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and cyanic acid (HCNO) and compounds derived from reactions with the anions
carbonate (CO
2
3
) and bicarbonate (HCO
3
−
).
Interestingly, warming is not the only climate phenomenon affected by the
carbon cycle. Inorganic carbon compounds also play a key role in acid rain. In fact,
normal, uncontaminated rain has a pH of about 5.6, owing largely to its dissolu-
tion of carbon dioxide, CO
2
. As water droplets fall through the air, the CO
2
in the
atmosphere becomes dissolved in the water, setting up an equilibrium condition:
↔
CO
2
(gas in the air)
CO
2
(dissolved in the water)
(7.1)
The CO
2
in the water reacts to produce hydrogen ions as
H
+
+
HCO
3
−
CO
2
+
H
2
O
↔
H
2
CO
3
↔
(7.2)
HCO
3
−
↔
2H
+
+
CO
3
2
−
(7.3)
10
−
4
atm, it is possible
to calculate the pH of water in equilibrium. Partial pressure can be thought of as
a concentration. That is, each of the gases in air exert a relative percentage of the
total pressure of the air. Since nitrogen molecules are the largest percentage of all
the air molecules, they exert the largest share of partial pressure in air. Likewise
they are the highest concentration of the air mixture. Such chemistry is always
temperature dependent, so let us assume that the air is 25
◦
C. We can also assume
that the mean concentration of CO
2
in the troposphere is 350 ppm (although 370
ppm may be a better estimate), but this concentration is rising by some estimates
at a rate of 1.6 ppm per year. The concentration of the water droplet's CO
2
in
water in equilibrium with air is obtained by inserting this partial pressure into
the Henry's law equation,
1
which is a function of a substance's solubility in water
and its vapor pressure:
×
Given the mean partial pressure of CO
2
in the air is 3.0
p
CO
2
=
K
H
[CO
2
]
aq
(7.4)
The change from carbon dioxide in the atmosphere to carbonate ions in water
droplets follows a sequence of equilibrium reactions:
K
a
1
←→
K
a
2
←→
K
H
←→
K
r
←→
CO
3(aq)
2
−
CO
2(g)
CO
2(aq)
H
2
CO
3(aq)
HCO
3(aq)
−
(7.5)
A more precise term for acid rain is
acid deposition
, which comes in two forms:
wet and dry.
Wet deposition
refers to acidic rain, fog, and snow. The
dry deposition
fraction consists of acidic gases or particulates. The strength of the effects depends
on many factors, especially the strength of the acids and the buffering capacity of
soils. Note that this involves every species in the carbonate equilibrium reactions
of equation (7.5) (see Fig. 7.2). The processes that release carbonates increase the
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