Chemical Kinetics

Types of complex chemical reaction, Reversible or opposing reactions, Consecutive and parallel reactions, Thermal chain reaction : Hydrogen and Bromine reaction

1. 02/10/2023 SYBSc Sem III Paper II - Chemical Kinetics Dr. Aqeela Sattar, Royal College

Chemical Kinetics
Reference books : Text Book of Physical chemistry – P. L. Soni
Advanced Physical chemistry 37
th
Edition – Gurdeep Raj
Chemical Kinetics –Laidler

Introduction to Reaction Mechanism:
Simple / Elementary reaction: Chemical reaction that take place in single step
without involving any intermediate step are called simple reaction.
A → B ( Product )
Complex Reaction : A complex reaction takes place when the reactants are
converted into products in more than one step. It involves multiple steps and
intermediates , a series of simple reaction that occurs in sequence leading to the
overall transformation of reactants into products.
Mechanism of reaction: The step-by-step sequence of elementary reactions by
which overall reaction occurs is called reaction mechanism. Its detailed pathway
consisting of a sequence of chemical changes on molecular level.
The reaction mechanism is a theoretical proposal that explain the reaction based
on known facts and can change if new experimental or theoretical data comes up.
Reaction mechanisms usually involve diagrams. These show a few things.
• The reactants.
• The products.
• The intermediates.
• The movement of electrons.
• The breaking and formation of bonds.
Intermediates are highly reactive, short-lived compounds that exist for a fraction
of a second in a chemical reaction. Once formed, they quickly react and turn into
more stable compounds.
Thus, a reaction intermediate is a chemical species that is formed in one
elementary step and consumed in a subsequent step.

COMPLICATING OR DISTURBING FACTORS IN REACTION KINETICS:
Only few reactions are simple i.e 1
st
, 2
nd
or 3
rd
order reactions. Most of the
reactions are complicated because of several reactions which take place
simultaneously along with the main reaction. Such reactions are known as
complicating (disturbing) factors.

2. 02/10/2023 SYBSc Sem III Paper II - Chemical Kinetics Dr. Aqeela Sattar, Royal College

Types of complex chemical reactions:
1. Competing or Side reactions or parallel reactions:
When a reactant [A] undergoes two or more independent reactions at the same
time, it is a case of side reaction.
Consider a reaction,

The reaction I is known as main reaction and the reaction II is known as side
reaction. It is possible to convert any of the side reactions into main reaction
by adjusting the favourable experimental conditions.




A
C 10%
B 90%
I
II

3. 02/10/2023 SYBSc Sem III Paper II - Chemical Kinetics Dr. Aqeela Sattar, Royal College

Example: 1 Following side reactions and main reaction occur when ethanol
is made to react under different conditions.

Example : 2

Example : . 3 Nitration of phenol


Example : 4 Nitration of benzoic acid. It gives a mixture of ortho, meta and
para nitrobenzoic acid.

4. 02/10/2023 SYBSc Sem III Paper II - Chemical Kinetics Dr. Aqeela Sattar, Royal College

2. Opposing reactions :
There are certain reactions in which the product of a chemical change react to
form the original reactants. Such reactions are known as Counter /
Reversible/ Opposing reactions.
In opposing reactions the net rate of the reaction will be influenced by both the
forward and backward rates and thus causes a serious disturbance in the
measurement of reaction rate.



