Page 56 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

40 Chapter 4
problems, and not R= 8.314 J K-' mol-',
e.g. for the reaction

2NOCl,,) + Cl2(,) + 2NO(,) K, = 3.75 x at 1069 K

+ Kp = K, (RqA''g = (3.75 x x ((0,08314 dm3 bar K-' mol-')
x (1069K))',
Au = (1 + 2) - (2) = 1, where Kp = (p(C12)' P(NO)~}/~(NOC~)~.
Hence, Kp = 3.33 x

Homogeneous and Heterogeneous Equilibria
(a) Homogeneous equilibrium.-This is an equilibrium involving
all species in the one phase, e.g. all gases, such as N2(g) + 3H2(,) +
2NH3,g).
(b) Heterogeneous equi1ibrium.This is an equilibrium when more
than one phase is involved in the process, e.g. if calcium carbonate
(limestone or marble) is heated, carbon dioxide gas is evolved [which
can be detected by bubbling it through calcium hydroxide, Ca(OH)2
(limewater), which turns milky], i.e. CaC03(,) + HEAT + CaO(,)
+C02(g). Two phases are involved here, the solid phase and the
gaseous phase, and hence this is an example of a heterogeneous
equilibrium reaction, where K, = ([CaO(,)] [C02(,)])/[CaC03(,)]. But
since the concentration of a solid is constant, this means K, = [C02(,)]
and Kp = p(C02). In fact, concentrations and pressures are approx-
imations of the activity, a, of a substance, and the activity of a pure
solid or pure liquid is unity, e.g. = 1.
For the equilibrium H20(1) * H,Oe), K, = [H20(g)], since the
activity a of pure H,O(,) = 1 and Kp = p(H20).

WORKING METHOD FOR THE SOLUTION OF STANDARD
EQUILIBRIUM TYPE PROBLEMS
1. Read the question carefully.
2. Identify all species involved, including their states, i.e. are they
(s), (1) or (g)? Identify the type of equilibrium, i.e. homogeneous
or heterogeneous equilibrium. Remember the activity, a, of a
solid or pure liquid is unity.
3. Write down the balanced chemical equation, with all states
included. This may not necessarily be given in the question.
This is the most important step in any equilibrium problem, as
it dictates the expression for K, and subsequent equilibrium