Regions of existence in a K-diagram

The K-diagram is usually limited to a triangular area below the straight line 

At this line the sum of the concentrations of Ai and Af equals the sum of the concentrations a at the beginning of the reaction. For this reason, in general, the diagram stays within this triangle given by the coordinates 0, 0 0, Uq and a , 0, Exceptions to this theorem can be expected if two reactants are produced within one step of a reaction. The sum of the concentrations of the reactants can no longer stay constant, if the mechanism contains the following partial step  

For this reason the above-mentioned straight line (eq. (2.79)) can no longer describe the above-mentioned limitation. The partial step, eq. (2.80), cannot be reversible since the system loses its linearity. 

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