Thermodynamic Activity

a variable characterizing the tendency of a substance to leave a solution. The activity of a component of a liquid or solid solution is proportional to the vapor pressure of that component above the solution, assuming an ideal gas phase. The proportionality factor is chosen so that the activity of the substance will be equal to its concentration in an ideal solution. Substitution of the activity for concentration in equations which define the conditions of phase, chemical, or electrochemical equilibrium for ideal solutions renders those equations applicable to real solutions. The activity coefficient, equal to the ratio of the activity to the concentration, is used in addition to the activity.

Ions cannot be separated from each other in solution but only in a combination in which electrical neutrality of the solution is preserved. For this reason the activity of a strong electrolyte must be considered as a whole. By definition this quantity is taken to be equal to the product of the activities of the ions into which the molecule dissociates electrolytically. The activity coefficient of a strong electrolyte is the geometric mean of the activity coefficients of its ions; the activity coefficients of the ion are assumed equal to the ratios of their activities to the concentration (as in the case of non-electrolytes).

Deviation from ideal behavior in dilute solutions of strong electrolytes is due exclusively to the electrostatic interaction between the ions. This fact is used as a basis for theoretical calculations of activity coefficients in such solutions.

The concepts “activity” and “activity coefficient” were introduced into chemical thermodynamics by the American scientist G. N. Lewis in 1907.