Crystal violet and sodium hydroxide lab report

Chemistry Lab Report

CHM 114 How Do We Determine the Rate Law for

the Crystal Violet Reaction? Introduction In this experiment, you will observe the reaction kinetics between crystal violet and sodium hydroxide. The equation for the reaction is:

CC

OH

OH –+

N(CH )

N(CH )

N(CH )

+ N(CH )

N(CH )

N(CH )3 3

3

3

3

32

2

2

2

2

2

A simplified version of the equation is:

CV+(aq) + OH�(aq) ĺ CVOH(aq) The rate law for this reaction is in the form:

Rate = k [CV+]m[OH�]n. 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 the hydroxide ion. In this experiment the hydroxide ion concentration, [OH�], will be in such large excess that the rate of the reaction will depend only on the concentration of CV+. In this situation we can combine the constants (the reaction rate constant, k, and the [OH]n term) into a new constant, k’:

k’ = k [OH�]n

Substituting k’, we can rewrite the rate law as:

Rate = k’ [CV+]m This special rate constant, k’, is sometimes referred to as the pseudo rate constant because it is a constant only when [OH�] is held constant. A true rate constant, k, is a constant that is independent of reactant concentrations and varies only with temperature. Because of the temperature dependence, rate constants are often reported with the temperature at which they were determined. Pseudo rate constants should be reported with both temperature and concentration of what is in excess. Your main objective is to determine the order (m) of the reaction with respect to crystal violet. As the reaction proceeds, the violet-colored CV+ reactant slowly changes to a colorless product, allowing the reaction to be monitored by colorimetry. If you have done any Beer’s law experiments in the past, you may remember that absorbance is proportional to concentration as illustrated by Beer’s law:

A = οbC where ο is the molar absorptivity constant and b is the path length, also a constant. This direct relationship between absorbance and concentration under a range of concentrations allows us to plot absorbance instead of concentration on the y-axis when studying concentration vs. time for a reaction.

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A convenient way to determine the order of this reaction with respect to CV+ is by monitoring absorbance over time using a large excess (>1000 fold) of hydroxide ion, OH�, so that its concentration does not change appreciably during the reaction. With the concentration of OH� ion held constant, absorbance versus time plots will allow you to determine the order with respect to crystal violet. *Bring a flash drive to lab so that you can remove the data from the LabQuest devises. OBJECTIVES In this experiment, you will

x Observe the reaction between crystal violet and sodium hydroxide. x Monitor the absorbance of the crystal violet solution as a function of time. x Determine the order of the reaction with respect to CV+. x Determine the pseudo rate constant, k’. x Determine the Beer’s Law relationship between absorbance and concentration for CV+ x Determine the half-life for this reaction with respect to CV+. x Determine the rate law for this reaction.

MATERIALS

Vernier LabQuest 0.10 M sodium hydroxide, NaOH, solution Vernier SpectroVis 2.5 × 10–5 M crystal violet solution Temperature Probe or thermometer 50 mL beaker Plastic cuvettes Two 100 mL beakers 1 liter beaker Disposable gloves 10 mL graduated cylinders

Getting started WEEK 1: Determination of Wavelength: Determine the wavelength of maximum absorbance by measuring absorbance as a function of wavelength. The point on the graph where absorbance is a maximum is the wavelength that should work best for your absorbance vs. time measurements. Calibrate at chosen Wavelength: Once you have determined the wavelength of light you will be using, it is important to calibrate the colorimeter at that wavelength. Use de-ionized water as your blank, because the reaction occurs in an aqueous medium. Determine Order with respect to CV+: To determine the order of the reaction with respect to CV+, start by mixing equal volumes of the 2.5 × 10–5 M CV+ solution and the 0.10 M NaOH solution. Collect absorbance vs. time data until the absorbance of the mixture falls to 0.20. Graphing this data with different functions of absorbance on the y-axis will help you determine the order with respect to CV+. If time permits, repeat the experiment using different initial concentrations of CV+, maintaining the same large excess of OH�. Once the order with respect to crystal violet has been determined, you will also be able to determine the pseudo rate constant, k’, and the half-life with respect to CV+ for this reaction. Beer’s Law Relationship between A and C: It is important to verify a Beer’s Law linear relationship between absorbance and concentration of CV+ at the wavelength chosen for the experiment. To generate a calibration curve, use at least four standard solutions that range in concentration from 1.25�10�5 M to 1.25�10�6 M. Conserve reagents by preparing volumes of 10.0 mL or less. If the data gives a linear graph, the equation for the line can be used to convert absorbance values to concentration values in your absorbance vs. time experiments.

