Lab Objectives:

·         Determine the order of a reaction for two reactants in a rate law

·         Determine the half life of a chemical reaction

 

 

Introduction

Order for the Dye: When nothing is know about the rate law for a reaction, the generalized rate law may be written to include the reactants with some unknown order. Thus, a guess for the rate law may be written rate=k[Dye]^n[ClO-]^m, where k is the rate constant, n is the order with respect to the dye and m is the rate order with respect to the hypochlorite ion.

 

 

Procedure:

 

Bleach stock solution: Obtain 10.00 mL of commercial bleach. Prepare three dilutions of the bleach in small-labeled beakers. The first should be a 1:10 dilution. The commercial bleach is 0.68 M ClO-. SO, the first dilution will yield 0.068 M ClO-. From this diluted solution, do two more dilutions to also obtain bleach solutions of 0.034 M and 0017 M ClO-.

 

Dye Stock Solution: Prepare a stock solution by placing about 1 drop in about 50 mL of water. The proper solution should have an absorbance between 1.5 and 2 at the peak when tested in the spectrometer. Do a quick test on the spectrometer and adjust the concentration until you get an appropriate dye solution. Print a spectrum of the dye solution with the wavelength limits set from 400 to 800 nm. Read the absorbance value at 630 nm for your final dye solution.

 

Kinetic Runs: To initiate the reaction, pour 10.0 mL of the dye solution and 10.00 mL of one of the bleach solutions into a 250 mL beaker and mix the solution thoroughly. Without delay, rinse the cuvette twice with the reaction mix and then fill it ¾ full. Wipe the corvette twice and place it in the spectrometer and start data collection. Absorbance readings should be taken about every 10 seconds. If the absorbance drops to less than 0.05, stop the data collection.

 

Repeat the reactions with the other concentrations of bleach. With the lowest concentration of bleach, readjust the data collection parameters for 1800 seconds.

 

 

 

 

Post-Lab Questions

1.      Determine the molar concentration of the dye in the stock dye solution that you used, as follows

a.       Read the spectrometer value, A_0, at the peak of the absorbance curve in the spectrum.    A₀=

b.      Use Beer’s Law to solve for [Dye]: A₀₌ abc where pathway (b) =1 cm and absorptivity (a) = 0.164 L/mg-cm at 630 nm and c=concentration

c.       Convert the concentration to moles/L (Calculate molar mass from structure)

2.      Based on the three trials you have run, what is the order of the reaction with respect to the dye? Explain?

3.      Complete the data table, presenting initial concentrations of reagent in the reaction after mixing. Determine the pseudo rate constants, k’, k’2 and k’3 for Run 1-3 using the slopes of the regression lines for the absorbance function which resulted in the straight line for the collection of graphs.

 

 

 

4.      Compare the pseudo rate constants for the trials. What is the order of the reaction with respect to [ClO-]? Discuss how you arrived at your answer.

 

 

5.      Write the complete rate law expression for the reaction including k, [Dye], [ClO-] and m

 

 

 

6.      Calculate the rate constant k for the reaction (note that k=k’/[ClO-] ^m)

 

 

7.      Using the graph of absorbance vs. time, determine the half-life of the reaction for the first trial. To do so, choose a convenient reference point near the beginning of the reaction. Find the point, where the absorbance decreased to half of the value that it was at the reference point. The time interval between those points is the half-life for that trial. Determine a second half-life, starting from the time at the end of the first half-life you have determined. Compare the two half-lives. Show your work.

First half life-

Second half-life

 

b) Calculate the pseudo rate constant, k’ from the average of the half-lives.

c) Compare your pseudo rate constant from #3 above to the pseudo rate constant determined form the graph (7b).

 

 

 

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