DETERMINATION of the EQUILIBRIUM CONSTANT for a CHEMICAL REACTION. (aq) + 2 HSCN(aq) Fe(SCN) 2 (aq) + 2 H 1+ (aq)

Transcription

1 DETERMINATION of the EQUILIBRIUM CONSTANT for a CHEMICAL REACTION I. THEORY In this experiment you will study the equilibrium properties of the reaction between iron(iii) ion, Fe 3+, and thiocyanic acid, HSCN. When containing, Fe 3+ and HSCN are mixed, two reactions can occur: Reaction 1. Fe 3+ (aq) + HSCN(aq) FeSCN 2+ (aq) + H 1+ (aq) or Reaction 2. Fe (aq) + 2 HSCN(aq) Fe(SCN) 2 (aq) + 2 H 1+ (aq) The expression for the equilibrium constant, K c, for reaction 1 is: [H 1+ ] [FeSCN 2+ ] Kc = [Fe 3+ ] [HSCN] and the expression for reaction 2: For this experiment, we will assume that reaction one is the more important of the two. We will determine a numerical value for the equilibrium constant, K c, for the reaction at two temperatures. We will also determine the enthalpy change ( H ) associated with the reaction from the values of the equilbrium constant at those two temperatures. In order to calculate a numerical value for K c you will need to determine the equilibrium concentrations of all the species in reaction equation 1. The FeSCN 2+ ion produced in reaction 1 is red in color and absorbs light at a wavelength of 447 nanometers (nm). The absorbance, A, of the ion is directly proportional to the concentration, M, of the ion in the : A M(FeSCN ion) A = k M(FeSCN ion) Ultraviolet/visible spectrophotometric techniques take advantage of this relationship to determine the concentration of the ion in equilibrium samples. The concentration of all species present at equilibrium can be calculated from the equilibrium concentration of the FeSCN 2+ ion and the initial concentrations of Fe 3+ and HSCN. Those concentrations can then be used to calculate a numerical value for K c for the reaction. You will use a spectrophotometer to determine the concentration of the FeSCN ion. You will need to make a calibration curve for the spectrophotometer in order to determine the FeSCN ion concentration in each. You will do this by first making a series of s of known concentration of the complex. Then you will study a series of samples which have gone to equilibrium.

2 In all the s to be prepared, the [H 1+ ] will be maintained at a constant value of 0.5 M and will be assumed to remain constant in all s. Therefore, in all calculations requiring [H 1+ ] you will use the value 0.5 M. II: Procedure A. Preliminary 1. Clean and dry with acetone 10 small test tubes, 1 large test tube, and a 50 ml, 100 ml, and 150 ml beaker. 2. From the reagent bottles on the front lab bench get 80 ml 0.50 M HNO 3 in the 150 ml beaker, 60 ml M Fe(NO 3) 3 in the 100 ml beaker, and 40 ml M HSCN in the 50 ml beaker. Place these beakers on a paper towel at your lab bench on which you have written the formula for each. 3. You have been given 6 pipets: o Rinse the ungraduated 5 ml pipet with the Fe(NO 3) 3, discard the in the pipet and place it by the beaker containing the Fe(NO 3) 3. o o Rinse the 5 & 10 ml graduated and the 25 ml ungraduated pipets. Rinse the other 5 ml graduated pipet with the HSCN, discard the in the pipet and place it by the beaker containing the HSCN. B. Preparation of Standard Solutions 1. Into the large test tube pipet the following s: 10 ml M Fe(NO3)3. Get this from the large bottle on the lab bench in the front of the room. 2.0 ml M HSCN. Get this from the beaker containing the HSCN at your lab bench ml 0.50 M HNO3. Get this from the beaker containing the HNO3 at your lab bench. Carefully shake the large test tube to mix the. 2. Rinse the remaining 10 ml graduated pipet with the you just prepared in the large test tube. Discard the in the pipet and use this pipet only for your standard. 3. Pipet a 10 ml portion of the from the large test tube into one of the small test tubes. This is a standard of FeSCN ion to be used in the preparation of the calibration curve. The concentration of the FeSCN ion in this test tube is l.0 x l0-4 M. Label this test tube A. 4. Pipet 8.0 ml of the in the large test tube into a second small test tube. Pipet 2.0 ml of 0.50 M HNO3 into this test tube. Mix this well. This is a second standard of FeSCN ion to be used in the preparation of the calibration curve. The concentration of the FeSCN ion in this test tube is 0.80 x 10-4 M. Label this test tube B. 5. Pipet 6.0 ml of the in the large test tube into a third small test tube. Pipet 4.0 ml of the 0.50 M HNO3 into this test tube. Mix the well. This is a third standard to be used in the preparation of the calibration curve. The concentration of the FeSCN ion in this test tube is 0.60 x 10-4 M. Label this test tube C. 6. Pipet 4.0 ml of the in the large test tube into a fourth small test tube. Pipet 6.0 ml of the 0.50 M HNO 3 into this test tube. Mix the well. This is a fourth standard to be used in the preparation of the calibration curve. The concentration of the FeSCN ion in this test tube is 0.40 x 10-4 M. Label this test tube D.

