Kinetics of Crystal Violet Fading

Product Details

Crystal violet is a common, beautiful purple dye. In strongly basic solutions, the bright color of the dye slowly fades and the solution becomes colorless. The kinetics of this “fading” reaction can be analyzed by measuring the color intensity or absorbance of the solution versus time to determine the rate law. In this advanced-inquiry lab, students use spectroscopy and graphical analysis to determine the rate law for the color-fading reaction of crystal violet with sodium hydroxide.

First, students construct a calibration curve of absorbance versus concentration for crystal violet using a series of known or standard solutions. This procedure provides a model for students to design experi­ments to determine the order of reaction with respect to both crystal violet and sodium hydroxide. Two dyes with similar structures, malachite green and phenolphthalein, are provided for optional extension or cooperative class inquiry studies.

Complete for 24 students working in pairs. A spectrophotometer or colorimeter as well as common laboratory equipment is required and available separately. A refill kit for the Kinetics of Crystal Violet Fading Advanced Inquiry Laboratory Kit is also available. A version of this lab is available as a Wet/Dry Advanced Inquiry Laboratory Kit for One Period (Catalog No. AP9475).

2024 CED Alignment:  

Unit 5 - Kinetics  

Topic 5.2 - Introduction to Rate Law  

Topic 5.3 - Concentration Changes Over Time 

Description of the Lab 

Students monitor the fading of a purple dye using visible spectroscopy. By analyzing concentration vs. time data, they determine the rate law and order of the reaction. The lab reinforces graphing, interpretation, and justification grounded in kinetic theory. 

Learning Objective 

• LO 5.2.A - Represent experimental data with a consistent rate expression. 

• LO 5.3.A – Identify the rate law expression of a chemical reaction using data that show how the concentrations of reaction species change over time. 

Skills Students Will Learn 

• 3.A – Represent chemical phenomena using appropriate graphing techniques, including correct scale and units. 
  In this lab: Students generate and compare linearized graphs to determine reaction order. 

• 5.B – Identify an appropriate theory, definition, or mathematical relationship to solve a problem. 
  In this lab: Students apply integrated rate laws to determine which mathematical model fits their data. 

• 6.F – Explain the connection between experimental results and chemical concepts, processes, or theories. 
  In this lab: Students interpret the rate law using molecular-level reasoning about collisions and kinetics.