In the last lesson students worked in small groups to create a draft of a procedure they had written for an experiment that tests the reaction rate of a calcium carbonate and hydrochloric acid reaction. In this lesson students will conduct a single trial of their experiment to see how the reaction works, and to see what revisions they may need to make to their experimental design.
This lesson aligns to the NGSS Disciplinary Core Idea of HS-PS1-5: Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs. It does this by giving students the chance to see how a reaction occurs when manipulating one of these variables or surface area.
It aligns to the NGSS Practice of the Scientist of Planning and carrying out investigations by giving students the chance to test run the experimental design they created in order to see how it works. This is typical in the scientific community—perfecting a procedure is often necessary before a full set of trials are conducted.
It aligns to the NGSS Crosscutting Concept of Stability and Change: Change and rates of change can be quantified and modeled over periods of time because this experiment will help students investigate the chemical change that takes place over periods of time that will vary according to the variable they are manipulating.
In terms of prior knowledge or skills, students should have already conducted this lesson on creating their experimental design.
The materials needed for this lesson include the following:
Do Now: Students begin class by taking out the experiment procedure that they worked on in class yesterday. In their notebook I ask them to make a data table for recording data for 3 different concentrations, temperatures, or surface areas.
I reason that this is a good way to start class because I want students to get their head back into their experimental design. They will not be able to make a data table without understanding the experiment. My hope is that most students can make a data table pretty easily, but if they cannot, then this is a warning to me that I should spend additional time reviewing the procedures with them. I do not want them going to work with chemicals if they do not understand what they are doing—a lack of understanding could lead to a waste of chemicals and time, and possible safety issues.
Activator: I ask for a volunteer from each of the three variable groups to show their data. While they run class I am free to walk around and inspect everyone’s data table. Students who do not have a data table must have one prior to starting the lab.
I have chosen this approach because I want students to feel supported, and I want everyone in each of the variable groups to be on the same page.
Mini-lesson: I project Experimental Procedure Trial Run slides and I begin by asking students to name the safety considerations for today. I am looking for them to name wearing gloves to protect against acid. They should have their hair tied up, and goggles on at all times. They should be very careful to not move unless they are certain there is no one that they might accidentally bump into. They should carry acid samples with two hands. They must dispose of their materials in waste beakers.
I then discuss materials. I remind them that most students will use 1M HCl, but the concentration group will also use 3M and 6M HCl. Reading the labels BEFORE they pour acids is critical. I note that there is only one place to measure out acid, and only 2 people should be there at a time. I note that every weigh station is equipped with calcium carbonate.
I note the locations where students can heat their acid. I remind students that if the acid overheats they can suffer serious injury, and the way to prevent this is to heat the acid slowly, and to monitor the temperature of the acid.
I then explain my role, which will be to watch for safety, to answer procedural questions, and to monitor how well the procedures work for students. I remind students that at after conducting 2-3 trials for their experiment that they should stop and reflect about how well their procedure worked, and how well they were able to follow the procedure. The question I want them to answer after testing their procedure is “What do I need to revise in my procedure?”
This instructional choice reflects my number one priority as a teacher, which is safety, and my second most important goal, which is that students learn science whenever possible by doing the work of the scientist.
Student Activity: After reviewing safety and procedures, I release students to do conduct trials of their experiment. Because I want to ensure that there is time left at the end of class for clean-up and debriefing, I set a timer that will go off 20 minutes before the end of my 65-minute class. I then walk around observing students, answering questions, and making mental notes about how well the procedures are being implemented and followed. I also make sure that every student is recording data in their data table.
In the event that a student is acting unsafe, I immediately discuss the problem with the student. I pay special attention to students who are heating acid, and those who are working with higher molarity acids. I want to make sure they are acting safe, and that they are confident while handling these potentially dangerous materials.
To wrap this lesson up I first make sure that all students have cleaned their work stations and tidied materials, which means wiping down their lab bench, properly disposing waste materials, and rinsing and putting their beakers and graduated cylinders on drying racks.
I then give students several minutes to complete the Reaction Lab Trial Run: Reflection. These questions include the following:
I then ask students to share some answers. The first three questions are designed to get students thinking about revisions they may need to make in their procedure. The fourth question is designed to lead into the next two lessons. How do they know if their answer is reasonable? The only way to know is to use stoichiometry to compare the mass of CO2 that the reaction produced with the theoretical yield of the reaction.
What I notice in reflection 1 is that the students need to really polish their procedure. The omitted a lot of detail, and they noticed this omission as they were conducting it; they knew what to do but they need to record what they did. Reflection 2 shows the need to re-teach stoichiometry, and I am pleased that they recognized that clumping of the powder limited the surface area. Reflection 3 makes me wonder if there is a measurement error because students should have seen carbon dioxide emissions of 1-2 grams.
I am especially pleased that I am able to have students teach a key part of this lab as shown in this procedure critique video. Students must know the starting and ending mass of their chemicals in order to know how much carbon dioxide was given off from the experiment.
One thing I recognize is that we have not discussed as a class how incorporate the idea of reaction rate into their data collection. That will come in a subsequent lesson.