The objective for this lab was to learn about the fascinating chemistry of oscillating chemical reactions and to be able to explain the chemistry that occurs in an oscillating reaction. More specifically, the main reaction that was studied was the Belousov-Zhabotinskii reaction, or the"BZ" reaction.

Experiment 1: (Shakhashiri, 7.2, pp. 257-261)

The third reaction studied was the Cerium-Catalysed Bromate-Malonic Acid Reaction. This reaction is also known as the Classic B-Z reaction.

"A clear colorless solution and a pale yellow solution are mixed, producing an amber solution, which becomes colorless after about 1 minute. Then a yellow solution is added, followed by a small amount of red solution, producing a green solution. the color of the solution gradually changes over a period of about 1 minute from green to blue, then to violet, and finally to red. The color then suddenly returns to green, and the cycle repeats more than 20 times. The electrical potential of the solution oscillates along with the color of the solution, and the range of these oscillations is about 200 mV" (2).

Hazards:

1. Bromates are strong oxidizing agents. Bromates mixed with finely divided organic materials metals, carbon, or other combustible materials are easily ignited.

2. Ingestion of potassium bromate (KBrO₃) can cause vomiting, diarrhea, and renal injury.

3. Sulfuric acid (H₂SO₄) is a strong acid and a powerful dehydrating agent, and can cause burns on contact. Spills should be neutralized with sodium bicarbonate (NaHCO₃) and rinsed down the drain with excess water.

4. Malonic acid (CH₂(COOH)₂) is a strong irritant to skin, eyes, and mucous membranes.

Procedure and Observations:

1.) To beaker A, 19 g. potassium bromate (KBrO₃) was dissolved in 500 mL distilled water in 1 L beaker.

- The total concentration of this solution was 0.23 M in KBrO₃

2.) To beaker B, 16 g. malonic acid (CH₂(COOH)₂) and 3.5 g. potassium bromide (KBr) were dissolved in 500 mL distilled water in a 1 L beaker.

- The total concentration of this solution was 0.31 M in malonic acid and 0.059 M in KBr.

3.) To beaker C, 5.3 g. cerium (IV) ammonium nitrate (Ce(NH₄)₂(NO₃)₆) was dissolved in 500 mL sulfuric acid (H₂SO₄) in a 1 L beaker.

- The total concentration of this solution was 0.019 M in Ce(NH₄)₂(NO₃)₆ and 2.7 M in H₂SO₄.

4.) Solutions A and B were poured into a 2 L beaker with a magnetic stir bar and was placed on a magnetic stirrer. The stirrer was adjusted to create a significantly sized vortex in solution.

- At this point, the solution was supposed to change into an amber color, but it remained colorless.

5.) Continuing on in the process, after the solution was colorless, solution C and 30 mL ferroin solution were added to the 2 L beaker solution.

- The solution became green and started to oscillate between a green and a blue color. Then, it started to move from a green to blue, and then to a violet, and a violet red. After time, it distinctly changed from violet to red.

- This experiment was much more successful than experiment 2, which was similar. This reaction did not have any noticeable turbidity and did not create any visible precipitate. The solution also remained clear and did not become cloudy. The colors were much more clear and the oscillations were not hard to recognize like the violet-red oscillation in experiment 2. In this experiment, there was a definite purple and a definite red color in the oscillations. Once the reaction neared an endpoint, the red was the first color to disappear and then the reaction worked its way to a green endpoint. Around the 25 minute mark, it stopped at a light green and ended at a light lime green color. The reaction lasted for approximately 25 to 30 minutes.

- The total concentration of the 2 L solution was 0.77 M BrO₃-, 0.10 M malonic acid, 0.020M Br^-, 0.0063 M Ce (IV), 0.90 M H₂SO₄, and 0.17 mM ferroin.

Disposal of Solution:

After the reaction stopped oscillating, the mixture was neutralized with NaHCO₃ and flushed down the drain with water.

