Association standard enthalpy

Enthalpy of Formation The ideal gas standard enthalpy (heat) of formation (AHJoqs) of chemical compound is the increment of enthalpy associated with the reaction of forming that compound in the ideal gas state from the constituent elements in their standard states, defined as the existing phase at a temperature of 298.15 K and one atmosphere (101.3 kPa). Sources for data are Refs. 15, 23, 24, 104, 115, and 116. The most accurate, but again complicated, estimation method is that of Benson et al. " A compromise between complexity and accuracy is based on the additive atomic group-contribution scheme of Joback his original units of kcal/mol have been converted to kj/mol by the conversion 1 kcal/mol = 4.1868 kJ/moL... 

Very recently, Zavitsas [44] expressed standard enthalpies of formation of alkanes as a simple sum in which each term consists of the number of hydrogen atoms of different types multiplied by an associated coefficient derived from the known enthalpy of formation of typical molecules. 

Where (dH ) is the standard enthalpy for the transfer of a solute from the mobile phase to the stationary phase, (AS ) the associated... 

The standard enthalpy of formation of monomeric HF is a hypothetical state that must be related to that of the real associated liquid, gas, or aqueous solution met in calorimetiy. Considerable difficulty has been encountered in allowing for the heat of association, which varies with temperature and pressure. For example, the presence of traces of water can affect the polymerization by entering into the hydrogen bonding (30) the treatment of results will depend on the association model adopted. The magnitude of corrections for gas imperfections has... 

The standard enthalpy of transition of a substance A from a phase a to a phase P (AtrsH°) is the enthalpy associated with the process ... 

Equation 2.67 indicates that the standard enthalpy and entropy of reaction 2.64 derived from Kc data may be close to the values obtained with molality equilibrium constants. Because Ar// is calculated from the slope of In AT versus l/T, it will be similar to the value derived with Km data provided that the density of the solution remains approximately constant in the experimental temperature range. On the other hand, the error in ArSj calculated with Kc data can be roughly estimated as R In p (from equations 2.57 and 2.67). In the case of water, this is about zero for most solvents, which have p in the range of 0.7-2 kg dm-3, the corrections are smaller (from —3 to 6 J K-1 mol-1) than the usual experimental uncertainties associated with the statistical analysis of the data. 

In the bomb process, reactants at the initial pressure pi and temperature 7 are converted to products at the final pressure pf and temperature Tf. The primary goal of a combustion calorimetric experiment, however, is to obtain the change of internal energy, Ac//°(7r), associated with the reaction under study, with all reactants and products in their standard states pi = pf = O.IMPa) and under isothermal conditions at a reference temperature 7r (usually 298.15 K). Once AC//°(298.15K) is known, it is possible to derive the standard enthalpy of combustion, AC77°(298.15K), and subsequently calculate the standard enthalpy of formation of the compound of interest from the known standard enthalpies of formation of the products and other reactants. 

Table 2.1 Bond dissociation enthalpies and standard enthalpies of formation of silanes, and enthalpies of associated silyl radicals (kJ/mol) ...

Abbreviations A H Huggins coefficient M, molar mass R, radius of gyration RD, core radius p, association number AmcH°, standard enthalpy of micellization, AmlcG°, standard Gibbs energy of micellization A2, second virial coefficient. Ru, hydrodynamic radius. 

The values of the CMC or CMT collected as a function of temperature or concentration can be used to extract the enthalpic and entropic contributions to the association process. For a closed association mechanism with relatively large aggregation number and a narrow distribution, the standard free energy and standard enthalpy of micelle formaMi nd AH°, per mole of the solute in the micelle) are related to the CMC and its temperature dependence in the form (Lindman and V fennerstrom, 1980 Zhou and Chu, 1994). 

The standard enthalpy of formation of a compound is the change in enthalpy associated with forming the compound from its constitutive elements, each in their standard state, at a given temperature. 

The standard enthalpy of formation of a compound, AHf, is defined as the increment in enthalpy associated with the reaction of forming a given compound from its elements, with each substance in its thermodynamic standard state at the given temperature.2 The thermodynamic cycle for the enthalpy of formation of methane (CH4) from the standard states of carbon and hydrogen (graphite and hydrogen molecules) is shown in Figure 1. 

H°) standard enthalpy. Compare with enthalpy change. A change in enthalpy associated with a reaction or transformation involving substances in their standard states. 

