AP Inquiry Lab 11

What is the Rate Law of the Fading Crystal Violet Reaction Using Beer’s Law?

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
(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).

You will be using a colorimeter for this lab. A colorimeter shines a light through the solution and checks how much light is absorbed by the solution. 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. Absorbance is proportional to the concentration of crystal violet (Beer’s law). Absorbance will be used in place of concentration in plotting the graphs.

Beer’s law is A = abc

where A = absorbance, a = molar absorptivity constant, b = path length, and c = concetration

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.

PreLab

Add this lab to your table of contents

Write a purpose for this lab

Create a table of reagents

Sketch a graph of concentration vs time, ln concentration vs. time, and 1/concentration vs. time for a zero, first and second order reaction.

Sketch a graph of the [CV+] and the [CVOH] over time during this reaction, reaction and write what you should visually see due this change in concentrations.

MATERIALS