Rate Law Determination of the Crystal Violet Reaction

Introduction

The purpose of this experiment was to determine if the reaction of crystal violet with sodium hydroxide was a first order, second order, or third order reaction. This was achieved by the measurement of the absorbance of two solutions of sodium hydroxide and crystal violet of different concentrations over a length of time. The absorbance and time data was plotted and analyzed to find which order best linearized the data. 

Procedure

Two 10 mL graduated cylinders were used to obtain 10 mL of 0.020 M sodium hydroxide and 10 mL of 2.0 x 10^-5 M crystal violet solution. The two solutions were combined in a beaker and stirred while data collection was started on Logger Pro. After three minutes of stirring the solution some of it was poured into a cuvette and placed into a calibrated colorimeter. The absorbance versus time data was collected every minute for twenty minutes. These steps were repeated using a dilute solution of 5.0 mL crystal violet, 5.0 mL water, and 10 mL sodium hydroxide. The data was exported into Microsoft Excel and was plotted and analyzed. The kinetic rate constant was was determined from the slope of the trendline of the plot that best linearized the data, then the half-life was determined.

Detailed procedures can be found in reference 1.

Results 

In Figures 1, 2, and 3 the blue data is of the first crystal violet solution, and the red data is the diluted solution. Absorbance versus time for both concentrations of crystal violet was plotted in Figure 1 to determine if the reaction was 0^th order. 

Figure 1. Absorbance versus time

The data sets of the natural log of absorbance versus time for both concentrations of crystal violet were plotted in Figure 2 to determine if the reaction was 1^st order. The data sets plotted are linear, which means that it was a 1^st order reaction.

Figure 2. Natural log of absorbance versus time

Figure 3 shows 1/absorbance versus time for both concentration of crystal violet, which was plotted to determine if the reaction was 2^nd order. The data is not linear so it is not a 2^nd order reaction.

Figure 3. 1/absorbance versus time 

The rate constant was found to be to be 0.0381 1/min.  The half-life was calculated to be 18.2 min.

Discussion

The plot that best linearized the data was the natural log of absorbance versus time, which means that it was a 1^st order reaction. The rate constant is not dependent of the concentration of crystal violet, because the rate constant is always constant. The rate constant was found to be to be 0.0381 1/min.  The half-life was calculated to be 18.2 min.

References

1. General Chemistry Experiments: A Manual for Chemistry 204, 205, and 206, Department of Chemistry, Southern Oregon University: Ashland, OR, 2010