Collision

11. Effect of temperature on reaction rate: the rate of a chemical reaction is significantly
affected by a change in temperature. For most of the chemical reactions, the rate increases
with increase in temperature. The rate usually becomes doubled for each 10º rise in
temperature.

a) Temperature coefficient: it is defined as the ratio of the rate constant of a reaction at two
different temperatures separated by 10 C. the two temperatures generally taken are 35 C and
25 C. thus,
Temperature coefficient =
35
25
C
C
k
k

For most of the homogeneous reactions, the value of temperature coefficient lies between 2
and 3.

b) Collision theory of reaction rate: this theory was proposed to explain the effect of
temperature on reaction rates. The salient features of the theory are as follows.

i) A reaction occurs only when the reactant molecules undergo collisions with one another.

ii) All collisions between the reaction molecules are not effective in producing a chemical
change. Only a fraction of total number of collisions is effective and leads to the formation f
products.

iii) The collisions between the reaction molecules are effective only when they acquire a
definite amount of energy. The minimum amount of energy which must be possessed by the
reacting molecules to make effective collisions is called threshold energy.

Effective collisions are those collisions which lead to the formation of products. The number
of effective collision is governed by the following two factors.
i) Energy barrier: the collisions are effective only when the molecules possess
energy greater than or equal to the threshold energy.

ii) Orientation barrier: the reactant molecules must collide with favorable orientation
in order to facilitate the breaking of old bond and formation of new bonds.


c) Qualitative of increase in reaction rate with temperature: the following figure shows
the energy distribution curves at temperature T
1
, T2 and T
3
such that T
3
> T
2
> T
1
. At
temperature T
1
, the fraction of molecules enclosed in the shaded are a b c d possesses kinetic
energy greater than or equal to the threshold energy and is capable of making effective
collisions. As the temperature increases from T
1
to T
2
. The fraction of molecules having
kinetic energy greater than the threshold energy increase. Thus effective collisions and hence
the rate of reaction increases. The fraction of such molecules becomes still larger as the
temperature increases from T
2
to T
3
resulting in a further increase in the rate of reaction.

Thus, an increase in temperature increases the number of effective collisions resulting in an
increase in the rate of reaction.
The fraction of molecules having energy greater than or equal to the threshold energy
increases appreciably even with a small rise in temperature. Hence, the rate of a reaction
increase appreciably even with a small rise in temperature.