Transition-State Theory and Enthalpy Diagrams

When reactants have collided with sufficient enthalpy of activation (AH ) and with the proper orientation, they pass through a hypothetical transition state in which some bonds are breaking while others may be forming. 

The relationship of the transition state (TS) to the reactants (R) and products (P) is shown by the enthalpy (energy) diagram. Fig. 3-2, for a one-step exothermic reaction A-t-B-+C + D. At equilibrium, formation of molecules with lower enthalpy is favored, i.e., CD. However, this applies only if AH of reaction predominates over T AS of reaction in determining the equilibrium state. The reaction rate is actually related to the free energy of activation, AG, where AG = AH - r A5.  

In multistep reactions, each step has its own transition state. The step with the highest-enthalpy transition state is the slowest step and determines the overall reaction rate. 

The number of species colliding in a step is called the molecularity (of that step). If only one species breaks down, the reaction is unimolecular. If two species collide and react, the reaction is bimolecular. Rarely do three species collide (termolecular) at the same instant. 

The rate equation gives molecules and ions and the number of each (from the exponents) involved in the slow step and in any preceding fast step(s). Intermediates do not appear in rate equations, although products occasionally do appear. 

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I am suggesting that often the applicability of Barkley-Butler type plots, that is, the linear relationship between the entropy and enthalpy of activation in a series may come about because of there being a distribution of reaction paths. Small variations in the importance of low activation energy, low probability paths could then account for the data in Dr. Taube s table. By contrast, transition state theory in its approximate application, invariably leads to diagrams of energy vs. reaction path which, in spite of all protest, one reaction path, whatever it is, one transition state, and one energy. 

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