Thermodynamics composition change

The coordinates of thermodynamics do not include time, ie, thermodynamics does not predict rates at which processes take place. It is concerned with equihbrium states and with the effects of temperature, pressure, and composition changes on such states. For example, the equiUbrium yield of a chemical reaction can be calculated for given T and P, but not the time required to approach the equihbrium state. It is however tme that the rate at which a system approaches equihbrium depends directly on its displacement from equihbrium. One can therefore imagine a limiting kind of process that occurs at an infinitesimal rate by virtue of never being displaced more than differentially from its equihbrium state. Such a process may be reversed in direction at any time by an infinitesimal change in external conditions, and is therefore said to be reversible. A system undergoing a reversible process traverses equihbrium states characterized by the thermodynamic coordinates. 

In practice, thermal cycling rather than isothermal conditions more frequently occurs, leading to a deviation from steady state thermodynamic conditions and introducing kinetic modifications. Lattice expansion and contraction, the development of stresses and the production of voids at the alloy-oxide interface, as well as temperature-induced compositional changes, can all give rise to further complications. The resulting loss of scale adhesion and spalling may lead to breakaway oxidation " in which linear oxidation replaces parabolic oxidation (see Section 1.10). 

In high pressure mixing devices each solvent is piunped separately in the proportions required by the gradient into a mixing chamber before being delivered to the column. Solvent compressibility and thermodynamic volume changes on mixing may Influence the accuracy of the composition delivered to the column. 

In general, thermodynamic properties of the components in a solution vary with composition because the environment of each type of atom or molecule changes as the composition changes. The change in interaction force between neighbouring atoms or molecules with the change in composition results in the variation of the thermodynamic properties of a solution. The thermodynamic properties that components have in a solution are called partial properties. 

Measurement of the interfacial tension can estimate the amount of surfactant present at the interface, called the surface excess. If the bulk liquids (A and B) extend to the interface SS with no compositional change (Figure 4.11), surface excess is present. The total thermodynamic energies of a system and the interface layer I for an open system are given by Equation (4.35a) and Equation (4.35b), respectively ... 

Figure 2. The thermodynamic situation upon solidification of a multemary system. The vertical lines designate principal reaction pathways, the dashed tangent lines illustrate the compositional changes arising from an equilibrium solidification at the respective pathways (interrupt lines on the vertical arrows). The narrow areas of existence designate stable phases with a finite phase width, the area designated metastable indicates the existence of a single phase solid which is unstable at ambient conditions.

In Chap. 6 we treated the thermodynamic properties of constant-composition fluids. However, many applications of chemical-engineering thermodynamics are to systems wherein multicomponent mixtures of gases or liquids undergo composition changes as the result of mixing or separation processes, the transfer of species from one phase to another, or chemical reaction. The properties of such systems depend on composition as well as on temperature and pressure. Our first task in this chapter is therefore to develop a fundamental property relation for homogeneous fluid mixtures of variable composition. We then derive equations applicable to mixtures of ideal gases and ideal solutions. Finally, we treat in detail a particularly simple description of multicomponent vapor/liquid equilibrium known as Raoult s law. 

Statistical thermodynamic treatments of defect populations have lead to an explanation of existence of grossly nonstoichio-metric crystals in terms of microdomains of ordered structure. The model considers that the nonstoichiometric matrix is made up of a mosaic of small regions of ordered defect-free structures, the microdomains. To account for stoichiometric variation, one can postulate that at least two microdomains with different compositions occur. However, compositional change might simply arise at the surface of the domain. For example, if there are compositionally identical microdomains, one of which is bounded by an anion surface and one by a cation surface, variation in the two populations can give rise to compositional variation. In a strict sense, as each microdomain is ordered, the concept of a defect is redundant, except for... 

So far we have not taken into account the chemical constitution of matter—a subject of central importance in chemical thermodynamics. We now discuss several fundamental issues, and in subsequent sections provide a systematic thermodynamic analysis of compositional changes. 

The full specification of a polymorphic system is specified by the thermodynamic properties of the phases involved. A solid phase has a uniform structure and composition throughout, is separated by other phases by defined boundaries, and undergoes a phase transition when a particular solid phase becomes unstable under a given set of environmental conditions. The course of these phase changes is dictated by differences in free energy at the transition that are associated with structural or compositional changes. Classical thermodynamics provides a basis for understanding the nature of these transitions. 

For an individual tray, the concept of efficiency often used is the Murphree vapor efficiency, defined as the ratio of the composition change in the vapor stream to the composition change if the vapor leaving the tray were in thermodynamic equilibrium with the liquid leaving. Thus,... 

Various mnemonics have been reported to help students to be familiar with thermodynamic relations [2-5]. Most of them are rather direct notation and demand their memorization. Teaching the pertinent thermodynamic relations to them could be consummated with a simple story displayed in the two-dimensional Cartesian coordinate system for a reversible change in a closed system without composition change in the absence of any other work except pressure-volume work. 

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