Stoichiometry

One of the first numerical problems encountered in introductory chemistry is that of "limiting reagents". The applet does not approach or solve the problem as it would be done with paper and pencil, on the blackboard, or in the textbook. Rather, it serves as a supplement to such calculations, providing imagery that helps students see beyond a route mathematical procedure. In lecture, the instructor may work through the problem on the blackboard, referring to the applet as a constant visual reminder of what is being calculated and why. Outside lecture, students may use the applet while solving problems, to generate and check intermediate results and help them discover and correct their own mistakes.

Classroom Use

The applet is meant to supplement the chalkboard solution of limiting reagent problems. 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. By clicking on the bars in the upper panel, the initial grams of the reactants can be entered. As the slider bar is moved to the right, the bar-chart will show 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 will show 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 what it means to be a limiting reagent.

The lecturer could then work through the calculation on the blackboard while referring to the applet to illustrate what is being calculating 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 easily predicable manner. 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. 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.