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WEEK 2: Determine Order with respect to OH�: To determine the order with respect to OH�, you will need to monitor the absorbance while changing the concentration of OH� (keeping [CV+] constant). Reactions with different initial concentrations of OH� will have different k’ values (why?). A comparison of two pseudo rate constants (k’) will allow you to determine the order of the reaction with respect to OH�. Remember that the change in the pseudo rate constants occurs proportionally to the change in reaction rate. Be sure to use calculated NaOH concentrations that result after dilution by mixing with the CV+ solution. _______________________________________________________________________ Precautions

x Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing. x Crystal violet is a biological stain. Avoid spilling it on your skin or clothing. Use gloves or wash

hands frequently, and clean up spills immediately. Thoroughly rinse all containers after using. Cuvettes must be thoroughly cleaned at the end of the lab.

x To correctly use cuvettes, remember:

x Wipe the outside of each cuvette with a lint-free tissue. x Handle cuvettes only by the top edge of the ribbed sides. x Dislodge any bubbles by gently tapping the cuvette on a hard surface. x Always position the cuvette so the light passes through the clear sides.

_________________________________________________________________________ Using the Spectrometer 1. Graph absorbance vs. wavelength to obtain the wavelength of maximum absorbance. Use this

wavelength for the remainder of the experiment. a. Thoroughly rinse a cuvette with de ionized water, and then rinse it twice with small amounts of 2.5 ×

10–5 M crystal violet solution. Fill the cuvette about 3/4 full with the crystal violet solution and place it in the Spectrometer.

b. Start data collection. A full spectrum graph of the solution will be displayed. Stop data collection. The wavelength of maximum absorbance (l max) is automatically identified.

2. Calibrate the Spectrometer at the wavelength of maximum absorbance. a. Use a USB cable to connect the Spectrometer to your LabQuest. Turn on the LabQuest. b. Calibrate the Spectrometer.

i. Place the blank cuvette (filled about ¾ full of deionized water) in the Spectrometer. ii. Choose Calibrate from the Sensors menu. The following message is displayed: “Waiting 90 seconds

for lamp to warm up.” After 90 seconds, the message will change to “Warm-up complete.” iii. Select Finish Calibration. When the message “Calibration completed” appears, select OK.

c. Follow the appropriate set of procedures from below for selecting the wavelength of maximum absorbance for this experiment.

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3. Set up the data-collection mode. a. On the Meter screen, tap Rate. Change the data-collection rate to 1 sample/second. b. Change the data-collection length to 200 seconds. Select OK. c. Proceed to Step 4.

4. Do this quickly! To initiate the reaction, simultaneously pour the accurately-measured crystal violet and

sodium hydroxide solutions into a beaker and stir the reaction mixture with a stirring rod. Empty the water from the cuvette. Rinse the cuvette twice with ~1 mL amounts of the reaction mixture, fill it 3/4 full, and place it in the Colorimeter. Close the lid. Start data collection.

5. Absorbance data should be collected for at least 200 seconds. You may stop data collection early if

absorbance falls below 0.2. Discard the beaker and cuvette contents as directed by your instructor. 6. Follow these directions to plot some function of Absorbance vs. time, such as lnA. You should choose

functions that will help you determine the order of the reaction with respect to CV+. a. Tap the Table tab to display the data table. b. Choose New Calculated Column from the Table menu. c. Enter the Name (ln Abs) and leave the Units field blank. Select the equation, Aln(X). Use

Absorbance as the Column for X, and 1 as the value for A. Select OK. d. A graph of ln absorbance vs. time will be displayed.

7. To see view and print any of your plots again:

a. Tap the vertical-axis label of the graph. b. Choose the name of the graph.

8. To generate an absorbance vs. concentration calibration curve, set the colorimeter to measure absorbance. Measure the absorbance of each of your prepared standard solutions that range in concentration from 1.25�10�5 M to about 1.25�10�6 M. Plot absorbance (y-axis) vs. concentration (x- axis) to identify the best-fit linear equation for you data. If the data gives a linear graph, use the linear equation to convert absorbance values to concentration values in your absorbance vs. time experiments.

The Report

x Show all data and calculations used to arrive at your results. Tabulate results when possible.

x Include printed graphs that you plotted to determine: (a) the order of the reaction with respect to CV+ and (b) the linear relationship between absorbance and concentration. For all linear plots, include the equation for the line and the correlation coefficient. In addition to identifying the order of the reaction with respect to CV+, you should determine the value of the pseudo rate constant, k’, including correct units and the temperature that it has this value. If multiple experiments were done using different concentrations of CV+, compare the k’ values and report an average. Since the k’ value is dependent upon [OH�], be sure to indicate the concentration of OH� used for the k’ value reported.

x Write a rate law expression in the form: Rate = k’ [CV+]m.

x Explain why absorbance values can be substituted for concentration values when determining order,

pseudo rate constant, and half life.

x Use your data table, graph, or k’ value to determine the half-life for the reaction with respect CV+.

If this is a 2-week investigation, also include the following: x Determine the order of the reaction with respect to OH�, and report the complete rate law expression

with the form: Rate = k [CV+]m[OH�]n

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