AP+Chem+Lab+10


 * The Determination of an Equilibrium Constant **

Chemical reactions occur to reach a state of equilibrium. The equilibrium state can be characterized by quantitatively defining its equilibrium constant, //K// // eq //. In this experiment, you will determine the value of //K// // eq // for the reaction between iron (III) ions and thiocyanate ions, SCN–.
 * Introduction:**

Fe3+(aq) + SCN–(aq) → FeSCN2+(aq) The equilibrium constant, //K// // eq //, is defined by the equation shown below.

// K //// eq // = [FeSCN2+] / [Fe3+][SCN−]

To find the value of //K// // eq //, 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 determine the concentration by measuring light that passes through a sample of the equilibrium mixtures. The amount of light absorbed by a colored solution is proportional to its concentration. The red FeSCN2+solution absorbs blue light, thus the Colorimeter users will be instructed to use the 470 nm (blue) LED. 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 FeSCN2+from solutions of varying concentrations of SCN–and constant concentrations of H+and Fe3+that are in stoichiometric excess. The excess of H+ions will ensure that Fe3+engages in no side reactions (to form FeOH2+, for example). The excess of Fe3+ions will make the SCN–ions the limiting reagent, thus all of the SCN–used will form FeSCN2+ions. The FeSCN2+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 FeSCN2+ions will slowly decompose. In Part II of the experiment, you will analyze a solution of unknown [SCN–] 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 Fe3+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.

In this experiment, you will
 * Objectives: **
 * Prepare and test standard solutions of FeSCN2+in equilibrium.
 * 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, //K// // eq //, for the reaction.


 * Materials: **
 * Lab Pro
 * Logger //Pro//
 * Colorimeter
 * plastic cuvette
 * four 10.0 mL pipettes
 * six 20 × 150 mm test tubes
 * test tube rack
 * eight 100 mL beakers
 * Beral pipets
 * 0.200 M iron (III) nitrate, Fe(NO3)3, solution in 1.0 M HNO3
 * 0.0020 M iron (III) nitrate, Fe(NO3)3, solution in 1.0 M HNO3
 * 0.0020 M thiocyanate SCN-
 * potassium thiocyanate, KSCN solution of unknown concentration
 * distilled water
 * 50 mL volumetric flask
 * tissue
 * Temperature Probe

For the solutions that you will prepare in Step 2 of Part I below, calculate the [FeSCN2+]. Presume that all of the SCN–ions react. In Part I of the experiment, mol of SCN–= mol of FeSCN2+. Thus, the calculation of [FeSCN2+] is: mol FeSCN2+÷ L of //total// solution. Record these values in the table below.
 * Pre-Lab Exercise: **


 * Beaker # ||  [FeSCN2+]  ||
 * 1 ||  0.00M  ||
 * 2 ||   ||
 * 3 ||   ||
 * 4 ||   ||
 * 5 ||   ||

__ Part I Prepare and Test Standard Solutions __ 1.Obtain and wear goggles. 2.Label five 100 mL beakers 1-5. Obtain small volumes of 0.200 M Fe(NO3)3, 0.0020 M SCN–, and distilled water. **//CAUTION://** **//Fe(NO//** **// 3 //****// ) //****// 3 //****// solutions in this experiment are prepared in 1.0 M HNO //****// 3 //****// and should be handled with care. //**// **Nitrile gloves are available as needed.** // Prepare four solutions according to the chart below (The fifth beaker is a blank.). Use a 10.0 mL pipet and a pipet pump to transfer each solution to a 50 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, //K// // eq //.
 * Procedure: **

** (ml) ** ||  ** H2O ** ** (ml) ** ||
 * ** Beaker # ** ||  ** 0.200 M Fe(NO3)3 (ml) **  ||  ** 0.0020 M SCN- **
 * 1 ||  5.0  ||  5.0  ||  40.0  ||
 * 2 ||  5.0  ||  4.0  ||  41.0  ||
 * 3 ||  5.0  ||  3.0  ||  42.0  ||
 * 4 ||  5.0  ||  2.0  ||  43.0  ||
 * Blank (5) ||  5.0  ||  0.0  ||  45.0  ||


 * Note: ** The fifth beaker is prepared for use as a “blank” for your colorimeter calibration. It will have a slightly yellow color due to the presence of Fe(NO3)3. By calibrating with this solution as your blank, instead of distilled water, you will account for this slight yellow color.

