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Chemistry 122
REACTION OF CRYSTAL VIOLET WITH NaOH:
A KINETIC STUDY
Revised 6/13/
Donald King and John Weyh
Western Washington University
LEARNING OBJECTIVES
The objectives of this experiment are to...
- study the reaction rate of crystal violet with NaOH using the Vernier LabPro interface and colorimeter.
- determine the reaction order with respect to each of the reactants.
- calculate the room temperature rate constant for the reaction.
BACKGROUND
Reaction Chemistry
Chemical kinetics is the study of reaction rates. In this experiment, the kinetics of the reaction between crystal violet, C 25 H 30 N 3 Cl, and NaOH will be studied. The LabPro colorimeter will be used to monitor the crystal violet concentration as a function of time. The net ionic equation of the reaction, including reactant and product structures and the reaction stoichiometry, is shown in Figure 1 on the next page.
All of the reactants and products shown in Figure 1 are colorless except for crystal violet which has an intense violet color. Thus, during the course of the reaction, the reaction mixture color becomes less and less intense, ultimately becoming colorless when all of the crystal violet has been consumed.
The crystal violet color is due to the extensive system of alternating single and double bonds which extends over all three benzene rings and the central carbon atom. This alternation of double and single bonding is termed conjugation , and molecules which have extensive conjugation are usually highly colored. Trace the conjugation in the crystal violet structure (Figure 1) and note that in the reaction product the central carbon-carbon double bond has been broken with the addition of the hydroxide ion, OH–. The reaction destroys the conjugation between the three rings, and hence, the product is colorless.
Figure 1. Net Ionic Equation of the Reaction between Crystal Violet and NaOH.
Kinetic Rate Laws
The net ionic reaction for the crystal violet (abbreviated CV) and the hydroxide ion, OH–, can be represented as: CV+^ + OH–^ → CVOH
The rate law for this reaction can be generalized as:
(1) Rate = t
CV
[ +^ ]
= k [OH–]x^ [CV+]y
In Equation 1, rate has the units M/min, k is the rate constant for the reaction, x is the reaction order with respect to OH–^ concentration, and y is the reaction order with respect to CV+ concentration. For this reaction, the values of x and y are expected to be 1 or 2; that is, the reaction is expected to be first or second order with respect to OH–^ and CV+. Following the experimental procedure outlined in the next section, you will collect data that will allow the determination of the order of the reaction with respect to OH–^ and CV+^ as well as the rate constant.
Your text outlines how to use graphical methods and integrated rate laws to determine the order of reactions with a single reactant. In this experiment, which has two reactants, the experimental and mathematical procedures must be modified from that presented in the text. The experimental modification is that the initial concentration of hydroxide ion, [OH–] 0 , is much greater than the
Determining the Reaction Order with Respect to [OH–]
After using the integrated rate law and performing the graphical analysis, you will have determined the value of k′ for one concentration of OH–. In part 2 of the experiment, you will obtain another k′ value by running the reaction using a different, large concentration of OH–. We now are going to use equation (3), k′ = k [OH–]x, to calculate the order of reaction with respect to OH–.
For the first run of the reaction let’s represent equation (3) as
k′ 1 = k [OH–^ run 1]x
where k′ 1 represents the pseudo rate constant for the first run of the reaction.
For the second run of the reaction let’s represent equation (3) as
k′ 2 = k [OH–^ run 2]x
where k′ 2 represents the pseudo rate constant for the second run of the reaction. Although it is not intuitive, we can take the ratios of these two equations, with the appropriate values, to solve for the single unknown value of x. Here is how you set up the ratio:
x
x
k OH run
k OH run k
k [ 1 ]
[ 2 ]
1
2 −
−
′
Things to keep in mind when using this ratio are: 1) k′ is the pseudo rate constant; 2) k is the rate constant; 3) the concentrations of OH–^ for each run need to be calculated prior to using them in this equation since you performed a dilution upon mixing the CV+^ with the OH–. Note that k is also unknown, but can be canceled out in the equation since it is in the numerator and the denominator.
Calculation of the Rate Constant
Calculation of the rate constant, k, is the final calculation that you need to perform in order to complete the rate law with appropriate values. Here again you can use equation (3). If you have performed the calculations described above, you now have the order of the reaction with respect to OH–^ (the value of x). Therefore, for each run of the reaction, the only unknown in equation (3) is the rate constant. Be sure to use the appropriate values of k′ and concentration when solving the equation.
Data Collection
In order to perform the calculations and data manipulations described above, we need to have data showing how the CV+^ concentration changes with time. This data will be collected with a computer program which uses the LabPro colorimeter with a beam of 565 nm green light. The light will pass through the solution containing CV and NaOH and fall on the system photocell. The photocell circuit will then produce a current (I), measured in microamps, which is directly proportional to the light intensity striking the photocell surface.