The enthalpy change on formation of Portland cement clinker cannot be calculated with high precision, mainly because of uncertainties associated with the clay minerals in the raw material. Table 3.1 gives data for the main thermochemical components of the reaction, almost all of which have been calculated from a self-consistent set of standard enthalpies of formation, and which are therefore likely to be more reliable than other values in the literature. The conversion of the clay minerals into oxides is an imaginary reaction, but valid as a component in a Hess s law calculation. Few reliable thermochemical data exist for clay minerals those for pyrophyllite and kaolinite can probably be used with sufficient accuracy, on a weight basis. 

The second step shown here combines the terms AHZ (for expanding the solvent) and AH3 (for solvent-solute interactions) and is called the enthalpy (heat) of hydration (AH d). This term represents the enthalpy change associated with the dispersal of a gaseous solute in water. Thus the standard enthalpy of solution for dissolving sodium chloride is the sum of AH and AH yd-... 

According to the Brpnsted definition, the acidity of a molecule is associated with its capacity to give up a proton Ph—NH2 — Ph—NH +H+. The change of standard enthalpy or free energy of this deprotonation reaction is a measure of the intrinsic acidity. As discussed above, in solution, the propensity of an aniline derivative is to accept a proton. The measured dissociation constant (pATa) is related to the basicity of the neutral molecule (or the acidity of the anilinium cations). As a consequence, relatively little is known about their acidity and/or the anilinide anions. However, the NH acidities have been well established in hydroxamic acids even though the latter usually behave as O-acids134. It is therefore of interest to get some insight into the deprotonation of aniline in the gas phase. 

I. Most chemical equilibria are associated with a finite standard enthalpy of reaction A// and are temperature dependent as shown by the Van t I loff equation given below... 

Comparing this result with the Arrhenius equation (equation (7.4.1)) it is clear that the major part of the temperature dependence of the rate constant is associated with the exponential term containing the standard enthalpy of activation, A H°. 

You probably know that two or more chemical equations can be combined algebraically to give a new equation. Even before the science of thermodynamics developed in the late nineteenth century, it was observed by Germaine Hess (1802-1850) that the heats associated with chemical reactions can be combined in the same way to yield the heat of another reaction. For example, the standard enthalpy changes for the oxidation of graphite and diamond can be combined to obtain AH° for the transformation between these two forms of solid carbon, a reaction that cannot be studied experimentally. 

In this chapter, a mathematical expression of the First Law will be discussed. The two functions internal energy, U, and enthalpy, H, will figure prominently. In addition, tabulations of standard enthalpy changes of formation, A//0 for a number of compounds, will be given. From such tables it is possible to derive the amount of thermal energy associated with any reaction, as long as all reactants and products are listed. 

H2O is a major detonation product. A simple exp-6 potential model, however, does not naturally represent the hydrogen-bonded tetrahedral structure of water. We find that an effective two-species model is effective in representing the equation of state of supercritical water over a wide range of conditions. In the two-species model, we represent water by two species in chemical equilibrium non-associated water (H2O) and associated water H20(a). The non-associated water has standard state parameters given by gaseous water. In associated water, however, standard state parameters are chosen closer to that of liquid water the standard enthalpy and entropy are both less than that of gaseous water. 

The standard enthalpy of formation, AH, of a substance is the enthalpy change associated with the reaction of the elements of the substance in their standard states to produce the substance in its standard state. Thus the enthalpy of formation of carbon monoxide is the enthalpy change associated with the reaction... 

TABLE 13. Measured bond dissociation energies for silanes and derived standard enthalpies of formation of associated radicals... 

Enthalpy change associated with the attachment of an electron A (MX, s) = Standard enthalpy of formation... 

The standard enthalpy of formation of synthetic pentlandite, Fe4 5Ni4 5Ss, and natural violarite, (Feo.294iNio.7059)3S4, were measured by solution drop calorimetry. Synthetic pentlandite was prepared from the elements and checked by X-ray diffraction analysis. First a stoichiometric mixture of FeS, NiS, Ni, and Fe was dropped from 298.15 K into a melt of Nio,6So,4 at 1100 K. In a second experiment synthetic pentlandite was dropped into the Nio,6So,4 melt. The enthalpy of formation of pentlandite from its elementary and binary compounds is obtained from the difference of the enthalpy effects associated with these two experiments, A,// = - (78.41 14.45) kJ mol for ... 

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