Thermodynamics and Chemistry

CHAPTER 9

9 Mixtures

A homogeneous mixture is a phase containing more than one substance. This chapter dis-
cusses composition variables and partial molar quantities of mixtures in which no chemical
reaction is occurring. The ideal mixture is defined. Chemical potentials, activity coeffi-
cients, and activities of individual substances in both ideal and nonideal mixtures are dis-
cussed.
Except for the use of fugacities to determine activity coefficients in condensed phases,
a discussion of phase equilibria involving mixtures will be postponed to Chap. 13.

9.1 Composition Variables

Acomposition variableis an intensive property that indicates the relative amount of a
particular species or substance in a phase.

9.1.1 Species and substances

We sometimes need to make a distinction between a species and a substance. Aspeciesis
any entity of definite elemental composition and charge and can be described by a chemical
formula, such as H 2 O, H 3 OC, NaCl, or NaC. Asubstanceis a species that can be prepared
in a pure state (e.g., N 2 and NaCl). Since we cannot prepare a macroscopic amount of a
single kind of ion by itself, a charged species such as H 3 OCor NaCis not a substance.
Chap. 10 will discuss the special features of mixtures containing charged species.

9.1.2 Mixtures in general

Themole fractionof speciesiis defined by

xi

def

D

ni

P

jnj

or yi

def

D

ni

P

jnj

(9.1.1)

(PD 1 )

whereniis the amount of speciesiand the sum is taken over all species in the mixture.
The symbolxiis used for a mixture in general, andyiis used when the mixture is a gas.

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