Chapter 6 - Kinetic Theory and Reaction Kinetics

Abstract

While reaction rates are empirical in nature, collision theory does provide a means to estimate the reaction rate. Collision theory gives rise to the correct reaction rate dependence on concentrations and temperature. Based on collision theory, most viable reactions are bimolecular. Reactions with more than three molecules are unfavorable. Therefore, an elementary reaction usually involves three or less molecules on either side of the reaction. Nonelementary reactions can be decoupled into elementary reaction steps or reaction pathways. The reaction system that forms a nonelementary reaction can be analyzed as one were dealing with a multiple reaction system. In many occasions, we find that simplification of the nonelementary reaction is necessary. Some reaction steps are fast than others. Some or all the reaction intermediates are present in the reaction system in only trace amount. These observations lead to two methods of simplifying reaction rate expressions: FES and PSSH. FES stands for fast equilibrium step approximation, while PSSH stands for pseudosteady state hypothesis. Active reaction intermediates are common as they are means to overcome the activation energies. Active intermediates can be active complex, transitional state (or activated molecules), and free radicals. Nonelementary reactions usually have complicated reaction rate expressions than the simple power law form. Fractional order (1/2, 3/2, ..) can be observed for free-radical reactions. Simple kinetic expressions can be obtained for otherwise a complicated problem when proper assumptions are made. For acid hydrolysis of polymers, the reaction kinetics may be approximated by three “first order reaction rate” relationships of lumped components despite a complicated composition of the reaction mixture.