Homogeneous versus Heterogeneous Chemical Reactions

In the sections that follow, we will learn about both homogeneous and heterogeneous chemical reactions. It is important to understand the difference between them. 

A homogeneous chemical reaction is a reaction where the reactants and products are all in one phase and the reaction can proceed anywhere throughout the volume of the considered phase/system. The hydrogen-oxygen combustion reaction is an example of a homogeneous chemical reaction (assuming that the water is produced in the gas phase). Many gas-gas and nuclear decay reactions are homogeneous. 

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In the case of heterogeneous catalysis, a DCKM or microkinetic model must incorporate the added dimension of adsorbed chemical species as well as active versus non-active sites. To obtain the full predictive capability from reactant influent to product effluent, all possible reactions in the system, both heterogeneous and homogeneous, must be accounted for. To properly understand the catalytic reaction sequence, it is possible that seemingly unimportant intermediates on the surface may be what initiate gas phase reactions. To begin this elucidation, the surface chemical species and their properties must be known. 

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