Facts and alternative facts in chemical kinetics: remarks about the kinetic use of activities, termolecular processes, and linearization techniques

This contribution examines three different controversial questions that arise in the everyday practice of chemical kinetics. First, it is argued that concentrations should be used as independent variables in rate equations rather than activities. The occasional use of activities in this context might be a result of a false analogy with well-established thermodynamic practices. Second, it is argued that termolecular elementary reactions might be more common than generally assumed. Formulas are derived to calculate the frequency of three-body collisions and it is pointed out that this frequency is often larger than the rates of third-order reactions. Finally, two experimental examples (exponential curves and Michaelis-Menten kinetics) are shown in order to demonstrate how misleading it can be to use linearized formulas for quantitative evaluation.