Phase enthalpy change

As described in Section 14-1. when AR and ZlS have the same sign, the spontaneous direction of a process depends on T. For a phase change, enthalpy dominates AG at low temperature, and the formation of the more constrained phase is spontaneous, hi contrast, entropy dominates AG at high temperature, and the formation of the less constrained phase is spontaneous. At one characteristic temperature, A G = 0, and the phase change proceeds in both directions at the same rate. The two phases coexist, and the system is in a state of d Tiamic equilibrium. 

Evaporation of the storage material. Evaporation is a phase change with usually large phase change enthalpy however the process of evaporation strongly depends on the boundary conditions ... 

Solid-solid phase changes. Solid-solid phase changes have the same characteristics as solid-liquid phase changes, but usually do not posses a large phase change enthalpy. However, there are exceptions. 

Large phase change enthalpy AH =s to achieve high storage density compared to sensible storage. 

The heat capacity cp is negligible compared to the phase change enthalpy (cp AT latent heat at the phase change temperature has to be considered. 

For non-pure materials, when the phase change temperature is not constant, the phase change enthalpy can be calculated using the following expression, which includes the temperature change (between Tmi and 7m2) during phase change ... 

Chickos, J.S., Acree, W.E., Jr., Liebman, J.F. (1999) Estimating solid-liquid phase change enthalpies and entropies. J. Phys. Chem. Ref. Data 28, 1535-1673. 

We now mm to derivatives of biphenyl. Our archives show a 12.8 6.3 kJmol-1 difference in the enthalpies of formation of the 2- and 4-amine as solids35. Is this difference due to strain in the former species One probe of the strain energy is to consider the enthalpies of reaction 12 for R = 2- and d-PhCgFU. 2 f°r the 2-isomer equals 1.5 2.6 kJ mol-1. We do not know what it is for the 4-isomer because we lack all phase-change enthalpy data for this species. Intuitively, this difference quantity should be 0 because no stabilizing or destabilizing effects are expected for this isomer. We thus conclude that 2-aminobiphenyl is essentially strain-free. 

We are being somewhat disingenuous here. If performed and interpreted correctly and with the appropriate ancillary phase-change enthalpy information, the enthalpy of formation of an arbitrary species by ion-molecule reaction chemistry and by combustion calorimetry must be the same. That the ionization potential of quinuclidine is higher than l,4-diazabicyclo[2.2.2]octane simply says that there is a stabilizing effect in the radical cation of the latter not found in the former. This information does not say that there is a stabilizing effect in the neutral molecular form of the latter not found in the former. After all, we trust the reader is not bothered by the fact that the ionization potential order of the cyclohexenes increases in the order 1,3-diene < 1,4-diene < 1-ene < 1,3,5-triene (benzene). 

In attempting a systematic analysis of related organolithium species, it would be advantageous to consider all of them in at least one common phase. Our preference is for the gas phase where intermolecular interactions are usually absent, although this may not hold true for organolithium species. Unfortunately, most of the desired phase change enthalpies for organolithiums are not found in the experimental literature. However, where the species of interest is a liquid, we can employ the published CHLP protocol for the estimation of the enthalpy of vaporization. ... 

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