This lesson is intended to be used as an introduction to the concept of limiting reactants. It is based on the Science Literacy Cycle as presented by the Sacramento Area Science Project.
By the end of this lesson, students should be able to
explain that the limiting reactant is completely consumed and determines the amount of product produced in a chemical reaction.
relate macroscopic observations to the atomic/molecular level and to symbolic chemical notation.
This lesson supports students’ understanding of
- Limiting reactant
Teacher Preparation: 15 minutes
Lesson: 45 minutes
For each group:
- sticky notes (4 different colors)
- test tubes (2)
- copper(II) chloride solution, 1 M
- aluminum foil
- test tube holder
- beaker (150 mL)
- spongy copper sample
- aluminum chloride solution, concentration doesn’t matter
- 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
- This is an interactive, engaging way for students to learn about limiting reactants.
- You can use each of these literacy strategies in different contexts, and you could construct a similar lesson using different strategies. The Sacramento Area Science Project has a resources part of their website that gives you many more choices for ways to engage students in learning. I encourage you to check it out, pick one from each category, and revamp an old lesson. I am not affiliated with SASP in any way, I just really like their stuff!
- The reaction in the explore section is exothermic and depending on the concentration of the copper(II) chloride solution, it can be quite violent.
- The lesson is infused with literacy: reading, speaking, listening, and writing. It addresses the CCSS science and technical subjects standard related to argument from evidence.
For the Student
Students should purposefully read the limiting reactants reading. They should be arranged in groups of four. Each student will be given 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. This is a summarizing activity called “Outside Circle-Inside Circle.” 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.
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 convince their peers that their note belongs in the middle, and they have to communicate with each other so they can put the best collection in the middle.
This part can be done as a demonstration or in lab groups by the students. Start with a flask of copper(II) chloride solution (1-M works well), pieces of aluminum foil, a flask of aluminum chloride solution (clear, colorless; concentration doesn’t matter), and a piece of spongy copper (it’s important that it be spongy copper and not a 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). 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, Cu = red-brown lumpy solid, AlCl3 = clear colorless liquid]. Write these physical properties under the appropriate chemical symbols in the balanced chemical equation, so it looks like this:
After students have seen all of the reactants and products and identified the relationship between their macroscopic properties and the chemical symbol, do the demonstration.
Add the piece of aluminum foil to the test tube with the copper(II) chloride solution. Hold the test tube with a test tube holder—the reaction is exothermic and the test tube could get hot quickly. Have students make observations about what happens. This could also be done by students in lab groups. In that case, have students carry out the reaction in a beaker, rather than a test tube.
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.”
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 like 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. I sometimes just shout out when to switch who’s talking, or ring a bell or 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 on the basis of 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 at the end of class or as a homework assignment. You can use the following template:
I know that __________________________ is the limiting reactant in this reaction because of my observations. First _______________________________________, so _______________________________________________________________ ___________________. Second, _______________________________________, so ________________________________________________________________. Third, ____________________________________________________________, so ________________________________________________________________ ________________________________. Limiting reactants___________________ and ______________________________________________________________.
Providing the template will dramatically increase the quality of the responses that you receive 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.