Enseignement/apprentissage de la chimie : conceptions des apprenants de 3ième en tle de l’enseignement secondaire général camerounais sur le concept de stœchiométrie

Abstract

Stoichiometry is fundamental concept to the quantitative study of chemical systems and requires, in the first instance, a flow between the macroscopic and microscopic registers. However, it turns out that many learners, from classes « 3ième » to « Tle », and even at university, have difficulty solving stoichiometry problems. We wanted to identify the causes of these difficulties. To achieve this, we explored three different avenues: the institutional avenue (analysis of the official textbook and syllabus of the « 3ième » class), the epistemological avenue (analysis of the evolution of ideas about stoichiometry) and the conceptual avenue (analysis of theories of levels of knowledge). The students' difficulties were described in the light of the theory of levels of meaning. We administered a paper-and-pencil questionnaire to a total of 239 learners in classes from « 3ième » to Tle, with the dual aim of determining the meanings they attributed to the various concepts in the conceptual network of stoichiometry and analysing the evolution of these meanings. The results show that some concepts in the conceptual network of stoichiometry, such as the stoichiometric coefficient, are at the crossroads of the three levels of meaning. The others are, for the most part, strongly constructed at the symbolic level, with a few allusions to the macroscopic level and very few meanings at the microscopic level. Also, the relationship between a compound's quantity of matter, its mass and its molar mass, which is still mobilised at a symbolic level by class « 3ième » learners, is the first to be called upon when they have to calculate the quantity of matter of a compound involved in a reaction knowing that of another, despite the mole-to-mole relationship between the said compounds. All this leads us to conclude that learners of « 3ième » of the general secondary education attribute varied and generally irrelevant meanings to the concepts in the conceptual network of stoichiometry. These irrelevant meanings, mostly at the symbolic level, seem to evolve much more towards the macroscopic than the microscopic levels as the learners progress through their course. In addition, certain misconceptions, such as equating the stoichiometric coefficient with the quantity of initial reactant or product formed, seem to increase as the level of students rises.