3 7. Pipet 2.0 ml of the in the large test tube into a fifth small test tube. Pipet 8.0 ml of the 0.50 M HNO 3 into this test tube. Mix the well. This is a fifth standard to be used in the preparation of the calibration curve. The concentration of the FeSCN ion in this test tube is 0.20 x 10-4 M. Label this test tube E. C. Preparation of reaction s 1. Label the remaining small test tubes Into each of the test tubes pipet 5.0 ml of M Fe(NO 3) Into each of the test tubes labeled 1-5 pipet the corresponding number of ml (1-5) of M HSCN. (Pipet 1.0 ml HSCN into test tube 1, 2.0 ml HSCN into test tube 2, etc.) 4. Into each test tube pipet enough 0.5 M HNO 3 to make the total volume equal to 10.0 ml in each test tube. ( 4.0 ml HNO 3 into test tube 1, 3.0 ml HNO 3 into test tube 2, etc.) 5. Mix each well using your stirring rod. Be sure to rinse the rod with distilled water and wipe dry before stirring each. You may wipe with a paper towel. 6. Clean and rinse with distilled water the 10 small cuvettes given to you. 7. These 10 cuvettes are to be rinsed and filled with the s in the small test tubes labeled A-E & 1-5. Fill each cuvette to the bottom of the green label on the cuvette. Be sure to keep these cuvettes in order and not get them mixed up. D. Measuring Absorbance 1. Measure the Absorbance of the five reaction s labeled 1-5 in the same manner as above. Record these values in Table II below. Be sure to record room temperature. It will be used in the calculation of the enthalpy change ( H ) for the reaction. 2. After the Absorbance of each has been measured at room temperature, place the cuvette in an ice/water bath. Re-measure the Absorbance of each after fifteen (15 minutes). Record these values in Table III below. Be sure to record the temperature of the ice/water bath. It will be used in the calculation of the enthalpy change ( H ) for the reaction. 3. After the Absorbance of all s has been determined, discard all reaction s, and rinse all test tubes and cuvettes well and leave them on your lab bench. 4. Return any unused portions of the s in the 3 beakers to the large bottles in the front of the room and rinse these beakers. IlI: DATA TABLES Table I: Standard Solutions Solution [FeSCN ion] Absorbance A 1.0 x 10-4 B 0.80 x 10-4 C 0.60 x 10-4 D 0.40 x 10-4 E 0.20 x 10-4

4 Table II: Reaction Solutions - Room Temperature Vol M Vol M Vol M Solution Fe(NO3)3 (ml) HSCN (ml) HNO3 (ml) Absorbance Table III: Reaction Solutions - Cold Vol M Vol M Vol M Solution Fe(NO3)3 (ml) HSCN (ml) HNO3 (ml) Absorbance IV. Calculations A. Excel File A template containing all data and calculations should be created. All required plots should be generated using Excel. B. Preparation of Calibration Curve A plot of the Absorbance vs. the concentration of FeSCN ion (in M units) will yield a straight line passing through the origin. Using your data from Data Table 1, plot the absorbance vs. the concentration of FeSCN ion for s A - E. Expand the axes on the plot to include the origin. Use Excel commands to do a linear least squares fit to the data. Be sure to display the slope, intercept, and R-squared values for the fit on the chart. The slope and the intercept will be used in subsequent calculations. C. Determination of [FeSCN ion] from Calibration Curve. Use the slope and the intercept from calibration curve prepared above, to determine the [FeSCN ion] for each of the s in test tubes 1-5 and record the values in Tables IV and V on the next page. If

