The thermochemical database

Up to now, we have assumed that we have a database large enough to calculate all the group energy values we need. Clearly that database has to come from somewhere. In fact, most of the enthalpies of formation we have (webbook.nist.gov) come from combustion studies. Just about all organic compounds bum. Many of them bum quantitatively to form only CO2 and H20. If a compound bums quantitatively, its enthalpy of formation can be determined from the heat given off, its heat of combustion. 

We have seen that Af//298 is the enthalpy change of a compound associated with its formation, in the standard state, from elements also in the standard state, In some cases, Af//298 can be measured directly. For example Af //298 (C02) is determined by burning graphite in oxygen. 

This reaction can be carried out under conditions that permit the temperature change of the surroundings to be measured and so to permit determination of the amount of heat given off per mole of C02(g) formed. [Although the actual combustion takes place at a high 

Although flow calorimetric experiments have been carried out at constant P which yield A Hm directly (for example, Wiberg and Fenoglio, 1968), combustion is usually carried out at constant volume in a closed container. Closed container or bomb calorimetric measurements of AT yield the energy of formation in the standard state, for example, AfE°29i (C02). This datum leads to AfH29g ( C02) through the defining equation 

Determining AfH°29S (hydrocarbons) by combustion, despite its seeming simplicity, is not an easy job. Combustion thermochemistry requires meticulous control of experimental conditions (Steele et al., 2002). The difficulty of this task and its expense in time and money mean that researchers in need of AfH°29% data are likely to find vast gaps in the thermochemical record. At present, chemists are synthesizing new compounds far more rapidly than thermochemical properties are being measured, hence the reliance in many contemporary studies and in industrial laboratories on computational methods. The simplest of these 

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The case of liquid solutions is more complicated because the conventions vary. These are always stated in introductory chapters of the thermochemical databases and deserve a careful reading. In most tables and in the present book, it is agreed that the standard state for the solvent is the pure solvent under the pressure of 1 bar (which corresponds to unit activity). For the solute, the standard state may refer to the substance in a hypothetical ideal solution at unit molality (the amount of substance of solute per kilogram of solvent) or at mole fraction x = 1. 

Several electrochemical techniques may yield the reduction or oxidation potentials displayed in figure 16.1 [332-334], In this chapter, we examine and illustrate the application of two of those techniques cyclic voltammetry and photomodulation voltammetry. Both (particularly the former) have provided significant contributions to the thermochemical database. But before we do that, let us recall some basic ideas that link electrochemistry with thermodynamics. More in-depth views of this relationship are presented in some general physical-chemistry and thermodynamics textbooks [180,316]. A detailed discussion of theory and applications of electrochemistry may be found in more specialized works [332-334],... 

The equilibrium distribution coefficient close to the melting point is also known as the partition coefficient. Since the partition coefficient controls the incorporation of impurities in the crystal during crystal growth and zone refining, it is one of the most important parameters that can be obtained from the thermochemical database. It is worth noting that the distribution coefficient determined by the ratio of volume concentrations, cm 3, can be related to the distribution coefficient by introducing the density ratio of liquid and solid silicon ... 

The assessed kinetic database covers the same system as in the thermochemical database and is schematically shown in Figs. 13.11 and 13.12, respectively. Diffusivities of Al, As, B, C, Fe, N, O, P, and Sb in both solid and liquid silicon have been extensively investigated. The assessment of the impurity diffusivity is basically the same as for the thermodynamic properties. Experimental data were first collected from the literature. Then, each piece of selected experimental information was given a certain weight factor by the assessor. The weight factor could be changed until a satisfactory description of the majority of the selected experimental data was reproduced. 

A more sophisticated approach for determination of the grain boundary segregation is similar to the determination of the surface tension of silicon melt. The novel approach of surface tension simulation has been successfully implemented in the thermochemical database. Hence, the assessment of the parameters for impurity segregation in solid silicon phase may greatly extend the application of the thermochemical database. The calculation results for C and O segregation are shown as dashed lines in Fig. 13.28. The McLean segregation isotherm can be reproduced using the approach similar to the surface tension simulation. 

