Limiting Reagents and Gases

student holding a syringe, reaction inside producing gas.

“What are we doing to help students achieve?”

I have always struggled as a teacher to find a good limiting reagents lab and activity that can be put into the hands of my students. Thanks to Dr. Bruce Mattson, I think I have found a winner I would like to share.

I had the pleasure of meeting Dr. Mattson this summer at Chem Ed 2019. He teaches at Creighton University in Nebraska and he has devoted much of his career to microscale gas chemistry as you can see on his website. His workshop on gases at Chem Ed was extremely helpful. It allowed people to see the immediate application to the high school classroom. The basis for many of the experiments is Mattson's unique method of reacting and collecting gases using a syringe which I demonstrated to students (see the video below).

 

Video 1: Microscale Preparation of Gases, Chad Husting's YouTube Channel, Aug 14, 2019 (accessed 2/2/20)

 

The first gas collection lab students attempted was the reaction of hydrochloric acid and magnesium. Each student team used 1.50 grams of about 1.8 M HCl. It is difficult to get an accurate small volume. Students find the mass of the HCl solution. Accuracy and precision is easier to obtain through finding the mass instead of the volume with small amounts. I provided the density of the HCl (1.048 g/ml). Students could then calculate to the volume and thus the moles of HCl used each time.

 

The students were perplexed to see the amount of hydrogen increase but then “flatline”.

Next, each student team had a different length of clean magnesium ribbon. The lengths varied from .5 cm to 8 cm. They measured out the length. I provided the centimeters of magnesium for every one gram of mass (I carefully measured the mass of 1 meter of Mg strip and then cut it into strips). They were able to calculate the moles of magnesium.

Each team from each class placed their data in a shared spreadsheet. All of the classes were responsible for developing a graph and then analyzing the graph in terms of excess and limiting reagents (figure 1). The students were perplexed to see the amount of hydrogen increase but then “flatline”. They also had to use the balanced equation of the reaction to determine the correct moles, grams and centimeters of magnesium and compare those values to the data on the graph.

 

Figure 1: Sample of a graph of the data collected by students.

 

Students immediately commented on the “sweet spot”. This is where the calculated value of the perfect stoichiometric amount of magnesium for the 1.50 mL HCl turned out to be about 2.4 cm. On the graph, this is the point between where the volume of hydrogen is increasing and after that volume has flatlined. This really hit home the idea of limiting and excess reagents and led to many rich discussions.

Overall, I liked this lab for several reasons. First, students got reasonably good data in a short period of time. Microscale is easy to prepare and cost effective. Dr. Mattson provided a great set of follow up questions to use with students. Students had to review dimensional analysis, proportional thinking, interpreting graphs and balancing equations. I would rate this lab as a “keeper”.

 

infrared image of gas collection process as reaction is taking place

Figure 2: Infrared image of the gas collection process.

 

As a bonus I was able to use the FLIR IR camera I just got. If you have not checked out I would highly recommend it. A student noticed the syringe was getting warm during the procedure. I showed him the IR picture to confirm his suspicions (figure 2).

The questions started flowing. “Why is it hot?” “What caused the heat?” “Is it the reactants or the products?” We got into a great conversation about bonding and heat well before we will ever cover it in class. It was a wonderful unscripted frontloading experience.

Overall, I plan on doing more with the gases. If you have not already, check out Dr. Mattson’s website. He has a treasure trove of great ideas that are student centered and teacher tested. You can even download the 2017 web version of his about the topic. Do you have a great idea? Please do not hesitate to share!

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General Safety

For Laboratory Work: Please refer to the ACS .  

For Demonstrations: Please refer to the ACS Division of Chemical Education .

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NGSS

Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data.

Summary:

Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data. Analyze data using tools, technologies, and/or models (e.g., computational, mathematical) in order to make valid and reliable scientific claims or determine an optimal design solution.

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Clarification:

Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.

Summary:

Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.

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Mathematical and computational thinking at the 9–12 level builds on K–8 and progresses to using algebraic thinking and analysis, a range of linear and nonlinear functions including trigonometric functions, exponentials and logarithms, and computational tools for statistical analysis to analyze, represent, and model data. Simple computational simulations are created and used based on mathematical models of basic assumptions. Use mathematical representations of phenomena to support claims.

Summary:

Mathematical and computational thinking at the 9–12 level builds on K–8 and progresses to using algebraic thinking and analysis, a range of linear and nonlinear functions including trigonometric functions, exponentials and logarithms, and computational tools for statistical analysis to analyze, represent, and model data. Simple computational simulations are created and used based on mathematical models of basic assumptions. Use mathematical representations of phenomena to support claims.

Assessment Boundary:
Clarification: