1iI1E. The Determination of 0 an Equilibrium Constant [LU. Computer

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1 Computer The Determination of 0 an Equilibrium Constant 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 (aq) + SCW (aq) FeSCN (aq) The equilibrium constant, Keq, is defined by the equation shown below. K eq [FeSCN j [Fe J{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 solution absorbs blue light, and it will be analyzed at 70 nm (blue light). Light 1iI1E Detector [LU Figure 1 In order to successfully evaluate this equilibrium system, it is necessary to conduct three separate tests. First, you will prepare a series of standard solutions of FeSCN from solutions of varying concentrations of SCN and constant concentrations of H and Fe that are in stoichiometric excess. The excess of H ions will ensure that Fe engages in no side reactions (to form FeOH, for example). The excess of Fe ions will make the SCN ions the limiting reagent, thus all of the SCN used will form FeSCN ions. The FeSCN 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 ions will slowly decompose. In Part II of the experiment, you will analyze a solution of unknown [SCN1 by using the same procedure that you followed in Part I. In this manner, you will determine the molar concentration of the SCN solution. Third, you will prepare a new series of solutions that have varied concentrations of the Fe ions and the SCN ions, with a constant concentration of W ions. You will use the results of this test to accurately evaluate the equilibrium concentrations of each species. Advanced Chemistry with Vernier 10-1

2 Computer 10 OBJECTIVES In this experiment, you will in equilibrium. Prepare and test standard solutions of FeSCN Test solutions of SCN of unknown molar concentration. 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 computer Vernier Colorimeter Temperature Probe (optional) plastic cuvette four 10.0 ml pipettes pipet pump or bulb six 0 x 150 mm test tubes 50 ml volumetric flask ) solution in 1.0 M HNO 0.00 M Fe(N M Fe(N M SCN solution in 0.10 M HNO KSCN solution of unknown concentration in 0.10 M HNO eight 100 ml beakers test tube rack plastic Beral pipets ) solution in 1.0 M HNO PRE-LAB EXERCISE J. For the solutions that you will prepare in Step of Part I below, calculate the [FeSCN Presume that all of the SCN ions react. In Part I of the experiment, mol of SCN = mol of. Thus, the calculation of [FeSCN FeSCN these values in the table below. j is: mol FeSCN L of total solution. Record Beaker number [FeSCN 1 O.OOM 5 PROCEDURE Part I Prepare and Test Standard Solutions 1. Obtain and wear goggles.. Label five 100 ml beakers 1-5. Obtain small volumes of 0.00 M Fe(N0), M SCN, and distilled water. CAUTION: Fe(N0 prepared in 1.0 MHNO to the chart below Use a 10 0 ml pipet and a pipet pump or bulb to transfer each solution to ) solutions in this experiment are and should be handled with care. Prepare four solutions according 10- Advanced Chemistry with Vernier

3 Beaker 0.00 M Fe(N0 ) M SCN H0 of one of the above solutions to use as the temperature for the equilibrium constant, Keq. a 50 ml volumetric flask. Mix each solution thoroughly. Measure and record the temperature Connect a Colorimeter to Channel 1 of the Vernier computer interface. Connect the interface. Start the Logger Pro program on your computer. Open the file 10 Equilibrium from the 5. Calibrate the Colorimeter. a. Prepare a blank by filling an empty cuvette / full with distilled water. Place the blank in CAL button, continue with this step to calibrate your Colorimeter. c. Choose Calibrate CH1: Colorimeter (%T) from the Experiment menu, then click Calibrate Now e. Type 0 in the edit box. f. When the displayed voltage reading for Input 1 stabilizes, click ZLZi. h. Type 100 in the edit box. i. When the voltage reading for Input 1 stabilizes, click then click Z_Z. 6. You are now ready to collect absorbance data for the standard solutions. Click to a. Empty the water from the cuvette. Using the solution in Beaker 1, rinse the cuvette twice c. Repeat Part b of this step to measure the absorbance of the solutions in Beakers,, d Click when you have finished collecting data to view a graph of absorbance vs Advanced Chemistry with Vernier 10 - press the CAL button, and proceed directly to Step 6. If your Colorimeter does not have a begin data collection. Note: Take readings within minutes of preparing the mixtures. with - ml amounts and then fill it / full. Wipe the outside with a tissue, place it in the Colorimeter, and close the lid. Wait for the absorbance value displaypd in the Meter window to stabilize. Click type the concentration of FeSCN (from your pre-lab calculations) in the edit box, and press the ENTER key. b. Discard the cuvette contents as directed. Rinse and fill the cuvette with the solution in Beaker. Follow the procedure in Part a of this step to measure the absorbance, and enter the concentration of this solution. data pair. concentration, Click the examine button, ji, and record the absorbance values for each and5. b. If your Colorimeter has a CAL button, set the wavelength on the Colorimeter to 70 nm, d. Turn the wavelength knob on the Colorimeter to the 0% T position. g. Turn the knob of the Colorimeter to the Blue LED position (70 nm). the cuvette slot of the Colorimeter and close the lid. Advanced Chemistry with Vernier folder. to the computer with the proper cable. number (ml) (ml) (ml) The Determination of an Equilibrium Constant

