Solution Thermodynamics

A solution may be defined as a homogeneous phase composed of different chemical substances, whose concentration may be varied without the precipitation of a new phase. It differs from a mixture by its homogeneity and from a compound by being able to possess variable composition. Raoult’s law is useful in discussing the properties of solutions. Solutions are classified into ideal and non-ideal groups depending on the condition whether they follow or deviate from Raoult’s law. The extent of deviation is accounted by introducing the concept of activity and activity coefficient. The molar fraction of concentration available for the reaction is called activity, i.e., the effective concentration. For an ideal solution, activity = atom/mole fraction. In order to account for any deviation from ideality a factor known as Raoultian activity coefficient is introduced. This factor may be greater or less than unity for a positive or negative deviation, respectively. The thermodynamic property of a particular component in a solution is discussed with the aid of partial molar quantities. Chemical potential and fugacity and their physical significance have been explained. Since calculation of excess free energy and integral molar free energy of a solution need activity as well as activity coefficient of all the components present in the solution, methods of estimation of activity of the second component by Gibbs-Duhem integration in a binary solution has been demonstrated. While discussing thermodynamics of dilute solutions attention has been focused on the validity of the Henry’s law, alternative standard states and their interrelationship as well as on interaction coefficients in multicomponent systems. A comparative assessment on regular solution model, Quasi-chemical model of solutions and the Darken’s formalism in binary metallic solutions has been presented in this chapter. Finally a brief discussion has been included on ionic melts.