Theory of sublimation

The processes of distillation and sublimation are closely related. There are three ways in which the vaporisation of a thermally stable substance may take place on heating — 

The condensation of the vapour of a stable substance may, likewise, take place in three ways — 

The term distillation is applied to vaporisation and subsequent condensation according to (i) it should also be applied to (ii) since it is really the liquid which is converted into vapour and is first formed by condensation. Strictly speaking, the term sublimation should be applied to changes according to (iii). However, in practice, a substance when heated may first melt and then boil, but on cooling it may pass directly from the vapour to the solid the process is then also called sublimation. Indeed the mode of vaporisation, whether directly from solid to vapour or through the intermediate formation of a liquid, is of secondary importance it is the direct conversion of vapour to solid which is really the outstanding feature of sublimation in the laboratory. 

To understand the conditions which control sublimation, it is necessary to study the solid - liquid - vapour equilibria. In Fig. 1,19, 1 (compare Fig. 1,10, 1) the curve T IF is the vapour pressure curve of the liquid (i.e., it represents the conditions of equilibrium, temperature and pressure, for a system of liquid and vapour), and TS is the vapour pressure curve of the solid (i.e., the conditions under which the vapour and solid are in equili-hrium). The two curves intersect at T at this point, known as the triple point, solid, liquid and vapour coexist. The curve TV represents the 

The normal melting point of a substance is the temperature at which solid and hquid are in equilibrium at atmospheric pressure. At the triple point, the pressure is the equilibrium vapour pressure of the system (solid liquid - vapour) and the temperature differs from the melting point. The difference is, however, quite small—usually only a fraction of a degree—since the line TV departs only slightly from the vertical within reasonable ranges of pressure. 

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The theory of sublimation, t.e. the direct conversion from the vapour to the sohd state without the intermediate formation of the liquid state, has been discussed in Section 1,19. The number of compounds which can be purified by sublimation under normal pressure is comparatively small (these include naphthalene, anthracene, benzoic acid, hexachloroethane, camphor, and the quinones). The process does, in general, yield products of high purity, but considerable loss of product may occur. 

The theory of sublimation will become clear by reference to Figure 2.7, which is the vapour pressure-temperature diagram for camphor. The vapour pressure of a solid increases with temperature in much the same way that the vapour pressure of a liquid does. 

After cooling, the solution was diluted with 1.5 liters of water and subjected to three extractions with ether. The amine was extracted from the ethereal solution with 2 N HCI and liberated therefrom by the addition of solid NaOH (while cooling). The alkaline solution was extracted with ether and the ethereal solution was dried with soiid NaOH. Distii-lation resulted in 10.6 grams (70% of the theory) of 1-aminoadamantane which, after sublimation, melted at 180° to 192°C (seal capillary). It is converted to the hydrochloride. 

The theory of the sublimation process has been discussed in detail elsewhere41 the following describes the practical aspects of this technique which is applicable down to a few milligrams of material. 

A great number of studies related to thermochemical properties of QDO and PDO derivatives have been recently described by Ribeiro da Silva et al. [98-103]. These studies, which have involved experimental and theoretical determinations, have reported standard molar enthalpies of formation in the gaseous state, enthalpies of combustion of the crystalline solids, enthalpies of sublimation, and molar (N - O) bond dissociation enthalpies. Table 5 shows the most relevant determined parameters. These researchers have employed, with excellent results, calculations based in density functional theory in order to estimate gas-phase enthalpies of formation and first and second N - O dissociation enthalpies [103]. 

My Becher s furnace answers much better than I expected, and I have succeeded in one important process, and am now upon another. No one, who has not tried, can conceive the extreme difficulty and care required as to practical details. I am sure the proper way is to begin young, have a laboratory, and learn all the ways of subliming, precipitating, revivifying, c c c. I know the theory now well enough, but the practical details bother me sorely with the many interruptions I have. Still, I might succeed at any time. 

The clear liquid yielded saltpeter upon drying. The juxtaposition of these two methods—distillation vs. precipitation —relayed Geoffroy s hope that the laws of rapports could mediate between the different chemical procedures to shape a uniform terrain of chemical theory. By going through a variety of processes that produced corrosive sublimates, Geoffrey illustrated that the same order of rapports could explain all the varieties of chemical operations in a consistent manner. The theory of the process was the same, no matter what the variations in practice were. Through the example of corrosive sublimates, Geoffroy thus presented concisely and powerfully the descriptive, predictive, and explanatory functions of his table of rapports. 

Berthollet s theory of causticity differed substantially from Macquer s, however. The importance of causticity for chemistry lay in the power bodies exhibited in decomposing one another by simple contact. As Bayen s investigations had shown, mercury was absolutely inactive in its natural state, but it became the most poisonous caustic in the form of corrosive sublimate. Its action was much more moderate in mild mercury... 

Some heats of sublimation of organic compounds calculated from the Clapeyron-Clausius formula ( 7.VIIIL) were in disagreement with Trouton s rule as calculated ( 14.VIIIL) for 4=4—4 and are of doubtful accuracy. They increased in the order ortho-, meta-, and para- compounds for dinitro-benzenes and nitroanilines, but the theoretical deductions made would be more interesting if greater confidence could be attached to the experimental results. The calculation of latent heats of sublimation from lattice theory is mentioned in 15.IXN. 

This equation permits the calculation of the lattice energy if we know F, the electron affinity of the halogen atom, S, the heat of sublimation of the alkali metal, /, the ionization potential of the metal and Z), the dissociation energy of the molecular halogen. These quantities are known, but to different orders of accuracy, and furthermore, the values should all refer to the same standard temperature, either absolute zero or room temperature, a condition which is not always fulfilled. However, the agreement between the calculated and observed values is sufficiently good to indicate that the theory developed for the lattice energy on the basis of ionic interaction is basically correct. 

The anthraquinone so obtained is purified by sublimation. Although various modifications of this process ltave been introduced, the basic principles remain the same.1"1 The yield of o-benzoyl benzoic acid is about 95 per cent of theory or 145 per cent of the weight of phthalic anhydride used. The overall yield of sublimed anthraquinone on the phthalic anhydride used is more titan 120 per cent by weight, or 85 per cent of theory. 

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