Activity and Activity Coefficients

Definition

The activity of a chemical entity is a dimensionless quantity derived by multiplying a correction factor (called an activity coefficient) by some measure of its molar abundance, such as molar concentration (molarity), molality, partial pressure, or mole fraction. See Eq. 1, where (a_i) is the activity of a species or component i, γ_i is the activity coefficient, and x_i is a measure of molar abundance.

$$ {a}_i={\gamma}_i{x}_i $$

(1)

Activities are thus used as “effective concentrations” and are necessary to the extent that chemical behavior is not ideal – i.e., strictly proportional to molar abundance – and so γ_i ≠ 1. Formally, a_i is defined by Eq. 2, where it described as a function of the difference between the chemical potential of i (μ_i) and its chemical potential in some standard state $\left({\mu}_i^{\ominus}\right)$, temperature (T), and the gas constant (R).

$$ {a}_i={e}^{\left({\mu}_i-{\mu}_i^{\ominus}\right)/ RT} $$

(2)

Molar Abundance and Ideal Behavior

Many...