Enthalpies sublimation

Experimentally one can determine the energy to separate an atom or molecule from the lattice. This is called the sublimation enthalpy The sublimation enthalpy is the 

On a real surface the atoms undergo reorientation processes, which lead to changes in the distribution of energies between different bonds. The bond of an atom in a kink site position (the calculation will be described in the next chapter) can differ from the ideal value. 

An approximate value of the sublimation enthalpy is the sum of enthalpy of fusion and the enthalpy of vaporization 

Values of the sublimation enthalpy of main group and transition metals are shown in Table 2.3 2 

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Equivalent expressions can be obtained for the (solid + vapor) equilibrium to give the vapor pressure of a solid, by substituting the sublimation enthalpy, AjubT/m, for Avap//m ... 

In this cycle, S is the sublimation enthalpy of Na, D is the dissociation enthalpy of Cl2, I is the ionization potential of Na, E is the energy released when an electron is added to a Cl atom, and U is the lattice energy. 

We used all of the Cis hydrocarbons in Pedley and derived their enthalpies of sublimation by subtracting the recommended enthalpies of formation of the solid and the corresponding gaseous species. There was considerable variation in the sublimation enthalpies, as seemingly befits the diverse choice of compounds (and associated crystal packing) including such species as naphthacene, 6,6-diphenylfulvene, 3,4,5,6-tetramethylphenanthrene, [3.3]paracyclophane and n-octadecane. 

The meaning of this observation is seen by considering Scheme 1. AH° (3) and AH0(4) are the enthalpies of hypothetical reactions of a family of compounds ML , where reactants and products are in the standard reference states and in the gas phase, respectively, and AHy are vaporization or sublimation enthalpies. 

On descending the sub-group the colour generally becomes lighter, indicating an increased separation between the frontier orbitals, which are believed to be predominantly associated with the metal frame, and therefore a decrease of the metallic character. The overall constancy of the values of the enthalpies of formation of these compounds, and the use of an estimated sublimation enthalpy of ca. 

If A is a gas at ambient temperature and pressure, AsoiVH° can be determined experimentally by calorimetric methods or from measurements of the solubility change with temperature [42-44], When A is a liquid or a solid, its solvation enthalpy in a given solvent is usually calculated from its standard solution enthalpy (Asin//°) and its standard vaporization or sublimation enthalpy ... 

Most of the methods for estimating reaction enthalpies are applicable only to the gas phase. Solvation enthalpy data are thus particularly important because they allow gas-phase estimates to be extended to reactions in solution—which is the most common medium for reactions of practical interest. However, solvation enthalpies are not very abundant and must often be estimated. Unfortunately, this can be a difficult exercise, especially when A is a solid, because sublimation enthalpies are scarce and hard to estimate. Thus, ASU, H°(A) is usually the unknown term in equation 2.44. The solution enthalpy term, Asi 7/°(A), is generally small and can often be predicted—or determined with a calorimeter. 

The enthalpy of reaction 2.45 cannot be determined directly. As shown in figure 2.5, it is calculated by using several experimental quantities the standard enthalpy of formation of the solid alkoxide, the standard sublimation enthalpy and the ionization energy of lithium, and the standard enthalpy of formation and the adiabatic electron affinity of gaseous methoxy radical (equation 2.47). 

A marginal but very important application of the drop calorimetric method is that it also allows enthalpies of vaporization or sublimation [162,169] to be determined with very small samples. The procedure is similar to that described for the calibration with iodine—which indeed is a sublimation experiment. Other methods to determine vaporization or sublimation enthalpies using heat flow calorimeters have been described [170-172], Although they may provide more accurate data, the drop method is often preferred due to the simplicity of the experimental procedure and to the inexpensive additional hardware required. The drop method can also be used to measure heat capacities of solids or liquids above ambient temperature [1,173],... 

Considering now the sublimation enthalpy of graphite (evaluated for the sublimation to monoatomic C gas) and the dissociation energy of hydrogen ... 

Verevkin, S.P. and Schick, C. Determination of vapor pressures, enthalpies of sublimation, enthalpies of vaporization, and enthalpies of fusion of a series of chloroaminobenzenes and chloronitrobenzenes. Fluid Phase Equillb., 211 (2) 161-177,2003. 

The enthalpy of formation in the gas phase was calculated by summing the enthalpy of formation of the solid species from Reference 50 with the sublimation enthalpy in Reference 1. 

There are enthalpies of formation for two phases of cumyl perbenzoate. However, the associated sublimation enthalpy that interconnects these phases, 43 kJ mol , is much too small. In that the enthalpy of sublimation must exceed the enthalpy of vaporization, and the latter is at least 75 kJmol using the CHLP protocol (from the number of carbons alone), the enthalpy of formation data loses credibility. Furthermore, lacking the enthalpy of formation of the corresponding cumyl benzoate (in any phase) disallows comparison. This would seem to be an altogether normal species until it is recognized that the archetype arylcarboxylate ester, methyl benzoate, has provided complications for the calorimetrist . 

A related study of 3,6-diphenyl-l,2,4,5-tetroxane resulted in enthalpies of formation of the sohd and gaseous species of 134.0 1.3 and 99.7 1.3 kJmor. Again, while computational theory and experiment are in good agreement, the sublimation enthalpy of 34.3 kJmoU seems too low. We would have suggested a lower bound of ca 71 kJmoU based on our enthalpy of vaporization estimation approach given above. 

Figure 9.63 Properties of REEs (from above dissociation energies of lanthanide oxides ( ), sublimation enthalpies of REEs (U) and transition energies for the transition of the 5d electron from 4fn 5d6s2...

The /3-diketonates are usually thermally stable and may be fused or volatilized with little or no decomposition. These complexes were tested by gas-liquid chromatography without evidence for on-column degradation.252 However, some retention of fluorodiketonato complexes was due to reaction in the stationary phase or with active sites.253 The sublimation enthalpy of tris(trifluoroacetylacetonate)vanadium(III) is 118 2kJmol-1 and the evaporation enthalpy is 77.8 0.8 kJ mol-1.254 At 300-470°C there is decomposition of [V(acac)3] acetone, CO and CO2 are the products.255... 

Summing the experimentally determined solid-phase enthalpies of formation of the olefin and the sublimation enthalpies predicted by equation 23 results in the desired gas-phase quantities of 358 and 473 kJmol-1 for the enthalpies of formation of tri- and tetraphenylethylene. Combined with the above enthalpies of formation of ethylene, styrene and ( )-stilbene, our new numbers allow us to generate a reasonably straight line as a function of the number of phenyl groups ... 

The formation enthalpies for gaseous fullerene hydrides were calculated above from quantum chemical calculations. Since the experimental sublimation enthalpies are available only for fullerene C60... 

Asii /P (C6qH36, 298.15K) = (195 + 10) kJmol-1 (Dorozhko et al. 2001) the sublimation enthalpies for the other fulleranes were estimated from an additive procedure ... 

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