Equilibrium Lesson Plan
Topic of Lesson: Equilibrium
Length of lesson: 3 – 50 minute class periods
Subject: ChemistryGrade Level: 11-12th Grade
Objectives:
-The student will explain the influence of temperature, surface area, agitation, and catalysts on the rate of a reaction. (Grade 9-12.Standard II.B.2)
Rationale/Purpose: The students should understand how forward and reverse reactions work to maintain equilibrium and what stresses can alter equilibrium, which can have real-world significance when dealing with pharmaceuticals, such as aspirin.
Preparation:
Day 1: The students need to have access to computers.
Day 2: Gather materials for the lab. Each group needs:
2 – large containers that are the same size (buckets, plastic boxes, etc.)
Water
Variety of beaker sizes
Day 3: Gather materials for the lab.
0.1 M FeCl3Concentrated nitric acid
0.1 M KSCNCopper mesh
0.1 M Fe(NO3)3Ice bath
0.1 M NaOHSix test tubes
0.1 M NaClTest tube stoppers
0.002 M Fe(NO3)30.002 M KSCN
Assessment:
Verbal answers given during the informal group discussion and individual written observations from the simulation on Day 1. Day 2 assessment will be answers on the lab worksheet. Day 3 assessment will be the answers on the lab worksheet.
Body of Lesson:
Day 1
Introduction:
0 min Set Context for Lesson (ENGAGE)
-Demonstration: Pour sodium chloride solution and silver nitrate solution into a beaker at the same time to produce a white precipitate of silver chloride.
-The demonstration was an example of a reaction that goes to completion, forming a precipitate. In this chapter, we are interested in reactions that do not go to completion. These reactions are reversible and reach equilibrium. Does anyone know what equilibrium means?
-Today we will use a computer simulation to observe and analyze
how a system reaches equilibrium and what happens when a stress is applied to the system.
Content/Activity:
5 min Set Up Activity (EXPLORE)
-Using the computer, show the students where they need to go to find the simulation. The web address is:
-Show the students which variables need to be selected and how the simulation works.
-Hand out worksheet. Give instructions to the students. Tell the students to try different rate combinations as well as changing the concentration, volume, and temperature variables. Does the reaction go to completion and stop or does it keep going? What happens when you change the variables?
30 min Gather Students to Discuss Activity (EXPLAIN)
-What did you observe?
- You saw molecules wandering around the beaker and changing color. What does it represent when the balls change color? Which are products and which are reactants? Does it matter if we are looking at equilibrium, since products react back to make reactants again?
-When the conditions remain the same, equilibrium is reached when the forward and reverse reactions occur at equal rates. At equilibrium it doesn’t matter which molecules are called the reactants or the products since the reaction is happening in both directions.
-What is happening in the graph? Does the reaction reach equilibrium? Are the concentrations equal at equilibrium? If not, what does equilibrium mean? Are the concentrations completely stable at equilibrium?
-The concentrations of the two molecules increase or decrease to a point where they are approximately equal to each other, where the reaction reaches equilibrium. The concentrations do not have to be equal at equilibrium, but the rates of the forward and reverse reactions are equal. The concentrations continue to fluctuate around equilibrium since the reaction does not stop once equilibrium is met.
-Why do the traces of each molecule eventual balance around a constant average? How come this value is an average and not a constant?
-The chemical reaction will occur even after equilibrium is reached. Chemical equilibrium is a dynamic system, meaning that reactants continue forming products and products continue re-forming reactants at the same rate.
-How does changing the concentrations of reactants and products in the system affect the equilibrium? Does it take more or less time to reach a stationary condition under various conditions?
-If you add a reactant, the system would favor the reaction that would consume the additional reactant, therefore more product would be made. If you removed a product, the reaction that produced more product would be favored.
-What is the effect of temperature on the equilibrium of the system compared to volume or concentration? Rate constants are calculated based on the temperature. We will talk about rate constants later.
-Le Chatelier’s principle: if a system is disturbed by applying stress, the system will readjust to a new equilibrium position to relieve the stress. Stress is a change in temperature or pressure. In an exothermic reaction, where heat is produced, lowering the temperature increases the rate of the forward reaction and raising the temperature increases the rate of the reverse reaction. Conversely, in an endothermic reaction, where heat is consumed, lowering the temperature increases the rate of the reverse reaction and raising the temperature increases the rate of the forward reaction.
