Page 85 - Chemical equilibria Volume 4

relative to the concentrations which depends on the molar volume of the
solvent: Molecular Chemical Equilibria 61
0 ∑
∏ C s s ν = K (II) ( ) s ν s [3.19a]
v
0
s
NOTE 3.4.– If the reaction takes place with the number of moles being
constant (i.e. ∑ ν = 0 ), we can regard the activities and concentrations as
s
s
one and the same thing, and write:
∏ C s s ν = ∏ a s s ν = K (II) [3.19b]

s s
Let us now examine the application of the law of mass action if we
choose to use reference (II) in solution, and compare it to the equilibrium
constant obtained in reference (I). By using relation [A1.13], which
introduces the Henry’s constant for each component, and substituting the
result back into relation [3.3], we obtain:

K (I) = ∏ a s s ν (I) = ∏ a s s ν (II) K sH s ν [3.20]
s s
This gives us the relation between the equilibrium constant relative to the
pure-substance reference (I) and that relative to the infinitely-dilute solution
reference (II):

K (I)
K (II) = [3.21]
∏ K sH s ν
s

NOTE 3.5.– for a perfect solution, given that the Henry’s constant of the
different components are all equal to one, the equilibrium constants K (II)
and K are identical. As, according to relation [3.17], the equilibrium
(I)
constants in reference (II) and reference (III) are identical, it follows that, for
a perfect solution, all the equilibrium constants are identical.