Enthalpy of sublimation

To a first approximation, the enthalpy of sublimation ISHs at constant temperature is ... 

The approximate calculation of the surface energies of metals as a function of crystal structure described earlier uses the enthalpy of sublimation, s, and the co-ordination number to calculate the energy as a function of the atomic concentration on the surface. The atomic areas of the principal configurations are as follows ... 

E8,6 Use the following vapor pressure data for solid palladium metal as a function of temperature,7 to calculate ASUb//m. the mean enthalpy of sublimation of palladium. 

Sublimation is the direct conversion of a solid into its vapor. Frost disappears on a cold, dry morning as the ice sublimes directly into water vapor. Solid carbon dioxide also sublimes, which is why it is called dry ice. Each winter on Mars, solid carbon dioxide is deposited as polar frost, which sublimes when the feeble summer arrives (Fig. 6.24). The enthalpy of sublimation, AHsub, is the molar enthalpy change when a solid sublimes ... 

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. 

Use the information in Tables 6.3, 6.7, and 6.8 to estimate the enthalpy of formation of each of the following compounds in the liquid state. The standard enthalpy of sublimation of carbon is +717 kJ-moF 1. (a) H20 (b) methanol, CH,OH (c) benzene, C6H6 (without resonance) (d) benzene, C6H6 (with resonance). 

Robert Curl, Richard Smalley, and Harold Kroto were awarded the Nobel prize in chemistry in 1996 for the discovery of the soccer-ball-shaped molecule C60. This fundamental molecule was the first of a new series of molecular allotropes of carbon. The enthalpy of combustion of C60 is —25 937 kj-mol, and its enthalpy of sublimation is +233 kj-mol There are 90 bonds in C60, of which 60 are single bonds and 30 are double bonds. Like benzene, C60 has... 

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 ). 

It has generally been assumed that the most important consideration in the surface enrichment of one metal in preference to another in a supported bimetallic cluster is based on differences in the enthalpies of sublimation of the metals which comprise the cluster. In most cases, the surface composition is enriched in the metal having the lower enthalpy of sublimation (1 ). 

The surface-catalyst composition data for the silica-supported Ru-Rh cuid Ru-Ir catalyst are shown in Figure 1. A similcir plot for the series of silica-supported Pt-Ru bimetallic catalysts taken from ref. P) is included for comparison purposes. Enthalpies of sublimation for Pt, Ru, Rh and Ir are 552, 627, 543, and 648 KJ/mole. Differences in enthalpies of sublimation (a<75 KJ/mole) between Pt and Ru cind between Rh and Ru are virtually identical, with Pt euid Rh having the lower enthalpies of sublimation. For this reason surface enrichment in Pt for the case of the Pt-Ru/Si02 bimetallic clusters cannot be attributed solely to the lower heat of sublimation of Pt. Other possibilities must also be considered. 

The lattice energy of a molecular compound corresponds to the energy of sublimation at 0 K. This energy cannot be measured directly, but it is equal to the enthalpy of sublimation at a temperature T plus the thermal energy needed to warm the sample from 0 K to this temperature, minus RT. RT is the amount of energy required to expand one mole of a gas at a temperature T to an infinitely small pressure. These amounts of energy, in principle, can be measured and therefore the lattice energy can be determined experimentally in this case. However, the measurement is not simple and is subject to various uncertainties. 

De Kruif, C. G. (1980) Enthalpies of sublimation and vapour pressures of 11 polycyclic hydrocarbons. J. Chem. Thermodyn. 12, 243-248. 

Rordorf, B. F. (1985b) Thermodynamic properties of polychlorinated compounds the vapor pressures and enthalpies of sublimation of ten dibenzo-p-dioxins. Thermochimica Acta 85, 435 438. 

Colomina, M., Jimenez, P, Turrion, C. (1982) Vapor pressures and enthalpies of sublimation of naphthalene and benzoic acid. J. Chem. Thermodynamics 14, 779-784. 

Li, X.-W., Shibata, E., Kasai, E., Nakamura, T. (2004) Vapor pressures and enthalpies of sublimation of 17 polychlorinated dibenzo-p-dioxins, and five polychlorinated dibenzofurans. Environ. Toxicol. Chem. 23, 348-354. 

Malaspina, L., Gigli, R., Bardi, G. (1973) Microcalorimetric determination of the enthalpy of sublimation of benzoic acid and anthracene. J. Chem. Phys. 59, 387-394. 

Murray, J.M., Pottie, R.F., Pupp, C. (1974) The vapor pressures and enthalpies of sublimation of five polycyclic aromatic hydrocarbons. Can. J. Chem. 52, 557-563. 

Oja, V., Suuberg, E.M. (1998) Vapor pressures and enthalpies of sublimation of polycyclic aromatic hydrocarbons and their derivatives. J. Chem. Eng. Data 43, 486 -92. 

Taylor, J.W., Crookes, R.J. (1976) Vapour pressure and enthalpy of sublimation of l,3,5,7-tetranitro-l,3,5,7-tetra-azacyclo-octane. J. Chem. Soc. Farad. Trans. 72, 723-729. 

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