Melting Point and Heat Capacity

In addition to size-dependent melting points, size-dependent specific heats have been observed. At high temperatures, there is an increase in the specific 

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The polymorphs of a substance can possess considerable different chemical and physical properties. Their melting points and heat capacities will be different. Their x-ray diffraction patterns will depend on the arrangement of molecules in the crystal lattice. In addition, vibrational spectra of the different polymorphic forms of a material will be different. These differences may be minor, but many times there are extensive differences, which can be used to identify the form and to understand its crystalline structure. 

First some illustrations are given to support the concept of thin gas-filled layers between crystalline domains and then several examples are given to show that the gaseous fraction which Eyring assumed to comprise a liquid metal agrees well with certain physical measurements which others have found by experiment. These examples include heats of fusion, liquid coordination numbers, coefficients of thermal expansion of liquids, the effect of pressure on the melting point, and heat capacities of liquids. 

Schmidt M, Kusche R, KronmuUer W, von Issendorff B, Haberland H (1997) Experimental determination of the melting point and heat capacity for a free cluster of 139 sodium atoms. Phys Rev Lett 79 99-102... 

Thermodynamic and physical properties of water vapor, liquid water, and ice I are given in Tables 3—5. The extremely high heat of vaporization, relatively low heat of fusion, and the unusual values of the other thermodynamic properties, including melting point, boiling point, and heat capacity, can be explained by the presence of hydrogen bonding (2,7). 

Kubaschewski and Evans (5). Between 300 and 582 K, which is the melting point, the heat capacity is obtained by linear interpolation. 

HEAT CAPACITY FROM 80 TO 300 K, MELTING POINT AND HEAT OF FUSION OF NITROETHANE. 

The Group 4—6 carbides are thermodynamically very stable, exhibiting high heats of formation, great hardness, elevated melting points, and resistance to hydrolysis by weak acids. At the same time, these compounds have values of electrical conductivity. Hall coefficients, magnetic susceptibiUty, and heat capacity in the range of metals (7). 

The physical properties of a flaimnable solid, such as hardness, texture, waxiness, particle size, melting point, plastic flow, tiiennal conductivity, and heat capacity, impart a wide range of cliaracteristics to tiie flanmiability of solids. A solid ignites by first melting and tiien producing sufficient vapor, which in turn mixes witii air to fonn a flaiimiable composition. 

If a solid is heated at a constant rate and its temperature monitored during the process, the melting curve as illustrated in Fig. 4.1 is obtained. Below the melting point, the added heat merely increases the temperature of the material in a manner defined by the heat capacity of the solid. At the melting point, all heat introduced into the system is used to convert the solid phase into the liquid phase, and therefore no increase in system temperature can take place as long as solid and liquid remain in equilibrium with each other. At the equilibrium condition, the system effectively exhibits an infinite heat capacity. Once all solid is converted to liquid, the temperature of the system again increases, but now in a manner determined by the heat capacity of the liquid phase. 

To calculate for this transition, it is necessary to have heat capacity data for both glassy and crystaUine glycerol from near 0 K to the melting point and the heat of fusion of both glass and crystal. Such data [7] lead to a ASm for Equation (11.7) of 19.2 J K mol. Thus, glassy glycerol cannot be assigned zero entropy at OK rather, it possesses a residual entropy of 19.2 J moP. ... 

Draw a molar heating curve for sodium similar to that shown for water in Figure 10.10. Begin with solid sodium at its melting point, and raise the temperature to 1000°C. The necessary data are mp = 97.8°C, bp = 883°C, AHvap = 89.6 kj/mol, and AHfusion = 2.64kJ/mol. Assume that the molar heat capacity is 28.2 J/(K mol) for both liquid and vapor phases and does not change with temperature. 

The low temperature heat capacity, 14.0-315 K was measured by Getting (7). Janz et al. (8) measured the heat content by drop calorimetry in the temperature range 630-1250 K, and gave enthalpy and heat capacity equations based on their measurements. The above information was used in a Shomate analysis in order to smooth the enthalpy and calculate heat capacity above 298.15 K. The values from the low and high temperature sources join smoothly at 298.15 K. The heat capacity was graphically extrapolated above the melting point. The entropy at 14.0 K was calculated from the extrapolated low temperature... 

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