Stressing Equilibrium

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Objective

SWBAT identify the effects of changing concentration or temperature on a reaction at equilibrium.

Big Idea

Reaction systems at equilibrium respond to relieve stress placed on the system.

Background

This lesson occurred during our PARCC testing week.  Due to the testing, the classes were shortened to 35 minutes.

After the students completed the Equilibrium Graphing lesson, I felt that we had covered general equilibrium enough to warrant a formal quiz.  Today begins with a short multiple choice quiz to assess where students are without their notes or help from other students.

The next topic is LeChatelier shifts, which finally gets to the heart of HS-PS1-6.  I am introducing this concept via a short PowerPoint, with fill-in style notes to speed up the process due to our shortened period.  Like stoichiometry, I believe LeChatelier shifts are something that needs to be engaged with and practiced to really get a full grasp on the concept.  The notes are meant to provide a brief, organized introduction to the topic.

We then have a short period of time to practice, with the practice organized by stress.  I think organizing the practice in this manner allows students to focus on the effects of one factor at a time.

Since we have not talked about gas pressure all year, I will mention pressure, but it is absent from our practice and assessments.  This is a local curricular decision meant to streamline the instruction.  If a student understands the concept of why stressed reactions must shift to restore equilibrium, they can apply the concept of gas pressure as a stress when they learn it in college or AP chemistry.

This lesson aligns with the following standards:

  • HS-PS1-6Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
  • Science and Engineering Practice 2  Develop and use models.
  • High School Energy and Matter Cross Cutting Concept: Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of, and within that system.
  • Middle School Stability and Change Cross Cutting Concept:  Systems in dynamic equilibrium are stable due to a balance of feedback mechanisms.

Opening Quiz

10 minutes

When students enter, I ask them to get out something to write with for our quiz.  There is a chorus of groans and please for time to study.  I remind them that it has been on our weekly schedule board all week, and that I moved it back a day to give them time to finish the graphing assignment.

When students are ready, I pass out the quiz.  They are relieved to see it is multiple choice.  I inform them to place it on my desk when they finish, and get a copy of the notes sheet for the day.

Students are maddeningly inconsistent on the quiz.  There are four questions that essentially ask what the definition of equilibrium is, with the rates of the forward and reverse reaction being equal as the answer.  All four questions are worded slightly differently.  Many students get two or three of the questions correct, but the fourth version wrong. 

This points to a need to continue to work on basic vocabulary concepts so students understand that "as fast as" means "equal" and "increases" means from "smallest to largest".  This is going to be a key component of cycled instruction for the coming school year.

LeChatelier Notes

15 minutes

Once all the students are done with the quiz and have picked up the LeChatelier Shifts- Student Outline I turn on the projector and we begin to take notes.

This student captured the first two slides perfectly.

As discussed in the background, I have the students record that pressure can stress a reaction at equilibrium, but place an X next to it so it is clear they do not need to focus it.

The second slide is the most important.  Shifts occur as a result of changes in the rate of the forward or reverse reaction.  As we have already studied how to speed up and slow down reactions by changing temperature and concentration, this is a new application of that information.

The LeChatlier’s Principle PowerPoint continues with an exploration of our two main stresses, temperature and concentration.  Discussion of temperature focuses on first finding where the heat is in the reaction: reactant side (endothermic) or product side (exothermic).  Once that is determined, then you must know if you are increasing or decreasing heat to know if you are shifting away or towards the heat.

Lastly we discuss concentration, with the same principle holding up.  You must find the chemical who's concentration is being changed.  Then determine if you are adding or removing that chemical to know if the shift will be away or towards the side with the chemical.

Students are still a little uncertain, so I ask them to keep the notes out to refer to, and prepare them to practice applying LeChatelier's principle.

LeChatelier Practice by Stress

10 minutes

I have a student pass out the LeChatelier by stress practice paper while I set up the document camera so we can work through it together.

Once all students have the paper, I explain the format, that there are three reactions we are looking at twice each.  The first time we will be concerned with how temperature changes affect the equilibrium.  The second time we will be focused on how changing the concentration of each chemical would affect the equilibrium.

Here is a student example of the temperature portion.

The first thing I had students do is find the heat in all three reactions and underline it.  They then labeled the reactions as exothermic or endothermic.

We then focused on the change that was occurring.  When heat was added, we were providing it to the right side, or reverse reaction.  Students know that when you heat something up, it increases the rate of the reaction, so they wrote the reason first: heat speeds up the reverse reaction.

Next I asked:

  • "If the reverse reaction is sped up, which way will the reaction go to get back to balanced?
  • "Left"
  • "So if the reaction shifts to the left, what happens to the amount of NaOH, do we make more or use it up and have less?"
  • "Make more"
  • "So does it increase or decrease?"
  • "Increase"
  • "What about the Na+ and OH-?"
  • "They would decrease, because they are being used while it goes left."

We then repeated this process for removing heat.

We performed the second reaction together as well, and then they practiced as a table on the third and we went over it together.

Before beginning the concentration section, I asked if the heat matters if we are talking about how much of each chemical we have.  Some students took the lead and replied that it did not, since we were only changing the concentrations.  I had the students then find the heat and put an X through it on each reaction for this section so it didn't confuse them.

Again, we followed a strict talk-through.

  • "How would adding NaOH, increasing its concentration, affect the forward reaction?"
  • "Speed it up"
  • "If the forward reaction is sped up, which way will the reaction go to get back to balanced?
  • "Right"
  • "So if the reaction shifts to the right, what happens to the amount of Na+, do we make more or use it up and have less?"
  • "Make more"
  • "So does it increase or decrease?"
  • "Increase"
  • "What about the OH-?"
  • "It would also increase, because the reaction is moving forward and making it."

Next I pointed out how adding a salt gives Na+, discussing how salt dissolves in water.  I let students try it on their own once they recognized what chemical was being changed.  We then went over it, and I let them practice on the rest of the example, as seen below.

Each class finished at a slightly different place.  Some got through all three examples, and some only finished this one.  Before class ended, I reminded them that we are doing a lab tomorrow to test our understanding of the shifts.  I asked students to keep their papers so that they could look them over ahead of the lab.