Page 47 - Chemical equilibria Volume 4

Physico-Chemical Transformations and Equilibria 23
However, as we are dealing with the case where there is only one
transformation per component, we necessarily have stoichiometric numbers
of 1:
dn (α) =− dn k (β) [1.98]
k
dt dt
and:

∑ N dn (α) =− ∑ N dn i (β) [1.99]
i
i= 1 dt i= 1 dt
Finally, we deduce from this:

x k 0(α) = x k 0(β) [1.100]

In the initial state, the molar fractions are the same in both phases.

Thus, the necessary and sufficient conditions for a phase change to be an
azeotropic transformation are that:
– the system starts in an equi-content initial state;

– the ratio of the transfer rates is constant over time.

NOTE 1.5.– The azeotropic nature of the transformation pertains only to the
compositions of the phases; it is independent of the external intensive
variables (temperature, pressure, etc.) insofar as the azeotropic nature of the
process only covers the compositions of phases; it is not dependent on
external intensive variables (temperature, pressure, etc.), because all the
kinetic laws of transition from one phase to another are identical functions of
these variables.

The results in this section never entail the hypothesis that the
transformations are at equilibrium; they are just as applicable for true
transformations as for reversible transformations. In the latter case, the rates
are null, and we are left with the condition of equi-content [1.100].

An example of azeotropic transformations, besides certain phase changes,
includes the transformation:

{ { HCl }} { { NH+ 3 }} =< NH Cl > [1R.4]
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