Thermodynamic activity

Thermodynamic activity is a dimensionless measure of a chemical species’ effective concentration relative to a reference state. It relates chemical potential to composition while accounting for nonideal interactions.

Chemical potential: The change in a system’s Gibbs energy when the amount of a component changes at fixed temperature, pressure, and other amounts. Activity enters chemical potential logarithmically, with a standard-state term.
Mole fraction: The ratio of a component’s amount of substance to the total amount in a mixture. Mole fraction is a common composition variable used to define activity in mixtures.
Reaction quotient: A ratio of product and reactant activities, each raised to its stoichiometric coefficient. It uses activities to track a reaction’s thermodynamic state.
Concentration-based equilibrium constant: An equilibrium constant expressed using concentrations instead of thermodynamic activities. It can depend on conditions when nonideality makes concentrations differ from activities.
Activity coefficient: A dimensionless factor relating a species’ activity to its concentration or composition measure. It captures deviations between a mixture’s activity and its ideal composition-based value.
Concentration: The amount of a constituent per specified volume or amount of mixture. Concentration-based activity definitions require a stated concentration scale.
Equilibrium constant: The value of the reaction quotient at chemical equilibrium for specified standard states. Thermodynamic equilibrium constants are formulated from dimensionless activities.
Colligative properties: Solution properties that depend chiefly on the number of dissolved particles rather than their identities. Simple concentration-based treatments work in dilute limits, while activity corrects nonideal behavior.
Chemical equilibrium: A state in which the forward and reverse rates of a chemical reaction are equal. Equilibrium conditions are expressed through activities in the reaction quotient.
Ideal solution: A solution whose components obey ideal-mixture relations, with no excess Gibbs energy of mixing. In the ideal limit, activity matches the chosen composition measure.

Linked from 11 pages