South Pasadena Honors Chemistry Name 8 Equilibrium Period Date D E T E R M I N A T I O N O F K e q L A B 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: K eq = [products]eq [reactants] 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 solution 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 solution has an absorbance of 0 because all of the light passes through). We don t know the solution s absorptivity, so we need to create a calibration curve by measuring the absorbance of solutions of FeSCN 2+ whose concentrations we know (called standard, or reference solutions), and plot the Absorbance vs. Concentration graph. Part 1: Preparing the Solutions We will be creating five reference solutions (with which we will create our calibration curve), and five test solutions (with which we will use in our K eq calculations). 1. Obtain the following materials: 30 ml of 0.200 M iron(iii) nitrate, Fe(NO 3) 3, solution 25 ml of 0.0020 M iron(iii) nitrate, Fe(NO 3) 3, solution 15 ml of 0.0020 M potassium thiocyanate, KSCN, solution 20 ml of 0.00020 M potassium thiocyanate, KSCN, solution Ten 50-mL beakers Stirring rod Distilled water Pipets Thermometer
2. Prepare the five reference solutions and five test solutions. a. Label the ten beakers 1 through 10. b. Use a separate pipet to transfer the appropriate volumes of each chemical. c. Mix each solution using a stirring rod. d. Rinse the stirring rod with distilled water and dry it between solutions. Standards 0.200 M Fe(NO 3) 3 0.00020 M KSCN Total Volume Reference solution #1 8.0 ml 2.0 ml Reference solution #2 7.0 ml 3.0 ml Reference solution #3 6.0 ml 4.0 ml Reference solution #4 5.0 ml 5.0 ml Reference solution #5 4.0 ml 6.0 ml Samples 0.002 M Fe(NO 3) 3 0.0020 M KSCN Distilled water Total Volume solution #6 5.0 ml 1.0 ml 4.0 ml solution #7 5.0 ml 2.0 ml 3.0 ml solution #8 5.0 ml 3.0 ml 2.0 ml solution #9 5.0 ml 4.0 ml 1.0 ml solution #10 5.0 ml 5.0 ml 0.0 ml 3. Measure the temperature of one of the solutions: Part 2: Measuring Absorbance We want to measure the absorbance of each solution. We will use the colorimeter with Vernier s Lab Quest 2. 1. Obtain the following materials: Vernier s LabQuest 2 Colorimeter Tissues or lens paper Eleven cuvettes 2. Using the LabQuest 2 and colorimeter, measure the absorbance of each solution. a. Fill one cuvette with distilled water; this is your blank or zero control. Fill each cuvette about 2/3 full with each solution. b. Connect the colorimeter to the LabQuest 2. c. Set the colorimeter to measure absorbance at 450 nm. d. 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. e. Record the absorbance for each solution. Data Table and Calculations Because we are mixing solutions, the concentrations of the species of each solution will be diluted. To determine the new concentration, we can use the dilution formula: M dilute = Mconcentrated Vconcentrated 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 solution #1 Reference solution #2 Reference solution #3 Reference solution #4 Reference solution #5 Use this data to create a Calibration Curve for Absorbance of FeSCN 2+ on a sheet of graph paper. The y-axis should be labeled Absorbance at 450 nm and the x-axis should be labeled Concentration of FeSCN 2+ ( 10 5 M). Plot the data points, and draw the best fit straight line. One of the points be the origin. Calibration Curve Calculations List the two points on the graph used to calculate the slope: Calculate the slope of the best-fit line. 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 solutions 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 Absorbance [FeSCN 2+ ] eq solution #6 solution #7 solution #8 solution #9 solution #10
K eq Calculations 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 solution. Deviation = average K eq calculated K eq. Calculate the average of the deviations. Solution Equilibrium Concentrations K eq Deviation solution #6 solution #7 solution #8 solution #9 solution #10 Average Keq: Average Deviation:
Discussion 1. Formally state the result of the laboratory by writing the chemical equation of the reaction, the equilibrium constant expression, the value of the equilibrium constant including average deviation, and the temperature. 2. Explain what is meant by an equilibrium constant. Was the value constant for all your experiments? Should it be constant? (Use complete sentences.) 3. 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.) 4. 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.) 5. Describe the possible sources of error in this experiment and their likely effect on the results. (Use complete sentences.)