Heat capacity data sources

Sources of heat capacity data at many temperatures include ... 

Standard entropies for many substances are available in tables such as Tables 11.2 through 11.6. Generally, the values listed are for 298.15 K, but many of the original sources, such as the tables of the Thermodynamics Research Center, the JANAF tables, or the Geological Survey tables, give values for other temperatures also. If heat capacity data are available, entropy values for one temperature can be converted to those for another temperature by the methods discussed in Section 11.4. 

Experimental (estimated if in brackets) entropy values based on first-mentioned source of heat-capacity data. 

The selection of entropy and heat capacity data for trigonal selenium in the temperature range 298.15 to 494.2 K and for the liquid in the temperature range 494.2 to 1500 K. The thermodynamic properties of the metastable monoclinic phase and the supercooled liquid are assessed for use as auxiliary data. The major source of information is the review by Gaur, Shu, Mehta, and Wunderlich [81GAU/SHU] which has been combined with other information for phase transformations. 

Figure 2.49 Comparison of heat capacity data for Y2Mn207 from different sources and that of the diamagnetic Y2Sn20y. Reprinted with permission from Gardner et al., 2010 [6]. Copyright (2010) American Physical Society...

Although these potential barriers are only of the order of a few thousand calories in most circumstances, there are a number of properties which are markedly influenced by them. Thus the heat capacity, entropy, and equilibrium constants contain an appreciable contribution from the hindered rotation. Since statistical mechanics combined with molecular structural data has provided such a highly successful method of calculating heat capacities and entropies for simpler molecules, it is natural to try to extend the method to molecules containing the possibility of hindered rotation. Much effort has been expended in this direction, with the result that a wide class of molecules can be dealt with, provided that the height of the potential barrier is known from empirical sources. A great many molecules of considerable industrial importance are included in this category, notably the simpler hydrocarbons. 

The development and application of the method can be illustrated by considering the problem of integrating the utilisation of energy between 4 process streams. Two hot streams which require cooling, and two cold streams that have to be heated. The process data for the streams is set out in Table 3.3. Each stream starts from a source temperature Ts, and is to be heated or cooled to a target temperature Tt. The heat capacity of each stream is shown as CP. For streams where the specific heat capacity can be taken as constant, and there is no phase change, CP will be given by ... 

The Geothermal Response Test as developed by us and others has proven important to obtain accurate information on ground thermal properties for Borehole Heat Exchanger design. In addition to the classical line source approach used for the analysis of the response data, parameter estimation techniques employing a numerical model to calculate the temperature response of the borehole have been developed. The main use of these models has been to obtain estimates in the case of non-constant heat flux. Also, the parameter estimation approach allows the inclusion of additional parameters such as heat capacity or shank spacing, to be estimated as well. 

Unless otherwise said, our preferred sources for enthalpies of formation of hydrocarbons are Reference 8 by Roth and his coworkers, and J. B. Pedley, R. D. Naylor and S. P. Kirby, Thermochemical Data of Organic Compounds (2nd ed.), Chapman Hall, New York, 1986. In this chapter these two sources will be referred to as Roth and Pedley , respectively, with due apologies to their coworkers. We will likewise also occasionally take enthalpies of fusion from either E. S. Domalski, W. H. Evans and E. D. Hearing, Heat Capacities and Entropies of Organic Compounds in the Condensed Phase , J. Phys. Chem Ref. Data, 13, 1984, Supplement 1, or E. S. Domalski and E. D. Hearing, J. Phys. Chem Ref. Data, 19, 881 (1990), and refer to either work as Domalski . 

A critically evaluated compilation of the heat capacities of pure liquid organic and some inorganic compounds. It covers data published between 1993 and 1999 and some data of 2000 as well as some data from older sources. This paper is an update of reference [24]. 

Data on a number of physical properties are also required. This includes vapour and liquid densities, latent heat of vaporisation and liquid specific heat capacity. These can usually be obtained frorrihiterature sources. Their measurement is beyond the scope of this Workbook. 

The interest in thermal data for hydrocarbons stems from two sources. The first relates to a need to establish the chemical potential (21) or the free energy (44) of pure compounds from measurements of the heat capacity from low absolute temperatures to the temperatures of interest. Such measurements and the third law of thermodynamics permit the evaluation of the free energy. The second industrial interest in thermodynamic properties arises from a need to evaluate the heat and work associated with changes in state of hydrocarbon systems. The measurements by Rossini (57), Huffman (17), and Parks (32, 53) are worthy of mention in a field replete with a host of careful investigators. Such thermal measurements have been of primary utility in predicting chemical equilib-... 

The methods used in predicting these thermodynamic properties employ (a) an equation of state, relating the pressure-volume-temperature characteristics of the fluids (b) ideal gas state heat capacities of the individual components and (c) binary interaction coefficients between the components. The development of these basic relationships is not within the scope of this paper. Technical literature sources of the thermodynamic equations and data are given in the references. 

It is possible, and perhaps generally believed, that the high reproducibility of an isotherm justihes the extraction of thermodynamic values from data that show hysteresis. However, hysteresis would still be a source of systematic error in the values. There is a poorly documented impression that small samples or thin films display less hysteresis. Hysteresis was not found for the heat capacity isotherm (Yang and Rupley, 1979), which may he taken as support for the view that meaningful free-energy information also can be derived from sorption isotherms. 

In the classical differential thermal analysis (DTA) system both sample and reference are heated by a single heat source. The two temperatures are measured by sensors embedded in the sample and reference. In the so-called Boersma system, the temperature sensors are attached to the sample pans. The data are recorded as the temperature difference between sample and reference as a function of time (or temperature). The object of these measurements is generally the determination of enthalpies of changes, and these in principle can be obtained from the area under a peak together with a knowledge of the heat capacity of the material, the total thermal resistance to heat flow of the sample and a number of other experimental factors. Many of these parameters are often difficult to determine hence, DTA methods have some inherent limitations regarding the determination of precise calorimetric values. 

Look It Up. When you need a value for a physical property of a substance—whether it be a density, vapor pressure, solubility, or heat capacity—there is a good chance that someone, somewhere has measured this property and published the result. Since experiments are usually costly and time consuming, a reliable source of physical property data is an invaluable asset in process analysis. Four excellent sources of data are the following ... 

The accuracy of calculated equilibrium states depends critically on the data sources used. Accurate predictions of heat capacities are often... 

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