PHYSICAL CHEMISTRY IN BRIEF

CHAP. 8: CHEMICAL EQUILIBRIUM [CONTENTS] 248

8.3.2 Dependence on pressure

• The equilibrium constants of reactions in which the standard state at the temperature
  of the system and an arbitrary standard pressureis chosen for all substances are
  independent of pressure. In general, however, the extent of reaction changes with
  pressure [see8.5.2].


• If we choose the standard state at the temperature and pressure of the system, then
  (
  ∂lnK
  ∂p

)

T

=−

∆rV•

RT

, (8.22)

where

∆rV•=

∑k

i=1

νiVm•,i

andVm•,is the molar volume of theithcomponent at the temperature and pressure of the system.

Note: If different standard states are chosen for different components of a system, the

summation is only performed over those system components for which the standard state

at the temperature and pressure of the system has been chosen.

8.3.2.1 Integrated form.

lnK(p 2 ) = lnK(p 1 )−

∫p 2

p 1

∆rV•

RT

dp. (8.23)

In condensed systems, ∆rV•can be mostly assumed to be independent of pressure. Then

lnK(p 2 ) = lnK(p 1 )−

∆rV•

RT

(p 2 −p 1 ), (8.24)

Note:The pressure dependence of the equilibrium constant in condensed systems is usually

very small.