Page 91 - Chemical equilibria Volume 4

Molecular Chemical Equilibria 67
In the example noted below, of the change of reference for the data
tables, the correction is generally very slight and is often ignored.
NOTE 3.7.– Certain transformations of pure phases or solutions can be
symbolically represented by a balance equation, which is generally very
simple. We can very easily extend the concept of the equilibrium constant
and law of mass action.
For the liquid–vapor balance, we can write a balance equation:

(A) = {A} [3R.1]

The application of the law of mass action (adopting the hypothesis of a
low vapor pressure which means that fugacity and pressure can be regarded
as the same thing) becomes:

P (vap ) = K ()(vapP ) [3.40]
P 0

We can see that the equilibrium constant is the vaporization constant
K ()(vapP ) . As the reference pressure is often 1 bar, the vaporization constant
is the saturating vapor pressure at the temperature in question: P 0 ( 0vap ) .

Thus, by the same type of reasoning, we would find that the constants of
vaporization, demixing and sharing of a solute between two solvents are
equilibrium constants.

3.1.8. Dissociative dissolution of a gas in a solid

To conclude this section on equilibrium constants, we are
going to discuss the case where the superposition of a phase equilibrium
is from dissociation, in the case of the dissociative dissolution of a gas in a
solid.

Let us envisage the dissolution of hydrogen in palladium. The experimental
results were interpreted by the dissociation of the hydrogen molecule at the
moment of dissolution and insertion of hydrogen atoms in interstitial sites <s>