Summary

In this activity, students will investigate how temperature, activation energy, initial amounts of products and reactants, and type of reaction (exo- or endothermic) effect the equilibrium position of a reaction using a simulation.

Grade Level

High School

NGSS Alignment

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

• HS-PS1-5: Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.
• 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
  • Engaging in Argument from Evidence

AP Chemistry Curriculum Framework

This activity supports the following units, topics, and learning objectives:

• Unit 5: Kinetics
  • Topic 5.6: Reaction Energy Profile
    • TRA-4.C: Represent the activation energy and overall energy change in an elementary reaction using a reaction energy profile.
• Unit 6: Thermodynamics
  • Topic 6.2: Energy Diagrams
    • ENE-2.B: Represent a chemical or physical transformation with an energy diagram.
• Unit 7: Equilibrium
  • Topic 7.8: Representations of Equilibrium
    • TRA-7.F: Represent a system undergoing a reversible reaction with a particulate model.
  • Topic 7.9: Introduction to Le Châtelier's Principle
    • TRA-8.A: Identify the response of a system at equilibrium to an external stress, using Le Châtelier's principle.

Objectives

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

• See how activation energy, initial concentrations, and temperature affect the value of equilibrium constants for endothermic and exothermic reactions.

Chemistry Topics

This lesson supports students’ understanding of:

• Thermodynamics
• Equilibrium
• Equilibrium constant
• Activation energy

Time

Teacher Preparation: 10 minutes

Lesson: 45 minutes

Materials

• Computer with access to the internet (per group)
• Student activity sheet (per student or per group, depending on how you want to do it)

Safety

No safety precautions need to be observed during this activity.

Teacher Notes

• Simulation can be found at http://phet.colorado.edu/en/simulation/reversible-reactions. It is recommended that you try the activity yourself first to get a sense of the timing and determining when equilibrium is reached, as it is a little tricky due to the constant fluctuations of the particles.
• Students should have been introduced to the ideas of equilibrium and thermodynamics, and how they are connected. They should know that the equilibrium constant should not change unless the temperature changes, regardless of what happens to the initial concentrations or the activation energy. This should be confirmed in the simulation. Trial 9 should be the only trial where K decreases because of the temperature increase and the fact that it is an exothermic reaction.
• Simulation timer is about 3x faster than real life.
• "Concentration" and "number of particles" will be treated interchangeably in this activity.
• Since the concentrations change so rapidly and continue to fluctuate even after a large amount of time, students may not always end up with the same exact equilibrium constant, though it should be close. (For example, maybe for most of the trials they get ~220 molecules of A at equilibrium but on one of the trials it ended up more like ~230 or ~210 molecules of A. That’s ok – since the simulation cannot show nearly as many particles as would be present in a real-life reaction, the equilibrium constant may shift a little due to the relatively small sample size.) Since for the first 6 trials the total number of molecules is 600, it should be easy to see if they are getting the same ratio as previous trials. Trials with a deviation in K of about ±0.2 can be considered the same K, within the limitations of the simulation.
• Similarly, since the fluctuations continue, students may end up with widely varying times. Students should probably run the first trial multiple times so they can get a sense of the timing and to be sure that they are finding a consistent K value before moving on to the next trial. A couple runs of the trial 1 conditions as a whole class might be useful.
• Trials 7 and 8 use different total molecule numbers to demonstrate that the ratios will be approximately the same even if the total number of particles varies. If you are running short on time, you can exclude these two trials and analysis question #3.
• Students could use the simulation to test the predictions they make in question #7.