2.3:
Thermodynamics: Chemical Potential and Activity
The activity of a species is a measure of its effective concentration and considers additional factors, like the effect of electrolytes on the chemical equilibrium.
So, the thermodynamic equilibrium constant is more precisely expressed in terms of activity than in terms of concentration.
Activity can be expressed as the product of the molar concentration and the activity coefficient of the species.
Activity is formally defined in terms of the chemical potential, also known as the partial molar Gibbs energy—the change in the Gibbs energy of the system contributed per mole of a species at a given temperature, pressure, and composition.
Recall that Gibbs energy is affected by both entropy and enthalpy, and a thermodynamically favorable reaction has a negative change in Gibbs energy.
As the composition of a mixture changes, so does the partial molar Gibbs energy. So, activity accounts for composition-related properties that affect favorability, such as ionic strength.
2.3:
Thermodynamics: Chemical Potential and Activity
The effective concentration of a species in a solution can be expressed precisely in terms of its activity. Activity considers the effect of electrolytes present in the vicinity of the species of interest and depends on the ionic strength of the solution. The activity of a species is expressed as the product of molar concentration and the activity coefficient of the species.
The thermodynamic equilibrium constant is more accurately defined in terms of activity rather than concentration. Activity is formally defined in terms of the chemical potential, also known as the partial molar Gibbs energy. The chemical potential of a system is the Gibbs energy change of the system per mole of a species in question, given that the temperature, pressure, and the number of moles of other species are held constant. For a chemical reaction to take place spontaneously, the chemical potential of the products must be less than that of the reactants. If the number of moles of other species varies in the solution—i.e., a change in the composition of the solution—the chemical potential of the solution changes, and so does the activity.