Page 90 - Chemical equilibria Volume 4

66 Chemical Equilibria
NOTE 3.6.– it must be carefully noted that relation [3.35] applies only to
mixtures of gases, and is not demonstrated for reactions in a condensed
solution.
3.1.7. Influence of the choice of reference pressure on the
equilibrium constant

The equilibrium constant of partial pressures does not depend on the
pressure in the experiment, but instead depends on the reference pressure.
However, we may be led to change the reference pressure, which leads to an
alteration of the value of K r () P . This occurs, in particular, when we
use tables of old thermodynamic data (before 1980), for which the
reference pressure was that of the standard state chosen theoretically
5
5
at 1 atmosphere (i.e. 1.129×10 Pa) instead of 1 bar (exactly 10 Pa), as is
used today.
If we know the general expression of the Gibbs energy function with
pressure, we can write:

P
∂ ln K r () = 1 ∂ Δ g 0 = − Δ v 0 0 [3.36]
r
r
∂ P 0 RT ∂ P 0 RT
However, for perfect gases, we have:

0
Δ v = RT ∑ ν i [3.37]
r0
P 0 i
Hence the variations of the constant with the reference pressure:

P
∂ ln K () ∑ ν i
r =− i [3.38]
∂ P 0 P 0
0
0
Between two reference pressures P and P , by integration we find:
(1) (2)
( ) ∑ k ν
0
2 P
0
K r (2) ⎛ P (1) k ⎞
( ) = ⎜ ⎜ P 0 ⎟ ⎟ [3.39]
0
2 P
K r (1) ⎝ (2) ⎠