Enthalpy of Vaporization and Fusion

As heat is taken from your skin to vaporize the water, you cool down. The heat required to vaporize one mole of a liquid is called its molar enthalpy (heat) of vaporization (AHvap). Similarly, if you want a glass of cold water, you might drop an ice cube into it. The water cools as it provides the heat to melt the ice. The heat required to melt one mole of a solid substance is called its molar enthalpy (heat) of fusion (AHfus). Because vaporizing a liquid and melting a solid are endothermic processes, their AH values are positive. Standard molar enthalpies of vaporization and fusion for five common compounds are shown in Table 15.4. 

Chemical Properties Handbook Vapor pressure, aqueous solubihty, Henry s law constant, and the octanol-water partition coefficient are hsted for over 1300 compounds along with other quantities such as enthalpies of vaporization and fusion, van der Waal s surface and volume. Compounds are indexed by CAS No. and formula. 

Calculate the molar entropies of fusion and vaporization for water at its normal melting point and boiling point. The molar enthalpies of vaporization and fusion of water are given in Tables 7.7 and 7.8, respectively. 

The reverse of vaporization is condensation and the reverse of fusion (melting) is freezing. The molar enthalpy changes are, respectively, the negative of the enthalpies of vaporization and fusion because the energy that is supplied (during heating) to vaporize or melt the substance is released when it condenses or... 

This relation is the origin of the obsolescent terms latent heat of vaporization and fusion for what are now termed the enthalpy of vaporization and fusion. 

An overview of some basic mathematical techniques for data correlation is to be found herein together with background on several types of physical property correlating techniques and a road map for the use of selected methods. Methods are presented for the correlation of observed experimental data to physical properties such as critical properties, normal boiling point, molar volume, vapor pressure, heats of vaporization and fusion, heat capacity, surface tension, viscosity, thermal conductivity, acentric factor, flammability limits, enthalpy of formation, Gibbs energy, entropy, activity coefficients, Henry s constant, octanol—water partition coefficients, diffusion coefficients, virial coefficients, chemical reactivity, and toxicological parameters. 

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. 

First law change in enthalpy, heat of formation, heat of reaction, Hess s Law, heats of vaporization and fusion, calorimetry... 

In some cases, a literature source of thermodynamic data may exist, allowing one to perform the conversion. Fortunately, standard references (such as Refs. 168 and 180) frequently tabulate both the condensed and gas phase thermochemistry values. When that is not the case, the following relations may be used, where the enthalpies and entropies of vaporization and fusion must be at 25 °C ... 

Define the molar enthalpy of fusion and the molar enthalpy of vaporization, and identify them for a substance by using a heating curve. 

Organic thermochemistry usually deals with molecules in the gaseous state in order to study their intrinsic stabilities in the absence of a crystal lattice, intermolecular bindings in the liquid state, or solvation forces. Therefore the determination of the enthalpy of vaporization or the enfrialpy of sublimation is an essential step in obtaining the enthalpy of formation in die gas phase. The enthalpy of sublimation can be obtained by combination of the enthalpy of vaporization and the enthalpy of fusion (equation (12)). The entibialpy of sion is easily and reliably obtained by DSC. 

The highest correlation coefficient (0.98) is for enthalpy of vaporization and standard enthalpy of atomization. For metals, the latter one is actually the standard sublimation enthalpy and is given, according to Hess law, by the sum of the standard enthalpies of fusion and vaporization. The enthalpies of vaporization are those at the boiling temperature and, therefore, lower than the standard ones. Other very close correlations are those between the boiling... 

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 ... 

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. 

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 types of values reported in the database standard enthalpies of formation at 298.15 K and 0 K, bond dissociation energies or enthalpies (D) at any temperature, standard enthalpy of phase transition—fusion, vaporization, or sublimation—at 298.15 K, standard entropy at 298.15 K, standard heat capacity at 298.15 K, standard enthalpy differences between T and 298.15 K, proton affinity, ionization energy, appearance energy, and electron affinity. The absence of a check mark indicates that the data are not provided. However, that does not necessarily mean that they cannot be calculated from other quantities tabulated in the database. 

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. 

Those heal effects can be easily calculated when the enthalpies of formation and the enthalpy-temperature relations are available for the substances considered. Usually, the heat of reaction is defined as the heat evolved by the process, and it is equal to the enthalpy change but opposite in sign, while heats of fusion or vaporization always refer to ihe heat adsorbed, and for heals of solution the usage varies. In order to avoid any confusion, it is recommended to express heat effects of chemical process by reporting the enthalpy change. AH. 

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