John F. Lefelhocz
Virginia
I
Commonwealth University Richmond, 23220
The Color Blind Traffit Light A n u n d e r g r a d u a t e kinetics experiment using a n oscillating reaction
Laboratory experiments in kinetics for first-year chemistry courses generally fall into one of two categories. The first requires a quantitative determination of a rate constant for a simple firstorder reaction and gives the student some insight into the mechanism of the studied reaction. The second more popular approach is to involve the student with a qualitative study of the influence of chemical and physical variables on the rate of a specific reaction. This paper deals with the latter approach and uses an oscillating reaction that has been reported by Zhabotinskii (I), was further studied by Degn ( B ) and Kasperek and Bruice (S), and has been used as a demonstration by Rossman (4). This experiment has been used successfully in a nonscience majors course, but it can easily be adapted for a science majors course. Regardless of their class level, science or nonscience majors should develop an understandig of the influence that changes in temperature and concentration have on an observed reaction rate. The reaction system used in this experiment is given below. HOOC-CHn-COOH
+ 6Ce4+ + 2H10
-
2COn
+
HCOOH + GCeS++ 6H+ 'OCe"
+ 2Br08- + l Z H t - 'OCe4+
+ Bra +
@)
The concentration of Ce4+in solution periodically increases and decreases with time and the change in the concentration of the Ce4+is followed by the repetitive color changes induced in the redox indicator Ferroin (violet to blue). Such color changes arouse student curiosity and interest. An oscillating reaction of this type allows the instructor to draw analogies between this cyclic system and the oscillatory systems which are found in nature, such as the heart beating, the lungs pumping, the Iirebs cycle, or any of the many so-called biological L'clocks." Observation of the test tube reaction can enable students to develop an insight into the chemical effects that are involved in biological "clocks." By using an oscillating reaction system to illustrate the effect of changes in temperature and concentration on a reaction rate, the instructor has a powerful tool that stimulates student interest and introduces interdisciplinary relevance into the study of chemical kinetics. Reagents a n d Solutions
Three reagent stock solutions and the redox indicator Ferroin (see Table 1) are prepared for the students. They then prepare approximately 100 ml of 3N H2SO4 and a base (parent) solution consisting of 50 ml each 312
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Table 1.
Solution identification
Stock Reagent Solutions
Concentration (molar)
A B
0.00045 M Ce(NH4),(SO4)..2HnOin 3 N HnSOI 0.090 M KBrOa in 3 N HPSO, 0.30 M Malonic acid in 3 N H a 0 4
Ferroin indicator
0 . 1 M Ferroin in 3 N HnSOl
C
Table 2.
Solution
Concentrations of Experimental Solutions
Ce4+
Malonic acid
Aciditv 3 N H2S04 3 N H&Ol 3 IV HISOl 3 N HB04 2 N HBO, 1 N HZSOI
of solutions A, B, and C. They record the time of mixing of the base solution and immediately prepare 30 ml of the five remaining test solutions, which in our laboratory are stored in loosely-stoppered 6-in. test tubes. Concentrations of all reactants in the six solutions are given in Table 2. We have found it helpful to em~hasizeto the students that the acidity is to be kept a't 3N in Solutions 1, 2, 3, and 4 , and i t is decreased to 2N in Solution 5 and 1N in Solution 6. An induction period of approximately 25 to 40 minis necessary before the oscillation of the Ce4+-Ce3+concentration will occur. Approximately 4 to 6 drops of the Ferroin indicator are generally required to see the desired color change when the experiment is carried out using a 6-in. test tube. The indicator has a tendency to decompose at elevated temperatures and it may be necessary to add more in the later stages of the temperature study. The color change can be observed if the solution is stirred with a magnetic stirrer, but if the solution is agitated or shaken violently, the color change cannot be observed as the indicator reverts back to a red color. We agree with Iiasperek and Bruice (3) that the amplitude of the oscillating reaction system decreases with time and that stirring ail1 restore it to its original intensity. The concentrations given in Table 2 facilitate the precise determination of the period of oscillation because the sky blue color appears rather abruptly and makes an excellent reference point for timing purposes. Presented in part at the Forty-ninth Annual Meeting of the Virginia Academy of Science, Chemistry Section, May 1971.
\
140--
0 U
m
110-
Y)
.5 1000
.-E
90-
0
:70-
Figure 2. The period of orciliation for rolutionr 1, 2, 3, ond 4, 01 25'C. ploned or m function of the BrOa- ion concentmtion.
40.3020100
5
10 15 20 25 30 35 40 45 50 55 60 Temperature *C
Figure 1. The period of owillotion for solution 1,0.00015 M Ce4+, 0.030 M B a a ; ond 0.10 M molonic acid in 3 N HzS04, plotted 03 a function of temperature.