The need to make available a comprehensive, internationally recognised and quality-assured chemical thermodynamic database that meets the modeling requirements for the safety assessment of radioactive waste disposal systems prompted the Radioactive Waste Management Committee (RWMC) of the OECD Nuclear Energy Agency (NEA) to launch in 1984 the Thermochemical Database Project (NEA-TDB) and to foster its continuation as a semi-autonomous project known as NEA-TDB Phase 11 in 1998. 

The thermochemical database for substituted naphthalenes is far less extensive than for substituted benzenes and is reproduced in Table 5. For a given R, there are two isomeric monosubstituted naphthalenes, the 1-R and the 2-R. The latter might be expected to be more stable because it lacks the 1.8-or peri-repulsion found in the former between the substituent and a small, but unavoidably nearby, hydrogen. For a methyl substituent, the difference between the enthalpies of formation of 1- and 2-me-thylnaphthalene, 113.5 1.7 and 110.7 1.7 kJ/mol, respectively, reveals a very small 2.8 2.4 kJ/mol peri effect. For identical substituents, there are the following naphthalene isomers 1.2-. 1.3-. 1.4-. 1,5-, 1.6-. 1.7-. 1.8-, 2,3-, 2.6-. and 2.7-isomers. There is no substituent for which all 10 isomers have been thermochemically characterized. Furthermore, there is only one substituent for which there are enthalpy of formation data for the isomer of greatest interest— 1,8-dimethylnaphthalene. Equation 20. analogous to eq 18, is now recast for mono- and disubstituted naphthalenes ... 

Delany, J. M. and S. R. Lundeen, 1989, The LLNL thermochemical database. Lawrence Livermore National Laboratory Report UCRL-21658. 

The values included in thermochemical databases (see appendix B) are normally referred to the substances in their standard states. The standard state notion, which is a consequence of the mathematical formalism used to describe the thermodynamics of reaction and phase equilibria [1], greatly simplifies the calculation of thermochemical quantities for the infinite variety of real processes, that is, those where one or more substances are not in their standard states. This situation will be exemplified in several chapters of the present book, but several case studies are discussed here. 

It must be stressed that the temperature is not included in the definition of standard state. Nevertheless, all modern thermochemical databases list the values at 298.15 K, so this is now regarded as a reference temperature. As shown in appendix B, very few data compilations give values at any other temperature. 

We have attempted to collect in appendix B a list of the most used thermochemical databases. Each one has been built with a particular class of substances and a specific set of properties in mind. We can find compilations of thermochemical values for gas-phase ions, for condensed and gas-phase pure organic compounds, for organometallic molecules, for gas-phase organic free radicals, for inorganic substances, and so on. Most are available in printed form, some are distributed in a software package, and a few can be used online, through the World Wide Web. 

Table B1 shows a selection of thermochemical databases that have been released over the past five decades.

The thermochemical consistency of the databases values (/). The absence of a check mark indicates nonconsistent data or that the comment does not apply. It may also mean that not all the database values are consistent. 

This is one of the most widely used thermochemical databases for inorganic compounds. The first and second editions of JANAF (Joint Army, Navy and Air Force) Tables date from 1964 and 1971, respectively. Supplements of the latter were released in 1974, 1975, 1978, and 1982. The third edition was published in 1985. 

This is, in our opinion, the primary source of thermochemical values and therefore it should be the starting point of all the other databases. The selections have been made by the Task Group on Key Values for Thermodynamics appointed in 1968 by the Committee on Data for Science and Technology (CODATA) of the International Council of Scientific Unions. Unfortunately, the number of species for which data are recommended in the final report is rather small ( 150). 

This has been, for many years, the main source of standard enthalpies of formation of neutral organic compounds. It is a classic work on thermochemistry and has set a standard for thermochemical databases. Superseded by Pedley s 1994 compilation [26]. 

This is one of the first thermochemical databases ever available. The standard enthalpies of formation are tabulated at 18 °C. 

The program, called Stanjan [8] (see Appendix I), is readily handled even on the most modest computers. Like the Gordon-McBride program, both approaches use the JANAF thermochemical database [1], The suite of CHEMKIN programs (see Appendix H) also provides an equilibrium code based on Stanjan [8],... 

