The Kinetics of Crystal Violet Fading Advanced Inquiry Lab Kit for AP* Chemistry requires using spectroscopy methods and graphical analysis to determine the rate law for the reaction of crystal violet with sodium hydroxide.
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Big Idea 4, Investigation 11, Primary Learning Objective 4.2. One of the most important considerations in a “green chemistry” analysis is how fast a chemical degrades. Crystal violet is a common, beautiful purple dye. In strongly basic solutions, the bright color of the dye slowly fades and the solution becomes colorless. The kinetics of this “fading” reaction can be analyzed by measuring the color intensity or absorbance of the solution versus time to determine the rate law. In this advanced inquiry lab, students use spectroscopy and graphical analysis to determine the rate law for the color-fading reaction of crystal violet with sodium hydroxide. Students begin by constructing a calibration curve of absorbance versus concentration for crystal violet. A series of known or standard solutions is prepared from a stock solution of crystal violet and the absorbance of each solution is measured at an optimum wavelength. A Beer’s law plot of absorbance as a function of concentration may be used to calculate the concentration of an “unknown” solution of the dye in a rate law experiment. This procedure provides a model for the guided-inquiry portion of the lab, during which students design experiments to determine the order of reaction with respect to both crystal violet and sodium hydroxide. Two dyes with similar structures, malachite green and phenolphthalein, are provided for optional extension or cooperative class inquiry studies. Complete for 24 students working in pairs. A spectrophotometer or colorimeter as well as common laboratory equipment is required and available separately.