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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.


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


Teacher Preparation: 10 minutes

Lesson: 45 minutes


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


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.

For the Student


Prelab Questions

  1. Sketch and label energy diagrams for an exothermic and an endothermic reaction. Identify each part of the diagrams that represent reactants, products, and activation energy.
  2. Define the following terms:
    1. Exothermic reaction
    2. Endothermic reaction
    3. Activation energy
    4. Reversible reaction
    5. Equilibrium
    6. Equilibrium position
    7. Equilibrium constant
  3. Circle the factor(s) below that will cause a change in the equilibrium constant (K) when that factor is changed. (Keep this in mind as you run your simulations.)
    1. Activation energy
    2. Initial concentrations
    3. Temperature


  1. Open the simulation at http://phet.colorado.edu/en/simulation/reversible-reactions. The blue-green line on the reactants side (left side) should be at 15 on the ruler, and the blue-green line on the products side (right side) should be at 8 on the ruler. Do not change these. The “hump” representing activation energy should be at 25. Drag this up or down according to the number in the “E a” column in the table below.
  2. Before you begin, hit the pause button at the bottom of the simulation. For each trial, using the tools in the upper right, add the appropriate number of molecules of “A” and “B” according to the table below. Then hit the “start” button at the bottom of the simulation to start the timer followed by the “play” button.
  3. Allow the simulation to run for several minutes (at least 200 s according to the simulation timer) until it appears equilibrium has been reached. Stop the timer once you think equilibrium has been reached. Then record the final concentrations of A and B at equilibrium and the time it took to reach it.
  4. Calculate the equilibrium constant in the rightmost column.
  5. For trial 9, add your molecules as before, but once you hit the play button, use the heat control box (just above the timer) to heat up the reaction. Drag the blue marker to “Add” and hold it there until the temperature gauge is around 600 K (Temperature will fluctuate, that’s ok). Let it reach equilibrium, then record your results and calculate K.
Ea Initial A Initial B Final A Final B Time of trial K (equilibrium constant)
1 20 300 300
2 23 300 300
3 26 300 300
4 20 150 450
5 20 450 150
6 20 600 0
7 20 200 200
8 20 200 100
9 (↑ temp) 20 300 300


  1. Compare your first three trials. What variable did you change between those trials and what remained the same? How did this change affect the results you recorded and the equilibrium constant?
  2. Compare trials 1, 4, 5, and 6. What variable did you change between those trials and what remained the same? How did this change affect the results you recorded and the equilibrium constant?
  3. Compare trials 1, 7, and 8. What variable did you change between those trials and what remained the same? How did this change affect the results you recorded and the equilibrium constant?
  4. Compare trial 1 and trial 9. What variable did you change between those trials and what remained the same? How did this change affect the results you recorded and the equilibrium constant?
  5. Is the reaction A → B in this simulation an endothermic or exothermic reaction? Explain.
  6. Which side of the reaction, A or B, is favored? Considering your answer to #5, why do you think that is?
  7. If the platforms representing A and B reversed positions, what type of reaction (endothermic or exothermic) would A → B be? What do you think would be different about K in general? What would differ in the results of trial 9? Justify your predictions with evidence gathered from this experiment and your understanding of equilibrium and thermodynamics.