The salt effect: kinetics and thermodynamics

The primary kinetic salt effect is reviewed and a generalized Brönsted–Bjerrum equation is presented. The classical Brönsted–Bjerrum equation predicts a unidirectional effect on the rate constant k with ionic strengh I. In the first part of this lecture we describe the classical Brönsted–Bjerrum theory. In the second part, we go beyond the simple Debye–Hückel ionic model and discuss the salt effect in the light of the Davies equation. We show the existence of a critical value of the ionic strength I_c at which the salt effect becomes maximum. The Davies equation also predicts a value of the ionic strength value I₀ at which we have no salt effect at all. More interestingly, beyond I₀ the salt effect inverts. By “inversion” we mean the reversal phenomenon that if the salt effect inhibits the rate for I<I₀, then for I>I₀ we have an acceleration of the ionic reaction, and vice versa. We fitted some kinetics experimental data that exhibit this inversion to the Davies equation and found good agreement. We close by discussing the salt effect on various relevant chemical equilibria including the ionization of weak acids and on the Nernst cell voltage in presence of spectator non-common ions. Thus, the goal here is to pedagogically review the primary salt effect and present its less-known inversion effect to physical chemistry students, researchers, and instructors. Graphical abstract: (Figure presented.).