Setting in the equation of mechanism - example

The setting in the model equation comprises several categories of ejqtressions, which we will examine by giving the equations, as examples, that correspond to the mechanism, which we gave above for the growth of MgO on magnesium. 

They result from the general form of equation [7.17] and the number of equations is equal to the number of intermediate species, that is, five in oiu example. 

Here (pi indicates the rate of reaction i, Jo and represent the flux of the oxygen vacancies and electrons, respectively, and they can be taken at the internal interface (int) or the external (ext) one. The amounts of matter of the various intermediate species are taken at the internal interface (int) or the external (ext) one, with areas s and 5.  

We thus have five equations, three on the external interface and two at the internal one  

We can write the reactivities of the elementary reactions by using relation [7.15], with 0 indicating the degree of covering of adsorbed oxygen, the equilibrium constant of reaction i and P the pressure of oxygen. In our case there are three steps, which are written as  

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