Space function of an elementary reaction

Each elementary step occurs in a single zone. In fact, if it involves two zones, then one part of the reaction will occur in one zone and the second part in the other. In this case, the reaction would no longer be elementary since it would occur in two steps and at least one intermediate species would be formed between both steps. 

In an elementaiy reaction, which occurs in a single zone, the space function of this elementary step is the ratio of the volume (or area) of the reaction zone to the reference amount chosen to define the fractional extent  

The space function is expressed in m per mole or m per mole. The notion of space function is not defined for a non-elementary reaction. 

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