Discussion:

This reaction was first discovered by Boris Belousov in 1958. "He mixed potassium bromate, cerium(IV) sulfate, and citric acid in dilute sulfuric acid and found that the ratio of concentration of the cerium(IV) and cerium(III) ions oscillated" (2). His discovery was rejected because his proposal meant that the reaction did not go directly to the thermodynamic equilibrium, like it was believed every reaction should do. Zhabotinskii studied the reaction and found that the oscillations still occurred when citric acid was replaced by any number of carboxylic acids with the common structural feature:

He also found that manganese ions could replace the cerium ions and still make the oscillations occur. The Overall reaction that occurs is the cerium-catalyzed oxidation of malonic acid of malonic acid by bromate ions in dilute sulfuric acid. "The bromate ions are reduced to bromide ions, while the malonic acid is oxidized to carbon dioxide and water" (2). This is represented as:

This overall equation, represents the major chemical transformations that occur during the experiment. The free-energy difference between the products and the reactants is what drives the reaction. This reaction shows the ultimate chemical transformations, but it does not show the most fascinating observations the oscillating demonstration has to offer. In explaining the oscillating reaction that is taking place, time must be spent in dealing with the periodic changes in the color of the solution as the reaction proceeds, the catalytic effect of the cerium, and the role played by the bromide ions that are added into the mixture.

In describing the reaction in detail, the mechanism must be split up into two different processes. Process A involves ions and the two-electron transfer steps. Process B involves ions and its two-electron transfer steps. The bromide ion concentration is important because it determines which process is dominant at any particular time during the mechanism. Process A is dominant when the bromide ion concentration rises above a certain critical level. Process B dominates the mechanism when the bromide ion concentration falls below a certain level. Oscillation is observed because Process A consumes bromide ions, which lead to the conditions which factor Process B. The bromide ions are indirectly liberated by Process B, so the reaction goes back to Process A. After solutions A and B are mixed, the transformation that takes place in Process A is represented by equation [2].

Equation [2] is the reduction of bromate ions by bromide ions through a series of oxygen transfers. The oxygen transfers are the two-electron reductions. After the solutions are mixed together and stirred with the magnetic stirrer, the bromine reacts with malonic acid, represented by equation [3].

Reactions [2] and [3] result in a decline in the bromide ion concentration. The rate for Process A falls to a negligible value after it has generated the necessary intermediates and consumed most of the Br^-.The result is the domination of Process B. The overall reaction effected by Process B is shown in equation [4].

This reaction is caused by reactions [5] and [6], which cause [HOBr] to increase autocatalytically. Autocatalysis is a catalytic reaction that is started by the products of a reaction that was itself catalytic.

At this point, it appears that autocatalysis is an essential feature of oscillating chemical reaction. Once the reactants are depleted, autocatalysis continues, and there is a second-order destruction of the autocatalytic species represented by equation [7].

Process B produces Ce(IV) and Br₂. Both of these react in part to oxidize organic material with the formation of bromide ion. As the concentration of Br^- produced by equation [7] increases, the rate of equation [2] (the net transformation of Process A) increases and surpasses the rate of equations [5] and [6] of Process B. This means that the reaction has started to follow the pathway of Process A again.

In this oscillating reaction, Process A and Process B result in a competition between bromide ions and bromate ions for bromous acid. "When the concentration of bromide ions is high, nearly all of the bromous acid reacts with it, following Process A" (2). During Process A, the bromide ion concentration decreases and the bromide ion becomes less and less successful at competing for the bromous acid. This means that Process B starts to take over again. At this point, Process B produces bromide ion indirectly, and eventually the concentration of bromide ion becomes high enough to cause a shift back to Process A. This competition and switching back and forth between processes cause the oscillations in the reaction to be observed.

"As the reaction oscillates between Process A and Process B, triggered by changes in the bromide ion concentration, other concentrations oscillate as well" (2). During Process A, the cerium ions are in their reduced state, Ce(III), because of their reaction with the bromomalonic acid. During Process B, some of the cerium ions are oxidized to Ce(IV). This means that the ration of [Ce(III)] to [Ce(IV)] will visually oscillate as well. The oscillations are viewed because of the use of an oxidation-reduction indicator, ferroin. As the ratio of [Ce(III)] to [Ce(IV)] decrease, the Ce(IV) oxidizes the iron in ferroin from iron(II) to iron(III). The color of the solution changes as the iron is oxidized because the iron(II) complex is red and the iron(III) complex is blue. When the [Ce(III)] to [Ce(IV)] ratio rises, the iron(III) is reduced to iron(II), and the solution returns to its original color, thus the oscillating colors. The color changes for this BZ reaction are more complex because there are changes due to the cerium ions as well as the iron complex. "The ferroin is known to catalyze the oscillating reaction in the absence of Ce ions, and it appears to enter the catalytic cycle involving Ce ions [5]" (2).

