What makes rain acidic
Oxidation of sulfur dioxide to sulfuric acid · Redox · Acid formation · Exothermic
Sulfur in coal once fell back to earth as sulfuric acid, dissolving statues and emptying lakes of fish.
Reactants approach
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ΔH = -463 kJ · exothermic, releases heat
ΔH per 2 mol SO2; overall of several atmospheric steps, product as pure liquid acid.
Meet the molecules
- Reactantgas
SO2
sulfur dioxide
- Reactantgas
O2
oxygen
- Reactantliquid
H2O
water
- Productliquid
H2SO4
sulfuric acid
What you notice
Unpolluted rain is already slightly acidic, around pH 5.6, because it dissolves carbon dioxide. In the 1970s and 1980s, rain downwind of coal-burning power plants often measured pH 4 to 4.5. That acid rain leached aluminium from soils, killed fish in lakes in Scandinavia and the north-eastern United States, damaged forests, and dissolved limestone and marble buildings. Most of the acid came from sulfur in coal, which burns to sulfur dioxide. Since scrubbers and low-sulfur fuels became law, sulfur dioxide emissions in the US and Europe have fallen by more than 80%.
Try it
Compare an old marble gravestone or statue with a newer one; rounded, blurred lettering often comes from decades of acidic rain dissolving the calcium carbonate.
Step by step
- 1
Sulfur burns
Sulfur impurities in coal and oil burn to SO2 in the furnace and leave through the smokestack.
- 2
Oxidation in the air
Over hours to days, SO2 is oxidized, mainly by hydroxyl radicals formed in sunlight or, inside cloud droplets, by hydrogen peroxide and ozone.
- 3
Water joins
The oxidized sulfur combines with water to form sulfuric acid, H2SO4, which dissolves in cloud droplets.
- 4
Acid falls
H2SO4 is a strong acid, so in rainwater it gives its protons to water molecules, raising the H3O+ concentration and lowering the pH.