Engineering Chemistry S Datta

REACTION DYNAMICS/CHEMICAL KINETICS 119
or log k 1
k2
= –
E
2.303R
L
NM
1
T
– 1 T
1 2
So, if we can determine the two rate constants k 1
and k 2
we can find out the value of E
from the above equation. If E for a reaction is known, we can determine the value of k 2
when k 1
is known.
Highlights:
• Temperature coefficient of a chemical reaction is defined as the ratio of two rate
constants of the reaction at two temperatures differing by 10°.
O
QP
Temperature co-efficient = k k
25°
35°
(usually)
• Plot of ln k vs. 1 T
gives a straight line so the equation of the straight line is:
ln k = a – b T
where k is the specific reaction rate and T is the absolute temperature.
• The exponential form of the above equation is known as Arrhenius equation
− E /RT .
a
k = Ae
where, k = Rate constant E a
= Energy of activation
R = Gas constant T = Absolute temperature
A = Another constant known as frequency factor.
Arrhenius equation describes how the rate constant for a reaction varies with
temperature and makes it possible to determine the activation energy for the
reaction.
Energy of Activation (E a
) of a Reaction: its Significance
The molecules of reactants, which take part in a chemical reaction should get activated
by gaining a minimum definite amount of energy, which is known as energy of activation
(E a
). For every reaction, the molecules should be raised to a state of sufficient energy, otherwise
the reaction will not be
Potential energy
E a
A+B
A+B
H
C+D
C+D
Progress of reaction
Fig. 5.3