Enthalpy of fusion vaporization

In spite of the wealth of information available on the preparative and structural aspects of the lanthanide chlorides (1-3), experimental thermodynamic, and, in particular, high-temperature vaporization data are singularly lacking. The comprehensive estimates of the enthalpies of fusion, vaporization, heat capacities and other thermal functions for the lanthanide chlorides by Brewer et ah (4, 5) appear internally consistent, but the relatively few experimental measurements (6-/2) do not permit confirmation of the estimates due to the narrow temperature ranges of study. Additionally, the absence of accurate molecular data for the gaseous species has hampered third-law treatment of the limited experimental vapor pressure data available. The one reported study (12) of the vaporization of EuC12 effected by a boiling-point method lacks accuracy for these reasons. 

We have shown in earlier work that it is possible to quantitatively relate a variety of liquid, solid and solution phase properties to the electrostatic potential patterns on the surfaces of the individual molecules [64-66]. Among these properties are pKa, boiling points and critical constants, enthalpies of fusion, vaporization and sublimation, solubilities, partition coefficients, diffusion constants and viscosities. For these purposes, we take the molecular surface to be the 0.001 au contour of the molecular electronic density p(r), following the suggestion of Bader et al [67],... 

AH change => enthalpy of fusion, vaporization, or sublimation for heat added... 

Temperature and Latent Enthalpies of Fusion, Vaporization, and Sublimation... 

Anderson and Wu, in Bulletin 606, include the following information for 832 compounds found in coal tar boiling temperature, the variation of boiling temperature with pressure, melting temperature, enthalpies of fusion, vaporization, and combustion, heat capacity, vapour pressure, and critical properties. 

Table 2.2 Enthalpies and temperatures of fusion and vaporization. Normal melting and boiling points and enthalpies of fusion and vaporization are tabulated by type of...

Benson and coworkers15, and the values differ somewhat. The differences probably reflect the small differences in the enthalpies of formation of the parent compounds used in the two cases (i.e., the differences between enthalpies of formation compiled by Benson and coworkers15 and those compiled by Pedley, Naylor and Kirby17) as well as different approaches to deriving the group contributions. The major differences probably arise from the different values used for the relevant enthalpies of fusion and vaporization. It should be emphasized that the derivation of group contributions is not purely quantitative in nature, and that subjective elements enter into the selection of molecules to be used to... 

Melting is, with only one known exception (helium), endothermic, and so all enthalpies of fusion (with the exception of that special case) are positive and are reported without their sign (see Table 6.3). The enthalpy of fusion of water at 0°C is 6.0 kj-mol to melt 1.0 mol H20(s) (18 g of ice) at 0°C, we have to supply 6.0 kj of heat. Vaporizing the same amount of water takes much more energy (more than 40 kj) because, when water is vaporized to a gas, its molecules must be separated completely. In melting, the molecules stay close together, and so the forces of attraction and repulsion are nearly as strong as those experienced in the solid (Fig. 6.22). 

Because enthalpy is a state function, the enthalpy of sublimation of a substance is the same whether the transition takes place in one step, directly from solid to gas, or in two steps, first from solid to liquid and then from liquid to gas. The enthalpy of sublimation of a substance must therefore be equal to the sum of the enthalpies of fusion and vaporization, provided that they are measured at the same temperature (Fig. 6.25) ... 

Self-Test 6.10A The enthalpy of fusion of sodium metal is 2.6 kj-mol 1 at 25°C, and the enthalpy of sublimation of solid sodium at that temperature is 101 kj-mol 1. What is the enthalpy of vaporization of sodium at 25°C ... 

FIGURE 6.25 Because enthalpy is a state property, the enthalpy of sublimation can be expressed as the sum of the enthalpies of fusion and vaporization measured at the same temperature. 

The enthalpies of fusion and vaporization affect the appearance of the heating curve of a substance. A heating curve is the graph showing the variation in the temperature of a sample as it is heated at a constant rate at constant pressure and therefore at a constant rate of increase in enthalpy (Box 6.1). 

The heat capacity of liquid iodine is 80.7 J-K -mol, and the enthalpy of vaporization of iodine is 41.96 kj-mol 1 at its boiling point (184.3°C). Using these facts and information in Appendix 2A, calculate the enthalpy of fusion of iodine at 25°C. 

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... 

C14-0133. The enthalpy of sublimation of Ice at 273.15 K Is not the simple sum of the enthalpies of fusion and vaporization of water, but it can be calculated using Hess law and an appropriate path that Includes fusion and vaporization. Devise such a path, show it on a phase diagram for water, and carry out the calculation, making reasonable assumptions If necessary (C(liquid water) = 75.3 7 mol K , and C(water vapor) = 33.6 K ). 

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. 

Figure 1.3 Standard enthalpy of aluminium relative to 0 K. The standard enthalpy of fusion (Afugffm) is significantly smaller than the standard enthalpy of vaporization (A vap// ).

Rordorf, B.F. Prediction of vapor pressures, boiling points and enthalpies of fusion for twenty-nine halogenated dibenzo-p-dioxins and fifty-five dibenzofurans by a vapor pressure correlation method. Chemosphere, 18(l-6) 783-788,1989. Rosen, J.D. and Carey, W.F. Preparation of the photoisomers of aldrin and dieldrin. 7 Agric. Food Chem., 16(3) 536-537,1968. Rosen, J.D. and Strusz, R.F. Photolysis of 3-(p-bromophenyl)-l-methoxy-l-methylurea, 7 Agric. Food Chem., 16(4) 568-573, 1968. 

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 data required for computing various heat effects involving explosives and explosions are standard heats (also called enthalpies) of formation, heats of detonation (or explosion), heats of fusion, vaporization and/or sublimation, heat conductivity, and specific heat. 

The enthalpies of formation of the saturated alcohols were taken from Reference 14 (2-, 3- and 4-heptanol) and K. B. Wiberg, D. J. Wasserman, E. J. Martin and M. A. Murcko, J. Am. Chem. Soc., 107, 6019 (1985) (1-methylcyclohexanol). The enthalpy of formation of 2-methylhex-l-ene-3-yn-2-ol is from Reference 32. The enthalpy of formation of solid 2,5-dimethylhexane-2,5-diol is from Reference 2 and the enthalpy of vaporization (100.7 0.5 kJmol-1) is from Reference 18 the liquid phase enthalpy of formation is derived as —664 kJmol-1 from an estimated enthalpy of fusion of 18 kJmol-1. 

To avoid the use of the ambiguous term "heat" in connection with "heat content," it is customary to use the term enthalpy. At a given temperature and pressure, every substance possesses a characteristic amount of enthalpy (H), and the heat changes associated with chemical and physical changes at constant pressure are called changes in enthalpy (AH) AHT is the enthalpy of transition. Two common enthalpies of transition are AHf = 1435 cal/mole for the enthalpy of fusion (melting) of ice at 0°C, and AH, = 9713 cal/mole for the enthalpy of vaporization of water at 100°C. 

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