Heats of formation from

Because the reaction takes place in the Hquid, the amount of Hquid held in the contacting vessel is important, as are the Hquid physical properties such as viscosity, density, and surface tension. These properties affect gas bubble size and therefore phase boundary area and diffusion properties for rate considerations. Chemically, the oxidation rate is also dependent on the concentration of the anthrahydroquinone, the actual oxygen concentration in the Hquid, and the system temperature (64). The oxidation reaction is also exothermic, releasing the remaining 45% of the heat of formation from the elements. Temperature can be controUed by the various options described under hydrogenation. Added heat release can result from decomposition of hydrogen peroxide or direct reaction of H2O2 and hydroquinone (HQ) at a catalytic site (eq. 19). 

Ammonium bicarbonate, sp gr 1.586, formula wt 79.06, is the only compound in the NH —CO2—H2O system that dissolves in water without decomposition. SolubiUty in 100 g of H2O ranges from 11.9 g at 0°C to 59.2 g/100 g of H2O at 60°C (8). The heat of formation from gaseous ammonia and carbon dioxide andUquid water is 126.5 kj/mol (30.2 kcal/mol). Ammonium bicarbonate is manufactured by passing carbon dioxide gas... 

The classical value is attained by most molecules at temperatures above 300 K for die translation and rotation components, but for some molecules, those which have high heats of formation from die constituent atoms such as H2, die classical value for die vibrational component is only reached above room temperature. Consideration of the vibrational partition function for a diatomic gas leads to the relation... 

Heat of formation from pure water and gas at 0° C kcal/mole solute... 

In our own validation sets, experimental heats of formation are preferentially taken from recognized standard compilations [38-40]. If there are enough experimental data for a given element, we normally only use reference values that are accurate to 2 kcal/mol. If there is a lack of reliable data, we may accept experimental heats of formation with a quoted experimental error of up to 5 kcal/mol. This choice is motivated by the target accuracy of the established semiempirical methods. If experimental data are missing for a small molecule of interest, we consider it legitimate [18] to employ computed heats of formation from high-level ab initio methods as substitutes. 

While it is straightforward to obtain theoretical heats of formation from processes which greatly disrupt bonding, e.g., the G3 recipe, it is also possible to make use of isodesmic reactions together with limited experimental data, or alternatively data from high-level quantum chemical calculations, to estimate heats of formation. Once in hand, these can be used for whatever thermochemical comparisons are desired. The key is to find an isodesmic reaction which is both uniquely defined, and which leads to products with known heats of formation. This is the subject of the present chapter. 

Calculating Heats of Formation from Bond Separation Reactions... 

Note In view of the uncertainties introduced into the calculation of heats of formation from heats of combustion of all but the simplest organic molecules, it has been found simpler and more reliable to measure directly the heats of certain types of reactions of theoretical interest, instead of... 

Table 5.2 Mean unsigned errors (kcal moL ) in predicted heats of formation from basic NDDO models...

In such cases, the salt is always more stable than the acid KC104, at moderate temperature, is a stable compound, but the acid is very unstable as well as explosive, even at low temperatures. In the salts, the stability is increased by the higher heat of complex formation the total heat of formation from the elements, that determines the stability of the complex against decomposition in lower oxides, may be negative for the acid, but positive for the salts of the alkali metals. 

Physical Properties.—Sulphuryl chloride is a colourless, fuming liquid, with an extremely pungent odour. Z)2 = 1-6074 ->t " = l-4437. It boils at 69-1° C. at 760 mm. pressure, and freezes at —46° C.5 The vapour density is normal at first, but when the chloride is kept, even at 100° C., its vapour commences to dissociate into sulphur dioxide and chlorine. At 200° C. dissociation is almost complete.6 When dissolved in benzene the substance shows a molecular- weight corresponding with S02C12. At ordinary temperatures the specific heat is 0-233, the latent heat of evaporation 32-4 calories per gram, and the heat of formation from the elements approximately 89,540 calories per gram-molecule.7 The dielectric constant at 20° C. is 8-5. As a solvent, the ebullioscopic constant of sulphuryl chloride has been found to have... 

The monochloride is soluble in various inert organic liquids, more particularly in benzene, chloroform, carbon tetrachloride and carbon disulphide, without undergoing chemical change. It is an exothermic compound, its heat of formation from gaseous chlorine and the amorphous modification of selenium being 22-1 Cals.1 Water causes a gradual decomposition of the chloride, selenium dioxide and selenium being formed 2... 

The resonance energy of carbon monoxide relative to the structure C=0 (which itself corresponds to resonance between + C 0 , C O , and C 0 ) can be found by comparing its heat of formation from atoms, 257 kcal/mole, with the ketone value of the doublebond energy, 174 kcal/mole.7 The very large difference, 83 kcal/mole,... 

The heat of formation of sodium orthoarsenate in aqueous solution from the elements is 381,500 calories,1 and that of the solid 360,800 calories.2 The heat of neutralisation (3Na0H.H3As04.aq.) is 35,920 calories,8 and the heat of formation from the oxides is 2... 

The molar heat of formation from solid arsenic and rhombic sulphur has been calculated 3 to be 28,900 calories. This value differs considerably from earlier determinations.4 From diatomic gaseous sulphur and solid arsenic the heat of formation is calculated 5 to be 51,430 calories. 

The molar heat of formation from solid arsenic and rhombic sulphur has been calculated to be 34,700 calories.9... 

Heats of formation from the elements in their standard states calculated from bond energies in Table 4-3, and AH° = 171.3 kcal for C(s)-- C(g). 

B. H. Wilsdon 6 studied the energy involved in the electronic shifts during the dissociation of ammonia. J. Thomsen gave for the heat of formation of ammonia (N,3H)=ll-89 Cals. F. Haber, 10-95 Cals. and M. Berthelot, 12-20 Cals. The subject was studied by R. de Forcrand. J. Thomsen gave (N,3H,Aq.)=20-3 Cals., and M. Berthelot, 21-0 Cals. A. T. Larson and R. L. Dodge calculated the following molar heats of formation from < 3,=4-5707(log jq—log P2)(Ti-1 Tz 1) 1 calories ... 

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