Thermodynamics and Chemistry

CHAPTER 9 MIXTURES

9.5 ACTIVITYCOEFFICIENTS INMIXTURES OFNONELECTROLYTES 257

Table 9.3 Reference states for nonelectrolyte constituents of mixtures. In each refer-

ence state, the temperature and pressure are the same as those of the mixture.

Chemical

Constituent Reference state potential

Substanceiin a gas mixture Pureibehaving as an ideal gasa refi (g)

Substanceiin a liquid or solid

mixture

Pureiin the same physical state as the

mixture

i

Solvent A of a solution Pure A in the same physical state as the

solution

A

Solute B, mole fraction basis B at mole fraction 1 , behavior extrapo-

lated from infinite dilution on a mole frac-

tion basisa

refx;B

Solute B, concentration basis B at concentrationc, behavior extrapo-

lated from infinite dilution on a concen-

tration basisa

refc;B

Solute B, molality basis B at molalitym, behavior extrapolated

from infinite dilution on a molality basisa

refm;B

aA hypothetical state.

9.5 Activity Coefficients in Mixtures of Nonelectrolytes

Anactivity coefficientof a species is a kind of adjustment factor that relates the actual
behavior to ideal behavior at the same temperature and pressure. The ideal behavior is
based on areference statefor the species.
We begin by describing reference states for nonelectrolytes. The thermodynamic behav-
ior of an electrolyte solution is more complicated than that of a mixture of nonelectrolytes,
and will be discussed in the next chapter.

9.5.1 Reference states and standard states

Areference stateof a constituent of a mixture has the same temperature and pressure as the
mixture. When speciesiis in its reference state, its chemical potentialrefi depends only
on the temperature and pressure of the mixture.
If the pressure is the standard pressurep, the reference state of speciesibecomes
itsstandard state. In the standard state, the chemical potential is thestandard chemical
potentiali, which is a function only of temperature.
Reference states are useful for derivations involving processes taking place at constant
Tandpwhen the pressure is not necessarily the standard pressure.
Table9.3describes the reference states of nonelectrolytes used in this book, and lists
symbols for chemical potentials of substances in these states. The symbols for solutes
includex,c, ormin the subscript to indicate the basis of the reference state.