In this activity, students will model equilibrium reactions using plastic chips to represent atoms. The goal of the lesson is to connect the symbolic model of an equilibrium reaction to its particle model.
High School (AP Chemistry)
AP Chemistry Curriculum Framework
- Big Idea 6: Any bond or intermolecular attraction that can be formed can be broken. These two processes are in a dynamic competition, sensitive to initial conditions and external perturbations.
- 6.1 The student is able to, given a set of experimental observations regarding physical, chemical, biological, or environmental processes that are reversible, construct an explanation that connects the observations to the reversibility of the underlying chemical reactions or processes.
- 6.2 The student can, given a manipulation of a chemical reaction or set of reactions (e.g., reversal of reaction or addition of two reactions), determine the effects of that manipulation on Q or K.
- 6.8 The student is able to use Le Chatelier’s principle to predict the direction of the shift resulting from various possible stresses on a system at chemical equilibrium.
This lesson will help prepare your students to meet the performance expectations in the following standards:
- HS-PS1-6: Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
- Scientific and Engineering Practices:
- Using Mathematics and Computational Thinking
- Developing and Using Models
- Engaging in Argument from Evidence
- Constructing Explanations and Designing Solutions
By the end of this lesson, students should be able to
- Explain the molecular changes that occur when an equilibrium system is disturbed
- Relate the symbolic representation to the particle representation for changes in an equilibrium system
This lesson supports students’ understanding of
- Equilibrium Constants
- Le Chatelier’s Principle
Teacher Preparation: 5 minutes to copy or construct beaker diagrams; If you want to sort the plastic chips into sets, this will take about 20 minutes.
Lesson: 45 minutes
Materials (per group)
- Large image of a beaker that can lay flat on the table (downloadable image available)
- You may need to adjust the paper size to match whatever modeling pieces you choose.
- It may be easiest to draw one on poster board.
- The pictures used in the answer key are made with 4 pieces of copy paper taped together and worked well.
- Alternatively, you can use the downloadable image and enlarge it on a poster printer.
- Chips of two different colors (as written: 18 red chips and 48 yellow chips)
- Many things can be substituted here, such as candies, small paper shapes, Legos, colored magnets, or parts from model kits. I chose plastic chips because they are inexpensive and because model kits may not contain enough parts of the same color.
- The chips used in photographs are “counting chips” available from Amazon.
- Flinn has them (Product # AP4585). These have wire around them so they can easily be gathered with a magnet (sold separately).
- Colored pencils or crayons, red and yellow
- No specific safety precautions need to be observed for this activity.
- This lesson was inspired by my experience as an AP Exam Reader. For more information, read the related article Taking Inspiration from the AP Chemistry Reading, in the November 2018 issue of Chemistry Solutions.
- This activity is meant to help students understand the processes involved in equilibrium reactions, while specifically pointing out that the language we use when modeling them can lead to incorrect assumptions. It will be useful to reinforce this throughout the lesson with individual groups.
- It is assumed that students have had a brief introduction to equilibrium prior to this activity. For simplicity, the equilibrium constant expression does not include “eq” as a subscript. Though the reaction quotient, Q, is not specifically addressed, you could certainly introduce it with the systems in Reaction 1. If you want to do this, you may also consider doing some Q/K calculations and analysis before the activity and then ask them to specifically use this in answering the questions. I included answers to these calculations in the answer key.
- For the manipulative part of the model, you can use a variety of objects, as noted in Materials, and should either enlarge the beaker diagram (available as a downloadable file) to an appropriate size for your objects or use poster paper and draw the beaker. You may choose to make up plastic bags with the appropriate number of chips (or other colored object) in them. Alternatively, you could ask students to go to a central location and count out the number of each color that they need. This activity is written with red and yellow, but any color could be used. You could edit the worksheet and leave a blank line for color if you don’t have enough of any two colors. This way, students can write in the colors they receive.
- In Reaction 1, students are told to round all equilibrium constants to whole numbers. With a sample size this small, that was the best way to allow two different systems to have matching values. Please reinforce this with students in the beginning or they will likely get confused when the constants don’t match.
- Reaction 2 uses smaller equilibrium constants, so students should NOT round to whole numbers.
For the Student
You have previously learned that equilibrium reactions are reversible reactions that result in a mixture of species. The equilibrium constant, K, for a given reaction is a number that allows us to predict relative amounts of reactant and product molecules in a system. The value of the equilibrium constant can be calculated if equilibrium concentrations are known. Following is an example of this calculation for a given reaction:
- The word equilibrium hints that something is “equal”. What is equal in an equilibrium system?