The simple physical approaches proposed by Mallard and Le Chatelier [3] and Mikhelson [14] offer significant insight into the laminar flame speed and factors affecting it. Modem computational approaches now permit not only the calculation of the flame speed, but also a determination of the temperature profile and composition changes throughout the wave. These computational approaches are only as good as the thermochemical and kinetic rate values that form their database. Since these approaches include simultaneous chemical rate processes and species diffusion, they are referred to as comprehensive theories, which is the topic of Section C3. 

Thermochemical data are also available from the Internet. Some examples are the NIST Chemical Kinetics Model Database (http //kinetics.nist. gov/CKMech/), the Third Millennium Ideal Gas and Condensed Phase Thermochemical Database for Combustion (A. Burcat and B. Ruscic, ftp //ftp. technion.ac.il/pub/supported/aetdd/thermodynamics/), and the Sandia National Laboratory high-temperature thermodynamic database (http //www.ca.sandia. gov/HiTempThermo/). 

Accurate self-consistent thermochemical data for the copper chlorides up to 200°C are required, in order to improve solubility calculations and electrochemical modelling capabilities for Aspen Plus and OLI software. Experimental work has been initiated at the University of Guelph, Canada and UOIT to determine a comprehensive thermochemical database, for solubility limits of OMIT, and aqueous cupric chloride versus chloride concentration and temperature using UV-VIS spectroscopy (Suppiah, 2008). The chloride ion is obtained by adding LiCl OMIT. The conditions of tests are primarily 25-200°C, up to 20 bars. Specialised equipment for this task is needed to reach elevated temperatures and pressures, because cupric chloride is chemically aggressive, and because changes in the solution concentrations must be made precisely. A titanium test cell has been custom made, including a UV-VIS spectrometer with sapphire windows, HPLC pumps and an automated injection system. The data acquired will be combined with past literature data for the cuprous chloride system to develop a self-consistent database for the copper (I) and copper (II) chloride-water systems. 

The FACTSage thermochemical database was used to identify the thermodynamically stable phases that could exist in a system comprised of a pure metal, oxygen, HC1 and Cl2 at 500°C (Suppiah, 2008). The predominant Fe, Ni, Cu and Cr phases in an 02/HCl/Cl2 environment were determined. The equilibrium reaction boundary was plotted as a function of the partial pressures of 02 and HC1, for a constant Cl2 partial pressure. The resulting predominance diagrams were plotted over an 02 and HC1 partial pressure range of 10-20 to 1 atm for Cl2 partial pressures between 10-6 and 1 atm. The predominant Ni and Cr species are solids, suggesting that a corrosion resistant protective layer could be formed on the metal. 

If either T or 7) happen to be equal to 298.15 K then the Cp m values required will be those tabulated in thermochemical databases. Frame 11 discusses further the use of heat capacity. 

As an addition a CD containing a demo version of the ICT-Database of Thermochemical Values and information about the ICT-Thermody-namic-Code ist attached to the book. The full version of the database contains detailed information of more than 14,000 substances, including structure formulae, oxygen balance, densities and enthalpies of formation. The Code may be used for calculating properties of formulations like the heat of explosion or specific impulse of explosives, propellants or pyrotechnics. Both programs, updated regularly, are available by the Fraunhofer ICT. 

Aluminum, boron, carbon, iron, nitrogen, oxygen, phosphorus, sulfur and titanium are the common impurities in the SoG-Si feedstock. Arsenic and antimony are frequently used as doping agents. Transition metals (Co, Cu, Cr, Fe, Mn, Mo, Ni, V, W, and Zr), alkali and alkali-earth impurities (Li, Mg, and Na), as well as Bi, Ga, Ge, In, Pb, Sn, Te, and Zn may appear in the SoG-Si feedstock. A thermochemical database that covers these elements has recently been developed at SINTEF Materials and Chemistry, which has been designed for use within the composition space associated with the SoG-Si materials. All the binary and several critical ternary subsystems have been assessed and calculated results have been validated with the reliable experimental data in the literature. The database can be regarded as the state-of-art equilibrium relations in the Si-based multicomponent system. 

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