Phase diagrams, reacting systems

Phase diagrams can be used to predict the reactions between refractories and various soHd, Hquid, and gaseous reactants. These diagrams are derived from phase equiHbria of relatively simple pure compounds. Real systems, however, are highly complex and may contain a large number of minor impurities that significantly affect equiHbria. Moreover, equiHbrium between the reacting phases in real refractory systems may not be reached in actual service conditions. In fact, the successful performance of a refractory may rely on the existence of nonequilibrium conditions, eg, environment (15—19). 

The cyclobutene-butadiene interconversion can serve as an example of the reasoning employed in construction of an orbital correlation diagram. For this reaction, the four n orbitals of butadiene are converted smoothly into the two n and two a orbitals of the ground state of cyclobutene. The analysis is done as shown in Fig. 11.3. The n orbitals of butadiene are ip2, 3, and ij/. For cyclobutene, the four orbitals are a, iz, a, and n. Each of the orbitals is classified with respect to the symmetiy elements that are maintained in the course of the transformation. The relevant symmetry features depend on the structure of the reacting system. The most common elements of symmetiy to be considered are planes of symmetiy and rotation axes. An orbital is classified as symmetric (5) if it is unchanged by reflection in a plane of symmetiy or by rotation about an axis of symmetiy. If the orbital changes sign (phase) at each lobe as a result of the symmetry operation, it is called antisymmetric (A). Proper MOs must be either symmetric or antisymmetric. If an orbital is not sufficiently symmetric to be either S or A, it must be adapted by eombination with other orbitals to meet this requirement. 

Up to this point, we have considered only one solid at a time. However, when two (2) or more solids are present, they can form quite complicated systems which depend upon the nature of each of the solids involved. To differentiate and to be able to determine the differences between the phases that may arise when two compounds are present (or are made to react together), we use what are termed "phase-diagrams to illustrate the nature of the interactions between two solid phase compositions. You will note that some of this material weis presented earlier in Chapter 1. It is presented here again to further emphasize the importance of phase diagrams. 

A phase diagram describes how a system reacts to changing conditions of pressure and temperature and consists of a field in which only one phase is stable, separated by boundary curves along which a combination of phases coexist in equilibrium. 

Halogen-donor Ligands. Xep2 reacts with excess VF5 at 90 °C to give XeF2,VF5 as colourless transparent crystals, m.p. 22—28°C. The i.r. spectrum of the vapour indicates that the compound is completely dissociated in this phase. The phase diagram for the HF—VF system has been presented. 

Fig. 8.2. Simulation of the isobaric phase diagram for the reacting system methylphosphinic acid + butanol -+ methylphosphinic acid butyl ester plus water based on COSMO-RS predictions [C28].

Valuable information on the corrosion process is provided by phase diagrams if they are available for the given system. They show whether the substances in question actually react mutually producing a melt, what is the respective saturated concentration (equilibrium melt composition), and if any new products are formed. However, phase diagrams are often not available for multicomponent systems the other characteristics determining the corrosion rate are usually also unknown. The resistance of refractories to corrosion is therefore in practice determined by tests providing useful Complex information which, however, holds exactly only for the test conditions. 

With the use of phase diagrams, we are also able to control the temperature and the viscosity at which phase separation occurs. The final morphologies of the three systems based on the same dicyanate monomer and modified with NFBN, ATBN, and PES are quite different and have different interfaces. When the additive can react with the monomer before network formation, a two-level structure is observed a primary structure (dispersed particles), and a substructure inside the dispersed particles. The complex morphology obtained in this case gives the best toughening effect. 

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