Summary

In this lesson, students will be introduced to the concept of limiting reactants by applying various literacy strategies to a one-page informational text and through a short demonstration (or lab). The reading and demo will help students create connections between the macroscopic, particulate, and symbolic representations of chemical reactions and limiting reactants.

Grade Level

High School

NGSS Alignment

This activity will help prepare your students to meet the following scientific and engineering practices:

• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence
    

Objectives

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

• Relate macroscopic observations to the atomic/molecular level and to symbolic chemical notation.
• Explain that the limiting reactant is completely consumed and determines the amount of product produced in a chemical reaction.

Chemistry Topics

This lesson supports students’ understanding of:

• Limiting reactants
• Observations
• Chemical reactions
  

Time

Teacher Preparation: 15 minutes

Lesson: 45 minutes

Materials

For reading activity:

• Sticky notes (4 different colors)
• Poster paper
• Student reading handout

For demo:

• 2 beakers (exact size doesn’t matter, between 400 mL and 600 mL)
• Hot hands (to handle hot beaker)
• Copper(II) chloride solution, ~1 M, ~100 mL
• Aluminum foil
• Copper sample (not sheet or wire, see teacher note in “Explore” section)
• Aluminum chloride solution (concentration and amount doesn’t matter, just enough for students to make observations)
• Student demo handout

Safety

• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
  

If doing the reaction as a lab instead of a demo:

• Always wear goggles when conducting an experiment in the lab.
• 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.
  

Teacher Notes

• This is an interactive, engaging way for students to learn about limiting reactants through both a literacy-infused reading component and a real-world chemical demonstration. This lesson is a good conceptual introduction to limiting reactants, as it does not ask students to do any limiting reactant calculations, but it does require them to use their observations of a reaction to determine which reactant is limiting and which is in excess.
• It would be helpful if students already know how to name compounds, balance equations, and predict products – if they do not, they can still complete this lesson but will need to be provided with the skeleton or balanced chemical equation in place of part a. of the prelab question.
• This lesson is based on the four-pronged Science Literacy Cycle (Reading, Writing, Inquiry, and Dialogue, in any order) as presented by the Sacramento Area Science Project (SASP). You can use each of the literacy strategies from this lesson in different contexts, and you could construct a similar lesson using different strategies. (SASP is not affiliated with ACS or AACT.)
• The lesson has students practice a number of literacy skills, including reading, speaking, listening, and writing. It addresses the CCSS science and technical subjects standard related to argument from evidence.

Engage

• Students should purposefully read the Limiting Reactants passage (individually) and then complete the “Outside Circle-Inside Circle” summarizing activity described below.
  • Arrange students into groups of four and provide each student with 3–5 sticky notes of a particular color that is different from the other members of the group (i.e., Jane will have 4 orange notes while Joe has 4 green notes, etc.).
  • While reading the passage independently, the students will write down the 3–5 most important ideas from the passage on their sticky notes.
  • After the students have read the passage and written their notes, they will place their notes around the outside edge of their group’s poster paper. As a group, they will then choose one note of each color to move to the center of the poster paper as a group summary of the key ideas.

• After the groups have discussed and chosen their summary key ideas, lead a group discussion about what they chose and why. Make sure that they did actually identify the key ideas. Use this opportunity to reinforce specific vocabulary terms such as limiting reactant and excess reagent. This is a great activity for summarizing skills because students have to communicate with each other to determine the best collection of notes to be put in the middle.

Explore

• Start by having students observe a beaker containing copper(II) chloride solution (1 M works well, approximately 100 mL, but you could use a smaller volume or a more dilute solution), pieces of aluminum foil, a beaker containing aluminum chloride solution (clear, colorless; concentration/amount doesn’t matter), and a piece of native copper or copper nugget*. Show each of the samples to the students and have them describe the samples based on their physical properties [CuCl₂ = bright blue clear solution, Al = shiny silver-colored metal, AlCl₃ = clear colorless liquid, Cu = red-brown lumpy solid]. Write these physical properties under the appropriate chemical symbols in the balanced chemical equation:

• • *Note that it’s important that the copper sample be “natural” or “native” copper or copper nugget, rather than a shiny sheet of copper metal or piece of copper wire – students need to see what it will look like as it is formed in the reaction. Ward’s Science sells one possible option for your copper sample. Alternatively, more widely available copper shot, such as this product from Flinn Scientific, could be used in a pinch. Finally, you could perform the reaction before you plan to use this lesson in class and filter out and save the resulting copper from the reaction. This will obviously be the closest in appearance to what students will see, as it is prepared the same way, but it does require preparing the sample in advance rather than just pulling it out of the storeroom.

