This document discusses chemical kinetics, which is the branch of physical chemistry concerned with understanding reaction rates. It defines chemical kinetics as the study of reaction rates and how reactant concentrations change over time. Reactions can occur in single or multiple steps. Kinetic measurements provide information about reaction mechanisms and whether reactions occur through one or more elementary steps. The document also discusses applications of chemical kinetics in pharmacy, defines reaction rates and how they are expressed, reaction order, molecularity, and provides an overview of zero-order reactions where the reaction rate does not depend on reactant concentration.

1. Chemical Kinetics
Presented by
Prodip Kumar Baral

2. Chemical Kinetics
• Chemical kinetics, the branch of physical chemistry that is concerned with
understanding the rates of chemical reactions.
• Chemical kinetics is the study of the rates of transformation of chemical
compounds from reactant species into products. The rate of a reaction is
defined to be the rate of decrease with time of the reactant concentration (in
number of moles or molecules per unit volume) due to chemical reaction(s),
or, equivalently, the rate of increase of the product concentration.
• Some chemical reactions go in a single step; these are known as elementary
reactions. Other reactions go in more than one step and are said to be
stepwise, composite, or complex. Measurements of the rates of chemical
reactions over a range of conditions can show whether a reaction proceeds by
one or more steps.
• If a reaction is stepwise, kinetic measurements provide evidence for the
mechanism of the individual elementary steps. Information about reaction
mechanisms is also provided by certain non-kinetic studies, but little can be
known about a mechanism until its kinetics has been investigated.

3. Application in pharmacy
• Understanding the drug degradation process
• Determination of Shelf life
• Production of more stable drug preparations, the dosage and
rationale of which may be established on sound scientific principle
• Find out the factors and optimum conditions for drug preservation
and activity, e. g. concentration, temperature, light, pH, and catalysts
• Selecting the proper container for dispensing (glass or plastic, clear or
opaque, cap liners)
• Anticipating drug excipient interactions

4. Reaction rate
• The rate of a reaction is defined in terms of the rates with which the
products are formed and the reactants (the reacting substances) are
consumed.
• The rate can then be defined as the concentration of a substance that is
consumed or produced in unit time. Sometimes it is more convenient to
express rates as numbers of molecules formed or consumed in unit time.
• r = dc/dt
• For reaction aA + bB + ------- = Products
• According to the law of mass action r =
1
𝑎
𝑑[𝐴]
𝑑𝑡
=
1
𝑏
𝑑[𝐵]
𝑑𝑡
= k[A]a[B]b--------

5. Order of Reaction
• The order of the reaction is defined as the manner in which the rate of a
reaction varies with the concentration of reactants.
• The order of a reaction determines the way in which the concentration of
a reactant or reactants influences the rate of a chemical reaction.
• The overall order is the sum of the exponents of concentration terms that
afford a linear plot.
• For the above reaction aA + bB = Products, the order of reaction = a + b

6. Molecularity of Reaction
• The molecularity of a reaction refers to the numbers of molecules,
atoms, or ions reacting in a elementary process to give the reactions.
• If only one type of molecules undergoes a change in to yield the
product , the product is said to be unimolecular.
• If two molecules undergoes to change yield the product, the reaction
is said to be bimolecular.
• Reaction that involves more than one steps ( complex reaction) may
have different molecularity and order of reaction.

9. Zero order reaction
The rate of the reaction doesn't depend on concentration of the
reagent(s).
The rate expression for chemical reaction, A B
Rate of reaction = - d[C]/dt = k
where, [C] indicates decreasing concentration of reagent & k indicates
rate constant.
Integrating of rate equation between initial concentration Co at t = 0 &
Ct, concentration after t = t we obtain,
Ct = C0 - kt

10. Zero order reaction
• When this linear equation is plotted with c on vertical axis against t
on horizontal axis, the slope of the line is equal to – k.
• Here the rate of reaction do not depend on the initial concentration
of reactants.
• T1/2 = 0.5C0/k