Estimation of Thermodynamic Properties of Organic Compounds in the Gas, Liquid, and Solid Phases at 298.15 K
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About one hundred years ago, the relationship between the boiling point of a substance and its heat of vaporization at the boiling point (Trouton’s Rule) was reported in the chemical literature. Since that time, much progress has been made in the correlation of physical and thermodynamic properties. Physical organic chemists in particular have been active in the development of correlation and estimation schemes linking the molecular structures of organic compounds and various thermodynamic properties. Some correlation and estimation schemes are more successful than others. Of the estimation schemes developed, the one put together by S.W. Benson and coworkers has had more universal acceptance because of its overall simplicity, ease of application, and general good agreement between estimated and experimental values. This scheme assigns individual group energy values for molecular fragments which are additive, account for nearest neighbor interactions, and give special consideration to corrections for steric strain or stereoisomerism. The primary focus of this scheme has been on organic molecules in the gas phase, although some applications to the liquid and solid phases have been reported.
Our recent development of the Benson approach to the estimation of thermodynamic properties (enthalpy of formation, heat capacity, and entropy) at 298.15 K has focused upon the liquid and solid phases. The gas phase has been included also for the sake of continuity and internal consistency of the calculation of thermodynamic properties among the three phases. Studies showing the predictive capability of this scheme toward estimating the thermodynamic properties of hydrocarbons and organic compounds containing the elements: CHO, CHN, and CHNO have already been carried out. Work is in progress to complete the scope of the predictive capability to cover organic compounds containing the elements: sulfur, phosphorus, fluorine, chlorine, bromine, iodine, and metals.
This paper provides a general overview of the NBS program in the estimation of the thermodynamic properties of organic compounds using the Benson approach and presents discussions of selected topics, such as: (1) relationships of the enthalpy of formation, heat capacity, and entropy at 298.15 K to other thermodynamic properties (Gibbs energy of formation, equilibrium constants, enthalpies and entropies of transition) which extend predictive capabilities, (2) descriptions of the Benson notation, group values and their application to the estimation of thermodynamic properties, and (3) explanations of unique solutions to the estimation of two selected classes of organic compounds.
KeywordsHeat Capacity Thermodynamic Property Hexanoic Acid Octanoic Acid Group Group
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