CHEM 1411 Spring 2016
Experiment 9: Decomposition of Aspirin
Background
Earlier in the semester, you carried out the synthesis of aspirin (acetylsalicylic acid) by reacting
salicylic acid with acetic anhydride in an esterification reaction. In this experiment, you will
investigate the kinetics of the reverse process—the decomposition of aspirin into salicylic acid
and acetic acid (see equation below).
O
OH
O
C
H
H
C
O
C
C
C
C
C
O
C
OH
C
CH3
H
H
C
C
H2O
C
H
aspirin (acetylsalicylic acid)
+
C
H3C
C
C
H
O
OH
C
OH
H
H
salicylic acid
This decomposition reaction requires water and elevated temperatures—the rate of the reaction is
very slow at room temperature. Because liquid water has a constant concentration, the rate of the
reaction only depends on the concentration of aspirin; the rate law for the reaction is:
rate = k [aspirin]m
(5)
Remember that k is the rate constant for the reaction, and the value of the exponent m will
determine the order of the reaction with respect to the aspirin concentration. To determine the
reaction order and the value of the rate constant for the decomposition of aspirin, you will carry
out two trials of the reaction at (or near) 80 °C; these two trials will have different initial aspirin
concentrations. Comparison of the reaction rates for the two trials will allow you to calculate the
value of m in the rate law and an average value for k for the reaction at 80 °C.
You will carry out the decomposition reactions for both trials for six minutes at the same time in
the same hot-water bath (to minimize any differences in temperature for the reactions). Only a
small fraction of the aspirin will be converted into salicylic acid during the reaction. To
determine the exact concentration of salicylic acid present after the reaction is stopped, you will
add a solution of Fe3+ ions to both reaction mixtures—this cation reacts with salicylic acid to
produce a strongly purple-colored iron salicylate complex. The absorbance of these solutions at
525 nm is directly proportional to the concentration of salicylic acid in the reaction mixtures. A
calibration curve will be provided that will allow you to determine the salicylic acid
concentrations from the measured absorbance values. Given the 1:1 stoichiometry for the
decomposition reaction, the concentration of salicylic acid is equal to the change in the
concentration of aspirin for each trial. The average reaction rate for both trials can then be found
using the equation given below:
average rate = –Δ[aspirin]
Δt
CHEM 1411 Spring 2016
Experiment 9: Decomposition of Aspirin
You will use these average reaction rates as estimates for the instantaneous initial reaction rates
for both trials. To improve the accuracy of these measured reaction rates, you will calculate the
average rates assuming a reaction time of five and one-half minutes (330 seconds) rather than the
actual six minute reaction time. Decreasing the reaction time by 30 seconds in the calculation of
the reaction rate will account for the fact that the reaction rate is much slower initially while the
reaction mixture is warming up to 80 °C and all of the aspirin has yet to dissolve into the water.
CHEM 1411 Spring 2016
Experiment 9: Decomposition of Aspirin
Experimental Procedure
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Eye protection must be worn at all times during the experiment.
Use caution while heating with the water bath—the beaker and hot plate will be very hot.
Measure masses on an analytical balance to the nearest 0.1 milligrams (mg).
A calibration curve (with the equation for the linear trendline) for determining the
concentration of salicylic acid in your reaction mixture will be provided.
Dispose of waste solutions down the drain with copious amounts of water.
Make a hot-water bath by filling a 600 mL beaker about two-thirds full with water and
placing the beaker on a hot plate set initially to about two-thirds power. Prepare an icewater bath in a 400 mL beaker. Obtain a stopwatch (if needed) and a digital thermometer.
Clean and dry two small test tubes. Transfer about 60.0 mg of the recrystallized aspirin to
one of the test tubes using a funnel. Record the exact mass of aspirin used on the Data and
Calculations page. Then add 10.0 mL of deionized water to the test tube, using the DI water
to rinse any aspirin in the funnel into the test tube. Note that not all of the aspirin will
dissolve in the water.
Transfer about 40.0 mg of the aspirin to the second test tube, again recording the exact
mass of aspirin that was measured. Add 10.0 mL of DI water to the test tube. Add a clean
stirring rod or spatula (that reaches all the way to the bottom) to both test tubes.
Monitor the temperature of the hot-water bath, adjusting the power setting as necessary to
bring the bath to a steady temperature that is close to 80 °C. (You can carefully add small
pieces of ice to cool the water in the bath if it gets too warm.) When the temperature of the
bath has remained near 80 °C for a few minutes, you can start the decomposition reaction.
Record the temperature of the water bath on your Data and Calculations page. Then
carefully place both test tubes in the hot-water bath while at the same time starting the
timer on the stopwatch. Carefully stir the reaction mixtures in both tubes continuously until
the aspirin in both tubes has completely dissolved. Stir the mixtures in the tubes
occasionally during the remainder of the reaction time.
After six minutes have elapsed, remove the test tubes from the hot-water bath and place
them in the ice-water bath to stop the decomposition reaction. Stir the reaction mixtures
while in the ice-water bath to help the mixtures cool. After one minute, remove both tubes
from the ice water and allow them to warm to room temperature.
Add 5.0 mL of 0.025 M Fe(NO3)3 solution (in 0.50 M HCl) to each test tube and stir the
solutions to mix. The solutions should turn purple on addition of the Fe(NO3)3.
Measure and record the absorbance of both solution at 525 nm using a spectrophotometer.
Make sure to clean and condition the cuvette with each solution before each absorbance
measurement.
Using the provided calibration curve, calculate the concentration of salicyclic acid present
in each reaction mixture.
CHEM 1411 Spring 2016
10.
Experiment 9: Decomposition of Aspirin
Use your experimental data to determine the rate of the decomposition reaction (in M/s) for
both reaction mixtures. Remember to use five and one-half minutes (330 seconds) as the
reaction time (instead of the actual six minutes) to account for the warmup period of the
reaction. Then use these reaction rates to determine the experimental rate law and the
average value of the rate constant, k, for the decomposition of aspirin. Include all of your
calculations in your Data and Calculations pages.
CHEM 1411 Spring 2016
Experiment 9: Decomposition of Aspirin
Data and Calculations Page
Record all of your group’s experimental data below. Include all data and calculations needed to
determine the rate for each of your reaction trials, the rate law for the reaction (with the
experimental value for the exponent, m), and the average value for the rate constant, k, for the
reaction. Attach additional pages as needed.