Page 23 - Basic physical chemistry for the atmospheric sciences

Chemical equilibrium 9

direction until Q =Kc. If Q > K0 the reaction will proceed in the re­
verse direction until Q = Kc.
Exercise 1.5. If . 80 mole of SOz{g), 0 . 3 0 mole of Oz(g), and 1 . 4
0
mole of S03(g) simultaneously occupy a volume of 2 Lat IOOOK, will
w
the mixture be in equilibrium? If not, in h at direction will it proceed
to establish equilibrium? Consider only the species S02(g), 02(g), and
S03(g) in the reaction
( 1 . 1 1 )

with Kc=2.8 x 102.
(
Solution. The reaction quotient for Reaction 1 . 1 1 ) is
2
[S03(g)]
Q = 2
[S02(g)] [02(g)J
To evaluate the initial value of Q we must determine their initial
molarities. These are: for S02=0.80/2 = 0 .40 M, for 02=0.30/2 = 0.15
M , and for S0 = 1 . 4/2 = 0 .70 M.
3
Hence,
2
(0.70) 1 5
Q = 2 20
(0.40) (0. )
Since this value of Q is not equal to Kc (namely, 2 . 8 x 1 0 2), the initial
(
mixture is not in equilibrium. Moreover, since Q <Kc, Reaction 1 . 1 1 )
will proceed in the forward direction.
Exercise 1.6. What are the equilibrium concentrations of S02(g),
0 (g), and S03(g) in Exercise 1 . 5?
2
Solution. If y moles (or� M) of S0 (g) are formed, it follows from
3

Reaction (I. 1 ) that y moles (or � M) of S02(g) and � moles (or � M) of
1
0 2(g) disappear. If this change establishes chemical equilibrium we
have

Reaction 2S02(g) + 0 2(g) � 2S03(g)
Initial concentrations 0.40 M 0.15 M 0.70 M
Change in concentra-
4
tions -�M -� M �M
Equilibrium concen- 2 2
trations (0.40-�)M (o.1s-�) M (o.70+�) M