Speeds and reactivities of reactions taking place in only a single zone

Speeds and reactivities of reactions taking place in only a single zone 

The absolute speed of this reaction depends on the volume of the reaction zone. It is convenient to introduce a reaction speed that does not depend on sizes of reaction spaces any more while dividing the absolute velocity by the volume of this space. We thus define that the voluminal speed is the absolute speed per unit of volume in a zone, which can be given as  

Now examine the case of diffusion of a species i in a three-dimensional zone. We can define the speed of diffusion by the flow of molecules of / that cross a surface (fictitious) perpendicular to the direction of diffusion. Then the flux of diffusion/, will be an areal speed because we have the following expression  

A zone of diffusion is a three-dimensional one, but taking into account the preceding expression of the voluminal speed, we will distinguish the diffusion length parallel with the flux, which is included in the expression of this flux, from the surface perpendicular to the direction of displacement. Thus, the areal speed of diffusion, that is, the flux, also depends on the geometry of the zone through the diffusion length. 

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