Heat of formation determination

C06-0080. Using standard heats of formation, determine A H for the foiiowing reactions ... 

The binding of counterions and van der Waals attraction make further contributions to the association. The heats of formation determined so far, however, are insufficient. More definite knowledge about the nature of the binding has been obtained by spectroscopic investigation. 

Historically, the hexahydro derivative (4) had its heat of formation determined by heat of combustion. The measured value was -24.5 0.86kcal/mol, as listed in the Table 5.5. The heat of hydrogenation of each of the compounds (1), (2), and (3) to compound (4) were then each separately determined. " These gave the values for the heats of hydrogenation (AHh ) in line 2 in Table 5.5. And from these values, the heats of formation of compounds (1), (2), and (3) were determined. " ... 

The heat of formation is the energy released as heat when atoms situated at theoretically infinite distance approach, bind, and form the molecule of interest. The core includes, by definition, the atomic nucleus and the electrons that do not participate in chemical bonds, that is, the nonvalence electrons. The semiempirical method PM6 estimates the heat of formation as the sum of the total repulsion energy of the cores and the total heat of formation of the atoms. Each semiempirical quantum mechanics method calculates, in its own manner, the energy of repulsion of the cores and utilizes a different set of values for the atomic heat of formation. Consequently, the values of the heat of formation determined for the same molecule by different semiempirical... 

In an early publication [108] our research group inadvertently published a wrong value for the heat of formation determined from bomb-calorimetric measurements. 

Heat of formation determined either empirically or semiempirically. 

Emphasis was put on providing a sound physicochemical basis for the modeling of the effects determining a reaction mechanism. Thus, methods were developed for the estimation of pXj-vahies, bond dissociation energies, heats of formation, frontier molecular orbital energies and coefficients, and stcric hindrance. 

The gasification is performed usiag oxygen and steam (qv), usually at elevated pressures. The steam—oxygen ratio along with reaction temperature and pressure determine the equiUbrium gas composition. The reaction rates for these reactions are relatively slow and heats of formation are negative. Catalysts maybe necessary for complete reaction (2,3,24,42,43). 

Thermodynamic Properties. The heat of formation of the thiosulfate ion, —5.75 kJ/g (—1.37 kcal/g), was determined by studying the equihbrium of the following reaction ... 

The solubilities of Li, Na, and Ca hypochlorites in H2O at 25°C ate 40, 45, and 21%, respectively. Solubility isotherms in water at 10°C have been determined for the following systems Ca(OCl)2—CaCl2, NaOCl—NaCl, and Ca(OCl)2—NaOCl (141). The densities of approximately equimolar solutions of NaOCl and NaCl ate given in several product bulletins (142). The uv absorption spectmm of C10 shows a maximum at 292 nm with a molar absorptivity of 350 cm ( 5)- Heats of formation of alkali and alkaline earth hypochlorites ate given (143). Thermodynamic properties of the hypochlorite ion ate ... 

Heats of formation, AH°, for individual chemicals involved in chemical reactions are important in determining the heat of reaction, AH°, and associated heating and cooling requirements. If AH° < 0, then... 

The heat of formation of 804X4 was determined to be AH°f = +163 kcal mor. " Thus 804X4 is even more endothermic than S4X4 (AH°f = +110 kcal mok ). The mean E-X bond energies in E4X4 were estimated to be 59 kcal mol (E = Se) " and 72 kcal mol (E = S) from the enthalpies of formation. 

MO studies (AMI and AMI-SMI) on the tautomerism and protonation of 2-thiopurine have been reported [95THE(334)223]. Heats of formation and relative energies have been calculated for the nine tautomeric forms in the gas phase. Tire proton affinities were determined for the most stable tautomers 8a-8d. Tire pyrimidine ring in the thiones 8a and 8b has shown a greater proton affinity in comparison with the imidazole ring, or with the other tautomers. In solution, the thione tautomers are claimed to be more stabilized by solvent effects than the thiol forms, and the 3H,1H tautomer 8b is the most stable. So far, no additional experimental data or ab initio calculations have been reported to confirm these conclusions. 

Nernst further remarked that the stability of a compound is measured by the quantity which can exist under given conditions, and the previous equations enable one to determine this, at least approximately, with the help of the heats of formation. A compound under given conditions is in general either very stable or very unstable—equilibria in which all the components exist in appreciable concentrations are the exception rather than the rule. A considerable heat of formation usually corresponds with marked stability (cf. HC1, HBr, HI). 

The heat of formation CH5 + is taken as 229 =fc 3 kcal./mole as determined recently in these laboratories (9). In all these energetic considerations we assume that all the energy of the reaction is concentrated in the ionic product species. Thus, the energies written are upper limits to the energies the ionic species actually contain. 

Equation can also be used to calculate the standard enthalpy of formation of a substance whose formation reaction does not proceed cleanly and rapidly. The enthalpy change for some other chemical reaction involving the substance can be determined by calorimetric measurements. Then Equation can be used to calculate the unknown standard enthalpy of formation. Example shows how to do this using experimental data from a constant-volume calorimetry experiment combined with standard heats of formation. 

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