6), helped by the NO2 catalyst, and dissolves in rain to form sulfuric acid, a major cause of acid rain. Acid rain damages limestone buildings, lakes, fish and trees. SO2 can be removed from power-station flue gases by reacting it with calcium carbonate or calcium oxide. In small amounts SO2 is a food preservative, because it is an antioxidant and kills bacteria. Industrially, SO2 is converted to SO3 in the Contact process to make sulfuric acid.
Burn a high-sulfur fuel in the simulator and follow the sulfur through the atmosphere to the rain, then watch it react with limestone.
| Step | Equation | S oxidation number |
|---|---|---|
| burning | S + O2 → SO2 | 0 → +4 |
| in water | SO2 + H2O → H2SO3 | +4 |
| oxidation | 2SO2 + O2 → 2SO3 | +4 → +6 |
| acid formed | SO3 + H2O → H2SO4 | +6 |
| attack on limestone | CaCO3 + H2SO4 → CaSO4 + H2O + CO2 | +6 |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the main source of sulfur dioxide pollution?
What is the oxidation number of sulfur in SO2?
Why is SO2 added to some foods and drinks?
Which equation shows how SO2 can be removed from flue gases?
Run the Contact process converter in the simulator and vary the conditions. Compare the result with what Le Chatelier's principle predicts.
Acid rain is caused by sulfur and nitrogen oxides, not by carbon dioxide; rain is naturally slightly acidic (about pH 5.6) from dissolved CO2, but SO2 and NO2 push it much lower. Remember the oxidation number of sulfur: +4 in SO2 and H2SO3, +6 in SO3 and H2SO4.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
What mass of calcium carbonate (Mr 100.1) is needed to remove 1.00 tonne of SO2 (Mr 64.1) by CaCO3 + SO2 → CaSO3 + CO2?
Which gas is NOT a significant cause of acid rain?
Why does acid rain damage buildings made of limestone?
Burning 1000 tonnes of coal containing 2.00% sulfur by mass produces how much SO2? (Ar S 32.1, Mr SO2 64.1)
This skill is now recorded on your profile. Keep the chain going.