Introduction to Reaction Engineering
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Introduction to Reaction Engineering
CHEMICAL KINETICS
The branch of physical chemistry which deals with
the rate of chemical reactions and the mechanism
through which they occur is called chemical
kinematics.
CLASSIFICATION OF CHEMICAL
REACTION
(1) Homogeneous and Heterogeneous
reaction
(2) Catalytic and Non-Catalytic reaction
(3) Based on Molecularity of Reaction
(4) Exothermic and Endothermic reaction
(5) Based on order of reaction
(6) Reversible and Irreversible reaction
Rate of chemical reaction
Rate of reaction can be define based on unit volume of
reacting fluid in case of homogeneous system based on
unit mass of solid in case of fluid-solid system and so on.
select one reaction component for consideration and
define the rate in terms of this component i.
If the rate of change in number of moles of this
component due to reaction is dNildt, then the rate of
reaction in its various forms can be determioned
Order of a reaction
The sum of the powers of concentration terms in rate
equation is known as order of reaction.
(or)
Order of given with respect to all participating reactants
in which case it is called order of reaction.
Consider a reaction mA + nB product
Rate eq (R)=k[A]m[B]n
Order = m + n
Zero Order Reaction
When the reaction rate is independent of
concentration of the reacting substance, it depends on
the zero power of the reactant and therefore is zero
order reaction.
The unit of K is concentration time-1
with typical units of mole L-1 s-1.
Half-life is given by equation;
t1/2 = C0/2k
The unit of K is concentration time-1
with typical units of mole L-1 s-1.
Half-life is given by equation;
t1/2 = C0/2k
First order Reaction
First order reaction consists of one reactant.
A product
R = k[A]1
Rate constant for first order reaction:
k = 2.303 * log(a/a-x)
t
where, a=initial conc. of A in mol/lit
a-x=change in initial conc. of A in
mol/lit in time interval ’t’
Half Life Time (t1/2)
Time taken for initial conc. of reactant to become half of
its volume
k= 0.693/t1/2
t1/2=0.693/k
Units of rate constant for first order reaction: sec-1
Examples of first order reaction
Thermal decomposition of N2O5
N2O5 N2O3+O2
Hydrolysis of hydrogen peroxide
H2O2 H2O +1/2O2
Decomposition of Calcium carbide
CaCO3 CaO + CO2
Second Order Of Reaction
Second order reaction consists of two reactants.
2A Products
A+B Products
Rate = k[A]2 ; n=2
Rate = k[A][B] ; n=1+1=2
Half Life Time
Half life time for second order reaction:
t1/2 = 1/a n-1
=1/a as n=2
Units:
k = mol -1 lit sec -1 (or)
= conc-1. sec -1
Examples Of Second Order Reaction
Thermal decomposition of Nitrous oxide
2N2O 2N2 +O2
Decomposition of NO2
2NO 2NO +O 2
Decomposition of Hydrogen Iodide
2HI H 2 + I2
Third Order Reaction
Third order reaction includes three reactants
3A Products
r = k [A] 3
A+B+C Products
r = k [A] [B] [C]
2A+B Products
r = k [A]2 [B]
Molecularity of the reaction
The number of ions or atoms or molecules involve in the
rate determining step or rate limiting step is called
molecularity .
The molecularity will be only whole number and never
be zero or fraction.
Molecularity can be determined by reaction mechanism
but it cannot be determined experimentally.
In no chemical reaction reactants are directly converted
into products in single step, initial reactants are converted
to final products occurred by different internal steps .
These steps are called elementary step reactions .
The sequential representation of elementary reaction step
in overall chemical reaction is called reaction mechanism.
Example:
A B
Reaction Mechanism:
A 2x
2x 2y
2y B (slow)
Among all three steps there is a elementary step withleast reaction . This step is called rate determining step orrate-limiting step.
Molecularity cannot exceed ‘3’ . If one molecule is
involved in elementary reaction then the reaction is uni-
molecular in nature.
CH2 CH2
2C2H4
CH2 CH2
In bimolecular reaction the two reactant
molecules(same or different) participate to cause a
chemical change.
2HI H2 + I2
In tri-molecular there will be collision of three
molecules.
2NO + O2 2NO2
Molecularity
It is defined as number of
molecules or atoms or
ions participating in rate
determining step.
It is a theoretical quantity,
which can be determined
through reaction
mechanism.
It is always whole
number or integer. It can
never be fraction or zero.
Order of reaction
Order is defined as sum of
powers of concentration
terms in a rate equation.
Order of reaction can be
determined
experimentally.
Order of reaction is zero ,
fraction , integer & whole
number.
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