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Calculation of thermochemical functions

Complete sets of basic data needed for the calculation of thermochemical functions are available only for a relatively small number of substances. Very often the data have to be completed by analysing values from different sources as well as by appropriate estimations before they can be used for calculations and they may well then be less accurate. Such estimates have been made especially for S (298.15) and Cp (T). [Pg.1895]

A pi cliininary text serves to introduce the subject of thermodynamics and includes a short description of the fundamental relations of chemical thermodynamics. The calculation of thermochemical functions is elucidated on the basis of these relations. This is followed by a description of the contents and struc-... [Pg.1895]

The majority of specialist publications report one or more properties of a substance or of a limited range of substances. It is rare that the complete sets of data necessary and sufficient for the calculation of thermochemical functions over a given temperature range are reported. [Pg.1967]

An important characteristic of ab initio computational methodology is the ability to approach the exact description - that is, the focal point [11] - of the molecular electronic structure in a systematic manner. In the standard approach, approximate wavefunctions are constructed as linear combinations of antisymmetrized products (determinants) of one-electron functions, the molecular orbitals (MOs). The quality of the description then depends on the basis of atomic orbitals (AOs) in terms of which the MOs are expanded (the one-electron space), and on how linear combinations of determinants of these MOs are formed (the n-electron space). Within the one- and n-electron spaces, hierarchies exist of increasing flexibility and accuracy. To understand the requirements for accurate calculations of thermochemical data, we shall in this section consider the one- and n-electron hierarchies in some detail [12]. [Pg.2]

Calculation of Thermodynamic Functions from Molecular Properties The calculation methods for thermodynamic functions (entropy S, heat capacities Cp and Cy, enthalpy H, and therefore Gibbs free energy G) for polyatomic systems from molecular and spectroscopic data with statistical methods through calculation of partition functions and its derivative toward temperature are well established and described in reference books such as Herzberg s Molecular Spectra and Molecular Structure [59] or in the earlier work from Mayer and Mayer [7], who showed, probably for the first time in a comprehensive way, that all basic thermochemical properties can be calculated from the partition function Q and the Avagadro s number N. The calculation details are well described by Irikura [60] and are summarized here. Emphasis will be placed on calculations of internal rotations. [Pg.12]

It is well known that it is impossible to compute the absolute values of thermochemical functions. Entropy is an exception to this rule, but again entropy does not suffice to calculate the equilibrium composition. Consequently, experimental, i.e. calorimetric measurement is essential for every determination of an equilibrium constant. In spite of that, all procedures may be classified by the nature of the work involved as experimental and calculatory ones. [Pg.199]

IVTANTHERMO consists of several databases and a set of algorithms and programs. The databases contain auxiliary data, for example, fundamental physical constants, atomic masses of elements etc. -, constants necessary for the calculations of thermodynamic functions primary experimental data on equilibrium constants of chemical reactions and on saturated vapor pressure of substances thermochemical constants tabulated values of thermodynamic properties for wide temperature intervals. [Pg.469]

Several surveys are available that address the performance of MNDO for specific properties or specific classes of compounds. In a recent review on the semiempirical calculation of thermochemical properties, the MNDO results have been compared with those from ab initio, density functional, and... [Pg.1602]

In order to construct the activity diagrams in a rigorous fashion, a certain amount of information must be available. Some experimental data for the mica-feldspar-kaolinite-gibbsite-montmorillonite relations are available. Data for the other minerals are often inferred from measurements of natural chemical parameters (K+, SiC, H+ concentrations in solutions) in situations where the different minerals are assumed to be stable. The relations between minerals can also be calculated as a function of K+, SiO and pH using thermochemical data for the participating phase (Hess, 1966) when they are known with precision. Frequently it is... [Pg.167]

In Section 1.1, parametric methods for improving tlie quality of correlated electronic-structure calculations were discussed in detail. Similarly, in Section 8.4.3, the mild parameterization of density functional methods to give maximal accuracy was described. Given that background, and the substantial data presented in diose earlier chapters, this section will only recapitulate in a rough categorical fashion tlie various approaches whose development was motivated by a desire to compute more accurate thermochemical quantities. [Pg.370]

Silicates. Thermochemical data are not well enough known as a function of temperature nor is the silicate mineralogy well enough understood to warrant any calculations as a function of temperature and Pco2. [Pg.260]

The critical thermochemical quantities for prediction of propeUant performance are the enthalpies of formation of the product and to a degree the reactant species. The enthalpy of formation is essential to the calculation of the enthalpy of reaction and since it also appears in the expression for the equilibrium constant, as it is the basis for relating the Gibbs functions of different species, influences the calculated product equilibrium compositions. It is most desirable to measure enthalpies of formation directly from calorimetric experiments, but often the enthalpies must be... [Pg.131]

Finally, significant advances in the techniques of both thermal and thermochemical measurements have come to fruition in the last decade, notably aneroid rotating-bomb calorimetry and automatic adiabatic shield control, so that enhanced calorimetric precision is possible, and the tedium is greatly reduced by high speed digital computation. Non-calorimetric experimental approaches as well as theoretical ones, e.g., calculation of electronic heat capacity contributions to di- and trivalent lanthanides by Dennison and Gschneidner (33), are also adding to definitive thermodynamic functions. [Pg.44]

The thermodynamic functions of the proton gas are calculated using the recent CODAT fundamental constants (2) and assuming that the proton is an ideal monatomic gas. Since there is no electron associated with this species, there is only a translational contribution to the thermochemical function. [Pg.992]


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Thermochemical calculations

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