GPS UNIT PLANCHEMISTRY: EQUILIBRIUM

Nancy Brim Grade Level: 10/11Lakeside High School Length of unit: 2 wks (non-block)Nancy_H_Brim@fc.deKalb.k12.ga.us

Brief Summary of the Unit:Students will discover that reactions can reach a state of equilibrium. Students will use Le Chatelier’s Principle to

explain how various factors affect chemical equilibria. Students will calculate equilibrium concentrations of reactants and products using the equilibrium constant expression.

STAGE 1 - PlanningStandard: Co-Requisite – Characteristics of ScienceSCSh1. Students will evaluate the importance of curiosity, honesty, openness, and skepticism in science.

a. Exhibit the above traits in their own scientific activities.b. Explain that further understanding of scientific problems relies on the design and execution of new experiments

which may reinforce or weaken opposing explanations.SCSh2. Students will use standard safety practices for all classroom laboratory and field investigations.

a. Follow correct procedures for use of scientific apparatus.b. Demonstrate appropriate techniques in all laboratory situations.c. Follow correct protocol for identifying and reporting safety problems and violations.

SCSh3. Students will identify and investigate problems scientifically.a. Collect, organize and record appropriate data.b. Graphically compare and analyze data points and/or summary statistics.c. Develop reasonable conclusions based on data collected.

SCSh4. Students will use tools and instruments for observing, measuring, and manipulating scientific equipment and materials.

a. Develop and use systematic procedures for recording and organizing information.b. Use technology to produce tables and graphs.

SCSh5. Students will demonstrate the computation and estimation skills necessary for analyzing data and developing reasonable scientific explanations.

a. Consider possible effects of measurement errors on calculations.b. Recognize the relationship between accuracy and precision.c. Express appropriate numbers of significant figures for calculated data, using scientific notation where appropriate.d. Solve scientific problems by substituting quantitative values, using dimensional analysis, and/or simple algebraic

formulas as appropriate.SCSh6. Students will communicate scientific investigations and information clearly.

a. Write clear, coherent laboratory reports related to scientific investigations.SCSh8. Students will understand important features of the process of scientific inquiry. Students will apply the

following to inquiry learning practices:a. Scientific investigators control the conditions of their experiments in order to produce valuable data.b. Scientific researchers are expected to critically assess the quality of data including possible sources of bias in their

investigations’ hypotheses, observations, data analyses, and interpretations.

CHEMISTRY ContentSC2. Students will relate how the Law of Conservation of Matter is used to determine chemical composition in

compounds and chemical reactions.

Big Idea: Chemical Equilibrium

Elements: f. Explain the role of equilibrium in chemical reactions.

Enduring Understandings:1. Chemical reactions reach equilibrium.2. Stresses placed on a system in equilibrium causes shifts in the system to relieve the stress.3. Chemical equilibrium is a state where the forward reaction and reverse reaction balance each other

because they have equal rates.

Essential Questions:1. What is chemical equilibrium?2. What is the equilibrium constant, Keq?3. What is the reaction quotient, Q, and how is it related to Keq?4. What is Le Chatelier’s Principle and how does it apply to Keq?5. What are the stresses that can cause a shift in a system?

Knowledge:1. Students will be able to define chemical equilibrium and a reversible reaction.2. Students will be able to explain Le Chatelier’s Principle as it applies to stress placed on a system at

chemical equilibrium.3. Students will be able to relate Keq to Q.4. Students will be able to relate relative amounts of reactants and products with the equilibrium constant.

Skills:1. Students will be able to write an equilibrium expression for a given chemical reaction.2. Students will be able to calculate Keq and/or the concentration of reactants or products for a chemical

reaction.3. Students will be able to calculate Q.4. Students will explore changing the concentration of a reactant or product to create a new

equilibrium.5. Students will explore how changes in the concentration and changes in temperature act as stressors in a

chemical reaction.

STAGE 2 – Evidence of LearningPerformance Tasks:

1. Students will explore equilibrium through manipulation of water and straws.2. Students will demonstrate competency in calculating Keq and/or the concentration of a reactant or

product in an equilibrium expression.3. Students will demonstrate competency in calculating Q and relating it to Keq.4. Students will demonstrate the effects of a change in the concentration or a change in the temperature on

a chemical system at equilibrium.

