Equilibrium, chemical heterogeneous

The kinetic factor is proportional to the energetic state of the system and (for heterogeneous catalytic systems) the number of active sites per unit volume (mass) of catalyst. The driving-force group includes the influence of concentration and distance from chemical equilibrium on the reaction rate, and the hindering group describes the hindering effect of components of the reaction mixture on the reaction rate. The kinetic factor is expressed as the rate constant, possibly multiplied by an equilibrium constant(s) as will be shown later. 

Most industrial catalysts are heterogeneous catalysts consisting of solid active components dispersed on the internal surface of an inorganic porous support. The active phases may consist of metals or oxides, and the support (also denoted the carrier) is typically composed of small oxidic structures with a surface area ranging from a few to several hundred m2/g. Catalysts for fixed bed reactors are typically produced as shaped pellets of mm to cm size or as monoliths with mm large gas channels. A catalyst may be useful for its activity referring to the rate at which it causes the reaction to approach chemical equilibrium, and for its selectivity which is a measure of the extent to which it accelerates the reaction to form the desired product when multiple products are possible [1],... 

In such an apparatus, a chemical reaction takes place with a conversion of compound A into the products B and C. Typically, a sharp pulse of component A is fed into the column. During the passage through the column, compound A is converted into the products B and C and the amount of component A decreases. Because of their different retention times, the products B and C are concomitantly separated from each other and component A. Due to the removal of the products from the reaction zone, chemical equilibrium is never reached and the reaction will ideally proceed until the total conversion of the compound A. The reaction may take place in the stationary and/or the mobile phase. Heterogeneous reactions maybe either catalyzed by the packed adsorbent or by an additional catalyst, which is mixed with the adsorbent. 

Fugacity. Accdg to Hackh s (Ref 1), it is the escaping tendency in a heterogeneous mixture, by which. a chemical equilibrium responds to altered conditions. In a dilute soln obeying the gas laws, the fugacity equals the osmotic pressure. In other solns it is the value of the pressure for which these equations are still valid... 

The oxidation of sulphur dioxide to trioxide is one of the oldest heterogeneous catalytic processes. The classic catalyst based on V2Os has therefore been the subject of numerous investigations which are amply reviewed by Weychert and Urbaneck [346]. These authors conclude that none of the 34 rate equations reported is applicable over a wide range of process conditions. Generally, these equations have the form of a power expression, in which the reverse reaction is taken into account within the limits imposed by chemical equilibrium, viz. 

The calculation of the equilibrium conversion of heterogeneous reactions is in most cases much more complicated then in the case of homogeneous reactions, because the calculations involve in general the solution of the conditions for chemical equilibrium and the conditions for phase equilibrium. In the following a relatively simple example is given. 

Chemical transport of solids is a well known preparative technique. As was pointed out by Schafer41, information on thermodynamic properties of heterogeneous systems can also be obtained from experiments involving chemical transport. In particular, the dependence of chemical equilibrium of a heterogeneous reaction of the type... 

The energy level diagrams, which can be compiled and used by any experimental chemist, represent a qualitative assessment of the dependence of chemical equilibrium composition of heterogeneous systems on the plasma parameters. The merit of such an approach is its simplicity. 

Sometimes the oxidized species can exist in two forms in chemical equilibrium, with one of them electro-inactive in the potential range where the electrochemical process occurs. This type of reaction pathway is known as a CE mechanism because a homogeneous chemical reaction (C) precedes the heterogeneous electrochemical process (E). If the chemical step is of first or pseudo-first order, the process can be expressed by the reaction scheme ... 

Homogeneous equilibrium refers to a chemical equilibrium among reactants and products that are all in the same phase of matter. Heterogeneous equilibrium takes place between two or more chemicals in different phases. 

For a heterogeneous equilibrium (a chemical equilibrium with components in different phases), reactants or products may be pure liquids or solids. The concentration of a pure liquid or solid in moles/liter cannot change. It is a constant property of the material, and these constants are incorporated into the equilibrium constant. Therefore the concentrations of pure liquids and solids are absent from equilibrium expressions for heterogeneous equilibria. 

We now provide a preliminary discussion of chemical equilibrium in ideal heterogeneous systems. This treatment is provisional since the generalized approach of Section 7 of Chapter 3 is more systematic and does not require a distinction to be made... 

In systems where heterogeneous chemical equilibria prevail, both chemical and phase equilibrium conditions must be simultaneously satisfied. In practice, this means that the chemical equilibrium condition—Eq. (51) in the discrete description, and Eq. (60) in the continuous one—must be satisfied in one phase, and the phase equilibrium condition [/( or fi(x) to be the same in all phases] must be satisfied this clearly guarantees that the chemical equilibrium condition is automatically satisfied in all phases. 

Understanding of sediment diagenesis and benthic fluxes has evolved with advances in both experimental methods and modeling. Measurements of chemical concentrations in sediments, their associated pore waters and fluxes at the sediment-water interface have been used to identify the most important reactions. Because transport in pore waters is usually by molecular diffusion, this medium is conducive to interpretation by models of heterogeneous chemical equilibrium and kinetics. Large chemical changes... 

CHEMICAL EQUILIBRIUM IN IDEAL HETEROGENEOUS SYSTEMS Lastly, we obtain 155... 

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