Engage - Limiting Reactants

For those of you all who are familiar with the 5E learning cycle, I would like to share an engage activity that I use when beginning a lesson on limiting reactants.

For those who would like to learn more about the engage phase and the overall 5E learning cycle, click here.

Within the 5E learning cycle, the engage phase is designed to promote student interest in the topic at hand while determining prior knowledge, assessing for misconceptions, and building a conceptual framework that is often couched within the context of a real-life example.

Therefore, this engage activity is designed accomplish the following goals:

• Help students tap into the conceptual framework they already possess related to limiting reactants

• Provide a real-life example that ties into specific student interests

• Raise and confront common misconceptions related to limiting reactants

• Introduce a framework that will help scaffold later vocabulary and calculations

Students are already familiar with the idea that of “limiting parts” from many scenarios in life. If a recipe for 30 cookies calls for 2 cups of chocolate chips, but they only have 1 cup, then they can only make 15 cookies. If a bicycle needs two wheels, and they have three…then, they can only build one complete bike. They have the framework, they just need to realize it and access it.

This year, since I have multiple students who are interested in planes and aerospace, I chose to do a plane assembly problem in which I show them a picture of a plane taking off and then a picture inside the plant where they are assembling a Boeing 747. We then create a chart with the following “part scenarios,” in which I provide students with different number of each parts. They then have to tell me how many complete planes I could make, the part that would be completely used up first, and the number of each part that would be left over.

As we are discussing and filling in the chart, I am very intentional in the language that I use. You will repeatedly hear me use the phrases, “limits how many planes we can produce…is used up first…is used up completely…parts left over...etc.” This language will directly translate to the formal definitions of limiting and excess reactant introduced later in the lesson.

A sample chart is shown below. Below certain columns, you can also see the “teacher notes” I have left myself to ask certain additional questions to address misconceptions. For example, one misconception is that the limiting reactant (or part in this case) is always the one present in the smallest amount. However, the second scenario column shows that this is not the case. To address this misconception, I ask students to predict which part they think would run out first if they knew nothing about the numbers of parts needed for one complete plane. They say that they would predict the wings to run out first since it has the smallest number of parts. However, we have already established that the wheels are actually the part that are completely used up first and stop our plane production.

So…I then ask students what else we have to take into account besides the initial number of each part. They say that we also have to look at the number or ratio of parts needed for each plane. From this point, we then write a chemical equation in which we assign made-up symbols to the parts (i.e. Tu for turbine) and write the number of parts needed for one complete plane as the coefficients for the symbols. This helps remind us that the ratio between parts aligns with the same way we use coefficients for mole ratios between reactants and products in a chemical equation.

To really make the entire lesson effective, find a way to keep parts of this engage discussion posted throughout so you can go back and explicitly refer to it to help students make connections between their real-world framework and how it mirrors what they are learning to do with products and reactants.