STAGE 3 – Learning Experience and InstructionDAY 1:

Objective: Discuss reversible reactions and chemical equilibriumActivity: LECTURE: Reversible reactions and chemical equilibrium

LAB: Establishing Equilibrium labPRACTICE: Writing equilibrium expressions

DAY 2:Objective: Describe and calculate the equilibrium constant and reaction quotient.Activity: LECTURE: Keq ans missing species

PRACTICE: Calculating Keq or finding the missing concentration

DAY 3:Objective: Explore chemical equilibriumActivity: LAB: Reversible Reactions

DAY 4:Objective: Explore the results of changes in temperature and concentration on a system at equilibriumActivity: DEMONSTRATION: Effect on temperature and concentration on equilibrium

LECTURE: Le Chatelier’s PrinciplePRACTICE: Le Chatelier’s Principle

DAY 5:Objective: Explore the results of changes in temperature and concentration on a system at equilibriumActivity: LAB: work on procedure for tomorrow’s lab

Practice: Le Chatelier’s PrincipleDAY 6:

Objective: Explore the results of changes in temperature and concentration on a system at equilibriumActivity: LAB: What happens to an Equilibrium system when it is disturbed?

DAY 7:Objective: Solve problems involving Keq, Q, and concentrations; Discussion of solution equilibriaActivity: ASSESSMENT: Calculating Keq, Q, and concentrations quiz

LECTURE: Solution equilibriaPRACTICE: Le Chatelier’s Principle

DAY 8:Objective: Review the objectives for the unitActivity: DISCUSSION/PRACTICE: Review of unit objectives

DAY 9:Objective: Display knowledge of unit objectivesActivity: ASSESSMENT: Unit test

DemonstrationsReversibility

14 g NaOH 500 mL water flask with stopper14 g Dextrose 1 mL methylene blue

Add all together. Shake vigorously until it turns blue. Let it sit – it will clear. Shake again. MUST MAKE DAY OF DEMO – NOT BEFORE.

Effect of Temperature of EQ100mL beaker 12 M HCl ice bath hot water bathCoCl2 2 test tubes

Add 10 mL of CoCl2 to beaker. Add 20mL 12M HCl. Add 20mL water. Then add 10mL HCl to get the purple color. Fill the test tubes. Put one in the ice bath and one in the hot water bath. Compare after a few minutes.

Effect of Concentration of EQ4-100mL beaker 1.0M solutions of chromate and dichromate solutions 3.0M HCl3.0M NaOH 1.0M Ba(NO3)2 pipets

Add 10-15mL of the chromate solution to a beaker. Do the same for the dichromate solution. Add drops of HCl to the yellow solution #1. Add drops of NaOH to the orange solution #2. Now switch and add HCl to #2 and NaOH to #1. Start again with two fresh beakers of solution. Add NaOH to the orange and then add Ba(NO3)2 (turns yellow). Add HCl to the yellow and then add Ba(NO3)2. Add NaOH to orange – precipitate appears. ASll HCl to yellow – precipitate appears.

Name __________________________________ Period ____ Date ________________

Observations: Left Right Left Right

Transfer Cylinder (mL) Cylinder (mL) Transfer Cylinder (mL) Cylinder (mL)

1. Which cylinder did you start the blue-tagged straw in each time? LEFT RIGHT

2. At equilibrium, which had a greater volume?

Forward Reaction REACTANTS (left) PRODUCTS (right)

Reverse Reaction REACTANTS (left) PRODUCTS (right)

4. What was the effect on equilibrium volumes when you started with no reactants and 25.0-mL of products compared to the other way around?

5. Graph your results for both experiments. In the first graph, plot the transfer number (1 through X) on the x-axis and the volumes (mL) for both reactants and products on the y-axis. In the second graph, use the second set of data. Plot the transfer number (1 through X) on the x-axis and the volumes (mL) for both reactants and products on the y-axis. Use different colors for products and reactants and provide a key. Be sure to label your axes as well as your graph.

ESTABLISHING EQUILIBRIUM

Analysis and Conclusion:

1. Was equilibrium established in both experiments? If it was, what can you conclude about equilibrium?

2. What variable determines whether there was a greater volume of reactants or products at equilibrium?

3. Why would this system be called a dynamic equilibrium?

ESTABLISHING EQUILIBRIUM

Procedure:1. Place one graduated cylinder on the left side and the other on the right. The Blue tagged straw is also on

the left (“forward”) and the other is on the right.2. Add a few drops of food coloring to a beaker of about 100 mL water. Mix well and add the colored water

to the left cylinder. Fill to the 25.0-mL mark.3. Place the forward/blue-tagged straw into the reactant cylinder. Place the other straw in the empty product

cylinder.4. Put your finger over the top of the blue-tagged straw. MAKE SURE THE STRAW GOES ALL THE

WAY TO THE BOTTOM OF THE CYLINDER. Transfer the colored water in the straw to the right cylinder.

5. Now put your finger over the other straw and transfer the colored liquid from the right cylinder to the left cylinder. MAKE SURE THE STRAW GOES ALL THE WAY TO THE BOTTOM OF THE CYLINDER.

6. Record the volume of each cylinder in the data table. Each time you transfer liquid from the left cylinder to the right cylinder and back from the right cylinder to the left cylinder constitutes ONE transfer.

7. Repeat steps 4, 5, and 6 until the volumes in the cylinders remain constant for at least five readings. 8. Repeat the entire experiment, but this time, begin with 25.0-mL of colored water in the right cylinder and

nothing in the left cylinder. Use the straws on the same side as when doing the forward reaction.

REVERSIBLE REACTIONS

In some chemical reactions, the reactants are not entirely converted to products. This is because as the products form, they react to re-form the reactants in a reverse reaction.

When the rate of a forward reaction is equal to the rate of the reverse, the system is said to be at equilibrium. At equilibrium, the forward and the reverse reactions proceed at the same rate, so the concentrations of the reactants and products do not change.

LeChâtelier’s principle states that if a system at equilibrium is subjected to a stress, the equilibrium will shift in a direction that will relieve the stress. One such stress is change in concentration. In this activity, you will see how changing the concentration of a reactant or product creates a new equilibrium.

Problem: How does the change in the concentration of a reactant or product affect a system at equilibrium?

Objective: Determine the shifts of equilibrium brought about by changes in concentration.

Materials: 12M HCl 0.1M CoCl2 test tubes goggles6M HCl NaCl solution dropper pipettes0.1M FeCl3 NH4Cl solution 10-mL graduated cylinder0.1M KSCN NH3 solution test tube rack

Safety Precautions:

Prelab Questions:1. State Le Chatelier’s Principle.

2. In which direction will a reaction shift if there is an increase in the concentration of a reactant?

3. In which direction will a reaction shift if there is a decrease in the concentration of a reactant?

4. Read the entire laboratory procedure. Form a hypothesis about how a stress on a system at equilibrium will cause a shift of the system. Record your hypothesis below and then transfer it IN INK when you get your lab sheet.

Procedure:PART A: CHLORIDE SOLUTION

1. Pour 3 mL of the saturated NaCl solution into a clean test tube. Add 6 drops of 12M HCl (Do this in the fume hood!) Record your observations in Data Table 1.

2. Pour 3 mL of the saturated NH4Cl solution into a clean test tube. Add 6 drops of 12M HCl (Do this in the fume hood!) Record your observations in Data Table 1.

PART B: IRON(III) CHOLRIDE AND KSCN SOLUTIONS1. Pour 5mL of FeCl3 into three test tubes and 5mL of KSCN into three test tubes.2. To the first test tube (one tube each of FeCl3 and KSCN), add 1mL of 0.1M KSCN solution.

Observe and record the color change in Data Table 1.3. To the second test tube (one tube each of FeCl3 and KSCN), add 1mL of 0.1M FeCl3 solution.

Observe and record the color change in Data Table 1.4. Use the third test tube of each as a control. Note and record the color of the solution.

PART C: COBALT CHLORIDE SOLUTION1. Pour 2mL of 0.1M CoCl2 solution in a clean test tube.

a. Add 3 mL of 12M HCl in the fume hood.b. Add water dropwise until the original color is restored.c. Record your observations in Data Table 1.

2. Pour 2 mL of 0.1M CoCl2 solution into a second clean test tube.3. Pour 2 mL of 0.1M CoCl2 solution into a third clean test tube.

a. Add about 1.5g NH4Cl to the test tube.b. Compare the colors of the contents of the second and third test tubes and record your

observations in Data Table 1.

