Stability, Displacement and Moderation of Chemical Equilibrium: Rediscovering Le Chatelier’s Principle

1 Department of Materials Science and Engineering, PennState University, PA 16802. Email: [email protected] Abstract – Engineering education often makes reference to historical “laws” dating back to a time when science was still at an early stage of development. An example is the principle of displacement and moderation of chemical equilibrium formulated by Le Chatelier in various forms between 1884 and 1933. The principle was never given a proper mathematical form, and later workers restated it in increasingly ambiguous verbal terms, subject to doubtful or plainly wrong interpretations. We show by an examination of Le Chatelier ́s works that in fact he dismissed as incorrect the version usually associated nowadays with his name. His definitive formulation was much more specific, referring explicitly to changes in temperature, pressure and composition of a system. We compare this version with the predictions obtained from rigorous thermodynamic considerations. The results show that Le Chatelier ́s approved principle is also incorrect as a whole, but can be accepted as two separate statements, one for temperature and pressure changes in isolated systems, and the other for mass perturbations in isothermal, isobaric ideal mixtures. Finally, we reflect on the disadvantages of using unsound or “loose” versions of this principle in engineering education instead of a formal treatment.