New Methods for Estimating the Heats of Formation, Heat Capacities, and Entropies of Liquids and Gases

Abstract

Using group additivity values to estimate Δvap H°(298) and the Kistiakowsky equation to estimate both Δvap S°(T b) and Δvap H°(T b) at the boiling point (T b), it is shown how an average value of Δvap C p° can be obtained which can then be used to calculate values of Δvap S°(298). This latter can then be used in conjunction with S°(g,298) to calculate S°(l,298). Alternatively, where values of S°(l,298) are available but not S°(g,298) the latter can be calculated. The method applies to regular liquids, even those with relatively large dipoles but not to H-bonded liquids. The accuracy of estimated values of S°(l,298) are 0.45 ± 0.16 cal/(mol K) with a maximum deviation of 1.0 cal/(mol K) for an assortment of 14 selected compounds and 0.3 ± 0.12 for another 17 liquids for which groups are not available but Δvap H°(298) and Δvap S°(298) are. Here the largest deviation is 1.9 cal/(mol K). Calculated values of C p°(l,298) are much less accurate, ±3 cal/(mol K) with a maximum deviation of 9.0 cal/(mol K). It is also shown that the best average value of C p° to use in calculating changes in ΔH° and ΔS° in a specified temperature interval, T 1 to T 2, is the arithmetic mean of the initial and final values, [C p°(T 1) + C p°(T 2)]/2. Changes are recommended in some of the group values for calculating Δvap H°(298) and also in the group O−(C)2 for calculating gas-phase entropies of ethers and for N−(C)2(H) for calculating entropies of secondary amines.

Calculation of Entropy and Heat Capacity of Organic Compounds in the Gas Phase. Evaluation of a Consistent Method without Adjustable Parameters. Applications to Hydrocarbons

“…Examples include direct extrapolation of data for related compounds [1][2][3] or, more generally, use of group increment methods. 1,[4][5][6][7][8][9][10][11][12] Several extensive applications of group increment calculations have been published; illustrative examples are those of Chao et al, 13 of Wilhoit and Zwolinski, 14 and of Scott 5 (using a different approach).…”

“…19 Many organic compounds of interest exist as mixtures of conformers. Two approaches have been used to get values of entropy and of heat capacity for conformer mixtures: (1) Use a model that simultaneously includes all rotational possibilities of the molecule; examples are the studies by Vansteenkiste et al [20][21][22][23] (2) Alternatively compute the energies and partition functions for all significant conformers individually; conformers are treated as individual molecules. Values of entropy and of heat capacity are computed as the sum of fractional values based on the Boltzmann fractions.…”

Reaction kinetics of phenol synthesis through one-step oxidation of benzene with N2O over Fe-ZSM-5 zeolite

“…The thermodynamic data of various substances, such as the standard formation enthalpy (∆ f H Θ m ), the standard Gibbs free energy (∆ f G Θ m ) and the heat capacity C Θ p, m , used in estimation of thermodynamic function changes of the reaction were found in the literature [29][30][31], or they were estimated using group additivity methods [32][33][34]. Hence, the heat and Gibbs function change of the reactions at different temperatures were estimated from their ∆ f H Θ m and ∆ f G Θ m values, as shown in Fig.…”

Enthalpy of Formation of CH3SO and CH3SO2: A Difficult Case in Quantum Chemistry

“…Theoretically, empirical group additivity methods such as those developed by Benson and co-workers 1 provide an easy way to estimate the thermochemistry of gas-phase reactions for species whose thermochemistry has not been measured. However, its weakness lies in its requirement to apply corrections for steric effects that are not always well determined, as well as for groups not experimentally studied.…”