Fundamental Objection Against Collision Theory

Although collision theory provides a useful formalism to estimate an upper limit for the rate of reaction, it possesses the great disadvantage that it is not capable of describing the free energy changes of a reaction event, since it only deals with the individual molecules and does not take the ensemble into consideration. As such, the theory is essentially in conflict with thermodynamics. This becomes immediately apparent if we derive equilibrium constants on the basis of collision theory. Consider the equilibrium 

Applying collision theory to the forward and the reverse reactions, and taking the ratio, we obtain the equilibrium constant  

we find a relation between K and the enthalpy of the reaction, instead of the free energy, and the expression for the equilibrium is in conflict with equilibrium thermodynamics, in particular with Eq. (32) of Chapter 2, since the prefactor can not be related to the change of entropy of the system. Hence, collision theory is not in accordance with thermodynamics. 

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