LECTURE A chemical kinetics of biological processes

Plan  Reaction rate.  Rate Law of chemical reaction. Concentration and temperature of reactants dependence rate.  The temperature dependence rate. The vant’-Hoff’s rule. Arrhenius’ equation.  Order and molecularity of reaction.  Compound reactions: parallel, series, conjugate, convertible, chain.  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.

The rate of chemical reaction is the change in 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:

Factors affection the reaction rate. 1. Nature of the reactants. 2. Concentration of the reactants. 3. Temperature. 4. Presence of Catalyst. 5. Surface area of the reactants. 6. Presence of light.

Nature of the reactants These reactions appear to be similar but the first is fast while the second is slow. This is because different amounts of energies are required for breaking of different bonds and different amounts of energies are released in the formation of different bonds.

Concentration of the reactants. Rate laws and rate constants. It is often found that the rate of reaction is proportional to the concentrations of the reactants raised to а power. A + B = C  = k[A][B] 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

Characteristics of rate constant  Rate constant is a measure of the rate of reaction. Greater is the value of the rate constant, factors is the reaction.  Each reaction has a definite value of the rate constant at particular temperature.  The value of the rate constant for the same reaction changes with temperature.  The value of the rate constant of a reaction does not depend upon the concentration of the reactants.  The units of the rate constant depend upon the order of reaction.

Order of reaction. The sum of the concentration terms on which the rate of а reaction actually depends as observed experimentally is called the order of the reaction. For example: A a +B b = C c  = k [A] a [B] b Order = a+b  = k

Molecularity The molecularity of an elementary reaction is the number of molecules coming together to react. In case of simple reactions (also called elementary reactions), the molecularity is simply the sum of the molecules of the different reactants as represented by the balanced chemical equation. (i) Decomposition of F 2 О 2 : F 2 О 2 = F 2 + O 2 Hence the molecularity of the reactions 1 and the reaction is called Unimolecular. (ii) Dissociation of HI: 2НI = H 2 + I 2 Hence the molecularity is 2 and the reaction is called Bimolecular. (iii) Reaction between NO and O 2 : 2NO + О 2 = 2NO 2 Неnсе the molecularity is 3 and the reaction is called Тermolecular.

Complex reactions Parallel Reversible Conjugate Chain

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

Parallel of reacting. For example: Phenol with nitric acid, so have been formed ortho- and pair -nitrophenol.

Series reactions Series the reactions are reaction which products firs step are reactants for second step: A  B  C  D  …. C 118 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 reactions which are flowing past in two parties: the forward reaction - conducts to formation reaction product and reverse reaction - decomposing reaction product on mother substances. k 1 A + B + C = A 1 + B 1 + C 1 k 2

Compound reactions. A + B = M (a) A + C = N (b) A - actor B - inducers C - promoter.

Temperature dependence The rate of reaction increases with increase of temperature. In most of the cases, the rate of reaction becomes nearly double for 10 К rise of temperature.  t 2 /  t 1 =  t 2 - t 1 /10

Arhenius equation or А - pre-exponential factor or the frequency factor; Е a - activation energy. е -Ea/RT - Boltzmann distribution  + G - activation Gibbs energy.

Presence of Catalyst. А catalyst generally increases the speed of а reaction without itself being consumed in the reaction. In case of reversible reactions, а catalyst helps to attain the equilibrium quickly without disturbing the state of equilibrium. Surface area of the reactants. For а reaction involving а solid reactant or catalyst, the smaller is the particle size i.е., greater is the surface area, the fast r is the reaction. Presence of light. Some reactions do not take place in the dark but take place in the presence of light e.g., Н 2 + С1 2 = 2НС1. Such reactions are called “photochemical reactions”

Catalyst is а substance which can change the speed of а chemical reaction without being used up in that reaction and the phenomenon is known as 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.

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

Heterogeneous Catalysis. If the catalyst is present in а different phase than that of 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

Enzymes Enzymes are proteins specialized to catalyze biological reac­tions. Some enzymes depend for activity only on their structure as proteins, while others also require one or more nonprotein components, called cofactors. The catalytically active enzyme-cofactor complex is called the holoenzyme. When the cofactor is removed, the remaining protein, which is catalytically inactive by itself, is called an apoenzyme. Cofactors are generally stable to heat, whereas most enzyme proteins (apoenzymes) lose activity on heating.