Effect of a Catalyst

Sulfur dioxide from the combustion of coal and from other sources appears to be a major cause of the marked increase in acidity of rain in the eastern United States in the past few decades.This acid rain has been shown to contain sulfuric and nitric acids.The SO is oxidized in moist, polluted air to H2SO4. Acid rain is discussed in the essay on acid rain at the end of Section 17.2. 

A catalyst is a substance that increases the rate of a reaction but is not consumed by it. The significance of a catalyst can be seen in the reaction of sulfur dioxide with oxygen to give sulfur trioxide. 

The equilibrium constant for this reaction is 1.7 X 10 , which indicates that for all practical purposes the reaction should go almost completely to products. Yet when sulfur is burned in air or oxygen, it forms predominantly SO2 and very little SO3. Oxidation of SO2 to SO3 is simply too slow to give a significant amount of product. However, the rate of the reaction is appreciable in the presence of a platinum or divanadium pentoxide catalyst. The oxidation of SO2 in the presence of a catalyst is the main step in the contact process for the industrial production of sulfuric acid, H2SO4. Sulfur trioxide reacts with water to form sulfuric acid. (In the industrial process, SO3 is dissolved in concentrated H2SO4, which is then diluted.)  

It is important to understand that a catalyst has no effect on the equilibrium composition of a reaction mixture. A catalyst merely speeds up the attainment of equilibrium. For example, suppose you mix 2.00 mol SO2 and 1.00 mol O2 in a 100.0-L vessel. In the absence of a catalyst, these substances appear unreactive. Much later, if you analyze the mixture, you find essentially the same amounts of SO2 and O2. But when a catalyst is added, the rates of both forward and revCTse reactions are very much increased. As a result, the reaction mixture comes to equihbrium in a short time. The amounts of SO2, O2, and SO3 can be calculated from the equilibrium constant. You find that the mixture is mostly SO3 (2.00 mol), with only 1.7 X 10 mol SO2 and 8.4 X 10 mol O2. 

A catalyst is useful for a reaction, such as 2SO2 + O2 2SO3, that is normally slow but has a large equilibrium constant. However, if the reaction has an exceedingly small equilibrium constant, a catalyst is of little help. The reaction 

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The oxidation of n-butane represents a good example illustrating the effect of a catalyst on the selectivity for a certain product. The noncatalytic oxidation of n-butane is nonselective and produces a mixture of oxygenated compounds including formaldehyde, acetic acid, acetone, and alcohols. Typical weight % yields when n-butane is oxidized in the vapor phase at a temperature range of 360-450°C and approximately 7 atmospheres are formaldehyde 33%, acetaldehyde 31%, methanol 20%, acetone 4%, and mixed solvents 12%. 

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 mechanism of the reaction A - B consists of two steps, with the formation of a reaction intermediate. Overall, the reaction is exothermic, (a) Sketch the reaction profile, labeling the activation energies for each step and the overall enthalpy of reaction, (h) Indicate on the same diagram the effect of a catalyst on the first step of the reaction. 

The effect of a catalyst is important in cationic copolymerizations. Epoxides and /3-lactones form random copolymers only with trialkyl aluminum catalysts. Unusual sequence distributions were observed in the cationic copolymerization of epoxides or lactones using Lewis acids175-177) have been attributed to the di-... 

In the presence of Au/C catalyst, the reaction pathway was studied concluding that glycerate/tartronate amounts represents the probe of path a and glycolate of path b [41c] (Scheme 1). The overall selectivity of the reaction is dictated by the balance of path a and b and represents the most valuable parameter to be considered for evaluating the effectiveness of a catalyst. 

The effect of a catalyst is to increase the rate at which a reaction will take place this is done by making available an alternative path of less energetic demand, often through the formation of a new, and more... 

In a catalysed system, the spontaneous reaction may also occur, but in many cases it is insignificant compared to the catalysed rate of reaction. The effect of a catalyst is to accelerate the rate of both the forward and reverse reaction, allowing equilibrium to be reached much more quickly. The concentration of the reactants is still important, as this affects the probability that the catalyst will interact with the reactants and trigger the reaction. 

Kinetics are an important part of catalysis after all, catalysis is concerned with accelerating reactions. In order to describe the effectiveness of a catalyst one would like to determine the acceleration that has been achieved in the... 

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. 

