RATE LAW DETERMINATION OF CRYSTAL VIOLET HYDROXYLATION

Rate Law Determination of Crystal Violet Hydroxylation
Revised 10/23/14
RATE LAW DETERMINATION OF
CRYSTAL VIOLET HYDROXYLATION
Adapted from "Chemistry with Computers"
Vernier Software, Portland OR, 1997
INTRODUCTION
In this experiment, you will investigate the kinetics of the reaction between crystal violet and
sodium hydroxide. The equation for the reaction is shown below:
A simplified (and less intimidating!) version of the equation is:
(1) CV+
crystal
violet
+
OH–
à
CVOH
hydroxide
ion
Kinetics is the study of the speed or rate of a chemical reaction. The differential rate law for the
hydroxylation of crystal violet is:
(2) rate = -Δ[CV+] = k [CV+]m [OH–]n
Δt
where k is the rate constant for the reaction, m is the order with respect to crystal violet (CV+),
and n is the order with respect to hydroxide ion. To determine the orders of reaction (m and n ),
the reaction will need to be done twice.
1
Crystal Violet Hydroxylation
Revised 10/22/14
Even though the balanced chemical reaction has a 1:1 mole ratio between CV+ and -OH, the actual
ratios of reactants used in lab will be much different. The concentration of -OH will be
approximately 1000 times that of concentration of CV+ in trial 1; and 500 times in trial 2. In both
trials, the hydroxide ion is in huge excess and can be assumed constant - neither will change
appreciably during the reaction. Therefore, the hydroxide's concentration term and reaction order
is grouped with the rate constant, k, to create the pseudo rate constants, k1 and k2. This allows for
the simplification of the rate law:
(3) rate1 = -Δ[CV+] = k1 [CV+]m
Δt
(4) rate2 = -Δ[CV+] = k2 [CV+]m
Δt
where k1 = k[OH–]1n; [OH–]1 is 0.020 M.
where k2 = k[OH–]2n; [OH–]2 is 0.010 M.
To find the reaction order of CV+, m, and the pseudo rate constants, k1 and k2, differential rate
laws expressed in equations 3 & 4 must be integrated. (You should review integrated rate laws
in your lecture text before continuing.) Integrated rate laws, when arranged in line equation form,
result in a mathematical function of concentration for the y axis and time on the x axis. The
mathematical function of concentration and the slope reveal the values of m, k1, and k2.
As the reaction proceeds, a violet-colored reactant will be slowly changing to a colorless product.
Using a Visible-Near InfraRed (Vis-NIR) spectrometer, the reduction of absorbance with time will
be monitored. Because the solutions used in this experiment are dilute, Beer's Law can be
invoked. Absorbance is proportional to the concentration of crystal violet (A = εl[CV+]) and can
be used instead of concentration when plotting data (A ≈ [CV+]). A blank is needed to calibrate
the spectrometer and correct for any impurities in the solvent (water) that may also be absorbing at
λmax.
Because, k, the actual rate constant, does not change (and is the same in both trials), the order of
reaction with respect to OH– (n) is found from the ratio of the pseudo rate constants and the ratio
of the concentrations of hydroxide ion used in trials 1 and 2:
(5) k = k1 / [OH–]1n = k2 / [OH–]2n
(6) k1 / k2 = ([OH–]1/ [OH–]2)n
To solve for n, apply the rule of logarithms:
(7) n = log (k1/k2) / log ([OH–]1/ [OH–]2)
2
Crystal Violet Hydroxylation
Revised 10/22/14
SAFETY
Wear safety goggles and lab aprons at all times in lab. Sodium hydroxide is caustic and can cause
burns. Wash any affected areas immediately with cold water. Crystal violet leaves stains when
spilled. Protect skin and clothing from contact and wash hands and glassware thoroughly when
experiment is finished.
PROCEDURE
Part A: Calibrate & Blank the VIS-NIR Spectrometer
Work in pairs. Wear safety goggles and lab aprons at all times. CAUTION: Sodium hydroxide
solutions are caustic. Avoid spilling it on your skin or clothing. Crystal violet is a biological
stain and it will stain skin, clothing and glassware. Avoid spills and wash hands thoroughly
before leaving lab.
1. Obtain a VIS-NIR spectrometer from the stockroom. Use the USB cable to connect the VISNIR Spectrometer to the LabQuest 2. Make sure the mode on the
window reads: Full
spectrum.
2. Then calibrate the spectrometer by clicking
. The calibration dialog box will display the
message: “Waiting….seconds for lamp to warm up.” (The minimum warm up time is one
minute.) Note: For best results, allow the spectrometer to warm up for at least three
minutes. Insert the blank cuvette (filled with deionized water) in the sample compartment.
This step will remove any background absorbance from the solvent (which is water). Click
Finish Calibration and then OK.
3. To create an absorption spectrum of crystal violet, fill a cuvette with ~2.0 x 10–5 M crystal
violet solution and place in the spectrometer. Click
once the data collection is complete.
