Page 179 - Modern physical chemistry
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8
Equilibria in
Condensed Phases
8. 1 The Chemical Potential Revisited
BY EQUATION (5.102) THE CHEMICAL potential fli is the contribution per mole of the ith
constituent to the Gibbs energy in a homogeneous region. For equilibrium to exist between
this region and a different homogeneous region, the chemical potentials must be equal fol-
lowing condition (6.102). In an ideal gas phase, fli. is related to the pressure of the ith con-
stituent by formula (7.40),
[8.1 ]
In another phase at equilibrium with the ideal one, the chemical potential satisfies
equation (7.42),
[8.2]
with fl? the value of fli when a; is 1, This is generally different from the fl? in equation
(8.1), In this chapter, the pertinent phase is a condensed phase-liquid or possibly solid.
In the interval over which the solute in this phase obeys Henry's law, we have
[8.3]
and the activity for (8.2) would be
[8.4]
Here Xi is the ith mole fraction.
Alternatively, one may express Henry's law in the form
Pi = kici, [8.5]
where c i is the molarity of i. Then in the very dilute solutions, we would have
[8.6]
Or, one may express Henry's law in the form
Pi = kimi, [8.7]
where m i is the molality of i. Then in the very dilute solutions, we would have
[8.8]
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