Single apparent activation energies

As explained before, a chemical reaction can seldom be described by a single elementary step, and hence we need to adapt our definition of activation for an overall reaction. Since we are not particularly interested in the effects of thermodynamics we define the apparent activation energy as... 

Dr. Adamson If there is a distribution of reaction paths, then the apparent activation energy should indeed change with temperature, and the effect would appear as a heat capacity of activation. However, it does not seem possible to distinguish this situation from that of a single reaction path where the transition state heat capacity is different from that of the reactants. That is to say, the formal thermodynamics would be identical for the two cases. 

Huang and Gallagher [87] carried out a comprehensive study, using TG and DSC, of the influence of / (HjO) on the kinetics of dehydration of samples of powder, pressed pellets and single crystals (platelets and cubic). The rate of dehydration of pellets lay between those of crystals (slow) and powders (fast). The apparent activation energies varied with a, fi om 220 kJ mol for powder at low ar, to 61 kJ mol for pellets at high a. Because the rate is influenced by self-generated water, the results for crystals are the most reproducible. 

Unlike the single crystal surface, characterized by a constant distance between neighbouring active sites (r), on the surface of amorphous oxides there should exist a wide distribution of the active site pairs with respect to the distances between them. As it follows from the results of Monte Carlo simulation of adsorption kinetics of Lennard-Jones gas on the amorphous solid surface represented by a normal distribution of the neighbouring active sites on the distances between them and with the account of repulsive lateral interactions described by the Lennard-Jones potential, apparent chemisorption activation energy depends but insignificantly on 0 at its low value (< 0.5), while over this value the energy increases abruptly [104]. From the Monte-Carlo simulation it follows that the dependence of apparent activation energy on 0 can be approximated as [80] ... 

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