Sublimation description

Phase changes, which convert a substance from one phase to another, have characteristic thermodynamic properties Any change from a more constrained phase to a less constrained phase increases both the enthalpy and the entropy of the substance. Recall from our description of phase changes in Chapter 11 that enthalpy increases because energy must be provided to overcome the intermolecular forces that hold the molecules in the more constrained phase. Entropy increases because the molecules are more dispersed in the less constrained phase. Thus, when a solid melts or sublimes or a liquid vaporizes, both A H and A S are positive. Figure 14-18 summarizes these features. 

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 Clapeyron equation, Equation (5.1), yields a quantitative description of a phase boundary on a phase diagram. Equation (5.1) works quite well for the liquid-solid phase boundary, but if the equilibrium is boiling or sublimation - both of which involve a gaseous phase - then the Clapeyron equation is a poor predictor. 

Equally precise and meticulous as an experimentalist, he devoted time during his days at Pittsburgh to write comprehensive articles on such practical techniques as crystallization, vacuum distillation, and sublimation, which were published in the Weissberger Techniques of Organic Chemistry series. His preoccupation with careful experimental techniques and then-accurate recording in the literature remained with him always. He abhorred vague descriptions of procedures, speculative interpretations not based on... 

Sublimation. Accdu to definition ziven in Perry (Ref 13, p660), it covers the physical changes encountered by a substance in passing from a solid phase to a gas and back to a solid phase. It is characterized by the absence of the liquid phase and is used for purification of volatile substances like iodine, camphor, etc. Detailed description of the process is given in Refs 12, 13, 15 18). 

Here is Albertus description of tuchia which was Usually an impure sublimate of zinc oxide mixed often with more or less of other metallic oxides, dust from the flues or domes of bronze or brass furnaces. 

The ninth book is devoted to arts connected with chemistry, as distillation and sublimation, the arts of coining and the manufacture of jewelry, mirrors of metal, etc. In this book appears a description of methods for extracting all gold or silver from the waste of the mines or sweepings of the mint. It is of especial interest because it describes, apparently for the first time, recovery of silver by the method of amalgamation, a process first apparently utilized on a large scale by the Spaniards in America later in that century. He says ... 

In the early alchemical texts (and in particular the Rosarium philosophorum ), this picture of soul alchemy is developed in parallel with descriptions of what was a sore physical process. Thus it was that the soul development of the alchemist went hand in hand with an actual physical operation, and this operation, the details of which have not been wholly lost, involved colour and form changes within a sealed flask, isomorphic to the inner changes of soul alchemy, described by these bird symbols. Thus we had a physical process which involved a blackening, a whitening a rapid iridescence of colours, a circular distillation stage, and a final sublimation. Part of the task of modern alchemy must be to rediscover this physical process, and explore its further ramifications. 

Another troublesome borderline area is that between ionic solids and three-dimensional polymers. The distinction cannot be made from the structure alone. Electrical conductivity in the molten state does not, as already mentioned, necessarily demonstrate the presence of ions in the solid state and such a test is inapplicable where, as often happens, the substance sublimes or decomposes before melting. There can rarely be any objective means of assigning a compound to one category or the other. We are often persuaded towards one description on aesthetic grounds. For example, the structure of sodium chloride cannot easily be rendered in terms of localised, electron-pair bonds (but this is true also of many unequivocally covalent compounds). Its structure is eminently plausible for an array of cations and anions, however. 

Gradient sublimation has been used for many years as a means to purify materials and many variations exist and have been proven effective for organic purification. A detailed description of the vapor phase transport method applied to organic materials was given by Ch. Kloc,... 

At p. 9 of Harris, there is a good description of the process of making 2 sublimate, which might be useful. 

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. 

The lattice energy is a crucial parameter to determine as the calculated value can be compared with the experimental sublimation enthalpy (Eq. 8) (13), as a check of the description of intermolecular interactions between the molecules by the defined potential function. 

Continuous freeze drying From the description of batch freeze drying, it can be seen that the utihty requirements vary considerably. During sublimation drying the requirements are 2 to 2.5 times the average requirement. To overcome this peak load and to meet the market request for high unit capacities, continuous freeze dryer designs have been developed. The special features are twofold ... 

One complication with this description is that a species can be present in a liquid mixture, though at the temperature and pressure of the mixture the substance would be a vapor or a solid as a pure component. This is especially troublesome if the compound is below its melting point, so that it is the solid sublimation pressure rather than the vapor pressure that is known, or if the compound is above its critical temperature, so that the vapor pressure is undefined. In the first case one frequently ignores the phase change and extrapolates the liquid vapor pressure from higher temperatures down to the temperature of interest using, for example, the Antoine equation, eqn. (2.3.11). For supercritical components it is best to use an EOS and compute the fugacity of a species in a mixture, as described in Section 2.5. 

But we must leave these more general descriptions of our Secret Fire, and lead up to the operation or practice, and it will therefore be well to note the different functions that the fire performs in the Work. It has not idly been said that the gold must be tried seven times in the fire, nor are we to imagine that the Solutions, Dissolutions, Putrefactions, Calcinations, Sublimations, Decensions and Coagulations mentioned, are but reiterations of an identical process. Exact details of them are, as we know only too well, not... 

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