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Chemistry 1B
Determination of the Rate Law for the Crystal Violet Reaction
The purpose of this laboratory study is to determine how the rate of disappearance of the crystal violet ion is related to its initial concentration and the initial concentration of hydroxide ion. As time elapses, the concentration of the crystal violet ion will be measured spectrophotometrically. The equation for the reaction is shown here:
Resonance Structures of Crystal Violet
A simplified version of the equation is: CV+^ + OH-^ → CVOH crystal violet hydroxide ion
The molar mass of crystal violet chloride (which is the salt we use) = 408.00 g/mol.
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. Since the hydroxide ion concentration is much larger than the concentration of crystal violet, [OH-] will not change appreciable during this experiment.
So rate = k[ CV+^ ] m[OH-]n^ = k'[CV+^ ] m, where k' = k[OH-] n^ and k' is called a pseudo rate constant.
As the reaction proceeds, the violet-colored reactant will be slowly changing to a colorless product. The crystal violet absorbs a broad band of colored light; the best wavelength to use is λmax, 589 nm.
You will monitor the absorbance of the crystal violet solution as a function of elapsed time. We will that absorbance is proportional to the concentration of crystal violet (Beer's law). Absorbance will be used in place of concentration in plotting the following three graphs:
- Absorbance vs. time: A linear plot indicates a pseudo zero order reaction (k' = -slope).
- ln Absorbance vs. time: A linear plot indicates a pseudo first order reaction (k' = -slope).
- 1/ Absorbance vs. time: A linear plot indicates a pseudo second order reaction (k' = slope).
The order of the reaction is that for which the graph of f(A) v. time has the most linear plot for the longest period of time.
Once the order with respect to crystal violet has been determined, you will also find n , the order with respect to hydroxide, and you will find the rate constant, k. To find n , you will do the experiment using a different [OH-], therefore having a different k' , while holding [CV+^ ] constant. [Recall that the order of a reaction with respect to a given reactant will be a rational number.]
rate 2 (^) = k[OH-] 2 n^ [CV+] m^ = k′ 2 = [OH-] 2 n rate 1 k[OH-] 1 n^ [CV+] m^ k′ 1 [OH-] 1 n
Find k (the actual rate constant), by using the equation, k' = k[OH-] n^. Calculate k twice; once for the data from run one, once from the data for run two. Your two values should be close to each other.
MATERIALS FOR EACH GROUP
FROM STOCKROOM
- 1-100 mL volumetric flask
- 1 or 2-5 mL pipets
- 3-l0 mL pipets
- 1-25 mL pipet
- pipet bulb
- cuvette for spectrophotometer
- timer
- ~90-100 mL 0.1 M NaOH, concentration known and recorded to 4 decimal place
- ~50 mL crystal violet solution, not to exceed .0160 g/L, conc. known to 3 SF
IN ROOM OR FROM LOCKER
- Sec 20 D+ spectrophotometer
- paper towels
- tissues to wipe cuvettes
- 6-100 or 150 mL beakers
- 1 or 2 stirring rods
- deionized water
- H 2 SO 4
- Phenolphthalein
Read the entire lab study before writing anything in your notebook.
Summary table for the two reaction mixtures "Run 1" and "Run 2":
Reagent distilled water .05xxx M NaOH crystal violet solution Run 1 15.00 mL 5.00 mL 10.00 mL Run 2 10.00 mL 10.00 mL 10.00 mL
PROCEDURE
- Clean the pipets and the volumetric flask. Rinse thoroughly; the final rinse being deionized water. Wash and dry 4-100mL beakers. Turn on the spectrophotometer by turning the Power Switch clockwise. Allow the spectrophotometer to warm up for at least 15 minutes before using. Set the desired wavelength (589 nm) with the Wavelength Control Knob. Set the Filter Lever to the "340-599 nm" position.
- Make 100.0 mL of a 0.05 M NaOH solution, the molarity known to four decimal places: Place about 20 mL of the standard 0.1 M NaOH in one of the 100 mL clean, dry beakers from your locker. Use this NaOH solution to rinse one of the 25 mL pipets by drawing several small portions of the solution into the pipet and rolling it around to touch all of the surfaces.
Discard the reaction mixture in the cuvette into your waste container, and completely clean the cuvette by shaking soapy water in it. Do not use a test tube brush, because we do not want to scratch the glass. If a faint purple stain remains, remove it by soaking with some of your dilute NaOH. Be sure to completely remove any NaOH that you use by washing several times.
- RUN TWO Read just the spectrophotometer by performing Steps 3 a-d again. In a different "reaction beaker'', repeat the steps of Run 1, but this time use 10. mL of water and 10.00 mL of the 0.05 M NaOH. Take readings of the timer and the spectrophotometer every 20-30 seconds. Again, stop when A< .15. (This will be faster for Run 2.)
When all measurements are completed, turn off the spectrophotometer by turning the Power Switch counterclockwise until it clicks. If you should spill anything on the instrument, be sure to clean and dry it immediately. It is always a good idea to write down the identification number of the instrument you used. Put the protective cover on the spectrophotometer before you leave.
- Disposal of reagents. Your used solutions are slightly basic. Neutralize with dilute sulfuric acid (phenolphthalein is no longer pink), then you may pour the solution in the sink. When you are sure you have finished, neutralize your standard NaOH solution and pour in the sink. The crystal violet may also be poured down the sink.
- The graphs: During another lab period we will use Microsoft Excel to plot the various functions, f(A), v. time. The order of the reaction with respect to crystal violet is that for which the graph of f(A) v. time has the most linear plot for the longest period.
- The report: Need: date, title, purpose (may be brief), method (may be brief), procedure (may refer to the handout by title), data pages (signed by instructor),
calculations, conclusion (would be the complete rate law). You will turn in separate computer graphs. Each run will take about one data page. Column headings for data and calculations could be: mL and concentrations of each reactant, followed by a large table for: Elapsed time, minutes, seconds, Absorbance
- Assigned Problem: To show that you are justified in using [OH-] as a constant using the concentrations from Run 1, show that the concentration of the NaOH has changed insignificantly when half of the crystal violet chloride has been consumed.
Report Summary for Crystal Violet Reaction Kinetics Laboratory
Name __________________________
Attach all data pages, supplemental calculations & graphs, labeled so your work can be followed.
Partners: Conclusion: Complete Rate Law Using Correct Units
Summary of how you obtained the above rate law:
Run 1 Run 2 Conclusions
[CV+] initial _______________^ _______________^ x
[OH-] initial _______________^ _______________^ x
time range used to obtain K' (from straightest graph)
_______________ _______________ x
order wrt CV+^ x x _______________
k' (give units consistent with order wrt CV+^ )
_______________ _______________ (^) x
order wrt OH-^ : Show calculation using (^) k′ 2 = [OH-] 2 n k′ 1 [OH-] 1 n
k (give units) use k = k′___ [OH-] n
_______________ _______________ (^) Kav_______________
± mean deviation
