Page 137 - Instant notes

E2
THERMODYNAMICS OF IONS IN
SOLUTION

Key Notes
The activity, a i , of an ion, i, is given by:

The increasing effect of the ionic atmosphere as the concentration
of the ion, c i is increased is reflected in the changing value of the
activity coefficient, γ i .
The ionic strength, I,

measures the overall degree of electrostatic interaction between
the ions at any concentration. It involves adding the contribution
of each ion, i, with concentration, c i, and charge z i . Multiply-
charged ions promote a larger degree of interaction than singly-
charged ions.
Activity coefficients can be calculated from the Debye-Hückel
limiting law (at lower I), Debye-Hückel law (at intermediate I) or
the Debye-Hückel extended law (at higher I), as appropriate.
These calculate the activity coefficient from the charge on the ion
and the ionic strength and are accurate up to around I=1 mol
−3
dm .
Related topics Free energy (B6) Non-electrolyte solutions (D1)
Fundamentals of equilibria Electrochemistry and ion
(C1) concentration (E5)

Ionic activity

The activity, a i, of an ion i (see Topic C1) is given by:

−3
where c i is the concentration of the ion in mol dm , is the standard concentration of 1
−3
mol dm and γ i is the activity coefficient of the ion. The activity coefficient quantifies
the effect of ionic interaction; at very high dilution, no ionic interaction occurs, γ i→1 and