1. Valdosta State University Experiment 2 Kinetics of Murexide Decomposition Valdosta State University

2. Objective Determine the rate constant and order of reaction for the decomposition of murexide using graphical methods. Valdosta State University Murexide (NH 4 C 8 H 4 N 5 O 6, or C 8 H 5 N 5 O 6.NH 3 ), also called ammonium purpurate or MX, is the ammonium salt of purpuric acid. Murexide in its dry state has the appearance of a reddish purple powder, slightly soluble in water. In solution, its color ranges from yellow in strong acidic pH through reddish-purple in weakly acidic solutions to blue-purple in alkaline solutions.

3. Valdosta State University Spectrophotometric investigation of the kinetics of decomposition of murexide in acid solutions. Ramaiah, N. A.; Gupta, S. L.; Vishnu. Indian Institute of Sugar Technol., Kanpur, Zeitschrift fuer Naturforschung (1957), 12b 189-95. A hypothesis referring to the formation of an intermediate complex (electrically uncharged) that decompd. unimolecularly to give the products, uramil and alloxan, was proposed. + H + /H 2 O

4. Valdosta State University Background As seen in experiment one, the rate of reaction can be described by the equation: Valdosta State University

5. Background As seen in experiment one, the rate of reaction can be described by the equation: Valdosta State University

6. Background As seen in experiment one, the rate of reaction can be described by the equation: Valdosta State University recall [ ] = mole/L

7. Valdosta State University Background - The rate for this reaction changes as the concentration changes. - The chemist is required to stop the reaction and measure the concentration. - A better approach would be to describe the rate in terms of the initial concentration. Valdosta State University

8. Background Rate Law For the reaction: A (aq) + B(aq)  C(aq) + D(aq) rate = k [A] q [B] r k = rate constant [A], [B] = concentration of reactants q, r = reaction orders Valdosta State University

9. Reaction Orders Order with respect to reactant Plot methodEquation of Line Zeroth[A] vs. Time[A] = -kt + [A] o Firstln[A] vs. Timeln[A] = -kt + ln[A] o Second1/[A] vs. Time(1/[A]) = kt + (1/[A]) o Slope -k k

10. Valdosta State University Background Valdosta State University Determining Reaction Orders One method is to run the reaction several times using different concentrations and observe the rate change.

11. Valdosta State University Background Valdosta State University Determining Reaction Orders One method is to run the reaction several times using different concentrations and observe the rate change. Consider: 2 I - (aq) + S 2 O 8 2- (aq)  I 2 (aq) + 2 SO 4 2- (aq) I - (aq) (M)S 2 O 8 2- (aq) (M)Initial Rate (mol/L*sec) 0.0800.04012.5 x 10 -6 0.040 6.25 x 10 -6 0.0800.0206.25 x 10 -6

12. Valdosta State University Background Valdosta State University Determining Reaction Orders A second method follows the concentration of one reactant with time. How the concentration decreases reveals the order of the reaction.

13. Valdosta State University Background Valdosta State University Determining Reaction Orders A second method follows the concentration of one reactant with time. How the concentration decreases reveals the order of the reaction. Consider: 2 C 4 H 6 (g)  C 8 H 12 (g)

14. Valdosta State University Background Valdosta State University Determining Reaction Orders 2 C 4 H 6 (g)  C 8 H 12 (g) Time (seconds)[C 4 H 6 ] 1951.6 x 10 -2 6041.5 x 10 -2 12461.3 x 10 -2 21801.1 x 10 -2 62100.68 x 10 -2

15. Valdosta State University Background Valdosta State University Determining Reaction Orders - The order of the reaction with respect to C 4 H 6 is determined by building several graphs. - The graph that yields the straightest line indicates the order of the reactant.

16. Valdosta State University Background Valdosta State University Determining Reaction Orders - The order of the reaction with respect to C 4 H 6 is determined by building several graphs. - The graph that yields the straightest line indicates the order of the reactant. Order with respect to reactant Plot methodSlopeEquation of Line Zeroth[A] vs. Time-k[A] = -kt + [A] o Firstln[A] vs. Time-kln[A] = -kt + ln[A] o Second1/[A] vs. Timek(1/[A]) = kt + (1/[A]) o

17. Valdosta State University Background Valdosta State University Determining Reaction Orders 2 C 4 H 6 (g)  C 8 H 12 (g) Time (seconds) [C 4 H 6 ] 1951.6 x 10 -2 6041.5 x 10 -2 12461.3 x 10 -2 21801.1 x 10 -2 62100.68 x 10 -2

18. Valdosta State University Background Valdosta State University Determining Reaction Orders Zeroth Order Plot – [C 4 H 6 ] versus Time

19. Valdosta State University Background Valdosta State University Determining Reaction Orders First Order Plot – ln[C 4 H 6 ] versus Time

20. Valdosta State University Background Valdosta State University Determining Reaction Orders Second Order Plot – [C 4 H 6 ] -1 versus Time

21. Valdosta State University Background Valdosta State University Determining Reaction Orders - The problem is finding a way to determine the concentration of the reactant. - If the compound has a color, the intensity of color can be related to the concentration.

22. Valdosta State University Colorimeter Transmittance (T) – the amount of light that penetrates a solution. T = I I / I 0 T = e -  bc A = -10 log(T) = 10 log(1/T) A = 10 log (100/%T) A =  b c  -molar absorptivity (L/mol cm) For a solution in a cuvette, with a ct. cell width, A  c A = Kc (Beer’s law)

23. Valdosta State University Background Valdosta State University Beer’s Law A =  bc A = absorbance  = molar absorbtivity b = path length c = concentration - The sample holder and solution characteristics are constant. - For this experiment, “a” and “b” are both constant.

