Chemical equilibrium catalysts effect

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. 

When a solid acts as a catalyst for a reaction, reactant molecules are converted into product molecules at the fluid-solid interface. To use the catalyst efficiently, we must ensure that fresh reactant molecules are supplied and product molecules removed continuously. Otherwise, chemical equilibrium would be established in the fluid adjacent to the surface, and the desired reaction would proceed no further. Ordinarily, supply and removal of the species in question depend on two physical rate processes in series. These processes involve mass transfer between the bulk fluid and the external surface of the catalyst and transport from the external surface to the internal surfaces of the solid. The concept of effectiveness factors developed in Section 12.3 permits one to average the reaction rate over the pore structure to obtain an expression for the rate in terms of the reactant concentrations and temperatures prevailing at the exterior surface of the catalyst. In some instances, the external surface concentrations do not differ appreciably from those prevailing in the bulk fluid. In other cases, a significant concentration difference arises as a consequence of physical limitations on the rate at which reactant molecules can be transported from the bulk fluid to the exterior surface of the catalyst particle. Here, we discuss... 

We note that particular catalysts or initiators used in chemical reactors change only the effective specific reaction rate and do not change the value of the chemical equilibrium constant. 

In practice, because of kinetic effects, the product distribution offered by different catalysts may exhibit deviations from the chemical-equilibrium analysis. However, the examination of numerous papers dealing with catalysis issues confirms the main trends operate at lower temperature and keep the H2/phenol ratio as low as possible to promote the formation of cyclohexanone. 

Experiments were executed in an autoclave at temperature between 130 and 180 °C, with alcohol/acid ratios between 1/9 to 27/1, as well as sulfated zirconia catalyst concentration up to 5 wt%. The experimental conditions preserved the chemical equilibrium constraint. Details are given elsewhere [2]. Two contributions in forming the reaction rate can be distinguished enhancement due to the solid catalyst and an autocatalysis effect by the fatty acid. Consequently, the following expression can be formulated for the overall reaction rate ... 

The Effect of Catalysts on Chemical Equilibrium. It is a consequence of the laws of thermodynamics—the impossibility of perpetual motion—that a system in equilibrium is not changed by the addition of a catalyst. The catalyst may increase the rate at which the system approaches its final equilibrium state, but it cannot change the value of the equilibrium constant. Under equilibrium conditions a catalyst has the same effect on the rate of the backward reaction as on that of the corresponding forward reaction. 

The methanol to DME ratio was found to increase with coke formation on the catalysts (Fig. 6b). This ratio was quite far from the ratio at chemical equilibrium, which was calculated at 425°C to be 0.47. The deviation from chemical equilibrium ratio was found to be larger at higher coke contents and on the externally precoked samples. This indicates that the methanol conversion to DME is not fast enough to reach equilibrium, probably due to the moderate external acidity of SAPO-34 and partly also due to the effect of diffusion of DME at the higher coke contents. MTO can be considered a simple sequence reaction Methanols DME- olefins... 

State LeChatelier s Principle. Which factors have an effect on a system at equilibrium How does the presence of a catalyst affect a system at chemical equilibrium Explain your answer. 

Skill 9.1 Analyzing the effects of concentration, pressure, temperature, and catalysts on chemical equilibrium and applying Le Chatelier s principle to chemical systems... 

Chemical equilibrium represents a balance between forward and reverse reactions. In most cases, this balance is quite delicate. Changes in experimental conditions may disturb the balance and shift the equilibrium position so that more or less of the desired product is formed. When we say that an equilibrium position shifts to the right, for example, we mean that the net reaction is now from left to right. Variables that can be controlled experimentally are concentration, pressure, volume, and temperature. Here we will examine how each of these variables affects a reacting system at equilibrium. In addition, we will examine the effect of a catalyst on equilibrium. 

On the basis of the parameters obtained in the HDN of DHQ [15], OPA [19] and THQ5 (Table 2), the complete HDN network of quinoline may be effectively modelled. The HDN of Q was performed at 623 K, 3,0 MPa and Ph2 6.5 kPa over NiMo/Al20j and NiMoP/Al203 catalysts. The transformation between Q and THQl was very fast, chemical equilibrium has usually been assumed for this reaction step [2-4],... 

