The Determination of an Equilibrium Constant Computer 10 Chemical reactions occur to reach a state of equilibrium. The equilibrium state can be characterized by quantitatively defining its equilibrium constant, Keq. In this experiment, you will determine the value of Keq for the reaction between iron (III) ions and thiocyanate ions, SCN. Fe 3+ (aq) + SCN (aq) FeSCN 2+ (aq) The equilibrium constant, Keq, is defined by the equation shown below. K eq 2 [FeSCN ] 3 [Fe ][SCN ] To find the value of Keq, which depends only upon temperature, it is necessary to determine the molar concentration of each of the three species in solution at equilibrium. You will use a colorimeter to help you measure the concentrations (see Figure 1). The amount of light absorbed by a colored solution is proportional to its concentration. The red FeSCN 2+ solution absorbs blue light, and it will be analyzed at 470 nm (blue light). Figure 1 In order to successfully evaluate this equilibrium system, it is necessary to conduct two separate tests. In Part I, you will calibrate the colorimeter by measuring the absorbance for three solutions of FeSCN 2+ by varying concentrations of SCN : (1) 0.2M Fe(NO3)3 solution, (2) using the standard solution given to you (45 ml 0.2M M Fe(NO3)3 and 5 ml of 0.002 M KSCN) and (3) a midpoint prepared by diluting the standard solution with distilled water. In these calibration solutions, H + and Fe 3+ are substantially constant and are in stoichiometric excess. The excess of H + ions will ensure that Fe 3+ engages in no side reactions (to form FeOH 2+, for example). The excess of Fe 3+ ions will make the SCN ions the limiting reagent, thus all of the SCN used will form FeSCN 2+ ions. The FeSCN 2+ complex forms slowly, taking at least one minute for the color to develop. It is best to take absorbance readings after a specific amount of time has elapsed, between two and four minutes after preparing the equilibrium mixture. Do not wait much longer than four minutes to take readings, however, because the mixture is light sensitive and the FeSCN 2+ ions will slowly decompose.. Advanced Chemistry with Vernier 10-1
Computer 10 In Part II, you will prepare a new series of solutions that have varied concentrations of the Fe 3+ ions and the SCN ions, with a constant concentration of H + ions. You will use the results of this test to accurately evaluate the equilibrium concentrations of each species. OBJECTIVES In this experiment, you will Calibrate a colorimeter with solutions of FeSCN 2+ in equilibrium. Determine the molar concentrations of the ions present in an equilibrium system. Determine the value of the equilibrium constant, Keq, for the reaction. MATERIALS Vernier computer interface 0.200 M iron (III) nitrate, Fe(NO3)3, solution computer in 1.0 M HNO3 Vernier Colorimeter 0.0020 M iron (III) nitrate, Fe(NO3)3, solution Temperature Probe (optional) in 1.0 M HNO3 plastic cuvette 0.0020 M thiocyanate, SCN, solution in four 10.0 ml pipettes 0.10 M HNO3 pipet pump or bulb six 20 150 mm test tubes 50 ml volumetric flask test tube rack eight 100 ml beakers distilled water plastic Beral pipets tissue PRE-LAB EXERCISE For the solutions that you will prepare in Step 2 of Part I below, calculate the [FeSCN 2+ ]. Presume that all of the SCN ions react. In Part I of the experiment, mol of SCN = mol of FeSCN 2+. Thus, the calculation of [FeSCN 2+ ] is: mol FeSCN 2+ L of total solution. Record these values in the table below. Cuvette number [FeSCN 2+ ] 1 0.00 M 2 3 PROCEDURE Part I Prepare and Test Standard Solutions 1. Obtain and wear goggles. 10-2 Advanced Chemistry with Vernier
