Experiment 1: The Kinetics of Methylene Blue Reduction by Ascorbic AcidIntroductionThe reaction of methylene blue and ascorbic acid is very convenient to study for the purpose of developing a rate equation because it reacts to form Leucomethylene blue and dehydroascorbic acid and the solution becomes colourless. The progress of the reaction may be monitored by measuring the absorbance of the 666nm band using a spectrophotometer. The rate law for this reaction is

Which by using excess ascorbic acid, can be simplified to:

The rate can then be determined using the integration rate method. Aim:To investigate the rate of reduction of methylene blue by ascorbic acid in an acid solution using a spectrophotometer and to use kinetic data to determine the rate law and rate constant for the reaction.Experimental:The experiment was conducted using the method outlined in the Laboratory Manual. There were no deviations from this method.ResultsTables 1-6 show the absorption of light with a 666nm wavelength with respect to time for ascorbic acid solution reacted with methylene blue. The progression of the reaction to completion was shown by the reduction of the absorption. The concentration was calculated by dividing the absorbance by the molar absorption coefficient for methylene blue at 666nm (7*104cm-1M-1)

Table 1: Sample i + stockTime (s)Absorbance[MB+]ln[MB+]1/[MB+]

00.007584081.02488E-07-16.09359757282

70.005517527.45611E-08-16.411613411815

140.004365325.89909E-08-16.645916951777

210.003120974.21753E-08-16.981423710560

280.003420264.62197E-08-16.889921635788

350.002600433.5141E-08-17.163928456824

420.003259264.4044E-08-16.938122704557

490.002523753.41047E-08-17.193829321500

560.002543253.43682E-08-17.186129096656

630.002393353.23425E-08-17.246930919050

700.00215482.9119E-08-17.351934341869

770.002561793.46187E-08-17.178928886087

840.002478243.34897E-08-17.21229859956

910.002289353.09371E-08-17.291332323626

980.002183242.95032E-08-17.338833894591

1050.0023633.19324E-08-17.259631316154

1120.002175492.93985E-08-17.342334015385

1190.002298333.10584E-08-17.287432197360

Table 2: Sample i+vTime (s)Absorbance[MB+]ln[MB+]1/[MB+]

0-0.00189662.56291E-08-17.479539018094

7-0.00167862.26833E-08-17.601644085320

14-0.00167932.2693E-08-17.601244066365

21-0.00202772.74009E-08-17.412736495088

28-0.00168812.28122E-08-17.59643836159

35-0.00199752.69929E-08-17.427737046827

42-0.00250333.3829E-08-17.201929560415

49-0.00219352.96421E-08-17.334133735794

56-0.0020062.71085E-08-17.423436888841

63-0.00197022.66238E-08-17.441537560440

70-0.00215832.9166E-08-17.350334286547

77-0.00243963.29671E-08-17.227830333259

84-0.00179942.43159E-08-17.532141125270

91-0.00296154.00201E-08-17.033924987461

98-0.00210162.84001E-08-17.376935211093

105-0.00241733.26659E-08-17.236930612996

112-0.0022052.97966E-08-17.328933560831

119-0.0021022.84057E-08-17.376735204190

Table 3: Sample iiTime (s)Absorbance[MB+]ln[MB+]1/[MB+]

00.143561691.94002E-06-13.1528515457.8

70.113840281.53838E-06-13.3848650033.5

140.088867331.20091E-06-13.6324832702

210.067959359.1837E-07-13.90071088886

280.051946037.01973E-07-14.16941424556

350.038128435.15249E-07-14.47861940809

420.028316283.82652E-07-14.77612613337

490.020689932.79594E-07-15.08993576619

560.014707231.98746E-07-15.43125031539

630.010782421.45708E-07-15.74176863025

700.008073291.09098E-07-16.0319166029

770.006059538.18855E-08-16.317912212170

840.004346425.87354E-08-16.650217025506

910.003521694.75903E-08-16.860621012667

980.002800763.78481E-08-17.089726421386

1050.002508773.39024E-08-17.199829496472

1120.001677152.26642E-08-17.602544122460

1190.001675622.26435E-08-17.603444162832

Table 4: Sample iiiTime (s)Absorbance[MB+]ln[MB+]1/[MB+]

00.131937441.78294E-06-13.2372560871.9

70.117787381.59172E-06-13.3507628250.6

140.105354321.42371E-06-13.4622702391.7

210.093204241.25952E-06-13.5848793955.3

280.082740171.11811E-06-13.7039894366.1

350.073398759.91875E-07-13.82371008192

420.06465258.73682E-07-13.95051144581

490.056210757.59605E-07-14.09051316474

560.049074166.63164E-07-14.22621507922

630.043295415.85073E-07-14.35151709188

700.038209385.16343E-07-14.47651936697

770.033168684.48225E-07-14.6182231021

840.029463363.98154E-07-14.73642511594

910.025536153.45083E-07-14.87952897853

980.023004733.10875E-07-14.98393216729

1050.02012212.7192E-07-15.11783677549

1120.017399692.35131E-07-15.26314252949

1190.016356622.21035E-07-15.32494524163

Table 5: Sample ivTime (s)Absorbance[MB+]ln[MB+]1/[MB+]

