Teaching Notes for "Riverside's Dilemma"
This problem is used in the second semester of an Honors general chemistry course as a context for the development of concepts associated with aqueous acid-base chemistry. Part 1 is used to introduce the equilibrium behavior of strong and weak acids and bases in solution. Part 2 deals with the concept of neutralization and its relationship to dissociation equilibria. In part 3, students encounter a more complex neutralization process, which requires them to pull together and apply the concepts developed in parts 1 and 2 to a more open-ended situation.
Part 1. Students in this course have a wide range of past experiences with acid-base chemistry. This problem is introduced after the students have completed a quick quiz (first as individuals and then in groups) that asks them to identify or define a series of terms associated with acid-base chemistry. They report out and compare their answers, but there is no formal instruction concerning any of these topics at this time - the purpose of the exercise is to jog memories, and to highlight areas of little common knowledge. (Ideas associated with Ka and Kb invariably fall in the latter category.)
Part 2. Students have a tendency to think that a neutralization reaction results in a solution of pH 7. This problem prompts them to discover the relationship between neutralization and dissociation reactions; when they recognize that a neutralization is the reverse of a weak dissociation process, the need to treat the neutralization as a separate, strongly-favored step in the pH calculation process becomes clearer.
Part 3. The third part of this problem is more difficult and open-ended. It requires students to pull together the material they've dealt with in Parts 1 and 2 in thinking about a more complex situation. They must recognize the various combinations of waste streams that are possible; decide which constitute neutralization processes; and of these, which have equilibrium constants appropriate for the desired result. They must then work backwards to decide on ratios of reactants that would results in the desired pH at the end.