Chapter 25 - Chemical Reaction Kinetics

Abstract

Within the fields of chemistry and chemical engineering, the term “kinetics” usually implies chemical reaction kinetics. Homogeneous chemical reactions occur within a single-phase fluid, while heterogeneous reactions encompass reactions that involve more than one phase. Solid state reactions occur when both the reactants and products are in the solid state. For example, the Johnson-Mehl-Avrami equation for crystal growth can be derived from solid state reaction kinetics assuming separate nucleation and growth processes. The evolution of each species is determined by the extent of reaction and its stoichiometric coefficient, and the rate of reaction is defined by the time derivative of the extent of reaction. Reactions can be first-order or higher order. The order of a reaction is typically determined empirically through experimental measurements of species concentrations at different stages of a reaction. Chemical reactions often involve more than one step. For reactions in series, there is typically a rate-limiting intermediate step, which is the bottleneck of the reaction. One goal of chemical reaction engineering is to identify this rate-limiting step and determine ways to optimize the process. As with many other kinetic processes, the temperature dependence of the reaction rate is described by the Arrhenius equation. The equilibrium among the reactants and products is governed by their difference in free energy and described by the equilibrium constant. Equilibrium is achieved for a given reaction when the detailed balance condition is satisfied, i.e., when the rates of the forward and reverse reactions are perfectly balanced.