Day 2: Turning Aluminum Foil into Copper -- Applying Stoichiometry in Lab

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Objective

SWBAT determine how many moles of copper should be produced from a measured amount of aluminum foil, and compare this theoretical yield to their actual yield from an experiment.

Big Idea

Stoichiometric calculations help us predict how much product is formed based on how much reactant we start with.

Why This Lesson?

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.  It is the second day of a two day lab investigation (the first lesson in the series is available here: Day 1: Turning Aluminum Foil Into Copper--Applying Stoichiometry in Lab).

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.  Students are investigating a simple chemical reaction and learning how to predict the yield of such a reaction.

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.  

Warm-Up

5 minutes

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: "Using the following data, calculate the number of moles of copper produced by the reaction.  

Mass of filter paper:  2.3 g

Mass of filter paper and copper:  2.6 g"

In this case, the warm-up is asking students to take sample data and perform the same calculations they will need to do in class today.  It is reinforcing that students can find the mass of copper product by subtracting the mass of the filter paper alone from the total mass of filter paper and copper.  This warm-up also gives students practice in converting from grams to moles.

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 releasing students to complete their data collection from yesterday's lab and the accompanying calculations.

Almost every student understood that in order to get the mass of the copper they needed to subtract the mass of the filter paper from the mass of the filter paper with the copper.  Then, they knew to convert those grams of copper to moles.  My students were successful in later applying this to lab calculations where they needed to follow the same process.

Finishing Data Collection

10 minutes

Student groups record the mass of their filter paper and copper, then complete the calculation to determine how many moles of copper their reaction produced.  I remind students that once they have collected the data, their calculation is the exact same as the one they did in the warm-up.

I allow 10 minutes here because I only have 2 weighing stations available and the groups need to take turns which can sometimes take a while.  I also want students to complete the calculation determining how many moles of copper was produced before moving on to the next segment of this lesson.

Whole Class Instruction

25 minutes

Students have learned how to grams to moles and moles to grams conversions, but they have not yet learned mole to mole conversions.  I use this lab as a reason for students to be able to convert from moles of one substance to moles of another.

This video shows how I explain using a balanced chemical reaction equation to create mole to mole conversion factors within the context of this lab reaction.

I make sure to explain that a balanced chemical reaction is in units of “moles”—that the coefficients in front of each compound tell us how many moles of reactant are used or how many moles of product is made.  I also explain that it is because of this fact that we have to convert from grams to moles in the first place.

Another great learning opportunity is to examine the mass of copper produced.  Some groups may produce more copper (in grams) than aluminum (in grams) they started with.  This is perfect for allowing students to examine why that is (copper has a much larger atomic mass than aluminum). 

Because the emphasis of this lab was to learn mole to mole ratios, I had students calculate how many moles of copper were produced during the previous day's lesson (here is the handout used: LAB - Stoichiometry).  In future lessons, we will predict how many grams of product would be expected based on masses of reactants used.  For this reaction, however, we are comparing moles of product expected to moles produced instead.

Group Work -- Finishing Lab Handout

15 minutes

Students should now have the tools to determine how many moles of copper they should have produced, and then compare that to how many they actually did.  I walk around as students work to complete the calculations and thoughtfully answer the conclusion question.  I also use this time to walk around and ask student lab groups what they found most interesting during the lab, what they liked most, or what they learned.  I also ask students if they can see a practical use for being able to predict how much product can be made.  In our conversations, I like to include drug companies as an example of when it is important to know how much reactant is needed to make a certain amount of product, especially if the reactants are expensive.  A lot of my students are Breaking Bad fans, and if pharmaceuticals are not helping them understand, I usually ask how Walter White knew how much methylamine they needed to steal in order to make enough of their “product” (methamphetamine).  This draws an immediate connection to something that those students can understand, and highlights the importance of understanding stoichiometry in drug production—albeit illegal.

Students turn in their labs today upon completion.

Sample student calculations:

 

Students were successful in the completion of these calculations (although several students mixed up the meaning of the last two questions: "How many moles of copper did you expect to produce?" and "How many moles of copper did you actually produce?").

 

Student Reflection

5 minutes

In student's Warm-Up/Reflection Books, students should spend about 3-5 minutes writing a response to the day's reflection prompt.  Prompts are designed to either help students focus on key learning goals from the day's lesson or to prompt deeper thinking.  The responses also allow me to see if there are any students who are missing the mark in terms of understanding.  The collection of responses in the composition books can also show a progression (or lack thereof) for individual students.  

Today's Reflection Prompt:  "What are some reasons that less product is made than expected?  What are some reasons that more product is made than expected?"

For this prompt, I really just want students to think about potential sources for lab error.  Most of my students probably will not be able to identify many of the sources I list here without guidance, but I want them to think about those potential sources of error individually without feeding them the answers.  In subsequent lab investigations, students will be expected to complete an error analysis, and this reflection is geared to help students start thinking in those terms.  Desired student responses could include:

Less:

  • some product is left in the beaker and not filtered
  • not all of the solid reactants reacted and they might weigh less than the product
  • some reactant was spilled
  • some product was spilled
  • masses were not recorded properly

More:

  • not all of the solid reactants reacted and they might weigh more than the product
  • product was not dry
  • product was not isolated properly
  • masses were not recorded properly