Reaction of Crystal Violet with NaOH - MicroLab · 12 REACTION OF CRYSTAL VIOLET WITH NaOH: A...

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Reaction of Crystal Violet with NaOHA Kinetic Study with Ea Calculation

INSTRUCTOR RESOURCES

The CCLI Initiative

Modified by Dale A. Hammond, PhD, Brigham Young University Hawaii

Learning Objectives

• study the rate of reaction of crystal violet with NaOH using the MicroLAB interface colorimeter.

• determine the order of reaction with respect to both reactants.

• calculate the value of the rate constant for the reaction at room temperature.

• Determine the activation energy, Ea, for the reaction from supplied data. (Students will not take thisdata since most general chemistry labs do not have the necessary equipment for doing theseexperiments.)

Procedure Overview

• the kinetics of the decomposition of crystal violet is studied using the MicroLAB colorimeter.

• the experimental data are plotted to determine the order of reaction in crystal violet.

• the reaction is studied at two different concentrations of hydroxide ion to determine the order of thereaction in hydroxide.

• students use the information provided in the table of data to calculate the average k, its standarddeviation, then plot lnk vs. 1/T (kelvin) to determine Ea for the reaction.

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Name ______________________________ Section _____________ Date ________________

REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY

Report Sheet

1. What is plotted on the y-axis for the graph which is linear?

2. What is the value of k' for the most linear plot? Include units and use appropriate significantfigures.

3. Attach a printout of your labeled graph and data points to this report sheet.

Determining the order of the reaction in NaOH

4. What is the value of k' when using 0.10 M NaOH? Include units and use appropriate significantfigures.

5. Determine the ratio of the two k' values. Round to the nearest integer.


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Report Sheet (page 2)

7. Calculate k using each k' value. Adjust [OH ] for dilution.

average8. Calculate k .

9. Calculate the molar absorptivity, ,, using the data from the first experimental run.

Summary

Species Order k' k' k k

Crystal Violet

OH --------------- ----------------- --------------- ----------------

averagek = (include units)

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Suggested Answers to Questions

Calculation of Ea for the reaction

1. Using the data in the following table, calculate the mean and standard deviation for each set oftemperature values.

Run / Temp. 25 °C 35 °C 45 °C 55 °C

Run 1 -0.00175 -0.00368 -0.00431 -0.00537

Run 2 -0.00160 -0.00387 -0.00437 -0.00727

Run 3 -0.00166 -0.00374 -0.00438 -0.00961

Mean value

Standard dev.

2. Examine the three graphs for the 55 °C data. How reliable is the data for the 3 run at that rd

temperature? Explain your answer.

3. Should the 55 °C data be used for determining the activation energy for this reaction? Why or why not?

4. Calculate the activation energy for this reaction from the equations given in the experimental write- up.You must show your work for the activation energy calculation.

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Questions to Answer

5. The following data were collected for a reaction of the type: A ==> Products

[A] Time (seconds)

1.00 x 10 M 0-3

6.07 x 10 M 5-4

3.68 x 10-4 M 10

2.23 x 10-4 M 15

1.35 x 10-4 M 20

a. Just by looking at the data (do not make a plot), determine whether the reaction is zero order in A.What must be true of the data for the reaction to be zero order in A?

b. Compute the natural logarithm for each of the concentrations and (without making a plot) determinewhether the reaction is first order in A.

c. Make the appropriate plot and calculate the value of k for the reaction.

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REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETIC STUDY

Tips and Traps

1. Crystal violet stains, so appropriate care should be taken in its handling.

2. NaOH can be used to remove the crystal violet stain from the solution cells.

4. One NaOH solution concentration must be exactly double the other. We recommend preparing 0.100M NaOH and diluting a portion exactly in half to obtain this ratio.

5. The NaOH is best dispensed using a calibrated (1.00 mL) plastic pipet that has a squeeze bulb at oneend.

6. Add the NaOH rapidly, cap the vial, rotate twice, place in the colorimeter chamber and begin theprogram immediately.

7. The colorimeter chamber should be covered with the canister.

8. Be sure to position the solution cell in the colorimeter in exactly the same way after measuring theblank.

9. Crystal violet is most easily dispensed by buret.

10. The best temperature for experimental conditions is 20 - 25 C.

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REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETIC STUDY

Suggested Answers for Report Sheet

1. What is plotted on the y-axis for the graph which is linear?

The linear graph of formed using ln(Abs) on the y-axis.

2. What is the value of k' for the most linear plot? Include units and use appropriate significant figures.

The value of k’ for the [OH ] = 0.050 plot is -0.0477 mol/L!

Experimental values may differ here as a function of the actual CV concentration and colorimeterparameters.


See screen image following.

Determining the order of the reaction in NaOH

4. What is the value of k' when using 0.10 M NaOH? Include units and use appropriate significantfigures.

The value of k’ for the [OH ] = 0.10 plot is -0.0936 mol/L!

Experimental values may differ here as a function of the actual CV concentration and colorimeterparameters.

