Heating enthalpy of formation

Considerable attention has been given here to heats (enthalpies) of formation, because there are extensive tabulations of these, e.g. [205] and papers on their calculation appear often in the literature, e.g. [201]. However, we should remember that equilibria [147] are dependent not just on enthalpy differences, but also on the often-ignored entropy changes, as reflected in free energy differences, and so the calculation of entropies is also important [206]. 

Let us look at a reaction and perform the calculations on the basis of the heats (enthalpies) of formation. 

Although it is difficult to predict activation energies, it is easier to measure or predict heats (enthalpies) of formation and hence heats of reaction. For an endothermic reaction, the activation energy must be at least as great as the endothermicity AH (Figure 2.5), so if the measured activation energy of reaction is less than the endothermicity of a proposed elementary reaction or an initial rate-determining step, that proposal can be ruled out. 

All chemical compounds have a heat (enthalpy) of formation , AH, which equates to the heat liberated or absorbed when 1 mole of the compound is formed from its constituent elements. An element in its standard state is defined as having zero heat of formation. The standard molar heat of formation, A//f, of a compound is then the change in enthalpy, positive or negative, when 1 mole of the compound is formed at standard conditions (298.15 K and 101.325 kPa) from the elements in their most stable physical forms (gas, liquid or solid). 

The heat (enthalpy) of formation of chemisorbed sulfur on nickel with respect to V2 82 is -247 kJ moT i.e., -158 kJ mol with H28 as the reference state for sulfur (the heat of formation of gaseous H28 is 89 kJ mol ). It is 75 kJ mol more exothermic than the enthalpy of formation of M382. This means that 8 adsorbed is energetically more stable than 8 in M3 82- A similar trend is observed for Fe. Indeed,... 

By combining our computed EDq values from either the CBS or G3B3 calculations, with the known heat of formation at 0 K for the element B, we can derive the heats (enthalpies) of formation Ayf° values at 0 K for the molecules in the gas phase. In this work, we use the value of Ay7/°(B) = 135.1 0.2 kcal/mol [75], and the rationale for this selection was discussed in our previous work [26, 76-78]. The heats of formation at 298 K are obtained by following the usual thermochemical procedures [79]. The calculated heats of formation at 0 K are used to evaluate the ionization energies and other energetic quantities. 

A/ij the lattice energy of sodium chloride this is the heat liberated when one mole of crystalline sodium chloride is formed from one mole of gaseous sodium ions and one mole of chloride ions, the enthalpy of formation of sodium chloride. 

There are many compounds in existence which have a considerable positive enthalpy of formation. They are not made by direct union of the constituent elements in their standard states, but by some process in which the necessary energy is provided indirectly. Many known covalent hydrides (Chapter 5) are made by indirect methods (for example from other hydrides) or by supplying energy (in the form of heat or an electric discharge) to the direct reaction to dissociate the hydrogen molecules and also possibly vaporise the other element. Other known endothermic compounds include nitrogen oxide and ethyne (acetylene) all these compounds have considerable kinetic stability. 

As expected from the enthalpy of formation, water is thermally very stable but when steam is heated to above 1300 K slight dissociation to the elements does occur. Pure water is almost a nonconductor of electricity but slight ionic dissociation occurs ... 

A variant on this procedure produces a first approximation to the molecular mechanics (MM) heat paiameters (Chapters 4 and 5) for C—C and C—H. Instead of atomization energies, the enthalpies of formation of propane and butane (—25.02 and —30.02 kcal mol ) are put directly into the b vector. The results (2.51 kcal mol and —3.76 kcal mol ) are not very good approximations to the heat parameters actually used (2.45 kcal mol and —4.59 kcal mol ) because of other factors to be taken up later, but the calculation illustrates the method and there is rough agreement. 

Determine the molecular mechanics heat parameters for C—C and C—H using the enthalpies of formation of n-butane and n-pentane, which are —30.02 and —35.11 kcal mol respectively. 

W hich leads to an isomeri/ation enthalpy of —1.7 kcal mol. ("Heat of fonnation should he taken to mean enthalpy of formation in this conte.st.) Entropy effects being... 

First, we would like to ehange the reference state from the isolated nuelei and eleetions to the elements in their standard states, C(graphite) and H2(g) at 298 K. This leads to the energy of formation at 0 K AfEo, whieh is identieal to the enthalpy of formation AfHo at 0 K. The energy and enthalpy are identieal only at 0 K. Next we would like to know the enthalpy ehange on heating propene from 0 to 298 K so as to obtain the enthalpy of formation from the isolated nuelei and eleetions elements This we will eonvert to from the elements in their standard... 

A particularly useful property of the PX monomer is its enthalpy of formation. Conventional means of obtaining this value, such as through its heat of combustion, are, of course, excluded by its reactivity. An experimental attempt was made to obtain this measure of chemical reactivity with the help of ion cyclotron resonance a value of 209 17 kJ/mol (50 4 kcal/mol) was obtained (10). Unfortunately, the technique suffers from lack of resolution in addition to experimental imprecision. It is perhaps better to rely on molecular orbital calculations for the formation enthalpy. Using a semiempirical molecular orbital technique, which is tuned to give good values for heat of formation on experimentally accessible compounds, the heat of formation of /5-xylylene has been computed to be 234.8 kj/mol (56.1 kcal/mol) (11). 

Values for the free energy and enthalpy of formation, entropy, and ideal gas heat capacity of carbon monoxide as a function of temperature are listed in Table 2 (1). Thermodynamic properties have been reported from 70—300 K at pressures from 0.1—30 MPa (1—300 atm) (8,9) and from 0.1—120 MPa (1—1200 atm) (10). 

An overview of some basic mathematical techniques for data correlation is to be found herein together with background on several types of physical property correlating techniques and a road map for the use of selected methods. Methods are presented for the correlation of observed experimental data to physical properties such as critical properties, normal boiling point, molar volume, vapor pressure, heats of vaporization and fusion, heat capacity, surface tension, viscosity, thermal conductivity, acentric factor, flammability limits, enthalpy of formation, Gibbs energy, entropy, activity coefficients, Henry s constant, octanol—water partition coefficients, diffusion coefficients, virial coefficients, chemical reactivity, and toxicological parameters. 

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... 

The amonnt of energy that can be released from a given chemical reaction is determined from the energies (enthalpies of formation) of the individnal reactants and prodncts. Enthalpies are nsnally given for snbstances in their standard states, which are the stable states of pnre snbstances at atmospheric pressnre and at 25°C. The overall heat of reaction is the difference between the snms of the standard enthalpies of formation of the prodncts... 

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. 

Enthalpy of formation. Magnesium ribbon reacts with oxygen to give MgO, a white solid, and 601.7 kJ of heat per mole of MgO formed. Hence AW MgO(s) = -601.7 kJ/mol. 

Enthalpies of formation for a variety of compounds are listed in Table 8.3. Notice that, with a few exceptions, enthalpies of formation are negative quantities. This means that the formation of a compound from the elements is ordinarily exothermic. Conversely, when a compound decomposes to the elements, heat usually must be absorbed. 

Table 9.1 Standard heat capacities, entropies, enthalpies of formation, and Gibbs free energies of formation at T = 298.15 K. ...

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