Equilibrium position catalyst effects

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. 

As A4> can amount to several volts, the electron deformation of the adsorbed molecules can be expected to influence their chemical behavior substantially. Therefore, when reactions are catalyzed via the intermediate formation of boundary layers on a catalyst, we may assume that the activation of the reacting molecules is frequently correlated to their polarization on the catalyst surface. There are two effects of polarization either it causes a strong but reversible adsorption, or the deformation of the electron shell of the adsorbed molecule is so thorough that the system—provided that it possesses sufficient activation energy— switches over irreversibly into a new quantized equilibrium position, forming a chemical bond (1) under liberation of energy. Intermediate states exist between these two extremes. 

A catalyst affects only the rate of a chemical reaction it has no effect on Ke 0r on the position of equilibrium at a given temperature. You cannot, therefore, increase the yield of a chemical reaction at a given temperature by adding a catalyst to the reaction mixture. Catalysts are, however, of great practical value because they may make an impractically slow reaction reach equilibrium at a practical rate, or may permit such a reaction to go at a practical rate at a lower temperature, where a more favorable equilibrium position exists. 

The equilibrium position of any given reaction is determined by the difference in energy between the starting materials and the products. The catalyst does not affect the stability of the starting materials or products involved in the reaction, and so cannot have any effect on the equilibrium position of a reaction. This means that a catalyst cannot be used to make a reaction proceed that would otherwise be thermodynamically impossible. A catalyst may only alter the rate of a reaction that is otherwise possible. 

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. 

Le Chatelier s principle allows us to predict the direction of the shift in equilibrium position. Most importantly, it helps research and industrial chemists create conditions that maximize yields. For the remainder of this section, we examine each of the three kinds of disturbances—concentration, pressure (volume), and temperature—to see how a system at equilibrium responds then, we ll note whether a catalyst has any effect. 

Let s briefly consider a final external change to the reacting system adding a catalyst. Recall from Chapter 16 that a catalyst speeds up a reaction by providing an alternative mechanism with a lower activation energy, thereby increasing the forward and reverse rates to the same extent. In other words, it shortens the time needed to attain the final concentrations. Thus, a catalyst shortens the time it takes to reach equilibrium but has no effect on the equilibrium position. 

Water is a poor nucleophile and therefore adds relatively slowly to a carbonyl group. The rate of the reaction can be increased by an acid catalyst (Figure 18.3). Keep in mind that a catalyst has no effect on the position of the equilibrium. A catalyst affects the rate at which the equilibrium is achieved. In other words, the catalyst affects the rate at which an aldehyde or a ketone is converted to a hydrate it has no effect on the amount of aldehyde or ketone converted to hydrate (Section 24.0). 

The slight difference in the average rotational energy of the two forms enhances the heat capacity due to the LeChatelier shift in the equilibrium position as the temperature is changed. This effect is exhibited in the curve for ot labeled e-H2. Equilibrium-hydrogen, e-H2, is hydrogen that is kept in the presence of a catalyst to ensure that the equilibrium between 0-H2 and P-H2 is established at all temperatures. The curve for e-H2 is typical of the heat capacity of a reactive mixture maintained in equilibrium as the temperature is changed. 

For these reactions, the equilibrium mixture will not have a lot of products present mostly reactants are present at equilibrium. If we define tbe change that must occur in terms of x as the amount (molarity or partial pressure) of a reactant that must react to reach equilibrium, then x must be a small number because fC is a very small number. We want to know the value of x in order to solve the problem, so we don t assume = 0. Instead, we concentrate on the equilibrium row in the ICE table. Those reactants (or products) that have equilibrium concentrations in the form of 0.10 — x or 0.25 + or 3.5 — 3x, etc., is where an important assumption can be made. The assumption is that because K 1, x will be small (x 1) and when we add x or subtract x from some initial concentration, it will make little or no difference. That is, we assume that 0.10 — X 0.10 or 0.25 + x 0.25 or 3.5 — 3x 3.5 we assume that the initial concentration of a substance is equal to the equilibrium concentration. This assumption makes the math much easier and usually gives a value of x that is well within 5% of the true value of x (we get about the same answer with a lot less work). When the 5% rule fails, the equation must be solved exactly or by using the method of successive approximations (see Appendix A1.4). 39. [CO2] = 0.39 M [CO] = 8.6 X 10 M [O2] = 4.3 x 10 M 41. 66.0% 43. a. 1.5 X 10 atmb. Pco = Pci = 1-8 X 10 atm Pcoci2 = 5.0 atm 45. Only statement d is correct. Addition of a catalyst has no effect on the equilibrium position the reaction just reaches equilibrium more quickly. Statement a is false for reactants that are either solids or liquids (adding more of these has no effect on the equilibrium). Statement b is false always. If temperature remains constant, then the value of K is constant. Statement c is false for exothermic reactions where an increase in temperature decreases the value of K. 47. a. no effect b. shifts left c. shifts right 49. H " + OH — H2O sodium hydroxide (NaOH) will react with the H " on the product side of the reaction. This effectively removes H " from the equilibrium, which will shift the reaction... 

The reactions catalysed by enzymes are equilibrium reactions. When the ratio of substrate to product has reached a constant value, the velocities of the forward reaction and the back reaction are equal. It does not mean tliat the substrate and product are present in the same concentration. Usually one or other predominates by a factor of several hundred. An enzyme may induce a reaction that cannot be appreciably detected in its absence, but it is unable to affect the equilibrium position. All that the enzyme does is to increase the rate at which the reaction proceeds to equilibrium. It is generally considered that enzymes combine with their substrates at three or more points. There is probably a simultaneous attack on the substrate by two groups of the enzyme, one withdrawing an electron from one position, whilst the other is donating an electron to a different atom of the substrate. This would explain why enzymes are much more effective then mono-functional catalysts like acids or bases. 

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. 

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)... 

Efficient enzymatic conversion can be achieved even though most of the reactants are present as solids, provided that there is a liquid phase in which the reaction can occur. This approach has been successfully used for carbohydrate ester synthesis with synthesis of glucose esters of fatty acids between C12 and C18 as typical examples [34]. It is important that the substrates dissolve during the reaction, and often the products precipitate as they are formed, which can be an advantage due to a favourable effect on the equilibrium position. Candida antarctica lipase B is an efficient catalyst in this system and solvents used (in moderate amounts) include ethyl methyl ketone, acetone or dioxane. In order to increase the ester yield, water formed in the reaction can be removed by azeotropic distillation and the solvent (e.g. ethyl metyl ketone) can after condensation be dried by pervaporation, giving a practically useful complete process [35]. 

To avoid the intrinsic instability of cyanohydrins and their silyl ether, Saa and coworkers reported catalytic asymmetric cyanophosphonylation reaction of aldehydes with commercially available diethyl cyanophosphonate [58]. In these works, Lewis acid-Lewis base bifunctional catalyst (65) prepared by mixing BI-NOLAM ligand with amino arms as Lewis base and Et2AlCl was found to work nicely (Scheme 6.46). Since a strong positive nonlinear effect was observed in this reaction, actual catalyst is in equilibrium with some oligomeric species of the aluminum complexes. Bifunctional catalyst (65) could also catalyze cyanosilylation of... 

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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