Effect of Pressure on Chemical Equilibrium in an Ideal Gas Mixture Nitrogen and hydrogen react to form ammonia in the presence of a catalyst,... 

In some cases it may be useful to reduce the catalyst activity level to facilitate the observation of early reaction steps. An example of this approach is shown by the two in situ studies illustrated in Fig. 28 [101]. When acetaldehyde-1,2- C was heated on a zeolite sample activated to 673 K, a complex product distribution was formed, which decomposed to CO, COj, and other products at higher temperatures. If a small amount of water was first adsorbed uniformly on the zeolite, acetaldehyde was converted almost quantitatively to crotonaldehyde by a similar in situ protocol. It seems that water levels the acidity of the zeolite in a manner analogous to that seen in nonaqueous acid-base chemistry. As an aside, note that the C chemical shifts of the carbonyl and 3 olefinic carbons are shifted downfield owing to the protonation equilibrium. This effect was discussed previously as a caveat to chemical shift interpretation. 

In (5.49), kf represents the effective reaction rate constant including internal transport resistances of the catalyst packing. stands for the catalyst volume installed in the column. For Da 3> 1 the chemical reaction approaches chemical equilibrium, for Da 1 the reaction is far from its equilibrium. 

Factors That Affect Chemical Equilibrium Changes in concentration can affect the position of an equilibrium state—that is, the relative amounts of reactants and products. Changes in pressure and volume may have the same effect for gaseous systems at equilibrium. Only a change in temperature can alter the value of equilibrium constant. A catalyst can establish the equilibrium state faster by speeding the forward and reverse reactions, but it can change neither the equilibrium position nor the equilibrium constant. 

The presence of a catalyst has no effect on the position of a chemical equilibrium. Figure 7.21 shows how a catalyst lowers the activation energy of a reaction. However, the effect is applicable to the values of both the forward and reverse reactions both values are reduced by the same amount. Consequently the presence of a catalyst increases the rate of the forward and reverse reactions equally. There is no change in the position of the equilibrium or the value of K. ... 

We have identified three factors that influence the rate of a chemical reaction. In relating these factors to equilibrium considerations, we will examine only concentration and temperature. These variables affect forward and reverse reaction rates differently. A catalyst, on the other hand, has the same effect on both forward and reverse rates. Therefore, a catalyst does not alter chemical equilibrium. A catalyst does cause a system to reach equilibrium more quickly. 

Although the existence of inert gas has without poison on the iron catalyst, from the perspective of chemical equilibrium, increasing the content of inert gas tp ) is equivalent to the reducing the effective pressure The relationship for operating pressure (po), effective pressure (pe) and content of inert gas (pi) is as follows ... 

Xhe presence of CO2 causes various kinetic effects it accelerates the reaction rate, enhances the selectivity, alleviates the chemical equilibrium, suppresses the unwanted total oxidation products, prevents the hot spots on the catalyst surface, poisons the non-selective sites of the catalysts, and the equilibrium yield of styrene dehydrogenation is much higher in the presence of CO2 than in that of steam. 

A catalyst is a substance that accelerates (or sometimes decelerates) the rate of approach to chemical equilibrium. In so doing, it is neither consumed nor is its effectiveness reduced unless it is deactivated in the course of reaction. Only heterogeneous solid catalysts are considered in this book. Catalysts are usually metals or metal compounds. The catalyst surface exposed to fluid reactants is responsible for the catalytic effect. It is natural then that the catalyst be made to have a high exposed surface area per unit weight. On the other hand, the reactor that contains the catalyst should be as small as practically possible. Therefore, the catalyst is usually spread on a substance of high surface area. Such a catalyst is called a supported catalyst. 

Most processes are catalyzed where catalysts for the reaction are known. The choice of catalyst is crucially important. Catalysts increase the rate of reaction but are unchanged in quantity and chemical composition at the end of the reaction. If the catalyst is used to accelerate a reversible reaction, it does not by itself alter the position of the equilibrium. When systems of multiple reactions are involved, the catalyst may have different effects on the rates of the different reactions. This allows catalysts to be developed which increase the rate of the desired reactions relative to the undesired reactions. Hence the choice of catalyst can have a major influence on selectivity. 

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