The different types of reversible reactions are:

a) Monomolecular reaction opposed by a monomolecular reaction
represented as




Eg. i. Conversion of ammonium thiocyanate into urea




ii. Muta-rotation of α -D-glucose into β-D-glucose
iii. trans-Azobenzol to cis Azobenzol



A B
k1
k-1
NH4CNO CO(NH2)2
k1
k-1

5. 02/10/2023 SYBSc Sem III Paper II - Chemical Kinetics Dr. Aqeela Sattar, Royal College

b) Monomolecular reaction opposed by a bimolecular reaction represented as






Eg. i. Thermal decomposition of PCl5



ii. Decomposition of ammonium chloride



c) Bimolecular reaction opposed by a bimolecular reaction represented as



d) Bimolecular reaction opposed by a unimolecular reaction represented as



3. Consecutive reactions :
In such reactions the products obtained in the 1st stage react with each other
or with the reactants to form new products. These reactions occur in sequence
and are represented by




A B + C
k1
k-1
PCl5 PCl3 + Cl2
k1
k-1
NH4Cl NH3 + HCl
k1
k-1
A
B
k1 k2
C

6. 02/10/2023 SYBSc Sem III Paper II - Chemical Kinetics Dr. Aqeela Sattar, Royal College


As the reaction proceeds
a) concentration of A falls continuously
b) concentration of C rises continuously
c) Concentration of B rises to a maximum
& then decreases with time.



Example 1: Hydrolysis of
diethylsuccinate in two stages


i. (CH2COOC2H5)2 + H2O HOOCCH 2 - CH2COOC2H5 + C2H5OH
ii. HOOCCH2 - CH2COOC2H5 + H2O HOOCCH 2 - CH2COOH + C2H5OH



Example 2 : Decomposition of ethylene oxide

Example 3 : Radioactive disintegration

7. 02/10/2023 SYBSc Sem III Paper II - Chemical Kinetics Dr. Aqeela Sattar, Royal College

Example 4 :


Chain Reactions :
A chain reaction takes place in a series of successive steps involving reactive
intermediates. The reactive intermediates are generated and consumed
continuously and are called chain carriers.
Step involved in chain reaction :
i) Chain initiation – In this step reactive intermediate atom or free radicals are
produced.
ii) Chain propagation – In this step intermediate react with other reactants to
produce new intermediates. Chain propagation is much faster & most of the
products are formed in these steps.
iii) Chain transfer : In some cases in chain propagation steps , new chain
carriers are generated. Thus initial chain carrier generates a new chain carrier by
reaction with some existing molecular species.
iv) Chain inhibition : In this step one of the reactive species attacks the product
molecules thus retarding the reaction. The chain inhibition may complete when
the concentration of product becomes significant.
v) Chain termination / breaking : In this step the active intermediate species
are removed or destroyed and are no longer available for chain initiation and
propagation.
Eg. Reaction between hydrogen and bromine is a thermal chain reaction
H2 + Br2 2HBr ( 200
0
C – 300
0
C )

The reaction is initiated by dissociation of Br2 molecule to atoms
i) Chain Initiation : Bromine molecules are broken down into individual
Br atoms by absorbing energy ( light or heat energy). This is known as
homolytic cleavage.
Br2 Br* + Br* -----(1)



k1

8. 02/10/2023 SYBSc Sem III Paper II - Chemical Kinetics Dr. Aqeela Sattar, Royal College

ii) Chain Propagation : In this step bromine atoms react with hydrogen
molecules to form hydrogen bromide and hydrogen radicals. Reaction
(1) is followed by a slow reaction (2),
Br* + H2 HBr + H* ---- (2)
And rapid reaction (3)
H* + Br2 HBr + Br ---- (3)
The intermediates in a chain reaction are called chain carriers.
iii) Chain Inhibition : In this step reactive species attacks the product
thereby decreasing the rate of its formation.
H* + HBr H2 + Br* ---- (4)

iv) Chain Termination / breaking :
Br* + Br* Br2 ---- (5)
H* + H* H2 ---- (5)

where k1 , k2, k3 , k4 and k5 , k6 represents specific rates of all the
reaction








********



k2

k3
k4
k5  
  ][][
22
22
HBr H k -BrHk HBrk HBr
dt
d
is HBr of formation ofRate Overall
BrHk HBrk HBr
dt
d
by given is HBr of formation of Rate
432
32
+=
+=
k6