Page 17 - Basic physical chemistry for the atmospheric sciences

Chemical equilibrium 3

and ocean sciences is that of dissolved carbon dioxide with liquid
water to form carbonic acid, H2C03(1),
(1.2)
Calculate the mass of carbonic acid that forms for every kilogram of
carbon dioxide that reacts with liquid water.
Solution. From the balanced chemical equation (1.2) we see that for
every mole of carbon dioxide that reacts with water one mole of
carbonic acid is formed. Since the molecular weight of C02 is 44.01 ,
the number of moles of C02 in I kg s 1000/44.01 =22.72. Therefore,
i
22.72 moles of carbonic acid will form for every kilogram of C02 that
reacts with water. The molecular weight of carbonic acid is 62.02,
therefore, the number of grams of carbonic acid in 22. 72 moles is
(22. 72 x 62.02) = 1409. Therefore, for every kilogram of C02 that reacts
with water 1.409 kg of carbonic acid are formed.

1.2 Equilibrium constants
A vapor is in equilibrium with its liquid when the rate of condensation
is equal to the rate of evaporation . An analogous state of equilibrium
exists in a chemical system when the rate at which the reactants
combine to form products is equal to the rate at which the products
decompose to form the reactants. For example , in the stratosphere
e
and mesospher , ozone (03) is formed by the reaction
O(g) + 02(g)____,. Oig) (l.3)
However, some of the ozone molecules so formed break up again

( 1 . 4)
Reaction ( l. 3) is called the forward reaction and Reaction (l .4) the
reverse reaction. Reactions (l .3) and (1.4) can be combined as follows

O(g) + 0 2 (g) � 03(g)
At every temperature there exists partial pressures of the gases for
which the forward and reverse reactions occur at the same rate; under
s
i
these condition , the system s said to be in chemical equilibrium.
A general chemical reaction can be represented by
aA +bB + ... �gG + h H + ... (l. 5)
where A, B, . .. and G, H , . . . represent the chemical reactants and

fJroducts, respectively, and a,b, ... and , h , ... their coefficients in the
g