Kinetics and Drug Stability Prepared By- Shubham Dubey Department of Pharmaceutical Chemistry Govt. Holkar science college Indore

1. Kinetics and Drug Stability
Prepared by
Shubham Dubey
Department of Pharmaceutical Chemistry
Govt. Holkar Science college Indore

2. Introduction

3. Introduction
(continued..)

4. Rate of reaction
 The rate of chemical reaction is defined as the velocity at which a reactant or
reactants undergo chemical change. The rate of reaction therefore can be
measured by measuring the change in the concentration of a reactant or product in
a particular period of time.
 The rate of reaction is given by-
 The + or – sign indicates an increase or decrease respectively in concentration dc
within a time interval dt.

5. Rate constant and order of reaction
 According to the law of mass action, the rate of chemical reaction is proportional
to the product of the molar concentration of the reactants each raised to power
usually equal to the number of molecules, a and b, of the substance A and B
undergoing the reaction.
 Thus in the reaction
Rate of reaction ∝ [ A]a [ B]b

6. Continued..
 K in the above equation is the rate constant also known as specific rate
constant.
 Order of reaction is the sum of powers of the concentration term involved in the
rate equation.
 Thus the order of above reaction is (a+b)
 The order of reaction determines the way in which concentration of reactant or
reactants influences the rate of a chemical reaction.

7. Molecularity of a Reaction

8. Molecularity of a reaction (continued...)

9. Zero order reaction
 If the rate of reaction is independent of the concentration of the reacting species,
the reaction is said to be a zero order reaction.
 The rate of zero order reaction is given by:
 Where, dA is the change in concentration with respect to time t, and '-' sign
indicates that the concentration is decreasing.

10. Zero Order reaction

11. Graphs of zero order kinetics

12. Half life of zero order reaction
 Half life of the chemical reaction is the time required for the initial concentration of
the reactant to get reduced to half.
 A0 =1/2At
Substituting this in the rate constant equation for zero order, we get,

13. Shelf life of a zero order reaction
 An expression of importance in the pharmaceutical field is t0.9 ie.,the time
required for the drug to decomposed by 10% ( ie, to 90% of its original
concentration). Thus ,

14. First order Reaction
 When the rate of reaction is directly proportional to the first power of the
concentration of a single reactant, the reaction is said to be of first order with
respect to the single reactant.
 In this type of reaction, the single reactant decomposes directly into one or more
products ie.,
A Products

15. First order reaction(cont..)

16. Graphs for first order reaction

17. Half life of first order reaction

18. Shelf life of first order reaction

19. Second order reaction

20. Second order reaction

21. Half life and shelf of second order reaction

22. Pseudo first order reaction
 A Pseudo first-order reaction can be defined as a second-order or bimolecular
reaction that is made to behave like a first-order reaction. This reaction occurs when
one reacting material is present in great excess or is maintained at a constant
concentration compared with the other substance.
 Example of pseudo first-order reaction is the inversion of cane sugar.
C12H22O11 + H2O → C6H12O6 + C6H12O6
Cane sugar Glucose Fructose
 When the other reactants are in excess, change in their concentrations does not affect
the reaction much, Therefore, now the reaction only depends on the concentration of
the isolated reactant. The concentrations of all the other reactants are taken as
constant in the rate law. Thus, the order of reaction becomes one.

23. Methods for determining the order of
reaction

24. Continued...

25. Important factors affecting rate of reaction
 1. Temperature
Generally the speed of many reactions can be increased two to three times with
each increase of 10 °C in temperature
The effect of temperature on reaction rate is given by Arrhenius equation which in
the exponential form is-

26. Continued...
 Expressing the Arrhenius equation in logarithmic form, we get,

27. Continued..
 2. Light-
 The rate of some photochemical reactions, which occur in presence of
light, increases with increase in the intensity of suitable light used. With
increase in the intensity, the number of photons in light also increases.
Hence more number of reactant molecules get energy by absorbing more
number of photons and undergo chemical change.
 E.g. The rate of photosynthesis is more on brighter days.

28. Continued..
 3. Solvent-
 The effects of solvents on the rate of decomposition of drugs is generally related
to the relative solubility of the reactants and the products in the given solvents.
 4. Ionic strength-
 An increase in ionic strength of the solution would tend to decrease the rate of
reaction involving interaction between oppositely charged ions and increase the
rate of reaction between similarly charges ions.
 5. Catalysis-
 Involvement on catalyst increases the rate of reaction.

29. Accelerated stability analysis

30. Prediction of shelf life
 Shelf life is the time period during which the dosage form is supposed to retains its
original qualities
 The prediction of shelf life is based on applying the Arrhenius equation which
give the effect of temperature on rate constant, k, of a chemical reaction.