The areal speed of an interface reaction

We see that all the preceding reactions are surface or interface reactions, taking place in zones with two dimensions. The volume of this zone is thus the product of its area and the thickness of the interface zone, that is, roughly the cell parameter of the solid. Thus, we can define an areal speed of such interface reaction as follows  

We will see later (in section 7.1.2.3) that this quantity will be the reactivity of the surface reactioa 

The expression of the reactivity will involve a constant instead of the speed constant and which we call the rate constant . It can be easily shown that  

These rate constants obey the Arrhenius law and the corresponding apparent energies of activation are andAj. 

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