00.30963654.18428E-06-12.3842238989.9

70.299641254.04921E-06-12.417246962

140.286000433.86487E-06-12.4636258740.9

210.276209033.73255E-06-12.4984267913

280.265032353.58152E-06-12.5397279211.2

350.254438673.43836E-06-12.5805290836.3

420.245473223.31721E-06-12.6164301458.5

490.23539993.18108E-06-12.6583314358.7

560.225311933.04476E-06-12.7021328433.6

630.215910292.91771E-06-12.7447342734.9

700.206577062.79158E-06-12.7889358219.8

770.196896052.66076E-06-12.8369375832.8

840.187392582.53233E-06-12.8864394892.9

910.179321962.42327E-06-12.9304412665.6

980.170448512.30336E-06-12.9811434148.7

1050.161923162.18815E-06-13.0325457006.9

1120.15413222.08287E-06-13.0818480107.3

1190.146362631.97787E-06-13.1335505593.5

Table 6: Sample iv (replicate)Time (s)Absorbance[MB+]ln[MB+]1/[MB+]

00.283276353.82806E-06-12.4732261229

70.272308653.67985E-06-12.5126271750.5

140.262745383.55061E-06-12.5484281641.5

210.252469933.41176E-06-12.5883293104.2

280.242990423.28365E-06-12.6266304538.8

350.23285763.14672E-06-12.6691317790.8

420.223624653.02195E-06-12.7096330911.6

490.213686882.88766E-06-12.7551346301.1

560.204944272.76952E-06-12.7968361073.8

630.195906482.64738E-06-12.8419377731.3

700.187426252.53279E-06-12.8862394822

770.178211932.40827E-06-12.9366415236

840.170183872.29978E-06-12.9827434823.8

910.162317082.19347E-06-13.03455897.8

980.154596912.08915E-06-13.0788478664.2

1050.146689031.98228E-06-13.1313504468.5

1120.140061531.89272E-06-13.1775528339.2

1190.132936311.79644E-06-13.2297556657.5

Figure 1

Figure 1: Shows the concentration of methylene blue in solution i with respect to timeFigure 2

Figure 2: Shows the natural log concentration of methylene blue in solution i with respect to timeFigure 3

Figure 3: Shows the inverse of the concentration of methylene blue in solution i with respect to timeFigures 2 is the straightest of figures 1-3 for solution i, indicating that the order of reaction with respect to methylene blue is a first-order reaction and this is supported by the linearity shown in the sample (i+v) through to sample iv plot (figure 4). The non-linear sections will be ignored for calculating the linear regressions in order to calculate the kobs.

Figure 4

Figure 4: Shows the natural log concentration of methylene blue in solutions (i+v)-iv with respect to timeTable 7Solution[Initial Solution][Ascorbic Acid in curvette]kobsk

i0.30M0.2830189M-1.52E-02-5.37E-02

ii0.15M0.1415094M-3.99E-02-2.82E-02

iii0.06M0.0566038M-0.0181-3.20E-01

iv0.03M0.0566038M-0.0064-1.13E-01

iv (repl)0.03M0.0283019M-0.0064-2.26E-01

Average-0.199

Figure 5

Figure 5: Shows kobs with respect to the Ascorbic Acid Concentration

Figure 7 shows that if the outlier for solution ii is ignored the kobs is proportional to the concentration of ascorbic acid so its order is 1. Table 7 shows the rate constants calculated from the various concentrations and their average. k=kobs/[ascorbic acid]1

Discussion/ ConclusionThe data obtained in this experiment was not satisfactory. The plots for solutions ii and i+v could not be satisfactorily transformed into straight lines so it was difficult to fit decent linear regression lines. As the rest of the analysis was dependant on the kobs derived from these plots, the rest of the results are considered unreliable, however when these data points were ignored in figure 5, so a conclusion could be reached.Potential sources for the unreliable data could be:-The methylene blue should have been added to the curvette when it was in the machine so there was not a delay in starting the absorption plotting.-Other variables that may not have been controlled carefully enough include the reactant temperature and the solution concentration.

References

AppendixExercises:1. The kobs changed significantly when the concentration of methylene blue changed (-1.52E-02 for 1.3*10-4 M methylene blue and -3.99E-02 for 6.5*10-5M methylene blue. This is what was expected as a change in the initial concentration should change the rate of reaction. The kobs is the slope of the concentration of methylene blue with respect to time i.e. the rate that methylene blue disappears. So if the rate changes the rate methylene is used changes so should the kobs 2)

3)

4) Solution iv0.03M Ascorbic Acid

Stock Solution1.3*10-4M Methylene Blue

Half life is 3828 seconds