Closure:
45 min Close
-Sum up key points. Students need to turn in their observation worksheets (EVALUATE). Remind students they will be doing a lab tomorrow.
Day 2
Introduction:
0 min Set Context for Lesson
-Yesterday looked at an equilibrium simulation.
-Today we are going to find equilibrium using a model.
Content/Activity:
5 min Perform the Demonstration (ELABORATE)
-Perform equilibrium demonstration with the help of two student volunteers. Have two identical containers on the front table, one filled halfway up labeled A and the other with only a little water in the bottom labeled B. Ask for two student volunteers. (If no one volunteers, call on students to assist in the demonstration.) Give each student a 50 mL beaker. Assign each student one of the containers. Tell the students the rules of the experiment: Transfer water from your container into the other container. Scoop as much water as possible into your beaker without tipping the container. Each student must dip and pour at the same time. Continue dipping until the water levels in the two containers remain constant. (Source of experiment: “Chemistry: Visualizing Matter.“ Myers, R. Thomas, Oldham, Keith B., and Tocci, Salvatore. Technology Ed. Holt, Rinehart, and Winston: Austin, 2000. p. 524.)
-Ask the students: What did you observe? Was equilibrium reached? If so, when?
-What would happen if we changed a variable? Have the students brainstorm several different variables that could be tested. (For example, different volumes in the container, different beaker sizes, etc.)
15 min Students Perform their Experiments (ELABORATE)
-Have the students form four groups and pick one of the variables to test. Each group should run three trials, changing only one variable.
-The students need to clean up their area when finished with the lab.
35 min Gather Class Together to Share Data
-Have each group tell the class what they tested. What were their findings? Was equilibrium reached?
Closure:
45 min Close
-Student worksheets are due tomorrow. Each group can turn in one worksheet for the whole group (EVALUATION). Read the Stimulus-Response lab for tomorrow.
Day 3
Introduction:
0 min Set Context for Lesson
-Yesterday we used a model to explore equilibrium.
-Today we are going to perform an experiment that will show us how a chemical reaction can move in both the forward and reverse directions.
-Assign partners for the students.
-Remind the students that they need to wear their safety goggles in the lab.
Content/Activity:
5 min Perform the experiment (ELABORATION)
-Students should follow the lab worksheet to perform the experiment.
(Source of experiment: CRISTAL Manual.)
-Students need to clean up their workstations when finished.
Closure:
45 min Close
-Lab worksheets are due tomorrow (EVALUATION). Each student needs to turn in his or her own worksheet.
Adaptations:
-Make sure everyone has a computer on Day 1. If there are not enough computers, assign partners.
-Make sure everyone has a group on Day 2.
-Make sure everyone has a partner on Day 3. If there are an odd number of students, make one group of three.
Name ______
Equilibrium Simulation
- Choose the values of Kb and Ku with appropriate sliders:
- Kb controls the rate of the forward reaction by which two green molecules turn bimolecularly into a single red molecule.
- Ku controls the rate of the reverse reaction, by which a red molecule turns unimolecularly into two green molecules.
- Having chosen appropriate values of the constants, press SETUP to clear the beaker and create an initial number of green molecules. (Note: we do not create red molecules initially, although this can be done in principal.)
Press RUN to start the simulation. Try several different rate combinations and pay attention to the plot of the concentrations. Record your observations.
- Set the size of the yellow box using the EDGE-SIZE slider. (If you would like to change the size while you are running a model. Press RUN to stop the model, adjust the EDGE-SIZE slider and redraw the box using the REDRAW BOX button. Resume the reaction by pressing RUN.) What happens when you change the size of the box?
- Use the other sliders and buttons to observe how concentration, volume, and temperature affect the equilibrium. Record your observations.
(A note on the temperature variable. Temperature changes have a unique effect on equilibrium compared with the other variables. You can observe this effect by toggling the TEMP-EFFECT button on or off and using the slider to set the temperature of the reaction in centigrade.)
- How do the stationary concentrations depend on the values of Kb and Ku? You can change Ku and Kb while the model is running. See if you can predict what the stationary concentrations will be with various combinations of Kb and Ku.
