2. The branch of chemistry which deals with study of rate
or speed of reaction is called as chemical kinetics
(where as Thermodynamics: which determines whether a reaction take place or not)
3. Slow reactions : These are the reaction which
proceed slowly and their rates can be
measured by conventional methods
For example inversion of Cane sugar in aqueous solution in
slow reaction
A large number of bonds have to be broken in reactant
molecules and a large number of new bonds have to be
form in the product molecules
4. Fast reactions : The rate of many reaction are too
fast to be measured by the conventional methods
these methods cannot deal with reaction whose
half lives are less than a second or so such
reactions are called as fast reactions
i) For example many ionic reaction such as neutralization of acid and
bases
ii) Organic substitution reaction
iii) The Explosive reaction of oxygen with hydrogen and hydrocarbons
The rate of reaction can be measured by employing special methods It
may be noted in case of simple ionic reaction no bonds are to be
broken the reactions are very fast
5. Reaction rate is the change in the concentration of a
reactant or a product with time
M N
rate = -
D[M]
Dt
rate =
D[N]
Dt
D[M] = decrease in concentration of M
at time t
D[N] = change in concentration of N
at time t
6. Postulates :
1. Molecules must come in physical contact (collision)
with each other
2. A collision results in reaction only if certain
threshold energy is exceeded
3. Reaction only when reactant molecules are properly
oriented
7. Factors that Affect the Reaction Rate Constant
1. Temperature: At higher temperatures, reactant molecules have
more kinetic energy, move faster, and collide more with greater energy
2. Concentrations of reactants : As the concentration of
reactants increases, number of collisions also increases
3. Surface area of a solid reactant : More area for reactants to
be in contact
4. Pressure of gaseous reactants or products : Pressure is
due to collision so increase pressure means Increased number of
collisions
8. • The rate law expresses the relationship of the rate of a
reaction to the rate constant and the concentrations of the
reactants raised to some powers.
• Lets consider a reaction
aA + bB mM + nN
• Thus rate law for a reaction is
Rate = k [A]m [B]n
- [A] & [B] represent the reactants.
- m + n is called “order of reaction”.
- k is called the rate constant.
9. Molecularity of reaction :
It is the sum of the number of molecules of
various reactant that take part in chemical
reaction is represented by a balance chemical
equation The reaction may be classified according
to their molecularity
•Uni molecular reaction: Unimolecular reaction is one in which only
one molecule of reactant is involved For example conversion of
maleic acid into fumaric acid
10. Bimolecular reaction: In which two molecules are
involved for example :
1. Inversion of Cane sugar
2. Hydrolysis of an Easter
Thus the molecularity of this reaction is two
11. The order of reaction is given by the
number of atoms or molecules whose
concentration alters during the chemical
change
OR
The order of reaction is given by the total
number of molecules are atoms whose
concentration determine the velocity of the
reaction
Order of reaction
12. • First order reaction: It is defined as one in
which the reaction rate is determined by the
variation of one concentration term only
R = KXCA
•Second order reaction: It is defined as one
in which the two concentration terms
determine the rate of reaction
R = KXCa2
R= KXCAXCB
13. • Third order reaction: A reaction is said to be third
order if three concentration terms determine its rate
R = KXCA3
R = KXCA2CB
R = KXCAXCBXCC
General order reaction : Consider the reaction
Aa + bB + cC product
Then rate of reaction α [A]a[B]b[C]c
Then the order of such reaction is equal to a + b + ------
14. Note : Order may be zero , that is rate is
independent of concentration of reactant
For example dissociation of HI into H2 and
I2 on surface of Gold
