Donald G. Hesse

Heat capacity corrections to a standard state: a comparison of new and some literature methods for organic liquids and solids

The estimation methods commonly used to correct phase change enthalpies to the standard state are compared where possible to experimental measurements. Heat capacity corrections for liquid-gas equilibria are found to correlate with molecular structure, and we suggest an improved method for estimating these corrections using group methods. A similar improvement for estimating heat capacity corrections for solid-gas equilibria using group methods is also proposed. Heat capacity corrections for liquid-solid equilibria are examined. These corrections were found to be comparable in magnitude to the experimental error associated with heat capacity measurements, so it was not possible to obtain any meaningful correlations.

A Group Additivity Approach for the Estimation of Heat Capacities of Organic Liquids and Solids at 298 K

A group additivity method is described which provides heat capacity estimates of the condensed phase. The data base consists of 810 liquids and 446 solids. Group values for carbon in various common substitution and hybridization states and for 47 functional groups are provided. The standard error of estimation using this approach on this data base is 19.5 (liquids) and 26.9 J/ (mole K) (solids). This can be compared to typical experimental uncertainties of 8.12 and 23,4 J/ (mole K) associated with these measurements, respectively. Experimental uncertainties were estimated from the numerical differences obtained for a given substance from multiple independent literature reports.

Determination of vaporization enthalpies of simple organic molecules by correlations of changes in gas chromatographic net retention times

A technique is described for determining vaporization enthalpies Δg1Hm(298 K) of organic compounds by high resolution gas chromatography. The technique correlates changes in net retention times of compounds whose Δg1Hm(298 K) are known with those of the compound(s) of interest. The best results are obtained when the reference compounds are structurally similar and in the same chemical family. Application to hydrocarbons and various hydrocarbon derivatives containing one functional group is documented in this report. Comparison with literature values of 102 compounds resulted in a standard deviation of 1.27 kJ mol−1. In most cases, the difference between the literature value and the value from our results is within the normal errors associated with vaporization enthalpy determinations. A linear correlation between the logarithms of experimental vapor pressure and reciprocal retention time was also observed for the compounds studied.

Sublimation enthalpies at 298.15 K using correlation gas chromatography and differential scanning calorimetry measurements

The sublimation enthalpies of 17 hydrocarbons are obtained by combining the technique of correlation gas chromatography (CGC), to evaluate vaporization enthalpies at 298.15 K, and differential scanning calorimetry (DSC) to measure fusion enthalpies. Vaporization enthalpies at 298.15 K obtained by CGC are compared to values measured directly from vapor pressure measurements at temperatures above the melting point by adjusting the experimental vaporization enthalpy for the effects of temperature. Vaporization enthalpies obtained by these two methods agree within3877 J mol ˇ1 . Fusion enthalpies are similarly adjusted for temperature. Sublimation enthalpies, obtained by combining temperature adjusted fusion, and vaporization enthalpies agree within2580 J mol ˇ1 . The sublimation enthalpies of azulene and 1,8-cyclotetradecadiyne are also measured by head-space analysis resulting in values of 76880 and 94348 J mol ˇ1 at 298.15 K, respectively. # 1998 Elsevier Science B.V.

An experimental test of the double solubility rule

Vapor pressures, fusion (δHfus), and sublimation (δHs) enthalpies are reported ford, dl-2,3-dibromobutane-1,4-diol, andd,dl-dimethyl 2,3-diacetyltartrate. Values of 8.1, 7.04, 6.9, and 6.6 kcal/mol for δHfus were measured by differential scanning calorimetry and values of 28.4, 27.3, 26.2, and 25.5 kcal/mol (not corrected to 25‡) were calculated for δHs from the temperature dependence of vapor pressure, respectively. The vapor pressure ofdl-2,3-dibromobutane-1,4-diol, which crystallizes as a conglomerate, exceeds the vapor pressure of thed form by more than a factor of 2 over the temperature range studied. The ratio of vapor pressures ofdl- andd-dimethyl 2,3-diacetyltartrate, which crystallizes as a racemic compound, is approximately 1.5.

Estimating Enthalpies of Sublimation of Hydrocarbons

A general technique for the estimation of sublimation enthalpies is described and applied to hydrocarbons. The vaporization enthalpy of a hydrocarbon is obtained by using a simple relationship based on the type and number of carbons present. Fusion enthalpies are estimated using a group additivity approach. Addition of vaporization and fusion enthalpies affords a sublimation enthalpy in good agreement with experiment. The standard error of this estimation technique is 2.6 kcal/mole (10.9 kJ/mole) and compares with a standard experimental error of 1.74 kcal/mole (7.28 kJ/mole) obtained from a statistical analysis of literature data.