Molar enthalpy of formation

Table 5.1 gives a sample calculation of the NHVj for toluene, starting from the molar enthalpies of formation of the reactants and products and the enthalpies of changes in state as the case requires. 

Generally speaking, intermolecular forces act over a short range. Were this not the case, the specific energy of a portion of matter would depend on its size quantities such as molar enthalpies of formation would be extensive variables On the other hand, the cumulative effects of these forces between macroscopic bodies extend over a rather long range and the discussion of such situations constitutes the chief subject of this chapter. 

The standard molar enthalpy of formation of a compound, AH , is equal to the enthalpy change when one mole of the compound is formed at a constant pressure of 1 atm and a fixed temperature, ordinarily 25°C, from the elements in their stable states at that pressure and temperature. From the equations... 

To find a numerical value for AHi, we need to know ArH° at one temperature, while evaluation of I requires ArG° at one temperature. The usual choice is to obtain ArH° and ArG° at T = 298.15 K from standard molar enthalpies of formation and standard molar Gibbs free energies of formation. Earlier in this chapter we referred to examples of these quantities. It is now time to define AfH° and AfG° explicitly and describe methods for their measurement. 

Standard molar enthalpies of formation, AfH°m, and standard molar Gibbs free energies of formation, Af(7, are useful, since they can be used to calculate ArH° and ArG°. The relationships are... 

P. A. G. O Hare, "Thermochemistry of Uranium Compounds XV. Calorimetric Measurements on UCI4, UOiCl . and UCFF . and the Standard Molar Enthalpy of Formation at 298.15 K of UC14", J. Chem. Thermodyn.. 17. 611-622 (1985). 

A molar enthalpy of formation B specific heat C heat of fusion D mass... 

L. Topor and O. J. Kleppa, Standard Molar Enthalpy of Formation of LaB6 by High-Temperature Calorimetry, Jour, of Chem. Thermo., 16, 993 (1984). 

A/Tf, the standard molar enthalpy of formation of a substance, is the enthalpy change for a reaction in which 1 mole of the substance in a specific state is formed from its elements in their standard states. 

The standard molar enthalpy of formation, A// , is the amount of heat absorbed when 1 mole of the substance is produced from its elements in their standard states. At 25°C, A// of liquid water is -285.8 kJ/mol and A// of water vapor is -241.8 kJ/mol. This means that more heat is released when liquid water is formed from its elements, then when gaseous water is formed from its elements. So, the formation reaction of liquid water is... 

The standard molar enthalpy of formation, AH, of elements in their standard states is zero. From the tabulated values of standard molar enthalpies in Appendix K, we can identify the standard states of elements. 

A classic method14 for examining the thermochemical regularity of an organic homologous series is plotting the standard molar enthalpies of formation versus the number of carbon atoms in the compounds. The linear relationship may be expressed as equation 1 where all the enthalpies of formation are in either the gaseous or a condensed phase, a is the slope, ft is the y-intercept and nc is the number of carbon atoms in the compound. 

Of particular importance is the standard molar enthalpy of formation,, ... 

These are derived by subtraction of the standard molar enthalpy of formation of the binary oxides, since standard enthalpies of individual reactions can be combined to obtain the standard enthalpy of another reaction. Thus,... 

Figure 6.1 The molar enthalpy of formation of NaCl as a function of the cube edge a of the NaCl crystal cubes.

Worked Example 3.11 The wood mentioned in our title question is a complicated mixture of organic chemicals so, for simplicity, we update the scene. Rather than prehistoric men sitting around a fire, we consider the calorific value of methane in a modem central-heating system. Calculate the value of A Hc for methane at 25 °C using molar enthalpies of formation AH. ... 

J. P. Leal, J. A. Martinho Simoes. Standard Molar Enthalpies of Formation of Lithium Alkoxides. J. Organometal. Chem. 1993, 460, 131-138. 

R. C. Santos, H. P. Diogo, M. E. Minas daPiedade. The Determination of the Standard Molar Enthalpy of Formation of 4-Chlorobenzoic Acid by Micro Rotating-Bomb Combustion Calorimetry.J. Chem. Thermodynamics 1999, 31, 1417-1427. 

