2 (aq) [FeSCN [Fe 3JSCN] Figure 1

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1 The Determination of an Equilibrium Constant Computer 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 (111) ions and thiocyanate ions, SCW. (aq) + SCN (aq) FeSCN Fe (aq) The equilibrium constant, Keq, is defined by the equation shown below. K j [FeSCN [Fe JSCN] 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 light, and it will be analyzed at 470 nm (blue light). solution absorbs blue fl Lght Detector [J 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 concentrations of SCN and constant concentrations of Fr and Fe excess. The excess of Ht ions will ensure that Fe for example). The excess of Fe the SCN used will form FeSCN 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 from solutions of varying that are in stoichiometric, engages in no side reactions (to form FeOH ions will make the SCN ions the limiting reagent, thus all of ions. The FeSCN ions will slowly decompose. complex forms slowly, taking at least one 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. ions Third, you will prepare a new series of solutions that have varied concentrations of the Fe 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. Advanced Chemistry with Vernier 10-1.

2 10- PRE-LAB EXERCISE 0 ml volumetric flask six 0 x 10 mm test tubes Vernier computer interface computer Temperature Probe (optional) plastic cuvette Vernier Colorimeter pipet pump or bulb four 10.0 ml pipettes M Fe(N0 Advanced chemistry with Vernier test tube rack to the chart below Use?10 0 ml pipet and a pipet pump or bulb to transfer each solution to 0000 M SCN, and distilled water CAUTION Fe(N0 prepared in 1.0 M HNOjand should be handled with care. Prepare four solutions according 1. Obtain and wear goggles. KSCN solution of unknown concentration 0.00 M Fe(N0 eight 100 ml beakers For the solutions that you will prepare in Step of Part I below, calculate the [FeSCN M SCN solution in 0.10 M HNO plastic Beral pipets Presume that all of the SCN ions react In Part I of the experiment, mol of SCN = mol of Part I Prepare and Test Standard Solutions : 1 O.OOM Beaker number [FeSCN FeSCN. Thus, the calculation of [FeSCN ino.1omhno ) solution in 1 0 M HNO ) solution in 1.0 M HNO these values in the table below. ]. MATERiALS Determine the molar concentrations of the ions present in an equilibrium system. Prepare and test standard solutions of FeSCN Test solutions of SCN of unknown molar concentration. Determine the value of the equilibrium constant, Keq, for the reaction. in equilibrium. J is: mol FeSCN L of total solution. Record ) solutions in this experiment are Label five 100 ml beakers 1-. Obtain small volumes of 0.00 M Fe(N0), In this experiment, you will OBJECTIVES PROCEDURE Computer 10

3 f%jpj k c The Determination ofan Equilibrium Constant 4 rc 0 ml volumetric flask. 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. Beaker 0.00 M Fe(N M SCN number (ml) (ml) (ml) ) H st: (vn) L O. o o. 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 Chemistiy with Vernier folder.. Calibrate the Colorimeter. a. Prepare a blank by filling an empty cuvette 4 full with distilled water. Place the blank in the cuvette slot of the Colorimeter and close the lid, b. If your Colorimeter has a CAL button, set the wavelength on the Colorimeter to 470 nm, press the CAL button, and proceed directly to Step 6. If your Colorimeter does not have a CAL button, continue with this step to calibrate your Colorimeter. Choose Calibrate CR1: Colorimeter (%T) from the Experiment menu, then click C. Calibrate Now Turn the wavelength knob on the Colorirneter to the 0% T position. Type 0 in the edit box, f. When the displayed voltage reading for Input 1 stabilizes, click g. Turn the knob of the Colorimeter to the Blue LED position (470 nm). h. Type 100 in the edit box. When the voltage reading for Input 1 stabilizes, click then click 6. You are now ready to collect absorbance data for the standard solutions. Click to begin data collection, Note: Take readings within 4 minutes of preparing the mixtures. a. Empty the water from the cuvette, Using the solution in Beaker 1, rinse the cuvette twice with -4 ml amounts and then fill it /4 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 (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, c. Repeat Part b of this step to measure the absorbance of the solutions in Beakers, 4, and. d. Click ss when you have finished collecting data to view a graph of absorbance vs. concentration. Click the examine button, and record the absorbance values for each data pair. fkee Advanced Chemistry with Vernier 10-

4 Computer JO 7. Clic1 the Linear Fit button, Z. 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.0 ml of the unknown into a clean and dry 100 ml beaker. Add precisely.0 ml of 0.00 M Fe(N0 and 40.0 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 /4 full. Wipe the outside with a tissue, place it in the Colorimeter, and close the lid. Watch the absorbance readings in the Meter window. Whç hereadins stabilize, record the aceaii1or your unknown in your data table. Remove and 10. Determine the concentration of the unknown SCN solution. 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 molll. Record this value in your data table. Part ill Prepare and Test Equilibrium Systems 11. Prepare five 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 M Fe(N0 ) in this test..,.. c I ) M SCW H0 Test tube M Fe(N0 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 -. To do this, subtract the absorbance reading for Test Tube 1 from the absorbance readings of Test Tubes -, and record these values as net absorbance in your data table Advanced Chemistry with Vernier

5 Beaker Absorbance. Parts I and II The Determination ofan Equilibrium Constant Advanced Chemistry with Vernier , Test tube number Absorbance Net absorbance Part Ill Best-fit line equation for the Part I standard solutions: Unknown, Part II 4 DATA TABLE

6 1. (Part II) Use the calibration equation from Item 1 and the absorbance reading for your unknown solution to determine [SCNI. DATA ANALYSiS 10-6 Advanced Chemistry with Vernier. Calculate the value of K for the reaction. Explain how you used the data to calculate Keq. [SCN9 [Fe Test tube number 4 ] tubes - in Part 111. Complete the table below and give an example of your calculations. 4. (Part Ill) Calculate the equilibrium concentrations for Fe and SCN for the mixtures in Test Test tube number 4 you prepared in Part Ill. Complete the table below and give an example of your calculations.. (Part Ill) Use the net absorbance values, along with the best fit line equation of the standard [FeSCN ] 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