This discussion of this BZ mechanism oversimplifies what actually occurs in the reaction mixture. The study of oscillating reactions is an active field of research, so this description just touches on the surface of all the details that are known about the system.

Experiment 2: (Shakhashiri, 7.4, pp. 266-269)

The second reaction studied was the Cerium-Catalysed Bromate-Ethylacetoacetate Reaction. This reaction is modified version of the BZ reaction.

"A clear colorless solution and a light yellow solution are mixed. Then a small amount of a red-orange solution is added, producing a blue mixture. A yellow solution is added to this blue mixture, resulting in a green solution which returns to blue in about 20 seconds. After several oscillations between blue and green, the color of the solution will start to oscillate from green, to blue, to violet, to red, and back to green, with an initial period of about 20 seconds. These oscillations will continue for about 20 minutes, by which time the period will have lengthened to about 20 seconds. The electrical potential of the solution oscillates along with the color, and these oscillations cover a range of about 180 mV" (2).

Hazards:

1. Bromates are strong oxidizing agents. Bromates mixed with finely divided organic materials metals, carbon, or other combustible materials are easily ignited.

2. Ingestion of potassium bromate (KBrO₃) can cause vomiting, diarrhea, and renal injury.

3. Sulfuric acid (H₂SO₄) isa strong acid and a powerful dehydrating agent, and can cause burns on contact. Spills should be neutralized with sodium bicarbonate (NaHCO₃) and rinsed down the drain with excess water.

4. Ethylacetoacetate (CH₃COCH₂COOC2H₅) is moderately irritating to skin and mucous membranes.

Procedure and Observations:

1.) To beaker A, 19 g. potassium bromate (KBrO₃) was dissolved in 1.5 M sulfuric acid (H₂SO₄) in 1 L beaker.

2.) To beaker B, 11 mL ethylacetoacetate (CH₃COCH₂COOC2H₅) was dissolved in 500 mL distilled water in 1 L beaker.

- ethylacetoacetate is also called acetoacetic ester or diacetic ether.

3.) To beaker C, 4.5 g. cerium (IV) ammonium nitrate (Ce(NH₄)₂(NO₃)₆) was dissolved in 500 mL 1.5 M sulfuric acid (H₂SO₄) in 1 L beaker.

4.) Solutions A and B were poured into a 2 L beaker with a magnetic stir bar and was placed on the magnetic stirrer. The stirrer was adjusted to create a significantly sized vortex in solution.

5.) 30 mL. ferroin solution was added to the mixture.

- The experiment called for a 0.50% ferroin solution, so a 0.01 M solution was used.

- Immediately after the ferroin solution was added, the solution changed to a deep blue color. The solution was also clear in appearance.

6.) Solution C was added to the 2 L beaker solution.

- After solution C was added into solution, the solution immediately turned green and oscillated between green and a blue for about 2 minutes. The solution then began to change from green to a grey-blue, to a purple-red, and back to the original green color. The solution also became cloudy immediately after the addition of solution C. As the reaction continued, there appeared to be a small, white precipitate forming and circulating at the bottom of the beaker. It was believed that the precipitate was the result of the turbidity.

- The reaction did not go quite as expected. It was believed that the solution would be clear and the color changes would be distinct, however, neither seemed to be the case. The solution was very cloudy and the color changes that were observed were not as expected. There was not a clear blue color and there was never a separate red color. The red always appeared as a violet-red. The addition of the cerium (IV) ammonium nitrate solution is what made the reaction cloudy.

- To try to make the color changes more distinct, more indicator (ferroin solution) was added slowly. As soon as the indicator was added (about 7 to 8 minutes into the reaction) the oscillating ceased and stayed in the green stage of the reaction. Up to an additional 15 mL. ferroin solution was added. Each time it was added, a dark-blue color appeared in the vortex, but the rest of the solution stayed green. After a while, this blue color in the vortex simply faded back into the green solution. A little more cerium (IV) ammonium nitrate was then added to see what the solution would do. As expected, the solution became discolored and turned into a darker, olive-green color.

- The total concentration of the original solution in the 2 L beaker was 0.077 M BrO₃-, 0.057 M ethylacetoacetate,
0.053 M Ce^4+, 1.0 M H₂SO₄, and 1.7*10^-4 M ferroin.

