INSTRUCTOR RESOURCES

Transcription

1 Reaction of Crystal Violet with NaOH A Kinetic Study with Ea Calculation INSTRUCTOR RESOURCES The CCLI Initiative Modified by Dale A. Hammond, PhD, Brigham Young University Hawaii Learning Objectives study the rate of reaction of crystal violet with NaOH using the MicroLAB interface colorimeter. determine the order of reaction with respect to both reactants. calculate the value of the rate constant for the reaction at room temperature. Determine the activation energy, Ea, for the reaction from supplied data. (Students will not take this data since most general chemistry labs do not have the necessary equipment for doing these experiments.) Procedure Overview the kinetics of the decomposition of crystal violet is studied using the MicroLAB colorimeter. the experimental data are plotted to determine the order of reaction in crystal violet. the reaction is studied at two different concentrations of hydroxide ion to determine the order of the reaction in hydroxide. students use the information provided in the table of data to calculate the average k, its standard deviation, then plot lnk vs. 1/T (kelvin) to determine Ea for the reaction. 9

2 Name Section Date REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY Report Sheet 1. What is plotted on the y-axis for the graph which is linear? 2. What is the value of k' for the most linear plot? Include units and use appropriate significant figures. 3. Attach a printout of your labeled graph and data points to this report sheet. Determining the order of the reaction in NaOH 4. What is the value of k' when using 0.10 M NaOH? Include units and use appropriate significant figures. 5. Determine the ratio of the two k' values. Round to the nearest integer. 6. Attach a printout of your labeled graph and data points to this report sheet. 10

3 Name Section Date REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY Report Sheet (page 2) 7. Calculate k using each k' value. Adjust [OH ] for dilution. 8. Calculate k average. 9. Calculate the molar absorptivity,, using the data from the first experimental run. Summary Species Order k' k' k k Crystal Violet OH k average = (include units) 11

4 REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY Calculation of Ea for the reaction Suggested Answers to Questions 1. Using the data in the following table, calculate the mean and standard deviation for each set of temperature values. Run / Temp. 25 C 35 C 45 C 55 C Run Run Run Mean value Standard dev. rd 2. Examine the three graphs for the 55 C data. How reliable is the data for the 3 run at that temperature? Explain your answer. 3. Should the 55 C data be used for determining the activation energy for this reaction? Why or why not? 4. Calculate the activation energy for this reaction from the equations given in the experimental write- up. You must show your work for the activation energy calculation. 12

5 REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY Questions to Answer 5. The following data were collected for a reaction of the type: A ==> Products [A] Time (seconds) x 10 M x 10 M x 10-4 M x 10-4 M x 10-4 M 20 a. Just by looking at the data (do not make a plot), determine whether the reaction is zero order in A. What must be true of the data for the reaction to be zero order in A? b. Compute the natural logarithm for each of the concentrations and (without making a plot) determine whether the reaction is first order in A. c. Make the appropriate plot and calculate the value of k for the reaction. 13

6 REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETIC STUDY Tips and Traps 1. Crystal violet stains, so appropriate care should be taken in its handling. 2. NaOH can be used to remove the crystal violet stain from the solution cells. 4. One NaOH solution concentration must be exactly double the other. We recommend preparing M NaOH and diluting a portion exactly in half to obtain this ratio. 5. The NaOH is best dispensed using a calibrated (1.00 ml) plastic pipet that has a squeeze bulb at one end. 6. Add the NaOH rapidly, cap the vial, rotate twice, place in the colorimeter chamber and begin the program immediately. 7. The colorimeter chamber should be covered with the canister. 8. Be sure to position the solution cell in the colorimeter in exactly the same way after measuring the blank. 9. Crystal violet is most easily dispensed by buret. 10. The best temperature for experimental conditions is C. 14

