Page 81 - Chemical equilibria Volume 4

Molecular Chemical Equilibria 57
NOTE 3.2.– the activities and values of the reference properties depend on
the convention chosen in the description of the solution. It follows that the
equilibrium constant will depend on the convention chosen for describing
the solution. Thus, a priori, we shall have three possible values for an
(I)
equilibrium constant, which we shall write as: K , K (II) and K (III) .
If we adopt the pure-substance reference (I), and if the solution is perfect,
the activity coefficient is equal to 1 and the law of mass action takes the
form of relation [3.3a], replacing the activities in that formula with the molar
fractions, as follows:

K r (I) = ∏ x i i ν [3.3a]
i
In that case, the Gibbs energy of the reference state is the standard Gibbs
0
energy of the reaction at the temperature at hand, i.e. Δ g . Remember that
r
the value of a function for a substance in the standard state at temperature T
is its value for a pure substance, in its normal state of condensation at a
pressure of 1 bar and at temperature T.

3.1.2. Different forms of the law of mass action
Depending on the values chosen to write the compositions of the multi-
component phases, the expression of the reaction quotient may take a variety
of forms, and therefore there are also different possible forms for the law of
mass action.

If we have a homogeneous system in the gaseous phase, it is preferable to
quantify the compositions by the partial pressures, and the law of mass
action will be expressed in terms of fugacity:

⎛ ⎜∏ f k ⎞ k ν = K () f [3.4]
k ⎝ P ⎠ 0 ⎟

The constant K () f is called the equilibrium constant relative to the
fugacities.