Course: Diploma

Subject: Applied science(Chemistry)

Unit:IV

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.

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 .

Rate of reaction= Rate of disappearance of A

= Rate of appearance of B

We express concentration in moles per

liter(mol/liter; mol/l; mol l-1 )

Mole/liter sec

Mole/liter min

Mole/liter hour

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

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.

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.

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

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)

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.

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.

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.

aA bB

-Δ[A]/Δt = k[A]1

Example

Example

Zero order rate laws

References

1. Essentials of physical chemistry by Bahl and tuli