Heats of Fusion and Vaporization

The amount of heat required to melt a substance in the soUd state is called its latent heat of fusion, and the amount of heat required to vaporize a Uquid is called the substance s latent heat of vaporization. Once again, water has relatively high latent heats of fusion (333 J/g at 0°C) and vaporization (2257 J/g at 100°C), which are especially high for such a small molecule. This is why it takes a lot of heat to boil water, and why water boils more easily in a pot with the lid on than in an open pot. 

Remember the saying a watched pot never boils Well, this is why without the lid on, the heat just escapes rather than staying to raise the water s temperature. This is also why steam can scald a person. All that heat that had to be absorbed to produce the steam is still present in the steam in the form of kinetic energy. If that steam touches your skin, all that extra energy is transferred back to your skin as heat. 

Water s high heat of vaporization is the principle behind the cooling effects of perspiring. For water on our skin to evaporate, the water has to absorb heat from the skin, which in doing so, cools down the skin. However, once again, waters high heats of fusion and vaporization are anomalous. 

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TABLE 2-190 Heats of Fusion and Vaporization of the Elements and Inorganic Compounds ... 

Pitzer, K.S., Guttman, L., Westrum, Jr., E.F. (1946) The heat capacity, heats of fusion and vaporization, vapor pressure, entropy, vibration frequencies and barrier to internal rotation of styrene. J. Am. Chem. Soc. 68, 2209-2212. 

Making and Using Graphs The heats required to melt or to vaporize a mole (a specific amount of matter) of matter are known as the molar heat of fusion (Hf) and the molar heat of vaporization (Hy), respectively. These heats are unique properties of each element. You will investigate if the molar heats of fusion and vaporization for the period 2 and 3 elements behave in a periodic fashion. 

Gross, K. D. Williamson, G. Waddington and H. M. Huffman Spiropentane Heat Capacity, Heats of Fusion and Vaporization, Vapor Pressure, Entropy and Thermodynamic Functions. J. Amer. chem. Soc. 72, 4664 (1950). 

Jones, W.M. and Giauque, W.F. The entropy of nitromethane. Heat capacity of solid and liquid. Vapor pressure, heats of fusion and vaporization, / Am. Chem. Soc., 69(5) 983-987, 1947. 

Thermochemical Properties mostly covers heats of formation, Gibbs free energy, entropies, and heat capacities. For many compounds, heats of fusion and vaporization are included. 

Flame temperatures can be measured directly, using special high-temperature optical methods. They can also be calculated (estimated) using heat of reaction data and thermochemical values for heat of fusion and vaporization, heat capacity, and transition temperatures. Calculated values tend to be higher than the actual experimental results, due to heat loss to the surroundings as well as the endothermic decomposition of some of the reaction products. Details regarding these calculations, with several examples, have been published [5]. 

Scottish chemist, physicist, and physician. Professor of chemistry at Glasgow. He clearly characterized carbon dioxide ( fixed air ) as the gas which makes caustic alkalies mild, and distinguished between magnesia and lime. He discovered the latent heats of fusion and vaporization, measured the specific heats of many substances, and invented an ice calorimeter. 

Henri-Victor Regnault, 1810-1878. French chemist and physicist He made precise measurements of specific heats and heats of fusion and vaporization, and of the velocity of sound, and contributed to the theory of organic radicals. Among his students may be mentioned Cannizzaro, Kekule, and Mendeleev. 

Physical properties of the solvent are used to describe polarity scales. These include both bulk properties, such as dielectric constant (relative permittivity), refractive index, latent heat of fusion, and vaporization, and molecular properties, such as dipole moment. A second set of polarity assessments has used measures of the chemical interactions between solvents and convenient reference solutes (see table 3.2). Polarity is a subjective phenomenon. (To a synthetic organic chemist, dichloromethane may be a polar solvent, whereas to an inorganic chemist, who is used to water, liquid ammonia, and concentrated sulfuric acid, dichloromethane has low polarity.)... 

The heats of fusion and vaporization.—From the lowering of the f.p. of cymene and toluene by the soln. of liquid hydrogen chloride, E. Beckmann and P. Wantig14 calculate the heat of fusion of hydrogen chloride as 10 3 cals, per gram of hydrogen bromide, 7 44 cals. and of hydrogen iodide, 413 cals. D. McIntosh and B. D. Steele calculate from Clapeyron and Clausius equation d log pjdT—XjRT 2, for the mol. ht. 

Oog and Hunt8711 found the heat of combustion of ethylene oxide to be 312 55 0.20 koal./mole. The heats of fusion and vaporization, determined with great precision by Giauque and Gordon,8 8 are 1236 and 6101 kcal./moie respectively. An approximate value for the strain energy of ethylene oxide has been computed by Nelson and Jeesup.mv The experimental heat of formation, derived by combustion calari-> metry, was subtracted from the calculated total bond energy, obtained... 

AsFb (c), AsFb (liq.). Ruff, Menzel, and Plant1 reported values for the heats of fusion and vaporization. 

The adiabatic flash pressure Pf, maintained slightly below the triple-point pressure, causes liquid to spontaneously vaporize and solidify. The ratio of solid to vapor is determined by the heats of fusion and vaporization for carbon dioxide about 1.7 moles of solid are formed for each mole vaporized. The solid, more dense than the liquid, falls through a liquid head and forms a loosely packed crystal bed at the bottom. The liquid head is about 10-12 feet, and increases the hydrostatic pressure on the solid to melter pressure Pm. The crystal bed depth is about two... 

EXAMPLE 3 The heat of sublimation of a substance (gas -> solid and vice verse) is the sum of the heats of fusion and vaporization of that substance at the same temperature. 

Valentine, R.H., Brodale, G.E., Giauque, W.F. (1962) Trifluoromethane Entropy, low temperature heat capacity, heats of fusion and vaporization, and vapor pressure. J. Phys. Chem. 66, 392-395. 

Aston, J.G., Sagenkahn, M.L., Szasz, G.J., Moessen, G.W., Zuhr, H.F. (1944) The heat capacity and entropy, heats of fusion and vaporization and the vapor pressure of trimethylamine. The entropy from spectroscopic and molecular data. J. Am. Chem. Soc. 66, 1171-1177. 

After freezing, the time to sublimate the solvent is given by the drying expressions in Tables 8.3 and 8.4, where the enthalpy of vaporization for drying is replaced by the enthalpy of sublimation. The enthalpy of sublimation is often equal to the sum of the heats of fusion and vaporization [16]. The enthalpy of sublimatian is also substituted for the enthalpy of vaporization in the Clausius Clapeyron equation (8.9) required for the calculation of the solvent partial pressure. The same rate determining steps of boundaiy layer mass transfer and heat transfer as well as pore diffusion and porous heat conduction are applicable in sublimation. 

Using the heats of fusion and vaporization for water, calculate the change in enthalpy for the sublimation of water ... 

In Chapter 8, along with tables of measured thermophysical data, we saw some fairly simple techniques for estimating these values when experimental results are not available. Among these techniques were Kopp s Rule for the heat capacity of both liquids and solids, and Trouton s ratio for latent heats of fusion and vaporization, along with Kistiakowski s temperature correction for the latter. 

Define both formally (in terms of internal energies and enthalpies) and in words a high school senior could understand the variables Cy(T) (heat capacity at constant volume), Cp(T) (heat capacity at constant pressure), AH (heat of fusion or heat of melting), A//v (heat of vaporization), standard heats of fusion and vaporization, and AHj (heat of solution or heat of mixing). 

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