Disposal of Solution:

The reaction solution was neutralized with NaHCO₃ and flushed down the drain with water. The organic residue on the beaker was dissolved in a 2 M solution of potassium hydroxide (KOH) in ethanol (C₂H₅OH) and flushed down the drain with water.

Discussion:

This experiment was just like the first one, except ethylacetoacetate was used in place of malonic acid. Both ethylacetoacetate and malonic acid contain a methylene group between two carbonyl groups, but when the malonic acid is oxidized in this BZ reaction, the products include carbon dioxide and formic acid. Nonvolatile oxidation products and CH₃COCBr₂CO₂CH₂CH₃ are produced when the ethylacetoacetate is reacted. "Because these reaction products are not oxidized further to gaseous carbon dioxide, the ethylacetoacetate substrate is particularly useful for studies in a closed-flow reactor, where the production of gas is undesirable because it changes the volume of liquid in the reactor" (2).

The mechanism to experiment is very similar to the mechanism that takes place in Experiment 1, where the malonic acid is used. Like the malonic acid, the ethylacetoacetate underwent enolization in solution, shown in equation [1]
 
 
 
 
 
 
 
 

The enol form of the ethylacetoacetate undergoes electrophilic attack by elemental bromine at the carbon-carbon double bond. The malonic acid also underwent electrophilic attack in

Experiment 1. "The enolization reaction is an equilibrium reaction, with most of the ethylacetoacetate in the dicarbonyl form in aqueous solution. The rate of the reaction with bromine is determined by the rate of enolization, which is much slower than the electrophilic attack by bromine" (2).

Experiment 3: (Shakhashiri, 7.6, pp. 273-275)

The first reaction studied was the Manganese-Catalysed Bromate-Malonic Acid Reaction. This reaction is modified version of the BZ reaction.

"Three white solids are added sequentially to a colorless solution being stirred in a beaker. After the third solid is added, the solution turns orange, and after about 75 seconds, it becomes colorless. The color then oscillates between colorless and orange, with an initial period of about 20 seconds. The oscillations will continue for about 10 minutes" (2).

Hazards:

1. Bromates are strong oxidizing agents. Bromates mixed with finely divided organic materials metals, carbon, or other combustible materials are easily ignited.

2. Ingestion of potassium bromate (KBrO₃) can cause vomiting, diarrhea, and renal injury.

3. Sulfuric acid (H₂SO₄) isa strong acid and a powerful dehydrating agent, and can cause burns on contact. Spills should be neutralized with sodium bicarbonate (NaHCO₃) and rinsed down the drain with excess water.

4. Malonic acid (CH₂(COOH)₂) is a strong irritant to skin, eyes, and mucous membranes.

Procedure and Observations:

1.) 750 mL distilled water put into a 1 L beaker

2.) 75 mL Con 18 M sulfuric acid (H₂SO₄)was carefully and slowly added to beaker

3.) stir bar added to beaker and placed in magnetic stirrer

4.) 9 g. malonic acid (CH₂(COOH)₂) was dissolved into solution

- malonic acid is also named methanedicarbonic acid or methanedicarboxylic acid.

5.) 8 g. potassium bromate (KBrO₃) was dissolved into solution

6.) 1.8 g. manganese (II) sulfate monohydrate (MnSO₄* H₂O) was dissolved into solution

- As soon as the manganese (II) sulfate monohydrate was added, the solution turned orange and immediately began to oscillate. For approximately 75 seconds, the solution oscillated between an orange color and a rusty orange color, working its way to oscillating into a colorless solution. After the 75 seconds, the solution was successfully oscillated between orange and a colorless solution. As time moved on, the time between oscillations increased. The solution oscillated for 20 minutes and then remained in the colorless stage.

- The total concentration of the solution was 0.1 M BrO₃-, 0.013 M Mn²⁺, and 1.6 M H₂SO₄.

Disposal of Solution:

After the reaction stopped oscillating, the mixture was neutralized with NaHCO₃ and flushed down the drain with water.

Discussion:

This experiment is very similar to Experiment 1. This experiment is different because the manganese ions are used as the catalyst instead of the cerium ions, there are no bromide ions that are used up in the preparation of the solutions, and the use of ferroin as an indicator, is omitted.