P. A. G. O Hare, S. Susman, K. J. Volin, S. C. Rowland. Combustion Calorimetry of Rhombohedral Sulfur in Fluorine A Question of Impurities. The Standard Molar Enthalpies of Formation ofSF (g) and SCpf (aq) at the Temperature of298.15 K. J. Chem. Thermodynamics 1992, 24, 1009-1017. 

It has been firmly established that thermochemical properties of a homologous series of compounds show a linear dependence on the number of carbon atoms in the alkyl group1,9 11. Especially useful is equation 1 which expresses the standard molar enthalpies of formation of a homologous series as a function of the total number of carbon atoms in the compound, nc. 

In Investigation 5-B, you used the reaction of oxygen with hydrogen to form water. Reactions like this one are known as formation reactions. In a formation reaction, a substance is formed from elements in their standard states. The enthalpy change of a formation reaction is called the standard molar enthalpy of formation, AH°f. The standard molar enthalpy of formation is the quantity of energy that is absorbed or released when one mole of a compound is formed directly from its elements in their standard states. 

Some standard molar enthalpies of formation are listed in Table 5.3. Notice that the standard enthalpies of formation of most compounds are negative. Thus, most compounds are more stable than the elements they are made from. 

Table 5.3 Selected Standard Molar Enthalpies of Formation...

Use the following standard molar enthalpies of formation to calculate the enthalpy change for this reaction. 

Q O The standard molar enthalpy of formation of calcium carbonate is -1207.6 kj/mol. Calculate the enthalpy of formation of calcium oxide, given the following equation. 

Enthalpy of Formation. Tables of enthalpies of reaction generally list the enthalpies of formation of various compounds in their standard states from the elements in their standard states at the specified temperature. Thus, if the standard molar enthalpy of formation, Af//, of CO2 at 25°C is given as —393.509 kJ mol , the following equation is implied ... 

Table 1 shows the aspect ratio values for the geometries of the stereoisomers of dendrimer 5 (G = 1) with Dj and Cj symmetries, obtained by semiempirical AMI optimizations. Other molecular characteristics, like their molar enthalpies of formation, entropies, and dipolar moments, which are relevant for a complete characterization of these isomers, are also given in Table 1. The aspect ratios of both isomers are very low, close to 1.3, indicating therefore spheroidal shapes in both cases. This fact is in complete agreement with the results previously described, suggesting that the first generations of both dendrimer series are close to the limit ones.

Since this chapter was prepared, the only chemical developments in compounds belonging to this class have involved artemisinin and its derivatives, and some of these will be summarized here. Using combustion calorimetry, Liu and co-workers have determined the standard molar enthalpies of formation of artemisinin 9a, artemether 28a and artesunate 31 as —1493, —2420 and —3320kJmoP respectively, and the thermal stabilities of these compounds were also investigated by thermal analysis in combination with FT-IR <2007MI1045>. 

Besides equilibriumconstants, additional thermodynamic data were included, if available, although little emphasis was put on their completeness. The data for primary master species comprise the standard molar thermodynamic properties of formation from the elements (AfG standard molar Gibbs energy of formation AfH°m standard molar enthalpy of formation ApSm- standard molar entropy of formation), the standard molar entropy (5m), the standard molar isobaric heat capacity (Cp.m), the coefficients Afa, Afb, and Afc for the temperature-dependent molar isobaric heat capacity equation... 

The conventional thermodynamic standard state values of the Gibbs energy of formation and standard enthalpy of formation of elements in their standard states are A(G — 0 and ArH = 0. Conventional values of the standard molar Gibbs energy of formation and standard molar enthalpy of formation of the hydrated proton are ArC (H +, aq) = 0 and Ar// (H +, aq) = 0. In addition, the standard molar entropy of the hydrated proton is taken as zero 5 (H+, aq) = 0. This convention produces negative standard entropies for some ions. 

Table 2.2 Standard molar Gibbs energies of formation and standard molar enthalpies of formation for some anions at 25 "C (in kJ mol - )...

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