|
An activity coefficient is a factor used in thermodynamics to account for deviations from ideal behaviour in a mixture of chemical substances. In an ideal mixture, the microscopic interactions between each pair of chemical species are the same (or macroscopically equivalent, the enthalpy change of solution and volume variation in mixing is zero) and, as a result, properties of the mixtures can be expressed directly in terms of simple concentrations or partial pressures of the substances present e.g. Raoult's law. Deviations from ideality are accommodated by modifying the concentration by an ''activity coefficient''. Analogously, expressions involving gases can be adjusted for non-ideality by scaling partial pressures by a fugacity coefficient. The concept of activity coefficient is closely linked to that of activity in chemistry. == Thermodynamic definition == The chemical potential, , of a substance B in an ideal mixture of liquids or an ideal solution is given by : where is the chemical potential in the standard state and xB is the mole fraction of the substance in the mixture. This is generalised to include non-ideal behavior by writing : when is the activity of the substance in the mixture with : where is the activity coefficient, which may itself depend on . As approaches 1, the substance behaves as if it were ideal. For instance, if , then Raoult's law is accurate. For and , substance B shows positive and negative deviation from Raoult's law, respectively. A positive deviation implies that substance B is more volatile. 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.〔R. DeHoff, ''Thermodynamic in Materials Science'', Taylor and Francis, 2006. pp230-1〕 Note that in general activity coefficients are dimensionless. Modifying mole fractions or concentrations by activity coefficients gives the ''effective activities'' of the components, and hence allows expressions such as Raoult's law and equilibrium constants to be applied to both ideal and non-ideal mixtures. Knowledge of activity coefficients is particularly important in the context of electrochemistry since the behaviour of electrolyte 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 electrolytes. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「activity coefficient」の詳細全文を読む スポンサード リンク
|