Page 22 - Basic physical chemistry for the atmospheric sciences

8 Basic physical chemistry
[0 (g)] M
.
3 or M- I , the umts of KP are
[O(g)][Oz(g)] (M)(M)
Po, atm
--- or = a tm- 1 •
(p o)( Po,) (atm)(atm)
Nevertheless, it is common practice in chemistry not to indicate the
units of equilibrium constants, with the understanding that when Kc is
used the concentrations are in molarity, and when KP is used the
s
partial pressures are in atmosphere .
i
Exercise 1.4. Ammonia, NH 3 (g), s produced commercially from the
n
n
reaction of hydroge , Hi{g), and atmospheric nitroge , Nz(g), at high
temperatures. If Hz(g), N2(g), and NH (g) attain equilibrium at 472°C
3
when their concentrations are 0. 1 2 M, 0.04 M , and 0.003 M, respec­
tively, calculate the values of Kc and K P for the reaction at 472°C.
Solution. The balanced chemical equation for the reaction is

Hence, from Eq. (1.6)
2
1 2
K = [NH3(g)] 2 (0.003) O
c [ Hz(g)] [ N (g)] (0. ) (0.04) . l
3
3
2
From Eq. ( 1 .9a)
Kc= K P (R�T),J.n
�
where Kc=O. l , R =0.0821 L atm deg- 1 mo1-1, T=745K and, from Eq.
( l .9c), �n=(3+ 1)- (2)=2. Therefore,

2 1 0.1
- '
K P = -3xto -
5)
(0.08 x7 4 2

1.3 Reaction quotient
If the general chemical reaction represented by Eq. (1.5) is no� in
equilibrium, we can still formulate a ratio of concentrations that has
the same form as Eq. 1 . 6). This is called the reaction quotient, Q
(
= [G]g[H]h .. . ( l . 1 0 )
Q [A]a[B]b .. .
Clearly, if Q =Kc, the reaction is in chemical equilibrium. If Q < Kc,
the reaction is not in equilibrium, and it will proceed in the forward