The computer program will collect photocell current readings during the reaction run, and will then calculate solution absorbance values for each reading using Equation (6) below
t
o t (^) I
I
A =log
where At is the reaction solution absorbance at any time t; Io is the DI water blank photocell current; and It is the current observed for the CV/NaOH reaction mixture at time t.
Crystal violet solutions obey Beer’s law. Thus, the relationship between the solution absorbance and the CV+^ concentration can be written as shown in Equation (7) below. In Equation 7, At is the reaction solution absorbance at time t; ε is the CV+^ molar absorptivity, 5.0 x 10^4 L /(cm mol); b is the cell path length (1.00 cm); and ct is the CV molar concentration at time t, [CV+]t. Beer’s law can be used to calculate [CV+]t from each absorbance value, At, obtained by the computer program during the reaction run.
(7) At = ε b ct
SAFETY PRECAUTIONS
Wear departmentally approved eye protection at all times in the laboratory. Follow all additional laboratory rules and regulations provided by your instructor. Know the location and proper use of all laboratory safety equipment (safety showers, eye washers, fire extinguishers, etc.). Dispose of all chemicals in the proper waste containers located in the Waste Hood.
A material safety data sheet (MSDS) for each chemical used in this experiment is located in a binder in the lab. You should be familiar with the hazards associated with each chemical, as well as the instructions on safe handling and appropriate disposal. Your instructor will be available to assist you in interpreting this information.
EXPERIMENTAL PROCEDURE
Obtain a colorimeter and a cuvette. Connect your colorimeter to channel CH1 on the side of the LabPro interface unit. Use the arrow keys on the colorimeter to select 565 nm, and let the system stabilize at this wavelength for at least 5 minutes. Turn on your computer, and open Logger Pro 3.6.1 by double-clicking on the Logger Pro icon on the computer desktop. Close the tips box, and open the experiment file by clicking File , Open , opening the Chemistry with Vernier folder and selecting 30 Rate Crystal Violet.cmbl.
Next, you can modify the experiment program ( 30 Rate Crystal Violet.cmbl ) to enable it to perform data collection automatically. Select Experiment , Data Collection from the Logger Pro menu bar. In the Data Collection pop-up window, select the Collection tab, and change the Mode to “Time Based” and the length to 20 minutes. The “Sample at Time Zero” box should be checked, and you should see the phrase “Triggering is disabled” to the right. Finally, change the Sampling Rate to 2 samples/minute; this should change the minutes/sample to 0.5. Double-check that the Samples to be Collected listed below reads 41.
- Next you will generate a plot of 1/[CV+]t (y-axis) versus time in minutes (x-axis). Double-click on the graph shown, and under Axes Options , change the y-axis to the 1/[CV+]t column. Under Graph Options , name the plot and make sure that “Point Protectors” is selected and “Connect Points” is not. If the data points do not appear on the graph, then click on the Autoscale icon.
- If the reaction between crystal violet and NaOH is second order in crystal violet, this plot will be linear. If it is not second order, this plot will be curved. Test for linearity with a linear regression by clicking the R= Linear Fit icon on the toolbar. Look at the actual data points. If they clearly follow a curved departure from the regression line, it should be concluded that the reaction is not second order in crystal violet.
- Repeat steps 2-4, substituting ln[CV]t for 1/[CV]t. Remember that ln indicates natural log. A linear graph in this instance would indicate a first order dependence on crystal violet. Closely examine your data as done in step 4 above.
- The plot with the best linear fit indicates the value of y, the reaction order with respect to CV. The absolute value of the slope of the straight line (shown in the linear fit box) is the best value of k′. Obtain a printout of the plot with the best linear relationship by selecting File , Print Graph. In the Printing Options window, select “Print Footer”, enter your names and click “OK”. Your printout should have a title and should include the linear regression curve fit line, and your correlation coefficient should be very near 0.999.
- Obtain a printout of the data table by selecting File , Print Data Table. Attach both your graph and data table to the report sheet.
Part 2 – Data Collection
Recall that the k′ just obtained is a pseudo rate constant, whose value depends upon the OH– concentration, i.e. k′ = k [OH–]x,. In this part of the experiment, the value of x will be determined as well as the value of the true rate constant, k.
In part 1 of the experimental procedure, 9.00 mL of 1.50 x 10-5^ M crystal violet and 1.0 mL of 0.050 M NaOH were combined to form the reaction mixture. A second kinetic run will now be made in exactly the same way except that the NaOH concentration will be doubled to 0.10 M.
- Carry out data collection steps 1 and 2 on page 6 using 1.0 mL of 0.10 M NaOH in place of 0.050 M NaOH. When you press the Collect icon to begin data collection for Part 2, the computer will erase and write over your Part 1 data, so be sure to print your Part 1 data table and graph before beginning Part 2****.
Part 2 – Data Analysis
- Carry out Data Analysis steps 1, 2, 6, 7 and 8 as listed on pages 6 and 7.
