El principio de Le Chatelier como regla cualitativa: un obstáculo epistemológico en el aprendizaje del equilibrio químico

Resumen

En este trabajo se demuestra que dificultades de aprendizajey errores conceptuales relacionados con la incoorrecta aplicación del principio de Le Chatelier pueden explicarse mediante la consideración de tres factores: a) Los problemas derivados de la generateón inductiva del conocimiento científico. b) Las dificultades emanadas del empleo del lenguaje científico. c) La demanda conceptual necesaria en el estudio de sistemas con diferentes variables a controlar. Como resultado, el propio principio actúa como un obstáculo epistemológico que impide un aprendizaje significativo de los conceptos relacionados con el mismo. Por ello, se hace necesaria la consideratició de un tratamiento metodológico altenativo de enseñanza.

Abstract

The paper demonstrates that difficulties and misconceptions related to the incorrect application of Le Chatelier's principle may be explained in terms of three factors: a) Problems originated in generating scientific knowledge inductively. b) Difficulties arising from using scientific language. c) The necessary conceptual demand in the study of multivariable systems of control. As a result the principle itself acts as an epistemological obstacle impeding meaningful learning of related concepts. An alternative methodological approach to teaching must therefore be considered.

Extended Summary

The article is concerned with the study of sources which may originate meaningless learning to students when dealing with the prediction of shifts in chemical equilibrium. To this end Le Chatelier's principle plays a preponderant role.

One of the greatest and most challenging aspects of chemical equilibrium has been the students' proper use of Le Chatelier's principle. However, the limited character of this principle has been sufficiently treated both in various articles and in advanced textbooks. Despite the broad attack on Le Chatelier's principle launched by some of these authors, and the eloquence of their works, their arguments have been largely ignored by authors of general chemistry texts. In many cases, textbooks illustrate the use of the principle with the help of the constant for equilibrium, but without explicitly controlling the variables involved. Furthermore, it is usually presented under a positivistic assumption as an infallible principle without limitations.

A second aspect pursued in this paper deals with the problems originated in the traditional formulation of the Le Chatelier's statement. One of the most outstanding features of the principle is its vague and ambiguous character, and the difficulty in being formulated in a more precise and general manner. To this, we must add the different attemps to find a didactic reformulation undertaken by authors of general chemistry textbooks. These statements have serious flaws, which essentially involve their correct interpretation and competence as a predictive principle. Thus, many statements are tortuous and difficult to understand, and words of a wide polysemic character are used. Hence, many of these words have different meanings in scientific and everyday contexts; other words seem to be expressed in a physical (mechanical) rather than a chemical context. In spite of all this, the principle is normally taught to students as a simple and easy rule to apply.

The inductivistic presentation of Le Chatelier's rule as an infallible principle, and the difficulties arising from the lack of a precise and clear formulation would be two of the sources that might originate some teachers' and students' misconceptions. There is a third factor to be added related with the difficulty of properly controlling the variables involved, and with the use of simple linear causal reasoning. Again, serious deficiencies reinforcing these difficulties are found in the usual teaching approach of Le Chatelier's principle. Normally, general chemistry textbooks do not control system variables, thus preventing making explicit statements on how disturbances to the chemical equilibrium takes place. In sum, a ‘functional reduction’ of variables takes place, which means that fewer variables than are necessary are considered, and generally only one. These circumstances propitiate different misconceptions, and linear causal reasoning would be the predominant method used reach the answer.

The study considers Le Chatelier's principle as an epistemological obstacle which prevents meaningful learning of related concepts. Thus, we reinforce the idea of abandoning the use of a qualitative rule. The alternative methodological teaching approach can be based on appropriate philosophical and psychological models for the selection and sequencing of instructional tasks, and is based on the constant expression for chemical equilibrium.

Palabras clave:

• Inductivismo
• lenguaje
• razonamiento causal
• cambio metodológico

Keywords:

• Induction
• language
• causal reasoning
• methodological change