The Routes of Complex Reactions

For instance, the reaction of ethylene with oxygen on the surface of silver results in the formation of ethylene oxide, carbon dioxide, and water vapor it is described by the equations 

Such equations will be called overall chemical equations, reactants and reaction products will be termed reaction participants.  

Along with reaction participants, the chemical equations of elementary reactions comprising the complex reaction include other species that do not appear in the overall equations. They are called intermediates.  

Overall reaction equations are linear combinations of chemical equations of stages, i.e., they are obtained by the addition of chemical equations of stages multiplied by certain numbers (positive, negative, or zero). The numbers must be chosen in such a way that the overall equations contain no intermediates. According to Horiuti, such numbers are called stoichiometric. To illustrate this notion by a simple example, let us assume that the mechanism of oxidation of S02 on the surface of a solid catalyst, e.g., platinum, is described by the following scheme  

In this scheme, as always in the following, Z denotes a site on the surface of the catalyst, the arrows denote elementary reactions, and in the overall equation the equality sign is employed. Stoichiometric numbers by which the stage equations should be multiplied to obtain the overall equation are given on the right side. In this instance intermediates are Z and ZO. Another scheme equivalent to the previous one may be used  

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