Nature of elementary steps

We will describe a model for the elementary step that is different fiom the elementary step in homogenous medium but one that leads to the same properties as given in section 4.1. 

Unlike the kinetics of homogenous reactions, where the most elementary step is the activated shock of two molecules, in heterogenous reactions, the simplest elementary step is the activated jutiq) of a particle from an equilibrium site to another in crystal lattices. Overall, these jumps are carried out in a direction normal to the interfaces that separate the phases [SOU 70a, p. 2032-2034] in a given zone. 

If we calculate the energy of a crystal based on the position of the ions, we note that the nodes of the crystal lattice constitute a local minimum of energy, which is coherent with the stability of ions in this position. The calculation also shows that by moving the particle, we meet a maximum of energy, where the particle is in unstable equilibrium compared with the initial and final positions. 

The jumps can have different natures, depending on the nature of the junping particle and the nature of the initial and final sites of the jmnp. From this last point of view, we distinguish three types of jrrmps  

Remark.- In a given phase, the sites of comparable nature must be on the same energy level. In fact, this is true only for those sites that are far from the surface because the proximity of a surface modifies the energy level of a well. 

---

It is recalled that the aim of chemical relaxation kinetics is the elucidation of reaction mechanisms number and nature of elementary steps identity and properties of reactants, intermediates, and products kinetic and thermodynamic parameters (rate constants, reaction enthalpy, and entropy) characterizing individual equilibria and reaction pathways. 

---