5. Determine the ratio of the two k' values. Round to the nearest integer.

-0.0936/-0.0477 = 1.96

The actual ratio should be very close to 2.00, indicating first order in [OH ], since doubling the!

[OH ] concentration doubled the rate of the second reaction. For the reaction to be second order,!

the rate should have quadrupled between experiments one and two.


See screen image following.

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Suggested Answers for Report Sheet (page 2)

7. Calculate k using each k' value. Adjust [OH ] for dilution.!

1 ml | 0.05 mmol | = 0.005 M 1 ml | 0.10 mmol | = 0.010 M10 ml |1 ml 10 ml |1 ml

k = k’/[OH ] ! 2

k = 0.0477 / (0.0050) = 1908 k = 0.0936 / (0.010) = 9362 2

average8. Calculate k .

1908 + 936 = 14222

9. Calculate the molar absorptivity, ,, using the data from the first experimental run.

, = A/bc = 0.7173 / 2.54 / 1.5 x 10 = 18827 L/cm/Mole-5

Summary

Species Order k' k' k k

Crystal Violet First 0.0477 0.0936 1908 936

OH Second --------------- ----------------- --------------- ----------------

averagek = 1422 L / mol sec (include units)2 2

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Suggested Answers for Report Sheet (page 3)

Calculation of Ea for the reaction

1. Using the data in the following table, calculate the mean and standard deviation for each set oftemperature values.

Run / Temp. 25 °C 35 °C 45 °C 55 °C

Run 1 -0.00175 -0.00368 -0.00431 -0.00537

Run 2 -0.00160 -0.00387 -0.00437 -0.00727

Run 3 -0.00166 -0.00374 -0.00438 -0.00961

Mean value -0.00167 -0.00376 -0.00435 -0.00742

Standard dev. 7.55 X 10 9.71 X 10 3.79 X 10 2.12 X 10-5 -5 -5 -3

2. Examine the three graphs for the 55 °C data. How reliable is the data for the 3 run at that rd

temperature? Explain your answer.

The data for the 3 run does not appear to be reliable because the original data does not follow ard

smooth curve, and the linear data does not plot on the regression line.

3. Should the 55 °C data be used for determining the activation energy for this reaction? Why or why not?

The 55 °C data should not be used in determining the activation energy because the standarddeviation (2.12 X 10 ) is on the same order of magnitude as the mean value (7.42 X 10 ) indicating-3 -3

significant error in those three measurements. However, as seen on the graphs of the data, the valuefor Ea differs very little when the 55 °C data point is included due to the spread of the data.

4. Calculate the activation energy for this reaction from the equations given in the experimental write- up.You must show your work for the activation energy calculation.

See attached graph plotting the data for 25, 35, and 45 °C data and the data for all for temperatures.

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Suggested Answers to Questions

1. In Figure 1, crystal violet is shown as a positive ion of charge 1+. Which atom in the crystal violetcation has a formal charge of +1? Explain thoroughly how you arrived at your answer.

The 1+ charge is located on the N atom that has four bonds to it.

2. The following data were collected for a reaction of the type: A ==> Products

[A] Time (seconds)

1.00 x 10-3 M 0

6.07 x 10-4 M 5

3.68 x 10-4 M 10

2.23 x 10-4 M 15

1.35 x 10-4 M 20

a. Just by looking at the data (do not make a plot), determine whether the reaction is zero order inA. What must be true of the data for the reaction to be zero order in A?

The reaction is not zero order in A. For the reaction to be zero order the changesin [A] must all be the same (since the time increments are all equal).

b. Compute the natural logarithm for each of the concentrations and (without making a plot)determine whether the reaction is first order in A.

The reaction is first order in A because the changes in ln A are the same (-0.49 units) for eachof the 5 second time increments.

c. Make the appropriateplot and calculate thevalue of k for thereaction.

The value of k is 0.10s-1.

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Sample Data

Sample data screen for the MicroLAB Crystal Violet Kinetics experiment.

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Laboratory Preparation (per student station)

Equipment

• MicroLAB interfaces’ colorimeter

• glass a solution cell (cuvette), 2.54 cm path

• ring stands

• buret clamp

• buret with at least 10 ml

• calibrated 1.0 ml plastic dropper

• 50 ml beaker

Supplies

• paper towels

• KimWipes

Chemicals

Actual quantities needed are given below. A 50% excess is recommended.

• 1.50 x 10 M crystal violet solution (18.00 mL)-5

• 0.050 M NaOH solution (1.0 mL)

• 0.10 M NaOH solution (1.0 mL)

Safety and Disposal

• crystal violet may cause skin and eye irritation and the NaOH solutions are corrosive.

• Make sure students wear goggles at all times

• all resulting solutions may be flushed down the drain with plenty of water

• have students wash hands with soap and water before leaving the lab.

Reaction of Crystal Violet with NaOH - MicroLab · 12 REACTION OF CRYSTAL VIOLET WITH NaOH: A...

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