Standard enthalpy of compounds

Standard Enthalpy of Compounds of Hydrogen and Oxygen at 25°C (kJ mole" )... 

Enthalpy of Formation. Once standard enthalpies are assigned to the elements, it is possible to determine standard enthalpies for compounds. For the reaction ... 

The standard enthalpy change, Aff°, for a given thermochemical equation is equal to the sum of the standard enthalpies of formation of the product compounds minus the sum of the standard enthalpies of formation of the reactant compounds. 

The standard enthalpy of combustion, AH°, is the change in enthalpy per mole of a substance that is burned in a combustion reaction under standard conditions. The products of the combustion of an organic compound are carbon dioxide... 

Use the information in Tables 6.3, 6.7, and 6.8 to estimate the enthalpy of formation of each of the following compounds in the liquid state. The standard enthalpy of sublimation of carbon is +717 kJ-moF 1. (a) H20 (b) methanol, CH,OH (c) benzene, C6H6 (without resonance) (d) benzene, C6H6 (with resonance). 

STRATEGY We write the chemical equation for the formation of HI(g) and calculate the standard Gibbs free energy of reaction from AG° = AH° — TAS°. It is best to write the equation with a stoichiometric coefficient of 1 for the compound of interest, because then AG° = AGf°. The standard enthalpy of formation is found in Appendix 2A. The standard reaction entropy is found as shown in Example 7.9, by using the data from Table 7.3 or Appendix 2A. 

For the thermodynamic factors Stull takes into account the decomposition temperature . This is defined as the temperature reached by the decomposition compounds of the particular substance when the latter decomposes into these constituent elements. It is therefore calculated using the standard enthalpy of formation of the compound. 

The standard enthalpy of formation AH°f of a compound is defined as the enthalpy change when one mol of the compound is formed from its constituent elements in the standard state. The enthalpy of formation of the elements is taken as zero. The standard heat of any reaction can be calculated from the heats of formation —AH of the products and reactants if these are available or can be estimated. 

As a thermodynamicist working at the Lower Slobbovian Research Institute, you have been asked to determine the standard Gibbs free energy of formation and the standard enthalpy of formation of the compounds ds-butene-2 and trans-butene-2. Your boss has informed you that the standard enthalpy of formation of butene-1 is 1.172 kJ/mole while the standard Gibbs free energy of formation is 72.10 kJ/mole where the standard state is taken as the pure component at 25 °C and 101.3 kPa. 

TABLE 1. Standard enthalpies of formation of organogermanium compounds (kJmol -i)a... 

TABLE 6. Standard enthalpies of formation of organolead compounds (kJ mol 1)... 

Standard enthalpy of formation, AHJ, is the enthalpy change when one mole of a compound is formed, under standard conditions, from its elements in their standard states. 

The compounds that have negative standard enthalpies of formation are more stable than the elements from which they are made. Those that have positive standard enthalpies of formation are less stable than the elements. It is unlikely that many compounds would have zero standard enthalpy of formation. Such compounds would be exactly as stable as the elements from which they are made. 

T. Suzuki, Empirical Relationship Between Lower Flammability Limits and Standard Enthalpies of Combustion of Organic Compounds, Fire and Materials (1994), 18 333-336. 

The standard enthalpy of formation, A fH, of a compound at 0 K reflects the strength of the chemical bonds in the compound relative to those in the constituent elements in their standard state. The standard enthalpy of formation of a binary oxide such as CaO is thus the enthalpy change of the reaction... 

Suzuki [Suzuki, Empirical Relationship between Lower Flammability Limits and Standard Enthalpies of Combustion of Organic Compounds, Fire and Materials, 18 333-336 (1994) Suzuki and Koide, Correlation between Upper Flammability Limits and Thermochemical Properties of Organic Compounds, Fire and Materials, 18 pp. 393-397 (1994)] provides more detailed correlations for the UFL and LFL in terms of the heat of combustion. 

We define the standard enthalpy of formation AH as the enthalpy change involved in forming 1 mol of a compound from its elements, each element existing in its standard form. Both T and p need to be specified, because both variables influence the magnitude of AH. Most books and tables cite AH at standard pressure p and at a temperature of 298 K. Table 3.1 cites a few representative values of AH. ... 

The standard enthalpy of formation AH is the enthalpy change involved in forming 1 mol of a compound or non-stable allotrope from its elements, each element being in its standard form, at s.t.p. 

Table 3.2 Standard enthalpies of combustion A7/c° for a few organic compounds (all values are at 298 K)...

The values of AHf (g) carry both the experimental uncertainty in the standard enthalpy of formation of the crystalline (or liquid) metal compound and the uncertainty (experimental or estimated) in the enthalpy of sublimation (or vaporization). 

