Rate Law Determination of the Crystal Violet Reaction
Recommended for grades 9–12.
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. A simplified version of the equation is:
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. Because 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. You will measure the color change with a Vernier Colorimeter or a Vernier Spectrometer. The crystal violet solution used in this experiment has a violet color, of course, thus the Colorimeter users will be instructed to use the 565 nm (green) LED. Spectrometer users will determine an appropriate wavelength based on the absorbance spectrum of the solution. 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.
In this experiment, you will
Observe the reaction between crystal violet and sodium hydroxide.
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.
Sensors and Equipment
This experiment requires each of the following Vernier sensors and equipment (unless otherwise noted):
Step-by-step instructions for computer-based data collection
List of materials and equipment
Note: The experiment preview of the computer edition does not include essential teacher information, safety tips, or sample data. Instructions for Logger Pro and other software (such as LabQuest App or TI handheld software, where available) are on the CD that accompanies the book. We strongly recommend that you purchase the book before performing experiments.