# Equilibrium and Le Chatelier's Principle

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## Objective

Students will be able to define chemical equilibrium and use Le Chatelier's Principle to predict how changes in pressure, concentration, and temperature affect systems in equilibrium through doing an activity, taking notes, watching videos, and performing practice questions.

#### Big Idea

Equilibrium occurs when the rates of the forward and reverse reactions are equal. Le Chatelier's Principle can be used to predict how changes in concentration, pressure, and heat affect these equilibrium systems.

## Introduction

In this lesson students are introduced to the concept of equilibrium and Le Chatelier's principle through performing an activity, taking notes, watching videos, doing whiteboard practice, and doing a practice worksheet.

• This lesson covers the Next Generation Science and Engineering Performance Expectation 1-6: Refine the design of a chemical system by specifying a change in conditions that would produce increased amount of products at equilibrium.  It does so because students are learning about equilibrium and Le Chatelier's Principle.
• This lesson aligns with several of the Next Generation Science and Engineering Practices including:
• 2: Developing and Using Models.  It does so because students are using models of chemical equations to determine the effect of changes on equilibrium.
• 4: Analyzing and Interpreting Data: It does so because students are interpreting the data from their engage activity to see what happens during equilibrium.
• 6: Constructing explanations: It does so because students are asked to explain how and why equilibrium reactions will shift when disturbed.
• This lesson aligns with several of the Next Generation Science and Engineering Crosscutting Concepts including:
•  4: Systems and System Models.  It does so because students are able to see how Le Chatelier's principle can be used as a model to see the effect on an equilibrium system.
• 7: Stability and Change.  It does so because students are directed to learn how equilibrium systems are stable with reactants and products being exchanged at equal rates, but that factors can stress the system to cause changes.

For this lesson each group of students needs 100 two-sided papers with Product on one side and Reactant on the other for the engage activity.

## Engage Part I

30 minutes

To begin the lesson I have students perform an activity with a partner where they are able to visualize what happens with reactants and products during an equilibrium reaction.

• For this activity each student needs an Equilibrium introduction activity paper and each group of students needs 100 2-sided papers with reactants on one-side and products on the other side.
• To make these papers I copy the last two pages of the Equilibrium introduction activity paper onto two different colors, glue together and then cut out with the paper cutter.
• I have students read the instructions at the top of the paper.  I tell them that this is very similar to the activity we did in lesson 1, but for this activity they are going to model a 2-way equilibrium reaction.
• I make sure that students have an elbow partner to work with to make a group of 2.  If there is an odd number class than I have one group of 3.   I then make sure that one student is the reactant and the other is the product.
• I then demonstrate to students how to do the activity by modeling that they will start with the 100 papers on the reactant side and that one partner will flip over 20% at each step to product.  I make sure to tell them to record the amount of reactant leftover and product made at each step.  I then explain how the other partner will be taking 30% of the product and converting back to reactant in each step, making sure to record the amount of product leftover and reactant produced in each step.  I also tell them that they are going to want to use a calculator to determine the % converted in each step.  This is a movie showing me doing this with my students.
• As students are working I walk around to ensure that they are on task and to help with any questions.   Here is a picture of what the chart should look like.  This is a picture of students working.
• As students begin to complete the first side of the paper I instruct them to flip to the back where they will be graphing.  I tell them to look at the front page to see how step# is going to be on the X axis and then amount of reactants and products is on the Y axis.  This is a picture of what the final graph should look like.

## Engage Part II

5 minutes

As a second part of the lesson to engage students I have them think about several questions on the first slide of the PowerPoint

The questions include:

1. What is activation energy? How does it look on a graph?
2. What are the four factors that affect reaction rates?
3. What is the difference between Exothermic and Endothermic reactions in terms of Energy of reactants and products?

I tell them to think about them on their own first and then to share with their table groups.

The goal of my doing this is to give students a chance to review what we learned in the previous lesson before learning new content.

I give students several minutes to do this and then call on several groups to share out their answers with the rest of the class.

For the most part students remember these concepts from the the first lecture, and if they forget than they know to look back at their notes and/or talk with their partners.

This is a copy of one student's filled in notes with the answer for this section at the top of the paper.

## Explain

20 minutes

This part of the lesson is where I teach students about equilibrium and Le Chatelier's Principle.  I present the notes to students on the unit9 lecture 2 PowerPoint and have them fill in information on their notes graphic organizer.

