Explore - Limiting Reactants

Today I would like to share an exploration activity that can be used when teaching about limiting reactants. The explore phase of the 5E learning cycle is designed to allow students to have time to explore concepts in a more concrete manner before scientific terminology/concepts are formally taught.

(Note: The Engage phase of this lesson has also been shared in a previous blog post.)

The goals of this explore activity are as follows:

• Give students the opportunity to perform two reactions in which they see physical evidence for reactants being used up first or remaining after the reaction is complete.

• Allow students to collect and analyze data that addresses the common misconception that the limiting reactant is the reactant present in the smallest amount.

• Have students make predictions about the mole ratios (coefficients) within the balanced chemical reaction based on data analysis.

• Have students model the reaction on the molecular level to build conceptual understanding.

Here’s how the activity goes:

The students first perform the following two reactions between Mg and HCl. This reaction is particularly useful for this activity since the lack of formation of hydrogen gas indicates that the reaction is complete. In Reaction 1, the Mg should completely dissolve and be the limiting reactant. In Reaction 2, the Mg should remain at the end of the reaction, thereby implying that the HCl was the limiting reactant.

(image from reich-chemistry.wikispaces.com)

Reaction 1

1. Find the mass of your ~2 cm strip of Mg ribbon. Write it here: ___________ g

2. Pour 4.00 mL of 1.5 M HCl into a small test tube. Then, use your tweezers to carefully drop the Mg into the test tube.

3. Observe the reaction and write down which reactant appears to be used up first. ____________

Reaction 2

1. Find the mass of your ~2 cm strip of Mg ribbon. Write it here: ___________ g

2. Pour 3.00 mL of 1.5 M HCl into a small test tube. Then, use your tweezers to carefully drop the Mg into the test tube.

3. Observe the reaction and write down which reactant appears to be used up first. ____________

Then, they calculate the number of moles of Mg used in each reaction and fill in the following table. Since my students had not yet studied solutions and molarity, I gave them the moles of HCl. If you have already covered solutions at this point, feel free to have them perform this calculation as well.

For both reactions, go back and calculate the initial number of moles of each reactant. Show your calculations neatly. (molar mass of Mg = 24.31 g/mol)

Next, they use the data to answer the following questions addressing a common misconception and then make predictions about the relative coefficients of the reactants, which they then confirm by writing out a balanced chemical reaction.

• Did the reactant with the least initial number of moles always get used up first? Give evidence.

• If you wrote a balanced chemical equation for the reaction between Mg and HCl, would you predict the coefficients of these reactants to be the same or different? If different, which reactant would have the larger coefficient? Give evidence to support your prediction. (*Note: DO NOT write out the equation…just think about it based on the data you have collected so far!)

• Now, write out the balanced chemical equation for the reaction between Mg and HCl and compare it with you prediction in the previous question. How does the stoichiometry of the reaction help to explain the results that you saw? (i.e. Does it help explain why a certain reactant was used up first in each reaction?)

Finally, they model the reaction on the molecular level using beads and pipe cleaners.

Microscopic modeling:

You will now use models to illustrate what is happening at the microscopic level during the reaction. Mg will be green beads, H will be white beads, and Cl will be orange beads. When atoms are together in a molecule, the beads will be joined on a pipe cleaner.

Rxn. 1 – You start with 3 HCl molecules and 1 Mg atom. Predict and model the products that will be formed and identify which reactant was left over and which reactant was used up first. Draw what your models look like here.

Rxn. 2 – You start with 3 HCl molecules and 2 Mg atoms. Predict and model the products that will be formed and identify which reactant was left over and which reactant was used up first. Draw what your models look like here.

• How does this modeling relate to the reactions you observed in the test tubes?

This activity generates a lot of great discussion and really challenges the students to think about what they are seeing at the macroscopic level. Then, going forward in the lesson, student experiences/thoughts from this activity can be referred to in order to help teach formal vocabulary and concepts.

If you would like a full copy of this activity handout, click here! I hope it helps give your students a great foundation for further learning about limiting and excess reactants!