Change in composition

If a mixing process or chemical transformation is brought about, spontaneously or by applying work on the system, the process will take place with entropy generation  

Indeed, for different molecules, which otherwise are nearly the same, such as isomers, or molecules of about the same size, polarity, or other properties, the thermodynamic probability of the mixed state at the same P and T is much larger than that of the respective pure states (in molar units)  

If the change is in composition only, at constant P and T, and confined to the system we wish to consider, for instance, in a mixer, separation column, or a reactor, then a system property G, the Gibbs energy, can be identified and has been defined as follows  

If we consider a chemical reaction that takes place in a homogeneous mixture, the equilibrium composition of the mixture can be found from the equality condition in Equation 2.19 if the dependence of G on the composition is known. 

If the process of mixing takes place with negligible change of the internal energy U and volume V, we speak of ideal mixing and it can be shown then that for 1 mol of mixture 

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In vapor-liquid equilibria, it is relatively easy to start the iteration because assumption of ideal behavior (Raoult s law) provides a reasonable zeroth approximation. By contrast, there is no obvious corresponding method to start the iteration calculation for liquid-liquid equilibria. Further, when two liquid phases are present, we must calculate for each component activity coefficients in two phases since these are often strongly nonlinear functions of compositions, liquid-liquid equilibrium calculations are highly sensitive to small changes in composition. In vapor-liquid equilibria at modest pressures, this sensitivity is lower because vapor-phase fugacity coefficients are usually close to unity and only weak functions of composition. For liquid-liquid equilibria, it is therefore more difficult to construct a numerical iteration procedure that converges both rapidly and consistently. 

At z in the curve, however (the minimum of vapour pressure), the solution and vapour are in equilibrium and the liquid at this point will distil without any change in composition. The mixture at z is said to be azeotropic or a constant boiling mixture. The composition of the azeotropic mixture does vary with pressure. 

Microscopic examination of cross sections through the paint layers gives definite information regarding the paint-layer sequence in the area from which the sample was taken (31,66). This information illustrates the artist s use of underlayers and glazes, superposition of compositional elements, and changes in composition. 

Table 8. Seasonal Change in Composition of Black Walnut Kernels ...

Ammonia combines with hydrogen sulfide, sulfur, or both, to form various ammonium sulfides and polysulfides. Generally these materials are somewhat unstable, tending to change in composition on standing. Ammonium sulfides are used by the textile industry. 

All reactor modes can sometimes be advantageously operated with recychng of part of the product or intermediate streams. Heated or cooled recycle streams serve to moderate undesirable temperature travels, and they can be processed for changes in composition before being returned. 

In distillation operations, separation results from differences in vapor-and liquid-phase compositions arising from the partial vaporization of a hquid mixture or the partial condensation of a vapor mixture. The vapor phase becomes enriched in the more volatile components while the hquid phase is depleted of those same components. In many situations, however, the change in composition between the vapor and liquid phases in equihbrium becomes small (so-called pinched condition ), and a large number of successive partial vaporizations and partial condensations is required to achieve the desired separation. Alternatively, the vapor and liquid phases may have identical compositions, because of the formation of an azeotrope, and no separation by simple distillation is possible. 

Assume that at t = 0 the feed rate to tray 23 is disturbed by increasing it by 30 percent to 130 moVmin without a change in composition. The resulting ethanol liquid mole frac tion on severaltrays is tracked in Fig. 13-109. Above tray 16, ethanol concentration remains veiy small. Below tray 9, ethanol concentration initially decreases fairly rapidly but... 

The presence of water vapour in the ingoing gas irrixmre has been found to suppress the formation of graphite and dins to favour diamond formation. The significant change in composition when water vapour is added, is the presence of carbon monoxide in about half the proportion of hydrogen atoms. 

