Reaction kinetics

Definition of reaction kinetics, law of mass action, rates of reaction- zero, first, second, pseudo zero & pseudo first order reaction, molecularity of reaction, determination of reaction order- graphic method, substitution method, half life method.

1. DRUG STABILITY-I CHEMICAL KINETICS Presented By: Mahewash Sana A. Pathan

2. CONTENTS: Reaction kinetics Law of mass action Rate of reaction Molecularity of reaction Order of reaction Determination of reaction order

3. Reaction kinetics/chemical kinetics “ Chemical kinetics is the branch of chemistry that is concerned with understanding the rates of chemical reactions” In 1864, Peter waage and Cato Guldberg pioneered the development of chemical kinetics by formulating law of mass action.

4. LAW OF MASS ACTION “ The rate of chemical reaction is proportional to the product of the molar concentration of the reactants each raised to a power equal to the number of molecules of the substance undergoing reaction.” mA+ nB Product Rate = K [A]m [B]n

5. RATE OF REACTION The speed or rate of a chemical reaction is the change in concentration of a reactant or product per unit time. Rate = dc/ dt

6. Molecularity of reaction Molecularity is number of molecules or atoms that must collide to give products. Stoichiometric equation is essential to decide molecularity . It must be small number & cannot be a fraction or zero.

7. ORDER OF REACTION “Order of reaction is defined as the number of concentration terms on which the rate of reaction depends when determined experimentally.” OR “ power dependence of the rate of reaction on the concentration of each reactant.” Types: Zero order reaction First order reaction Second order reaction Pseudo zero order reaction Pseudo first order reaction

8. Zero order reaction A reaction in which the rate does not depend on the concentration terms of the reactants. -dc/ dt = K0 Or - dA / dt = K0 E.g. Colour loss of liquid multisulfonamide preparations Oxidation of vit -A in oily solution Photochemical degradation of chlorpromazine

9. FIRST ORDER REACTION A reaction in which the rate of reaction depends on the concentration of one reactant. -dc/ dt = K 1 C E.g. decomposition of hydrogen peroxide catalysed by KI.

10. SECOND ORDER REACTION A reaction in which the rate depends on the concentration terms of two reactants each raised to the power one. Rate= K2 [A] [B] E.g. Alkaline hydrolysis of esters, hydrolysis of chlorbutanol

11. PSEUDO ZERO ORDER REACTION A reaction which may be a first order but behaves like a zero order, depending on experimental conditions. E.g. Suspensions -d [A]/ dt = K1 [A]

12. PSEDO FIRST ORDER REACTION A reaction which is originally a second order reaction but is made to behave like a first order reaction. E.g. Hydrolysis of ester catalyzed by H+ ions, base catalyzed oxidative degradation of prednisolone in aqueous solution -dc/ dt = K2 [A] [B]

13. DETERMINATION OF REACTION ORDER Graphic method Substitution method Half life method Initial rate method Software tools

14. 1. GRAPHIC METHOD The kinetic experiment is conducted & data are collected on the time course of changes in concentration of reactants. A graph is plotted as per the general principles of each reaction order. A straight line that gives best fit is identified, the reaction is considered to be of that order.

15. 2. SUBSTITUTION METHOD A kinetic experiment is conducted & data are collected on the time course of changes in concentration of reactants. Data are substituted in integral rate equations to get K value. The order which gives constant K value at different time periods within the experiment is identified. The reaction is of that order.

16. 3. HALF LIFE METHOD The half life are calculated for each time period in the kinetic study.