<p>In this article, we illustrate the limitations of expressing rates of reaction in terms of chemical activities instead of concentrations of reactants using the observed effect of the ionic strength on the rates of chemical reaction. We find that the proposal by J. N. Brønsted of including activity coefficients of not only those of reactants but also that of the union of them played a crucial role in the development. The discussion also sheds light on a few basic aspects of activated complexes, which also illustrates the intuitive capacity of Brønsted. We show that the rate constants, corresponding to when rates are expressed in terms of activities, depend on the medium’s ionic strength. However, the thermodynamic equilibrium constant, even when expressed as the ratio of the activities-based rate constants of forward and reverse reaction, remains independent of the ionic strength of the medium as it should be. Also, in nonequilibrium thermodynamics, when a chemically reacting system is close to equilibrium, it is shown that the reaction rate is linearly related to its chemical affinity in conformity with the Onsager theory. However, a practicing chemical kineticist generally asserts that the reaction rate does not depend on its chemical affinity but is determined by its Gibbs free energy of activation. We discuss the truth of both these assertions.</p>