The Foundations of Chemistry

EXAMPLE 17-15 Kcand KPfor Heterogeneous Equilibrium

Write both Kcand KPfor the following reversible reactions.

(a) 2SO 2 (g)O 2 (g)^34 2SO 3 (g)

(b) 2NH 3 (g)H 2 SO 4 (
) 34 (NH 4 ) 2 SO 4 (s)

(c) S(s)H 2 SO 3 (aq) 34 H 2 S 2 O 3 (aq)

Plan

We apply the definitions of Kcand KPto each reaction.

Solution

(a) Kc KP

(b) Kc[NH 3 ]^2 KP(PNH 3 )^2

(c) Kc KPundefined; no gases involved

EXAMPLE 17-16 Heterogeneous Equilibria

The value of KPis 27 for the thermal decomposition of potassium chlorate at a given high

temperature. What is the partial pressure of oxygen in a closed container in which the following

system is at equilibrium at the given temperature? (This can be a dangerous reaction.)

heat

2KClO 3 (s)888n2KCl(s)3O 2 (g)

Plan

Because two solids, KClO 3 and KCl, and only one gas, O 2 , are involved, we see that KPinvolves

only the partial pressure of O 2 , that is, KP(PO 2 )^3.

Solution

We are given

KP(PO 2 )^3  27

Let xatmPO 2 at equilibrium. Then we have

(PO 2 )^3  27 x^3 x3.0 atm

This tells us that the partial pressure of oxygen at equilibrium is 3.0 atm.

You should now work Exercises 80 and 91.

RELATIONSHIP BETWEEN 
G^0 rxnAND THE

EQUILIBRIUM CONSTANT

Let us consider in thermodynamic terms what may happen when two substances are mixed

together at constant temperature and pressure. First, as a result of mixing, there is usually

an increase in entropy due to the increase in disorder. If the two substances can react with

17-12

[H 2 S 2 O 3 ]



[H 2 SO 3 ]

1



(PNH 3 )^2

1



[NH 3 ]^2

(PSO 3 )^2



(PSO 2 )^2 (PO 2 )

[SO 3 ]^2



[SO 2 ]^2 [O 2 ]

738 CHAPTER 17: Chemical Equilibrium

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 20.9, Thermodynamics and the
Equilibrium Constant.