Chemical Oscillation and Waves
Purposes: To determine the sensitivity of front velocity to concentration of ferroin, [BrO3]0
and [H+]0. To calculate a value for k (the rate constant) using these data. To observe the
phenomenology of waves. To determine the rate-limiting step and the rate law describing it.
Note: You will not be able to do this experiment without reading this article: Pojman, John A.,
et al., Chemical Oscillations and Waves in the Physical Chemistry Lab., Journal of Chemical
Education, 1994, 71:84-90.
Reagents: These reagents will be supplied:
Solution A:
Solution B:
Solution C:
Solution D:
100 ml 0.6 M NaBrO3/H2SO4
40 ml 0.48 malonic acid, i.e., propanedioic acid, HO2CCH2CO2H
25 mM ferroin, i.e., tris [1,10 phenanthroline] iron(II) sulfate, (C12H8N2)3Fe2+(aq)
0.97 M NaBr
Include safety information for these materials in your notebook. NaBr and NaBrO3 can be
irritants, and the latter is a potent oxidizer. Wear gloves (and goggles, naturally) throughout this
experiment. Discard the reagents in labeled waste bottles. Please do not discard anything down
the drain.
Procedure:
1.
Into a small Erlenmeyer flask introduce 7 ml solution A, 3.5 ml solution B, and 1 ml
solution D. The solution should be orange.
2.
Immediately cover the flask with Parafilm™ and stir on a magnetic stirrer under the
hood until all the (yellow-brown) bromine color disappears, about 15 minutes.
3.
When the solution has cleared completely, add 0.5 ml of solution C and stir briefly.
4.
Using a pipet, transfer the solution to a petri dish. Try to keep the solution as a thin film
on the surface of the dish. Be sure it covers the bottom of the dish. Swirl if necessary.
Place the dish on a white background (like a piece of paper) to
see the waves more clearly. Don't disturb the solution!
5.
You will notice spreading rings of blue forming in the orange
solution. Notice how rapidly the blue color spreads. Calculate
how long a molecule would take to diffuse just 1 cm [D  105
cm2/s, distance = ½Dt.] Can simple diffusion account for the
speed of color propagation?
6.
You will see bubbles form. What are they? Look at the reaction. If too many bubbles
form, swirl the dish around; that disturbs the solution, of course, so start any timing over.
7.
What happens when two waves collide? How does this differ from water waves?
For the next series of experiments, prepare the reaction mixture as in steps 1-5
above.
8.
To make a spiral, slowly move a pipette tip through the center of a target pattern. In
which direction does the spiral rotate?
9.
Repeat the spiral experiment. Use graph paper to measure the change in the radial
distance from the center of a target pattern as a function of time. The slope of the line
drawn through these data will provide the wave speed. Also measure the distance
between fronts, i.e., the wave length.
10.
Repeat step #9 at least three times to get the mean, standard deviation, and relative
standard deviation. Does the velocity remain constant with time?
11.
Repeat the spiral experiment three times each for the following solutions. Calculate the
mean, standard deviation, and relative standard deviation. Does the velocity remain
constant with time for these solutions?
Solution A1: Take 40 ml solution A and dilute to 50 ml.
Solution A2: Take 20 ml solution A and dilute to 25 ml.
Solution A3: Take 10 ml solution A and dilute to 20 ml.
12.
Using 1.0 ml solution C and the same volumes of the other reagents, make velocity and
wavelength measurements for solutions A1, A2, and A3. Does the velocity remain
constant?
13.
Plot the velocity as a function of the square root of the product of [BrO3]0 and [H+]0. Fit
a line. Using the slope and the diffusion coefficient of HBrO2, calculate the rate constant
of the autocatalysis reaction. Compare it to the reported value of 20 M2 s1. Does the
velocity depend on the ferroin concentration?
For the next series of experiments, you will measure the concentration of bromine
spectrophotometrically at 960 nm. Calculate and record the concentration of each
species in solution.
14.
Add 2 ml of solution A into a Spec-20 cuvette. Add 2 ml of solution B and 0.4 ml of
solution D into the same cuvette. Cover the cuvette with Parafilm immediately.
15.
Invert the cuvette a few times, then place it into the Spec-20.
16.
Begin recording the absorbance at 15-second intervals until the absorbance remains
constant for about 3 minutes. Begin the stopwatch when you place the cuvette into the
Spec-20.
17.
Repeat steps #14-16 with 0.8 ml solution D.
18.
Repeat steps #14-16 with 1.6 ml solution B, 0.4 ml water (to keep the concentrations of
the other species constant,) and 0.4 ml solution D.
19.
Repeat steps #14-16 with 1.2 ml solution B, 0.8 ml water, and 0.4 ml solution D.
20.
Plot absorbance vs. time. Is the plot linear? If so, fit a line and determine the linear
equation and r2. The slope is the important quantitative result. If your plot is exponential,
take the natural logarithm of the absorbance data and then fit a line.
21.
Determine the rate-limiting step of the reaction and the kinetics of that step.
22.
Calculate the rate constant k with its correct units.
Notebook Hints: Set up the well-organized data tables that you will need. You should
record the various chemical equations in a convenient format. This way you will not forget to
record any data or observations you need when writing your report. As you perform the
experiment, record specifically what you are doing and how you are doing it in your notebook.
Report Hints: You will not be able to do this lab report without the reference article! You
may exceed four typed pages for this report.
Introduction: In one or two paragraphs, summarize in your own words the reference article
given above. Exclude the experiments we did not do.
Results: Include all your data in tables. Graph your data (radial distance vs. time, and velocity
vs. square root of [BrO3]0 and [H+]0.) Include the mean, standard deviation, and relative
standard deviation. Show a sample calculation for velocity. Include your graph and calculation
of the rate constant for step #13 above.
Discussion: Answer all the questions in the procedure. There will be some overlap with your
notebook, so if you do a good job there, your report will be easier to write. Discuss errors
inherent in the lab, any mistakes you made, and how those would affect your data.
Photo by Fritz Goro taken from page 144 of
P.W. Atkins', Atoms, Molecules, and Change,
W.H. Freeman, 1991.
Turn this page on its side to read the time
progression of this experiment. And don't get
paranoid if your waves aren't as visible.
These photos have been color saturated!
(The clue is the non-white, "white" paper on
which those petri dishes rest.) The actual
colors are closer to those in the figure on the
first page in which I rebalanced the colors to
turn the background paper white. Goro
cheats.
With the paper turned –90º, the progression
of pictures will be:
Top row:
1 2 3 4 5
Bottom row: 6 7 8 9 10
Note that photo 4 is a "flopover" or mirror
image of what it should be! With that in
mind, 3 and 5 make sense. 