Page 180 - Chemical equilibria Volume 4

156 Chemical Equilibria
A1.2.3. Convention (II)
Convention (II) (known as the infinitely-dilute solution reference)
distinguishes, amongst the components of the solution, the substance (or
substances) present in high proportion, known as the solvents, and those
which are present in smaller proportions – the solutes. The reference state is
different for these two categories of components:

– For a solvent, we choose its pure state (in the same state of aggregation
as the solution) as a reference, and therefore its chemical potential obeys
relation [A1.9]. Convention (II) is identical to convention (I) in the case of a
solvent.
– For a solute, the reference state is an imaginary solution in which all the
solutes are infinitely dilute. The reference chemical potential is, therefore, that
of the solute in that imaginary solution, and is written as μ s ∞ for a solute s.
The activity coefficient of the solute s in that imaginary solution is equal to 1.
Thus, for solute s, the chemical potential is written as follows, agreeing that
the activity tends toward 1 if the molar fraction tends toward zero:
μ s = μ s ∞ + Rlnγ (II) x s [A1.10]
T
s

A1.2.4. Relation between conventions (I) and (II)

The activity coefficients of a solute in reference (I) (pure substance) and
reference (II) (infinitely-dilute solution) are linked to one another because
the chemical potential of the solute is independent of the convention chosen.
Therefore, we can write:

0
μ s = g + Rlnγ s (I) x = μ s ∞ + Rlnγ s (II) x s [A1.11]
T
T
s
s
If we let K represent Henry’s constant, defined by the relation:
iH
μ ∞ − g 0
ln K = s s [A1.12]
iH
RT
from this, we deduce:
γ (I) μ ∞ − g 0
ln s = s s = ln K [A1.13]
γ s (II) RT iH