- Without adding additional reactants or products and with the temperature effect in the off position, note that more red product molecules accumulate when the volume decreases. Can you explain why?
Name ______
Equilibrium Lab
Equilibrium reactions do not start out at equilibrium. Instead, they begin with more reactants than products and eventually reach a state of equilibrium, where the amount of product and reactants remains constant, but typically not equal. In this activity, you will see firsthand how a process reaches equilibrium and how changing conditions will affect the forward and reverse reactions.
Materials: 2 – large containers that are the same size
Water
Beakers in a variety of sizes
Procedure:
- Get two identical containers and label one container “Products” and the other one “Reactants”.
- Set up your experiment based on the variable you are going to test.
- Make sure you dip and pour at the same rate.
- Run the experiment three times, changing only one variable at a time.
Questions:
- What variable are you testing?
- What happens to the amount of reactants? What happens to the amount of products? Record your observations for each trial.
- Does the system reach a state of equilibrium?
Name ______
Stimulus-Response
Exploration
Problem:
How can a chemical reaction be made to move in both a forward and a reverse direction?
Materials:
0.1 M FeCl3Concentrated nitric acid
0.1 M KSCNCopper mesh
0.1 M Fe(NO3)3Ice bath
0.1 M NaOHSix test tubes
0.1 M NaClTest tube stoppers
0.002 M Fe(NO3)30.002 M KSCN
Hazard Warning:
Wear safety goggles and lab aprons. Nitric acid and sodium hydroxide are extremely corrosive to skin and eyes. Wash your hands thoroughly before leaving the laboratory. Your teacher will instruct you on how to properly dispose of used concentrated nitric acid.
Procedure:
In this experiment, you will investigate what happens when the conditions of a certain type of reaction are changed by placing a “stress” on the system. In this special type of reaction, it is possible for the reaction to move in both a forward and reverse direction. Your task in this experiment is to discover how to make this forward and reverse process happen.
Part I: The first reaction you will be studying is shown below. As you can see the products have a red color while the reactants are yellow. By observing the color changes that occur you can determine whether the reaction is moving toward the products or toward the reactants.
Fe+3 (aq) + SCN- (aq) < - - - - - > Fe(SCN)+2 (aq)
yellow colorlessred
Follow the procedure outlined below. Read it before going into the lab. Construct your own data table in which to record all your data. Be sure to label each column clearly. Don’t forget to wear your safety goggles! Record all observations in your data table.
- Label six clean, medium size test tubes #1 through #6.
- To each test tube add 5 mL of 0.002 M KSCN and 5 mL of 0.002 M Fe(NO3)3.
- Leave test tube #1 as your color control.
- To test tubes #2-6, add 10 drops of the following solutions, (one solution per
test tube):
0.1 M FeCl3
0.1 M KSCN
0.1 M Fe(NO3)3
0.1 M NaOH
0.1M NaCl
- Record the resulting colors in your data table.
Part II: The second equilibrium you will investigate is shown below. This will be created by reacting copper with nitric acid. The resulting brown gas will be collected and studied.
N2O4 (gas) + heat < - - - - - > 2NO2 (gas)
colorless brown
Follow the procedure below to set up this equilibrium situation.
- Obtain a small sample of copper mesh and place it into a medium sized test tube. Under the fume hood, add one drop of concentrated HNO3. Keep your goggles on! Place a stopper on the test tube and observe any changes.
- Place the stoppered test tube into an ice water bath for a few minutes. Record what happens.
- Remove the tube from the ice bath and warm it with your hands. Record what happens.
Summing Up:
- In which test tube in Part I did a chemical reaction occur? How could you tell?
- In Part I, how could you tell when the concentration of Fe(SCN)+2 increased? How could you tell when it decreased?
- Study the chemical formulas for the solutions you added to test tubes #2-4 in Part I. Compare these to the chemical equations provided. What relationship can you find between these formulas and the yellow to red reaction?
- In Part II, what happened to the concentration of NO2 gas when the test tube was placed into the ice bath? When warmed?
- Can equilibrium equations go in both a forward and a reverse direction? How can you tell?
- In general, what are some of the things you can do to cause a reaction to go back and forth?