PHYSICAL CHEMISTRY IN BRIEF

CHAP. 8: CHEMICAL EQUILIBRIUM [CONTENTS] 231

Example

The reaction

0 =SO 3 (g)−SO 2 (g)−

1

2

O 2 (g).

is again under way in a closed system. What are the partial pressures of the individual components

of the system at equilibrium if initially there is only SO 2 and oxygen in the system at constant

temperature and volume? The initial partial pressure of SO 2 is p◦SO 2 , the partial pressure of

oxygen isp◦O 2.

Solution

Substance Initial partial pressure Partial pressure at equilibrium

pi, 0 pi,eq

SO 2 pSO 2 , 0 pSO 2 , 0 −∆p

O 2 pO 2 , 0 pO 2 , 0 −^12 ∆p

SO 3 0 ∆p

If the extent of reaction isξthen

∆p=ξ

RT

V

.

The initial pressure is

p 0 =pSO 2 , 0 +pO 2 , 0 ,

and the equilibrium pressure is

p=pSO 2 +pO 2 +pSO 3 =pSO 2 , 0 +pO 2 , 0 −

1

2

∆p.

From the preceding two equations we obtain

∆p= 2(p 0 −p).

By way of measuring the total pressure we may calculate the partial pressures of all com-
ponents of the mixture.