In this experiment, "manganese ions are used as the catalyst instead of cerium ions, no bromide ions are used in the preparation of the solutions, and the redox indicator, ferroin, is omitted" (2). The changes that are made do not change the chemical reactions greatly, but they do change the appearance of the experiment. The manganese ions act similarly to the cerium ions and take part in a reaction that involves BrO₂ radicals. This is shown in equation 1.

The reaction is a one-electron transfer in which the cerium ions are oxidized. This means that the manganese ions will do the same. This can be shown in Equation 2.

Mn(II)(aq) + BrO₂(aq) + H⁺(aq) Mn(III)(aq) + HBrO₂(aq) (2)Equation [2] shows the conditions of the experiment, however, no Mn (III) have been detected yet. "The standard reduction potentials of the two couples are similar [2]" (2).

This means that the two metals can be inner changed because the can fulfill thermodynamically similar roles. The slight difference simply means that at any given point, the ration of [Ce(IV)] to [Ce(III)] is greater than that of [Mn(III)] to [Mn(II)]. "This suggests that the concentration of Mn(III) may not reach a detectable level in the oscillating reaction mixture, according for the failure to observe it in the mixture" (2).

Bromide ions are necessary for oscillating reactions to work and are produced in this experiment in a series of reactions that occur within the mixture. The BrO₃^- ions react in a series of steps with the Mn(II) ions, producing HOBr. This is demonstrated in equation 3.

The HOBr that is formed reacts with malonic acid to form bromomalonic acid, shown in equation 4.

The bromomalonic acid is oxidized by the Mn(III) and releases bromide ions:

This reaction is analogous to the reaction described in the opening of chapter 7 in Shakhashiri, where Ce(IV) is the oxidant. Equation [5] shows how the bromide ions are introduced into the solution to create the oscillatory appearance. The oscillatory appearance and disappearance of the orange color of the elemental bromine are shown without the use of the ferroin indicator.

It was very important that all of the equipment was clean at the beginning of the experimental because any small amount of chloride ions in the solution would interfere with the mechanism of the BZ reaction. Chloride that is entered in the solution inhibits the oscillations.

Side Notes:

1.) At the end of the lab experimentation, a little time was spent also spent on looking at the static effect of the BZ reaction in petri dishes and small beakers. Not much time was spent on the static reaction, however, good film clips were taken of the reaction and will be placed on the internet. Further researching of this reaction, as well as other oscillating reactions, will also come in the future.

2.) Experiment 1 was run three different times. The first and second times were during lab hours, but the third run was completed during a chemistry seminar on April 9, 1999. The outline for that presentation can be found at http://inst.augie.edu/~gjhonsbr/BZ_pres.html

3.) The second and the third times Experiment 1 were run, turned out different than the first time due to chloride contamination in the PVC pipes of the college laboratory. These demonstrations were run the same as the first demonstration, in the same conditions, but oscillated from a green, to a blue, and to a purple. The reaction then stayed in the purple stage for approximately 15-20 minutes before it changed back to green. The chloride that was entered into the system due to contamination inhibited the reactions for the second and third runs.

4.) The static reaction, run without stirring, worked because the solutions were taken from pre-made demonstration bottles from the stock room.

(1) Hall, Nina, Exploring Chaos: A Guide to the New Science of Disorder, W. W. Norton & Company, New York, 1991, pp. 108-121.

(2) Shakhashiri, Bassam Z., Chemical Demonstrations: A Handbook for Teachers of Chemistry, University of Wisconsin Press, Madison, Wisconsin, 1985, Vol. 2, pp. 232-307.
 
 

(3) http://www.musc.edu/~alievr/rubin.html

(4) http://ink.yahoo.com/bin/query?p=belousov-zhabotinskii+reaction&z=2&hc=0&hs=0

(5) http://www.musc.edu/~alievr/belous.html

(6)  http://www.unipa.it/~chimica/MAGIA/cilindro.htm

(7)  http://www.pc.chemie.uni-siegen.de/pci/versuche/v57-3.html

(8)  http://www.chem.monash.edu.au/Docs/DGHewitt/POWERPNT/Chap10/sld033.htm

(9)  http://www.asms.net/~lilly/sq91/Abrams.html

(10)  http://www.nrcam.uchc.edu/vcell_mathFramework.html

(11)  http://ink.yahoo.com/bin/query?p=zhabotinskii&z=2&hc=0&hs=0

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