Introduction

So far, we have considered rather simple reactions, i.e., elementary reactions and reactions involving a few steps in the reaction mechanisms. The variety of real chemical reactions is far wider. In this lecture, we shall consider a few major groups of these complex reactions. A common theme in all these is the involvement of several steps, and often, repeating steps.

When certain steps repeat very fast, liberating a lot of heat, explosions occur. Several accidents occur all over the world in chemical laboratories, not only while exploring new reactions, but when old reactions are carried out in regimes which are unsafe. When one scales up a reaction from the laboratory scale ( milligrams to grams) to the industrial scale (kilograms to tons) one can not just increase the amounts of reactants 'n' fold. This becomes a problem of chemical engineering wherein mass transfer, heat transfer, diffusion, stirring and so on need to be considered in detail. Another area of wide applicability is catalysis wherein catalysts can be used to enhance the reaction rates and reduce the time and costs for carrying out reactions. Catalysts also alter the reaction mechanisms and thus add a new dimension in chemical kinetics.