7 REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETIC STUDY Suggested Answers for Report Sheet 1. What is plotted on the y-axis for the graph which is linear? The linear graph of formed using ln(abs) on the y-axis. 2. What is the value of k' for the most linear plot? Include units and use appropriate significant figures. The value of k for the [OH ] = plot is mol/l Experimental values may differ here as a function of the actual CV concentration and colorimeter parameters. 3. Attach a printout of your labeled graph and data points to this report sheet. See screen image following. Determining the order of the reaction in NaOH 4. What is the value of k' when using 0.10 M NaOH? Include units and use appropriate significant figures. The value of k for the [OH ] = 0.10 plot is mol/l Experimental values may differ here as a function of the actual CV concentration and colorimeter parameters. 5. Determine the ratio of the two k' values. Round to the nearest integer / = 1.96 The actual ratio should be very close to 2.00, indicating first order in [OH ], since doubling the [OH ] concentration doubled the rate of the second reaction. For the reaction to be second order, the rate should have quadrupled between experiments one and two. 6. Attach a printout of your labeled graph and data points to this report sheet. See screen image following. 15

8 Name Section Date REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY Suggested Answers for Report Sheet (page 2) 7. Calculate k using each k' value. Adjust [OH ] for dilution. 1 ml 0.05 mmol = M 1 ml 0.10 mmol = M 10 ml 1 ml 10 ml 1 ml 2 k = k /[OH ] 2 2 k = / (0.0050) = 1908 k = / (0.010) = Calculate k average = Calculate the molar absorptivity,, using the data from the first experimental run. -5 = A/bc = / 2.54 / 1.5 x 10 = L/cm/Mole Summary Species Order k' k' k k Crystal Violet First OH Second k average = 1422 L / mol sec (include units) 16

9 REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY Calculation of Ea for the reaction Suggested Answers for Report Sheet (page 3) 1. Using the data in the following table, calculate the mean and standard deviation for each set of temperature values. Run / Temp. 25 C 35 C 45 C 55 C Run Run Run Mean value Standard dev X X X X 10 rd 2. Examine the three graphs for the 55 C data. How reliable is the data for the 3 run at that temperature? Explain your answer. rd The data for the 3 run does not appear to be reliable because the original data does not follow a smooth curve, and the linear data does not plot on the regression line. 3. Should the 55 C data be used for determining the activation energy for this reaction? Why or why not? The 55 C data should not be used in determining the activation energy because the standard -3-3 deviation (2.12 X 10 ) is on the same order of magnitude as the mean value (7.42 X 10 ) indicating significant error in those three measurements. However, as seen on the graphs of the data, the value for Ea differs very little when the 55 C data point is included due to the spread of the data. 4. Calculate the activation energy for this reaction from the equations given in the experimental write- up. You must show your work for the activation energy calculation. See attached graph plotting the data for 25, 35, and 45 C data and the data for all for temperatures. 17

10 REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY Suggested Answers to Questions 1. In Figure 1, crystal violet is shown as a positive ion of charge 1+. Which atom in the crystal violet cation has a formal charge of +1? Explain thoroughly how you arrived at your answer. The 1+ charge is located on the N atom that has four bonds to it. 2. The following data were collected for a reaction of the type: A ==> Products [A] Time (seconds) 1.00 x 10-3 M x 10-4 M x 10-4 M x 10-4 M x 10-4 M 20 a. Just by looking at the data (do not make a plot), determine whether the reaction is zero order in A. What must be true of the data for the reaction to be zero order in A? The reaction is not zero order in A. For the reaction to be zero order the in [A] must all be the same (since the time increments are all equal). changes b. Compute the natural logarithm for each of the concentrations and (without making a plot) determine whether the reaction is first order in A. The reaction is first order in A because the changes in ln A are the same (-0.49 units) for each of the 5 second time increments. c. Make the appropriate plot and calculate the value of k for the reaction. The value of k is 0.10 s-1. 18

11 REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY Sample Data Sample data screen for the MicroLAB Crystal Violet Kinetics experiment. 19

12 REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY Laboratory Preparation (per student station) Equipment MicroLAB interfaces colorimeter glass a solution cell (cuvette), 2.54 cm path ring stands buret clamp buret with at least 10 ml calibrated 1.0 ml plastic dropper 50 ml beaker Supplies paper towels KimWipes Chemicals Actual quantities needed are given below. A 50% excess is recommended x 10 M crystal violet solution (18.00 ml) M NaOH solution (1.0 ml) 0.10 M NaOH solution (1.0 ml) Safety and Disposal crystal violet may cause skin and eye irritation and the NaOH solutions are corrosive. Make sure students wear goggles at all times all resulting solutions may be flushed down the drain with plenty of water have students wash hands with soap and water before leaving the lab. 20