Visualization to get beyond recipes: Stoichiometry applet

The goal of this applet is to get students to see beyond the recipe for solving limiting reagent problems.  The students often learn this as a stepwise procedure. In the initial stages of learning, students will often have correct intermediate results written down but not understand what these really represent and so are not able to complete the problem.  Even once they learn the procedure, without deeper understanding, retention is bound to suffer. 

The applet can be found at (http://ir.chem.cmu.edu/irProject/applets/stoich/ and click on applet) Consider using the applet while solving the following problem, Example 4.5 of Chang:^[9]

Urea [(NH₂)₂CO] is used as fertilizer and as animal feed and is used in the polymer industry. It is prepared by the reaction between ammonia and carbon dioxide:

2NH₃ + CO₂ --> (NH2)₂CO + H₂O

In one process, 637.2g of NH₃ are allowed to react with 1142 g of CO₂. (a) Which of the two reac­tants is the limiting reagent? (b) Calculate the mass of (NH₂)₂CO formed. (c) How much of the excess reagent (in grams) is left at the end of the reaction?

By clicking on the bars in the upper panel, the initial grams of the reactants may be entered. As the slider bar is moved to the right, the bar-chart shows that the moles of NH₃ decrease twice as fast as those of CO₂, giving a visible image of the meaning of the stoichiometric coefficients. Pulling the slider bar across the screen shows the reactants disappearing and products appear­ing. When the slider is positioned all the way to the right, the reaction is complete and it is clear that the reaction has ended because the NH₃ has run out. The applet thus provides a visual image of the limiting reagent calculation which may be useful both in lecture and during homework.

The limiting reagent applet is meant for reactions that go to completion. An extension of this applet builds in the idea that the equilibrium position of a reaction is set by the equilibrium constant. (http://ir.chem.cmu.edu/irProject/applets/equil/) In this applet a dot has been added to the slider to indicate the equilbrium position of the reaction.  The common visual theme will encourage students to make connections between this new material and that covered a few months previously, possibly even by a different instructor in the first semester of a two-semester introductory course.

Uses in Lecture

The lecturer can begin by using the applet to illustrate the concept of a limiting reagent in a detailed and concrete manner without the need to perform any explicit mathematical calculations.  He or she may then work through the calculation on the blackboard while referring to the applet to illustrate what is being calculated and why. The conversion from grams to moles can be motivated by using the applet to stress that it is the moles of the reagents that change in an manner that can be easily derived from the stoichiometric coefficients. After calculating a number on the blackboard, the lecturer can pull the progress bar to the relevant position and illustrate what has just been calculated so that students can see how that number fits into the overall scheme. Finally, the conclusion of which reagent is limiting and how much of each product is produced can be confirmed. Throughout, the applet serves as a constant reminder of what is being calculated and why.

Note that the applet provides an alternative representation that complements what is usually presented on the board.  It does not approach or solve the problem as it would be done with paper and pencil. Rather, it serves as a supplement to such calcula­tions, with the goal of providing students deeper insight into the qualitative nature of the system and the calcula­tion. While the specific approach to calculations varies between textbooks and faculty members, our goal is to “simulate” the system in a manner that can accommodate any correct approach.

Uses outside the classroom

Students can use the applet while performing practice problems. It can be used to check their answers and, when they disagree, provide an instrument for them to discover and correct their own errors. For instance, if a problem involves a number of intermediate steps, students can check these intermediate steps. If a student begins doing manipulations and gets confused as to what it is they are calculating, comparing their numbers to those on the applet may help bring the student back on track. For instance, the introduction of an intermediate variable representing the number of moles consumed is not explicitly included in the applet.