Thermodynamics of Solutions: Properties, Models, and Applications

1. Partial Molar Property


A partial molar property is the change in a thermodynamic property of a solution when an infinitesimal amount of a component is added, keeping the amount of other components constant.


It helps in understanding how each component in a mixture contributes to the overall properties of the solution.

2. Chemical Potential


The chemical potential (𝜇) is the change in the Gibbs free energy of a system when the number of particles of a component is increased by one, keeping temperature, pressure, and other component amounts constant.


It determines the direction of chemical reactions and phase equilibria. A system is in equilibrium when the chemical potential is equal for all phases.

3. Methods of Determining Partial Molar Property

Graphical Method:

  1. Plot the total property (e.g., volume, enthalpy) of the solution against the composition.
  2. The slope of the tangent to this curve at a given composition gives the partial molar property.

Interception Method:

  1. Measure the property of mixtures of different compositions.
  2. Use linear regression or graphical methods to determine the intercepts and slopes that correspond to partial molar properties.

4. Tangent Intercept Method


Used to determine the partial molar properties by analyzing the graphical representation of a thermodynamic property as a function of composition.


  1. Plot the total property (e.g., volume, enthalpy) versus composition.
  2. Draw tangents to the curve at different points.
  3. The intercept of these tangents with the axes provides values related to the partial molar properties.


Commonly used in thermodynamics to find partial molar volumes and enthalpies from experimental data.

5. Consistency Tests for VLE Data


To validate the accuracy and consistency of experimental Vapor-Liquid Equilibrium (VLE) data.

Common Tests:

  • Gibbs-Duhem Test: Checks if the experimental data satisfy the Gibbs-Duhem equation, ensuring thermodynamic consistency.
  • Herington Test: Evaluates the consistency of VLE data by comparing the calculated activity coefficients with experimental values.
  • Redlich-Kister Test: Assesses the quality of VLE data by analyzing the deviation of experimental data from an ideal behavior predicted by the Redlich-Kister equation.

6. Effect of Temperature and Pressure on Equilibrium Constant


Endothermic Reactions:

  • Increase in Temperature: Equilibrium constant (K) increases.
  • Explanation: According to Le Chatelier’s Principle, adding heat favors the endothermic direction (products), increasing K.

Exothermic Reactions:

  • Increase in Temperature: Equilibrium constant (K) decreases.
  • Explanation: Adding heat favors the endothermic direction (reactants), decreasing K.


Gaseous Reactions with Unequal Mole Numbers:

  • Increase in Pressure: Favors the side with fewer gas molecules.
  • Explanation: Le Chatelier’s Principle states that the system will adjust to reduce the pressure.

Gaseous Reactions with Equal Mole Numbers:

  • Effect: No change in the equilibrium constant.
  • Explanation: Changes in pressure have no effect if the number of gas molecules on both sides is equal.

7. UNIQUAC Model

UNIQUAC (Universal Quasi-Chemical) is a model used in chemical engineering to predict the thermodynamic properties of liquid mixtures.


UNIQUAC is designed to predict activity coefficients in liquid mixtures, facilitating the calculation of phase equilibria.


The model considers both combinatorial (size and shape of molecules) and residual (energy interactions between molecules) contributions to the activity coefficients.


UNIQUAC is widely used in the design and optimization of separation processes in industries like petrochemicals and pharmaceuticals due to its ability to handle a wide range of mixtures, including highly non-ideal systems.

8. Vapor Absorption System


  • Absorber: Absorbs refrigerant vapor into a liquid absorbent.
  • Generator: Heats the solution from the absorber to separate the refrigerant.
  • Condenser: Condenses the separated refrigerant into a liquid.
  • Expansion Valve: Reduces the pressure of the liquid refrigerant.
  • Evaporator: Absorbs heat from the space to be cooled, causing the refrigerant to evaporate.


  • Uses a heat source (e.g., steam, hot water) to drive the refrigeration cycle.
  • Typically employs a solution like lithium bromide-water or ammonia-water.

9. UNIFAC Model

UNIFAC (Universal Quasi-Chemical Functional Group Activity Coefficients) is a model used in chemical engineering to predict the activity coefficients of mixtures.


UNIFAC is designed to predict the non-ideal behavior of mixtures, which is essential for calculating phase equilibria, such as vapor-liquid and liquid-liquid equilibria.

Functional Group Contributions:

The model breaks down molecules into functional groups and calculates their contributions to the overall activity coefficient. This approach allows for the estimation of interactions between different types of molecules in a mixture.

Predictive Nature:

Unlike empirical models that require extensive experimental data, UNIFAC can predict activity coefficients using a database of group interaction parameters, making it useful for systems where experimental data is scarce or unavailable.


It is widely used in process design, simulation, and optimization in industries like petrochemicals, pharmaceuticals, and environmental engineering to model the behavior of complex mixtures.