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Exp 5 Lab Report - Determination of a reaction rate law...

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Determination of a reaction rate law using the technique of flooding the system along with spectroscopy and the percent of alcohol in a sample using titrations Jubilee Ngo TA: Christine Huang Section: A00 Chemistry 100A at University of California, San Diego 9500 Gilman Drive, La Jolla, California 92093 25 July 2006 Abstract Consisting of two parts, the overall purpose of this experiment was the determination of the rate law of a reaction involving the oxidation of ethanol by dichromate ion in acidic solution and of the percent of alcohol in a sample. In the first part, six runs were carried out in which the system was “flooded” with excess, but known amounts of two of the three reagents (ethanol and sulfuric acid), leaving the concentration of last one (dichromate) to be tracked using spectrophotometry with the pseudo rate law, rate = k obs [Cr 2 O 7 2- ] y . After the λ max value of 440nm was determined, %T readings were taken at 10-second intervals for 5 minutes after the substances were mixed and allowed to react during each run. Then, three plots of the absorbance values (A vs. t, ln A vs. t, and 1/A vs. t) were generated and their corresponding trendlines and average R-squared values indicated that order (y) of the dichromate ion is 1. Afterwards, using the plots and the equation k obs = k [C 2 H 5 OH] x [H + ] z , average x and z values, orders of ethanol and H + respectively, were calculated to be 1.40 ± 0.14 and 2.45 ± 0.21 and rounded to 1 and 2. Based on these values and using the same k obs equation, k values were determined, with the average rate constant with average values of x and y is 0.00066 ± 0.00004 M -3 s -1 and the average rate constant with rounded values of x and y is 0.00111 ± 0.00016 M -3 s -1 . Hence, the rate law turned out to be Rate = (0.00066) [C 2 H 5 OH] 1.40 [Cr 2 O 7 2- ] 1 [H + ] 2.45 or Rate = (0.00111) [C 2 H 5 OH] 1 [Cr 2 O 7 2- ] 1 [H + ] 2 In the second part of the experiment, titrations of FEDS, K 2 Cr 2 O 7 , and ethanol were carried out in order to calculate the concentration of unknown ethanol (unknown #54), which is 0.015 ± 0.002 M and the average of percent weight by volume of ethanol in the sample is 0.07 ± 0.01 % (w/v). Result Part A. Kinetics In Table 5 (see the Appendix), the volumes of each substance and their concentrations used in each run are recorded. This table also includes the ratios of sulfuric acid to potassium dichromate and ethanol (alcohol) to potassium dichromate, which are huge values because these runs are performed when the system is “flooded” with two of the three reactants (sulfuric acid and ethanol) in order to focus on only one (potassium dichromate). The plot of the absorbance spectrum of the dichromate ion is shown below in Figure 1.
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Figure 1. Absorbance of dichromate ion at different wavelengths Absorbance vs. Wavelength (λ) 0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350 400 410 420 430 440 450 460 470 480 490 500 Wavelength (λ), nm Absorbance Based on this graph, the wavelength at which the absorbance of the dichromate ion is greatest, or λ max , is 440 nm. (See Table 6 in the Appendix for the data). For all 6 runs of the experiment in Part A, the data table and plots for zero order, first order, and second order reactions are made. Each plot also has
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