PART D: AMMONIA SOLUTION1. Pour 5mL of ammonia solution into another clean test tube

a. Add 10 drops of 6M HCl and stir the solution.b. Record your observations in Data Table 1.

CLEAN UP AND DISPOSAL1. Dispose of chemicals by pouring down the drain with lots of running water.2. Clean test tubes thoroughly and DRY them.3. Return all lab equipment to its proper place.4. Wash your hands thoroughly with soap and water before leaving the lab.

Name ____________________________________ Period _____ Date ____________

Partner ___________________________________

REVERSIBLE REACTIONS

Hypothesis (in ink):

Data and Observations:DATA TABLE

Step NumberObservation

Part A: 1

2

Part B: 2

3

4

Part C: 1

3

Part D: 1

Analysis and Conclusion:1. Collecting and Interpreting Data

a. In Part A, step 1, which ion concentration change _____________ is responsible for the equilibrium shift?

b. In Part A, step 2, which ion concentration change _____________ is responsible for the equilibrium shift?

c. In Part B, step 2, which ion concentration change _____________ is responsible for the equilibrium shift?

d. In Part B, step 3, which ion concentration change _____________ is responsible for the equilibrium shift?

2. Explain the meaning of a control, as used in Part B, step 4.

3. Collecting and Interpreting Data

a. In Part C, step 1, which ion concentration change _____________ is responsible for the equilibrium shift?

b. In Part C, step 3, which ion concentration change _____________ is responsible for the equilibrium shift?

c. In Part D, step 1, which ion concentration change _____________ is responsible for the equilibrium shift?

4. Drawing a Conclusion. The equilibrium system for the cobalt chloride solution may be expressed as follows:

4Cl-(aq)

+ Co(H2O)6+2 (aq) ↔ 6 H2O (l) + CoCl4

-2(aq)

Explain what happened to the concentration of each of the following ions when hydrochloric acid is added.

a. Cl-

b. Co(H2O)6+2

c. CoCl4-2

5. Predicting. Predict the effect of adding sodium hydroxide in place of hydrochloric acid to a saturated solution of sodium chloride. (See Part A, step 1.)

6. To what extent are accuracy and precision factors in this experiment?

7. In the Haber process, nitrogen and hydrogen are combined to form ammonia according to the following reaction: N2 (g) + H2 (g) ↔ 2 NH3 (g) Explain what effect an increase in pressure would have on the yield of ammonia.

According to your text, Le Chatelier’s Principle describes the effect that applying various types of stresses will have on the position of equilibrium. That is, whether it will shift to increase or decrease the concentration(s) of the products in the equilibrium system. These stresses include things like variation in the concentrations of reactants and products, the temperature of the system, and (for reactions involving gases), the pressure.

Some investigations are done with systems in a water solution. Here, unless gases are involved in the reaction, the volume of the solution generally defines the volume of the system, and pressure is of little or no consequence. This sort of system permits us to simplify Le Chatelier’s Principle to read:

For any system at equilibrium in solution: if anything is added to the system, it will try to replace what was removed. Note that the word “anything” refers to energy (heat) as well as to any of the reactants or products shown in the reaction equation.

Purpose: To observe for yourself what Le Chatelier’s Principle means. Your investigation will deal with two complex ions, both containing cobalt (II); they are Co(H2O)6

2+ and CoCl42-.

Materials: 50 mL beaker 24 well plate thin-stem pipets 12M HClgoggles hot plate ice calcium chloridecobalt(II) chloride deionized water ethanol 0.1 M silver nitrate

Safety Wear goggles. Use extreme caution. Hydrochloric acid is caustic and should be kept off skin and clothing. Wash any contact with copious amounts of water. Perform any step with the HCl in the fume hood. Silver nitrate will stain your hands and clothes!

Goal: To write up the procedure and then perform the steps to show four equilibrium shifts for these compounds using the materials that you have available to you.

Procedure:1. Place a small amount of solid cobalt (II) chloride in well A1 of your 24 well plate. You should

have enough to be able to visually tell the color of the compound. Note both the color and the formula for the compound. This is for comparison purposes only.

2. Measure 10 mL of ethanol into a 100 mL beaker using the markings on the beaker. This doesn’t have to be really accurate.

