Formulation of the total collision rate

Encounters with impact parameter b rA + rB count towards the collision rate those with b rA + rB do not. The total number of collisions per unit time per unit volume between A and B can be found from kinetic theory, and this collision rate is often called the collision frequency, Zab- This quantity is proportional to the numbers of molecules of A and B per unit volume, nA and nB  

ZAb is the constant of proportionality, called the collision number, and this can be found from kinetic theory  

Certain combinations of masses turn up in mechanical arguments when discussing systems of more than one object. Such combinations are called the reduced mass for the system. One such combination is the reduced mass, ji, for the approach of two molecules to each other, given by 

The calculated collision rate was found to be very much greater than observed rates, suggesting that some factor limits the effectiveness of collisions in causing reaction viz., only collisions which have at least a critical energy can result in reaction. 

In simple collision theory, for a bimolecular reaction this critical energy, eo, is the kinetic energy of relative translational motion along the line of centres of the colliding molecules, loosely described as the violence of the collision. 

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