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PHYSICAL CHEMISTRY 2 LABORATORY REPORT Experiment No.6 Title: Kinetics of Chemical Reaction – Iodination of cyclohexanone Date of expt: 23 July 2008 Name: Cheam Yee Ping Matrix No: SEC070015 Lab Patner: Chan Wei Kuen Matrix No: SEC070014 Group: C Laboratory: Physical chemistry 2nd year Lecturer: Dr. Azizah binti Mainal Date of submission report: 6 August 2008 1 Expeiment 6: Kinetics of chemical reaction – iodination of cyclohexanone Objective: To determine the rate constant and the rate of reaction for the reaction of iodination of cyclohexanone. Background: There are some factors that determine the rate of reaction. One of them is the concentration of reactant. The increase of concentration causes the number of moles per unit volume increase. So, the number of collisions per unit time in that volume increase and the effective collisions increase. Therefore, the rate of reaction increase. Another factor is temperature. When temperature increase, the average kinetic energy of molecules increases. Then, the number of molecules with the energy equal or more then activation energy increase and cause the number of effective collisions increase. Thus, the reaction rate increase. Pressure is one of the factors that determine the rate of reaction. When pressure increase, volume will be decrease. The increase of number of molecules per unit volume cause more collisions per second. The number of effective collisions increase so that the reaction rate also increases. The presence of catalysis will increase the rate of reaction. Catalyst provide an alternative path way which has lower activation energy. Therefore, the rate of reaction increase. Experimental Procedure: The following solutions are to be prepared from the stock solutions: Volume of stock solution used/cm3 I II III Cyclohexanone 20 20 20 HCl 10 10 6 Distilled Water 14 16 20 KI3 6 4 4 -3 [Cyclohexanone] stock = 0.230 mol dm [HCl] stock = 0.500 mol dm-3 [KI3] = 0.0360 mol dm-3 IV 14 10 22 4 The acid, ketone and water were mixed together in test tube. The test tube was placed in the constant temperature bath. The test tube and its content were allowed to equilibrate for at least 15 minutes. The KI3 stock solution was thermostat as well. The reaction was started by pipetting the required amount of the KI3 solution into the test tube. As soon as the pipette has drained, the timer was started. Immediately, a sample was transferred from the test tube to a spectrophotometer sample cell and its absorbance was measured. Further samples were taken and measured at intervals of 2 minutes, until sufficient points were obtained for giving a good plot of absorbance A versus time. The same procedure was carried out for each of the mixture I, II, III, and IV. The gradient of the graph was determined. 2 Results and calculations: Temperature of water bath: 29.9°C Wavelength of light: 565nm Concentration of stock solution: [S] stock = 0.2305 M [HCl] stock = 0.506 M [KI3] stock = 0.0360M where [S] = [cyclohexanone] For absorbance of KI3 stock solution Readings 1 2 Absorbance 2.000 2.000 Time,t (min) 0.45 3.10 5.05 7.00 9.00 11.05 13.05 15.20 Mixture I 0.280 0.241 0.199 0.165 0.148 0.110 0.069 0.033 Time,t (min) 0.50 3.55 5.45 7.45 9.40 11.45 13.35 Time,t (min) 0.35 2.30 4.25 6.30 8.25 10.25 12.25 14.25 16.25 18.30 