Phase Changes and Thermochemistry

The phase changes of water result in the transitions between the solid, liquid, and gas states  

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M. Hilled, Methods Of Calculating Phase Diagrams , in Calculation of Phase Diagrams and Thermochemistry of Alloys, Y. A. Chang and J. F. Smith, Eds., Proc. Conf. AIME Fall Meeting, Sept. 17-18, 1979, Milwaukee, 1979, The Metallurgical Society, pp. 1-13 (1979). 

Thermochemistry is the study of heat changes that accompany chemical reactions and phase changes. In thermochemistry, the system is the reaction or process being studied, and everything outside the system is called the surroundings. The universe is defined as the system plus the surroundings. 

Thermochemistry is concerned with the study of thermal effects associated with phase changes, formation of chemical compouncls or solutions, and chemical reactions in general. The amount of heat (Q) liberated (or absorbed) is usually measured either in a batch-type bomb calorimeter at fixed volume or in a steady-flow calorimeter at constant pressure. Under these operating conditions, Q= Q, = AU (net change in the internal energy of the system) for the bomb calorimeter, while Q Qp = AH (net change in the enthalpy of the system) for the flow calorimeter. For a pure substance. 

J. S. Chikos, W. E. Acree Jr., and J. F. Liebman, in Computational Thermochemistry, K. K. Irikura and D. J. Frurip, Eds., ACS Symposium Series 677, American Chemical Society, Washington, DC, 1998, pp. 63-91. Estimating Phase-Change Enthalpies and Entropies. 

As described in the Introductory Chapter, attention was focused [1] prior to 1961 mainly on the morphology of the cool-flame and ignition regions, rates were followed by pressure change, and essentially chemical techniques were used for product analysis. The acceptance of free radicals, followed by the masterly and elegant Semenov theory [2], which established the principles of branched chain reactions, provided the foundation for modern interpretations of hydrocarbon oxidation. This chapter builds on these early ideas, and pioneering experiments such as those carried out by Knox and Wells [3] and Zeelenberg and Bickel [4], to provide a detailed account of the reactions, thermochemistry and detailed mechanisms involved in the gas-phase chemistry of hydrocarbon oxidation. 

Calorimetry is the basic experimental method employed in thermochemistry and thermal physics which enables the measurement of the difference in the energy U or enthalpy of a system as a result of some process being done on the system. The instrument that is used to measure this energy or enthalpy difference (At/ or A//) is called a calorimeter. In the first section the relationships between the thermodynamic functions and calorimetry are established. The second section gives a general classification of calorimeters in terms of the principle of operation. The third section describes selected calorimeters used to measure thermodynamic properties such as heat capacity, enthalpies of phase change, reaction, solution and adsorption. 

Data of heats of formation of compounds for the purpose of deter-mining the enthalpies (heat outputs) of pyrochemical reactions, together with other pertinent data (on phase changes, melting points, etc.), are found in Circular 500 NBS and Metallurgical Thermochemistry by Kubaschewski and Evans, Lange s Handbook contains reprinted data from the former. ... 

From the above relationships, it should be apparent that, if there are no phase changes involved, we can characterize the thermochemistry of a substance if we know AfH and S at some temperature (tables and compilations list either AfHo, AfH2m or AfH298)... 

J. D. Cox and G. Pilcher, Thermochemistry of Organic and Organometallic Compounds, Academic Press, London New York, 1970. The major feature is a tabulation of experimental measurements of A H°, heats of reaction, for organic and organometallic compounds. Values of AfH° are derived for each experimental result where phase changes are involved measurements of AH (heat of vaporization) are tabulated. Experimental uncertainties are assessed. Introductory chapters discuss the basics of thermochemistry, the types of experimental measurements involved, their accuracies and limitations. Additional chapters examine theoretical aspects of thermochemistry and various schemes for relating thermochemical properties to structural parameters. 

Energetics. Regardless of the gas phase combustion kinetics and thermochemistry, burning will only be possible if the energy balance is favorable. The first law of thermodynamics for a constant pressure gas phase process in which all of the work is pressure-volume (P-V) work states that the internal energy change dLT is related to the change in heat content dQ ... 

A new volume of Landolt-Bomstein appeared in 1961. This deals with calorimetric quantities and is concerned with elements, alloys, and compounds, and with reaction enthalpies. Subjects covered include the experimental and theoretical basis of thermochemistry, standard values of molar enthalpies, entropies, enthalpies of formation, free energies of formation, and enthalpies of phase change. Planck, Einstein, and Debye functions, anharmonicity, and internal rotation are considered. The final section presents thermodynamic data for mixtures and solutions. 

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