CHM2 Lab(total 2 lab)

CV+ + OH– CVOH

(crystal violet) (hydroxide)

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 more than 1000 times as large as the concentration of crystal violet, [OH-] will not change appreciably during this experiment. Thus, you will find the order with respect to crystal violet (m), but not the order with respect to hydroxide (n).

You will be using a colorimeter for this lab. A colorimeter shines a light through the solution and checks how much light is absorbed by the solution. As the reaction proceeds, a violet-colored reactant will be slowly changing to a colorless product. Using the green (565 nm) light source of a computer-interfaced Colorimeter, you will monitor the absorbance of the crystal violet solution with time. Absorbance is proportional to the concentration of crystal violet (Beer’s law). Absorbance will be used in place of concentration in plotting the graphs.

Beer’s law is A = abc

where A = absorbance, a = molar absorptivity constant, b = path length, and c = concetration

Once the order with respect to crystal violet has been determined, you will also be finding the rate constant, k, and the half-life for this reaction.

PreLab

Add this lab to your table of contents

Write a purpose for this lab

Create a table of reagents

Sketch a graph of concentration vs time, ln concentration vs. time, and 1/concentration vs. time for a zero, first and second order reaction.

Sketch a graph of the [CV+] and the [CVOH] over time during this reaction, reaction and write what you should visually see due this change in concentrations.

MATERIALS

Power Macintosh or Windows PC	0.020 M NaOH
Vernier computer interface	2.0 X 10–5 M crystal violet
Logger Pro	distilled water
Vernier Colorimeter	stirring rod
one plastic cuvette	two 10-mL graduated cylinders
250-mL beaker

Write a PROCEDURE For Graphically determining the order of the reaction

The following bits of information will avoid excess waste, and help you with your procedure.

1. To set up the program, Click or tap Mode to open Data Collection Settings. Change Rate to 1 samples/s and End Collection to 200 s. Click or tap Done.

2. Use 10.0 mL of 0.020 M NaOH solution. Use 10.0 mL of 2.0 X 10–5 M crystal violet solution. CAUTION: Sodium hydroxide solution is caustic. Crystal violet is a biological stain. Avoid spilling either on your skin or clothing.

3. Remember to Calibrate the Colorimeter, set the wavelength on the Colorimeter to 565 nm (Green).

4. Do this quickly! To initiate the reaction, simultaneously pour the 10 mL portions of crystal violet and sodium hydroxide into a 250 mL 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 device. Close the Colorimeter lid. Click or tap Collect to start data collection.

5. To keep the solution from warming inside the Colorimeter, the cuvette should be removed from the Colorimeter between readings. However, make sure your absorbance is stable before clicking keep. That is too say don’t rush too much putting it and clicking keep or you will have error.

6. Data collection will end after 200 s.

7. Create a calculated column, ln Absorbance, and add a linear curve fit to the graph ln Absorbance vs. time:

a. Click or tap View, , and select Graph and Table.

b. In the Absorbance column header in the table, click or tap More Options, , and choose Add Calculated Column.

c. Enter ln Absorbance as the Name and leave the Units field blank.

d. Click or tap Insert Expression and choose Aln(X) as the expression.

e. Enter 1 as Parameter A and select Absorbance as Column X.

f. Click or tap Apply. A graph of ln absorbance vs. time is displayed. Double-click the graph to autoscale the graph.

g. To see if the relationship is linear, click or tap Graph Tools, , and choose Apply Curve Fit.

h. Select Linear as the curve fit and Dismiss the Curve Fit box.

i. Record the slope as the rate constant, k, and dismiss the Linear curve fit box.

8. Create a calculated column, 1/Absorbance, and then plot a graph of 1/Absorbance vs. time:

a. In the data table, click or tap More Options, , in the Absorbance column header, and then choose Add Calculated Column.

b. Enter 1/Absorbance the Name and leave the Units field blank.

c. Click or tap Insert Expression and choose A/X as the expression.

d. Enter 1 as Parameter A and select Absorbance as Column X.

e. Click or tap Apply.

f. Click or tap the y-axis label and select only 1/Absorbance to display a graph of 1/Absorbance vs. time.

g. To see if the relationship is linear, click or tap Graph Tools, , and choose Apply Curve Fit.

h. Select Linear as the curve fit and Dismiss the Curve Fit box.

i. Record the slope as the rate constant, k, and dismiss the Linear curve fit box.

9. To see any of the three graphs again, click or tap the y-axis label and choose the column you want to display.

10. Write up and explain your procedure to your instructor before beginning. Be prepared to answer questions.

PostLAb

1. Was the reaction zero, first, or second order, with respect to the concentration of crystal violet? Explain.

2. Calculate the rate constant, k, using the slope of the linear regression line for your linear curve (k = –slope for zero and first order and k = slope for second order). Be sure to include correct units for the rate constant. Note: This constant is sometimes referred to as the pseudo rate constant, because it does not take into account the effect of the other reactant, OH-.

3. Write the correct rate law expression for the reaction, in terms of crystal violet (omit OH-).

4. Using the printed data table, estimate the half-life of the reaction; select two points, one with an absorbance value that is about half of the other absorbance value. The time it takes the absorbance (or concentration) to be halved is known the half-life for the reaction. (As an alternative, you may choose to calculate the half-life from the rate constant, k, using the appropriate concentration-time formula.)

5. Print a copy of the graphs “Absorbance”, “ln Absorbance”, or “1/Absorbance”, and attach them to the lab

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