PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 366

11.4 Chemical potential, activity and activity coefficient in electrolyte solutions

This section describes thermodynamic behaviour of electrolyte solutions in the absence of an
external electric field. We will focus on the description of the chemical potential, activities and
activity coefficients.
Electrolyte solutions represent a special case of mixtures whose components may be divided
into three groups:

1. Solvents, most often water. The quantities relating to the solvent will be denoted using
   the subscript 1.


2. Undissociated electrolytes,e.g. undissociated molecules of a weak acid. This component
   is missing in the solutions of strong electrolytes.


3. Ions.

11.4.1 Standard states

11.4.1.1 Solvent

For the chemical potential of a solvent, the standard state of a pure substance at the temperature
and pressure of the system is chosen [see equations (6.72) and section6.5.3]. For the solvent
activitya 1 , equation (6.99) applies,a 1 =x 1 γ 1 , wherex 1 is the molar fraction of the solvent
andγ 1 is its activity coefficient.

Note:For the thermodynamic description of non-ideal behaviour of a solvent, the osmotic

coefficient defined in6.5.5is sometimes used instead of the activity coefficient.

The activity of a solvent will be approximated by one, if not otherwise stated

a 1 = 1. (11.36)

The reason for this approximation is that solvents are usually present in solutions in a large
excess and therefore they behave almost as pure substances.