Effect of a Catalyst on Equilibrium

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. 

When Cases Are Present Effect of a Change in Temperature on Equilibrium Effect of a Catalyst on Equilibrium... 

Briefly describe each of the following ideas or phenomena (a) dynamic equilibrium (b) direction of a net chemical change (c) Le Chatelier s principle (d) effect of a catalyst on equilibrium. 

Catalysts increase the rate of reactions. It is found experimentally that addition of a catalyst to a system at equilibrium does not alter the equilibrium state. Hence it must be true that any catalyst has the same effect on the rates of the forward and reverse reactions. You will recall that the effect of a catalyst on reaction rates can be discussed in terms of lowering the activation energy. This lowering is effective in increasing the rate in both directions, forward and reverse. Thus, a catalyst produces no net change in the equilibrium concentrations even though the system may reach equilibrium much more rapidly than it did without the catalyst. 

The effect of a catalyst on the time required for equilibrium to be established for the reaction... 

Consider the equilibrimn A v B in which both the forward and reverse reactions are elementary (single-step) reactions. Assmne that the only effect of a catalyst on the reaction is to lower the activation energies of the forward and reverse reactions, as shown in Figure 15.14. Using the Arrhenius equation (Section 14.5), prove that the equilibrium constant is the same for the catalyzed reaction as for the uncatalyzed one. 

Catalysts increase the rates of both reversible and irreversible reactions. Catalysts do not alter the position of equilibrium (Chapter 7), they only increase the rate at which equilibrium is achieved. In other words, the presence of a catalyst does not increase the yield of products but increases the rate of their production. This is because a catalyst lowers the activation energy barrier, E, for both the forward and reverse reactions, increasing the rates of both forward and reverse reactions to the same degree. (This effect of a catalyst on forward and reverse energy barriers is known as the principle of microscopic reversibility.) Hence, to find a good catalyst for a particular reaction it is sufficient to look for a good catalyst for the reverse reaction. 

Franke [47] undertook a comprehensive electroanalytical study of K2S207 mixtures with K2S04, which is formed by Eqs. (47) and (48) and V2Os, a widely-used oxidation catalyst for S02. Pure pyrosulfate under N2 or air (Fig. 38a,b) shows only the reduction to S02 and sulfate, Eq. (48) (all potentials are vs. Ag/Ag+). When S02 is added, a new reduction and oxidation peak appear (Fig. 38c,d). When the electrolyte was pre-saturated with K2S04 (ca. 4 wt.%) (Fig. 39) the gas composition had no direct effect on the voltammetry. Although the equilibrium for Eq. (49) lies well to the right at this temperature, 400 °C, the kinetics are quite slow in the absence of a catalyst. The equilibrium between pyrosulfate and sulfate, Eq. (47), lies well to the left (K = 2 x 10-6), but will proceed to the right in the absence of S03. Thus, the new peaks are sulfate oxidation, Eq. (43), and S03 reduction to sulfite ... 

Table 7.2 summarizes the effect of a catalyst, and other effects of changing conditions, on a system at equilibrium. The Sample Problem that follows provides an opportunity for you to use Le Chatelier s principle to predict the equilibrium shift in response to various conditions. 

Effect of Addition of a Catalyst on the Position of Equilibrium as Measured by the Equilibrium Constant, K... 

The formulations of catalysts developed for the steam reforming of naptha fractions included high nickel content, but catalysts subsequently developed for the commercial exploitation of natural gas were reduced in nickel content and operated at higher temperatures. Commercial reaction conditions now vary in the ranges of 600-900"C and 5-40 bar. A high temperature culminating in a reactor exit temperature of 800-900"C is necessary to overcome the adverse effect of lower temperatures on equilibrium yields, but these conditions are not reflected in all experimental programmes. 

Understand Le Chatelier s principle, and predict the effects of concentration, pressure (volume), temperature, and a catalyst on equilibrium position and on K ( 17.6)... 

Le Chatelier s principle states that if a stress is applied to a system at equilibrium, the equilibrium will shift in a tendency to reduce that stress. A stress is something done to the system (not by the equilibrium reaction). The stresses that we consider are change of temperature, change of pressure, change of concentration(s), and addition of a catalyst. Let us consider the effect on a typical equilibrium by each of these stresses. 

A catalyst has no effect on the position of equilibrium. A catalyst increases the rate at which equilibrium is attained. As discussed in the Reaction Rates chapter, a catalyst provides an alternative route of lower activation energy. Because the rates of both the forward and backward reactions are increased, there is no change in the position of equilibrium. In industry, the presence of a catalyst allows a process to be carried out at a lower temperature (thereby reducing heat energy costs) whilst maintaining a viable rate of reaction. 

The effect of pressure, temperature and a catalyst on both the rate of a reaction and the position of an equilibrium is illustrated in the conditions employed for the manufacture of ammonia, NH3. 

At a given temperature, a reaction will reach equilibrium with the production of a certain amount of product. If the equilibrium constant is small, that means that not much product will be formed. But is there anything that can be done to produce more Yes, there is— through the application of Le Chatelier s principle. Le Chatelier, a French scientist, discovered that if a chemical system at equilibrium is stressed (disturbed) it will reestablish equilibrium by shifting the reactions involved. This means that the amounts of the reactants and products will change, but the final ratio will remain the same. The equilibrium may be stressed in a number of ways changes in concentration, pressure, and temperature. Many times the use of a catalyst is mentioned. However, a catalyst will have no effect on the equilibrium amounts, because it affects both the forward and reverse reactions equally. It will, however, cause the reaction to reach equilibrium faster. 

You know from previous work that the presence of a catalyst has no effect on the position of equilibrium. It therefore follows that a catalyst has no effect on the numerical value of the equilibrium constant K. 

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