9 Equilibrium. Aubrey High School PreAP -Chemistry. Name Period Date / /

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1 Aubrey High School PreAP -Chemistry 9 Equilibrium Name Period Date / / 9.2 Determination of Keq Lab - Equilibrium Problems Lab Overview In a reversible reaction, equilibrium is the state at which the rates of forward and reverse reactions are equal. At that point, the concentrations of the reactants and products stop changing. A reaction mixture is at equilibrium when the ratio of the concentrations of products to that of reactants reaches the equilibrium constant, K eq: The purpose of this lab is to determine the value of the equilibrium constant, K eq, for the following reaction: Fe 3+ (aq) + SCN (aq) FeSCN 2+ (aq) pale yellow colorless blood-red To do so, we need to find the concentrations of Fe 3+, SCN, and FeSCN 2+ in equilibrium mixtures. [FeSCN 2+ ] eq K eq = [Fe 3+ ] eq[scn ] eq The relationship between the concentration of a and its absorbance is given by Beer s Law: A = a b c in which A is the absorbance (unitless), a is the absorptivity of the substance (which is a constant and is reported in L mol 1 cm 1 ), b is the length of the cuvette (the test tube, which is usually 1 cm), and c is the concentration of the sample (in molarity). Since a and b are constant, the absorbance is directly related to the sample s concentration: A = k c Notice that this is a linear relationship that theoretically goes through the origin (i.e. a clear has an absorbance of 0 because all of the light passes through). We don t know the s absorptivity, so we need to create a calibration curve by measuring the absorbance of s of FeSCN 2+ whose concentrations we know (called standard, or reference s), and plot the Absorbance vs. Concentration graph. The value for the equilibrium constant, K eq, is temperature-dependent: it changes at different temperatures. Le Châtelier s Principle states that when a stress is placed on system at equilibrium, the reaction will shift in the direction that reduces the stress and re-establishes equilibrium. This idea helps to predict whether a reaction is endothermic or exothermic. Because K eq is temperature dependent, when the temperature changes, a reaction shifts because the value of K eq changes: if the value of K eq increases, the reaction will shift to the right; if the value of K eq decreases, the reaction will shift to the left. When the temperature is raised, the reaction will absorb the extra heat and shift in the endothermic direction. When the temperature is lowered, the reaction will release more heat and shift in the exothermic direction. In this lab, we also will determine whether the following reaction is endothermic or exothermic: Fe 3+ (aq) + SCN (aq) FeSCN 2+ (aq) pale yellow colorless blood-red To do so, we will find the value K eq at various temperatures, and determine how the reaction shifts when the temperature is changed.

2 Part 1: Preparing the Solutions We will be creating five reference s (with which we will create our calibration curve), and five test s (with which we will use in our K eq calculations). 1. Obtain the following materials: 30 ml of M iron(iii) nitrate, Fe(NO 3) 3, 25 ml of M iron(iii) nitrate, Fe(NO 3) 3, 15 ml of M potassium thiocyanate, KSCN, 20 ml of M potassium thiocyanate, KSCN, Ten 50-mL beakers Stirring rod Distilled water Pipets 2. Prepare the five reference s and five test s. a. Label the ten beakers 1 through 10. b. Use a separate pipet to transfer the appropriate volumes of each chemical. c. Mix each using a stirring rod. d. Rinse the stirring rod with distilled water and dry it between s. Standards M Fe(NO 3) M KSCN Total Volume Reference #1 8.0 ml 2.0 ml Reference #2 7.0 ml 3.0 ml Reference #3 6.0 ml 4.0 ml Reference #4 5.0 ml 5.0 ml Reference #5 4.0 ml 6.0 ml Samples M Fe(NO 3) M KSCN Distilled water Total Volume #6 5.0 ml 1.0 ml 4.0 ml #7 5.0 ml 2.0 ml 3.0 ml #8 5.0 ml 3.0 ml 2.0 ml #9 5.0 ml 4.0 ml 1.0 ml # ml 5.0 ml 0.0 ml 3. Measure the temperature of one of the s: Part 2: Reference Solutions We want to measure the absorbance of each. We will use the colorimeter with Vernier s Lab Quest Obtain the following materials: Vernier s LabQuest 2 Colorimeter Tissues or lens paper Eleven cuvettes 2. Set the mode for the LabQuest 2 to Events with Entry. Connect the colorimeter to the LabQuest 2 and set it to measure absorbance at 450 nm. Use a clean cuvette filled with distilled water to calibrate the colorimeter by pressing the Cal button.