The Determination of an Equilibrium Constant Cuvette Solutions Used number 1 ¾ full 0.2M Fe(NO3)3 2 ¾ full standard solution 3 ¾ full of diluted standard solution (exactly equal volumes of standard solution and distilled water) Standard solution made by combining 90 ml of 0.2M Fe(NO3)3 with 10 ml of 0.002M KSCN 3. Connect a Colorimeter to Channel 1 of the Vernier computer interface. Connect the interface to the computer with the proper cable. 4. Start the Logger Pro program on your computer. Open the file 10 Equilibrium from the Advanced Chemistry with Vernier folder. 5. Calibrate the Colorimeter. a. Prepare a blank by filling an empty cuvette ¾ full with distilled water. Place the blank in the cuvette slot of the Colorimeter and close the lid. b. Choose Calibrate CH1: Colorimeter from the Experiment menu, then click. c. Turn the wavelength knob on the Colorimeter to the 0% T position. d. Type 0 in the edit box. e. When the displayed voltage reading for Reading 1 stabilizes, click. f. Turn the knob of the Colorimeter to the Blue LED position (470 nm). g. Type 100 in the edit box. h. When the voltage reading for Reading 2 stabilizes, click, then click. 6. You are now ready to collect absorbance data for the calibration solutions. Click to begin data collection. Note: Take readings within 4 minutes of preparing the mixtures. b. Obtain 0.2M Fe(NO3)3 from the instructor. Use this to obtain an absorbance reading from cuvette # 1 CAUTION: Fe(NO 3 ) 3 solutions in this experiment are prepared in 1.0 M HNO 3 and should be handled with care. Mix each solution thoroughly. Measure and record the temperature of one of the above solutions to use as the temperature for the equilibrium constant, Keq. a. Using each solution in the table above, fill the cuvette ¾ full. Wipe the outside with a tissue, place it in the Colorimeter, and close the lid. Wait for the absorbance value displayed in the Meter window to stabilize. Click, type the concentration of FeSCN 2+ (from your pre-lab calculations) in the edit box, and press the ENTER key. b. Discard the cuvette contents into a beaker in the fume hood. Rinse with distilled water and fill the cuvette with the second solution in the table above. Follow the procedure in Part a of this step to measure the absorbance, and enter the concentration of this solution. c. Repeat Part b of this step to measure the absorbance of the third solution in the table above. d. Click when you have finished collecting data. Click the Examine button,, and record the absorbance values for each data pair. Advanced Chemistry with Vernier 10-3
Computer 10 7. Click the Linear Fit button,. A best-fit linear regression line will be shown for your five data points. This line should pass near or through the data points and the origin of the graph. (Note: Another option is to choose Curve Fit from the Analyze menu, and then select Proportional. The Proportional fit has a y-intercept value equal to 0; therefore, this regression line will always pass through the origin of the graph). Leave the graph and best fit line displayed and proceed to Step 8. Part II Prepare and Test Equilibrium Systems 8. Prepare four test tubes of solutions, according to the chart below. Follow the necessary steps from Part I to test the absorbance values of each mixture. Record the test results in your data table. Note: You are using 0.0020 M Fe(NO3)3 in this test. Test tube number 0.0020 M Fe(NO3)3 (ml) 0.0020 M SCN (ml) H2O (ml) 1 3.00 0.00 7.00 2 3.00 2.00 5.00 3 3.00 3.00 4.00 4 3.00 5.00 2.00 12. To get good data for the calculation of Keq, you must determine the net absorbance of the solutions in Test Tubes 1-4. To do this, subtract the absorbance reading for Test Tube 1 from the absorbance readings of Test Tubes 2-4, and record these values as net absorbance in your data table. 10-4 Advanced Chemistry with Vernier
The Determination of an Equilibrium Constant DATA TABLE Part I Cuvette Absorbance 1 2 3 Best-fit line equation for the Part I calibration solutions: Part II Test tube number Absorbance Net absorbance 1 2 3 4 Advanced Chemistry with Vernier 10-5
Computer 10 DATA ANALYSIS 1. (Part III) Use the net absorbance values, along with the best fit line equation of the calibration solutions in Part I to determine the [FeSCN 2+ ] at equilibrium for each of the mixtures that you prepared in Part III. Complete the table below and give an example of your calculations. Test tube number 2 3 4 [FeSCN 2+ ] 2. (Part III) Calculate the equilibrium concentrations for Fe 3+ and SCN for the mixtures in Test tubes 2-4 in Part III. Complete the table below and give an example of your calculations. Test tube number 2 3 4 [Fe 3+ ] [SCN ] 5. Calculate the value of Kc for the reaction. Explain how you used the data to calculate Kc. 10-6 Advanced Chemistry with Vernier