5 absorbance is the y-value, and the slope and the intercept are known, then the concentration (the x- value) can be calculated by rearranging the general equation for a straight line. Table IV. [FeSCN ion] in reaction s Room Temperatue Test Tube # [FeSCN ion] Test Tube # [FeSCN ion] 1 x x x x x 10-4 Table V. [FeSCN ion] in reaction s Cold Test Tube # [FeSCN ion] Test Tube # [FeSCN ion] 1 x x x x x 10-4 D. Calculation of K c for the Reaction at Room Temperature. 1. For each of the s in test tubes 1-5, calculate the number of moles of Fe3+ in each test tube from the initial concentration and the volume of Fe3+ used. Since this is constant for all 5 test tubes, it need be done only one time. Record these values in TableVI. 2. For each of the s in test tubes 1-5, calculate the number of moles of HSCN in each test tube from the initial concentration and volume of HSCN used. These values will be different for each. Record these values in Table VI. 3. Assuming that the correct formula for the FeSCN ion is FeSCN2+, calculate the number of moles of FeSCN2+ in each 10 ml using the data from Table IV. This is the number of moles of FeSCN2+ present at equilibrium in each of the 5 reaction s. Record these values in Table VI. 4. From equation 1 on the first page we see that when 1 mole of FeSCN2+ is formed, 1 mole of Fe3+ and 1 mole of HSCN is consumed. From this and the number of moles of FeSCN2+ present in each 1-5 calculated in step 3, we can calculate the number of moles of Fe3+ and HSCN present at equilibrium in each 1-5. Subtract the number of moles of FeSCN2+ present at equilibrium in each 1-5 from the initial number of moles of Fe3+ and the initial number of moles of HSCN present in each 1-5. Record these values in Table VI. 5. Using the number of moles of Fe3+ and the number of moles of HSCN present at equilibrium from step 4 and the volume of each 1-5 being 10 ml, calculate the [Fe3+] and [HSCN] at equilibrium. Record these values in Table VI.

6 6. From the equilibrium concentrations of Fe3+, HSCN, and FeSCN2+ in Table VI and noting that the [H1+] in all s l-5 is 0.50 M calculate the value of Kc for each 1-5 using equation 3 on the first page. 7. From the five values of Kc calculated above calculate an average value of Kc. E. Calculation of K c for the Reaction at Reduced Temperature. 1. Initial number of moles of Fe 3+ and HSCN are the same as in the previous calculations. Transfer these values from Table VI to Table VII. 2. Repeat ALL the calculations described in Section D above using the concentrations of Fe(SCN) 2+ in the cold s in Table IV. Record the results of these calculations in Table VII. The final result will be an average value for K c the reaction at the reduced temperature. F. Determination of Enthalpy Change ( H ) for Reaction. 1. Use the two average values for the equilibrium constants (K c) in Tables VI and VII, and their associated temperatures to calculate the enthalpy change ( H ). ( R = 8.31 J/mol K) ln (K c/k c) = ( H /R)(1/T - 1/T ) 2. Report the value of H in kj/mol. Note the sign of H and comment as to whether the sign is consistent with the change in the observed aborbance as the samples are cooled. Table VI: Room Temperature C Initial # of Moles Equilibrium # of Moles Equilibrium Concentrations Col Soln Fe 3+ HSCN Fe 3+ HSCN FeSCN 2+ [Fe 3+ ] [HSCN] [FeSCN 2+ ] [H + ] K c 1 x10-6 x10-6 x10-6 x10-6 x10-6 x10-4 x10-4 x Avg. Column 9: given Column 1: calculate from concentration and volume used of Fe 3+ stock and final volume of reaction Column 2: calculate from concentration and volume used of HSCN stock and final volume of reaction Column 8: use calibration curve to determine from absorbance of equilibrium s

7 Column 5: calculate from column 8 and final volume of reaction Column 3: column 1 - column 5 Column 4: column 2 - column 5 Column 6: calculate from column 3 and final volume of reaction Column 7: calculate from column 4 and final volumn of reaction Column 10: calculate from columns 6-9 Table VII: Low Temperature C Initial # of Moles Equilibrium # of Moles Equilibrium Concentrations Col Soln Fe 3+ HSCN Fe 3+ HSCN FeSCN 2+ [Fe 3+ ] [HSCN] [FeSCN 2+ ] [H + ] K c 1 x10-6 x10-6 x10-6 x10-6 x10-6 x10-4 x10-4 x Avg. Column 9: given Column 1: calculate from concentration and volume used of Fe 3+ stock and final volume of reaction Column 2: calculate from concentration and volume used of HSCN stock and final volume of reaction Column 8: use calibration curve to determine from absorbance of equilibrium s Column 5: calculate from column 8 and final volume of reaction Column 3: column 1 - column 5 Column 4: column 2 - column 5 Column 6: calculate from column 3 and final volume of reaction Column 7: calculate from column 4 and final volume of reaction Column 10: calculate from columns 6-9