(445g) Teaching Degree-of-Freedom Analyses to MEB Students with "Specify the Specifications" Problems.

Performing a degree-of-freedom analysis is an essential problem-solving step for Chemical Engineers in Material and Energy Balances and beyond. Before deriving and solving equations, the problem-solver must first ensure that the number of constraints matches the number of unknowns— the problem has multiple contradictory solutions if there are too many constraints, but the problem is not solvable if there are too few constraints. Despite this importance, novice Chemical Engineering students sometimes skip this step and assume (sometimes incorrectly!) that any problem assigned for coursework must be appropriately specified.

To encourage students to learn, use, and practice this essential skill, an original genre of Material and Energy Balance problems are presented here in which many potential constraints are presented in list form, as opposed to being explicitly given in the problem statement. Students are instructed to accept these potential constraints as true only until the quota of constraints (as defined by the number of unknowns from the degree-of-freedom analysis) has been met. They must disregard all further constraints as superfluous.

Due to the uncertainty about which information should be accepted as true, these type of “specify your specifications” problems can be quite stressful for students lacking in self-confidence. Hence, these problems do not work well for high-stakes individual assessments, such as exams. However, they work very well for in-class problem-solving sessions in small groups, where spirited debate among peers can help students to understand the importance of a degree-of-freedom analysis and gain confidence. These problems also work well for homework assignments, where students may experience the agony of attempting to solve an over- or under-specified problem without the time pressure of an exam.