1. Start the Logger Pro program on your computer. Open the file “10 Equilibrium” from the Advanced Chemistry with Vernier folder. Wait for the program to adjust and detect the colorimeter. 2.Prepare a “//blank”// by filling a cuvette 3/4 full with the solution from beaker #5. To correctly use cuvettes, remember: 3.To calibrate the Colorimeter, place the “blank” cuvette into the cuvette slot of the Colorimeter, adjust the wavelength to 470nm. Push the CAL button. Calibration only takes a few seconds and the red flashing light will stop when calibration is complete. 4.Data collection can now be started. Be sure to check your default data collection time to account for the entire experimental process. 5.Using the solution in Beaker 1, rinse the cuvette twice with ~1 mL amounts and then fill it 3/4 full. Wipe the outside with a tissue, place it in the Colorimeter. 6.Wait for the absorbance value displayed in the Meter window to stabilize. Click KEEP, type in the concentration of FeSCN2+ (from your pre-lab calculations) in the edit box and click OK. 7.Collect absorbance-concentration data for the five standard equilibrium mixtures by repeating the previous steps. 8. Click STOP when you have finished collecting data. Click the Examine button (or you can use it through the pull down menu), and write down the absorbance values for each data pair in your data table. 9. Choose the Linear Fit function. A best-fit linear regression equation will be plotted for your data. This line should pass near or through the data points //and// the origin of the graph. Leave the equation in place on the graph and continue with the testing the unknown solution.
 * 1) Wipe the outside of each cuvette with a lint-free tissue.
 * 2) Handle cuvettes only by the top edge of the ribbed sides.
 * 3) Dislodge any bubbles by gently tapping the cuvette on a hard surface.
 * 4) Always position the cuvette so the light passes through the clear sides.

__ Part II Test an Unknown Solution of SCN ____ – __ 1.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 0.200 M Fe(NO3)3and 40.0 mL of distilled water to the beaker. Stir the mixture thoroughly. 2.Using the solution in the beaker, rinse a cuvette twice with ~1 mL amounts and then fill it 3/4 full. Place the cuvette of unknown in the device. Read the absorbance value displayed in the meter. **//(Important: The reading in the meter is live, so it is not necessary to click COLLECT to read the absorbance value.)//** When the displayed absorbance value stabilizes, record its value in your Data and Calculations table for Parts I and II. 3.Determine the concentration of the unknown SCN–solution.
 * 1) Choose Interpolate from the Analyze menu. A vertical cursor now appears on the graph. The cursor’s concentration and absorbance coordinates are displayed in the floating box.
 * 2) Move the cursor along the regression line until the absorbance value is approximately the same as the absorbance value you recorded in Step 2 (part II). The corresponding concentration value is the concentration of the unknown solution, in mol/L. Record the value in your Data Table for Parts I and II.

__ Part III Prepare and Test Equilibrium Systems __
 * 1) Prepare four test tubes of solutions, according to the chart below. Repeat Steps 1 and 2 from Part II to test the absorbance values of each mixture. Record the test results in your data table. **//Note//**//: You are using 0.0020 M Fe(NO// // 3 //// ) //// 3 //// in this test. //

** (ml) ** ||  ** 0.0020 M SCN- ** ** (ml) ** ||  ** H2O ** ** (ml) ** ||
 * ** Test tube # ** ||  ** 0.0020 M Fe(NO3)3 **
 * 1 ||  3.00  ||  2.00  ||  5.00  ||
 * 2 ||  3.00  ||  3.00  ||  4.00  ||
 * 3 ||  3.00  ||  4.00  ||  3.00  ||
 * 4 ||  3.00  ||  5.00  ||  2.00  ||

__ Part I and II __
 * Data Tables **
 * ** Beaker # ** ||  ** Absorbance **  ||
 * 1 ||   ||
 * 2 ||   ||
 * 3 ||   ||
 * 4 ||   ||
 * Unknown, Part II ||   ||
 * Concentration Unknown ||   ||

__ Part III __


 * ** Test Tube # ** ||  ** Absorbance **  ||  ** Net Absorbance **  ||
 * 1 ||   ||   ||
 * 2 ||   ||   ||
 * 3 ||   ||   ||
 * 4 ||   ||   ||


 * Calculations and Post-Lab Questions **
 * 1) (Part II) Compare your experimental [SCN–], of your unknown, with the actual [SCN–]. Suggest reasons for the disparity.
 * 2) (Part III) Use the net absorbance values, along with the best fit line equation of the standard solutions in Part I to determine the [FeSCN2+] at equilibrium for each of the mixtures that you prepared in Part III. Complete the table below and give an example of your calculations.

3. (Part III) Calculate the equilibrium concentrations for Fe3+and SCN–for the mixtures in Test tubes 2-5 in Part III. Complete the table below and give an example of your calculations.
 * ** Test tube # ** ||  ** 1 **  ||  ** 2 **  ||  ** 3 **  ||  ** 4 **  ||
 * [FeSCN2+] ||   ||   ||   ||   ||
 * ** Test tube # ** ||  ** 1 **  ||  ** 2 **  ||  ** 3 **  ||  ** 4 **  ||
 * [Fe2+] ||   ||   ||   ||   ||
 * [SCN-] ||   ||   ||   ||   ||

4. Calculate the value of //K// // eq // for the reaction. Explain how you used the data to calculate //K// // eq //.