- Explain what an equilibrium value of 3.1 x 10-5 indicates about the relative number of reactant molecules and product molecules.
Upon completion of this activity, you will be able to explain the molecular changes that occur when equilibrium systems are established or disturbed.
- Large image of a beaker that can lay flat on the table
- Chips of two different colors (18 red chips and 48 yellow chips)
- Lay out the beaker image on the table.
- Gather your chips and be sure you know which color represents each atom.
- Follow the directions in each section to create your models and to answer the questions.
- For equilibrium calculations, treat each modeled molecule as 1-molar
- *All equilibrium values should be rounded to whole numbers unless specified!
Reaction 1: System 1
- In the beaker diagram, use the colored chips to represent 18 molecules of reactant. This is your “initial conditions”. Draw a model of this in your “Initial” box below.
- Using the chips, decompose your XY2 molecules into the designated products until you have enough products to give you an equilibrium value of 3.
- This is your “equilibrium conditions”. Draw a model of this in your “Equilibrium” box below.
- When we talk about the “left side” of the equation, what are we talking about?
- Under initial conditions, is it possible for only what is represented by the “left side” of the equation to be in the beaker? Explain.
- When we talk about equilibrium conditions, which side of the reaction represents what is in the beaker?
- Under equilibrium conditions, is it possible for only what is represented by the “left side” of the equation to be in the beaker? Explain.
- When the reaction was at initial conditions, how did we know that the reaction would proceed in the forward, rather than the reverse direction?
Reaction 1: System 2
A reaction in equilibrium has a certain number of molecules of each reactant and product. This number can change if conditions are changed.
Change in concentration
Question to explore: If additional molecules are added, can the reaction find equilibrium again?
- To your beaker model from Reaction 1: System 1, use additional chips to add six more molecules of Y2 to the mixture.
- This is your new “initial conditions”. Draw this in the “Initial” box below.
- The reaction will now shift to the left. Simulate this by reacting X2 molecules with Y2 molecules until the equilibrium value again equals 3.
- This is your “equilibrium conditions”. Draw a model of this in your “Equilibrium” box below.
- Under initial conditions for this system, which side of the reaction was represented in the beaker?Explain.
- How did we know that the reaction would shift left in this system?
- How was the reaction able to reach equilibrium again, after the original addition of molecules?
- When discussing this kind of change, we often say that we “add molecules to the right side of the equation”.As you see in the beaker, there is no “right side” portion of the beaker!Use what you did with your model to find a better way to state the change that was made.
- When discussing this kind of equilibrium shift, we often say that “the reaction shifted to the left to relieve the stress”.Again, from your beaker model, you should realize that there are no “sides” of a real reaction!Use what you did with your model to find a better way to state the change that occurs.
- Clear your beaker of the chips used in reaction 1 and start with an empty beaker.
- An equilibrium system at room temperature has been shown to contain the following mixture:
10 A; 2 B2; 8 AB
- Use the chips to model this equilibrium on your beaker image.
- This is your “initial conditions”. Draw a model of this in your “Initial” box below.
- When heated to a certain temperature and maintained, this causes 4-molar of the AB to decompose into A and B2.
- Use the chips to model this change on your beaker image.
- This is your “final conditions”. Draw a model of this in your “Final” box below.
Do NOT round equilibrium values to whole numbers for this set.
- How many total A atoms are present in the initial beaker? How many B atoms?
- How many total A atoms are present in the equilibrium beaker? How many B atoms?
- Why is the total number of A and B atoms the same in each beaker?
- How many total particles (molecules or unbonded atoms) are present in the initial beaker? In the equilibrium beaker?
- How is it possible that the number of particles in the equilibrium beaker is not equal to the number of particles in the initial beaker?
- Both the initial and the final beakers were at equilibrium.
- What condition changed from initial to final?
- Why did the reaction shift to the left when this change occurred?
- What is the value of the equilibrium constant in each of the beakers?
- What is the effect on the equilibrium constant of adding heat to an exothermic reaction?
When we use models to explain chemical changes, we often use words that apply to the model, but not to the actual system. Write a one-paragraph summary of how the language used when modeling with chemical equations differs from that used when modeling with physical representations of atoms in a beaker.