1. Chemistry

2. Chemical Kinetics – 1

3. Session Objectives Rate of reaction Factors affecting rate constant
Molecularity
Order of a reaction
Kinetics of zero order reaction
Kinetics of first order reaction
Pseudo first order reaction

4. Rate of reaction For a reaction, The rate may be expressed as
Unit of rate of reaction =

5. Rate of reaction For general reaction Rate of reaction
According to law of mass action
Where k is rate constant.

6. Rate constant Measure of rate of reaction
Different for different reactions
Independent of concentration of reactants
Depends on the temperature

7. Graphical representation of rate
The slope of graph at any instant gives us the rate of reaction at that moment and is called instantaneous rate of the reaction.
The average of these slopes over a period gives us rate of the reaction which is called as average rate of the reaction.

8. Illustrative Example
The reaction, 2N2O5(g)  4NO2(g) + O2(g), takes place in a closed
container, it was found that the concentration of NO2 increases by
1.6 x 10-2 moles/litre in four seconds. Calculate the rate of the
reaction and the rate of change of the concentration of N2O5.
Solution:
2N2O5(g)  4NO2(g) + O2(g)
rate of change of concentration of N2O5

9. Rate law
It is a mathematical expression that gives the true rate of reaction in terms of concentration of the reactants which actually influence the rate.
Rate expression is experimentally determined.
Consider a general reaction

10. Molecularity For an elementry reaction, molecularity
is the total number of molecules taking
part in reaction.
It has no significance for complex reaction

11. Order of reaction
The number of molecules which actually react and undergo a change in concentration is known as order of reaction. For example,the dissociation of N2O5 is a first order reaction
There can be a reaction of fractional order
Consider a general reaction

12. Molecularity of Reaction
Molecularity vs. Order
Order of reaction is for overall reaction.
The overall molecularity of complex reaction has no significance. Individual step has its own molecularity.
It can even have fractional values.
It is always a whole number.
It is determined experimentally.
It is a theoretical concept.
Order of reaction can be zero.
Molecularity of reaction
Cannot be zero.
It is the sum of the power of concentration terms on which the rate of reaction actually depends or it is the sum of powers of the concentration terms in the rate law equation.
It is the number of atoms, ions or molecules that must collide with one another simultaneously so as to result into a chemical
reaction.
Order of Reaction
Molecularity of Reaction

13. Illustrative Example
For a reaction, aA + bB + cC  proucts, rate expression can be expressed as rate = k[A]1[B]-3/2[c]1/2, Write the order of the reaction with respect to ‘A’ , ‘B’, and ‘C’. What is the overall order of the reaction?
Solution :
Order of the reaction w.r.t. ‘A’ = 1
Order of the reaction w.r.t. ‘B’ = -3/2
Order of the reaction w.r.t. ‘C’ = 1/2
Overall order of the reaction = 1 + (-3/2) + 1/2 = 0

14. Illustrative Example
The rate of the reaction, A  P, increases by a factor of 5.18 when the concentration of A is tripled. What is the order of A?
Solution:
Assuming the order of the reaction w.r.t. ‘A’ is n, then
the rate expression can be written as
or n = 1.5

15. Illustrative Problem For the reaction A + B Products
The following initial rates were obtained at various given intial concentrations
S. No [A] [B] Rate (mol lt–1 sec–1)
Write rate law and find the rate constant of the above reaction.

16. Solution

17. Zero order reaction A Product Rate of reaction
where k = rate constant for zero order reaction
[A] =- kt + C where C = Integration Constant.
When t = 0, then [A] =A0 hence C = A0

18. Zero order reaction
Half life of a reaction :- Time taken for the reactant concentration to decrease to one half of its initial value
Half life a initial concentration

19. Zero order reaction Half life a initial concentration

20. Zero order reaction

21. First order reaction
A Product
Unit of rate constant is time-1

22. First order reaction
Half life of first order reaction does not depend upon initial concentration
Amount of substance left after
n half lives

23. First order reaction
For 75% completion of reaction

24. First order reaction
Slope of line =

25. Illustrative Example
For a first order reaction, A  P, half life is 69.3 min. Calculate the time taken to decay the amount of ‘A’ to 80%.
Solution:

26. Alternate Solution

27. Illustrative Example
The half-life of a substrate in a certain enzyme-catalyzed first order reaction is 138 s. How long will it require for the initial concentration of substrate, which was mmol/L, to fall to mmol/L?
Solution:

28. Pseudo - first order reaction
Reactions which are not truly of the first order but under certain conditions reactions become that of first order are called pseudo unimolecular reaction.
For example: Hydrolysis of ester in presence of acid
CH3COOC2H5 + H2O  CH3COOH + C2H5OH
From this reaction, the rate expression should be
r = k [ester] [H2O]
Since, hydrolysis takes place in the excess of H2O and concentration change of H2O is negligible practically.
therefore, r = k’ [ester]
Where k’ = k[H2O].

29. Thank you