Heat of Formation. Thermodynamic Functions

The heat capacity C , thermodynamic functions S , -(G -Hj.)/T, H - Hj., and the equilibrium constant K for the formation of PH as an ideal gas from the elements have been calculated for a standard-state pressure of 1 atm and tabulated for 298.15 K and between 0 and 6000 K at 100 K intervals [2, 4]. The JANAF data with T = 298 K are based on early spectroscopic data [7, 8] and semiempirical estimates for Dq and the electronically excited states [3]. In the third edition of the JANAF Tables [1], the old values were only converted to Joule units and to a standard pressure of 0.1 MPa. In the Russian compilation [4], values for a reference temperature T = 0 K are given that are based on a partition function established by using spectroscopic data for the X a A, A rij states [9,10] and theoretical data for the b and c 11 states [3, 11]. Selected values from [4] are  

Polynomial expansions of the partition function and the equilibrium constant at T=1000 to 

10000 K based on the spectroscopic data compiled by Huber and Herzberg [12] are of 

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Dissociation Energy. Heat of Formation. Thermodynamic Functions... 

Physical Properties. Sulfur dioxide [7446-09-5] SO2, is a colorless gas with a characteristic pungent, choking odor. Its physical and thermodynamic properties ate Hsted in Table 8. Heat capacity, vapor pressure, heat of vaporization, density, surface tension, viscosity, thermal conductivity, heat of formation, and free energy of formation as functions of temperature ate available (213), as is a detailed discussion of the sulfur dioxide—water system (215). 

However, even the best experimental technique typically does not provide a detailed mechanistic picture of a chemical reaction. Computational quantum chemical methods such as the ab initio molecular orbital and density functional theory (DFT) " methods allow chemists to obtain a detailed picture of reaction potential energy surfaces and to elucidate important reaction-driving forces. Moreover, these methods can provide valuable kinetic and thermodynamic information (i.e., heats of formation, enthalpies, and free energies) for reactions and species for which reactivity and conditions make experiments difficult, thereby providing a powerful means to complement experimental data. 

The heat of formation (AH ) for 1,2,4-thiadiazole has been reported <1990JPR885> while other thermodynamic functions (entropy, heat capacity, free energy) have not been reported. 

However, the obscure choice of frequencies in the visible and UV regions in the original calculations may have been guided by a desire to fit experimental heats. In fact, the Debye rotational and translational crystal frequencies relate to sublimation energies of the lattice, and, together with internal molecular vibrations, can be used to calculate thermodynamic functions (16). An indirect connection between maximum lattice frequencies (vm) and heats of formation may hold because the former is inversely related to interatomic dimensions (see Section IV,D,1) ... 

This interconversion can also be performed in solvents, and the rate of the isomerization is dependent on the solvent used. In the dipolar aprotic solvent DMSO the rate of the reaction is fast, but in methanol, acetone, or dioxane the rate is low. However, the value of the equilibrium constant is scarcely influenced by the solvent ( 134/133 = 6-10) (75JHC985).This is not too surprising, since the equilibrium position is controlled by the relative thermodynamic stability of the isomers, which is a function of their heats of formation and of solvation. Undoubtedly, the heat of formation is the more important factor to the thermodynamic stability (75JHC985). 

Thermodynamic parameters The tribochemical energy for the stable compounds of tribofilm comes from the heat of formation AHf and work function WF (AHf - WF). Using the data from Table 5.11, compare the tribochemical energy for iron compounds and explain formation of the tribofilms of FeS and FeO in spite of positive tribochemical energy. 

The heat of formation of oxazole, calculated by the MINDO/3 method, is -1.82 kJ mol-1. The thermodynamic functions of heat capacity, heat content, free energy and entropy have been calculated for gaseous oxazole and compared with those for isoxazole, thiazole, isothiazole and 1,3,4-thiadiazole (68SA(A)36l). For calculated and experimental ionization potentials of oxazole and benzoxazole, see Section 4.18.2.3.7... 

Bondi and Simkin presented a similar calculation for liquids, using the heat of vaporization (AHv) (241, 242). The purpose was to obtain an estimate of but in the process a value of the H bond increment, 5(OH) is derived. This increment is .. . a measure of (but not identical with) the heat of formation of the H bond,. . Values for 5(OH) are very similar to enthalpy values, though they are not intended as thermodynamic functions. Bondi gives an extensive discussion of their use with simple and polyhydric alcohols and a few other compound classes. Table 7-V compares 6(OH) and AH values. 

The possibility of calculating thermodynamic functions from infrared and Raman frequencies has been exploited for Fe(05115)2, Ni(OsH5)2, and Ru(OsH6)2 (124)- The vapor-pressure curve of ferrocene was also reported soon after its discovery (112). Vapor-pressure measurements for dicyclopentadienyl manganese are also available (216). The heats of formation of Fe(05Hs)2 and Ni (05115)2 from the O5H6 radical and the... 

SiC(a), also called hexagonal II or 6H, is one of the more common of many hexagonal forms which arise from various possible stacking sequences of the hexagonal SiC layers (1 ). The properties of these phases are so similar that they have not been adequately differentiated thermodynamically. It has frequently been assumed that cubic SiC(B) transforms to alpha at about 2300 K, but this seems unlikely since both phases have been prepared over temperature ranges of 1700-3000 K (1 4). Heat of formation and equilibrium data Indicate that alpha is less stable up to 2000 K. The adopted functions suggest that this is the case at all temperatures however, the stability difference is small. 

Values of thermodynamic functions (heats of formation and entropies) are known for many compounds, typically at 298.15° K = 25° C (see e.g. [17]). However, for even more compounds these values are not known and must be estimated. 

Another procedure based only in part on the additivity properties of the values for AH and AS° is based on correlation equations for a series of homolog compounds or of compounds containing a repetitive unit (such as polymers). Since the compounds in a homolog series differ by the same fragment, it can be expected that the values in a series is linear The data used for generating the correlation equation can be either experimental or calculated values of a thermodynamic parameter. As an example, a set of experimental and calculated values for the heats of formation AHf for the series of saturated normal hydrocarbons CnH2n.,2 are plotted as a function of carbon number in Figure 2,2.4. 

The standard free energy of formation, AG°, of caesium monoxide has been calculated from experimental results as -308.42 1.18 kcal mol-1. To permit practical thermodynamic calculations to be made at higher temperatures, experimentally determined heat capacities at 5—350 K have been extrapolated to 763 K, the m.p. of Cs20, and a table of extrapolated thermodynamic functions, including AG°, is now available up to 763 K.162... 

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