Chemical activity

Chemical activity is an effective concentration that accounts for interactions among particles in a nonideal mixture. It enters thermodynamic equations in place of concentration or pressure.

Chemical potential: Chemical potential is the change in a system’s Gibbs free energy when a component is added at constant temperature and pressure. Activity expresses each component’s chemical potential relative to a chosen standard state.
Concentration: Concentration measures the amount of a component relative to the volume or amount of a mixture. Activity corrects concentration-based descriptions when mixture interactions matter.
Ionic strength: Ionic strength is a measure of the concentration of ions weighted by the squares of their charges. It helps estimate ionic activity coefficients in electrolyte solutions.
Partial pressure: Partial pressure is the pressure a gas component would exert if it alone occupied the mixture’s volume at the same temperature. It approximates gas activity only when the gas mixture behaves ideally.
Activity coefficient: An activity coefficient is the factor relating a species’ activity to its concentration or composition measure. It quantifies how particle interactions make activity differ from concentration.
Molarity: Molarity is the number of moles of solute per liter of solution. A common solution activity is defined from molarity relative to a standard concentration.
Debye–Hückel theory: Debye–Hückel theory approximates ionic activity coefficients in dilute electrolyte solutions using electrostatic interactions. It provides a first model for calculating activities from ionic strength.
Mass action law: The law of mass action relates reaction rates or equilibrium composition to products of reactant concentrations or activities. Its thermodynamic equilibrium form uses activities, while simplified forms often substitute concentrations.
Standard state: A standard state is a reference condition used to define thermodynamic properties of a substance or species. Activity is dimensionless because it is defined relative to a standard state.
Molality: Molality is the number of moles of solute per kilogram of solvent. Molality-based activities avoid volume changes with temperature that affect molarity.

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