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Showing 241–270 of 1709 reference entries

Physics · Earth & cosmosNuclear binding energy

A bound nucleus has less rest mass than its separated nucleons.

ModelPhysics · Earth & cosmosExponential radioactive decay

A fixed decay probability per time gives a population timescale.

ModelPhysics · Earth & cosmosFission, fusion and reaction energy

Nuclear rearrangement can release or require energy.

ModelPhysicsScattering cross section

Normalize a scattering rate by incident flux.

ModelPhysicsStandard Model matter families

Quarks and leptons form three generations.

ModelPhysicsStandard Model interactions

Gauge fields mediate three interactions; the Higgs is a scalar sector.

ModelPhysicsInteraction-specific quantum-number balances

A balance rule must name the interaction where it applies.

ModelPhysics · ChemistryBloch states and electronic bands

A periodic crystal potential organizes energies by wavevector and band.

ModelPhysics · ChemistrySemiconductor electrons and holes

Band occupancy and doping control mobile carriers.

ModelPhysicsBose and Fermi thermal occupations

Indistinguishable particles have different equilibrium state populations.

ModelChemistryElectron screening and effective nuclear attraction

Other electrons modify the attraction experienced by an electron in an atom.

ModelChemistryMultielectron ground-state configurations

Orbital occupations connect atomic electronic structure with valence chemistry.

ModelChemistryLocalized bonds and resonance descriptions

Localized orbital overlap and resonance are complementary chemical descriptions.

ModelChemistryMolecular orbitals and bonding occupation

A molecular orbital description distributes electronic states across a molecule.

ModelChemistryIntermolecular interactions

Dispersion, dipole interactions and hydrogen bonding influence molecular materials.

ModelChemistryBonding classes of solids

Ionic, metallic, molecular and network solids organize common bonding patterns.

DefinitionChemistryPolymers and macromolecular architecture

Polymers are substances composed of macromolecules with recurring structural units.

DefinitionChemistryCoordination compounds and ligands

Ligand donor atoms surround a central metal in a coordination sphere.

DefinitionChemistryActivities and chemical potentials

Activity expresses composition relative to a specified thermodynamic standard state.

DefinitionChemistrypH and hydrogen-ion activity

pH is an activity-based quantity realized through measurement conventions.

DefinitionChemistryBuffers and finite buffering capacity

An acid–base pair can resist limited changes in solution pH.

ModelChemistrySolubility products and precipitation

An equilibrium product constrains dissolution of a specified solid phase.

ModelChemistryReference electrodes and redox potentials

Electrode potentials are specified relative to a reference half-cell.

DefinitionChemistryThe Nernst equilibrium relation

Reversible electrochemical potential depends on activities and temperature.

ModelChemistryElementary steps and reaction mechanisms

A mechanism proposes steps that account for an overall chemical transformation.

ModelChemistryArrhenius temperature dependence

Many rate constants follow an exponential temperature model over a limited range.

ApproximationChemistryHess's law and thermochemical accounting

Reaction enthalpies can be combined because enthalpy is a state function.

ModelChemistryPhase diagrams and coexistence

Phase diagrams describe equilibrium states under stated conditions.

ModelChemistryCalibration, standards and matrix effects

Quantitative analysis needs a validated relation between response and analyte amount.

MethodChemistryChromatographic retention and separation

Differential interactions with mobile and stationary phases separate mixture components.

Method
DefinitionSource checked

Activities and chemical potentials

Chemistry

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.

  1. Quantities, Units and Symbols in Physical Chemistry, 3rd edition, 2nd printing: sections 2.11 and 2.13.1E. Richard Cohen and the international IUPAC Green Book author teamReference guide · PDFiupac.orgRead source

Evidence and review

IUPAC Green Book section 2.11 standard-state and chemical-potential relations.

2026-10-10 · Checked dimensionless activity and compatible standard-state scope.

Related reference entries

Explore relationships within this area of study and its connections to other areas.

Showing 1–7 of 7 relationships

Scientific relationships · 7 / 7

The relationship type describes the connection. Its linked statements, sources and conditions define the scope.

  1. Related to
    Relationship sources · 1

    Linked statements

    • 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.

  2. Related to
    Relationship sources · 1

    Linked statements

    • 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.

  3. Uses definition
    Relationship sources · 1

    Linked statements

    • 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.

    Reaction quotients use dimensionless activities.

  4. Related to
    Relationship sources · 1

    Linked statements

    • 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.

    Activities extend ideal mole-fraction behaviour.

  5. Related to
    Relationship sources · 1

    Linked statements

    • 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.

  6. Related to
    Relationship sources · 2

    Linked statements

    • 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.

  7. Related to
    Relationship sources · 2

    Linked statements

    • 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.

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chem-activities-chemical-potential
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