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.