Summary

In this lab, students react copper(II) chloride with aluminum and determine the limiting reactant. They will identify the limiting reactant both theoretically through calculations and experimentally through their observations of the chemical reaction. They then determine which product to isolate to determine the percent yield of the reaction.

Grade Level

High School

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-7: Using mathematical representation to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence

Objectives

By the end of this lab, students should be able to:

• Determine the limiting reactant given the initial masses of two reactants.
• Separate a mixture of products after a chemical reaction is complete.
• Calculate percent yield and consider potential sources of error.

Chemistry Topics

This lab supports students’ understanding of:

• Stoichiometry
• Limiting reactants
• Separating mixtures
  
• Percent yield

Time

Teacher Preparation: 15 minutes

Lesson: 2 days of 50-minute periods

Materials

For each group:

• Copper(II) chloride dihydrate
• Aluminum foil
• 250-mL Erlenmeyer flask
• Balance
• Graduated cylinder
• Distilled water
• Filter paper
• Funnel
• Beaker
• Safety goggles

For the class:

• Drying oven (optional)

Safety

• Always wear safety goggles when working with chemicals in a laboratory setting.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• A side reaction that occurs in this activity produces small amounts of hydrogen, so it should be conducted away from any sources of heat or flame. Since the quantities of reactants students are using are fairly small, the amount of hydrogen produced does not present a significant hazard, but be sure students do not use more than the amounts listed in the procedures, or complete in a fume hood.

Teacher Notes

• This lab only involves one chemical reaction and small amounts of chemicals, but it allows students to practice numerous technical and mathematical skills, including making qualitative observations, measuring mass, writing and balancing chemical equations, determining which reactant is limiting and which is in excess, determining which products can be more easily isolated, separating mixtures by filtration, calculating theoretical yield and percent yield, and identifying possible sources of experimental error.
• Assign each group a group number prior to beginning the activity. This number will determine the masses of their two reactants (see the table in step 2 of the procedures), as they will be different for each lab group.
• Part of the prelab involves students reading the procedures and creating an appropriate data table. If time is limited or students would struggle with this task, you could develop the data table as a whole class, or you could provide students with a data table (see answer key).
• This lab requires substances to be dried overnight, so it must take place over two days. Depending on the level of independence of your students and how pressed for time you are, the second day could very quick, just enough time to allow students to measure the mass of their product, and then they could do the rest of the calculations and questions for homework. Alternatively, you could allow students to work in class with their lab groups on their percent yield calculations and analysis questions.
  • If you do not have access to a drying oven, you could allow the products to air dry, but you may want to wait more than one day before measuring the final mass to ensure they dry completely. You could also place products in a desiccator to dry.
  • You may wish to remind students to subtract the mass of filter paper or beaker when determining final mass of product.
• Some students may need some guidance on how to identify which product to isolate in analysis question 5. Encourage them to think about what materials remain in the flask after the reaction is complete and what states those substances are in. All of the groups’ reactions will have a solid product (copper) and an aqueous product (aluminum chloride) as well as the excess reactant – either solid aluminum or the aqueous copper(II) chloride solution, depending on the group’s reactant amounts. In either case, the products will be separated by filtration.
  • If there are two solids in the flask (i.e. when aluminum is the excess reactant), it will be easier to isolate the aqueous product, aluminum chloride, by evaporating the water from the filtrate and discarding the solids caught by the filter. If both copper and aluminum solids are present, measuring the combined mass of the solids would not give a useful data point.
  • If there is only one solid in the flask (i.e. when aluminum is the limiting reactant), it will be easier to isolate the solid product, copper, after the filtration and discard the filtrate. If two aqueous substances are present in solution, evaporating the water from the filtrate would only allow students to measure the combined mass of those two substances. Having students draw particle diagrams before and after filtration might help them make this distinction.
• Students are told to isolate their identified product once their reaction is complete – if they have trouble identifying when the reaction is complete, ask them to think about what their limiting reactant (LR) is, and to observe the contents of their flask for signs that either the LR has been used up or the LR is still present. (If aluminum is the LR, is there any grey aluminum metal left in the flask or is it all gone? If the copper(II) chloride dihydrate is the LR, is the solution still blue in color or has the color faded away?) You may also wish to encourage them to stir the contents of their flask periodically to make sure the reactants are well mixed.
• Note that the reaction observed in this lab, a single replacement reaction between copper ions and aluminum atoms, is not as simple as it appears to be at first glance. Because aluminum foil has a very thin aluminum oxide coating on it from exposure to oxygen in the air, the aluminum atoms are not immediately accessible when the foil is placed in the copper(II) chloride solution. The presence of chloride ions in the solution disrupts this oxide layer, allowing the copper(II) ions to interact directly with the neutral aluminum atoms beneath this layer. Other copper compounds that do not contain chloride ions will not produce a noticeable reaction because the aluminum oxide layer will remain intact, so be sure not to substitute a different copper salt.
  • See Flinn Scientific’s “Foiled Again” demonstration procedures and/or video for a more in-depth look at this phenomenon.
• Additionally, because a solution of copper(II) chloride is slightly acidic, some of the aluminum atoms will react with the acidic hydrogen ions rather than copper(II) ions, forming hydrogen gas. Students may be confused about where the bubbles are coming from, as the chemical equation they wrote in the prelab did not include products they would recognize as a gas. This side reaction could impact the reaction’s theoretical vs. actual yield if using a precise enough balance, and students could reference this mysterious bubbling that is not explained by their balanced chemical equation in their discussion of sources of error. (Again, the Flinn resources referenced above contain useful information about the production of hydrogen gas.)
  • As hydrogen gas is produced, the reaction should be conducted away from any sources of heat or flame. Since the quantities of reactants students are using are fairly small, the amount of hydrogen produced does not present a significant hazard, but be sure students do not use more than the amounts listed in the procedures, or complete in a fume hood.
• Thanks to Meegan Hammond from Saunders Trades and Technical High School for supplying student data for the answer key.