Catalysts lower activation energy

Use of the statement catalysts lower activation energy should be discouraged. Catalysts do not lower the activation energy for the original reaction. This remains unchanged. A catalyst provides an alternative route, or reaction pathway, that has lower activation energy. 

The use of a catalyst affects the rate of reaction by enabling the products to form by an alternative route. Each stage has lower activation energy than the uncatalyzed reaction. 

Compared with uncatalyzed reactions, catalysts introduce alternative pathways that, in nearly all cases, involve two nr more consecutive reaction steps. Each of these steps has a lower activation energy than does the uncatalyzed reaction. We can nse as an example the gas phase reaction of ozone and oxygen atoms. In the homogeneons uncatalyzed case, the reaction is represented to occur in a single irreversible step that has a high activation energy ... 

A catalyst is a substance that increases the rate of a reaction without being consumed by it It does this by changing the reaction path to one with a lower activation energy. Frequently the catalyzed path consists of two or more steps. In this case, the activation energy for the uncatalyzed reaction exceeds that for any of the steps in the catalyzed reaction (Figure 11.11). 

A homogeneous catalyst is one that is present in the same phase as the reactants. It speeds up the reaction by forming a reactive intermediate that decomposes to give products. In this way, the catalyst provides an alternative process of lower activation energy. 

Yet the reaction is quite slow, even at high temperatures. Evidently the rate is controlled by a high activation energy. In fact, the practical use of reaction (19) depends upon the presence of a catalyst to provide a reaction path with a lower activation energy. The two important commercial methods for manufacture of H2S04 differ principally in the choice of catalyst for this step. 

A catalyst speeds up a reaction by providing an alternative pathway—a different reaction mechanism—between reactants and products. This new pathway has a lower activation energy than the original pathway (Fig. 13.34). At the same temperature, a greater fraction of reactant molecules can cross the lower barrier of the catalyzed path and turn into products than when no catalyst is present. Although the reaction takes place more quickly, a catalyst has no effect on the equilibrium composition. Both forward and reverse reactions are accelerated on the catalyzed path, leaving the equilibrium constant unchanged. 

FIGURE 13.34 A catalyst provides a new reaction pathway with a lower activation energy, thereby allowing more reactant molecules to cross the harrier and form products. 

Catalysts participate in reactions but are not themselves used up they provide a reaction pathway with a lower activation energy. Catalysts are classified as either homogeneous or heterogeneous. 

Another way to make a reaction go faster is to add a substance called a catalyst. A catalyst functions by changing the mechanism of a reaction in a manner that lowers activation energy barriers. Although the catalyst changes the mechanism of a reaction, it is not part of the overall stoichiometiy of the reaction. A catalyst always participates in an early step of a reaction mechanism, but when the reaction is over, the catalyst has been regenerated. When we write a net reaction that is influenced by a catalyst, we write the formula of the catalyst above or below the reaction arrow. 

Catalysts are substances which increase the reaction rate when added to a system by providing an alternative mechanism with a lower activation energy. Although a catalyst may enter into a reaction, it does not appear in the overall balanced equation it is a reactant in one step and a product in another. It is not consumed during a reaction. 

Catalysts increase the rate of a reaction, but remain chemically unchanged at the end of the reaction. Catalysts provide an alternative reaction route of lower activation energy. 

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. 

A catalyst provides an alternative route (1) with a lower activation energy (1). It is incorrect to state that a catalyst lowers the activation energy . 

Homogeneous catalysts are catalysts that are in the same phase or state of matter as the reactants. They provide an alternate reaction pathway with a lower activation energy. 

A catalyst can increase the rate of this reaction by providing an alternative mechanism with lower activation energy. A possible mechanism for the catalyzed reaction is shown below. 

The transition metal atoms or Ions on the surface of the active sites of the catalyst form weak bonds with the reactant molecules. It Is thought that the presence of unpaired d electrons or unfilled d orbitals allows Intermediate complexes to form. The effect of this is to weaken the covalent bonds Inside the reactant molecules and, as these reactant molecules are now held in a favourable position, they are more susceptible to attack by molecules of the other reactant. The overall effect is that an alternative reaction pathway with a lower activation energy is provided and so the rate of the reaction Is Increased. 

Another reason that transition metals are good catalysts may be because they have variable oxidation states. Again, this allows the transition metal to provide an alternative reaction pathway with a lower activation energy, so speeding up the reaction. 

An important characteristic of a catalyst s action is that the mechanism of the reaction is altered in a manner that allows for a lower activation energy. In a number of nonenzymatic examples, a reactant or product can act as a catalyst as well, and the definition must be altered to include substances that appear in the overall rate expression with a power higher than the corresponding... 

D) A catalyst is a substance that increases the rate of a chemical reaction withont itself being consumed. The catalyst may react to form an intermediate, but it is regenerated in a subsequent step of the reaction. The catalyst speeds up a reaction by providing a set of elementary steps (reaction mechanisms) with more favorable kinetics than those that existed in its absence. Choices A and C, even thongh they are true statements, are the resnlts of a lowered activation energy. 

C) A catalyst has no effect on equilibrium. By adding a catalyst, the activation energy of both the forward and reverse rates will be lowered, but there will be no effect on equilibrium. 

Since the catalyzed path requires lower activation energy, more molecules will have sufficient energy to react effectively than in the case of the uncatalyzed path. In homogeneous catalysis, this is generally achieved by the reaction between the catalyst and one or more reactants to form an unstable chemical intermediate, which subsequently reacts to produce the final product. The catalyst is regenerated in the final step. For example, if reactant A reacts with B to form the product (P) in the presence of a catalyst (C), a possible reaction scheme is... 

In heterogeneous catalysis, the catalyst provides an alternative pathway of lower activation energy generally through... 

A catalyst is a substance that increases the rate at which a chemical reaction approaches equilibrium, while not being consumed in the process. Thus, a catalyst affects the kinetics of a reaction, through provision of an alternative reaction mechanism of lower activation energy, but cannot influence the thermodynamic constraints governing its equilibrium. 

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