Chapter 10. Enthalpy Changes on Solution of Gases in Liquids

Abstract

Solution chemistry is at the heart of many research areas in the pure and applied sciences, and quantitative experimental investigations of physical properties of nonelectrolyte solutions and of corresponding phase equilibria, in particular vapour (V)/liquid (L) equilibria, have always held a prominent position in physical chemistry. In this context, we note that infinite-dilution properties are of considerable theoretical interest as well as of practical importance in chemical engineering, biophysical chemistry, ecology and biomedical research. This review is on the enthalpy changes on solution of nonreactive gases in liquids with the focus being on aqueous solutions, since water is the most important solvent on earth. High-precision measurements of the solubility of gases (solute 2) in liquids (solvent 1) yield Henry fugacities h2,1(T,Pσ,1), also known as Henry's law constants, where Pσ,1=Pσ,1(T) is the vapour pressure of the solvent at temperature T. Determining h2,1 over extended temperature ranges, and subjecting these data to a van't Hoff-type analysis, i.e., essentially determining the temperature dependence of the Henry fugacity, yields the molar enthalpy change on solution , where HL,∞2 is the partial molar enthalpy of solute 2 at infinite dilution in liquid solvent 1, and is the molar enthalpy of the pure (*) solute in the perfect gas (pg) state. Using sophisticated calorimeters, a small number of laboratories has succeeded in measuring directly enthalpies of solution ΔsolH2 of slighly soluble gases in liqids. For the solutions investigated so far, these calorimetry-based results agree excellently with the van't Hoff-based results.