Learning about stoichiometry: from students’ preconceptions to the concept of limiting reactant

Abstract

We have studied students’ previous conceptions and the effects of the usual teaching about the concept of limiting reactant. Previous work revealed two apparently contradictory conceptions held by students: both reactants are totally converted at the end of the transformation whatever the proportions, and only one reactant is converted whatever the proportions, with an active agent/passive object representation. We examined students’ explanations about various experimental problems to see whether one kind of problem leads preferentially to one conception or the other. We investigated grade 10 students at the beginning and at the end of the school year in order to study the impact of teaching stoichiometry on students’ conceptions. The results show that the conception both reactants are totally converted is quite strong in those problems where reactants are in the same physical state, and is more in competition with the conception: only one reactant is totally converted when the reactants are in a different physical state. It seems that teaching has little effect on wrong answers, but mainly leads to a shift from no answers to good answers. [Chem. Educ. Res. Pract., 2007, 8 (4), 362-375.]