STOICHIOMETRY WITHOUT TEARS
RICHARD M. FELDER
North Carolina State University Raleigh, NC 27695
Students who are about to take stoichiometry fear it, and many who are currently taking it hate it. The homework never ends, and you can spend hours on a single problem without getting anywhere. It’s the weedoutcourse—30%, or 50%, or 70% flunk it, depending on the institution, the class size, and who is teaching. So what’s in this killer stoichiometry course? “What goes in either comes out or stays in,” that’s what—and usually we never get to the part where it stays in, leaving us with Input = Output. Not exactly intellect-stretching stuff. Of course, there’s more— gas laws (PV = nRT: given three variablevalues, solve for the fourth), simple vapor-liquid equilibrium relations [yAP = pA*(T): given a vapor pressure correlation and two of the variables yA, P, and T, solve for the third variable], and energy balances (Q =∆H: given feed and outlet conditions, calculate ∆H by integrating heat capacities and adding latent heats, and then solve for Q). That’s about it. The energy balances give the studentstheir first brief immersion in the alphabet soup of thermodynamics, but only up to U and H—and most of those who go down in the course are lost well before they get there. What defeats many of them, I believe, is the simplicity of the subject matter. The course starts off with deceptively easy material: units and dimensions, definitions of process variables, and material balance problems that canbe solved with college freshman and even high school methods. We
Richard M. Felder is professor of ChE at N.C. State, where he has been since 1969. He received his BChE at City College of New York and his PhD from Princeton. He has worked at the AERE, Harwell, and Brookhaven National Laboratory, and has presented courses on chemical engineering principles, reactor design, process optimization, andradioisotope applications to various American and foreign industries and institutions. He is coauthor of the text Elementary Principles of Chemical Processes (Wiley,1986).
Copyright ChE Division ASEE 1990
Educational psychologists tell us that you never ...teach anyone how to do anything by telling them how to do it. Rather, you teach them by showing them how, and then having them try itthemselves and giving them corrective feedback. I believe in this principle... give sermons about carrying units, drawing and labeling flow charts, doing the problem bookkeeping or degree-of-freedom analysis before plunging into the math, but they don’t believe us—and sure enough, they get the right answers doing it their way. Then the game changes. The problems get longer, and we keep throwing moreinformation into the pot. We give them multiple process units, recycle and purge, single and multiple reactions, volumetric flow rates instead of mass or molar flow rates, and relative saturations or dew points instead of mole fractions—and the problems that used to take them thirty minutes start taking an hour, then two hours. They write equation after equation, but never seem to have quite enoughinformation to solve for the quantities they are trying to calculate. Some begin to believe that there may be a point, after all, in being systematic about setting up problem solutions, and save themselves; others resist to the bitter end and fail. I don’t recall ever failing a student in stoichiometry who really understood how to draw and label a flow chart and to use it systematically in thecourse of a problem solution. Consequently, since I began teaching the course twenty years ago I have directed more and more of my efforts toward motivating the students to do just that. It seems to work. Only about ten percent of the students who take the course from me these days fail it, and most of those give up early in the semester. Also, the attitudes of those who pass are neutral to...