Rate Law Determination of the Crystal Violet Reaction, Lab Reports of Chemistry

Download Rate Law Determination of the Crystal Violet Reaction and more Lab Reports Chemistry in PDF only on Docsity! Computer 30 Chemistry with Vernier 30 - 1 Rate Law Determination of the Crystal Violet Reaction In this experiment, you will observe the reaction between crystal violet and sodium hydroxide. One objective is to study the relationship between concentration of crystal violet and the time elapsed during the reaction. The equation for the reaction is shown here: CC OH OH –+ N(CH ) N(CH ) N(CH ) + N(CH ) N(CH ) N(CH )3 3 3 3 3 32 2 2 2 2 2 A simplified (and less intimidating!) version of the equation is: CV+ + OH–  →  CVOH (crystal violet) (hydroxide) The rate law for this reaction is in the form: rate = k[CV+]m[OH–]n, where k is the rate constant for the reaction, m is the order with respect to crystal violet (CV+), and n is the order with respect to the hydroxide ion. Since the hydroxide ion concentration is more than 1000 times as large as the concentration of crystal violet, [OH-] will not change appreciably during this experiment. Thus, you will find the order with respect to crystal violet (m), but not the order with respect to hydroxide (n). As the reaction proceeds, a violet-colored reactant will be slowly changing to a colorless product. Using the green (565 nm) light source of a computer-interfaced Colorimeter, you will monitor the absorbance of the crystal violet solution with time. We will assume that absorbance is proportional to the concentration of crystal violet (Beer’s law). Absorbance will be used in place of concentration in plotting the following three graphs: • Absorbance vs. time: A linear plot indicates a zero order reaction (k = –slope). • ln Absorbance vs. time: A linear plot indicates a first order reaction (k = –slope). • 1/Absorbance vs. time: A linear plot indicates a second order reaction (k = slope). Once the order with respect to crystal violet has been determined, you will also be finding the rate constant, k, and the half-life for this reaction. Ev al ua tio n co py Computer 30 30 - 2 Chemistry with Vernier OBJECTIVES In this experiment, you will • Observe the reaction between crystal violet and sodium hydroxide. • Use a Colorimeter to monitor the absorbance of the crystal violet solution with time. • Graph Absorbance vs. time, ln Absorbance vs. time, and 1/Absorbance vs. time. • Determine the order of the reaction. • Determine the rate constant, k, and the half-life for this reaction. MATERIALS computer 0.020 M NaOH Vernier computer interface 2.0 × 10–5 M crystal violet Logger Pro distilled water Vernier Colorimeter stirring rod one plastic cuvette two 10 mL graduated cylinders 250 mL beaker PROCEDURE 1. Obtain and wear goggles. 2. Use a 10 mL graduated cylinder to obtain 10.0 mL of 0.020 M NaOH solution. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing. Use another 10 mL graduated cylinder to obtain 10.0 mL of 2.0 × 10–5 M crystal violet solution. CAUTION: Crystal violet is a biological stain. Avoid spilling it on your skin or clothing. 3. Connect the Colorimeter to the computer interface. Prepare the computer for data collection by opening the file “30 Rate Crystal Violet” from the Chemistry with Vernier folder of Logger Pro. 4. Prepare a blank by filling an empty cuvette 3/4 full with water. Seal the cuvette with a lid. To correctly use a colorimeter cuvette, remember: • All cuvettes should be wiped clean and dry on the outside with a tissue. • Handle cuvettes only by the top edge of the ribbed sides. • All solutions should be free of bubbles. • Always position the cuvette with its reference mark facing toward the white reference mark at the top of the cuvette slot on the Colorimeter. 5. Calibrate the Colorimeter. a. Open the Colorimeter lid. b. Holding the cuvette by the upper edges, place it in the cuvette slot of the Colorimeter. Close the lid. c. If your Colorimeter has a CAL button, Press the < or > button on the Colorimeter to select a wavelength of 565 nm (Green) for this experiment. Press the CAL button until the red LED begins to flash. Then release the CAL button. When the LED stops flashing, the calibration is complete. 6. To initiate the reaction, simultaneously pour the 10 mL portions of crystal violet and sodium hydroxide into a 250 mL beaker and stir the reaction mixture with a stirring rod. Click . Empty the water from the cuvette. Rinse the cuvette twice with ~1 mL amounts of the reaction mixture and then fill it 3/4 full. Do not put the cuvette in the Colorimeter yet. To