Standard molar enthalpy formation

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). 

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. 

Ha Hi AHr AH m AH°fi AHS, AHvaP A I see Dimensionless Groups Henry s law constant for species Pa m3 mol-1, equation 9.2-8 enthalpy of reaction for reaction as written, J standard enthalpy of reaction, J standard enthalpy of combustion of species i J mol-1 standard enthalpy of formation of species i, J mol-1 standard molar enthalpy of reaction with respect to species i, J mol-1 enthalpy of vaporization, J mol-1 standard molar enthalpy of activation (TST), J mol-1 initiator (species) inhibitor, inert species... 

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,... 

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 11 Standard molar enthalpies of combustion, sublimation, and formation for 1,3-dithiane and its 1 -oxide and 1,1-dioxide at 298.15 K...

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... 

Standard Molar Enthalpy (Heat) of Formation of Substances at 25 °C in kJ/mol... 

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 - )...

The enthalpies of formation of aqueous ions may be estimated in the manner described, but they are all dependent on the assumption of the reference zero that the enthalpy of formation of the hydrated proton is zero. In order to study the effects of the interactions between water and ions, it is helpful to estimate values for the enthalpies of hydration of individual ions, and to compare the results with ionic radii and ionic charges. The standard molar enthalpy of hydration of an ion is defined as the enthalpy change occurring when one mole of the gaseous ion at 100 kPa (1 bar) pressure is hydrated and forms a standard 1 mol dm-3 aqueous solution, i.e. the enthalpy changes for the reactions Mr + (g) — M + (aq) for cations, X (g) — Xr-(aq) for monatomic anions, and XOj (g) —< XO (aq) for oxoanions. M represents an atom of an electropositive element, e.g. Cs or Ca, and X represents an atom of an electronegative element, e.g. Cl or S. 

O Hare, P.A.G. (1993) Calorimetric measurements of the specific energies of reaction of arsenic and of selenium with fluorine. Standard molar enthalpies of formation Af7/°m at the temperature 2.98.15 K of AsFs, SeF6, As2Se3, AS4S4, and As2S3. Thermodynamic properties of AsFs and SeF6 in the ideal-gas state. Critical assessment of AfH°m (AsF3, 1)), and the dissociation enthalpies of As-F bonds. Journal of Chemical Thermodynamics, 25, 391-402. 

O Hare, P.A.G., Lewis, B.M., Susman, S. and Volin, K.J. (1990) Standard molar enthalpies of formation and transition at the temperature 298.15 K and other thermodynamic properties of the crystalline and vitreous forms of arsenic sesquiselenide (As2S3). Dissociation enthalpies of As-Se bonds. Journal of Chemical Thermodynamics, 22, 1191-206. 

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