PHYSICAL CHEMISTRY IN BRIEF

CHAP. 8: CHEMICAL EQUILIBRIUM [CONTENTS] 244

8.2.9 Heterogeneous reactions

In the case of heterogeneous reactions, different standard states are often chosen for individual
components of the reacting system. For condensed components, this is most often the standard
state of a pure component at the temperature and pressure of the system, for gaseous compo-
nents it is the standard state of a pure gaseous component in the state of an ideal gas at the
temperature of the system and a pressure of 101.325 kPa.

Example

Write the equilibrium constant for the reaction

0 =Si(s) + 2H 2 O(l)−SiO 2 (s)− 2 H 2 (g).

Solution

It holds that

K=

a^2 H 2 OaSi

aSiO 2 a^2 H 2

.

If we choose the standard state of a pure substance at the temperature and pressure of the system

for condensed components, their activities equal 1 because neither Si nor SiO 2 is soluble in water.

For hydrogen, we choose the standard state of a pure substance in the state of an ideal gas at a

standard pressure. Then

K=

1

a^2 H 2

=

(

pst

φH 2 pH 2

) 2

.

Decomposition temperatureis the temperature at which the sum of the partial pressures
of gaseous components originating by the decomposition of a solid reaches the value of the
ambient pressure. It is the lowest temperature at which a reaction can be performed at a
sufficient rate because below this temperature the rate of reaction is determined by the rate of
diffusion of gaseous products toward the surface (an analogy to boiling).