LECTURE 6 A chemical kinetics of biological processes associate. prof. Yevheniya B. Dmukhalska

Plan  Reaction rate.  Complex reactions: parallel, series, conjugate, convertible, chain.  Rate Law of chemical reaction. Concentration and temperature of reactants dependence rate.  Catalysts. Homogeneous and heterogeneous catalysis.  Enzymes as biological catalysts.

Definition Chemical kinetics is that branch of chemistry, which deals with the study of the the rates of chemical reactions, the factors affecting the rates of the reactions and the mechanism by which the reactions proceed.

4 Classification of chemical reactions according to the quantity of stages (phases). Simple reactions go in a one elementary chemical act Complex(compound) reactions go in several stages

Chain reactions Primary process – chain initiating step (stage): h  Cl 2 === 2С1. chlorine molecule absorbs one quantum of light (h  ) and dissociates to give Cl atoms. Secondary process – chain propagating step (stage): 1. Cl. + Н 2 = HCl + H. 2. H. + Cl 2 = HCl + Cl Third process – chain terminating step (stage): Сl. + Cl. = Сl 2 H. + Cl. = HCl H. + H. = Н 2

Parallel reactin are concurrent chemical reactions with at least one common reactant. For example: Phenol with nitric acid, so have been formed ortho-, and pair-nitrophenol.

Series reactions are ( are reaction which products firs step ( stage ) are reactants for second step ( stage ): A  B  C  D  …. C 18 H 32 O 16 + HOH = C 12 H 22 O 11 + C 6 H 12 O 6 Raffinose disaccharide monosaccharide C 12 H 22 O 11 + HOH = C 6 H 12 O 6 + C 6 H 12 O 6 Monosaccharides

Reversible the reactions These two reactions are occurring simultaneously, which means that the reactants are reacting to yield the products, as the products are reacting to produce the reactants. k 1 A + B + C = A 1 + B 1 + C 1 k 2

Classification of the chemical reaction according to the quantity of the reacting phases Homogeneous: N 2 (g) + H 2 (g) → NH 3 (g) Heterogeneous: Mg (s) + HCl (l) → MgCl 2 (l) + H 2(g) Topochemical reactions (occurs at the boundary of solid phases) СuO + C Cu + CO

14 Classification of chemical reactions according to the molecularity Unimolecular (monomolecular): Н 2 СО 3 → Н 2 О + СО 2 Bimolecular: CuO + CO → Cu + CO 2 Termolecular: 2 NO + O 2 = 2 NO 2 The molecularity of an elementary reaction is the number of molecules coming together to react

The rate of a reaction is the speed at which a reaction happens. reaction

The rate of chemical reaction is the change the concentration of any one of the reactants or products per unit of time. For example A + В  С concentrations of the participants are [А], [В], and [С]. The rate of consumption of the reactant А is the rate of formation of the product С is:

The Kinetic curves of the rate reaction’ mean value Initial substances Product of reaction Сonc entra tion t α Veritable (true) rates: dC = v·dt;v = tgα

Reaction Rate and Stoichiometry C 2 H 5 Br (aq) + H 2 O(l)  C 2 H 5 OH(aq) + HBr(aq) we know In general for aA + bB  cC + dD Reaction Rates

FACTORS WHICH INFLUENCE RATES OF CHEMICAL REACTIONS 1.Concentration of the reacting species. 2.Temperature of the system. 3.Nature of the reactants and products. 4.Presence of a catalyst. 5.Surface area. 6.Exposure to radiation.

1.Concentration of the reactants. The concentration of the reactants is maximum at the start of the reaction and, therefore, the rate of change is also maximum. As the concentration of the reactants decreases, it brings about the corresponding decrease in the rate of the reaction. The rate of a reaction is directly proportional to the concentration of the reactants. Rate law expression

THE LAW OF MASSACTION (The Rate laws) The rate of reaction is proportional to the concentrations of reactants raised to а power. A + B = C;  = k[A][B] equation of the rate laws 3H 2 +N 2 =2NH 3 ;  = k[H 2 ] 3 [N 2 ] The coefficient k is called the rate constant for the reaction or velocity constant. The rate constant is independent of the concentrations but depends on the temperature. [A] = [B] = 1 mole/liter, then rate = k

2. Nature of reactants and the products. A chemical reaction involves the breaking of old bonds and formation of new bonds. The reactivity of a substance can therefore, be related to the ease with which the specific bonds are broken or formed and the number of such bonds involved. For example:

Mechanisms with an Initial Fast Step 2NO(g) + Br 2 (g)  2NOBr(g) Consider the following mechanism

3. Temperature of the system. In general, an increase in the temperature increases the rate of almost all chemical reactions. This effect is observed for exothermic as well as for endothermic reactions. A general approximate rule for the effect of temperature on reaction rates is that the reaction rate for most of the chemical reactions becomes almost double, for every 10 0 C rise in temperature. This is also called temperature coefficient. It is the ratio of rate constants of the reaction at two temperature differing by 10 0 C Thus

or Vant-Hoff’s rule : where T 2  T 1 Temperature coefficient of reaction:

Arrhenius Equation It is a well-known fact that raising the temperature increases the reaction rate. E a = activation energy R = [ J · mol -1 · K -1 ] T = absolute temperature in degrees Kelvin A = pre-exponential or frequency factor A = p · Z, where Z is the collision rate and p is a steric factor. Z turns out to be only weakly dependant on temperature. Thus the frequency factor is a constant, specific for each reaction.

4. Presence of a catalyst. A catalyst is a substance which influences the rate of a reaction without undergoing any chemical change itself. It has been observed that many reactions are made to proceed at an increased rate by the presence of certain catalysts. 5. Surface area. The large the surface area of the reactants, the faster is rate of reaction. It has been observed that if one the reactants is a solid, then the rate of the reaction depends upon the state of sub-division of the solid. 6. Exposure to radiation. In some cases, the rate is considerably increased by the use of certain radiations. For example, reaction of hydrogen and chloride takes place very slowly in the absence of light. However, in the presence of light, the reaction takes place very rapidly.

Catalysis. 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.

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

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 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 3. Chemical environment (pH). 4. The concentration of substrate.

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