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Unformatted text preview: 1 Nick Benzschawel Sarah Weinreis Chem 563 Sec 6 4/25/09 Experiment 91 Kinetics of Reduction of Toluidine Blue by Sulfite Abstract The effect of the ionic strength on the rate of reaction of toluidine blue with sodium sulfite was monitored and it was observed that as concentration of sulfite increased, k 1 increased. Further, it was observed that as concentration of NaCl increased, k 1 decreased. In this experiment, Debye Hckle theory was employed to get the A parameter, which was determined to be 0.3740.031 (L/mol) 1/2 . Specifically, a plot of log k 2 versus I 1/2 was used to calculate this parameter. Also from this plot, the rate constant at infinite dilution, 2 k , was calculated to be 32.21(1.70)-1-1-2 s M 10 . Introduction The purpose of this experiment was to investigate reaction rates as a function of reactant concentration, ionic strength, temperature and solvent conditions in order to better understand activated- complex theory, Debye-Hckel theory and the kinetic salt effect. To do this, various reactions between toluidine blue and sodium sulfite were carried out and either the concentration of sodium sulfite or the ionic strength was changed, but never both during the same reaction. After the reactions were initiated the progress of a particular reaction was monitored by taking absorbance measurements using the Ocean Optic Spectrophotometers. The overall reaction between toluidine blue and sodium sulfite is: H SO TBH O H SO TB 2 4 2 2 3 (1) The rate law that describes the overall reaction from Equation 1 is written as: ] ][SO [TB k R 2 3 2 (2) If the concentration of 2 3 SO is kept much larger than the concentration of TB , then the rate R can be expressed in terms of k 1 , the first order rate constant: ] [TB k dt ] d[TB R 1 (3) ] [SO k k 2 3 2 1 (4) Integration of Equation 3 results in a useful relationship between the measurement of [TB + ] at various times throughout the reaction and the first order rate constant k 1 : 2 t k ] [TB ] [TB ln 1 O (5) t time of the reaction The first order rate constant k 1 can be determined by measuring the absorbance of the reaction mixture of toluidine blue and sodium sulfite vs. time. During the reaction of toluidine blue and sodium sulfite, a transition state complex designated as ] SO [TB 3 is assumed to be in equilibrium with the pure reactants since the formation of the transition state is the slow step (rate limiting step). The modified overall reaction (taken from Equation 1) that includes the activated complex of the transition state is: H SO TBH ] SO [TB SO TB 2 4 O H 3 2 3 2 (6) Since the formation of the transition state is in equilibrium, the rates of formation of the transition state and breakdown back to reactants are equal....
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