3. Add some crystals of the cobalt (II) chloride solid to the 10 mL of ethanol in the 50 mL beaker until a blue color results. Stir the solution or swish it around to get the cobalt(II) chloride to dissolve. You want a blue color, not a pink or purple color. Make sure all the solid has dissolved. This is your basic solution with which to run all your reactions. Ethanol dehydrates the pink cobalt.

4. Using a thin stemmed pipet, transfer one fifth of the blue solution to four of the wells in a 24 well plate, B1, B2, B3, and B4. Be sure to leave a small amount of solution in the beaker. You can now perform your experiments on the solution in these wells. Perform all experiments on the solutions in the well plate holes except for the ones involving a temperature change.

5. Now, using HCl, deionized water, solid CaCl2, AgNO3, heat, and/or ice, cause the equilibrium to shift at least four times. You are starting at the blue form. You can then shift it to pink and back to blue, and/or you can shift it to pink in different ways. At least one of the shifts should involve a temperature change and one should involve an addition of a compound. If you are going to use

What Happens To An Equilibrium System When It Is Disturbed?

heat or ice, then use the solution in the beaker. When using ice, please place the beaker in an ice water bath. Don’t place the ice in the solution.

The net ionic equation for the equilibrium reaction you are investigating is:Co(H2O)6

2+ + 4 Cl- CoCl42- + 6 H2O

Pink blue

6. Discard all solutions down the sink with plenty of water. Return any solids to your teacher.

YOUR LAB REPORT

The lab report that you will submit should include the following:1. Title Page – be sure to following the instructions for the correct format. Refer to your laboratory

report instruction sheet given out in the fall.2. Procedure - List ALL the steps involved in your procedure. Write down every step that you did

even if it did not show a shift. Make sure that I could replicate your experiment from the instructions that you write. You can start your procedure where mine left off. No need to repeat what I wrote.

3. Data and Observations – use a table like the one below. The step # should match the procedure #. Show observations for every step that you added something or changed the temperature of it.

Procedural Step Observation

STEP ______

STEP ______

4. Conclusions - Answer the questions below in your conclusion section.A. On the reagent bottle, the formula for solid cobalt(II) chloride is CoCl2 6 H2O. What name

do we give to compounds which have water molecules bound to their structure?B. For the following questions, answer those that pertained to the experiments that you

performed. You will only have to do four since you only had four shifts. 1. Which cobalt complex (pink or blue) was favored by adding deionized water? Use Le

Chatelier’s to explain why the color change occurred.2. Which cobalt complex was favored if you added HCl? Use Le Chatelier’s Principle to

explain why the color change occurred.3. Which cobalt complex was favored when you added calcium chloride? Use Le

Chatelier’s Principle to explain why the color change occurred.4. Which cobalt complex was favored when you added silver nitrate? Use Le Chatelier’s

Principle to explain why the color change occurred.5. Which cobalt complex was favored when the temperature was increased? Use Le

Chatelier’s Principle to explain why the color change occurred6. Which cobalt complex was favored when the temperature was decreased? Use Le

Chatelier’s Principle to explain why the color change occurred.

GRADING YOUR LAB

Title Page 5 title, partner, name, date, class4 missing one part3 missing two parts2 missing three parts1 missing four parts0 no title page present

Procedure 30 Experiment could be replicated perfectly.25 Missing some descriptors which would make procedure clearer.20 Missing some steps and/or a shift. 15 Missing some steps and/or two shifts. 10 Missing many steps and/or three shifts.0 Procedure missing completely

Data Table 25 Data table lists all steps reported/complete observations of each change20 Data table is basically complete, but is missing a few descriptor (observations)15 Data table is missing a step/shift or observations.10 Data Table is missing steps/shifts and several observations.5 Data table is missing steps/shifts and many observations.0 No data table present.

ConclusionsPart A 5 Answered correctly

0 Incorrect answer

Part B 35 All equilibrium shifts are described. Explanation of why includes what was added (or taken away) and how that effected the reaction.

30 All equilibrium shifts are described. Explanation is missing for one shift. 25 All equilibrium shifts are described. Explanation is missing for two shifts. 20 All/Some equilibrium shifts are described. Explanation is missing for all/some

shifts. 15 Some equilibrium shifts are described. Explanation is missing for all shifts

0 No conclusion stated.