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Andre
LAUGIER et Alain DUMON
ABSTRACT: The subject of this work concerns the construction of a representation of a chemical reaction in macroscopic and microscopic forms by secondary-school pupils in the 15-16 year age range. All the surveys on this subject have revealed many unsolved difficulties among pupils. The balanced equation enables a chemical reaction to be represented in a microscopic form, but it can be read at a macroscopic level (that of the observed phenomena) or at a microscopic level (that of particles and the imagined phenomena). A study of the history of chemistry reveals that the necessity of representing a chemical reaction at these two levels presented chemists with an intellectual (epistemological) obstacle until the end of the nineteenth century. Our study revealed such difficulties among pupils and the recurrence and persistence of some of these difficulties enabled us to identify them as genuine epistemological obstacles which will require much work to overcome. Pupils must have educational experiences designed to allow them to move freely between these levels to appreciate the significance of the balanced equation. [Chem. Educ. Res. Pract. Eur.: 2000, 1, 61-75] KEY WORDS: chemical reaction; representation in equations; phenomenology; macroscopic level; microscopic level; epistemological obstacle RESUME: Ce travail concerne la construction d'une representation de la reaction chimique, dans les registres macroscopique et microscopique, par des eleves en classe de seconde (15-16 ans). Toutes les etudes sur ce sujet ont fait apparaitre chez les eleves, l'existence de difficultes, nombreuses et non resolues. L'equation bilan permet, dans le registre symbolique, de representer une reaction chimique, mais sa lecture peut se faire au niveau macroscopique (celui de la phenomenologie observee) ou au niveau microscopique (celui des particules et de la phenomenologie imaginee). L'etude historique revele que la necessite de representer la transformation chimique dans ces deux niveaux a constitue pour les chimistes un obstacle epistemologique qui n'a pu etre franchi qu'a la fin du XIXe siecle. Notre etude didactique montre que lorsque les eleves sont face a une phenomenologie macroscopique et doivent imaginer une phenomenologie microscopique explicative, ils heurtent eux aussi a de nombreux obstacles. L'enseignement doit leur proposer un curriculum adapte pour leur permettre de circuler entre ces deux niveaux de representation grace a l'equation bilan. MOTS CLES : reaction chimique; niveau de representation; phenomenologie; niveau macroscopique; niveau microscopique; obstacle epistemologique
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