2.6:
Factors Affecting Activity Coefficient
The activity coefficient of an ion depends on the ionic strength of the solution.
As indicated by the Debye–Hückel theory, when the ionic strength approaches zero, the activity coefficient approaches unity.
As the ionic strength increases from 0 to 0.1 moles per liter, the activity coefficient value decreases.
However, with an increase in ionic strength above 0.1 moles per liter, the activity coefficient may increase.
The influence of ionic strength on the activity coefficient is associated with the charge of the ion, which also directly influences the activity coefficient.
With an increased charge on the ion, the decrease in the activity coefficient is even more pronounced.
For a particular ionic strength, ions with the same charges have approximately the same activity coefficients.
Another significant factor affecting the activity coefficient of an ion is the ion size parameter—the effective diameter of the hydrated ion.
Smaller ions with a high charge have larger effective diameters and deviate less from ideality than similarly charged ions with smaller diameters do.
2.6:
Factors Affecting Activity Coefficient
The extended Debye-Hückel equation indicates that the activity coefficient of an ion in an aqueous solution at 25°C depends on three partially interdependent properties: the ionic strength of the solution, the charge of the ion, and the ion size.
The activity coefficient value for an ion is close to one when the solution has almost zero ionic strength, i.e., when the solution shows close to ideal behavior. As the ionic strength of the solution increases from 0 to 0.1 mol/L, a decrease in the activity coefficient value is observed.
Solutions with an ionic strength above 0.1 mol/L are not well represented by the Debye-Hückel equation. In such solutions, the activity coefficient of an ion may exceed unity. The ionic strength of the solution is closely related to the charge of the ions present in the solution, thereby affecting their activity coefficient. For a particular ionic strength, ions with less charge have activity coefficient values higher than ions with a higher charge. In other words, the deviation from ideality is more pronounced for multiply charged ions than for singly charged ions.
Ions with similar charges often have similar activity coefficients. The dominant factor in the coefficient discrepancies here is the ion size parameter. Ion size describes the effective diameter of a hydrated ion, and even ions with similar charges tend to have different extents of hydration and therefore different ion sizes. Ions with similar charges can be hydrated to different extents, resulting in different effective diameters. These diameters are known as the ion size parameters. Ions with smaller ion size parameters deviate more from ideality, resulting in a lower activity coefficient than those with larger ion size parameters but the same charge.