From molecular potential energy to rates of reaction

2 From molecular potential energy to rates of reaction 

Potential energy surfaces or profiles are descriptions of reactions at the molecular level. In practice, experimental observations are usually of the behaviour of very large numbers of molecules in solid, liquid, gas or solution phases. The link between molecular descriptions and macroscopic measurements is provided by transition state theory, whose premise is that activated complexes which form from reactants are in equilibrium with the reactants, both in quantity and in distribution of internal energies, so that the conventional relationships of thermodynamics can be applied to the hypothetical assembly of transition structures. 

for the general reaction given in Equation 9.1, the concentration of the transition structures, TS, is governed by the equilibrium constant Ki , with no implication as to how TS is formed from A and B  

The rate of reaction is the concentration of TS multiplied by the frequency, v, of their decomposition into product, usually given by v = k T/h Equation 9.2 then applies  

Under this treatment, the reaction order of an elementary unimolecular or bimolecular reaction must identify with molecularity, and K is related in Equation 9.3 to the standard molar free energy difference, AG, between reactants and transition state (a hypothetical construct comprising one mole of transition structures, see below)  

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