Αcid rain

Αcid rain

Acidic gases in the atmosphere dissolves in water to contribute to the acidity. Two important
factors are

(i) Dissolution of acidic gases in water
(ii) Strength of the resulting acid

In this context even though CO2 levels are high, their contribution to the acidity is very low

(pH 5.1 - 5.8) and it is not considered as acid rain. But the SOx and NOx even though they

are present in smaller quantities have a higher contribution resulting in a pH of 4 - 5.

• Reactions of atmospheric SO2 (g)

(i)

$S{ O }_{ 2 }(g)+H2O(l)\longrightarrow { H }_{ 2 }S{ O }_{ 3 }(aq)$   .
${ H }_{ 2 }S{ O }_{ 3 }(aq)+{ H }_{ 2 }O(l)\longrightarrow { H }_{ 3 }{ O }^{ + }(aq)+\quad HS{ O }_{ 3 }^{ - }(aq)$.
$HS{ O }_{ 3 }^{ - }(aq)+{ H }_{ 2 }O(l)\longrightarrow { H }_{ 3 }{ O }^{ + }(aq)+S{ O }_{ 3 }^{ 2- }(aq)$.


(ii) Oxidants in the atmosphere can oxidize SO2 to SO3.

$S{ O }_{ 2 }\longrightarrow S{ O }_{ 3 }$ .


O2(g), O(g), OH(g) and peroxides can serve as oxidants. Some salts can catalyse the
oxidation. Then SO3(g) dissolves in H2O to form H2SO4.

(iii) Both SO2 and the oxidant (normally O2) can dissolve in a rain drop. Rain drop facilitates
the oxidation process by bringing two chemicals together.

$2S{ O }_{ 2 }(aq)+2{ H }_{ 2 }O(l)+{ O }_{ 2 }(aq)\longrightarrow 2H2SO4(aq)$  .
$H2S{ O }_{ 4 }(aq)+2{ H }_{ 2 }O(l)\longrightarrow 2{ H }_{ 3 }{ O }^{ + }(aq)+S{ O }_{ 4 }^{ 2- }(aq)$ .

• Similarly NOx reacts in the atmosphere.

$2NO(g)+{ O }_{ 2 }(g)\longrightarrow 2N{ O }_{ 2 }(g)$ .

$4N{ O }_{ 2 }(aq)+2{ H }_{ 2 }O(l)+{ O }_{ 2 }(aq)\longrightarrow 4HN{ O }_{ 3 }(aq)$.

$HNO3(aq)\longrightarrow { H }^{ + }(aq)+N{ O }_{ 3 }^{ - }(aq)$ .

• Acid rain damages plants and causes the death of fish in the lakes. Acids such as sulphuric
acid and nitric acid dissolve aluminium from aluminosilicate materials of soil giving free Al3+ to
water. It interferes with the operation of fish gills.
• Acid rain water, draining through soils washes out nutrients and liberates aluminum ions.
The roots of the trees may take up the aluminum ions instead of essential nutrients

eg. ${ Ca }_{  }^{ 2+ }$  and ${ Mg}_{  }^{ 2+ }$

Change of the composition of earth’s surface due to acid rain
• Dolomite, limestone or marble are soluble in acidic water.
• Under mild acidic conditions;

$CaC{ O }_{ 3 }(s)+{ H }^{ + }(aq)\longrightarrow C{ a }^{ 2+ }(aq)+HC{ O }_{ 3 }^{ - }(aq)$ .
$MgC{ O }_{ 3 }(s)+2{ H }^{ + }(aq)\longrightarrow M{ g }^{ 2+ }(aq)+{ H }_{ 2 }O(l)+C{ O }_{ 2 }(g)$ .
$CaC{ O }_{ 3 }.MgC{ O }_{ 3 }(s)+4{ H }^{ + }(aq)\longrightarrow C{ a }^{ 2+ }(aq)+M{ g }^{ 2+ }(aq)+2{ H }_{ 2 }O(l)+2C{ O }_{ 2 }(g)$ .

Here, insoluble substances become soluble.

Under strong acidic conditions;

$CaC{ O }_{ 3 }(s)+{ 2H }^{ + }(aq)\longrightarrow C{ a }^{ 2+ }(aq)+HC{ O }_{ 3 }^{ - }(aq)+H{ O }_{ 2 }O(l)$ .

$MgC{ O }_{ 3 }(s)+2{ H }^{ + }(aq)\longrightarrow M{ g }^{ 2+ }(aq)+{ H }_{ 2 }O(l)+C{ O }_{ 2 }(g)+H{ O }_{ 2 }O(l)$ .
$CaC{ O }_{ 3 }.MgC{ O }_{ 3 }(s)+4{ H }^{ + }(aq)\longrightarrow C{ a }^{ 2+ }(aq)+M{ g }^{ 2+ }(aq)+2{ H }_{ 2 }O(l)+2C{ O }_{ 2 }(g)+H{ O }_{ 2 }O(l)$ .

• Many other salts in the rocks and sand also dissolve in the acid rain. Soil becomes gradually

more acidic in the natural course of events. Cations are removed from the soil solution by

plants and replaced by H+ ions. Minerals such as sulphides are oxidized to form acids. At low

pH, hydrogen ions displace other cations from soil. Not only Al3+, Mg2+, Ca2+ but heavy

metal ions are also displaced by H+ ions. The leaving of these ions deprives plants of the

nutrients required for healthy growth. The acidic water passing through the soil, causes to

leaching Al3+ and other minerals and also weathering of rocks. The Ca2+ and Mg2+

concentration increase in water, and hardness of water also increases. The acidity, salinity and

nitrogen concentration also increase insurface water. Concentration of the heavy metal ions

also increases in the surface water.