The Teaching of Consecutive First-Order Reaction Kinetics

An instructional sequence has been developed to increase conceptual understanding of a special topic in chemical kinetics, first-order consecutive reactions: A→k1B→k2C where k1 and k2 are the rate constants associated with each step, using a sequence of qualitative and quantitative exercises. The qualitative exercise involves aperture-limited water flow through tubes in series to represent interconversion of species involved in the consecutive reaction. Adjusting the ratios of the aperture diameters permits exploration of the limits, k1 >> k2 and k2 >> k1. The quantitative activity involves employing a “dice game” to generate random kinetics “data.” The data was subsequently fitted using a preprogrammed Excel workbook using integrated rate laws to determine the relevant rate constants. The focus of this instructional sequence was to link changes in the time-dependent concentrations of species A, B, and C to changes in k1 and k2, and to create an environment that allowed discussion of the steady-state approximation without the use of complex mathematics. The instructional sequence was piloted during Cohort III of the Information and Technology in Science (ITS) Center. A similar exercise is now implemented as part of the Texas A&M University First-Year Chemistry Program as part of the laboratory experience. We found that the average participant score from pre- to post-test showed marked improvement (25% to 75%); however, even after instruction some misconceptions remained.