Enthalpy data sources

With regard to enthalpy changes, the simplest manner for computing is via tabulated enthalpy data. Data can be found in the literature [1,2]. Typical enthalpy data for air and water are given in Table 2.2. The Steam Tables are the most frequently used sources of enthalpy data for water. 

Tables of AH° for compounds are the most important data source for thermochemistry. From them it is easy to calculate AH° for reactions of the compounds, and thereby systematically compare the energy changes due to bond rearrangements in different reactions. Appendix D gives a short table of standard enthalpies of formation at 25°C. The following example shows how they can be used to determine enthalpy changes for reactions performed at 25°C and 1 atm pressure.

Ascertain the reference state for enthalpy values from the data source... 

Tables 4.3 and 4.4 list typical enthalpy data for the combustion gases. Some sources of enthalpy data are listed in Table 4.5. The most common source of enthalpy data for water is the steam tables which are reproduced in Appendix Cl and on a sheet that can be found inside the back cover.

Table A-47 Calculation of the standard enthalpy of sources of the data are indicated. 

Vapor-liquid equilibrium data and enthalpy data needed to describe this system.were taken from several sources.16,18,20... 

The Barin tables are far more complete in coverage than any of the sources described above. All of the natural elements and their compounds are included. In addition to the substance types listed in USBM Bull 677, the Barin tables include a large number of ternary oxides - aluminates, arsenates, borates, chromates, molybdates, nitrates, oxy-halides, phosphates, titanates, tungstates, selenates, vanadates, zirconates, etc. - as well as cyanides, hydroxides, complex silicates and inter-metallic compounds. The only substances not included by Barin, for which tables can be found elsewhere, are the ionized-gas species and a limited number of gas species important only at very high temperatures, which are listed in the JANAF tables. For each table Dr. Barin gives references for each of the major thermochemical values employed (enthalpy of formation and entropy at 298 K, and heat capacity). Like the USBM Bulletins, no attempt is made to discuss the choice between conflicting data sources. 

The primary data sources are individual publications reporting the results of experimental investigations and supplementary theoretical calculations. The bases for these are calorimetric determinations of enthalpies of reaction, phase transformation and heat capacities, vapor pressure determinations, emf measurements, equilibrium determinations etc. 

Kinetic studies of pyrolysis processes in the gas phase have been an important source of bond dissociation enthalpy data for organometaiiic neutral molecules. In the case of very low pressure pyrolysis (VLPP), a precursor molecule... 

The main advantage of this method is that no additional correlations and routes have to be implemented in the simulator. Only the parameters for the heat of vaporization are affected. Moreover, the procedure has become a consistency test. If the data for vapor pressure, cp , Cp, and Ah are correct, the equation of state used gives reasonable values for (H-h ), and no extrapolations of any correlation take place during the parameter adjustment, it should be possible to represent both the liquid heat capacity and the enthalpy of vaporization. If the procedure does not work at once for a substance, all the input data should again be checked carefully. In most of the cases, ambiguous data sources or raw data based on bad estimations can be detected. After correction, the problems often disappear. 

This book contains enthalpy data on 4000 compounds and 5000 chemical processes involving organic and organometallie compounds. Much of the processing of the data was done by computer (see Item [150]). Properties which are presented Include enthalpies of combustion, formation, sublimation, vaporization, and reaction. It Is also a revision of the values published by Cox and Pilcher (item [28]). There are 450 references to the sources from which the data were taken. 

This article Is a detailed review of the thermodynamics, kinetics, and structural characteristics of adenosine and the adenine nucleotides In solution. Both log K and enthalpy data are tabulated for protonation and metal-ion binding reactions to adenosine and the adenine nucleotides. The conditions are given under which the tabulated data are applicable, namely, Ionic strength, temperature, supporting electrolyte, pH, method of measurement, as well as references to the original data source. 

These volumes contain extensive tabulations of physical data relevant to concentrated solutions of binary systems, both organic and inorganic. The properties that are tabulated include dielectric constant, viscosity,. equivalent conductivity, surface tension, diffusion and thermal diffusion coefficients, vapor pressure, specific heat, electrochemical data, enthalpy of combustion, enthalpy of dilution and solution, transition enthalpies, and other properties. These books contain extensive tabulations of data pertinent to water and electrolyte solutions. The data are well organized and there is a general compound index as well as references to the original data sources. 

In the above example it was not necessary to know how the specific heats varied with temperature, at what temperature the phase change occurred and the latent heat of vaporisation at that temperature. By working in enthalpies it was sufficient to know the enthalpy of the feed stream and the enthalpy of the product stream. Thus, whenever possible, engineers extract enthalpy data from reference sources or computer databases in preference to specific heat capacity and latent heat data. 

The originally measured data for each single system are then listed together with some comment lines if necessary. The data are usually given as published, but temperatures are always given in K. Pressures are sometimes recalculated into kPa or MPa. Enthalpy data are always recalculated into J or kj, if necessary. Mass fraction-based Henry s constants are calculated from pubhshed specific retention volumes, if such data are not provided in the original source. They are always tabulated in MPa. 

Thermodynamic Parameters For the majority of the lead(ll) polymeric hydrolysis species, enthalpy data are available from more than one source. The average of these values has been retained and an uncertainty assigned to span the range in the values and their respective assigned uncertainties ... 

Reid, Prausnitz, and Sherwood (1977) provide ideal gas Cp data for 468 organic molecules as four-term polynomials. The valid temperature range of these polynomials is not mentioned, nor is the data source, presumably mostly Thinh et al (1971). The absolute enthalpy polynomial (probably in the range 273-1000 or 298-1500 K) can also be found since the value of A// 298 is mentioned. The entropy cannot be calculated since the integration constant is not given. However, as mentioned earlier, these polynomials are useless anyway for high-temperature combustion calculation since they cannot be extrapolated far outside the range where they were fitted. 

Enthalpy of Formation The ideal gas standard enthalpy (heat) of formation (AHJoqs) of chemical compound is the increment of enthalpy associated with the reaction of forming that compound in the ideal gas state from the constituent elements in their standard states, defined as the existing phase at a temperature of 298.15 K and one atmosphere (101.3 kPa). Sources for data are Refs. 15, 23, 24, 104, 115, and 116. The most accurate, but again complicated, estimation method is that of Benson et al. " A compromise between complexity and accuracy is based on the additive atomic group-contribution scheme of Joback his original units of kcal/mol have been converted to kj/mol by the conversion 1 kcal/mol = 4.1868 kJ/moL... 

Data on the gas-liquid or vapor-liquid equilibrium for the system at hand. If absorption, stripping, and distillation operations are considered equilibrium-limited processes, which is the usual approach, these data are critical for determining the maximum possible separation. In some cases, the operations are are considerea rate-based (see Sec. 13) but require knowledge of eqmlibrium at the phase interface. Other data required include physical properties such as viscosity and density and thermodynamic properties such as enthalpy. Section 2 deals with sources of such data. 

Extensive tabulations of standard enthalpies of formation and related thermodynamic data can be found in the literature.5 Table 9.1 summarizes selected values from these sources. 

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