The Reaction of a Food Colorant with Sodium Hypochlorite: A Student-Designed Kinetics Experiment
Josefina Arce, Rosa Betancourt, Yamil Rivera, and Joan Pijem University of Puerto Rico, Rio Piedras Campus, Department of Chemistry, P. O. Box 23346, San Juan, Puerto Rico 00931-3346
The kinetics of the reaction of the food colorant FD&C Blue #1 with sodium hypochlorite (Clorox) is described in a student-designed experimental format. In this format, students are guided- by means of questions- to make decisions regarding concentration of reagents, choice of equipment, and actual laboratory procedures to be followed. This format provides an opportunity for students to apply the concepts and skills learned in class and in previous laboratory sessions to a new problem. We have found that this experience helps students gain depth of understanding of all concepts involved. The reaction (with a large excess of NaOCl) is followed with a Spectronic 20 at the Blue #1 colorant lmax of 630 nm. The %T is measured over time and three graphs: A vs time, ln A vs time and 1/A vs time are plotted to find that the second one is linear and thus first order with respect to the Blue #1. When the concentration of NaOCl is reduced to one-half the original value, it is found that the rate is reduced by one-half, indicating that the reaction is first order with respect to NaOCl and second order overall. The rate constant of the reaction is determined from the slope of the curve and the mean obtained by our students is 17 M-1 min-1 at room temperature (about 28° C).
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