Le Châtelier's principle applied to the temperature dependence of solubility

Abstract

Le Chatelier's principle is a well known generalization stating how changes of condition affect systems at equilibrium (1), In a previous paper I proposed a brief, yet unambiguous way of phrasing it (2):A system at equilibrium resists attempts to change its temperature, pressure, or concentration of a reagent.General chemistry texts often cite the principle when discussing the temperature dependence of solubility. It predicts that solubility will increase with temperature if dissolution absorbs heat and, conversely, that solubility will decrease with temperature if dissolution liberates heat.The validity of the principle when applied to ionic compounds in water has been questioned by Bodner (3). He points out that the enthalpies of solution usually tabulated in reference books refer to infinitely dilute solutions, and he cautions that such values have little relevance to the situation involving a solute in equilibrium with saturated solution. Bodner properly focuses attention on enthalpies of solution at saturation. Even then, his examination of literature data does not confirm the principle for many compounds. He offers several possible explanations for the apparent anomalies. One explanation, which is not explored, refers to the complicating factor that some ionic compounds exist as hydrated solids when equilibrated with saturated solution.Subsequent papers by Fernandez-Prini and by Brice defend the principle with largely theoretical arguments (4). Both authors reiterate that the parameter of most importance is enthalpy of solution at saturation. Furthermore, in the case of ionic compounds in water, they point out that enthalpy of solution varies unpredictably as one proceeds from lower to higher concentration. Except for a few examples, this matter is not pursued in any comprehensive fashion. This paper extends the work of previous authors by conducting a detailed examination of experimental results available in the literature. Le Chatelier's principle is tested, using appropriate thermodynamic data, for predicting the temperature dependence of solubility for ionic compounds in water. Care is taken to identify correctly the actual hydrate solid equilibrated with saturated solution and to base predictions solely upon its enthalpy of solution at saturation.

Thermodynamic modeling of crystal deposition in humans

“…This rule states that the enthalpies of dissolution become progressively more endothermic with extent of hydration, since the enthalpies of dehydration, corresponding to e.g. COT → COD + H 2 O(aq) or COD → COM + H 2 O(aq) are always positive [38]. The enthalpies of solution derived from our solubility products [4] given in Table 5 obey this rule (Table 6) while those of refs.…”

Solubility of the Three Calcium Oxalate Hydrates in Sodium Chloride Solutionsand Urine-Like Liquors

“…, are always positive [21] (a rule which has already been discovered 120 years ago [22]), the enthalpies of dissolution should become progressively more endothermic with increasing extent of hydration. for COM, COD, and COT, respectively).…”

Thermal Coefficients and Heat Capacities in Systems with Chemical Reaction: The Le Châtelier-Braun Principle

“…This equation shows that in the G representation the natural variables in a closed system with chemical reaction are (T, P, q). Thus, from the condition of eq 2 relative to the conjugate variables si and c, it takes the form G"> 0 rrp (5) From the equalization of the second mixed partial derivatives of G, we get the results, i(3h' (7) which are the analogue of the Maxwell relations (11) for the variables si and %, written in terms of the heat of reaction ArH and reaction volume A, V. A,-does not imply a difference between final and initial states, but rather the operator d/D£^t hat is, an instantaneous rate of change with respect to the extent of the reaction E, (11)(12)(13). In particular, ArH represents a differential heat of reaction, and also, (9) (TO) where one gets these changes as a sum of two contributions, the first coming from the variation in the absence of a chemical reaction, and the second coming from the presence of a chemical reaction.…”

“…Although considerable attention has been dedicated to the discussion and applications of the Le Chatelier's principle (3)(4)(5)(6)(7)(8)(9)(10), no similar study has appeared in the peda-gogical literature dedicated to the so-called Le Chatelier-Braun principle, which allows a more subtle interpretation of the way in which a system returns to the equilibrium state (11, 12): while a system tries to reduce the external perturbation, other processes are atso induced which lead to the same result.…”