4 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). Part II Leave the graph and best fit line displayed and proceed to Step 8. Test an Unknown Solution of SCW 8. Obtain about 10 ml of the unknown SCN solution. Use a pipet to measure out 5.0 ml of the unknown into a clean and dry 100 ml beaker, Add precisely 5.0 ml of 0.00 M Fe(N0 ) and 0.0 ml of distilled water to the beaker. Stir the mixture thoroughly. 9. Using the solution in the beaker, rinse a cuvette twice with 1 ml amounts and then fill it ¾ full. Wipe the outside with a tissue, place it in the Colorimeter, and close the lid. Watch the absorbance readings in the Meter window. When the readings stabilize, record the absorbance value for your unknown in your data table. Remove and clean the cuvette. 10. Determine the concentration of the unknown SCN solution. Part Hi a. With the linear-regression curve still displayed on your graph, choose Interpolate from the Analyze menu. b. A vertical cursor now appears on the graph. The cursor s concentration and absorbance coordinates are displayed in the floating box. c. Move the cursor along the regression line until the absorbance value is approximately the same as the absorbance value you recorded in Step 9. The corresponding concentration value is the concentration of the unknown solution, in mol/l. Record this value in your data table. Prepare and Test Equilibrium Systems 11. Prepare five test tubes of solutions, according to the chart below. Follow the necessary steps from Part Ito test the absorbance values of each mixture. Record the test results in your data table. Note: You are using M FeNO ) in this test. Test tube M Fe(N0 ) M SCW H0 number (ml) (ml) (ml) To get good data for the calculation of Keq, you must determine the net absorbance of the solutions in Test Tubes -5. To do this, subtract the absorbance reading for Test Tube 1 from the absorbance readings of Test Tubes -5, and record these values as net absorbance in your data table. 10- Advanced Chemistry with Vernier

5 The Determination of an Equilibrium Constant DATA TABLE Parts I and II Beaker Absorbance : Unknown, Part II Best-fit line equation for the Part I standard solutions: Part III Test tube number Absorbance Net absorbance 5 Advanced Chemist,y with Vernier 10-5

6 I 1. (Part II) Use the calibration equation from item 1 and the absorbance reading for your unknown solution to determine [SCN]. DATA ANALYSIS 10-6 Advanced Chemistry with Vernier 5. Calculate the value of Keq for the reaction. Explain how you used the data to calculate Keq. [SCN9 [Fe Test tube number tubes -5 in Part III. Complete the table below and give an example of your calculations.. (Part III) Calculate the equilibrium concentrations for Fe and SCN for the mixtures in Test ] [FeSCN st tube number you prepared in Part III. Complete the table below and give an example of your calculations.. (Part III) Use the net absorbance values, along with the best fit line equation of the standard solutions in Part Ito determine the [FeSCN J at equilibrium for each of the mixtures that. (Part II) Compare your experimental [SCN], of your unknown, with the actual {SCN]. Suggest reasons for the disparity. Computer 10 r I :