Thermodynamic spontaneity-equilibrium criteria require that in a single-reaction system, reactions in either the forward or reverse direction at equilibrium be nonspontaneous. Conversely, the concept of dynamic equilibrium holds that forward and reverse reactions both occur at equal rates at equilibrium to the extent allowed by kinetic considerations. This paper discusses the resolution of this apparent conundrum. Qualitatively, it is argued that thermodynamics deals only with the macroscopic behavior of large systems, not with the microscopic details that lead to the observed macroscopic events. Since the observed events are measurable changes in the system composition without the necessity to do work on the system, any set of microscopic events that lead to the same macroscopic observations are compatible with thermodynamics. Quantitatively, it is shown that when considered statistically, thermodynamics shows that microscopic reactions in both forward and reverse directions are expected to occur at all points along the reaction coordinate provided only that both reactants and products are present. At equilibrium, the statistical treatment predicts that forward and reverse microscopic reactions have equal probability. Consequently, the concept of a dynamic equilibrium and the spontaneity-equilibrium constraints of thermodynamics are completely compatible.
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