Activity coefficient

A measure of deviation from ideal behavior in a mixture

Activity coefficient

The activity coefficient is a factor used in thermodynamics to account for deviations from ideal behavior in a mixture of chemical substances. In an ideal solution, the interactions between molecules are the same as those in the pure components, and the activity coefficient is equal to one. However, in real solutions, interactions can differ, leading to activity coefficients that deviate from unity.

Definition

In a mixture, the activity \( a_i \) of a component \( i \) is related to its concentration \( c_i \) by the equation:

\[

a_i = \gamma_i c_i

\]

where \( \gamma_i \) is the activity coefficient of component \( i \). The activity coefficient quantifies the non-ideal behavior of the solution.

Importance

Activity coefficients are crucial in chemical thermodynamics for calculating the chemical potential of a species in a mixture. They are used in the Gibbs free energy calculations and are essential for understanding phase equilibria, reaction equilibria, and electrolyte solutions.

Models for Activity Coefficients

Several models exist to estimate activity coefficients, including:

• Debye–Hückel theory: Used for dilute electrolyte solutions.
• UNIQUAC (Universal Quasi-Chemical): A model for predicting activity coefficients in non-electrolyte solutions.
• NRTL (Non-Random Two-Liquid): A model that accounts for non-randomness in liquid mixtures.

Applications

Activity coefficients are used in various fields such as:

• Chemical engineering: For designing separation processes like distillation and extraction.
• Environmental science: To model the behavior of pollutants in natural waters.
• Pharmaceuticals: In the formulation of drugs to ensure proper solubility and stability.

Related pages

• Chemical potential
• Gibbs free energy
• Phase equilibria
• Electrolyte solution

Gallery

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  Graph showing UNIQUAC regression for chloroform and methanol.

Activity Coefficient

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