Rate Law Determination of the Crystal Violet Reaction

25 March 2020 Experiment #6: Rate Law Determination of the Crystal Violet Reaction Abstract: The purpose of this experiment is to understand first, second and third order chemical reactions based on the absorbance of a crystal violet and sodium hydroxide solution. After testing the solution, it was found that the reaction is first order. Introduction: The experiment was performed by taking the absorbance of a Crystal violet and sodium hydroxide solution every thirty seconds using a colorimeter. The experiment was conducted by simultaneously mixing crystal violet and sodium hydroxide. After twenty minutes of taking the absorbance every thirty seconds the data was graphed on excel and analyze to see if it fit first, second or zero order reaction.      0.020M NaOH 2 x 10-5 M crystal violet Distilled water Stirring rod A10-mL graduated cylinder      Materials: Computer Vernier computer interface LoggerPro software Vernier colorimeter One plastic cuvette A 250-mL beaker  Experimental Procedures: Procedure: ** Safety precautions: Avoid spilling Sodium hydroxide and Crystal violet on your skin or clothing. Wear lab coat and safety goggles. 1. Use a 10 mL graduated cylinder to measure out 10 mL of 0.020 M NaOH. 2. Wash the 10 mL graduated cylinder and measure out 10 mL of 2.0 x 10-5 M crystal violet solution. 3. Connect the Colorimeter to the computer. On the computer open file “30 Rate Crystal Violet” found in the Chemistry with Vernierfolder of LoggerPro. 4. Review how to “prepare a cuvette” instructions and prepare a blank by filling an empty cuvette ¾ full with water. Then Seal the cuvette with a lid. 5. Calibrate the Colorimeter. Then empty the cuvette. 6. Simultaneously pour the 10-mL samples of crystal violet and sodium hydroxide into a 250-mL beaker and stir the reaction mixture with a stirring rod. 7. Rinse the cuvette twice with ~1 mL amounts of the reaction mixture. Then refill until it is ¾ full and observe the absorbance. Click Collect to start the clock. If the absorbance is less than 0.34 click Collect. If not, remove the cuvette from the Colorimeter. Observe the absorbance at 30 second intervals until it is less than 0.34. 8. Remove the cuvette from the Colorimeter between readings. 9. When the absorbance is less than 0.34 and stable, click Keep. This saves absorbance and time are saved. Remove the cuvette from the Colorimeter. 10. After 45 seconds, return the sample to the Colorimeter, wait for the absorbance to stabilize, and click Keep. Remove the cuvette again. Repeat these steps and continue collecting data once every minute, for a total of 20 minutes. Save. 11. Discard the solutions in the labeled waste bottle under the hood. 12. Analyze the data graphically to see if the reaction is zero, 1st, or 2ndorder with respect to crystal violet. Then plot the data using Excel. Data and Results: Rate constant for first order reaction: A=[A]0e-kt 0.02=0.20 e-k(19.06) 0.01= e-k(19.06) Ln(0.1)=-k(19.06) -4.6= (-19.06)k 0.24=k K=2.4x10-1 Half-life for first order reaction: t1/2=0.693/k t1/2=0.693/2.4x10-1 t1/2= 2.9mins Table A1: Data showing time, transmittance and absorbance of the Crystal violet and Sodium Hydroxide solution in the zero order. Time (min) 0.126666667 0.975 Transmittance (%T) 62.28980225 63.40585693 Absorbance 0.205583048 0.197870624 2.29 3.26 4.181666667 5.135 6.051666667 6.975 7.396666667 8.408333333 9.273333333 9.385 10.33666667 11.27833333 12.425 13.055 14.10666667 15.07 16.04 17.06666667 18.09 19.06166667 67.52130859 69.33489746 71.60188354 74.00837646 74.9151709 76.69388306 78.33308838 79.51889648 82.13464966 81.36736206 81.96026611 82.79730713 