 Vernier Software & Technology

Rate Law Determination of the Crystal Violet Reaction

Introduction

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: ${\text{C}}{{\text{V}}^{\text{ + }}}{\text{(aq) + O}}{{\text{H}}^{\text{ - }}}{\text{(aq)}} \to {\text{CVOH(aq)}}$

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.

Objectives

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 features the following Vernier sensors and equipment.

Option 4

You may also need an interface and software for data collection. What do I need for data collection?

Chemistry with Vernier

See other experiments from the lab book.

 1 Endothermic and Exothermic Reactions 2 Freezing and Melting of Water 3 Another Look at Freezing Temperature 4 Heat of Fusion of Ice 5 Find the Relationship: An Exercise in Graphing Analysis 6 Boyle's Law: Pressure-Volume Relationship in Gases 7 Pressure-Temperature Relationship in Gases 8 Fractional Distillation 9 Evaporation and Intermolecular Attractions 10 Vapor Pressure of Liquids 11 Determining the Concentration of a Solution: Beer's Law 12 Effect of Temperature on Solubility of a Salt 13 Properties of Solutions: Electrolytes and Non-Electrolytes 14 Conductivity of Solutions: The Effect of Concentration 15 Using Freezing Point Depression to Find Molecular Weight 16 Energy Content of Foods 17 Energy Content of Fuels 18 Additivity of Heats of Reaction: Hess's Law 19 Heat of Combustion: Magnesium 20 Chemical Equilibrium: Finding a Constant, Kc 21 Household Acids and Bases 22 Acid Rain 23 Titration Curves of Strong and Weak Acids and Bases 24 Acid-Base Titration 25 Titration of a Diprotic Acid: Identifying an Unknown 26 Using Conductivity to Find an Equivalence Point 27 Acid Dissociation Constant, Ka 28 Establishing a Table of Reduction Potentials: Micro-Voltaic Cells 29 Lead Storage Batteries 30 Rate Law Determination of the Crystal Violet Reaction 31 Time-Release Vitamin C Tablets 32 The Buffer in Lemonade 33 Determining the Free Chlorine Content of Swimming Pool Water 34 Determining the Quantity of Iron in a Vitamin Tablet 35 Determining the Phosphoric Acid Content in Soft Drinks 36 Microscale Acid-Base Titration

Experiment 30 from Chemistry with Vernier Lab Book Included in the Lab Book

Vernier lab books include word-processing files of the student instructions, essential teacher information, suggested answers, sample data and graphs, and more.