Academic journal article The Science Teacher

Bill's Box: A Simple and Effective Approach to Stoichiometry

Academic journal article The Science Teacher

Bill's Box: A Simple and Effective Approach to Stoichiometry

Article excerpt

Stoichiometry, the branch of chemistry dealing with the relative quantities of reactants and products in a chemical reaction, is challenging for students new to chemistry. This article describes an effective method of teaching stoichiometry to students at the secondary and college levels. Students find this method helpful because it starts with familiar analogies and a simple organizing grid. Using the approach, students quickly grasp the basic concepts of quantifying chemical reactions.

Chemical problem-solving and stoichiometry

The typical introductory chemistry class comprises a vast diversity of students. Therefore, teachers should use instructional methods that broaden their approaches to difficult, abstract concepts like stoichiometry. This article presents one such method.

Students can easily learn the basics of this approach in one class period. They can later build on it throughout their study of stoichiometry. Eight categories of questions (or problem types) are ultimately presented to students. These questions guide students to an understanding of stoichiometric concepts and help them develop the skills necessary to solve related problems. Unknowns involved in chemical reactions that can be calculated using this approach include moles, mass and volume of reactants and products, limiting reactant, excess reactant, moles and mass of excess reactant, molarity of solutions, molar mass, gas pressure, gas temperature, the ideal gas constant, and coefficients of a chemical equation.

Introduce stoichiometry with a simple analogy and an organizing grid

Many real-world analogies have been successfully used to convey the concept of whole-number relationships in chemical reactions: making sandwiches, s'mores, fruit baskets/salads, nuts/bolts/washers, cakes/cupcakes, coins, construction blocks, dresses, and student teams (Haim et al. 2003; Herron and Clausen 1975). Analogies that are most familiar to students and allow for hands-on experience may be the most effective regardless of the students' cognitive level (Herron and Clausen 1975; McMinn 1984; Toth 1999). Whatever analogy the instructor chooses can be substituted for the author's analogy presented here. First, a nonchemistry question is posed to students. This form of problem-solving scenario is an effective way for students to develop long-lasting conceptual understanding (Farrell, Moog, and Spencer 1999; Steiner 1986; Witzel 2002).

Problem Type 1

1. The ACME Tricycle Co. needs to plan its next production run.

2. Each tricycle is made from one frame, three wheels, and two handlebar grips.

3. Using only symbols and numbers, write an equation that represents this process.

4. How many frames, wheels, and grips are needed to produce 6,000 tricycles?

5. Try to solve this without your calculator.

6. Write your answers.

7. Most students will answer this quickly. Their equation should resemble: F + 3 W + 2 G [right arrow] [FW.sub.3][G.sub.2].

Next, model the construction of a three-row organizing grid, drawn underneath the equation, with one column aligned with each reactant and each product. Label the bottom line "count."

When I taught students how to use this strategy, I used alliteration and named the tool after myself, "Bill's Box," because it was easy for my students to remember. The Bill's Box activity has become the method for solving even the most arduous of stoichiometry problems.

Identify the starting point (*). For each part, ask, "Will the count be the same, more, or less?" Have different students come forward and insert their answers. When Problem Type 1 is completed, Bill's Box will look like Figure 1. Bold numbers represent the final answers.

FIGURE 1

Completed grid after answering Problem Type 1.

Using the coefficients, calculations along the bottom line
relate the desired tricycle count to counts for grips, wheels,
and frames to produce 6,000 tricycles. … 
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