Cohesive energy enthalpy

Molecules have forces of attraction between them, and these intermolecular forces are responsible for many of the properties of liquids. There is a cohesion energy that holds the molecules together. The energy necessary to overcome these forces to vaporize a mole of liquid is known as the cohesion energy of the liquid or the energy of vaporization. It is related to the enthalpy of vaporization by the equation... 

The evaporation enthalpy AH was taken as the parameter of the cohesion energy between molecules minus the thermal energy needed to separate them (RT) divided by the molar volume Vm. So, equation (13.1) can be re-written as ... 

Table XI gives the room-temperature, atmospheric pressure crystal structures, densities, and atomic volumes, along with the melting points and standard enthalpies of vaporization (cohesive energies), for the actinide metals. These particular physical properties have been chosen as those of concern to the preparative chemist who wishes to prepare an actinide metal and then characterize it via X-ray powder diffraction. The numerical values have been selected from the literature by the authors.

We define the cohesive energy Ecoh (Johansson, Skriver ) as the difference between the energy of an assembly of free atoms in their ground state (see Table 1 of Chap. A) and the energy of the same assembly in the condensed phase (the solid at 0 °K), (this definition yields a positive number for Ecoii). It coincides with the enthalpy of sublimation AHj (see Chap. A) (which is usually extrapolated at room temperature). 

Solvent-polymer compatibility problems are often encountered in industry, such as in the selection of gaskets or hoses for the transportation of solvents. A rough guide exists to aid the selection of solvents for a polymer, or to assess the extent of polymer-liquid interactions. A semi empirical approach has been developed by Hildebrand based on the principle of like dissolves like. The treatment involves relating the enthalpy of mixing to a solubility parameter, S, and its related quantity, 8, called the cohesive energy density. 

Tekac, V., et al., Enthalpies of Vaporization and Cohesive Energies for Six Monochlori-nated Alkanes. J. Chem. Thermodyn., 1981 13, 659-662. 

When a solute is mixed with the solvent, one can assume that two opportunities forsolute-solvent interactions exist when both a solute-solute and a solvent-solvent interaction are broken. Therefore, the enthalpy of mixing per mole of solute is assumed to be proportional to the differences in cohesive energy densities ... 

The theoretical partial molal enthalpy of mixing resulting from mixing two compounds with different cohesive energy densities was estimated by ... 

The cohesive energy density of a pure compound can be estimated by different techniques (Scatchard, 1949 Hoy, 1970 Fedors, 1974). One method (Hildebrand et al., 1970) relates this parameter to the molar enthalpy of vaporizaMiflv, of the same compound ... 

A satisfactory theory of metallic bonding must account for the characteristic properties of high electrical and thermal conductivity, metallic lustre, ductility and the complex magnetic properties of metals which imply the presence of unpaired electrons. The theory should also rationalise the enthalpies of atomisation A/f tom of metallic elemental substances. A/f tom is a measure of the cohesive energy within the solid, and we saw in Chapter 5 how it plays an important part in the thermochemistry of ions in solids and solutions. The atomisation enthalpies of elemental substances (metallic and nonmetallic) are collected in Table 7.1. There is a fair correlation between A/Z tom an(J physical properties such as hardness and melting/boiling points. 

Consider now a metallic assembly of n Be atoms, using for the meanwhile only the 2s orbitals. As for Li(s), we obtain a band of n MOs but we now have In electrons, so that the band should be filled. Not only should this crystal have no metallic properties but it should also have zero cohesive energy since the occupancy of antibonding MOs should cancel out the effect of the occupied bonding MOs. But Be(s) does crystallise as a metallic solid, whose enthalpy of atomisation is twice as great as that of... 

Where U. = AHv l - RT is the molar interaction energy for particles of solute 1 and can be approximated by the molar enthalpy of vaporization AHV j expressed at temperature T. is the molar volume of the pure liquid substance 1. The cohesive energy density is expressed by cn which describes the interaction between the particles of the pure liquid 1. In a two component system it is assumed that the interaction... 

Enthalpy of mixing, AH, can be correlated to cohesive energy density, i.e., solubility parameter (5) as... 

A curious thermodynamic result is connected to the cohesive energy of an ionic liquid. From calorimetric experiments, performed to determine the heat of vaporization along an homologous family of cations, the increment in enthalpy of vaporization per -CH2- group was calculated [25] and it closely matches that observed for the H-alkanes, suggesting that, in the ionic liquid phase, the alkyl chains exist in an alkane-like environment. 

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