The historical development of the basic ideas concerning chemical equilibrium

Abstract

This paper outlines the historical evolution of the concept of chemical affinity that eventually supported the concept of chemical equilibrium. It provides the theoretical roots that grounded the following four key basic ideas of chemical equilibrium: ‘reversibility’, ‘incomplete reaction’, ‘equilibrium constant’ and ‘dynamic equilibrium’. The starting point of this historical reconstruction is the state of the art in the elaboration of the first affinity tables, based on the concept of elective affinities, during the eighteenth century. It is discussed how Berthollet reworked this idea, considering that the amount of the substances involved in a reaction was an essential factor determining its direction, which accounted for the early concepts of incomplete reaction and reversibility. Guldberg and Waage considered the concentrations of the chemicals involved, instead of their mass and formulated the first mathematical equations accounting for chemical equilibrium reactions. Afterwards, it is presented how Pfaundler provided the seminal ideas concerning a dynamical molecular interpretation of the macroscopic properties of equilibrium reactions. This material is intended to serve as a supplementary reading for an introductory course in physical chemistry.