This document discusses key concepts in chemical kinetics including: 1. Chemical kinetics is the study of reaction rates and factors that affect rates. A rate is a change in concentration per unit time. 2. The rate of a reaction can be expressed as the change in concentration of reactants or products over time. Rate laws show the relationship between reaction rate and reactant concentrations. 3. Reactions can be elementary (single step) or complex (multiple steps). The overall rate of a complex reaction is governed by the slowest elementary step, known as the rate determining step. The molecularity of a reaction is defined by the number of reactant molecules in the rate determining step.

1. Course: Diploma
Subject: Applied science(Chemistry)
Unit:IV

2.  Chemical kinetics is the study of
reaction rates, the changes in
concentrations of reactants (or
products) as a function of time.
 Chemical kinetics is the study of rates
of chemical reactions, the factors
that affect reaction rates, and the
mechanisms (the series of steps) by
which reactions occur.

3.  A rate is a change in some variable per
unit of time
 The rate of reaction :the change in
concentrations or amounts of either
reactants or products per unit time.”
Consider a general reaction, A B .

4.  Rate of reaction= Rate of disappearance of A
 = Rate of appearance of B

5.  We express concentration in moles per
liter(mol/liter; mol/l; mol l-1 )
 Mole/liter sec
 Mole/liter min
 Mole/liter hour

6. A rate law shows the relationship between
the reaction rate and the concentrations
of reactants.
•For a general reaction,
the rate law has the form

7.  exponents m and n, called the reaction orders,
define how the rate is affected by reactant
concentration.
 The overall reaction order can be found by
adding the exponents on the reactants in the
rate law.

10.  Two types:
 1. Elementary reaction: single step
 2. Complex reaction: two or more step.
1. Molecularity of an Elementary reaction:
 Molecularity : the number of molecules that must collide
for the elementary reaction to occur.
 A unimolecular reaction - one reactant molecule.
 A bimolecular reaction - the collision of two reactant
molecules.
 A termolecular reaction - the collision of three
reactant molecules.

11. 1. Br2 2Br
Maleic Acid Fumaric Acid
O3  O2 + O rate = k [O3]
2. 2HI H2 + I2
Ethyl Acetate + water Acetic Acid + Ethyl
Alcohol
NO2 + NO2  NO3 + NO rate = k [NO2]2
3. 2NO + O2 2NO2
2NO + Cl2 2NOCl

12.  Each step - an elementary reaction.
 Each elementary step - its own rate.
 Some elementary steps - slow while others may be fast.
 The overall rate of a complex reaction is governed by the
rate of the slowest elementary step .
 rate determining step .
 The number of reacting species taking part in the slowest
elementary step decides the Molecularity of the reaction.
(i.e., rate determining step)

13.  For example, the reaction between nitric oxide and
hydrogen is a complex reaction.
2 NO + H2→ N2 + 2 H2O
This reaction is supposed to take place in the following
two steps.
Step 1 : 2 NO + H2→ N2 + H2O2(slow)
Step 2 : H2O2 + H2→ 2 H2O (fast)
step 1 : rate determining step.
 the Molecularity of the slowest step - 3 and is actually
taken as the Molecularity of the overall reaction.
 Molecularity and order of a reaction may be similar in
some cases, yet the two are quite different from each
other.

14.  Total no. of molecules, atom : Molecularity of reaction.
 Sum of the power to which the concentration are raised- order
of reaction.
 2A + B products
 Molecularity of the reaction – 3
 Reaction order – 3
 Molecularity and order for an elementary reaction – equal.

15. Molecularity of a reaction Order of a reaction
1. number of reacting species
undergoing simultaneous
collision in the reaction.
1. sum of the powers of the
concentration terms in the rate
law expression.
2. theoretical concept 2. determined experimentally.
3. Whole number. 3. fractional values .
4. cannot be zero. 4.can be zero.
5. does not tell us anything
about the mechanism of a
reaction.
5. slowest step in the mechanism
and hence it gives some clue
about mechanism of the
reaction.

16.  aA bB
-Δ[A]/Δt = k[A]1

23. Example

26. Example

28. Zero order rate laws

30. References
1. Essentials of physical chemistry by Bahl and tuli