Melting, solid reactant

Melting. Loss of crystallinity of a solid reactant through melting, or eutectic formation, or dissolution in a product often results in an enhanced rate of decomposition as a consequence of the relaxation of the bonding forces responsible for lattice stabilization. The appearance of a... 

MW irradiation conditions [80]. More recently, however, Varma and Kocevar s group have shown that a solvent-free and catalyst-free reaction of hydrazines with carbonyl compounds is possible upon MW irradiation (Scheme 6.24) [81]. Interestingly, the general reaction proceeds smoothly even for solid reactants and is completed below the melting points of the two reactants possibly via the formation of a eutectic. The reactions have been conducted in household MW oven and the control experiments are conducted concurrently in separate open beakers the reactions can be essentially followed by visual observation when a melt is obtained [82],... 

Synthesis of CHDM/BA and CHDM/PDA Polyester Resins Described in Tables III and IV. CHDM, brassylic acid (or dodecandioic acid) and "Fascat 4100 in a mol ratio of 2/1/0.005 and 5 wt % of xylene were placed in a three-necked, 250-mL round-bottomed flask fitted with line, a thermometer, an Ace temperature controller, a heating mantle, a mechanical stirrer, a steam-heated Allihn partial condenser with a xylene-filled Barrett moisture test receiver below the Allihn condenser and a total condenser above it. The mixture was heated to about 140 without stirring to melt the reactants and stirring was started. Heating was continued to 205 C. A slow N sweep was maintained throughout the reaction. About 95 to 100% of the theoretical water was collected in the Barrett receiver. Upon cooling the reaction mixtures partially solidified to white waxy solids which liquified when heated to about 40 C. Acid numbers were < 1 mg of KOH/g of resin. 

Since both Li and SFg are solid at room temperature, external heat is needed to melt these reactants in order to initiate their exothermic reaction. When the reaction between Li and SFg occurs in a closed chamber, the reaction products of LiF and Li2S are obtained in liquid form due to the heat generated. 

Both solid-solid and solid-gas types of reactions lead from solid reactants to a solid product without the use of solvents. Solvent-less processes, however, are not necessarily solid-state processes. Indeed, it has been argued [8d,e] that many solid-state syntheses cannot be regarded as bona fide solid-solid reactions because they occur with the intermediary of a liquid phase, such as a eutectic phase or a melt, or may require destruction of the crystals prior to reaction. This latter situation is often observed, for instance, in the case of reactions activated by co-grinding, since the heat generated in the course of the mechanochemical process can induce local melting at the interface between the different crystals, or when kneading, i.e. grinding in the presence of small amounts of solvent, takes place (vide infra). 

Although the concentration of fluorine is the most important quantity in the control of the reaction rate and must be maintained within certain limits, in practice the stoichiometry, the molecular fluorine to substrate H-atom molar ratio, is used to determine the reaction parameters leading to a successful and efficient perfluorination. AF is most successful when sublimable solids are introduced into the hydrocarbon evaporator unit of the aerosol fluorinator as solutions by a syringe pump. This now common procedure emphasizes the individual molecule s isolation as it is fluorinated using AF. No intermolecular reactions between solute and solvent have been observed Choice of the solvent is important as it must not boil at a temperature below the melting point of the solute in order to prevent solid deposition in the tubes feeding the evaporator. It must also fluorinate to a material easily separable from the solid reactant after perfluorination. In most cases it has been found that aliphatic hydrochlorocarbons are excellent choices, but that carbon tetrachloride and chloroform and other radical-scavenging solvents are not (sec ref 6). 

The second model (Fig. 20c) assumes that upon melting of reactant A, a layer of initial product forms on the solid reactant surface. The reaction proceeds by diffusion of reactant B through this layer, whose thickness is assumed to remain constant during the reaction (Aleksandrov et al., 1987 Aleksandrov and Korchagin, 1988). The final product crystallizes (C) in the volume of the melt after saturation. Based on this model, Kanury (1992) has developed a kinetic expression for the diffusion-controlled rate. Using this rate equation, an analytical expression for the combustion wave velocity has been reported (Cao and Varma, 1994)... 

In this case, the combustion front propagates due to the interaction between the solid reactant and the gas infiltrated to the reaction zone from the surrounding atmosphere (see Section IV,D). For metal-nitrogen reactions, this type of combustion was observed for low initial sample density (-0.2) in the Ta-N2 system, where the metal does not melt in the combustion wave (Pityulin et al, 1979 Ku-... 

Reactions in melts, vitreous materials, polymers, etc. can justifiably be analyzed by equations based on a concentration dependence of rate. Some reactions proceeding in vitreous reactant phases have been shown to conform to second or even third-order rate equations. Progressive melting of a solid reactant during decomposition results in acceleratory behaviour [52,71-73] and comprehensive melting before dehydration was observed to result in an approximately constant rate of water evolution [74,75]. 

Microscopy is the most appropriate technique for studying the kinetics of nucleation. The shapes, sizes, textures and distributions of nuclei can be determined and the kinetics of nucleation can be distinguished fi om the kinetics of growth. Details of the intranuclear material, which is often porous with small crystallites separated by fine channels that provide routes for escape of product gas, may be discemable. Changes in particle-size, topochemical relationships and the possibility of melting of the solid reactant can also be recognized. 

As can be seen from Table 8.3, regardless of the difference in temperatures, the decomposition rates for AgN03 and Cd(N03)2 in the solid and molten states appear to be about the same. (With a temperature difference of 100 K, a rate increase by 2-3 orders of magnitude could be expected.) For some reason the decompositions of the solid reactants slow down after their melting. The magnitude of the molar enthalpy (parameter E) rises by about 20 kJ mol ... 

The molar enthalpies of decomposition of both molten nitrates (Table 16.39) appear to be 20 kJ moP higher compared to the solid nitrates, which is in full agreement with the CDV mechanism involving partial transfer of the condensation energy to the reactant in the zone of the reaction between the solid reactant and solid product. If there is no such zone (in particular, in the case of reactant melting), the enthalpy of the decomposition (Ari7y/i/) should correspond to the enthalpy of the straight vaporization process calculated from the thermochemical data. The experi-... 

In fusion reactions, pulverized mixtures are heated and usually the reactant in greater proportion melts completely and the binary reaction occurs then in the interface between a solid and a melt. Such interfacial reactions also occur between an excess of the solid reactant and minimal amounts of the fusible partner to give rise to a so-called ciypto fusion reaction. 2 An example is found in the behavior of lithium nitrate (m.p. circa 300 C) toward soda-lime glass. There is an exchange of sodium atoms of the sodium silicate for lithium atoms and the glass becomes dull. Since even slight quantities of lithium can bring about this effect, the latter may serve as a test for lithium. 

As a first approach, a thermochemical analysis of the complex reaction system was carried out. This means that transport of reactants in the fluid phase was neglected, but a fast reaction kinetics was assumed. The system under study comprised the gas atmosphere, the boron oxide melt, solid and liquid GaAs, the crucible and the graphite heaters. Figure 9.24 shows a scheme of the system. 

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