Acid Rain – General Information
¾Rainwater
is slightly acidic naturally because of
the absorption of carbon dioxide:
H2O(l) + CO2(g) W H+(aq) + HCO3-(aq)
¾The
pH of natural rainwater is about 5.6. The
term “acid rain” is applied to rainwater that is more
acidic than this.
¾Acid rain is usually produced by either nitric acid
or sulfuric acid which has entered the environment
through human activities.
Nitric Acid (HNO3)
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When air is heated sufficiently, NO2(g) can be
produced:
N2(g) + 2 O2(g) → 2 NO2(g)
This reaction occurs naturally in lightning storms, but
artificial sources include high-temperature combustion
processes in power plants and internal combustion
engines.
Nitric acid can then form with atmospheric moisture by
the reaction
3 NO2(g) + H2O(l) → 2 HNO3(aq) + NO(g)
Nitric acid accounts for about ¼ of the acidity of acid
rain.
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Sulfuric Acid (H2SO4)
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Traces of SO2(g) are found naturally due to biological
decay and volcanic activity, but artificial sources, such
as combustion of sulfur-containing coal or refining of
sulfide ores, are significant.
S(s) + O2(g) → SO2(g)
The SO2(g) can then undergo further reactions in the
atmosphere to produce sulfuric acid:
2 SO2(g) + O2(g) → 2 SO3(g)
SO3(g) + H2O(l) → H2SO4(aq)
Sulfuric acid accounts for well over ½ of the acidity of
acid rain.
Buffering by Carbonates (CO32-)
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The effect of acid rain can be buffered by naturally
occurring carbonates in the environment:
CO32- + H+ W HCO3HCO3- + H+ W H2CO3 W H2O(l) + CO2(g)
Regions in which limestone (CaCO3) or carbonaterich soils are found are buffered from the effects of
acid rain.
Carbonate-containing construction materials, such as
marble and concrete, are dissolved by acid rain – the
acid is consumed, but damage is done in the process.
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INCO Smokestacks in Sudbury
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Reference: Curbing Acid Rain, Ontario Hydro, 1989.
Reference: Curbing Acid Rain, Ontario Hydro, 1989.
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