Kinetics of the Decomposition of Crystal Violet in Acidic Solutions
SCHA-265 Experiment
Kinetics of the Decomposition of Crystal Violet in Acidic Solutions
Note: A requirement for this lab you will plot your raw data into your lab notebook as it is being collected. This will assist you in seeing the trends in the data before you make your Excel plots.
A characteristic of organic dyes is that they change color with changes in pH as the dye is protonated or deprotonated (usually weak bases). We have already seen this with phenolphthalein. Crystal violet belongs to the roseaniline series of triphenylmethane dyes and was one of the earliest synthetic coloring materials.
In neutral solutions, the fully conjugated molecule is a brilliant violet color. In solutions of pH = 1 or less, it turns a pale yellow as the extended conjugation is disrupted. Upon exposure to light, this dye undergoes a reaction and will bleach to a colorless solution.
In acid solution, the CV+ ion (violet) is converted to the HCV2+ ion which is yellow. Addition of another acid proton to this species creates the HCV3+ ion which is colorless.
In this experiment we will follow the rate of the bleaching process:
HCV2+ + H+ = HCV3+
The reaction will be followed using a Spec 20 since the HCV2+ species absorbs light in the visible region (yellow) and the HCV3+ ion does not. Monitoring the absorbance of the 2+ ion vs. time will give you data that will enable you to determine the order and rate constant for this reaction.
PROCEDURE
- Turn on your Spec 20
2.Measure 5.00 mL of the 1 x 10-4 M crystal violet solution and 5.00 mL of 0.400 M HCl in separate vials. (The excess H+ will make the reaction independent of the hydrogen ion concentration.) (Explain why in your report)
3.Set the Spec 20 for 420 nm the wavelength of maximum absorbance, max, for the HCV2+ species. Zero the absorbance with 0.200 M HCl. Prepare a clean, dry cuvette and zero the stopwatch.
4.WHEN YOU MIX THE TWO SOLUTIONS, the time is t=0 so be sure everything is ready. You will start the stopwatch when the solutions are mixed. Pour some of this solution into the cuvette and take absorbance readings for 20 minutes. Readings should be taken at least every 2 minutes. The time spacing is not as important as the fact that you know the exact time that each reading was taken. (Experiment A)
5.From a plot of your data determine if this reaction zero, first or second order with respect to the crystal violet. (Called pseudo-order reaction because of keeping the concentration of one reactant constant throughout the reaction) Once the order of the reaction is determined, the appropriate linear plot can be made and the rate constant (k) determined. A more exact computer generated plot of your data will be done (using Excel), to determine the precise slope of the line!
6.Repeat steps two through five except mix 2.50 mL of the crystal violet with 2.50 mL of water and then mix that solution with the 5.00 mL of 0.400 M HCl. (Experiment B)
7.Repeat steps two through five except mix 1.00 mL of the crystal violet with 4.00 mL of water and then mix that solution with the 5.00 mL of 0.400 M HCl. (Experiment C)
8.In an attemptto determine the order of the reaction with respect to H+, reduce the initial H+ concentration to 0.001 M, use 5.00 mL of the acid and proceed as in step 2 above. Repeat the reading with 5.00 mL of a 0.002 M HCl solution. (Note: We will try different concentrations of acid – it is important the color intensity change as a function of time.) skip 20052
9.Give the correct rate expression for this reaction. Verify your order determination from your inititial rates from experiments A, B and C.
10.For your first set of data, plot [HCV2+] vs. time. Determine the instantaneous reaction rate at two different times. Calculate the rate constant from each of these tangents and compare to that found from your plot in step 5. You will need to determine the slope of the line at various points. Find the equation of the line and then differentiate this to get your slope.
Yourreport will include several data tables and graphs (Remember that these are labeled / legended as Tables (roman numerals) and Figures(Arabic numerals)). Make sure all calculations are clearly presented. You must also write an abstract and discussion with this lab report.
Kinetics LabPage 1Printed 11/21/2018