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: A simplified (and less intimidating!) version of the equation is: CV+ + OH-   ® CVOH 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  computer-interfaced spectrophotometer, 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 = e [CV+] Therefore, 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. MATERIALS Solutions of crystal violet and NaOH, spectrophotometer, cuvet, two small (10 mL) beakers, 25 mL beaker. PROCEDURE   Each student in a group will measure kinetics of the reaction at different concentration of sodium hydroxide. Before starting kinetic measurements, you should measure the spectrum of the crystal violet in the aqueous solution and establish the wavelength suitable for absorption measurements. Of course, this wavelength is the same for the whole class, so there is no reason to establish it once and once again. 1.  Prepare spectrophotometer for kinetics measurements (separate instructions will be provided for each group). 2. Pipet  5.0 mL of  NaOH solution into 10 mL beaker. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing.  Pipet  5.00 mL of 2.0 X 10-5 M crystal violet solution. CAUTION: Crystal violet is a biological stain. Avoid spilling it on your skin or clothing.  3. To initiate the reaction, simultaneously pour the 5-mL portions of crystal violet and sodium hydroxide into a 25-mL beaker and stir the reaction mixture.  Rinse the cuvette  with ~1-mL amounts of the reaction mixture and then fill it 3/4 full. Place the cuvette in the cuvette slot of the spectrophotometer, and take click on "Collect" button. THe program will collect the absorbance data for you. 4. Analyze the data graphically to decide if the reaction is zero, first, or second order with respect to crystal violet. You will see three windows on display. • Zero Order: If the current graph of absorbance vs. time is linear, the reaction is zero order.  • First Order: To see if the reaction is first order, it is necessary to plot a graph of the natural logarithm (ln) of absorbance vs. time. If this plot is linear, the reaction is first order. • Second Order: To see if the reaction is second order, plot a graph of the reciprocal of absorbance vs. time. If this plot is linear, the reaction is second order. PROCESSING THE DATA 1. Was the reaction zero, first, or second order, with respect to the concentration of crystal violet? Explain. 2. Calculate the rate constant, k, using the slope of the linear regression line for your linear curve (k = –slope for zero and first order and k = slope for second order). Be sure to include correct units for the rate constant. Note: This constant is sometimes referred to as the pseudo rate constant, because it does not take into account the effect of the other reactant, OH-. 3.Estimate the half-life of the reaction. 4. Compare the reaction orders a nd rate constants received with your group for various concentrations of NaOH. 5. Write the correct rate law expression for the reaction