This lesson is an application of stoichiometry and will serve as a "how-to" for students to complete their second culminating project (assigned in about 2 weeks) that will be used as a summative assessment for this unit. Students will be applying what they have learned thus far about grams and moles conversions, and through the process of completing the lab analysis, will also learn about mole to mole ratios. Before this lesson, students should already know how to balance chemical reaction equations and how to convert from grams to moles and vice versa.
This lesson addresses multiple Science and Engineering Practices. First, students will be carrying out an investigation (SEP 3). Students will analyze and interpret their collected data (SEP 4). As they are completing grams to moles conversions and then completing mole to mole conversions, they will need to engage in mathematical thinking (SEP 5). Finally, as students draw conclusions based on their data and calculations, they will need to construct explanations for what the data means and why their outcome is what it is (SEP 6).
This student investigation directly addresses HS-PS1-2. Students are investigating a simple chemical reaction and learning how to predict the yield of such a reaction. A second Performance Expectation, HS-PS1-7, is also covered as students are learning conservation of mass as it pertains to this reaction. They learn that the atoms themselves are conserved during chemical reactions, and later can make the connection that mass is also conserved (which will be critical during our study of nuclear reactions).
Finally, there are two Crosscutting Concepts that are addressed. Students are using a chemical reaction equation to represent the observed chemical reaction, which addresses XC-SPQ-MS-4, a middle school crosscutting concept that is still particularly important for students at the high school level to master. Students also must clearly demonstrate understanding that matter is conserved during a chemical reaction which directly addresses XC-EM-HS-1.
Lab Prep BEFORE class:
While I take attendance, students do a warm-up activity in their composition Warm-Up/Reflection books. I use warm-ups to either probe for students' prior knowledge about the day's upcoming lesson or to have them bring to mind and review what they should have learned previously. (To read more about Warm Up and Reflection Books, please see the attached resource.)
Today's Warm-Up: "How many moles of aluminum are in 0.05 g of Al?"
In this case, the warm-up is asking students to recall from yesterday's lesson how to convert from grams of a particular substance to moles of that substance. It is also preparing students to participate in similar conversions in today's investigation.
If time permits, I walk around with a self-inking stamp to stamp the completed warm-ups indicating participation, but not necessarily accuracy. On days when there is too much business keeping, I do not stamp. Students have been told that warm-ups are occasionally immediately checked and other times not. At the end of each unit, Warm-Up/Reflection Books are collected and spot-checked. Today, I deliberately make sure to check composition books to gauge student understanding on this topic, informing me of how much time I will spend explaining how to complete this calculation at the whiteboard before moving on to Pre-Lab.
Right away, as I am reading student responses I see that the most common mistake is a simple decimal error. Most students remembered from the previous day's work that converting grams to moles involves using atomic/molecular mass as a conversion factor to cancel moles and leave grams.
I do a quick wrap-up at the white board in front to demonstrate and review the conversion, although most students correctly completed the conversion themselves.
Please note: As described in the section "Why This Lesson?" there is significant pre-lab preparation that must be done by the instructor.
First, I hand out the LAB - Stoichiometry (aluminum to copper). I ask students to skim through the lab and to name the different sections that they will be completing (Procedure, Observations, Data, Analysis, and Conclusion). After reminding students that they are expected to document what happens in a variety of ways (recording their observations and collecting quantitative data), I ask students to take a closer look at the Procedure section. I ask the class what we will be doing first, second, etc., until we have briefly discussed the instructions for the entire lab. I call on specific students who do not appear to be actively participating in the discussion to make sure that everybody understands what they will be doing in lab.
I make sure to highlight a few key points:
Students are directed to record their observations. Some sample student work:
As students are working to complete the lab procedure, I am walking around both to ask probing questions and to provide the copper (II) sulfate solution when groups are ready (after recording the mass of their foil).
I am focused on asking two main questions:
Once students are ready to filter, I am walking around to help them do so. This is the first time that they will be filtering in my class, so it is really important that I help guide them in the process. One thing that they will undoubtedly encounter is difficulty getting all of the copper out of their reaction beaker. I show them how to take the filtered solution and pour some back into their reaction beaker with copper remaining, swirl to get copper suspended, and re-filter.
After filtration, the paper with solid needs to dry (so that the remaining solution is not counted in the product's mass). Students are instructed where to store wet filters with filtrate for drying. (Day 1 Foil to Copper result)
As groups finish filtering, I tell students to leave their filters on their lab station paper with everybody’s name in the group written on the paper. I ask them why we are leaving them overnight to dry to make sure that they understand we do not want to add the mass from the remaining solution to our data. I ask students why it was important to record the mass of the dry filter paper before filtering. I expect students to understand that in order to get just the mass of the filtered copper, we need to know how much the filter alone weighs so we can subtract it from the mass of the filter plus copper. I explain that this is easier than trying to remove the copper from the filter paper and that some products will actually embed into the surface of the paper making it really difficult to remove for weighing.
I also take this time to make sure that students understand that as aluminum moves from solid to aqueous form, copper moves from aqueous to solid form. At this point in our curriculum, students have not learned about oxidation-reduction reactions and would have no frame of reference for understanding the transfer of electrons that is happening. I keep the discussion centered around the fundamental understanding of conservation of matter.
I tell students that the next time we meet, we will be discussing our results and learning a way to figure out how much copper we should have gotten based on how much aluminum we started with. I tell students that I expect them to have finished the following parts of the Analysis section of their lab handouts before our next class:
"How many moles of copper did you expect to produce?" is a question we will be able to answer during the next class session. Also, calculations for the actual amount of copper produced can be done as soon as the filter plus copper has been weighed.
This lab is used to give a frame of reference and applied purpose to learning mole to mole ratios.
Here are examples of student work (keeping in mind that the calculations for actual yield and theoretical yield were added during the Day 2 lesson):
The sample above demonstrates ability to pick out reactants and products, but this student still needs to understand that the reactants do not "disappear" persay; in this case, the aluminum foil reactant is moving from solid to aqueous form. The sample below shows perhaps more understanding regarding the transformation during the chemical reaction, but less ability to use the reaction equation to identify the reactants and products. (Both students mixed up the two final calculation questions in the Analysis section.)