A quantitative expression developed by Albery and Knowles to describe the effectiveness of a catalyst in accelerating a chemical reaction. The function, which depends on magnitude of the rate constants describing individual steps in the reaction, reaches a limiting value of unity when the reaction rate is controlled by diffusion. For the interconversion of dihydroxacetone phosphate and glyceraldehyde 3-phosphate, the efficiency function equals 2.5 x 10 for a simple carboxylate catalyst in a nonenzymic process and 0.6 for the enzyme-catalyzed process. Albery and Knowles suggest that evolution has produced a nearly perfect catalyst in the form of triose-phosphate isomerase. See Reaction Coordinate Diagram... 

Any (bio)chemical reaction is accompanied by energy conversion, most often in the form of heat production, the amount of heat produced being proportional to that of substance converted. Therefore, heat is a highly nonspecific expression of a (bio)chemical reaction but can be used as indicative for a given substance when this is selectively converted (e.g. by effect of a catalyst, particularly an enzyme). This section discusses three types of sensors based on the use of as many types of devices for measurement of the heat involved in a biochemical reaction, namely fibre optics, polymer films and thermistors. 

A variety of concave pyridines 3 (Table 1) and open-chain analogues have been tested in the addition of ethanol to diphenylketene (59a). Pseudo-first-order rate constants in dichloromethane have been determined photometrically at 25 °C by recording the disappearance of the ketene absorption [47]. In comparison to the uncatalyzed addition of ethanol to the ketene 59a, accelerations of 3 to 25(X) were found under the reaction conditions chosen. Two factors determine the effectiveness of a catalyst basicity and sterical shielding. Using a Bronsted plot, these two influences could be separated from one another. Figure 4 shows a Bronsted plot for some selected concave pyridines 3 and pyridine itself (50). 

The effect of a catalyst on the rate of the reaction with a mechanism... 

To understand the significant effect of catalyst nature, a better understanding of the main reactions, peracetic acid decomposition, and its reaction with acetaldehyde was needed. A literature -survey showed that the kinetics were not well studied, most of the work being done at very low catalyst concentration 1 p.p.m.), and there is disagreement with respect to the kinetic expressions reported by different authors. The emphasis has always been on the kinetics but not on the products obtained, which are frequently assumed to be only acetic acid and oxygen. Consequently, the effectiveness of a catalyst was measured only by the rates and not by the significant amount of by-products that can be produced. We have studied the kinetics of these reactions, supplemented by by-product studies and experiments with 14C-tagged acetaldehyde and acetic acid to arrive at a reaction scheme which allows us to explain the difference in behavior of the different metal ions. 

In some solid rocket proplnts, the effectiveness of a catalyst is reinforced by employing so-called synergist catalysts. These may include phenothiazine and other substances listed under Refs 9 12 Refs 1)J. Linsk R.W. Todd, USP 2936 225... 

Over 90% of industrial processes use catalysts. A catalyst is a substance which can alter the rate of a reaction without being chemically changed itself. In the laboratory, the effect of a catalyst can be observed using the decomposition of hydrogen peroxide as an example. 

The important effect of a catalyst is to help a reaction occur more quickly than it could without the catalyst. The way that a catalyst accomplishes this is to lower the activation energy of the reaction. In Figure 16.8, notice the decreased height of the curve, indicating the lowered activation energy. 

The polymerization rate of propylene and the amorphous polymer content of the polymer produced were used as criteria for the effectiveness of a catalyst combination. In this connection it seemed worthwhile to try to produce a polypropylene of either very low or very high amorphous content, the former being of interest as a plastic and the latter possibly as a material with elastic properties. The molecular weight of the polymer was an additional factor to be considered. 

The C5+ hydrocarbon yield together with chromatographic data and bromine number has been used to assess the effectiveness of a catalyst for catalyzing iso-paraffin/olefin alkylation. This yield is defined as the grams of C5+ hydrocarbon produced per gram of olefin converted. For butene feed, if only alkylation occurs, the C5+ yield would be 2.04 g C5+/g C4- converted however, if only polymerization occurs, the Cs" " yield would be 1.0 g C5+/g 04 converted. The 04 WHSV is defined as g C4=/g catalyst/hour. 

Effect of a catalyst on the number of reaction-producing collisions. Because a catalyst provides a reaction pathway with a lower activation energy, a much greater fraction of the collisions is effective for the catalyzed pathway (b) than for the uncatalyzed pathway (a) (at a given temperature), This allows reactants to become products at a much higher rate, even though there is no temperature increase. 

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