Fusion enthalpies

According to Trouton s rule, the entropy of vaporization of an organic liquid is a constant of approximately 85 J-mol 1 -K 1. The relationship between entropy of fusion, enthalpy of fusion, and melting point is given by... 

The terms enthalpy of fusion, enthalpy of vaporization, enthalpy of combustion, and many more cause some students to believe that there are many different kinds of enthalpies. There are not. These names merely identify the processes with which the enthalpy term is associated. Thus, there are processes called fusion (melting), vaporization, sublimation, combustion, and so forth. The corresponding enthalpy changes are called by names that include these descriptions. 

The heat capacity, Cp, a useful parameter in evaluating vaporization, sublimation, and fusion enthalpies with temperature, was determined by DSC measurements for 1,3-oxathiane 3,3-dioxide to be 148.1 2.6 J moC at... 

What can be said about the thermochemistry of methylated cyclic ketones in any phase other than liquid In that we would have discussed gas-phase species if the data were available, what we are really asking is what can be said about the species of interest as solids , Let us generalize this to see what can be said about the thermochemistry of cyclic ketones as solids. In that the cyclic ketones in the previous section were all formally quite strain-free derivatives of cyclopentanone or cyclohexanone, let us now consider only such species and their cycloalkane analogs. There are no enthalpy-of-fusion data available for cyclopentanone. For cyclohexanone, the temperature-uncorrected fusion enthalpy is about 1.3 kJmol-1—we arbitrarily ignore in this discussion enthalpies of any crystal - different crystal phase enthalpy. The same treatment for cyclohexane discloses the fusion enthalpy of 2.7 kJmol-1. Accordingly, the earlier reported value of 638(6) = 104.4 for the gas phase is sequentially modified to ca 115 kJ mol-1 for the liquid and to 113 kJmol-1 for the solid. For 2-norbomanone, adamantanone and diamantanone (39a, 42 and 43), the gas-phase differences of 115.7 6.8, 96.0 5.1 and 90.6 3.2 become for the solids 124.4 6.8, 116.9 4.5 and 97.8 3.1 kJmol-1. Very roughly, 639( ) is some 10 kJmol-1 more positive for solids than for gases and 639(8) is about 113 kJmol-1 for all of the solids discussed here. 

More Precise Name Enthalpy of fusion Enthalpy of vaporization Enthalpy of sublimation... 

Molar Gibbs free energy of component i Convective heat transfer coefficient Microhardness Heat of fusion Enthalpy of component i Preexponential constant for kinetics Permeability coefficient, ktji Length of sample Lewis number, 2/a... 

The enthalpies of transition and fusion are also poorly defined. Kantor, et al. ( ) has measured the enthalpy of the crystal and liquid. No data points were obtained in the beta phase region. As a result, we can only derive a combined value for the enthalpy of transition and fusion. Following the suggestion of Abey 12), the enthalpy values of Loasby and Dearden (10) and Abey (1 ) have the same value for the ratio of the transition enthalpy to fusion enthalpy, even though the actual values for each enthalpy are quite different. Thus, using this ratio and the sum of the enthalpies, as determined by Kantor et al. ( ), we calculate the Individual enthalpies of transition and fusion. A much more detailed discussion is in progress. 

Fusion enthalpy (kJ moP7 Crystallization entropy (J mor He (kJ mor7 Fusion entropy (J moF ... 

There are numerous isomers of the multiply methylated anilines, most of which have been ignored by the thermochemical community. Of the six possible ring-only methylated species, the sole species to be studied is the 2,4,5-trimethylaniline (the earlier-named pseudocumidine) in the very old Reference 23. The solid-phase enthalpy of formation is —110 k mol 1. The enthalpy of formation of the corresponding hydrocarbon, 1,2,4-trimethylbenzene (the earlier-named pseudocumene), is —25 kJ rnol 1 using a temperature-uncorrected fusion enthalpy. The resulting amino/hydrogen difference is 85 kJmol-1, an altogether unreasonable value. 

J. S. Chickos, W. E. Acree, Jr. and J. F. liebman,./. Rhys. Chem. Ref. Data, 28, 1535 (1999). Strictly, use of these fusion enthalpies are estimates. The quantities should be corrected to 298 K from the melting point. However, the error is generally small because changes in heat capacities of solids and liquids as functions of temperature are generally small. 

Pure substances and eutectic mixtures solidify and melt at definite temperatures dE, which differ from substance to substance and are barely dependent on the pressure. The best known example of this is water which at atmospheric pressure, freezes at E = 0 °C. This releases the fusion enthalpy of hE = 333kJ/kg. When a solid body melts the enthalpy of fusion must be supplied to the solid as heat. 

During the time interval dt the phase interface moves a distance of ds. This releases the fusion enthalpy... 

The simple method based on Equation 10.13, however, is not commonly used because 100% pure crystalline samples for most polymers are not available. One alternative approach is to use the fusion enthalpy, the latent heat of melting, of chemical repeated units (A Hu) to replace A/fioo in the calculations. AHu can be calculated using the I dory relationship for the depression of the equilibrium melting temperature of a homopolymer due to the presence of low molecular mass diluents. The AHu values of some polymers are available in literature. 

For the calculation of the phase diagrams using coupled analysis of thermodynamic and phase diagram data, the thermodynamic data represent enthalpies of fusion, enthalpies of mixing, heat capacities, and all other data that are available from the literature. The phase diagram data are the measured temperatures of primary crystallization, temperatures of secondary crystallization, etc. as well as the temperatures of the eutectic temperatures. 

The results obtained for water content, fusion enthalpy. Toilet and Tpea, are presented in Table 27.2. Water content varied from 44.17 to 44.27% and... 

Chickos, J.S., Nichols, G. and Ruelle, P. (2002) The estimation of melting points and fusion enthalpies using experimental solubilities, estimated total phase change entropies, and mobile order and disorder theory. /. Chem. Irf. Comput. Sci, 42, 368-374. 

Around the melting point of the eutectic, 620 - 640 K, the DTA signal shows a small dip for all mixtures, representing the fusion enthalpy. From this point on, the catalyst immediately shows combustion activity, as can be seen from the rising DTA signal. 

What can be said about either series when studied as solids From Eqs. 29 and 30 we see that our question can be answered in terms of the enthalpies of fusion. Consider the first series. For benzene, the enthalpy of fusion is 9.9 kJ mol" while it is but 6.6 kJ mol" for toluene. Extrapolating to 3.3 for the xylenes seems wrong if for no other reason than we would further extrapolate to impossible zero or negative values for a species with four or more methyl groups. Indeed, the fusion enthalpies for the three xylenes are 13.6,11.6, and 17.1 kJmol" for the o-, m-and p-isomers, respectively. Homology is lost, at least suspect, or said more optimistically, one must be tolerant of greater disparities. 

Gibbs free energy of formation Latent heat of fusion Enthalpy change at the triple point Heat of vaporization Enthalpy of vaporization Volume change for liquid-solid transformation... 

---