Chemical Kinetic
The
branch of physical chemistry which deals with the rate of reactions, their
mechanism, conditions and the factors influencing the rate of reactions.
Rate of Reaction:
The
rate of reaction is defined as change in concentration of reactant or product
per unit time.
Rate = Change in
Conc. of reactant of product
Time taken
Let us consider a
simple reaction.
A B
The
concentration of A decreases and that of “B” increases as time passes and hence
the rate of reaction may be equal to the rate of disappearance of A which is
equal to rate of appearance of B.
The
rate of a chemical reaction is not uniform and as the reaction going on, the
rate of formation of product become slow therefore overall rate of reaction is
the average of whole rate of reaction.
Unit of Rate of Reaction:
Rate = Dx
=
mole dm-3
= mole dm-3 sec-1
Dt Sec
 |
Determination
of Rate of Reaction:
There
are two methods for determining the rate of a reaction.
1-
Physical Method:
In this method the reaction is followed by measuring the change in
physical properties of reactants and products for e.g. PH method, colorimetric
method, spectroscopic method, polarimetric method.
2-
Chemical Method:
In this method samples are drawn from the reacting vessel at
a regular interval of time. Reaction is stopped at that particular moment by
suddenly chilling the sample. The amount of reactant and product at that time
is determined by titrating it with proper reagent.
Rate Law:
According to Law
of “Mass Action”.
“The rate of reaction
is directly proportional to the active mass of the reactant.”
Rate µ Active Mass.
Rate = K
(Active Mass)
Active
mass mean molar concentration having exponent as degree of activity or “order
of reactant”. If order is represented by “n” than Rate Law is given by.
Rate = K[R]n
Consider a rate
reaction.
aA
+ bB Product
Rate µ [A]a
[B]b
Rate = k [A]a
[B]b
Where a & b are the order of
reactant A & B respectively.
Order
of reaction =
n = a + b
Rate Constant:
“It is the rate of
reaction when concentration of reactants
are unit at a given
temperature”.
Rate = k [R]n
k = Rate
[R]n
If concentration of reactants is unit.
k
= Rate
“Unit of Rate Constant”
k = Rate
[R]
(a) If Zero order (n = 0)
k = Rate
[R]o
k =
Rate
Where
rate ml dm-3, sec-1
 |
k =
mole dm-3 s-1
(b)
If 1st order( n = 1)
k = Rate
[R]
k
= moledm-3 sec-1
moledm-3 |
k = Sec-1
(c) If 2nd order (n = 2)
k = Rate
[R]2
k = mole
dm-3 sec-1
(mole dm-3)
(mole dm-3)
 |
k = dm3
mole-1 sec-1
(d) If 3rd order (n = 3)
k = Rate
[R]3
k = mole
dm3 sec-1
(mole dm-3)3
k = mole
dm3 sec-1
(mole dm-3)
mole2 dm-6
 |
k = dm6
mole-2 sec-1
Energy Of Activation:
“The excess energy which is required
for the reaction in addition to the average energy of the molecules is called
Energy of Activation”. It is denoted by Ea and is measured in K.J/mole.
According
to the “collision theory” chemical reaction can only be possible when the
effective collision among the molecules takes place and it happens only when
the molecules of reactants it happens only when the molecules of reactants
acquire threshold energy.
“The
minimum energy which required for the reaction is called Threshold Energy”. It
is denoted by El.
The
energy graph of a reaction showing the formation of product from reactant is
given below.
Here,
Point “C” is the region where
effective collision occurs among the molecules of reactant. It is called threshold
point or transition state. The energy between point C & B is called
activation energy and denoted by “Ea”.
The energy between point B & A
is called average Kinetic energy of the reacting molecule. It is denoted by Ea.
From
the above graph we can derive following Equation.
Ea
= El
- E
The rate of reaction depends upon
the energy of activation depends upon the energy of activation of the reaction.
The reaction which have low value of energy of activation are called fast
reaction. Reactions whose energy of activation is high are called slow
reactions.
Energy
of activation depends upon average energy of reacting molecules which is depend
upon temperature, and threshold energy which is depend upon bond energy of the
reacting molecules.
Collision Theory:
The frequent collision
among the molecules of reactants may breaks the existing bond and initiate the
chemical reaction, this is called as effective collision of molecules.
All
the collision of molecules are not effective only a fraction of molecule which
have energy greater than the average energy and effective symmetry of the
molecules.
Effective
Collision:
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Ineffective
Collision:
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FACTORS INFLUENCING THE RATE OFCHEMICAL REACTIONS:
Following factors effect the rate of
reaction.
1.
Concentration
2.
Temperature
3.
Catalyst
4.
Pressure
5.
Surface Area
6.
Radiation
1. Concentration:
According Guldberg Law “The rate of reaction is directly
proportional to the “Active Mass” of the reactant” The greater the conc.
greater the active mass and higher the rate of the reaction.
Consider
the rate equation.
A +
B Product
Here.
·
If conc. of A or B are double the rate of
reaction will also be doubled.
·
If conc. of both A and B are double the rate of
will be four times increased.
The increase in the concentration of reacting molecules cause
more crowding and more collision of molecules which results in an increase in
the rate of formation of products.
2. Temperature:
The increase in temperature increase the kinetic energy of the
molecules which decrease the energy of the activation and increase the fraction
of the molecules possessing threshold energy.
In general the rate of a reaction doubles for every 10 K rise
in temperature it is due to increase in the frequency of collisions.
The graph of Kinetic Energy verses fractions of molecules at
different temperature is given below.
Effect Of Temperature On
Kinetic Energy
  |
K.E
The
shaded area of the curve of T2 is double than T1 shows
that a rise of temperate increase the rate of reaction.
3. Catalyst:
Catalyst is a substance which provide new path for the reaction
by changing the threshold energy.
The catalyst which decrease the threshold energy by
increasing the effective collision per unit time are known as positive
catalyst.
The catalyst which
increase the threshold energy and energy of activation by decreasing the
effective and energy of activation by decreasing the effective collision per
unit time are known as negative catalyst also known as “Inhabitor”
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4. Pressure:
The reactions in which the reactants are gases are influence by
the pressure. The increases in the pressure decrease the volume of the reacting
mixture. The molecules come close each other which increase in the collision of
the reacting molecules and increase the rate of formation of product.
5. Surface Area of Reactant:
In a heterogeneous reaction in which one or more
reacting species are in different phase such as solid, the surface area of
solid reactant effect the rate of reaction.
Greater the surface area the more is the frequency of
collision of molecules, and the higher the rate of reaction.
Consider
the reaction.
CaCO3(s) + 2HCl(l) CaCl2(g) + H2O(g)
+ CO2(g)
If we
take solid chunk of marble CaCO3, the reaction rate is slow. But
finely divided marble reacts vigorously because it offers greater surface area
for HCl to act upon.
6. Radiation:
Some chemical reactions proceed only under the influence of
light. These are called photochemical reactions.
The rate of these reaction does not depend upon rate of
reaction therefore these reaction are also called “Zero order reaction”
e.g. CH4 + Cl2 hv CH3Cl + HCl
The rate of these reactions are depend upon the intensity of
light or no. of photon absorb.
