DeterminationofthePseudoRateConstantforthe：的伪速率常数的测定.docVIP

Determination of the Pseudo Rate Constant for the Reduction Reaction of Methylene Blue David Mork, Concordia College INTRODUCTION: A schematic representation of the chemical reaction of methylene blue and ascorbic acid is shown below. Methylene blue (MB+) is reduced to leucomethylene blue (LB+) by ascorbic acid (H2A). Ascorbic acid is also called Vitamin C. Ascorbic acid is oxidized to dehydroascorbic acid (D). We can symbolize this reaction as MB+ + H2A ( LB+ + D. The mechanism for this reaction has never been fully investigated, but the kinetics of the reaction are easy to follow. The dark blue of methylene blue disappears as the reaction progresses. The absorbance of light decreases as the reaction goes to completion. You will record absorbance versus time using the Ocean Optics spectrophotometer and computer program, and you will analyze your results using Excel. You and your partner will be finding the pseudo rate constant,(k’), for methylene blue. The rate law for this reaction has the form Rate = k [MB+]m{H2A]n[HCl]p The concentration of HCl and ascorbic acid are much higher than the concentration of methylene blue. Over the length of our experiment, these two concentrations will not change and so we can treat them as constants. The rate law becomes Rate = k’ [MB+]m where k’ = k[H2A]n[HCl]p. Because we have not studied the kinetics of hydrochloric acid or ascorbic acid, we will not be able to write a complete rate law for this reaction or determine the true rate constant, k. EXPERIMENT: You will follow the change in absorbance (which correlates to concentration) over time for two different concentrations of methylene blue. If the spectrophotometer is busy, perform one experiment and analyze your data in a computer lab. 0.10 M HCl 0.030 M Ascorbic Acid 1.0 x 10-4 M Methylene blue 1 1.5 mL 1.0 mL 0.50 mL 2 1.5 mL 1.0 mL 0.25 mL You may not need to set up the computer program unless you are the first user. Regardless, you should verif