Using Limiting-Excess Stoichiometry to Introduce Equilibrium Calculations: A Discrepant Event Laboratory Activity Involving Precipitation Reactions

Abstract

The transition between limiting and excess stoichiometry and equilibrium calculations is demonstrated in a laboratory activity for introductory college chemistry students. Students are presented with two precipitation reactions and asked to predict the identity of any excess reactants in the filtrate, on the basis of mole calculations they have already learned. They find that in one of the reactions there are two excess reactants instead of only the one that they originally predicted. This "discrepant event" provides a motivating laboratory opportunity in which to introduce the concept of equilibrium, equilibrium calculations, and Ksp as reasonable explanations of the results.