PHYSICAL CHEMISTRY IN BRIEF

CHAP. 8: CHEMICAL EQUILIBRIUM [CONTENTS] 256

Example

Assess the change in the extent of the reaction

0 =N 2 O 4 (g)− 2 NO 2 (g)

at temperature 298 K if pressure changes fromp 1 = 101 kPa top 2 = 600 kPa. Assume ideal

behaviour of both gases.

substance G◦m(298)/kJmol−^1

NO 2 − 38. 4

N 2 O 4 − 81. 6

Solution

The total Gibbs energy equals

G=ξ

(

G◦N 2 O 4 +RTln

pξ

pst(2−ξ)

)

+ 2(1−ξ)

(

G◦NO 2 +RTln

2 p(1−ξ)

pst(2−ξ)

)

The course of the dependenceG=G(ξ)for both pressures is shown in Figure 8.3. Due to

stoichiometry, the equilibrium extent of the reaction increases from about 0.92 at 101 kPa to

0.95 at 600 kPa in this case.

8.5.2.1 Reactions in condensed systems.

are not very susceptible to a change of pressure because the effect of pressure on the equilibrium
constant [see8.3.2] and the activity coefficients [see6.5.5] is rather small.

8.5.3 Effect of temperature.

The change of temperature affects the value of the equilibrium constant [see8.2.1].

• If the reaction isexothermic, both the equilibrium constant and the equilibrium extent
  of the reactiondecreasewith increasing temperature.