It is also called pre-exponential factor. It is a constant specific to a particular reaction. R is gas constant and E a is activation energy measured in joules/mole (J mol – ). It can be understood clearly using the following simple reaction H I 2HI g According to Arrhenius, this reaction can take place only when a molecule of hydrogen and a molecule of iodine collide to form an unstable intermediate (Fig.
. ). It exists for a very short time and then breaks up to form two molecules of hydrogen iodide. Fig.
. : Formation of HI through the intermediate Intermediate Fig. . : Diagram showing plot of potential energy vs reaction coordinate Fig.
. : Distribution curve showing energies among gaseous molecules The energy required to form this intermediate, called activated complex (C), is known as activation energy ( E a ) . Fig. .
is obtained by plotting potential energy vs reaction coordinate. Reaction coordinate represents the profile of energy change when reactants change into products. Some energy is released when the complex decomposes to form products. So, the final enthalpy of the reaction depends upon the nature of reactants and products.
All the molecules in the reacting species do not have the same kinetic energy. Since it is difficult to predict the behaviour of any one molecule with precision, Ludwig Boltzmann and James Clark Maxwell used statistics to predict the behaviour of large number of molecules. According to them, the distribution of kinetic energy may be described by plotting the fraction of molecules ( N E / N T ) with a given kinetic energy (E) vs kinetic energy (Fig. .
). Here, N E is the number of molecules with energy E and N T is total number of molecules. The peak of the curve corresponds to the most probable kinetic energy, i.e., kinetic energy of maximum fraction of molecules. There are decreasing number of molecules with energies higher or lower than this value.
When the Fig. . : A plot between ln k and /T In Fig. .
, slope = – and intercept =