CATALYSTS INCREASE THE RATE OF CHEMICAL REACTIONS

Ss discussed in the previous section, a chemical reaction can be made to go fester by increasing the concentration of the reactants or by increasing the temperature. A third way to increase the rate of a reaction is to add a catalyst, which is any substance that increases the rate of a chemical reaction by lowering its activation energy. The catalyst may participate as a reactant, but it is then regenerated as a product and is thus available to catalyze subsequent reactions. 

Atomic chlorine lowers the energy barrier of this reaction by providing an alternate pathway involving intermediate reactions, each having a lower activation energy than the uncatalyzed reaction. This alternate pathway involves two steps. Initially, the chlorine reacts with the ozone to form chlorine monoxide and oxygen  

The chlorine monoxide then reacts with another ozone molecule to re-form the chlorine atom as well as produce two additional oxygen molecules  

Although chlorine is used up in the first reaction, it is regenerated in the second reaction. As a result, there is no net consumption of chlorine. At the same time, however, a total of two ozone molecules are rapidly converted to three oxygen molecules. The chlorine is therefore a catalyst for the conversion of ozone to oxygen because the chlorine increases the speed of the reaction but is not consumed by the reaction. 

Chlorine atoms in the stratosphere catalyze the destruction of Earths ozone layer. As we explore further in Chapter 17, evidence tells us that chlorine atoms are generated in the stratosphere as a by-product of human-made chlorofluoro-carbons (CFCs), once widely produced as the cooling fluid of refrigerators and air conditioners. Destruction of the ozone layer is a serious concern because of the role this layer plays in protecting us from the sun s harmful ultraviolet rays. One chlorine atom in the ozone layer, it is estimated, can catalyze the transformation of 100,000 ozone molecules to oxygen molecules in the 1 or 2 years before the chlorine atom is removed by natural processes. 

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Catalysts increase the rates of chemical reactions by lowering activation energies. 

As we know, catalysts increase the rate of chemical reactions but are not used up in the process. Although a catalyst actually participates in a chemical reaction, it is not permanently modified and may be used again and again. Enzymes are true catalysts that speed... 

Catalysis Catalytic reagents (as selective as possible) are superior to stoichiometric reagents. Catalysts increase the rates of chemical reactions but are not consumed in the reactions. As such, a catalyst may decrease the temperature at which a reaction can be performed economically, thus saving energy costs. Because a catalyst will accelerate the rates of certain reactions more than others, a catalyst can also be used to effect the selectivity in a sequence of reactions. In some reactions, stoichiometric reagents are used to promote a particular pathway or to influence a specific reaction step. Substitution of a catalytic material, particularly one that can easily be recovered, enhances the performance of the reaction in terms of speed and decreases the consumption of materials. Chapter 6 covers some of the basic catalytic tools used in typical chemical reactions and in industry. 

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. 

Enzymes are a subset of proteins whose role in life is catalysis. Catalysts are agents that increase the rates of chemical reactions without undergoing chemical change themselves. Let s consider a model reaction m which some substrate, S, is converted into some product, P. We can write this simply as S P. The point is the difference in the rate of this reaction in the absence and presence of a catalyst. If we make the necessary measurements, we will find that the rate m the presence of the catalyst is greater, perhaps very much greater, than in its absence. At the same time, the catalyst. 

Most nickel is used to make stainless steel. Compounds of nickel combined with many other elements, including chlorine, sulfur, and oxygen, exist. Many of these compounds dissolve fairly easily in water and have a characteristic green color. Nickel and its compounds have no characteristic odor or taste. Nickel compounds are used for nickel plating, to color ceramics, to make some batteries, and as substances known as catalysts that increase the rate of chemical reactions. 

Catalysts increase the rate of a reaction by allowing the reaction to proceed by a different pathway, which has a lower energy transition state. Although the catalyst affects the rate at which an equilibrium is estabhshed, it does not alter the position of the equilibrium. The addition of a catalyst, which is not chemically changed during the reaction, allows many reactions to take place more quickly and at lower temperatures. 

A substrate is the substance upon which an enzyme acts in an enzymatic reaction. Enzymes are biological catalysts that increase the rate of chemical reactions by decreasing the activation energy required for that reaction. An enzyme catalyzes a chemical reaction converting a snbstrate reactant to a product. An individnal enzyme generally has more than one snbstrate and may be specific to several reaction intermediates that are part of an overall reaction. 

A variety of inorganic, organometallic substances Increase the rate of chemical reactions One catalyst can facilitate multiple reactions Could require high temperature/pressure... 

Proteins are linear chains of amino acids that fold into complex 3-dimensional structures. They function in the maintenance of cellular and tissue structure and the transport and movement of molecules. Some proteins are enzymes, which are catalysts that enormously increase the rate of chemical reactions in the body. Chapters 6 and 7 describe the amino acids and their interactions within proteins that provide proteins with a flexible and functional 3-dimensional structure. Chapters 8 and 9 describe the properties, functions, and regulation of enzymes. 

Enzymes are proteins that act as catalysts, compounds that increase the rate of chemical reactions (Fig. 8.1). Enzyme catalysts bind reactants (substrates), convert them to products, and release the products. Although enzymes may be modified during their participation in this reaction sequence, they return to their original form at the end. In addition to increasing the speed of reactions, enzymes provide a means for regulating the rate of metabolic pathways in the body. This chapter describes the properties of enzymes that allow them to function as catalysts. The next chapter explains the mechanisms of enzyme regulation. 

Enzymes, in general, provide speed, specificity, and regulatory control to reactions in the body. Enzymes are usually proteins that act as catalysts, compounds that increase the rate of chemical reactions. Enzyme-catalyzed reactions have three basic steps ... 

Thinking it Through Catalysts are substances that increase the rate of chemical reactions without themselves being consumed in the reaction. What we re looking for in the sequence of steps is a substance that is used in one step and regenerated in a later one. Step 1 uses CH3CH2Br and AlBr3. AlBr, is produced by step 3, so there is no net loss of AlBr, in the overall reaction. It serves as the catalyst. Choice (A) is the correct answer. Every other choice is a reaction intermediate. 

Like all catalysts, enzymes increase the rate of chemical reactions without altering the equilibrium constant of the catalyzed reaction and without... 

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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