Stoichiometric liquid component

Activities, stoichiometric liquid component, Group III-V materials, 286-88... 

In equation 33, the superscript I refers to the use of method I, a T) is the activity of component i in the stoichiometric liquid (si) at the temperature of interest, AHj is the molar enthalpy of fusion of the compound ij, and ACp[ij] is the difference between the molar heat capacities of the stoichiometric liquid and the compound ij. This representation requires values of the Gibbs energy of mixing and heat capacity for the stoichiometric liquid mixture as a function of temperature in a range for which the mixture is not stable and thus generally not observable. When equation 33 is combined with equations 23 and 24 in the limit of the AC binary system, it is termed the fusion equation for the liquidus (107-111). 

Here a. (T) is the activity of component i in a stoichiometric liquid at the liquidus temperature, T, AS (IC) is the entropy of fusion of compound IC at the melting temperature, T, and AC (IC) is the difference in heat capacity between themstoichiom6tric liquid and the solid compound. This sequence is the same as that proposed for binary III-V systems by Vieland (5.). 

Each of the above three methods employs a different data base. Most of the property values required for the evaluation of in Equations 7-9 have been experimentally determined for III—V systems and these three relationships can be used as a test for thermodynamic consistency. The first method, Equation 7, is most reliable at or near the binary compound melting temperature. As the temperature is lowered below the melting one, uncertainties in the extrapolated stoichiometric liquid heat capacity and component activity coefficients become important. The second method, Equation 8, is limited to the temperature range in which an experimental determination of AG. is feasible (e.g., high temperature galvanic cell). Method II is also valuable for "pinning down" the low temperature values of 0yp. Method III is the preferred procedure when estimating solution model parameters from liquidus data. Since the activity coefficients of the stochiometric liquid... 

Liquid Solution Behavior. The component activity coefficients in the liquid phase can be addressed separately from those in the solid solution by direct experimental determination or by analysis of the binary limits, since y p = 1. Because of the large amount of experimental effort required to study a ternary composition field and the high vapor pressures encountered in the arsenide and phosphide melts, a direct experimental determination of ternary activity coefficients has been reported only for the Ga-In-Sb system (26). Typically, the available binary liquidus data have been used to fix the adjustable parameters in a solution model with 0,p determined by Equation 7. The solution model expression for the activity coefficient has been used not only to represent the component activities along the liquidus curve, but also the stoichiometric liquid activities needed in Equation 7. The ternary melt solution behavior is then obtained by extending the binary models to describe a ternary mixture without additional adjustable parameters. In general, interactions between atoms in different groups exhibit negative deviations from ideal behavior... 

The breaking up of azeotropic mixtures. The behaviour of constant boiling point mixtures simulates that of a pure compound, because the composition of the liquid phase is identical with that of the vapour phase. The composition, however, depends upon the pressure at which the distillation is conducted and also rarely corresponds to stoichiometric proportions. The methods adopted in practice will of necessity depend upon the nature of the components of the binary azeotropic mixture, and include —... 

Xi = mole fraction of component i in the liquid phase yi = mole fraction of component i in the vapor phase Vi = stoichiometric coefficient of component i (negative for reactants, positive for products)... 

Heats of Reaction. Chemical reactions absorb or liberate energy, usually in the form of heat. The heat of reaction, h.Hn, is defined as the amount of energy absorbed or liberated if the reaction goes to completion at a fixed temperature and pressure. When > 0, energy is absorbed and the reaction is said to be endothermic. When /sHr < 0, energy is liberated and the reaction is said to be exothermic. The magnitude of Is.Hr depends on the temperature and pressure of the reaction and on the phases (e.g., gas, liquid, solid) of the various components. It also depends on an arbitrary constant multiplier in the stoichiometric equation. 

The binary systems we have discussed so far have mainly included phases that are solid or liquid solutions of the two components or end members constituting the binary system. Intermediate phases, which generally have a chemical composition corresponding to stoichiometric combinations of the end members of the system, are evidently formed in a large number of real systems. Intermediate phases are in most cases formed due to an enthalpic stabilization with respect to the end members. Here the chemical and physical properties of the components are different, and the new intermediate phases are formed due to the more optimal conditions for bonding found for some specific ratios of the components. The stability of a ternary compound like BaCC>3 from the binary ones (BaO and CC>2(g)) may for example be interpreted in terms of factors related to electron transfer between the two binary oxides see Chapter 7. Entropy-stabilized intermediate phases are also frequently reported, although they are far less common than enthalpy-stabilized phases. Entropy-stabilized phases are only stable above a certain temperature,... 

When two or more substances are mixed together in a manner that is homogeneous and uniform at the molecular level, the mixture is called a solution. The component (usually a liquid) that is present in much larger quantity than the others is called the solvent the other components are the solutes. The concentration of a solution describes the amount of solute present in a given amount of solution. When a solution is involved in a reaction, the stoichiometric calculations must take into account two quantities not previously discussed the concentration of the solution, and its volume. 

In general, for a trickle-bed reactor, a material balance is required for each of the components present, taken over each of the gas and liquid phases. In the example, however, pure hydrogen will be used and the volatility of the other components will be assumed to be sufficiently low that they do not enter the vapour phase. This means that the material balance on the gas phase can be omitted. It also means that there will be no gas-fllm resistance to gas-liquid mass transfer. In the liquid phase, material balances are required for (i) the hydrogen (reactant A), and (ii) the thiophene (reactant B). The amounts of each of the reactants consumed will be linked by the stoichiometric equation ... 

The mixture with the maximum boiling point is called maximum bailing azeotrope and behaves as if it is a pure chemical compound of two components, because it boils at a constant temperature and the composition of the liquid and vapour is the same. But the azeotrope is not a chemical compound, because its composition is not constant under conditions and rarely corresponds to stoichiometric proportions. 

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