Activity expresses composition relative to a specified thermodynamic standard state.
Key knowledge
Chemical activity relates a species' chemical potential to its standard-state value through μ = μ° + RT ln a. The activity is dimensionless; standard states and composition scales must be specified. Activity coefficients describe departures from the chosen ideal behaviour.
Conditions and scope
The species, phase, temperature and standard-state convention must agree.
Concentration or molality alone is not generally the activity.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
pH is defined as the negative base-ten logarithm of hydrogen-ion activity. Individual ionic activities cannot be measured independently, so practical pH realization uses conventions and standards. Replacing activity by a concentration ratio is a dilute-solution approximation.
The pH definition uses hydrogen-ion activity with a specified convention.
A distribution coefficient relates one species' concentrations in two specified coexisting phases. For mineral/melt partitioning the usual convention is solid over liquid. The value depends on species, phase composition and physical conditions; a constant coefficient is an assumption within a model rather than an element's universal property.
Equilibrium exchange depends on phase-specific chemical potentials.
For a balanced reaction with signed stoichiometric coefficients νᵢ, the reaction quotient is the product of activities raised to those coefficients. At equilibrium Q equals the dimensionless equilibrium constant K for the specified temperature and standard states.
For an ideal liquid solution in equilibrium with an ideal vapour, Raoult's law gives each component's partial pressure as its liquid mole fraction times its pure-liquid vapour pressure at the same temperature. Nonideal liquid interactions require activity corrections; the total pressure is the sum of partial pressures.
Species transport is driven by chemical and electrical contributions to electrochemical potential. Replacing activity with concentration is justified only by the adopted activity model; in concentrated electrolytes, thermodynamic factors and cross-species constraints matter.
Extends chemical potential by electrical work for a charged species.
A mineral assemblage at specified pressure, temperature and bulk composition minimizes Gibbs energy subject to component balance. Nonideal solution activities require calibrated mixing models. Gridded minimization approximates continuous compositions and needs refinement near phase boundaries.
Conceptual association supported by the recorded research scope; this relation does not assert a proven derivation or mandatory prerequisite.
Reactive transport couples species fluxes to kinetic/equilibrium reactions while enforcing elemental and charge balance. PHREEQC supports different activity formulations, but its database consistency and nonideal solid-solution coverage are limited; successful convergence is not chemical validation.
Conceptual association supported by the recorded research scope; this relation does not assert a proven derivation or mandatory prerequisite.