• I start with showing students slide 2 of the PowerPoint to review what we will be learning about in class.
• I then show students slides 3 and 4 where I define equilibrium.  As I do this I refer back to the modeling lab that students did at the beginning of class and discuss how equilibrium was established when they were passing back the papers at the same rate (give 12, take 12) and that the amount of reactants and products is constant, but that they are not necessarily the same as each other (61 for reactants compared to 39 for products).
• I then introduce Le Chatelier's Principle on slide 5 and go over examples of how this works for concentration (slide 6), pressure/volume (slides 7 & 8), and temperature (slides 9 & 10).
• For concentration I stress that equilibrium shifts AWAY from what is ADDED and TOWARDS what is TAKEN.  Students always get confused with "which side is it being added or taken from?"   Therefore I make sure to show them that it is added or taken from the side in which it is already on in the equation.
• For pressure I tell students to make sure to only look at GASES.  I also review Boyle's Law with pressure and volume having and inverse relationship and explain that if the pressure decreases you want to have MORE gas, while if it increases the equilibrium will shift to have LESS gas.
• For temperature I tell students how they first need to out heat on the left for endothermic reactions or the right for exothermic reactions and then think about how heat changes.  Students get confused about putting heat on a side so I make sure to reiterate that you place it there to think about how temperature is affecting the reaction.
• This is an example of one student's filled in notes

## Elaborate Part I

10 minutes

For this section of the lesson I have students perform practice questions of Le Chatelier's principle which are on the last 3 slides of the PowerPoint.

Students are still very unsure of how to do Le Chatelier's Principle so I make sure to go through each of the questions with them one at a time.

For example I first show them slide 12 and have them try questions #1.  I then go over the answer and then have them try question #2, etc.

Here is a copy of one student's notes which include the answer to the practice questions at the bottom of the second page.

## Elaborate Part II

15 minutes

To help reinforce Le Chatelier's principle I have students watch the videos for two chemical reactions.  I choose to do the videos versus real-life demonstrations because it is much easier and less time consuming with the end of the year crunch.

I start by passing out the Equilibrium Video Questions paper.

I then tell students how we are going to first focus on the first reaction and to look over the questions that we will be answering.  I tell them to take a second and predict the answer before the video.

I then show students the first video LeChateliers-Cobalt Equilibrium movie

I show them the entire video one time.  I then start it over again and during the second time I pause for them to answer the questions.  The final question "is the reaction exothermic or endothermic" is not on the video, but I tell students they should be able to figure it out by looking at the equation.

Next, I tell students that we are going to focus on the second equation.  I tell them to look over the questions and to try to predict the answers before the video.  After several minutes I show students the second video NO2-N2O4Equilibrium-Pressure movie.  I play the entire video one time.  Then during the second time I stop so that they can answer the first question.

I then show students the third video Demo-NO2-N2O4- temperature movie.  I show them the entire video one time.  I then start it over again and during the second time I pause for them to answer the last two questions.

This is a copy of the answer key for the answers I expect and a copy of one student's answers to the questions.

## Evaluate

As a final part of this lesson I have students perform homework.

I pass out the homework worksheet and let them know that the paper will be due at the next class.

At the next class period I check their homework for completion by stamping and then go over the answers using the answer key.

This is a copy of one student's paper with their answers to the questions.

## Standard Mediated Instruction (SMI) Time

20 minutes

At the end of each class my students have SMI time.  For this SMI period students worked on test corrections from the last exam (Unit 8- Water Quality).

When students perform test correction in my class I have them use a paper where they write down each incorrect answer, what the correct answer is and an explanation of why that answer is correct.  For their explanation they can write a sentence for the "wordy" fact-based questions, and must show their work for the "mathy" equation-based questions.

When I grade students test corrections I grade it using a check (100%) where students completely explain all of their incorrect answers, check minus (75%) where students are missing a question or two or missing their justification, and check minus minus system (50%) where students are missing several of their incorrect answers.

This is a copy of a student who earned a check.  If you notice they explain why the correct answer is correct and show their work for the answers that that are "mathy" like solving for ppm in #9.

This is a copy of a student who earned a check minus.  Notice how this student does not completely explain WHY the answer is correct in #13 and has incorrect work for #19.