Unformatted Attachment Preview
Rate Determination
and Activation Energy
An important part of the kinetic analysis of a chemical reaction is to determine the activation
energy, Ea. Activation energy can be defined as the energy necessary to initiate an otherwise
spontaneous chemical reaction so that it will continue to react without the need for additional
energy. An example of activation energy is the combustion of paper. The reaction of cellulose
and oxygen is spontaneous, but you need to initiate the combustion by adding activation energy
from a lit match.
In this experiment you will investigate the reaction of crystal violet with sodium hydroxide.
Crystal violet, in aqueous solution, is often used as an indicator in biochemical testing. The
reaction of this organic molecule with sodium hydroxide can be simplified by abbreviating the
chemical formula for crystal violet as CV.
CV+ (aq) + OH– (aq) → CVOH (aq)
As the reaction proceeds, the violet-colored CV+ reactant will slowly change to a colorless
product, following the typical behavior of an indicator. You will measure the color change with a
Vernier Colorimeter or a Vernier Spectrometer. You can assume that absorbance is directly
proportional to the concentration of crystal violet according to Beer’s law.
The molar concentration of the sodium hydroxide, NaOH, solution will be much greater than the
concentration of crystal violet. This ensures that the reaction, which is first order with respect to
crystal violet, will be first order overall (with respect to all reactants) throughout the experiment.
You will monitor the reaction at different temperatures, while keeping the initial concentrations
of the reactants the same for each trial. In this way, you will observe and measure the effect of
temperature change on the rate of the reaction. From this information you will be able to
calculate the activation energy, Ea, or the reaction.
OBJECTIVES
In this experiment, you will
•
•
•
React solutions of crystal violet and sodium hydroxide at four different temperatures.
Measure and record the effect of temperature on the reaction rate and rate constant.
Calculate the activation energy, Ea, for the reaction.
MATERIALS
computer
Vernier computer interface*
Logger Pro
Colorimeter or Spectrometer
Temperature Probe or thermometer
5 plastic cuvettes
1 liter beaker
*No interface is required if using a Spectrometer
0.10 M sodium hydroxide, NaOH, solution
2.5 × 10–5 M crystal violet solution
ice
two 10 mL graduated cylinders
two 100 mL beakers
50 mL beaker
watch with a second hand
PROCEDURE
NOTE: You will be preparing a series of water baths during this experiment. Measure the
temperature of the water baths in whatever way you wish, but make sure that your measurements
are as accurate as possible.
Both Colorimeter and Spectrometer Users
1. Obtain and wear goggles.
2. Prepare a blank by filling a cuvette 3/4 full with distilled water. To correctly use cuvettes,
remember:
•
•
•
•
Wipe the outside of each cuvette with a lint-free tissue.
Handle cuvettes only by the top edge of the ribbed sides.
Dislodge any bubbles by gently tapping the cuvette on a hard surface.
Always position the cuvette so the light passes through the clear sides.
Spectrometer Users Only (Colorimeter users proceed to the Colorimeter section)
3. Use a USB cable to connect the Spectrometer to your computer. Choose New from the File
menu.
4. To calibrate the Spectrometer, place the blank cuvette into the cuvette slot of the
Spectrometer, choose Calibrate ►Spectrometer from the Experiment menu. Wait for the
Spectrometer to warm up, and then click
.
5. Determine the optimum wavelength for examining the crystal violet solution and set up the
mode of data collection.
a. Empty the blank cuvette and rinse it twice with small amounts of 2.0 × 10–5 M crystal
violet solution. Fill the cuvette about 3/4 full with the crystal violet solution and place it in
the spectrometer.
a. Click
. A full spectrum graph of the solution will be displayed. Note that one area
of the graph contains a peak absorbance. Click
.
b. To save your graph of absorbance vs. wavelength, select Store Latest Run from the
Experiment menu.
c. To set up the data collection mode and select a wavelength for analysis, click Configure
Spectrometer Data Collection icon, , on the toolbar.
d. Click Absorbance vs. Time (under the Set Collection Mode). The wavelength of
maximum absorbance ( max) will be selected. Click
. Remove the cuvette from
the spectrometer and dispose of the crystal violet solution as directed. Save the cuvette for
Step 8.
Colorimeter Users Only
3. Connect the Colorimeter to the computer interface. Prepare the computer for data collection
by opening the file “35 Activation Energy” from the Advanced Chemistry with Vernier folder
of Logger Pro.
4. Open the Colorimeter lid, insert the blank, and close the lid.
5. To calibrate the Colorimeter, press the < or > button on the Colorimeter to select the
wavelength of 565 nm (Green). Press the CAL button until the red LED begins to flash and
then release the CAL button. When the LED stops flashing, the calibration is complete.
Remove the cuvette from the Colorimeter and save it for Step 8.
Both Colorimeter and Spectrometer Users
6. Prepare a 25°C water bath in a 1 liter beaker. The bath can be shallow because you will be
using it to condition the two 10.0 mL samples of reactants.
7. Prepare the NaOH and crystal violet solutions for the first trial.
a. Use a 10 mL graduated cylinder to obtain 10.0 mL of 0.10 M NaOH solution. Transfer the
solution to a 100 mL beaker. CAUTION: Sodium hydroxide solution is caustic. Avoid
spilling it on your skin or clothing.
b. Use another 10 mL graduated cylinder to obtain 10.0 mL of 2.5 × 10–5 M crystal violet
solution. Transfer the solution to a second 100 mL beaker. CAUTION: Crystal violet is a
biological stain. Avoid spilling it on your skin or clothing.
c. Place the two 100 mL beakers of reactants in the water bath. Make sure that the level of
the solutions on the beakers is below the level of the water bath. The beakers should be in
the water bath for at least one minute.
8. Conduct the first trial. Be ready to time the reaction.
a. Check the temperature of the water bath; it should be holding steady at or near 25°C.
b. When you are certain that the reactants are 25°C, pour the NaOH into the beaker of crystal
violet solution. Start timing the reaction using the second hand on your watch. You will
allow the reaction to run for one minute before proceeding to Part c of this step.
c. After one minute, record the temperature of the reaction mixture.
d. Fill a clean, dry cuvette about 3/4 full with the reaction mixture. Place the cuvette in the
device (Colorimeter or Spectrometer). Close the lid on the Colorimeter.
e. Click
. The default settings are 1 sample per second for 200 seconds. You may
click
to end the data collection early. Observe the progress of the reaction.
f. When data collection is complete, carefully remove the cuvette from the spectrometer.
Dispose of the contents of the beaker and cuvette as directed.
9. Because the reaction is first order with respect to crystal violet, you can determine the rate
constant, k, by plotting a graph of ln Absorbance vs. time.
a. Choose New Calculated Column from the Data menu.
b. Enter “ln Absorbance” as the Name, and leave the unit blank.
c. Enter the correct formula for the column into the Equation edit box by choosing “ln” from
the Function list, and selecting “Absorbance” from the Variables list. Click
.
d. Click on the y-axis label. Choose ln Absorbance. A graph of ln absorbance vs. time should
now be displayed. Change the scale of the graph, if necessary.
e. Click the Linear Regression button, . Write down the slope value in your data table as
the rate constant, k.
f. Close the Linear Regression box by clicking the X in the corner of the box.
10. To prepare for the next trial, choose Store Latest Run from the Experiment menu.
11. Repeat the necessary steps to complete trials 2-4. The next trials will be at ~20°C, ~15°C,
and ~10°C. Use small amounts of ice to cool the water bath to the desired temperature.