Thermodynamics of mixtures

Introduction to Thermodynamics of Mixtures

Thermodynamics of mixtures is a branch of thermodynamics that deals with the behavior of mixtures of different substances under different conditions. A mixture is formed when two or more substances are combined physically, but not chemically, to form a new substance. The behavior of mixtures is different from that of pure substances, and it is important to understand the concepts of thermodynamics of mixtures to predict and control their behavior.

The thermodynamics of mixtures is important in various fields such as chemical engineering, material science, and environmental science. In chemical engineering, the thermodynamics of mixtures is used to design and optimize separation processes such as distillation, extraction, and adsorption. In material science, the thermodynamics of mixtures is used to understand the properties of alloys and polymers. In environmental science, the thermodynamics of mixtures is used to study the behavior of pollutants in the environment.

Key Concepts and Definitions

The key concepts and definitions in thermodynamics of mixtures include the following:

  • Mole fraction: It is the ratio of the number of moles of a component to the total number of moles in a mixture. It is denoted by the symbol x and is expressed as x = n_i / n_tot, where n_i is the number of moles of the ith component and n_tot is the total number of moles in the mixture.
  • Partial molar property: It is the change in a thermodynamic property of a mixture when one mole of a component is added to the mixture while keeping the temperature, pressure, and the number of moles of other components constant. It is denoted by the symbol bar{V_i}, bar{H_i}, bar{S_i}, etc., for volume, enthalpy, entropy, etc., respectively.
  • Mixing enthalpy: It is the enthalpy change when two or more substances are mixed together. It can be either positive or negative, depending on whether the mixture is exothermic or endothermic.

Thermodynamic Properties of Mixtures

The thermodynamic properties of mixtures include the following:

  • Gibbs free energy: It is the energy that is available to do work at constant temperature and pressure. The Gibbs free energy of a mixture is given by the equation G = sum_i n_i mu_i, where mu_i is the chemical potential of the ith component.
  • Enthalpy: It is the heat absorbed or released when a system undergoes a constant pressure process. The enthalpy of a mixture is given by the equation H = sum_i n_i bar{H_i}, where bar{H_i} is the partial molar enthalpy of the ith component.
  • Entropy: It is a measure of the disorder or randomness of a system. The entropy of a mixture is given by the equation S = -sum_i n_i R ln x_i, where R is the gas constant and x_i is the mole fraction of the ith component.

Example: Calculating Enthalpy of Mixing

The enthalpy of mixing can be calculated using the following equation:

Delta H_{mix} = sum_i sum_j n_i nj bar{H{ij}}

where bar{H_{ij}} is the partial molar enthalpy of mixing of the ith and jth components. The partial molar enthalpy of mixing can be obtained experimentally or calculated using thermodynamic models such as the regular solution model or the excess Gibbs energy model.

For example, consider a mixture of ethanol (C_2H_5OH) and water at a temperature of 25°C. The mole fraction of ethanol is 0.4 and the mole fraction of water is 0.6. The partial molar enthalpies of ethanol and water are 44.1 kJ/mol and 55.5 kJ/mol, respectively. The partial molar enthalpy of mixing of ethanol and water is -5.9 kJ/mol.

The enthalpy of mixing can be calculated as follows:

Delta H_{mix} = (0.4)(0.4)(-5.9) + 2(0.4)(0.6)(44.1) + (0.6)(0.6)(-5.9) + 2(0.6)(0.4)(55.5) = 2.26 kJ/mol

This means that the mixing of ethanol and water is exothermic, releasing 2.26 kJ of energy per mole of mixture.