The standard enthalpies of formation of the gaseous compounds and the enthalpy of disruption derived therefrom are given in Table 13. An interesting problem arises as to how these results are to be evaluated. If the value of AHf [M(CO)s, g] derived15,1 ) from electron impact measurements on M2(CO)io (M = Mn, Re) is used, then as outlined earlier this will be expected to give an upper limit to the value of D(M-M). It has been shown16) that for all values of Z)(M-M) below specified upper limits the following relation holds... 

Adiabatic detachment energy [7]. Abbreviations used rcoy (Em) = covalent radius of element E in a trivalent compound BE(E-E) = bond enthalpy of a single E-E bond D°298(E2) = dissociation enthalpy of the E2 molecule at standard conditions IE = ionization enthalpy EA = electron affinity AHf°(E2 g) = standard enthalpy of formation of the gaseous E2 molecule. 

Enthalpies of reaction can also be calculated from individual enthalpies of formation (or heats of formation), AHf, for the reactants and products. Because the temperature, pressure, and state of the substance will cause these enthalpies to vary, it is common to use a standard state convention. For gases, the standard state is 1 atm pressure. For a substance in an aqueous solution, the standard state is 1 molar concentration. And for a pure substance (compound or element), the standard state is the most stable form at 1 atm pressure and 25°C. A degree symbol to the right of the H indicates a standard state, AH°. The standard enthalpy of formation of a substance (AHf) is the change in enthalpy when 1 mol of the substance is formed from its elements when all substances are in their standard states. These values are then tabulated and can be used in determining A//°rxn. 

The standard enthalpy of formation of a compound, AHf, is the enthalpy change when 1 mol of the substance is formed from its elements and all substances are in their standard states. 

It is obvious from the definition of standard enthalpy of formation that these quantities do not represent the absolute enthalpic stability of compounds. They merely reflect their enthalpic stability relative to that of the chemical elements in standard reference states (to which AfH° = 0 has been arbitrarily assigned). It is thus unreasonable to state that a given substance is more stable than another just because it has a lower standard enthalpy of formation. We can only use AfH° values to make such direct comparisons when we are assessing the relative stability of isomers. 

To calculate the standard enthalpy of formation of the crystalline compound, our imaginary author would have used the best available values at the time, which were quoted from the widely adopted NBS Circular 500 [22] AfH° (C02, g) = -394.907 kJ mol-1 and Af77°(H20, 1) = -286.131 kJ mol-1. The final value reported in his publication was, therefore (see equation 2.7), Af//°(Ci4Hio, cr) = -6959.4 - X kJ mol-1. 

The choice of a given database as source of auxiliary values may not be straightforward, even for a thermochemist. Consistency is a very important criterion, but factors such as the publication year, the assignment of an uncertainty to each value, and even the scientific reputation of the authors or the origin of the database matter. For instance, it would not be sensible to use the old NBS Circular 500 [22] when the NBS Tables of Chemical Thermodynamic Properties [17], published in 1982, is available. If we need a value for the standard enthalpy of formation of an organic compound, such as ethanol, we will probably prefer Pedley s Thermodynamic Data and Structures of Organic Compounds [15], published in 1994, which reports the error bars. Finally, if we are looking for the standard enthalpy of formation of any particular substance, we should first check whether it is included in CODATA Key Values for Thermodynamics [16] or in the very recent Active Thermochemical Tables [23,24],... 

Having thus settled on Pedley s tables for the pure organic compounds, we have then decided to use NBS Tables to derive the solution enthalpies in figure 2.1. The values can be easily evaluated from the differences between the standard enthalpies of formation of the compounds in solution and the standard enthalpies of formation of pure substances, viz. 

The important point to be noted here is that the calculation uses NBS data only. Had we combined Pedley s standard enthalpies of formation for the pure compounds with the NBS values for the solutions, we would have obtained incorrect results. 

In summary, we selected one database (Pedley s) to quote the standard enthalpies of formation of the pure organic compounds and another database (NBS) to derive the solution enthalpies. Although these databases are not mutually consistent, that did not affect our final result because the experimental enthalpies of solution were calculated with NBS data only. The exercise illustrates the sort of caution one should keep in mind whenever two or more nonconsistent databases are used. 

The so-called Laidler scheme was developed as a tool to estimate standard enthalpies of formation of organic compounds [90], It relies on the bond-additivity concept, that is, it assumes that the standard enthalpy of atomization of a given molecule in the gas phase (Aat//°, defined as the standard enthalpy of the reaction where all the chemical bonds are cleaved, yielding the gaseous ground-state atoms) can be evaluated by adding the relevant bond enthalpy terms. For instance, in the case of phenol, its standard enthalpy of atomization, or simply its enthalpy of atomization, refers to reaction 5.28 at 298.15 K ... 

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