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Unformatted text preview: to sulfur trioxide by reactions that are currently
being intensely studied. The effects of light and ozone on this oxidation are mentioned in the text. The net
oxidation reaction is
2SO2 (g) + O 2 (g) → 2SO3 (g)
In the final step, sulfur trioxide combines with water to form sulfuric acid. Sulfur trioxide is converted to
sulfuric acid by rainwater.
SO3 (g) + H 2 O(l) → H2 SO4 (aq)
Streams and lakes show the most dramatic effects of acid rain. It is known that natural waters with a low
pH can kill fish eggs, salamander eggs, and frog eggs. The extent of change in acidity of a lake or stream when
under the stress of acid rain is determined mainly by the buffering capacity of the surrounding soil. Watershed
soils containing limestone are alkaline and can resist rapid changes in pH, making them less susceptible to
Trees are also susceptible to acid rain. Acid rainfall causes damage to leaves and the growing tissues of
trees. The needles of firs, spruces, and pines turn yellow and fall off. Among the possible causes are acid rain
and ozone from polluted air. These pollutants damage the cell membranes of needles, allowing nutrients to
Acid precipitation can acidify the soil, interfering with nutrient availability. To grow normally trees require
adequate supplies of 16 elements. Several of these, notably Ca, Mg, and K are taken up by tree roots as cations
from aqueous solution in the soil. When sulfuric acid is deposited in soil by precipitation, nutrient cations can
be leached from the root zone. This can greatly affect the health of forests.
Sulfur dioxide emissions can be reduced by removing SO2 after combustion, but before it leaves the stack
and is released into the atmosphere. Powdered limestone (CaCO3 ) is injected into the hot gases leaving the
combustion zone. Heat causes the carbonate to decompose into quicklime (CaO). The quicklime then reacts with
SO2 to form calcium sulfite (CaSO3 ).
CaCO3 (s) → CaO(s) + CO2 (g)
CaO(s) + SO 2 (g) → CaSO3 (s)
About half of the SO2 is removed by contact with the dry CaO. The remaining SO...
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This note was uploaded on 09/15/2009 for the course CHEM 102 taught by Professor Bastos during the Spring '08 term at Adelphi.
- Spring '08