Greater deviations which are occasionally observed between two reference electrodes in a medium are mostly due to stray electric fields or colloid chemical dielectric polarization effects of solid constituents of the medium (e.g., sand [3]) (see Section 3.3.1). Major changes in composition (e.g., in soils) do not lead to noticeable differences of diffusion potentials with reference electrodes in concentrated salt solutions. On the other hand, with simple metal electrodes which are sometimes used as probes for potential controlled rectifiers, certain changes are to be expected through the medium. In these cases the concern is not with reference electrodes, in principle, but metals that have a rest potential which is as constant as possible in the medium concerned. This is usually more constant the more active the metal is, which is the case, for example, for zinc but not stainless steel. 

The reason for this simple relationship is that the concept of minimum reflux implies an infinite number of stages and thus no change in composition from stage to stage for an infinite number of stages each way from the pinch point (the point where the McCabe-Thiele operating lines intersect at the vapor curve for a well-behaved system, this is the feed zone). The liquid refluxed to the feed tray from the tray above is thus the same composition as the flash liquid. 

Furthermore, K is related to k, and by the requirement that no net change in composition occur at equilibrium ... 

Partial Condensing - The relief requirement is the difference between the incoming and outgoing vapor rates at relieving conditions. The incoming vapor rate should be calculated on the same basis as stated earlier. If the reflux is changed in composition or rate, the incoming vapor rate to the condenser should be determined for the new conditions. 

Equation 6-16 makes it possible to express changes in composition in terms of the change in mass fraction of a single component. Thus,... 

Just as the solid/liquid phase equilibria in the systems HX/H2O show several points of interest, so too do the liquid/gas phase equilibria. When dilute aqueous solutions of HX are heated to boiling the concentration of HX in the vapour is less than that in the liquid phase, so that the liquid becomes progressively more concentrated and the bp progressively rises until a point is reached at which the liquid has the same composition as the gas phase so that it boils without change in composition and at constant temperature. This mixture is called an azeotrope (Greek a, without zein, to boil rpo7tr, trope, change). 

Adsorption is the property of certain extremely porous materials to hold vapors in the pores until the desiccant is either heated or exposed to a drier gas. The material is a solid at all times and operates alternately through drying and reactivation cycles with no change in composition. Adsorbing materials in principal use are activated Alumina and silica gel. Molecular sieves are also used. Atmospheric dew points of minus 1000°F are readily obtained using adsorption. 

Some metals and alloys have low rates of film dissolution (low /p) even in solutions of very low pH, e.g. chromium and its alloys, and titanium. In these cases the value of /p is quite low, and although it increases as the temperature increases, a maximum is reached when the solution boils. The maximum current is below and breakdown does not occur. However, in certain alloys, e.g. Cr-Fe alloys, the protective film may change in composition on increasing the anode potential to give oxides that are more soluble at low pH and are therefore more susceptible to temperature increases. This occurs in the presence of cathode reactants such as chromic acid which allow polarisation of the anode. 

These three diagrams show clearly how a very small change in the relative position of the AG-concentration curves for the different phases can have a dramatic effect on the composition and stability of the solid phases in the Fe,j,-Zn( system. A detailed discussion of these diagrams can be found elsewhere In addition, Fig. 7.86 shows that any physical effects in the solid phases, which can be expressed in terms of energy, can be added to the AG -concentration curves. Thus the curves can be corrected for these effects and the resulting new equilibria from the shift of the AG-concentra-tion curves (i.e. change in composition of phase, their stability, etc.) can be predicted simply by the use of the tangency rule. 

The mechanism of the corrosion reaction is not clear, particularly in view of the changes in composition of the HNOj that take place during the 48 h period of the test. Streicher reports that the corrosion potential of the... 

The main problem in the study of the role of these parameters in electrolyte conductivity is their interdependence. A change in composition of a binary solvent changes viscosity, along with the permittivity, ion-ion association, and ion solvation, which may be preferential for one of the two solvents and therefore also changes the Stokes radii of the ions. 

To discuss the effectiveness of internal standards, it is helpful to recall the absorption and enhancement effects the standards are intended to compensate. In the present discussion, we shall ignore the incident beam even though this simplification is not always justified. We have indicated above that an internal standard will hot be completely effective if a change in composition influences the intensity ratio (Equation 7-12). Such influences could arise from differences in the extent to which the two analytical lines are absorbed or from differences in the extent to which the two lines are excited. Let us examine the way in... 

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