Learning Chemistry from Good and (Why Not?) Problematic Results: Kinetics of the pH-Independent Hydrolysis of 4-Nitrophenyl Chloroformate

摘要

The determination of kinetic data is central to reaction mechanism; science courses usually include experiments on chemical kinetics. Thanks to PC-controlled data acquisition and availability of software, the students calculate rate constants, whether the experiment has been done properly or not. This contrasts with their experience in, e.g., organic synthesis, where a broad melting point indicates an impure product. In order to teach quality kinetics and link theory to experiment, we used a multistep project, based on the (convenient) pH-independent hydrolysis of 4-nitrophenyl chloroformate in aqueous organic solvents. The steps of the project included statement of the experiment's objective; a quiz on reaction mechanism and experimental techniques in chemical kinetics; students' decision on the organic solvent to be employed; extraclass activity to assess their choice of the solvent; carrying out the experiment; and discussion of the results obtained. We have applied the constructivist approach to illustrate that controlling the experimental conditions (solution temperature and homogeneity) is a prerequisite for obtaining quality kinetic data. The students' evaluation was highly positive because they participated in the different steps of the project.