Thermodynamica: chemisch potentieel en activiteit

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.