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catalyst

catalyst, substance that can cause a change in the rate of a chemical reaction without itself being consumed in the reaction; the changing of the reaction rate by use of a catalyst is called catalysis. Substances that increase the rate of reaction are called positive catalysts or, simply, catalysts, while substances that decrease the rate of reaction are called negative catalysts or inhibitors.

Mechanism of Catalysis

Catalysts work by changing the activation energy for a reaction, i.e., the minimum energy needed for the reaction to occur. This is accomplished by providing a new mechanism or reaction path through which the reaction can proceed. When the new reaction path has a lower activation energy, the reaction rate is increased and the reaction is said to be catalyzed.

If the activation energy for the new path is higher, the reaction rate is decreased and the reaction is said to be inhibited. Inhibitors can provide an interesting challenge to the chemist. For example, because oxygen is an inhibitor of free-radical reactions, many of which are important in the synthesis of polymers, such reactions must be performed in an oxygen-free environment, e.g., under a blanket of nitrogen gas.

In some reactions one of the reaction products is a catalyst for the reaction; this phenomenon is called self-catalysis or autocatalysis. An example is the reaction of permanganate ion with oxalic acid to form carbon dioxide and manganous ion, in which the manganous ion acts as an autocatalyst. Such reactions are potentially dangerous, since the reaction rate may increase to the point of explosion.

Some substances that are not themselves catalysts increase the activity of a catalyst when added with it to some reaction; such substances are called promoters. Alumina is a promoter for iron when it is used to catalyze the reaction of hydrogen and nitrogen to form ammonia. Substances that react with catalysts to reduce or eliminate their effect are called poisons.

Types and Importance of Catalysts

Enzymes: Natural Catalysts

Enzymes are the commonest and most efficient of the catalysts found in nature. Most of the chemical reactions that occur in the human body and in other living things are high-energy reactions that would occur slowly, if at all, without the catalysis provided by enzymes. For example, in the absence of catalysis, it takes several weeks for starch to hydrolyze to glucose; a trace of the enzyme ptyalin, found in human saliva, accelerates the reaction so that starches can be digested. Some enzymes increase reaction rates by a factor of one billion or more.

Enzymes are generally specific catalysts; that is, they catalyze only one reaction of one particular reactant (called its substrate). Usually the enzyme and its substrate have complementary structures and can bond together to form a complex that is more reactive due to the presence of functional groups in the enzyme, which stabilize the transition state of the reaction or lower the activation energy. The toxicity of certain substances (e.g., carbon monoxide and the nerve gases) is due to their inhibition of life-sustaining catalytic reactions in the body.

Laboratory and Industrial Catalysts

Catalysis is also important in chemical laboratories and in industry. Some reactions occur faster in the presence of a small amount of an acid or base and are said to be acid catalyzed or base catalyzed. For example, the hydrolysis of esters is catalyzed by the presence of a small amount of base. In this reaction, it is the hydroxide ion, OH-, that reacts with the ester, and the concentration of the hydroxide ion is greatly increased over that of pure water by the presence of the base. Although some of the hydroxide ions provided by the base are used up in the first part of the reaction, they are regenerated in a later step from water molecules; the net amount of hydroxide ion present is the same at the beginning and end of the reaction, so the base is thought of as a catalyst and not as a reactant.

Finely divided metals are often used as catalysts; they adsorb the reactants onto their surfaces (see adsorption), where the reaction can occur more readily. For example, hydrogen and oxygen gases can be mixed without reacting to form water, but if a small amount of powdered platinum is added to the gas mixture, the gases react rapidly. Hydrogenation reactions, e.g., the formation of hard cooking fats from vegetable oils, are catalyzed by finely divided metals or metal oxides. The commercial preparation of sulfuric acid and nitric acid also depends on such surface catalysis. Other commonly used surface catalysts, in addition to platinum, are copper, iron, nickel, palladium, rhodium, ruthenium, silica gel (silicon dioxide), and vanadium oxide.

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Catalyst

Catalyst

A catalyst is any agent that functions to speed up a reaction or process without being used up or changed itself.

In chemical reactions, molecules are changed by moving or rearranging atoms or clusters of atoms. For each reaction, to achieve these chemical transitions from one molecule to an altered molecule, a certain amount of energy is normally required to prepare the molecule to undergo change. This is referred to as the activation energy. Activation energy can be thought of as a barrier that prevents molecules from changing from one form to another.

In a chemical reaction, catalysts function to hold a molecule in a certain position or influence the strength of the individual chemical bonds that undergo change during the reaction. Catalysts speed up reactions by lowering the activation energy necessary for the reaction to take place. In living systems, most chemical reactions are catalyzed by proteins called enzymes.

Catalysts can be homogeneous or heterogeneous. A homogeneous catalyst is one that exists in the same phase (gas, liquid, or solid) as the reacting chemical. In biology, for example, enzymes are distributed in the liquid environment inside of cells, and the reacting chemicals are dissolved in the liquid state there as well. In contrast, heterogeneous catalysts exist in a different physical state than the reacting chemicals. For example, in automobiles, the catalytic converter is a solid phase platinum-based catalyst found in the exhaust system, but the reacting chemicals are found in the exhaust gases that pass through after combustion of the gasoline.

Catalysts can be slowed when various inhibitors or poisons are present. Inhibitors are agents that physically interact with the surface of a catalyst to slow or interfere with a chemical reaction. Often, molecules that act as inhibitors for a certain catalyst have shapes and structures very close to the chemical that normally interacts with the catalyst. The inhibitors differ chemically from the reacting chemical, however, so that they are unable to be chemically altered by the normal action of the catalyst. In the case of enzymes, specific inhibitors may often be used in drugs, such as the popular statin drugs used to lower cholesterol. In the example of the catalytic converter, heavy metals such as lead function as poisons by irreversibly combining with the catalytic surface of the platinum, destroying its catalytic properties.

Among the many catalysts used in forensic testing, scientists use inorganic catalysts in the analysis of paint samples and biological catalysts when analyzing DNA .

see also Chemical equations; Endothermic reaction; Exothermic reactions.

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catalyst

catalyst A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. The catalyst provides an alternative pathway by which the reaction can proceed, in which the activation energy is lower. It thus increases the rate at which the reaction comes to equilibrium, although it does not alter the position of the equilibrium. Enzymes are the catalysts in biochemical reactions; they are highly specific in the type of reaction they catalyse.

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catalyst

catalyst Substance that speeds up the rate of a chemical reaction without itself being consumed. Many industrial processes rely on catalysts, such as the Haber process for manufacturing ammonia. Metals or their compounds catalyse by adsorbing gases to their surface, forming intermediates that then readily react to form the desired product while regenerating the original catalytic surface. The metabolism of all living organisms depends on biological catalysts called enzymes.

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catalyst

cat·a·lyst / ˈkatl-ist/ • n. a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. ∎ fig. a person or thing that precipitates an event.

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catalyst

catalyst An agent that participates in a chemical reaction, speeding the rate, but itself remains unchanged. Catalysts are used, for example, in the hydrogenation of vegetable oils. Enzymes and coenzymes are biological catalysts.

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catalyst

catalyst (kat-ă-list) n. a substance that alters the rate of a chemical reaction but is itself unchanged at the end of the reaction. The catalysts of biochemical reactions are the enzymes.

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catalyst

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