Reaction of Crystal Violet with NaOH
A Kinetic Study
INSTRUCTOR RESOURCES
The CCLI Initiative
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.
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 the
reaction in hydroxide.
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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 significant
figures.
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 significant
figures.
5.
Determine the ratio of the two k' values. Round to the nearest integer.
6.
Attach a printout of your labeled graph and data points to this report sheet.
6
Name ______________________________ Section _____________ Date ________________
REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY
Report Sheet (page 2)
7.
Calculate k using each k' value. Adjust [OH ] for dilution.
8.
Calculate kaverage.
9.
Calculate the molar absorptivity, ,, using the data from the first experimental run.
Summary
Species
Order
k'
k'
k
k
---------------
-----------------
---------------
----------------
Crystal Violet
OH
kaverage = ____________________
(include units)
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Name ______________________________ Section _____________ Date ________________
REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY
Questions
1.
In Figure 1, crystal violet is shown as a positive ion of charge 1+. Which atom in the crystal violet
cation has a formal charge of +1? Explain thoroughly how you arrived at your answer.
2.
The following data were collected for a reaction of the type:
[A]
A ==> Products
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 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)
determine whether 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 KINETICS 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 0.100
M 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 one
end.
6.
Add the NaOH rapidly, cap the vial, rotate twice, place in the colorimeter chamber and begin the
program 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 the
blank.
9.
Crystal violet is most easily dispensed by buret.
10.
The best temperature for experimental conditions is 20 C.
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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?
The linear graph is 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/Lsec
Experimental values may differ here as a function of the actual CV concentration and colorimeter
parameters.
3.
Attach a printout of your labeled graph and data points to this report sheet.
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 significant
figures.
The value of k’ for the [OH! ] = 0.10 plot is -0.0936 mol/Lsec
Experimental values may differ here as a function of the actual CV concentration and colorimeter
parameters.
5.
Determine the ratio of the two k' values. Round to the nearest integer.
-0.0936/-0.0477 = 1.96
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 1 and 2.
6.
Attach a printout of your labeled graph and data points to this report sheet.
See screen image following.
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Name ______________________________ Section _____________ Date ________________
REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY
Report Sheet (page 2)
7.
Calculate k using each k' value. Adjust [OH! ] for dilution.
1 ml | 0.05 mmol | = 0.005 M
10 ml |1 ml
1 ml
| 0.10 mmol | = 0.010 M
10 ml |1 ml
k = k’/[OH! ]2
k = 0.0477 / (0.0050)2 = 1908
8.
k = 0.0936 / (0.010)2 = 936
Calculate kaverage.
1908 + 936 = 1422
2
9.
Calculate the molar absorptivity, ,, using the data from the first experimental run.
, = A/bc = 0.7173 / 2.54 / 1.5 x 10-5 = 18827 L/cm/Mol
Summary
Species
Order
k'
k'
k
k
Crystal Violet
First
0.0477
0.0936
1908
936
OH
Second
---------------
-----------------
---------------
----------------
kaverage = 1422 L2 / mol2 sec
(include units)
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REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY
Suggested Answers to Questions
1.
In Figure 1, crystal violet is shown as a positive ion of charge 1+. Which atom in the crystal violet
cation 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]
a.
A ==> Products
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
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?
The reaction is not zero order in A. For the reaction to be zero order there must be no
change in [A].
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
each of the 5 second time increments.
c.
Make the appropriate
plot and calculate the
value of k for the
reaction.
The value of k is 0.10
s-1.
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REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY
Sample Data
Sample data screen for the MicroLAB Crystal Violet Kinetics experiment.
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REACTION OF CRYSTAL VIOLET WITH NaOH: A KINETICS STUDY
Laboratory Preparation (per student station)
Equipment
•
MicroLAB interfaces’ colorimeter
•
glass solution cell (cuvette), 2.54 cm path
•
ring stand
•
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 10-5 M crystal violet solution (18.00 ml)
•
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.
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