Actiivsuskoefitsienti

Actiivsuskoefitsienti (activity coefficient) is a thermodynamic parameter that quantifies the deviation of a species in a mixture from ideal behavior. In an ideal solution, the chemical potential of a component depends only on its mole fraction. In real solutions, interactions between molecules cause the chemical potential to differ from this ideal prediction. The activity coefficient corrects for these interactions by relating the actual activity of a species to its concentration or mole fraction.

Mathematically, the activity a_i of species i is expressed as a_i = γ_i x_i, where x_i is the mole fraction and γ_i is the activity coefficient. When γ_i equals one, the species behaves ideally. Values of γ_i greater than one indicate positive deviation (the species behaves as if it is more concentrated than it actually is), while values less than one indicate negative deviation.

The activity coefficient depends on temperature, pressure, composition, and the nature of intermolecular forces. It is employed in calculations of solution equilibria, such as solubility, vapor‑liquid equilibrium, and electrochemical potentials. Various models exist for estimating γ_i, including the Debye–Hückel theory for electrolytes, the Wilson equation for non-electrolytes, and the Pitzer equations for highly concentrated ionic solutions.

Accurate determination of activity coefficients is essential for predicting the behavior of solutions in fields ranging from chemistry and chemical engineering to environmental science and metallurgy.