2 - Chemical Kinetics of Elementary Steps

Abstract

This chapter discusses chemical kinetics of elementary steps. A reaction is an elementary step if it takes place in a single irreducible act at the molecular level, just the way it is written in the stoichiometric equation for reaction. The act of reaction is irreducible when no intermediate between reactants and products can be detected by any available technique. The word irreducible has an operational meaning: what appeared at one time to be elementary, today may well be broken down into two or more simpler steps. Simplicity has been elevated to the rank of principle, that of the least change of structure (Muller, 1886). The act of reaction will normally be a simple one, where only one bond is made or broken. Also simple is the situation where one bond is broken while another one is made. More rarely, two bonds are broken and two new ones are made in what is commonly called a four-center reaction. While more complicated rearrangements are known to occur in one step consisting of a series of concerted motions, they must be looked upon with suspicion in the absence of independent evidence that shows conclusively that they are truly irreducible. The chapter discusses transition-state theory, the rate of an elementary step, and thermodynamic formulation of rates. It also focuses on the collision theory and the equation of Arrhenius. According to the simple views of the collision theory of reaction rates, molecules are considered as hard spheres.