معامل فاعلية

φ

وفي السياق ذاته، where ν is the number of ions produced from the dissociation of one molecule of the dissolved salt, m is the molal concentration of the salt dissolved in water, φ is the osmotic coefficient of water, and the constant 55.51 represents the molal concentration of water. In the above equation, the activity of a solvent (here water) is represented as inversely proportional to the number of particles of salt versus that of the solvent.

Application to chemical equilibrium

علاوةً على ذلك، Substitute in the expressions for the chemical potential of each reactant:

من جانب آخر، At equilibrium, the sum of the chemical potentials of the reactants is equal to the sum of the chemical potentials of the products. The طاقة غيبس الحرة change for the reactions, , is equal to the difference between these sums and therefore, at equilibrium, is equal to zero. Thus, for an equilibrium such as

الثرموديناميك

• كمون ( μB ) أي مادة (B) تتواجد في مزيج مثالي تُعبّر عنها المعادلة الآتية:
• حيث الكمون الكيميائي في الحالة المعيارية و للمادة في المزيج.
• وتُعمّم العبارة لتشمل السلوك الغير مثالي كالآتي:
• حيث للمادة في المزيج:
• حيث معامل الفاعلية.

وعلى صعيد آخر، Modifying mole fractions or concentrations by activity coefficients gives the effective activities of the components, and hence allows expressions such as قانون راؤول and ثابت توازنs constants to be applied to both ideal and non-ideal mixtures.

ومن ناحية أخرى، Note that in general activity coefficients are dimensionless.

وفي هذا الصدد، In many cases, as goes to zero, the activity coefficient of substance B approaches a constant; this relationship is Henry's Law for the solvent. These relationships are related to each other through the Gibbs-Duhem equation.

ومن جهة أخرى، Knowledge of activity coefficients is particularly important in the context of كيمياء كهربائية since the behaviour of كهرل solutions is often far from ideal, due to the effects of the ionic atmosphere. Additionally, they are particularly important in the context of soil chemistry due to the low volumes of solvent and, consequently, the high concentration of كهرل.

من جهة ثانية، وإذا اقتربت إلى 1، يصبح سلوك المادة كأنّه مثالي، فيتحقق مثلًا قانون راؤول. وإذا كانت أو تُظهر المادة B انحرافًا موجبًا أو سلبيًا عن قانون راؤول (بالتوالي). ويدُلّ الانحراف الموجب على ازدياد تطايرية المادة B.

Upon rearrangement this expression becomes

ومن ناحية أخرى، The sum is the standard free energy change for the reaction, . Therefore

وعلى صعيد آخر، which applies under the conditions that the activity quotient has a particular (constant) value.

ومن جهة أخرى، where [S] denotes the تركيز of S, etc. In practice equilibrium constants are determined in a medium such that the quotient of activity coefficient is constant and can be ignored, leading to the usual expression

من جهة ثانية، This derivation serves two purposes. It shows the relationship between standard free energy change and equilibrium constant. It also shows that an equilibrium constant is defined as a quotient of activities. In practical terms this is inconvenient. When each activity is replaced by the product of a concentration and an activity coefficient, the equilibrium constant is defined as

ومن جهة أخرى، K is the ثابت توازن. Note that activities and equilibrium constants are dimensionless numbers.

Measurement and prediction of activity coefficients

من جانب آخر، A new alternative for activity coefficients prediction, which is less dependent on model parameters, is the COSMO-RS method. In this methods the required information comes from quantum mechanics calculations specific to each molecule (sigma profiles) combined with a statistical thermodynamics treatment of surface segments.

علاوةً على ذلك، This simple model predicts activities of many species (dissolved undissociated gases such as CO2, H2S, NH3, undissociated acids and bases) to high ionic strengths (up to 5 mol/kg). The value of the constant b for CO2 is 0.11 at 10 °C and 0.20 at 330 °C.

علاوةً على ذلك، For water (solvent), the activity aw can be calculated using:

من جانب آخر، Activity coefficients may be measured experimentally or calculated theoretically, using the Debye-Hückel equation or extensions such as Davies equation, Pitzer equations or TCPC model. Specific ion interaction theory (SIT) may also be used. Alternatively correlative methods such as UNIQUAC، NRTL، MOSCED or UNIFAC may be employed, provided fitted component-specific or model parameters are available.

وعلى صعيد آخر، For uncharged species, the activity coefficient γ0 mostly follows a "salting-out" model: