Page 18 - Basic physical chemistry for the atmospheric sciences

4 Basic physical chemistry

balanced chemical equation. If Reaction (1.5) is in chemical equilib­
m
riu , and if the reactants and products are ideal gases or are present
4
as solutes in an ideal solution, then
g
[G] [H]h . . .
a
[A] [B] b .. . Kc ( 1 . 6)
where [A], [B]. . . . and [G], [H], . . . represent the equilibrium concen­
trations of the reactants and products, and K c is called the equilibrium
constant for the forward reaction (or, simply, the equilibrium constant
for the reaction . 5 Equilibrium constants for some chemical reactions
)
are given in Appendix IV. The value of Kc for a chemical reaction
depends only on temperature (see Section 2.2), not on the concentra­
tions of the chemical species or the volume or pressure of the system.
The concentrations in Eq. ( 1 . 6) may be expressed in molarity (M).
For a gas the molarity is the number of moles of gas per liter of air; for
a solution, it is the number of moles of solute per liter of solution. If
any of the reactants or products are pure liquids or pure solids, their
concentrations (i.e. , densities) are essentially constant, compared to
the large changes that are possible in the concentrations of the gase .
s
Therefore, the concentrations of liquids and solids are incorporated
into the value of Kc. The practical consequence of this is that the
concentration of any pure liquid or pure solid may be equated to unity
in Eq. ( 1 .6).
Exercise 1.2. At 2000°C the value of Kc for the reaction
( 1 . 7 )

i s 1 . 0 x 10-4• If the equilibrium concentrations of 0 (g) and NO(g) are
2
50 Mand 0 . 030 M, respectively, what is the equilibrium concentration
of N2(g)?
Solution. Application of Eq. ( 1 . 6 ) to Re-action (1 . 7) yields
[NO(g)] 2
K= -----
c [N z(g)] [ Oz(g)]
Therefore,

-4 _ (0.030)2
l.O x JO -
[Nig)](50)
and,