Entropy and the Absolute Rate of Chemical Reactions I. The Steric Factor of Bimolecular Associations

The equilibrium constant for a bimolecular association may be expressed in terms of the energy change, AB, and standard entropy change, Delta S-o, on association. On account of the well-known relation between the equilibrium constant, and the rate constants of the bimolecular association and its reverse, the corresponding unimolecular decomposition, the values of these rate constants could be determined separately, if one could divide each of the terms, AB and Delta S-o, into two parts, in the proper way. The proper method of dividing Delta E is known; this paper is concerned with the division of Delta S-o. Considered from a statistical point of view, the entropy of a system depends upon the volume in phase space available to the system under fixed thermodynamic conditions. The separate rate constants will depend upon the fraction of the phase space in which it is possible for the reaction under consideration to take place. Application of this principle leads to an interpretation of the collision number and the steric factor of bimolecular association reactions. The known bimolecular associations have been discussed from this point of view.