In this paper it is performed a historical account of the theoretical roots that grounded the following four key basic ideas of chemical equilibrium: ‘incomplete reaction’, ‘reversibility’, ‘equilibrium constant’ and ‘molecular dynamics’. These notions developed in nineteenth-century as a consequence of the evolution of the concept of chemical affinity. The discussion begins with the presentation of the earliest affinity table [‘Table des rapports’] published in 1718 by Geoffroy. Afterwards, it is examined Bergman’s compilation. The theory supporting this arrangement assumed that (...) chemical displacement reactions were complete and could not be reversed. It is analysed how Berthollet’s model showed the inadequacy of this view. His new conceptual considerations established 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 specified the role of mass in chemical equilibrium reactions as they considered the concentrations of the chemicals involved, formulating the first chemical equilibrium mathematical equation that approximates what is called nowadays chemical equilibrium constant. Finally, it is presented how Pfaundler was the first to consider the kinetic theory in the interpretation of the macroscopic properties of equilibrium reactions. (shrink)

After briefly presenting the first formulation of the equilibrium constant stated by Guldberg and Waage, this study examines the early thermodynamic and kinetic deductions of the equilibrium law. Firstly, it is discussed how Horstmann applied the concept of entropy to chemical equilibrium reactions, which meant the first thermodynamic explanation of the Guldberg-Waage law of mass action proposed in 1864. A different theoretical derivation of the equilibrium constant came from the works of van’t Hoff. This study analyses the first accurate kinetic (...) treatment of equilibrium reactions as well as van’t Hoff’s imaginative thermodynamic foundation of the constant ratios in chemical equilibrium systems. (shrink)