The rate of a reaction is the amount of chemical change occurring per unit time. It can be expressed as the decrease in concentration of a reactant or increase in concentration of a product over time. The rate is influenced by temperature, concentration of reactants, nature of reactants, presence of catalysts, and radiation. Reactions can be zero order, first order, or second order depending on how the rate depends on the concentration of reactants. The order and molecularity of a reaction are different concepts. Chain reactions involve initiation, propagation, and termination steps that regenerate reactive intermediates to sustain the reaction.

1. Rate of the Reaction
The rate of reaction i.e. the velocity of a reaction is the
amount of a chemical change occurring per unit time.
The rate is generally expressed as the decrease in
concentration of a reactant or as the increase in
concentration of the product.
If C the concentration of a reactant at any time t is, the
rate is –dC/dt or if the concentration of a product be x
at any time t, the rate would be dx/dt .
Unit of Rate of Reaction is moles/litre/second.

2. Factors influencing the rate of reaction
(i) Temperature – For an Increase of temp to 100o rate may
doubled or trippled
(ii) Concentration of the reactants : Number of reacting
molecules undergo effective collision may increase and
rate enhances
(iii)Nature of reactants : Nature of bonds in reacting
molecules. Less bond rearrangement faster the reaction.
(iv) Catalysts enhances rate by descreasing the activation
energy
(v) Radiation – Rate of photochemical reactions enhanced
by light absorption

3. Order of the Reaction

4. Order of the Reaction

5. Molecularity of a Reaction

6. Differences between Order and Molecularity
For Elementary Reactions

7. Zero Order Reactions

8. Zero Order Reactions

9. First Order Reactions

10. Or

12. Examples of First Order Reactions

13. Second Order Reactions

16. Units of Rate Constant

17. Units of Rate Constant

19. The rate law expression for zero order reaction is,
[A]t= −kt[A]0
Where [A]0 and [A]t are the concentration of the reactant at time t =0 and
after time t respectively.
Half Life Period of a Zero Order Reaction

20. Half Life Period of a First Order Reaction

21. Half Life Period of a Second Order Reaction

24. Theory of Reaction Rates

33. Transition State Theory or Activated Complex
Theory or Theory of Absolute Reaction Rates

34. PE diagram for Exothermic Reaction PE diagram for Endothermic Reaction

36. Rate
Constant = K

41. Complex Reactions

42. Classifications

49. Reaction Mechanism

51. The steady-state approximation is a method used to derive a rate
law. The method is based on the assumption that one
intermediate in the reaction mechanism is consumed as quickly as
it is generated. Its concentration remains the same in a duration of
the reaction. Thus, the system has reached a steady-state.
In cases where the reactants are investigated under such conditions
that the slowest rate-determining step does not exist, one assumes
the steady-state approximation, for the transient, or short-lived,
intermediate species.
Reactants I1 → I2 → I3……..In
The rate of formation of an intermediate is equal to the rate of its
decomposition so that
Steady state approximation is used to check the consistency of
the rate law.
Steady State Approximation

52. Chain Reactions
The main types of steps in chain reaction are of the following types.
1. Initiation (formation of active particles)
2. Propagation (may comprise several elementary steps in a cycle,
where the active particle through reaction forms another active particle
which continues the reaction chain by entering the next elementary
step).
3. Termination with product formation
A chain reaction is a sequence of reactions where a reactive product
or by-product causes additional reactions to take place.
Eg: The pyrolysis (thermal decomposition) of acetaldehyde
CH3CHO (g) → CH4 (g) + CO (g)

53. Chain initiation: The chain is initiated (started) by UV light breaking a
chlorine molecule into free radicals.
Cl2 → 2Cl.
Chain propagation reactions : These are the reactions which keep the
chain going.
CH4 + Cl. → .CH3 + HCl
.CH3 + Cl2→ CH3Cl + Cl.
Chain termination reactions: These are reactions which remove free
radicals from the system without replacing them by new ones.
2Cl. → Cl2
.CH3 + Cl. → CH3C l
.CH3 + .CH3→ CH3 CH3
Photochemical Chlorination of methane
CH4 + Cl2 → CH3Cl + HCl

54. The reaction H2 + Br2 → 2 HBr proceeds by the following mechanism:[
Initiation
Br2 + hν → 2 Br• (photochemical) each Br atom is a free radical,
representing an unpaired electron.
Propagation (here a cycle of two steps)
Br• + H2 → HBr + H•
H• + Br2 → HBr + Br•
(The sum of these two steps corresponds to the overall reaction H2 +
Br2 → 2 HBr, with catalysis by Br•
Termination
H. + Br. → HBr
2 Br• → Br2
Chain Reaction : Hydrogen Bromine Reaction