Summary

In this activity students, through their physical movement, will model the law of conservation of mass during a chemical reaction. Students will also explore the concepts of limiting and excess reactants as well as balancing a chemical equation. Through this activity they also develop a means of representing particles at the molecular level.

Grade Level

Middle or high school

Objectives

By the end of this activity, students should be able to

• develop a working model for the conservation of mass at the molecular level.

Chemistry Topics

• Chemical reactions
• Conservation of mass
• Limiting & excess reactants

Time

Teacher Preparation: minimal

Lesson: 20 minutes

Materials

• Small white boards and markers or drawing paper and markers.
• Suitable space for students to move around (may need to move desks or tables aside, or move into an open hallway)

Safety

• No specific safety precautions need to be observed for this activity.

Teacher Notes

• This activity is designed to occur at the very beginning of a chemical reactions unit, as an opening/introduction activity. It can be especially helpful at the middle school level, when students do not have a lot of experience with reactions yet.
• Begin this activity by questioning students about their prior knowledge: including the terms reactants and products and how these might be represented in a chemical equation. Record these ideas on a class white board, tablet or under a document camera for students to refer to throughout the activity.
• Announce to the students that they will each be representing chemical reactants in this activity.
• Give students an example of a representation of a chemical reaction such as:
  • one boy reactant with one girl reactant makes a product. (This example may produce a few giggles but generally keeps them interested.)
  • If boys are assigned a value of one gram as a reactant and girls a value of two grams as a reactant, the pairing would have a final value of three grams as a product.
  • Display the value of boy=1 g, girl=2 g on a class white board, tablet or under a document camera.

• Next have students all stand, away from tables, chairs etc. Ask all “reactants” to move to a designated side of the room.
• Announce the task is to make a product that has a blue eyed person with a brown eyed person. “One, two, three, react.”
• Immediate discussion: Is there anyone that was not able to react? Why not?” Explain how this corresponds to the terms limiting reactant and excess reactant. Display these terms on a class white board, tablet or under a document camera for later reference by students.
• Question one or more groups about what their assigned numerical gee values was as reactants and what it is now as a product (referring to the initial example given). Relate this to a mass in grams for a typical chemical reaction.
• A reaction doesn’t always have to be two reactants making one product. Students should again become individual particles as reactants. The next example could be a reaction that includes a taller student with a shorter student and with a student with glasses. “One, two, three, react.”
• Again have a short class discussion: “Is there anyone that was not able to react? What is limiting the reaction?” (It’s always whatever reactant you run out of. It is used up.) “In real chemical reactions it’s often designed that way. Discuss with your current product partners (and any excess reactants in groups of two or three) why this might be so.” Elicit responses. (Responses will vary, but typically someone comes up a limited availability of one or higher price for one.) Relate to the real world context in the chemical industry.
• Sit down together with your current product group and discuss how you could simply represent each of you as reactant particles? How could you differentiate yourself from the other reactants? How could you represent the product you formed?
• Using the drawing materials supplied, students draw a model representing their reaction.
• After review some or all of the models presented, discuss similarities and differences. Then suggest for the sake of clarity and understanding of future activities, the class should come to a consensus of how they would be represented. Have them record what is agreed upon for future reference in their notebooks.
• From here I take my students to a Conservation of Mass lab activity sheet.
• Follow-up to the lab activity could include:
  • Video regarding Lavosier’s work and the Law of Conservation of Mass (~6mins)
  • Limiting reactant reading activity to check for understanding.
  • Limiting reactant video animation that shows the particle level (1 min, 34 secs)
• Further extensions:
  • Students are used to represent specific atoms, ions, or molecules to depict reaction types—single displacement, double displacement, synthesis, decomposition, and combustion. Other possible combining particles could be a group of students with the same birthday month decomposing into two products, determined by gender.
  • Students are used to represent elements becoming ions in an ionic bond through the transfer of one or more electrons.
  • AACT’s simulation Balancing Chemical Equations.