Thermodynamique : coefficient d'activité

Activity is the measure of effective concentration and equals the product of molar concentration and activity coefficient. In ideal solutions, activity equals concentration. However, as ionic strength increases, the activity coefficient decreases below unity, making activity lower than the actual concentration. This deviation from ideality becomes more pronounced in solutions with higher ionic strength.

The activity coefficient depends directly on ionic strength. In solutions with very low ionic strength, the activity coefficient approaches unity, indicating nearly ideal behavior. As ionic strength increases beyond 0.1 mol/L, the activity coefficient decreases below unity, indicating deviation from ideality. In very high ionic strength solutions, the activity coefficient may exceed unity. The relationship is expressed through the extended Debye-Hückel equation.

The activity coefficient is a dimensionless quantity because it represents a ratio—the deviation of actual behavior from ideal behavior. It multiplies molar concentration to yield activity, which is also unitless. This dimensionless nature allows the activity coefficient to serve as a universal correction factor across different solution systems and concentrations.

The Debye-Hückel limiting law is a simplified form of the extended Debye-Hückel equation used for extremely dilute solutions with very low ionic strength, typically below 0.1 mol/L. In these conditions, uncertainties in ion sizes have negligible effects, and the solution behaves nearly ideally. The limiting law provides accurate activity coefficient calculations without requiring detailed ion size parameters.

The activity coefficient of uncharged molecules is approximately unity at all ionic strengths less than 0.1 mol/L, meaning their activity essentially equals their molar concentration. This occurs because neutral species lack the electrostatic interactions that affect ionic species. Therefore, uncharged molecules exhibit nearly ideal solution behavior across a wide range of dilute conditions.

The extended Debye-Hückel equation relies on the effective diameter of hydrate ions, which varies widely with charge and ion geometry. These uncertainties in ion size parameters can significantly affect calculated activity coefficients, particularly in concentrated solutions. The equation works best in extremely dilute solutions where these size uncertainties have minimal impact on results.

Activity represents the effective concentration of species in equilibrium systems and is essential for accurate equilibrium calculations. Using activity instead of molar concentration accounts for deviations from ideal behavior caused by ionic interactions. This correction becomes increasingly important in solutions with higher ionic strength, ensuring more accurate predictions in chemical equilibria systematic approach to equilibrium calculations.