Page 86 - Chemical equilibria Volume 4

62 Chemical Equilibria
3.1.3. Use of solution models and application of the law of mass
action
The use of the law of mass action to predict equilibrium states for
reactions where all the components are in the same solution poses two
problems:
– The first is the knowledge of the activity coefficients. They cannot be
determined experimentally, because we are unable to take measurements on
the solution, other than in conditions of equilibrium, because of the reaction
itself which is taking place. Hence, in general, we need to use physical
models or mathematical models obtained by extrapolation from
measurements taken in other conditions.

– The second problem stems from the fact that the calculation of the
activity coefficients, using models, requires us to know the concentrations at
equilibrium – concentrations which we can only obtain by dosages or
by application of the law of mass action if we know those activity
coefficients.

In the latter case, we proceed iteratively. Beginning with a perfect system
or an ideal dilute system with activity coefficients of one, we calculate a first
series of values of the concentrations, starting with a known initial state.
Then, using that model, we calculate the activity coefficients at the
concentrations thus obtained. Again applying the law of mass action gives us
a second series of values for the concentrations; on the basis of those values
we calculate a second series of activity coefficients, and so on until we
obtain a correct convergence.
Numerous reactions take place in polyphase media and involve
components belonging to different phases. Thus, we may simultaneously
encounter gases, species in solution and/or substances in pure phases
(essentially solids), which are involved in the same equilibrium. Thus, the
expression of the law of mass action will involve the fugacities (or partial
pressures) for the gases, the activities (or the molar fractions) for the species
in solution. There will be no term relating to a component in a pure phase.
Of course, the equilibrium constant will contain all the terms relating to all
the components in the reaction in the definition of the form [3.2] as a
function of the Gibbs energy of the chosen reference state.