Exploring Solid-State Kinetics through Anion Exchange in Hydroxy Double Salts: An Undergraduate Laboratory Experiment

Abstract

An integrative upper-division undergraduate laboratory experiment that incorporates inorganic synthesis, solid-state chemistry, and chemical kinetics is presented. Adapted from published research on anion exchange in layered metal hydroxides, the experiment has students synthesize zinc hydroxy nitrate, [Zn5(OH)8](NO3)2·nH2O (ZHN), and follow the release of its interlayer nitrate during exchange with chloride ions over the temperature range 30–70 °C. Reaction progress is monitored simultaneously using ultraviolet–visible (UV–vis) spectroscopy to track nitrate release in solution and powder X-ray diffraction (PXRD) to characterize structural transformations in the solid phase. Complementary attenuated total reflectance–Fourier transform infrared (ATR-FTIR) spectroscopy confirms the exchange process. Students apply and critically compare three kinetic models (first-order, parabolic diffusion, and Avrami–Erofe’ev) to evaluate possible mechanistic pathways and use Arrhenius analysis to extract apparent activation energies from the UV–vis and PXRD data. Because the exchange proceeds through several coupled transport and reaction steps, students encounter difficulty directly assigning a mechanism to a reaction in a layered solid; the scaffolded, inquiry-based design develops their capacity to interpret nonideal kinetic systems and to integrate structural, kinetic, and thermodynamic analysis.