Spontaneity and Equilibrium: Why "AG < 0 Denotes a Spontaneous Process" and "AG = 0 Means the System Is at Equilibrium" Are Incorrect

• Autores: Lionel M. Raff
• Localización: Journal of chemical education, ISSN 0021-9584, Vol. 91, Nº 3, 2014, págs. 386-395
• Idioma: inglés
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• Resumen

  • The fundamental criteria for chemical reactions to be spontaneous in a given direction are generally incorrectly stated as ΔG < 0 or ΔA < 0 in most introductory chemistry textbooks and even in some more advanced texts. Similarly, the criteria for equilibrium are also misstated as being ΔG = 0 or ΔA = 0. Following a brief review of the thermodynamic equations related to reaction spontaneity and equilibrium in systems involving a single reaction, this paper addresses the nature of these errors by first discussing the conceptual problems thereby introduced. This qualitative discussion is followed by a quantitative treatment of the 2NO2(g) → N2O4(g) reaction conducted both under constant temperature and volume and under constant temperature and pressure conditions. The results provide clear examples of the conceptual problems introduced by using ΔG < 0 or ΔA < 0 as criteria for reaction spontaneity and ΔG = 0 or ΔA = 0 as the corresponding criteria for equilibrium. It is shown that ΔG < 0 or ΔA < 0 are necessary conditions for a transformation from state A to state B to be spontaneous, but they are not suffcient conditions. If ξ denotes the reaction coordinate, the correct criteria for a spontaneous forward reaction are (∂G/∂ξ)T,p < 0 or (∂A/∂ξ)T,V < 0 when the process can be characterized by a single reaction coordinate. The correct criterion for equilibrium is either dG = 0 or dA = 0. The paper concludes with some briefly stated recommendations as to the manner in which textbooks should be altered. The pedagogical problem of presenting the correct criteria for spontaneity and equilibrium to beginning students not versed in calculus and thermodynamics is also addressed, and some recommendations are made.