Day 1: Turning Aluminum Foil into Copper -- Applying Stoichiometry in Lab (Cornerstone #2)

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:

• Approximately 0.1-0.2 M copper (II) sulfate solution (enough for each group to use about 20 mL) -- add a small spoonful of sodium chloride for every 500 mL of solution (this helps remove the coating on the aluminum foil)
• Aluminum foil (enough for each group to use a 1 to 1 1/2 inch square)
• Vessel for reaction (beakers) for each group
• Funnel (groups can share if necessary, taking turns filtering)
• Filter paper (one for each group -- I cut brown paper towels into circles)
• Balance(s) for weighing station(s)

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 must wear safety goggles.  No, glasses do not count.
• Students need to record everything, from qualitative observations to filling in the masses for listed in the Data section.
• Students should be thinking about what is happening as the reaction takes place.  Is the foil actually disappearing?  Where is the copper coming from?
• Students should fold the filter paper into fourths (I demonstrate this since we have never filtered before.), and place it into the filter tray to remove the copper from solution.
• We will not be weighing the filter paper with copper in it today (I ask students why not, they should understand that the wet paper would have "extra" mass from the solution, changing our data.)

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:

1. Is the foil disappearing?If students answer yes, then I ask Is that how science works?  Let’s think about where the foil could be going?If students answer no, then I ask Where is the aluminum going if it is not disappearing?If students are having difficulty articulating that the aluminum is moving from being a solid to being an ion in solution, I move on to ask about the copper (question 2) and try to draw similarity between the aluminum moving into solution and the copper moving out of solution.
2. Where is the copper coming from?If students can articulate that the copper is coming from the solution and somehow becoming solid, then I know that they have a basic understanding of this reaction.  If students are struggling to make that connection, I ask What is happening to the blue color in your solution?  Students should be able to observe the blue color lightening over time as the copper ions are pulled out of solution and become solid metal.  If students do articulate that the blue is going away somehow, then I can ask If the blue is going away, and the copper metal is appearing, is there a connection between those two observations?  At this point, students should be able to draw the correct conclusion.

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.