Empirical estimates of the activation energy

Approximate calculations of this activation energy have been made in a number of examples using the quantum theory of molecular binding, by making assumptions concerning the structure and partition functions of the transition state molecule. 

The entropy of activation may be estimated from experimental data for gaseous molecules, and in the special case of HI formation, which may be regarded as involving the intermediate formation of the dimer H2l2, using the general empirical relationship 

S°(dimer)/S°(monomer) = 1.52 0.05 J mol-1 K-1 This yields a value of the entropy of formation of the transition state molecule 

An estimate of the enthalpy change which corresponds to the activation energy of the collision theory analysis of 167kJmol 1 may be made by assuming that the formation of the dimer from two molecules of the monomer is energetically equivalent to the dipole-dipole and dispersion interactions of two HI molecules. These exothermic sources of interaction are counterbalanced 

Another empirical equation due to Semenov relates the activation energy to the formation energy of the product molecule reaction A//298 For two gram-molecules of HI this is 600 kJ, and substituting in the equation 

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