24. Valdosta State University Technique - Colorimeter Experimental Setup 1. Boot up the computer. When the computer boots up, turn on the Pasco Scientific interface. 2. Connect the serial plug of the converter to the serial connection on the interface, then connect the USB plug to the USB port on the computer (be sure the colorimeter is plugged into channel B) 3. Start DataStudio Software. 4. On the opening screen, select “Create Experiment”. 5. Click and drag the “Colorimeter” sensor to Channel B of the interface box shown in the experimental setup window. 6. Double click on the colorimeter icon. Click on “Slow”, then click on the “+” sign until the “Periodic Samples” displays “5 s”. Click on “Ok”. 7. Click on the “Options” button. Click on the “Automatic Stop”. Click on time and enter “301” then click “Ok” to exit.

25. Valdosta State University Technique - Colorimeter Calibration of Colorimeter 1. Fill a plastic cuvet full of 0.1 M HCl. 2. Double click on the colorimeter icon. Click on the calibration tab. 3. Put the cuvet with the 0.1 M HCl into the colorimeter with the ribbed side facing toward you. Close the lid. 4. Make sure the colorimeter display reads “Please Calibrate”. On the computer screen, click on the “Take Reading” in the Low Point box. 5. Press the “Select” and the “Start / Stop” buttons on the colorimeter at the same time. 6. Press “Select” on the colorimeter. 7. Press “Select” on the colorimeter two more times, or until the digital display on the colorimeter shows “Green 565 nm” 8. Press “Start” on the colorimeter. Click on “Take Reading” in the High Point box. Click on “Ok”. 9. Fill a clean, dry cuvet with the 0.100 g / L murexide solution. Put the cuvet in the colorimeter with the ribbed side facing toward you. Close the lid of the colorimeter. 10. The digital display on the colorimeter will display the %T. Record the data on the report sheet. 11. Drag the table icon from the “Displays” menu and drag it onto the “% Transmittance, Ch B” icon. 12. Double click on the “% Transmittance, Ch B” icon. Click on the “Numeric” tab. Click on “Fixed Decimals”. Change the number in the box from 0 to 1. Click on “Ok”.

26. Valdosta State University Technique - Colorimeter Running the Experiment 1. Measure out 2.5 mL of murexide solution and put the solution into a cuvet. 2. The following steps must be done quickly: a. Add 10 drops of 0.1 M HCl to the murexide in the cuvet. b. Quickly cap the cuvet, mix the solution and insert the cuvet (ribbed side toward you) into the colorimeter. c. Close the cover and double click on the “Start” button. 3. When the timer reads zero the computer has finished taking data, and you may remove the cuvet from the colorimeter. 4. Repeat the experiment two additional times.

27. Valdosta State University Example Data Valdosta State University Determining [murexide] - molecular formula: C 8 H 8 N 6 O 6 - formula weight of murexide: 284.19 g/mol - solution concentration is 0.100 g/L

28. Valdosta State University Example Data Valdosta State University Determining [murexide] - molecular formula: C 8 H 8 N 6 O 6 - formula weight of murexide: 284.19 g/mol - solution concentration is 0.100 g/L

29. Valdosta State University Example Data Valdosta State University Converting %T into A - use equation 2.5 pg 19 from the laboratory manual.

30. Valdosta State University Example Data Valdosta State University Converting %T into A - use equation 2.5 pg 19 from the laboratory manual.

31. Valdosta State University Example Data Valdosta State University Determining  - use equation 2.6 pg 19 from the laboratory manual.

32. Valdosta State University Example Data Valdosta State University Determining  - use equation 2.6 pg 19 from the laboratory manual.

33. Valdosta State University Example Data Valdosta State University Time ( s )TransmittanceAbsorbance 04.561.341 54.781.321 104.881.312 154.781.321 204.881.312 254.981.303 304.981.303 355.191.285 405.281.277 455.401.268 505.601.252 555.701.244 605.921.228 655.921.228 706.011.221 756.321.199

34. Valdosta State University Example Data Valdosta State University Use Beer’s Law to determine [murexide] at each time - use equation 2.5 pg 19 from the laboratory manual or A =  c

35. Valdosta State University Example Data Valdosta State University Use Beer’s Law to determine [murexide] at each time Time ( s )TransmittanceAbsorbance[murexide] 04.561.3410.00033 54.781.3210.0033 104.881.3120.00032 154.781.3210.00033 204.881.3120.00032 254.981.3030.00032 304.981.3030.00032 355.191.2850.00032 405.281.2770.00032 455.401.2680.00031 505.601.2520.00031 555.701.2440.00031 605.921.2280.00030 655.921.2280.00030 706.011.2210.00030 756.321.1990.00030 ln[murexide]1/[murexide]

36. Valdosta State University Example Data Valdosta State University Zeroth Order graph - plot [murexide] versus time - be sure to include a trendline with the R 2 and the linear regression equation

37. Valdosta State University Example Data Valdosta State University First Order graph - plot ln[murexide] versus time - be sure to include a trendline with the R 2 and the linear regression equation

38. Valdosta State University Example Data Valdosta State University Second Order graph - plot 1/[murexide] versus time - be sure to include a trendline with the R 2 and the linear regression equation

39. Valdosta State University Safety The 0.1 M HCl is corrosive. If you spill some on you, wash the affected area for five minutes. If the acid gets in the eyes, wash the eyes for ten minutes. Seek immediate medical attention! Neutralize any acid spills with baking soda. Murexide is a mild irritant. If you spill some on you, wash the affected area with soap and water. If the murexide gets in your eyes, wash the eyes for ten minutes. Seek immediate medical attention! Valdosta State University

40. Waste Disposal All waste materials may be disposed of in the sink, flushed with plenty of water. Valdosta State University