Gaseous equilibrium

AGf° = +58.07, +65.86, and +62.97 kj-mol, respectively. In the presence of a suitable metal catalyst, these three compounds can be interconverted to give an equilibrium gaseous mixture. What will be the percentage of each isomer present at 25°C once equilibrium is established ... 

Figures A.l and A.2, and the equilibrium calculations of Appendix A, lead to huge pressure ratios for the required isothermal concentration cell circulators. These are not existing, developed devices, but are ideas made necessary by Figures A.l and A.2. The incorporation of fully developed circulators in a practical non-equilibrium gaseous cell, distinct from Figure A.l, would involve their operation at a reduced, more practical pressure ratio, relative to the operating point of the cell on its V// characteristic. Figure 6.5, rather than relative to the zero-current point of equilibrium at the top of the figure. The initial steep slope of the figure results from readjustment from zero-flow equilibrium at the zero point to irreversible gas diffusion at the operating point. The latter explanation is unique to the author.

In CO atmosphere the equilibrium gaseous phase will have a considerable amount of CO2 in accordance with reaction (7) (at 1053 K P(C02) -- 0.1 atm). The concentration of the oxide particles in the salt melt will be low (Fig. 11) and comparable with that in the CO2 atmosphere. The reducting atmosphere causes (reaction 5) a high amount of the alkaline metal (Na, K) appear in the melt ( 10 mole fractions at 1053 K) and the gaseous phase ( 10 atm at 1053 K). 

We are going to consider three types of equilibrium gaseous phase homogeneous, liquid phase homogenous and solid phase homogenous equilibria. 

Nitrous acid, HNO2, has not been isolated as a pure compound, although it is observed in equilibrium gaseous mixtures. It is a planar molecule, apparently preferring the trans structure shown in Structure 11.8. 

A third definition of surface mobility is essentially a rheological one it represents the extension to films of the criteria we use for bulk phases and, of course, it is the basis for distinguishing states of films on liquid substrates. Thus as discussed in Chapter IV, solid films should be ordered and should show elastic and yield point behavior liquid films should be coherent and show viscous flow gaseous films should be in rapid equilibrium with all parts of the surface. 

To proceed fiirther, to evaluate the standard free energy AG , we need infonnation (experimental or theoretical) about the particular reaction. One source of infonnation is the equilibrium constant for a chemical reaction involving gases. Previous sections have shown how the chemical potential for a species in a gaseous mixture or in a dilute solution (and the corresponding activities) can be defined and measured. Thus, if one can detennine (by some kind of analysis)... 

The value of the standard free energy AG depends on the choice of reference state, as does the equilibrium constant. Thus it would be safer to write the equilibrium constant K for a gaseous reaction as... 

Water in contact with either the atmosphere or carbonate-bearing sediments contains dissolved or free CO2 that exists in equilibrium with gaseous CO2 and the aqueous carbonate species H2CO3, HCO3A and. The concentration of free... 

S. C. Sharma, "Equilibrium Water Content of Gaseous Mixtures," Ph.D. dissertation. University of Oklahoma, Norman, 1969. Available from University Microfilms, Ann Arbor, Mich., order 69-8601. 

The basic thermodynamic data for the design of such reactions can be used to assess the dissociation energies for various degrees of dissociation, and to calculate, approximately, tire relevant equilibrium constants. One important source of dissociation is by heating molecules to elevated temperamres. The data below show the general trend in the thermal dissociation energies of a number of important gaseous molecules. 

In most circumstances, it can be assumed diat die gas-solid reaction proceeds more rapidly diaii die gaseous transport, and dierefore diat local equilibrium exists between die solid and gaseous components at die source and sink. This implies diat die extent and direction of die transport reaction at each end of die temperature gradient may be assessed solely from diermodynamic data, and diat die rate of uansport across die interface between die gas and die solid phases, at bodi reactant and product sites, is not rate-determining. Transport of die gaseous species between die source of atoms and die sink where deposition takes place is die rate-determining process. 

Botli reactions involve the formation of a vapour-uatisporting species from four gaseous reactant molecules, but whereas the tetra-iodide of zirconium is a stable molecule, the nickel teU acarbonyl has a relatively small stability. The equilibrium constatits for these reactions are derived from the following considerations ... 

The anticipated content of impurities in the refined metal may be calculated a priori by assuming thermodynamic equilibrium at both metal/gas interfaces, and using the relevant stabilities of tire gaseous iodides. Adequate thermodynamic data could provide the activities of the impurities widr that of zirconium close to unity, but tire calculation of tire impurity transport obviously requires a knowledge of activity coefficients in the original impure material, which are not sufficiently well known. 

This is a 4 2 reaction, and is thus pressure dependent. However, it is necessary to compute the equilibrium partial pressure of some alternative gaseous species, such as SiCls, and other hydrocarbons such as C2H2 and for this a Gibbs energy minimization calculation should be made. 

A simple example of the analysis of multicomponent systems will suffice for the present consideration, such as the calculation of the components in a gaseous mixture of oxygen, hydrogen and sulphur. As a first step, the Gibbs energy of formation of each potential compound, e.g. S2, H2S, SO, SO2, H2O etc. can be used to calculate the equilibrium constant for the formation of each compound from the atomic species of the elements. The total number of atoms of each element will therefore be distributed in the equilibrium mixture in proportion to these constants. Thus for hydrogen with a starting number of atoms and the final number of each species... 

When one of the elements is solid, as in tire case of carbon in the calculation of the partial pressures of tire gaseous species in the reaction between methane and air, CO(g) can be used as a basic element together widr hydrogen and oxygen molecules, and thus the calculation of the final partial pressure of methane must be evaluated using the equilibrium constant for CH4 formation... 

These multicomponent calculations are now computerized, and complicated systems, such as tire Si-C-H-Cl quaternaty, may be solved by the use of commercially available software, e. g. the IVTAN database. The solution to this multicomponent system which is obtained by this means is somewhat subjective, since, at the time of writing for example, data are available for 72 gaseous species in the quaternary system Si-C-H-Cl. Choosing 19 of the most probable of tlrese, and using tire IVTAN software to solve this multicomponent equilibrium, yields the following results for tire most probable species (see Table 3.2). 

The principle of Le Chatelier shows that when the pressure applied to a gaseous system is increased, dre equilibrium composition will chairge in order to reduce tire number of gaseous molecules. In the case of tire steam reforming of metlrane, the partial pressures of methane and steam will increase as the pressure is increased. In the water-gas reaction, where tire number of molecules is the same on both sides of the equation, the effect of increasing... 

It follows that the position of thermodynamic equilibrium will change along the reactor for those reactions in which a change of tire number of gaseous molecules occurs, and therefore that the degree of completion and heat production or absorption of the reaction will also vaty. This is why the external control of the independent container temperature and the particle size of the catalyst are important factors in reactor design. 

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