Chapter 6: Thermodynamics of Solutions

Thermodynamics of Solutions begins by defining the fundamental properties of ideal and non-ideal solutions, utilizing Raoult's law to describe the relationship between vapor pressure, mole fraction, and activity in ideal scenarios. The text elaborates on the deviations from ideality—both positive and negative—driven by the attractive or repulsive forces between component atoms, quantified through the thermodynamic parameter known as the activity coefficient. A significant portion of the discussion is dedicated to partial molar quantities, explaining how extensive properties like volume, enthalpy, and entropy change when components are added to a solution, and introducing the Gibbs-Duhem equation as a vital mathematical tool for relating these changes. The chapter further explores the integral molar quantities of mixing, detailing the free energy, enthalpy, and entropy changes associated with the formation of solutions, noting that ideal solution formation involves zero enthalpy change and zero volume change. The concept of excess functions is introduced to measure the difference between real and ideal solution behaviors. Additionally, the text defines regular solutions, where the entropy of mixing is ideal but the heat of mixing is non-zero. The discussion extends to dilute solutions governed by Henry's law, contrasting it with Raoultian behavior, and details the use of the one weight percent standard state, which is particularly relevant for industrial steelmaking calculations. Finally, the chapter addresses dilute multicomponent solutions, employing Wagner's interaction coefficients to quantify how various solutes influence the activity coefficients of others in complex alloy systems like liquid steel.