Entalpia

Accepted Manuscript
Title: Total Phase Change Entropies and Enthalpies. An update on Fusion Enthalpies and Their Estimation Authors: James S. Chickos, William E. Acree Jr. PII: DOI: Reference: To appear in: Received date: Revised date: Accepted date: S0040-6031(09)00223-8 doi:10.1016/j.tca.2009.05.008 TCA 75095 Thermochimica Acta 25-2-2009 12-5-2009 14-5-2009

Please cite this article as: J.S.Chickos, W.E. Acree Jr., Total Phase Change Entropies and Enthalpies. An update on Fusion Enthalpies and Their Estimation, Thermochimica Acta (2008), doi:10.1016/j.tca.2009.05.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting,typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Total Phase Change Entropies and Enthalpies. An update on Fusion Enthalpies and Their Estimation James S. Chickos Department of Chemistry University ofMissouri-St. Louis Saint Louis MO 63121 and William E. Acree, Jr. Department of Chemistry University of North Texas Denton TX 76203 Abstract:

measurements. A group additivity method developed to estimate the total phase change entropies and enthalpies of organic solids is updated, applied to the new data and the results are compared. mol-1 for total phase change entropies and enthalpies,respectively, are similar in magnitude to alon with references are available as supplementary material.

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those reported previously. Experimental and estimated fusion entropies and fusion enthalpies

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The uncertainties associated with the 1016 new measurements, 18.5 J mol-1 K-1 and 7.6 kJ

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This compendium summarizes the fusion enthalpies of approximately 1000 newus

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INTRODUCTION Fusion enthalpy is an important physical property of the solid state. The magnitude of the fusion enthalpy influences solute solubility in both an absolute sense and in its temperature dependence. correcting thermochemical data to a standard state when combined with other thermodynamic properties. Techniques for estimating fusion enthalpies canplay several useful roles. Perhaps most importantly, they provide a numerical value that can be used in cases when there are no in cases where two or more values are in significant disagreement. experimental data. Estimations are also useful in selecting the most probable experimental value This property is an important consequence of molecular packing in crystals and can be useful in

It isprecisely because of the unpredictability of various phase changes that can occur in the solid state that the direct estimation of fusion enthalpy has had only limited success. An alternative phase transitions from a rigid crystal at T = 0 K to an isotropic liquid at the melting point, T = Tfus. It has been previously shown that the total phase change entropy is a more reliable group property thanfusion entropy.1 Once the total phase entropy is calculated and the fusion approach that we have selected has been to model the total entropy change associated with all

Large positive differences between estimated and experimental values ( T0fus H tpce calc - Hfus) are useful in identifying systems exhibiting either dynamic or complex phase behavior in the solid state. Plastic crystals exhibitmolecular motion in the solid state while those forming liquid crystals can exhibit non-isotropic molecular motion in the liquid phase. Both have associated with these phenomena, additional phase transitions or unusually large heat capacities that can attenuate the enthalpy and entropy associated with fusion.2 Large negative differences between estimated and experimental values on the other hand...