Stoichiometric conversions and analysis are likely the most pervasive topic in an introductory chemistry course. Whether a basic high school course or a more math-heavy honors course, you can bet that at some point the idea of a mole-to-mole conversion will be discussed. And for the more math-heavy course, gram-to-gram conversions are a sure bet. The most difficult and dreaded task of stoichiometry may be the so-called "limiting reactant problem". How does a student approach a situation in which the amounts of two or more reactants are given and the mass of product must be calculated? This Concept Builder targets this very task.

The Limiting Reactants Concept Builder is comprised of 36 questions. The questions are organized into nine Question Groups and spread across three different activities. In the first activity, The Perfect Match, students complete a table in which they relate a given quantity of a reactant and product (in moles) to the number of moles of the other species in the balanced chemical equation. There are six rows in the table and thus six situations to analyze. The calculations presume reactants are present in ratios that match the ratios of the coefficients in the balanced chemical equation. In the second activity, Moles on ICE, students solve four problems in which they must determine the moles of reactant reacting and the moles of product being formed. The reactants are present in ratios that do not match the ratios fo the coefficients in the balanced chemical equation. Students must identify the limiting reactant and determine the moles of excess reactant remaining. The information is organized in an ICE Table. (I=Initial amount; C=Change in amount; E=Ending amount). The final activity, Mass on ICE, is similar to the second activity with the exception that the analysis is done in terms of grams. The amount of mass initially present is a "kind number", typically a multiple or a simple fraction of the molar mass of the species. The exact questions are shown below.

We view this Concept Builder as being most valuable in the early stages of teaching and learning about limiting reactants. The use of graphic organizers and "kind numbers" allow students to focus on the sensibility of limiting reactant problems. The design is intended to help students understand the logic of how to approach such a problem and to gain some confidence in their analysis. Once this understanding is developed, it will be easier to teach a strategy like "perform two calculations of the amount of product and use the calculation that produces the least amount of product" (or whatever strategy you present).

Student answers are considered correct if they fall within 2% of the keyed answer. We recommend that students use at least 3 significant figures for their molar mass values and enter their answers to at least three significant digits. Feedback is provided after each submission. If a student's answer is correct, the answer field turns green. If it is not correct, then the answer field turns pink. Answers can be entered directly from a device keyboard. Those who are using mobile devices or touch screens may prefer our built-in number pad. A tap on the number pad icon next to a field will pop up a built-in number pad that does not cover any part of the question.

This Concept Builder was intended as an in-class activity. Teachers using the Concept Builder with their classes should preview the activity (or view the Questions in a separate file) in order to judge which activities would be most appropriate for their students.

 

The most valuable (and most overlooked) aspect of this Concept Builder is the Help Me! feature. Each question group is accompanied by a Help page that discusses the specifics of the question. This Help feature transforms the activity from a question-answering activity into a concept-building activity. The student who takes the time to use the Help pages can be transformed from a guesser to a learner and from an unsure student to a confident student. The "meat and potatoes" of the Help pages are in the sections titled "How to Think About This Situation:" Students need to be encouraged by teachers to use the Help Me! button and to read this section of the page. A student that takes time to reflect upon how they are answering the question and how an expert would think about the situation can transform their naivete into expertise. 

 

 

Additional Resources:

 

Visit: Concept Builder | Questions (for Task Tracker Teachers only)