The solutions will oscillate for well over 3 hr after mixing. In the demonstration by Rossman (4) a color change from green to red (the traffic light) is observed by increasing the Ce4+ concentration tenfold and by careful adjustment of the indicator coucentration. This color change, however, is too gradual for timing purposes and the solution oscillates no longer than 45 min to 1hr after preparation. Experiment a n d Results
The experiment as performed in our laboratories consists of a required three-part study of temperature, dilution, and acidity effects on the per,iod of oscillation and a fourth, optional, open-ended study. We have found that efficient use of the induction period can be made if the student prepares the six solutions first and then attends the "pre-lab" lecture. Solution 1is used for the investigation of the influence of temperature on the period of oscillation. It is cooled to 15°C and the period of oscillation is recorded a t 5" intervals as the solution is warmed until the period of oscillation is too short for precise observation. Figure 1 is a typical plot of the period of oscillation versus the solution temperature from actual student data obtained in our nonscience majors course. Demonstration that the plot is logarithmic is made to these students. The dilution study consists of noting the period of oscillation for Solutions 1, 2, 3, and 4. A plot of the period of oscillation as a function of the concentration of any one of the components, malonic acid, Ce4+ or BIG- is then made. A plot of the period of oscilla-
tion of the Br03- concentration from actual student data is shown in Figure 2. In the acidity study the student observes the period of oscillation of Soutions 4, 5, and 6. The observed periods of oscillation from student data are recorded in Table 3. We have found that the student appreciates the value of the temperature study dataif he determines the temperature to which Solution 5 must be heated in order for it to have an oscillation period comparable to that observed for Solution 1a t room temperature. The optional open-ended part of the experiment allows the student, with some supervision, to "do his thing" in studying the reaction system. We offersome suggestions to direct our students' thoughts into possible areas of investigation. The addition of bromine to the oscillating solution delays formation of ceric ion, but after this delay the normal period resumes (3). One of our suggestions is a study of the inhibition time as a function of the amount of bromine added to a given solution. The effect of mixed solvent systems such as alcohol-water or acetone-water is another possibility for study. Discussion a n d Conclusions
A major advantage of this system is that an excellent temperature study can be performed using one solution (see Fig. 1) thus eliminating dilution and timing errors which are frequently encountered in the temperature study of an irreversible reaction. The effect of temperature on the rate of oscillation can be further discussed by the introduction of the Arrhenius equation. Even though the mechanism of this system is not fully understood (however, see the companion piece by Field (7) for a discussion of the latest workon this reacTable 3. Acidity Study. The Effect of the Hi concentration on the Period of Oscillation Solution 4 5 6
H+ Concentration 3N 2N 1N
Time of Oscillation
,,,
45 60 sec Too long to 'measure accurately
All solutions are 0.00005 M Ce'+, 0.010 M BIO; and 0.03 M rndonic acid. Volume 49, Number 5, May 1972
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tion and the article by Degn (8) for a review of the theory of oscillating reactions in general), the students in an advanced first year course could calculate an activation energy for the reaction. Kasperek and Bruice (8) have shown that the reaction system is sensitive to shaking or stirring and have concluded that colloidal particles are present. They suggest that the farmation and dissolution of these colloidal particles or a reaction that is dependent on their surface area explains the oscillation. We have noted that violent shaking of our reaction system prevents the observation of the color change, but when laminar flow exists as when a magnetic stirrer is used, the color change is seen. The standard state oxidation potentials of the Ce4+-Cea+and the BrOa--Brz half cells predict that the reaction as written in eqn. (2) will not proceed when all reactants and products are in the standard state. The fact that BrOa- does oxidize Cea+ in the laboratory may he used to introduce the students to the Nernst equation and the relationship it draws between emf values, standard state emf values, and reaction coefficients. The interested student may investigate the effect of various concentrations of HC1 or HNOa'as part of the open-ended study since the Ce4+-Cea+half cell potential is also dependent on the type of acid used (5). Higgins (6) has formulated a general theoretical method which can he applied to the kinetic analysis of
314 / Journal of Chemical Education
any type of reaction scheme that will differentiate fast reactions from slow reactions, and weak control from strong control. He points out that the essential requirement for oscillation in any system is the existence of appropriate feedback in the reaction pathways. It has been observed in this laboratory that the addition of Ferroin to the system of Kasperek and Bruice (8) does increase the frequency of the Ce4+-Cef+ oscillation. This further complicates the explanation of the oscillatory nature of this reaction. A further investigation of this phenomenon is now being carried out in this laboratory. Often the knowledge that there is a need for further investigation into the complexity of a reaction motivates students. All too often, in firstyear courses particularly, students are given experiments that seem to them to leave no avenues for further investigation. This experiment should definitely eliminate the feeling that there are no areas of chemistry where questions need to he postulated and answers sought. Literature Cited za&sorn*sarr.A. M..Biofrr;ko. 9,308(1904). Dma. H.,Nature (London), 213, 589 (1907). K A ~ P E R EG. K . J., A N D BRUICE.T.C., ln07v. Chblll., 10, 382 (1971). R O S ~ M AG. N . R., private communication. Co~rorr,F. A,. AND WILKINBON. G., ''Inorm.ni~Chamiatry" (2nd Ed.). John Wilev & Sons, Inc., 1966, p. 1008. Hraams, J. J.,Ind. En.. Chem., 59, 18 (1907). FIBLD.R. J., J. CEEM.EDUC.,49, 308 (1972). DEYN.H m e , J. CIEY,EDUO.. 49. 302 (1972).