3 Aubrey High School PreAP -Chemistry 9 Equilibrium Name Period Date / / 9.2 Determination of Keq Lab - Equilibrium Problems 3. Using the LabQuest 2 and colorimeter, measure the absorbance for the Reference Solutions only. a. As you begin, tap the Collect button ) on the LabQuest 2. b. Clean cuvettes with a tissue. Handle cuvette at the top so no fingerprints are in the light path. Place cuvette in the colorimeter so the transparent sides are in the path of the light. c. When you ve inserted the cuvette with a new, tap the Keep button and enter the concentration. d. After collecting the data for all the s, tap the Stop button. e. Record the absorbance for each. f. On the LabQuest 2, go to the Analyze menu, select Curve Fit. Use Proportional and record the equation of the line. Part 3: Solutions 4. Measure and record the absorbance for each of the Solutions. (You do not need to keep these measurements.) Part 4: Solutions at Different Temperatures 5. In addition to the colorimeter, connect a temperature probe to the Lab Quest device. 6. Prepare an ice water and a warm water bath. 7. Place the labeled and covered cuvettes with the Solutions in the ice water bath or hot water bath for at least five minutes. Record the temperature of the water bath. 8. After five minutes, quickly dry off the cuvette, and find the absorbance of the.

4 Data Table and Calculations Because we are mixing s, the concentrations of the species of each will be diluted. To determine the new concentration, we can use the dilution formula: M concentrated V concentrated M dilute = V dilute In the Data Tables, find the new concentrations, showing all your work, and the record the measured absorbance. Data Table 1 Reference Solutions For the Reference Solutions, there is an excess of Fe 3+, which means that virtually all of the SCN was used up to form FeSCN 2+. Use the dilution formula to find the [SCN ] used, which is also the [FeSCN 2+ ] formed. [FeSCN 2+ ] Absorbance Reference #1 Reference #2 Reference #3 Reference #4 Reference #5 Write the equation in slope-intercept form that relates the Absorbance and the Concentration. Data Table 2 Solutions Calculate the new concentrations of Fe 3+ and SCN after all the s are mixed to create each sample, using the dilution formula. Record the absorbance measured for each sample. Use the equation from the Calibration Curve to estimate the [FeSCN 2+ ] eq. [Fe 3+ ] initial [SCN ] initial #6 #7 #8 #9 #10

5 Aubrey High School PreAP -Chemistry 9 Equilibrium Name Period Date / / 9.2 Determination of Keq Lab - Equilibrium Problems Part 3: K eq at Room Temperature Transfer your data from Data Table 2 into the ICE Box, and fill in all values. Use the equilibrium concentrations from the ICE Box to calculate the K eq value. Calculate the average of the K eq values. Calculate the deviation of the K eq values for each test. Deviation = average K eq calculated K eq. Calculate the average of the deviations. Temperature: C Solution Abs [FeSCN 2+ ] eq Equilibrium Concentrations K eq Deviation #6 #7 #8 #9 #10 Average Keq: Average Deviation:

6 Temperature: C Solution Abs [FeSCN 2+ ] eq Equilibrium Concentrations K eq Deviation #6 #7 #8 #9 #10 Average Keq: Average Deviation:

7 Aubrey High School PreAP -Chemistry 9 Equilibrium Temperature: C Name Period Date / / 9.2 Determination of Keq Lab - Equilibrium Problems Solution Abs [FeSCN 2+ ] eq Equilibrium Concentrations K eq Deviation #6 #7 #8 #9 #10 Average Keq: Average Deviation:

8 Discussion 1. State the values of K eq at the different temperatures studied, from lowest to highest. Fe 3+ (aq) + SCN (aq) FeSCN 2+ (aq) [FeSCN 2+ ] eq K eq = [Fe 3+ ] eq[scn ] eq at C at C at C 2. Determine whether the reaction is exothermic or endothermic. Explain. 3. Consider the results of the trials at room temperature. Define equilibrium constant. Was the value constant for all your trials at room temperature? Should it be constant? (Use complete sentences.) 4. What does the calculated value of the equilibrium constant, K eq, indicate about the extent or degree of completeness of the reaction? (i.e. at equilibrium, are there mostly reactants, products, or relatively large amounts of both?) (Use complete sentences.) 5. The average deviation describes the precision of the results. Does the precision indicate that the equilibrium constant is indeed a constant for this reaction? Explain. (Use complete sentences.) 6. Describe the possible sources of error in this experiment and their likely effect on the results. (Use complete sentences.)