Entropy of activation determines the preexponential factor  of the Arrhenius equation for temperature dependence of reaction rates. The relationship depends on the molecularity of the reaction: 
* for reactions in solution and unimolecular gas reactions
* while for bimolecular gas reactions
In these equations  is the base of natural logarithms,  is the Planck constant,  is the Boltzmann constant and  the absolute temperature. ' is the ideal gas constant in units of (bar·L)/(mol·K). The factor is needed because of the pressure dependence of the reaction rate.  (bar·L)/(mol·K).
The value of  provides clues about the molecularity of the rate determining step in a reaction, i.e. the number of molecules that enter this step.  Positive values suggest that entropy increases upon achieving the transition state, which often indicates a dissociative mechanism in which the activated complex is loosely bound and about to dissociate. Negative values for  indicate that entropy decreases on forming the transition state, which often indicates an associative mechanism in which two reaction partners form a single activated complex.