Molecular kinetics. Catalysis. Plan 1.Mechanism of Catalytic reactions 3. Characteristics of catalysts. 4. Types of catalysis. 5. Enzymes. Assistant Kozachok S.S. prepared

A substance which changes the speed of a reaction without being used itself is called a catalyst. The phenomenon of increasing the rate of reaction by the use of catalyst is called catalysis. If а catalyst increases (accelerates) the speed of а reaction, it is called а positive catalyst and the phenomenon is called positive catalysis. On the other hand, if а catalyst decreases (retards) the speed of а reaction, it is called а negative catalyst and the phenomenon is called negative catalysis. The catalysts may be of two main types:

1. Homogeneous catalysts. If the catalyst is present in the same phase as the reactants, it is called а homogeneous catalyst and this type of catalysis is called homogeneous catalysis. NO(g) 2 SO 2 (g) + О 2 (g) ===== SO 3 (g) Н + (aq) CH 3 COOC 2 H 5 (l) + Н 2 О(1) ===== СН З СООН(1) + C 2 H 5 OH(1) Н + (aq) С 12 Н 22 О 11 (aq) + Н 2 О (1) ====== С 6 Н 12 О 6 (aq) + С 6 Н 12 О 6 (aq) Sucrose Glucose Fructose

2. Heterogeneous catalysts. If the catalyst is present in а different phase than the reactants, it is called а heterogeneous catalyst and this type of catalysis is called heterogeneous catalysis. Pt, С 4NH 3 + 5O 2 ======== 4NO + 6Н 2 O

Mechanism of Catalytic reactions 1. Homogeneous catalytic reactions. These reactions can be explained on the basis of intermediate compound formation. According to this theory, the catalyst combines with one of the reactants to form an intermediate. Intermediate compound being unstable combines with the other reactant to form product. For example, the combination of SO 2 and O 2 to form SO 3 is a slow process. However, in the presence of NO (catalyst), the reaction becomes fast. It is believed that in this reaction nitric oxide combines with one of the reactants to form intermediate compound (NO 2 ). This intermediate (NO 2 ) combines readily with SO 2 to form SO 3 and the catalyst NO is regenerated in the last step.

2. Mechanism of heterogeneous catalytic reactions. The heterogeneous catalysis is a surface phenomenon. It involves the following steps: - Diffusion of the reactants at the surface of the catalyst. - Adsorption of the molecules of the reactants at the active sites. - Occurrence of the chemical reactions on the surface of the catalyst. - Desorption of products molecules from the surface. - Diffusion of products away from the surface of the catalyst. The role of heterogeneous catalysts can be explained in terms of adsorption of reactants on the surface of the catalyst. The adsorption helps the reaction in the following ways: 1. Adsorption increases the concentration of reactant on the surface of the catalyst. Due to increased concentration of the reactants, the reactions proceed rapidly. 2. Adsorbed molecules get dissociated to form active species like free radicals which react faster than molecules. 3. The adsorbed molecules are not free to move about and therefore, they collide with other molecules on the surface. 4. The heat of adsorption evolved acts as energy of activation for the reaction.

Theory of heterogeneous catalysis 1. Adsorption theory 2. Multiplet’s theory 3. Тhe theory of the active ensembles (migrated atoms) 4. Electro-chemical theory (of free electrons and free valency)

Types of catalysis PositiveNegativeAutocatalysis Homogeneous HeterogeneousEnzyme Acid-base specificAcid-base unspecific

Characteristics of catalysts: 1. Activity. The ability of a catalyst to increase the rate of a chemical reaction is called activity. A catalyst may accelerate a reaction to as high as times. 2. Selectivity. The ability of the catalyst to direct a reaction to give a particular product. 3. Small quantity. Only small quantity is need for a reaction. 4. Specific. One catalyst is need for specific reaction only 5. Physical properties may change during a reaction but no it does not take part in the reaction. 6. Catalyst doesn’t influence on the general stoichiometric coefficients. 7. Catalysts decrease activation energy thus increase the chemical rate. 8. Catalysts don’t influence on the equilibrium constant. They only reduce time of reaching the equilibrium and increase the rate of forward and back reaction.

