Summary

In this activity, students will view an animation that explores equilibrium through the examples of water and a piece of chalk (CaCO₃). They will see the relationship of H⁺ and OH^– with water molecules, as well as the autoionization of water. They will also see that once equilibrium is established, there is no net change in the ions in solution, as when one set of particles dissolves, another set of particles will recombine.

Grade Level

High School

Objectives

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

• Explain the concept of dynamic equilibrium.

Chemistry Topics

This activity supports students’ understanding of:

• Equilibrium

Time

Teacher Preparation: minimal
Lesson: 10-30 minutes

Materials

• Computer and projector with internet access
•  https://teachchemistry.org/classroom-resources/equilibrium-animation
• Student handout

Safety

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

Teacher Notes

• All of the animations that make up the AACT Animation collection are designed for teachers to incorporate into their classroom lessons. Intentionally, these animations do not have any spoken explanations so that a teacher can speak while the animation is playing and stop the animation as needed to instruct.
• If you assign this to students outside of class time, you can create a Student Pass that will allow students to view the animation (or any other video or ChemMatters article on the AACT website).
• We suggest that a teacher pause this animation at several points or watch it more than once to give students the opportunity to make notes, ask questions, and test their understanding of the concepts presented. The animation is a little over 1 minute long and moves quickly, so students will likely require pausing or multiple viewings to successfully complete the student activity sheet if you choose to use it. Here are some of the points at which you might want to pause the video to allow students to answer the questions on the activity sheet:

• You can use this resource either as a first introduction to the concept of equilibrium or as a visual reinforcement of this concept after you have first introduced the topic.
• This animation introduces students to the idea that pure water is not simply H₂O molecules, but also contains small amounts of H⁺ and OH^– ions, particularly in the segment from 0:29-0:41. You could introduce the phrase “autoionization of water” to describe how water self-ionizes in this segment. In this animation, that process is shown as H₂O ⇌ H⁺ + OH^– and H⁺ + OH^– ⇌ H₂O but another common way to show this is H₂O + H₂O ⇌ H₃O⁺ + OH^– since the H⁺ ions do not exist free in solution. You can relate this concept to K_w, acids and bases, K_a and K_b, the pH scale, etc. if you have covered or plan to cover acids and bases in detail.
• A key takeaway for students from this animation is that when a system reaches dynamic equilibrium, the individual particles are not static but the overall ratio of ions in solution (or gases, or whatever is in a state of equilibrium) is consistent and doesn’t change (unless some external factor causes it to do so, though these scenarios are not presented in this animation). So, for example, in 0:30-0:35 when one H₂O molecule separates into H⁺ and OH^– ions, somewhere else in the solution an H⁺ ion and an OH^– ion recombine. Similarly, at the end of the animation (0:45-0:55), once the chalk (CaCO₃) reaches equilibrium, some of the dissolved Ca²⁺ particles and CO₃^2– particles recombine and deposit back on the stick of chalk, and another pair of particles separate shortly thereafter. The individual particles change (“dynamic”), but the overall ratios remain constant (“equilibrium”).
• This animation shows how particles behave at equilibrium, but it does not go into what happens when stress factors (such as changing concentrations, temperature, or pressure) are introduced to the system. Many of the resources below dive into Le Châtelier’s principle, reaction quotient, and equilibrium constant on a more quantitative level and are a good way to delve deeper into equilibrium after this more qualitative introduction.
• Some useful resources that address the topics in the extension questions include:
  • For Question 9: Annenberg Learner animations on Hemoglobin and Carbon Monoxide Poisioning
    • For this question, it may help if students have already learned about equilibrium on a more quantitative level, to include equilibrium constants.
  • For Question 10: ACS Climate Science – Ocean Chemistry
• Related classroom resources from the AACT Library that may be used to further teach this topic:
  • Simulation: Predicting Shifts in Equilibrium: Q vs K
  • Unit Plan: Equilibrium Unit Plan
  • Activity: Equilibrium Introduction
  • Activity: Equilibrium in a Beaker
  • Lab: LeChâtelier’s Soda
  • Lab: A Greener LeChâtelier’s Principle Lab
  • Activity: How do Pollutants Affect our Oceans?