Page 32 - Chemical equilibria Volume 4

8 Chemical Equilibria
By substituting this back into relation [1.21], we obtain an expression of
the affinity as a function of the chemical potentials of the components
involved in the transformation:
A =− ∑ νμ i [1.23]
i
i
The affinity of a transformation therefore depends only on the chemical
potentials of the components involved in that transformation.

Using expression [1.22], we can write:

ν i ⎜ ∑ ⎛ G ⎞ ∂ ⎟ = ∑ ν i i G = Δ G = ∑ ν μ i [1.24]
r
i
i ⎝ n ∂ i ⎠ PT i i
,, j n
Thus, by comparing this with relation [1.23]:

Δ G =− A [1.25]
r
The affinity of a transformation is thus the opposite of the Gibbs energy
associated with that transformation.

These results can easily be generalized to any general Gibbs energy using
the generalized chemical potentials which correspond to it. For example, for
the electrochemical Gibbs energy and the electrochemical potentials, an
expression such as [1.23] will give the electrochemical affinity of an
electrochemical reaction.

1.3.4. Affinity, reaction quotient and activities

If, in relation [1.23], we explicitly state the chemical potentials of the
species in solution in the form:

0
μ = μ + Rlna i [1.26]
T
i
i
for the transformation [1R.1], we find the expression of the affinity as a
function of the activities:
∑
A =− νμ i 0 − RT ∏ a i i ν [1.27]
i
i i