Label any significant peaks with wavelengths. (The wavelength with the maximum
absorbance should be labeled λmax. All absorbances should be recorded at this same
wavelength.) Remove the "rainbow" background spectrum by double clicking the rainbow
background. E-mail this data to your ELN.
3
Crystal Violet Hydroxylation
Revised 10/22/14
Part B: Monitoring Change in Absorbance over Time, Trial 1
1. Prepare the computer to collect absorbance versus time data.
§
Set λmax: Go to the
window, click on the mode and change it from full spectrum to time
based. Change the duration to 24 minutes and interval to 1 min/sample. Click OK. The Abs
should be measured @ the maximum wavelength (λmax) found in Part A, #3. If not, click on
the red box labeled “USB:Abs@...” and change the wavelength to the desired λmax.
2. The stockroom will follow these procedures to create aqueous solutions of NaOH and crystal
violet:
•
Place ~0.8 g NaOH in a 1L volumetric flask, dilute to volume with DI H2O.
•
Place ~0.01 g crystal violet in a 2L volumetric flask, dilute to volume with DI H2O.
Set up the calculation before coming to lab, but determine the exact solution concentrations
with the more accurate masses provided on the labels of the reagent bottles.
3. Obtain 5.0-mL of both solutions in graduated cylinders.
4. To initiate the reaction, simultaneously pour the 5-mL portions of crystal violet and sodium
hydroxide into a 100-mL beaker and stir the reaction mixture with a stirring rod. Click
.
Important Note: Because initial data is sometimes sporadic, readings are not taken until 3
minutes have passed. Empty the water from the cuvette. Rinse the cuvette twice with ~1-mL
amounts of the reaction mixture and then fill it 3/4 full. Do not put the cuvette in the
spectrometer yet.
5. After about three minutes have passed since combining the 2 solutions, wipe the outside of the
cuvette, place it in the cuvette slot of the spectrometer. Data collection will end after 24
minutes. When you leave the cuvette in the spectrometer, it warms up the solution. What does
that do to the reaction you are monitoring? Place the solution in a 100 or 250 mL beaker for
neutralization at the end of lab.
6. Analyze the data graphically using excel to decide if the reaction is zero, first, or second order
with respect to crystal violet:
•
Zero Order: If the current graph of absorbance vs. time is linear, the reaction is zero order.
•
First Order: To see if the reaction is first order, plot the natural logarithm (ln) of
absorbance vs. time. If this plot is linear, the reaction is first order.
•
Second Order: To see if the reaction is second order, plot the reciprocal of absorbance vs.
time. If this plot is linear, the reaction is second order.
4
Crystal Violet Hydroxylation
Revised 10/22/14
Part C: Monitoring Change in Absorbance over Time, Trial 2
1. Prepare 10.00 mL of ~0.010 M NaOH by diluting the NaOH solution used in Trial 1. (Your
procedure should clearly show your calculations, the exact NaOH concentration, and the
glassware used to perform the dilution.) Repeat steps 2-6 in Part B using 5.00 mL of the
~0.010 M NaOH solution just made.
2. Combine Trial 2’s and Trial 1’s solution. Add small amounts of dilute HCl(aq) solution and
check the solution with pH paper. Once pH ≤ 8, pour the neutralized solution down the drain.
Make sure to clear your email address and password of the LabQuest2 so others can’t access
your email account. Shutdown the LabQuest2 and not simply put it to sleep. To shutdown the
LabQuest2: press the home key, select System à Shut Down à OK.
DISCUSSION & CALCULATIONS
(1) What wavelength should be used to measure the absorbances for the kinetics trials?
(2) Examine your plot. What is the order of reaction (m) with respect to crystal violet?
(3) Calculate the pseudo rate constant, k1, using the slope of the linear regression line.
(Remember, k1 or k2 = – slope for zero and first order reactions and k1 or k2 = + slope for
second order reactions).
(4) Write the correct rate law expression for the reaction, in terms of crystal violet only (omit
OH–).
(5) Calculate the half-life of the reaction (in minutes) using the pseudo rate constant, k1, and the
appropriate half-life equation.
(6) Calculate the pseudo rate constant, k2, using the slope of the linear regression line from the
graph from Part C.
(7) Find the order of reaction (n) with respect to hydroxide ion:
n = log (k1 / k2 ) / log ([OH–]1/ [OH–]2)
5
Crystal Violet Hydroxylation
Revised 10/22/14
Do not round off value for n if it is a decimal or fraction. Report this value to 2 sig figs.
(The concentration of OH– was 0.020 M in Part A and 0.010 M in Part B.)
(8) Now round off the value for n to an integer (0, 1, 2, etc.). Write the complete rate law
expression using both CV+ and OH– in the expression with their appropriate orders.
QUALITATIVE ERROR ANALYSIS
1. What modifications could be made to the procedure to better account for random
(indeterminate) errors?
2. List three potential systematic (instrumental, methodological, or personal) errors that could be
made in this experiment. (Note: Be specific, systematic errors are in the details. For example,
losing your solution because you knocked over the cuvette is not a systematic error – it’s a
gross one.)
3. Did any gross errors occur? Did you mess up? Did the equipment or instrumentation fail?
6