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Approaches to the Treatment of Equilibrium Perturbations
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Abstract
Perturbations from equilibrium are treated in the textbooks by a combination of Le Châtelier's principle, the comparison of the equilibrium constant K with the reaction quotient Q,and the kinetic approach. Each of these methods is briefly reviewed. This is followed by derivations of the variation of the equilibrium value of the extent of reaction, ξeq, with various parameters on which it depends. Near equilibrium this relationship can be represented by a straight line. The equilibrium system can be regarded as moving on this line as the parameter is varied. The slope of the line depends on quantities like enthalpy of reaction, volume of reaction and so forth. The derivation shows that these quantities pertain to the equilibrium system, not the standard state. Also, the derivation makes clear what kind of assumptions underlie our conclusions. The derivation of these relations involves knowledge of thermodynamics that is well within the grasp of junior level physical chemistry students. The conclusions that follow from the derived relations are given as subsidiary rules in the form of the slope of ξeq, with T, p, et cetera. The rules are used to develop a visual way of predicting the direction of shift of a perturbed system. This method can be used to supplement one of the other methods even at the introductory level.
Keywords (Audience):
Upper-Division UndergraduateKeywords (Domain):
Physical ChemistryKeywords (Feature):
Research: Science and EducationKeywords (Subject):
EquilibriumCiting Articles
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This article has been cited by 3 ACS Journal articles (3 most recent appear below).
The Adverse Effects of Le Châtelier's Principle on Teacher Understanding of Chemical Equilibrium
Derek CheungJournal of Chemical Education2009 86 (4), 514The Adverse Effects of Le Châtelier's Principle on Teacher Understanding of Chemical Equilibrium
Derek CheungJournal of Chemical Education2009 86 (4), 514Although the scientific inadequacy of Le Châtelier's principle has long been documented in the literature, the principle is still treated as a central concept of chemical equilibrium by textbook writers and teachers in many countries. In the past, ...
Effect of a Perturbation on the Chemical Equilibrium: Comparison with Le Châtelier's Principle
Emilio Martínez TorresJournal of Chemical Education2007 84 (3), 516Effect of a Perturbation on the Chemical Equilibrium: Comparison with Le Châtelier's Principle
Emilio Martínez TorresJournal of Chemical Education2007 84 (3), 516This article develops a general thermodynamic treatment to predict the direction of shift in a chemical equilibrium when it is subjected to a stress. This treatment gives an inequality that relates the change in the perturbed variable and the change that ...
Equilibria That Shift Left upon Addition of More Reactant
Jeffrey E. LacyJournal of Chemical Education2005 82 (8), 1192Equilibria That Shift Left upon Addition of More Reactant
Jeffrey E. LacyJournal of Chemical Education2005 82 (8), 1192Most textbook presentations of Le Châtelier's principle in general and physical chemistry do not include a discussion of constant pressure conditions for which addition of a reactant can shift the equilibrium to the left. We propose presentations of ...
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- Received: August 03, 2009
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