Question

Distinguish between homogeneous and heterogeneous reactions. Which ones are described
by boundary conditions and which ones manifest themselves in the differential equations

Answer 1

There are many ways of classifying chemical reactions. In chemical reaction engineering probably the most useful scheme is the breakdown according to the number and types of phases involved, the big division being between the
homogeneous and heterogeneous reaction.

1. Homogeneous reactions: A reaction is homogeneous if it takes place in one phase alone.
2. Heterogeneous reaction: A reaction is heterogeneous if it requires the presence of at least two phases to proceed. It is immaterial whether the reaction takes place in one, two, or more phases; at an interface; or whether the reactants and products are distributed among the phases or are all contained within a single phase. All that counts is that at least two phases are necessary for the reaction to proceed.

Sometimes this classification is not clear-cut as with the large class of biological reactions, the enzyme-substrate reactions. Here the enzyme acts as a catalyst in the manufacture of proteins and other products. Since enzymes themselves are highly complicated large-molecular-weight proteins of colloidal size, 10-100 nm, enzyme-containing solutions represent a gray region between homogeneous and heterogeneous systems. Other examples for which the distinction between homogeneous and heterogeneous systems is not sharp are the very rapid chemical reactions, such as the burning gas flame. Here large nonhomogeneity in composition and temperature exist. Strictly speaking, then, we do not have a single phase, for a phase implies uniform temperature, pressure, and composition throughout. The answer to the question of how to classify these borderline cases is simple. It depends on how we choose to treat them, and this in turn depends on which description we think is more useful. Thus, only in the context of a given situation can we decide how best to treat these borderline cases.

Now heterogenous reaction are described by boundary conditions where as homogenous reaction manifest themselves in the differential equations form.