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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