• This is written as a teacher-led demonstration, but if you have enough time and supplies, it could be done on a smaller scale in lab groups by the students. In that case:
• Have students use smaller amounts of copper(II) chloride (~10-20 mL) and small pieces of aluminum foil, and consider using smaller beakers or test tubes.
• Since the reaction is rather exothermic, consider testing more dilute concentrations of copper(II) chloride, such as 0.5 M or 0.1 M, to see what level of reaction you would be comfortable with your students performing themselves.
• Be sure to let them know that the glass could get hot as the reaction occurs.
• Depending on how much of each reactant students use, they could end up with a different limiting reactant than their classmates, which could lead to some good conversation between groups as they try to understand what their observations tell them about their limiting and excess reactants.
• The precise amount and concentration of reactants is not critical in this demo, but if 100 mL of 1.0 M copper(II) chloride solution is used as listed in the materials section, 1.80 g aluminum would provide exactly enough aluminum to react with all of the copper(II) chloride. You can choose which you want to be the limiting reactant – aluminum should be the limiting reactant if less than 1.80 g of aluminum foil is used, and copper(II) chloride should be the limiting reactant if more than 1.80 g of aluminum foil is used, according to the following calculations:

Explain

• When the reaction is complete, the students will engage in a strategy called “Paired Verbal Fluency.” Explain the strategy first, and then lead them as they participate. They will need to work in groups of two, and identify a partner A and a partner B. It works like this:
  • In pairs – the teacher will show you a prompt and will give you one minute to read and think about the prompt.
  • Partner A then has 15 seconds to talk about the prompt while Partner B listens silently.
  • Partner B then has 15 seconds to talk while Partner A listens.
  • After a 15 second pause, each partner will be given 30 seconds to respond.
• Use the following prompt for their discussion: “Looking at the reaction mixture, what was the limiting reactant? Support your argument with as many observations as you can. Try to link your observations to the chemical equation.” (This prompt can be written on the board, projected as a presentation slide, printed out for individual students, etc.)
• During the 15 second pause, you can remind them what they are supposed to be talking about, encourage them to think of something else they haven’t yet discussed, etc. When they respond to each other they can continue where their partner left off, ask their partner questions about what they said, correct their partner if they think there was a mistake, etc. While they are talking, you should walk around to get a sense of what’s going on and where there is any confusion, and to assure they are on task. You can adjust the times (15 seconds, 30 seconds) to meet the students’ needs – if it feels too long, use a shorter time, and vice versa. You want students to reach that uncomfortable point where they think they’ve thought of everything, but they have to stop and think some more, so make sure you don’t cut it too short. When it is time for partners to switch roles, you can tell them verbally, ring a bell, use a whistle, etc.
• After they have finished the discussion, have the groups share what they talked about so you can make sure you address any misconceptions, and they correctly identify the limiting reactant based on their observations.

Elaborate & Evaluate

• The students will create a written record of what they’ve learned about limiting reactants. This strategy is called a “Framed Paragraph.” It is a template for the thinking that they need to do about science so they can think about the science instead of thinking about how to write a paragraph. This can be done individually at the end of class or as a homework assignment. The template is on the last section of the student handout.
• Providing the template will dramatically increase the quality of the responses that you receive, especially for students who struggle to express their thoughts in writing, because it eliminates incomplete answers, focuses their minds on the science concepts, and provides a structure that makes it easier for the teacher to understand what the student is trying to say. You could consider making it an open-ended question if students are more comfortable writing freehand responses to short answer questions.
• You could ask students to use their observations and explanations to try to predict what they might have seen if the other reactant had been the limiting reactant. If time allows, you could do the demo a second time using significantly less of the reactant that was in excess the first time around (or more of the reactant that was limiting) to see if students’ predictions were correct.