Chapter 6 - Vapor Pressures

Abstract

Vapor pressure is required in developing equations of state, in studying first-order and second-order vapor–liquid phase transitions, in obtaining the other thermodynamic properties of substances, in deriving the enthalpy and entropy in the two-phase region, and in describing binary and multicomponent mixtures, hence the objective of extensive studies based on both theoretical ideas and empirical approaches has been to describe the vapor pressure as a function of temperature along the entire vapor–liquid coexistence curve. The Clausius–Clapeyron equation proposed the exact thermodynamic relation between the saturated vapor pressure p, the latent heat or enthalpy of vaporization M-/, the temperature T, and the volume change AV or the compressibility-factor change AZ accompanying vaporization. According to the modern understanding of critical phenomena from critical scaling laws, there is a theoretical value for not only the leading non-analytic critical exponent but also the corresponding amplitude; specifically, certain ratios of amplitudes are universal constants. An overview of Clausius–Clapeyron, Antoine modified the Clausius–Clapeyron equation, Frost and Kalkwarf proposed equation, and many other theories opposed by various scientists are shown with their shortcomings, finally the equation proposed by Xiang and Tan is shown to satisfy certain criteria and had good generalization, and is applicable to simple, quantum, nonpolar, polar, hydrogen-bonding, and associating molecules. The Xiang–Tan equation was applied to a diverse set of substances including simple, nonpolar, polar, quantum, hydrogen-bonding, and associating substances, which have been experimentally investigated in detail to obtain reliable experimental data according to the table given. The three parameters of some substances are revised from the table along with the critical temperature and critical pressure. A comparison has been made between the vapor–pressure values calculated from the Xiang–Tan equation and the experimental data, and then Xiang–Tan equation is found to be accurate in many ways.