86.07571777 86.21522461 86.45936157 87.78467651 87.85442993 88.9007312 89.56338867 94.82977173 0.170559149 0.159048122 0.145075553 0.130719123 0.125430225 0.115239273 0.106054751 0.099529655 0.085473591 0.089549764 0.08639664 0.081983788 0.065119347 0.064416036 0.063187976 0.056581287 0.056236335 0.051094667 0.047869483 0.023055295 Figure A1: Data from chart A1 in a scatter plot. Chart A2: Data from the Crystal violet and Sodium Hydroxide solution in first order. Time (min) 0.126666667 0.975 2.29 3.26 4.181666667 5.135 6.051666667 6.975 7.396666667 8.408333333 Absorbance ln (abs) 0.20558304 8 -1.581905201 0.19787062 4 -1.620141878 0.17055914 9 -1.768673125 0.15904812 2 -1.838548466 0.14507555 3 -1.930500617 0.13071912 3 -2.034704358 0.12543022 5 -2.076005648 0.11523927 3 -2.160744677 0.10605475 1 -2.243799804 0.09952965 -2.307299635 9.273333333 9.385 10.33666667 11.27833333 12.425 13.055 14.10666667 15.07 16.04 17.06666667 18.09 19.06166667 5 0.08547359 1 0.08954976 4 0.08639664 0.08198378 8 0.06511934 7 0.06441603 6 0.06318797 6 0.05658128 7 0.05623633 5 0.05109466 7 0.04786948 3 0.02305529 5 -2.459547828 -2.41296079 -2.448806491 -2.501233761 -2.731533584 -2.74239267 -2.761641254 -2.872076969 -2.878192202 -2.974075152 -3.039277075 -3.769859824 Figure A2: Data from chart A2 in a scatter plot. ln (abs) 0 Absorbance -2 -4 0 2 4 6 8 10 12 14 16 18 20 f(x) = − 0.04 x − 1.79 R² = 0.16 -6 -8 -10 -12 Time (min) Chart A3: Data from the Crystal violet and Sodium Hydroxide solution in second order. Time (min) 0.126666667 0.975 2.29 3.26 4.181666667 5.135 6.051666667 6.975 7.396666667 8.408333333 9.273333333 9.385 10.33666667 11.27833333 12.425 13.055 14.10666667 15.07 16.04 17.06666667 18.09 19.06166667 Absorbance 1/absa 0.205583048 4.864214293 0.197870624 5.053807291 0.170559149 5.863068637 0.159048122 6.287405252 0.145075553 6.892960108 0.130719123 7.649990131 0.125430225 7.972560016 0.115239273 8.677597257 0.106054751 9.429092003 0.099529655 10.04725672 0.085473591 11.69952015 0.089549764 11.16697531 0.08639664 11.57452417 0.081983788 12.19753352 0.065119347 15.35641932 0.064416036 15.52408471 0.063187976 15.8257958 0.056581287 17.6736878 0.056236335 17.78209766 0.051094667 19.57151419 0.047869483 20.89013577 0.023055295 43.37398442 Figure A3: Data from chart A3 in a scatter plot. 1/absa 19 f(x) = 0.79 x + 2.97 R² = 0.39 17 Absorbance 15 13 11 9 7 5 3 1 0 2 4 6 8 10 Time (min) 12 14 16 18 20 Discussion: Questions: 1. Was the reaction zero, first, or second order, with respect to the concentration of crystal violet? Explain. Answer: The reaction was first order with respect to the concentration of crystal violet because the slope of the first order graph had the best fit when graphed compared to the zero order graph and the second order graph’s slope. 2. Estimate the half-life of the reaction; select two points, one with an absorbance value that is about half of the other absorbance value. The time it takes the absorbance (or concentration) to be halved is known the half-life for the reaction. (As an alternative, you may choose to calculate the half-life from the rate constant, k, using the appropriate concentration-time formula.) Answer: t1/2=2.9 Conclusion: The experiment proved that the reaction with respect to the crystal violet concentration is first order. According to figure A2 depicting the first order reaction graphed the slope is the best fit. The first order reaction gave a slope of 0.9. Taking a look at figure A3 depicting the second order reaction the line does not match the slope as perfect as the first order reaction. The first order reaction data gave a rate constant, k of 2.4x10-1 and a half life of 2.9.