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Joan
Josep SOLAZ 1 and Juan
QUILEZ 2
ABSTRACT: Many thermodynamics and physical chemistry textbooks provide a quantitative formulation of Le Chatelier's principle that restricts its applicability to conditions involving changes in T at P constant and changes in P at T constant. Both equations give the variation of the extent of reaction, x. However, most textbooks do not present a similar discussion about the change of x in an open equilibrium mixture. These possible disturbances are usually solved by using one of the many available qualitative statements of Le Chatelier's principle. But, these cases involve some situations in which Le Chatelier's principle is limited. Furthermore, all its qualitative statements are vague and ambiguous, and they are usually nonequivalent. Thus, both high school and college chemistry students, and also many chemistry teachers hold a wide range of misconceptions when they try to predict the possible evolution of a chemical equilibrium system that may have been perturbed by changing its mass. The aim of this paper is not to establish didactic guidelines for treating these problems, but to fill the gap resulting from the lack of a relevant advanced thermodynamic discussion, and to find the mathematical expressions of the variation of x, with the infinitesimal variation in the mass of a chemical equilibrium mixture. [Chem. Educ. Res. Pract. Eur.: 2000, 2, 303-312] KEY WORDS: Le Chatelier's principle; extent of reaction; change in the mass; chemical equilibrium condition; chemical equilibrium disturbance; chemical equilibrium shift
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