Homogeneous catalytic kinetics

The kinetic scheme of homogeneous catalytic reactions is often discussed in terms of a two-step sequence 

A generalization of the two step sequence for acid-base catalysis still keeping the second step as irreversible is given below 

As an example consider an acid catalysis when X in (5.5) can be written as SH+ 

The second step is considered to be irreversible. In eq. (5.6) the proton is considered to be transferred to solvent (water) and is an example of a protolytic mechanism. In a prototropic mechanism, a proton is transferred to the solute (SH++B= P+BH+). 

The concentration of the BH+ species is defined from the protonation equilibrium BH+H20 =B + H3O 

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