Rate Law Determination of Crystal Violet

Performed by: Caleb Winscott

Partner: Hunter Lowe

Instructor: Dr. Irene Zegar

        Date Performed: February 24, 2017

Date Report Submitted: March 10, 2017

Purpose:

This report discusses an experiment to study and observe the reaction of Crystal violet and sodium hydroxide and to determine the rate law for the reaction. To determine the rate law, you must use the formula: rate=k[CV+]m[OH-]n where k is the rate constant, [CV+] is the concentration of Crystal violet, m is the order with respect to Crystal violet, [OH-] is the concentration of hydroxide molecules, and n is the order with respect to hydroxide. Use a pseudo rate method to determine the order with respect to Crystal violet.

Principles:

The chemical equation (CV+) + (OH-) → (CVOH) has the rate law of rate=k[CV+]m[OH-]n, where k is the rate constant for the reaction, m is the order with respect to Crystal violet, and n is the order with respect to the hydroxide ion. Since the hydroxide concentration is so much larger than the concentration of Crystal violet, the concentration of hydroxide at any given time will barely change, therefore, we will use the pseudo rate method to determine the order with respect to Crystal violet. To determine the order with respect to hydroxide, we have to plug in all the information at the end of the experiment. K is determined by using the Arrhenius equation K=Ae–Ea/RT. A is the Arrhenius constant, Ea is the activation energy, R is the gas constant, and T is temperature. We will be using a colorimeter to determine the absorbance rate of Crystal violet. Knowing the proportionality constant we can determine the concentration of Crystal violet. Using that information, we can determine the correct rate using [CV+] vs. Time for a zero order, ln[CV+] vs. Time for a first order, and 1/[CV+] for a second order reaction.

Procedure:

 Part 1

Using the provided 20.0 Micromolar stock solution of Crystal violet, and test tubes from your drawer, prepare 10 ml CV+ solution with concentrations of 0.0, 1.0, 2.0, 4.0, 6.0, 8.0, and 10.0 mM. Label the test tubes. Then, prepare a blank by filling an empty cuvette ¾ full with distilled water.

Use the LabQuest to collect and record data for each of the concentrations.

Part 2:

Tap the Meter tab found on the top left corner of the LabQuest, then choose New from the File menu. Next, tap the Mode field and choose Time Based. Type 0.5 sample/s for Rate and tap done. Type 2s/sample for the interval and tap done. Then, type 600 s for the duration of the run. With these settings, data collection will last for 10 minutes. Tap okay

To initiate the reaction, simultaneously pour 9.0 ml of 20 mM CV+ and 1 mL of 0.2M NaOH into a beaker and stir the reaction mixture with a stirring rod. Empty the solution from the cuvette. Rinse the cuvette with a bit of the solution to wash away any foreign particles. Then, tap collect to record data. Repeat for .1 M NaOH. Analyze and plot data.

Results:

We can determine from the experiment the absorbance rates of Crystal violet at different concentrations. In order to process the data, we have to know the concentration of Crystal violet. To find that, you divide the absorbance value by the slope of the standard curve. Then, using that data, you can find out the second and zero rate laws.

Part 1 

0.2M

Figure 1: Standard curve of absorbance of Crystal Violet at 565 nm as a function of Crystal Violet concentration.

[pic 1] [pic 2]

From this graph, we can determine the slope of the equation of the line which represents the molar absorbtivity of Crystal violet. Since the y-intercept (R2) is within 0.005 we know that this data is an accurate fit.

Part 2:

Figure 2: The concentration of Crystal violet as a function of time

[pic 3]

From this graph, we can determine whether or not the order with respect to Crystal violet is a zero order reaction, with k having a negative slope. Due to the fact that the y-intercept, in this case 0.9888, is not the closest result to 1.00, we can conclude that the order with respect to Crystal violet is not a zero order reaction.

Figure 3: One divided by the concentration of Crystal violet as a function of time.

[pic 4]

Using this graph, we can determine whether or not the order with respect to Crystal violet is a second order reaction with k having a negative slope. Since the R2 value is not the closest to 1.00, we can conclude that the order with respect to Crystal violet is not a second order reaction.

Figure 4: The natural logarithm of the concentration of Crystal violet as a function of time.

[pic 5]

From this graph, we can determine whether or not the order with respect to Crystal violet is a first order reaction. Due to the fact that the y-intercept (R2) is the closest out of the calculated results to 1.00, we can conclude that the order with respect to Crystal violet is a first order reaction.

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