What is Equilibrium?

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SWBAT explain chemical equilibrium in terms of equal rates of forward and reverse reactions.

Big Idea

Chemical reactions are dynamic processes in both the forward and reverse direction.


After studying reversible reactions via the PhET simulation, there still seemed to be some gaps in student understanding, so I decided to provide a formal "sit-and-take-notes" day.

Since there weren't any special diagrams or pictures I wanted students to have, I allowed them to write the notes freestyle, trying to emphasize that they should only write what they need, based on the things that they hear and think "That's new!" or "I didn't remember that, I should write it down."

One of the purely new concepts here is endothermic and exothermic reactions.  This is introduced to set up how students will treat energy when we begin to perform LeChatelier shifts later in the unit.

To this point, we had used overly simplistic models for our reversible reactions.  At the end of the notes, I provided students a list of actual chemical reactions that are reversible.  Students then had to think about the forward and reverse reactions and identify the reactants and products in each direction.  I wanted them thinking about this prior to teaching LeChatelier shifts, as I planned on talking in terms of the rate of the forward or reverse reaction, and need them to be able to easily identify both.

This lesson aligns to 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.
  • High School Stability and Change Cross Cutting Concept: Much of science deals with constructing explanations of how things change and how they remain stable.
  • Science and Engineering Practice 2: Develop and use models


5 minutes

To begin the period, I pass back the students' reversible reaction sheets from the day before, and collect those who kept it overnight to complete the graph.  I mention that while the student data collection and conclusions were quite good, we are going to need to improve our graphing skills; particularly in terms of building an appropriate scale and labeling our axes.

I apologize briefly to my morning classes, I was more than a little frustrated at their lack of graphing ability.  I did not realize how much it was now absent from our Algebra I, Geometry, and freshman Geography curricula.  It used to be a staple of all three courses, so I anticipated much higher skills right off the bat.

I then tell the students that we will be taking notes today, and students need to get out loose leaf paper for their binder, or their notebook.  While they are getting out materials, I take attendance and get the PowerPoint and projector set up.

Equilibrium Notes

30 minutes

When students have their materials ready, we begin the Equilibrium Overview PowerPoint.  Below are the highlights of my narrative beyond the slides themselves.

What is Equilibrium?  I used this reaction since we did it during our periodic trends unit, and I refer back to and describe the reaction to help students remember it.

What is Equilibrium?   Now I refer to the equilibrium games and PhET simulations as examples where they saw the rates become equal to establish equilibrium.

What is Equilibrium?   On this slide I particularly reference the PhET simulation, as students were able to observe the particles still in motion and reacting once it reached equilibrium.

What Equilibrium Isn't.   For each point on this slide, I ask students to provide me examples from the work we've done that proves we do not need equal amounts of reactants and products, or the reaction to stop.  Students refer to the blue bottle demo, the water games and the PhET simulation for them.

Types of Equilibrium.   Later in the day I removed the examples of physical equilibrium.  This was partially as I needed to conserve a little time, and also because I wanted students to focus on the chemical side of equilibrium.  However, if we teach states of matter again next year, I will refer to it as a physical equilibrium to seed the term and concept early in the year.

Representing Chemical Equilibrium.   I was thrilled on this slide, as many students just wrote the two styles of the reversible arrows.  I explained that depending on the publisher of the text or lab we look at, either arrow may be used.

Heat and Chemical Reactions.   I make a big deal of the word root here.  EN = in.  I use the example of chemical cold packs, which most students have used before.  If I had more time in the lesson, the reaction of barium hydroxide and ammonium chloride is outstanding as an example.

Heat and Chemical Reactions.   Before moving to this slide, I ask students to predict the opposite of endothermic.  Three of the four classes remembered "exothermic" from last semester.  Again I stress the prefix "EXO" = Exit.  The heat exits the reaction.  We have had lots of reactions that heated up this year, so I ask them to remind me of examples.

Heat and Reversible Reactions.  Before clicking to the text, I ask students what they notice about the graphs.  The symmetry is mentioned, and that they are opposites.  I let students explain that before telling it to them.

I encourage students to keep out their notes to assist them with our next task.

Reversible Reactions

15 minutes

I have a student pass out the Breaking Down Reversible Reactions practice sheet while I transition to the document camera and set up to do an example with students.

We do the first reaction together, with students identifying the reactants and products of the forward reaction quite easily.  Since we did not do chemical naming this year, we refer to each chemical purely by its formula.  I use both, so students hear the names, even though it is not expected that they know them.  They do tend to remember most of the pure elements.

To do the reverse reaction, I ask them to draw an arrow pointing in reverse, and ask "What chemicals did you start with?"  By adding the extra arrow, and thinking about where you start and end in reverse, students quickly were able to identify the reactants and products for the reverse reaction.

I then ask, "What do you notice about the four answers?"

  • "The reactants for the forward reaction are the products of the reverse reaction."
  • "The products for the forward reaction are the reactants of the reverse reaction."

For the second reaction, I only do the endothermic and exothermic portion with them to get them oriented to the type of question.  I let students work independently with their notes available. 

While students work, I circulate the room.  All students process the forward reactions just fine, but looking in reverse trips up some students.  When I catch them making errors in reverse, I make them go back to drawing the single arrow for reverse to help them orient to where a reaction is starting from.

This student had to write out the arrows on the top of the page to help herself, but then did a beautiful job, only confusing a couple of the endo-/exothermic labels.

When students finish, I collect the papers to assess overnight if we are ready to move on to the next assignment.  Students did very well with processing the reactions in each direction.  However, it too many of them until the fourth reaction to have confidence in their understanding of the differences between the forward and reverse reaction.

Overall, I am pleased that I took this time to cement this understanding for my students before moving on.  Equilibrium can be confusing enough, but it is doubly so when you don't have a strong foundation of understanding the reverse reaction process.