JChemEd Activity Supports Student Understanding of the Limiting Reactant

AACT members have access to 50 ACS Publications articles each year. We are highlighting one article or activity each month that you might consider downloading through this member benefit. This month, we are featuring a hands-on student activity from the July 2014 issue of the Journal of Chemical Education titled A Hands-on Activity Incorporating the Threefold Representation on Limiting Reactant by Angélica M. González-Sánchez, Edgardo L. Ortiz-Nieves, and Zuleika Medina.

The concept of the limiting reactant for a chemical reaction is one of the most difficult for high school students to fully understand. Many of them assume that the reactant with the smallest mass will run out first, failing to understand that the moles of reactants must be examined and compared rather than the mass. The activity described in this article includes three different representations for students to consider, macroscopic, submicroscopic, and symbolic.

In this lab-based activity, the reaction between copper (II) sulfate and sodium hydroxide is represented in three different ways. Each of the representations is carried out at its own lab station and should take students about 20 minutes to complete. After working through each station, the lab groups use their findings to answer a series of questions. A short video of the reaction can be found here for reference.

The first station (macroscopic) requires students to make careful observations as two different volumes of copper (II) sulfate are reacted with two different concentrations of sodium hydroxide. Students transfer samples of the resulting solution, without the precipitate, to further examine by adding additional reactants. They then predict the identity of the limiting and excess reactants based on those observations.

At the second station (submicroscopic), students use Play-Doh models to represent the two reactants in the reaction. Using those models, they construct each of the products and analyze how many of each reactant is required to form each product in the reaction. They use this information to once again predict the limiting and excess reactants of the original reactions.

Finally, at the third station (symbolic), students use conclusions from the first two stations to write a molecular equation and then calculate the moles of each of the reactants and the solid product formed. The results of these calculations are used to positively identify the limiting and excess reactants before the follow-up questions are answered.

The supporting information available for this activity includes student instructions, instructor background information, preparation instructions, and labels to use at each of the three lab stations. An answer key is also included in the instructor background document. All items can be downloaded as Word documents so that you can edit them to meet the needs of your students.

AACT has several resources in our high school library to help with student understanding of limiting reactants. They can be downloaded in the sidebar of this page. If you have a favorite ACS Publication article that you use in your classroom, please share it with us at AACTconnect@acs.org.