20.35 22.30 24.20 Mixture III 0.241 0.230 0.208 0.182 0.162 0.148 0.130 0.104 0.080 0.063 0.043 0.020 0.005 Time, t (min) 0.35 2.25 4.30 6.30 8.30 10.30 12.30 14.25 16.30 18.30 20.25 22.20 3 2.000 Average 2.000 Mixture II 0.195 0.193 0.096 0.065 0.063 0.029 0.005 Mixture IV 0.214 0.208 0.168 0.155 0.131 0.109 0.089 0.067 0.043 0.025 0.011 0.002 3 SUMMARY OUTPUT Regression Statistics Multiple R 0.997737 R Square 0.995478 Adjusted R Square 0.994725 Standard Error 0.006102 Observations 8 ANOVA df Regression Residual Total Intercept X Variable 1 1 6 7 Coefficients 0.288266 -0.01661 Significance SS MS F F 0.049184 0.0491845 1320.976 2.89E-08 0.000223 3.723E-05 0.049408 Standard Error 0.004239 0.000457 t Stat 67.99627 -36.34524 P-value 6.81E-10 2.89E-08 Upper Lower 95% 95% 0.277892 0.298639 -0.01772 -0.01549 4 SUMMARY OUTPUT Regression Statistics Multiple R 0.9822963 R Square 0.96490601 Adjusted R Square 0.95788721 Standard Error 0.0146383 Observations 7 ANOVA df Regression Residual Total Intercept X Variable 1 1 5 6 Coefficients 0.20583942 -0.0155401 SS MS F 0.029458029 0.029458 137.4745 0.001071399 0.000214 0.030529429 Standard Error 0.011153769 0.001325387 t Stat 18.4547 -11.725 P-value 8.59E-06 7.93E-05 Significance F 7.93E-05 Upper Lower 95% 95% 0.177168 0.234511 -0.01895 -0.01213 5 SUMMARY OUTPUT Regression Statistics Multiple R 0.9991494 R Square 0.9982995 Adjusted R Square 0.9981449 Standard Error 0.0034049 Observations 13 ANOVA df Regression Residual Total Intercept X Variable 1 SS MS F 1 0.07486724 0.074867 6457.671 11 0.000127529 1.16E-05 12 0.074994769 Standard Coefficients Error t Stat 0.2489772 0.001816215 137.0858 -0.010155 0.000126367 -80.3596 P-value 3.9E-19 1.38E-16 Significance F 1.38E-16 Upper Lower 95% 95% 0.24498 0.252975 -0.01043 -0.00988 6 SUMMARY OUTPUT Regression Statistics Multiple R 0.9991494 R Square 0.9982995 Adjusted R Square 0.9981449 Standard Error 0.0034049 Observations 13 ANOVA df Regression Residual Total Intercept X Variable 1 1 11 12 Coefficients 0.2489772 -0.0101548 SS MS F 0.07486724 0.074867 6457.671 0.000127529 1.16E-05 0.074994769 Standard Error t Stat 0.001816215 137.0858 0.000126367 -80.3596 P-value 3.9E-19 1.38E-16 Significance F 1.38E-16 Upper Lower 95% 95% 0.24498 0.252975 -0.01043 -0.00988 7 From graph: mixture I II III IV Gradient, dA/dt ( minˉ¹) -0.0166 -0.0155 -0.0102 -0.0103 Concentration for 20 0.230 [s]I = 50 = 0.092 M 20 0.230 [s]II = 50 = 0.092 M 20 0.230 [s]III = 50 = 0.092 M 14 0.230 [s]IV = 50 = 0.0644 M 10 0.500 50 = 0.10 M 10 = 0.500 50 = 0.10 M 6 = 0.500 50 = 0.06 M 10 = 0.500 50 = 0.10 M [H+]I = [H+]II [H+]III [H+]IV where [s] is the concentration of cyclohexanone [I3-]I [I3-]II [I3-]III [I3-]IV 6 0.0360 50 = 0.00432 M 4 0.0360 = 50 = 0.00288 M 4 0.0360 = 50 = 0.00288 M 4 0.0360 = 50 = 0.00288 M = According to Beer-Lambert Law, A(I3- ) = l [I3-] Where A = absorbance = molar absorption coefficient [I3-] = concentration l = optical path length 8 l = A (I3- ) / [I3] 2.000 = 0.0360 = 55.56 M-1 The differential rate equation for the reaction is -d[A]/dt =kl [s]a [I3]b [H+]c Where k = rate constant, a, b,and c are the orders of reaction with respect to S, I3- and H+ respectively Determining the value of a, b and c For a, Take solution II and IV for constant [H+] and [I3] but different [s] dA dA log log dt II dt IV a = log S II log S IV 0.0155 