Enzymes Substance that acts as a catalyst in living organisms, regulating the rate at which life's chemical reactions proceed without being altered in the process. Enzymes catalyze all aspects of cell metabolism, including the digestion of food, in which large nutrient molecules (including proteins, carbohydrates, and fats) are broken down into smaller molecules; the conservation and transformation of chemical energy; and the construction of cellular materials and components. Enzymes are mainly proteins. In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, called the products.

Like all catalysts, enzymes work by lowering the activation energy (Ea) for a reaction, thus dramatically increasing the rate of the reaction. Most enzyme reaction rates are millions of times faster than those of comparable un- catalyzed reactions. As with all catalysts, enzymes are not consumed by the reactions they catalyze, nor do they alter the equilibrium of these reactions. However, enzymes do differ from most other catalysts by being much more specific. Enzymes are known to catalyze about 4,000 biochemical reactions.

Structure of proteins: Most all enzymes are proteins; many depend on a nonprotein cofactor, either a loosely associated organic compound (e.g., a vitamin; see coenzyme) or a tightly bound metal ion (e.g., iron, zinc) or organic (often metal- containing) group. The enzyme-cofactor combination provides an active cpnfiguration, usually including an active site into which the substance (substrate) involved in the reaction can fit. Many enzymes are specific to one substrate.

Mechanism of enzyme’s action: "Lock and key" model Enzymes are very specific, and it was suggested by Emil Fischer in 1894 that this was because both the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another. This is often referred to as "the lock and key" model. However, while this model explains enzyme specificity, it fails to explain the stabilization of the transition state that enzymes achieve. The "lock and key" model has proven inaccurate, and the induced fit model is the most currently accepted enzyme-substrate-coenzyme figure.

Substrate entering active site of enzyme Enzyme/su bstrate complex Enzyme/pro ducts complex Products leaving active site of enzyme Substrate Active site Products

Enzymes are classified by the type of reaction they catalyze: 1.Oxidation-reduction 2.Transfer of a chemical group 3.Hydrolysis 4.Removal or addition of a chemical group 5.Isomerization 6.Polymerization Influence on the activity of enzymes: 1. Enzyme activity can be affected by other molecules. Inhibitors are molecules that decrease enzyme activity; If a competing molecule blocks the active site or changes its shape, the enzyme's activity is inhibited. If the enzyme's configuration is destroyed (denaturated), its activity is lost.

Activators are molecules that increase activity. Many drugs and poisons are enzyme inhibitors. 2. Activity is also affected by temperature As is true for most chemical reactions, the rate of enzyme- catalyzed reactions generally increases with temperature, within the temperature range in which the enzyme is stable and retains full activity. The rate of most enzymatic reactions approximately doubles for each rise in temperature. Although enzyme-catalyzed reactions often appear to have an optimum temperature, the peak in such a plot of catalytic activity. Looking at the graph slope goes down it is temperature results because enzymes, being proteins, are denatured by heat and become inactive as temperature is raised beyond a certain point. Most enzymes are inactivated at temperatures above about 55 to

Some are quite stable and retain activity at much higher temperature.Some are quite stable and retain activity at much higher temperature.

3. Chemical environment (pH). 4. The concentration of substrate. Application of ensymes: Some enzymes are used commercially, for example, in the synthesis of antibiotics. In addition, some household products use enzymes to speed up biochemical reactions (e.g., enzymes in biological washing powders break down protein or fat stains on clothes; enzymes in meat tenderizers break down proteins, making the meat easier to chew).

The optimal meaning of рН for enzymes EnzymeSubstrateрН  -fructofuranozydaza Urease Papain Pepsin Arginase Saccharose Urea Protein Arginine 4,5-6,6 6,7 5,0 1,5-2,0 9,5-9,9

Using enzymes in medicine The actionThe name They make better the digestion function Pepsin, mezym, phestal, pancreatyn AntiviralDezoxy-ribonuclease AntisepticTrypsin, lidaza, hemotrypsyn