log 0.0103 = 0.0920 log 0.0644 = 1.1458 dA dt II Let x = dA dt IV 0.0155 = 0.0103 = 1.5049 dA dA d d dx dt IV dt II x dA dA dt II dt IV dx 0.0034 0.0006 1.5049 0.0155 0.0103 dx 0.4178 9 dx x 0.4178 = 1.5049 = 0.2776 da a = 1.15 ± 0.20 ≈1 For b, Take solution I and II for constant [H+] and [s] but different [I3]. dA dA log log dt I dt II b= log I 3 I log I 3 II 0.0166 log 0.0155 = 0.0432 log 0.02880 = 0.1691 dA dt I Let y = dA dt II 0.0166 0.0155 1.0710 dA dA d d dy dt I dt II y dA dA dt I dt II dy 0.0001 0.0003 1.0710 0.0166 0.0155 dy 0.272 dy db y 0.0272 = 1.0710 =0.03 10 b = 0.1691 ± 0.03 ≈0 For c, Take solution II and III for constant [I3] and [s] but different [H+] dA dA log log dt II dt III c= log H II log H III 0.0155 log 0.0102 = 0.1 log 0.06 = 0.8192 dA dt II Let z = dA dt III 0.0155 = 0.0102 = 1.5196 dA dA d d dz dt II dt III z dA dA dt II dt III dz 0.0003 0.0003 1.51`96 0.0155 0.0102 dz 0.0741 dz z 0.0741 = 1.5196 = 0.0488 dc c = 0.8192 ± 0.05 ≈0 11 Determining the rate constant, k The above equation can be written as k = ___-d[A]/dt___ l [s] [H+] k I = __- ( -0.0166 )________ 55.56 x 0.092 x 0.10 = 0.0325 k II = _- ( - 0.0155)___________ 55.56 x 0.092 x 0.10 = 0.0303 0.0102 55.56 0.0920 0.0600 = 0.0333 k III 0.0103 55.56 0.0644 0.1000 = 0.0288 k IV 0.0325 0.0303 0.0333 0.0288 4 = 0.0312 mol-1min-1dm3 Average of rate constant, k = Determining the standard deviation of k Solution k i (mol-1min-1dm3) I II III IV 0.0325 0.0303 0.0333 0.0288 ki k ki k (mol-1min-1dm3) 0.0013 -0.0009 0.0021 -0.0024 (mol-2min-2dm6) 1.69 x 10-6 0.81 x 10-6 4.41 x 10-6 5.76 x 10-6 12.67 x 10-6 ki k k i k 2 Standard deviation, s = n 1 1 2 2 2 12.67 10 6 3 = 0.0021 mol-1min-1dm3 = 12 Standard uncertainties, σ = = s n 0.0021 4 = 0.0011 mol-1min-1dm3 Uncertainty of k = 2σ = 2 (0.0011) = 0.0022 mol-1min-1dm3 Rate constant, k = ( 0.0312 ± 0.0022 ) mol-1min-1dm3 Discussion: From the above experiment, it is noticed that the rate of bromination of ketone is same as the rate of iodination. This shows that the rate of reaction depends on the concentration of acid and cyclohexanone since acid hydrochloride acts as catalyst while cyclohexanone is the reactant in the experiment. The rate of reaction does not depend on the concentration of iodide ion. Scheme A from the proposed mechanisms satisfies the rate equation since the first step in Scheme A is the slowest step, the rate determining step. Therefore, it controls and determines the overall rate of the reaction. During the experiment, we could observe that the colour of the mixture change from brown to colourless. It is because the reaction is taking place to decolourise the brownish colour of KI3. From the pictures above, the colour of the mixture change from brownish to yellowish and finally become colourless. Some precautions can be carried out in the experiment: o Spectrophotometer has to be calibrated each time with distilled water before measuring the absorbance for the next sample. o Temperature must be kept constant throughout the experiment o Spectrophotometer has to be switch on for at least 20 minutes before absorbance measurements begin 13 Conclusion: Rate constant, k = (0.0312 ± 0.0022) mol-1min-1dm3 Order of reaction for 1. cyclohexanone, a = 1 2. iodide ion, b = 0 3. hydrogen ion, c = 1 Reference: 1. Atkins, P.W (1998), Physical chemistry, 7th ed. Oxford 14