Chemical vibrations. History of the discovery of oscillatory reactions
Oscillatory chemical reactions
In this course work I will consider a special case of a problematic experiment, oscillatory chemical reactions. Oscillatory reactions are a whole class of oxidation reactions of organic substances with the participation of a catalyst with redox properties. This process occurs cyclically, that is, it consists of multiple repetitions.
Oscillatory chemical reactions were discovered and scientifically substantiated in 1951 by the Soviet scientist Boris Petrovich Belousov. B.P. Belousov studied the oxidation of citric acid during its reaction with sodium bromate in a solution of sulfuric acid. To enhance the reaction, he added cerium salts to the solution. Cerium is a metal with variable valence (3+ or 4+), so it can be a catalyst for redox transformations. The reaction is accompanied by the release of CO 2 bubbles, and therefore it seems that the entire reaction mixture is “boiling.” And against the background of this boiling, B.P. Belousov noticed an amazing thing: the color of the solution periodically changed - it became yellow, then colorless. Belousov added a complex of phenanthroline with ferrous iron (ferroin) to the solution, and the color of the solution began to periodically change from purple-red to blue and back.
This is how the reaction that became famous was discovered. Now it is known all over the world, it is called the “Belousov-Zhabotinsky reaction”. A. M. Zhabotinsky did a lot to understand this amazing phenomenon. Since then, a large number of similar reactions have been discovered.
History of the discovery of oscillatory reactions.
IP Belousov made the discovery of the oscillatory chemical reaction while trying to create a simple chemical model of some stages of the system of key biochemical transformations of carboxylic acids in the cell. However, the first message about its discovery was not published. A reviewer for a chemical journal doubted the fundamental possibility of the reaction described in the article. Most chemists in those years believed that purely chemical oscillations did not exist, although the existence of oscillatory reactions was predicted in 1910 by A. Lotkoy on the basis of the mathematical theory of periodic processes.
The second attempt to publish the research results was made by the scientist in 1957, and again he was refused, despite the works of the Belgian physicist and physical chemist I.R. Prigogine that appeared at that time. These works demonstrated the possibility and probability of oscillatory chemical reactions.
Only in 1959 was a short abstract about the discovery of a periodically active oscillatory chemical reaction by B.P. Belousov published in the little-known publication “Collection of Abstracts on Radiation Medicine.”
And the whole point is that when B.P. Belousov made his discovery, periodic changes in the concentration of reagents seemed to be a violation of the laws of thermodynamics. In fact, how can a reaction go either in the direct or in the opposite direction? It is impossible to imagine that the entire huge number of molecules in a vessel would be in one state or another (sometimes all “blue”, sometimes all “red”...).
The direction of the reaction is determined by the chemical (thermodynamic) potential - reactions are carried out in the direction of more probable states, in the direction of decreasing the free energy of the system. When a reaction in a given direction is completed, this means that its potential has been exhausted, thermodynamic equilibrium is achieved, and without the expenditure of energy, spontaneously, the process cannot go in the opposite direction. And here... the reaction goes first in one direction, then in the other.
However, there was no violation of laws in this reaction. There were fluctuations—periodic changes—in the concentrations of the intermediates, rather than the initial reactants or final products. CO 2 does not turn into citric acid in this reaction; this is in fact impossible. The reviewers did not take into account that while the system is far from equilibrium, many wonderful things can happen in it. The detailed trajectories of a system from the initial state to the final state can be very complex. Only in recent decades have these problems been addressed by the thermodynamics of systems far from equilibrium. This new science became the basis of a new science - synergetics (the theory of self-organization).
Belousov's reaction, as noted above, was studied in detail by A. M. Zhabotinsky and his colleagues. They replaced citric acid with malonic acid. The oxidation of malonic acid is not accompanied by the formation of CO 2 bubbles, so changes in the color of the solution can be recorded without interference by photoelectric devices. It later turned out that ferroin, even without cerium, serves as a catalyst for this reaction. B.P. Belousov, already in his first experiments, noticed another remarkable property of his reaction: when stirring stops, the color change in the solution spreads in waves. This propagation of chemical vibrations in space became especially clear when in 1970 A. M. Zhabotinsky and A. N. Zaikin poured a thin layer of the reaction mixture into a Petri dish. Bizarre figures are formed in the cup - concentric circles, spirals, “vortices”, spreading at a speed of about 1 mm/min. Chemical waves have a number of unusual properties. So, when they collide, they are extinguished and cannot pass through each other.
The essence of oscillatory reactions. Mechanism and kinetics of oscillatory reactions.
Content
- INTRODUCTION…………………………………………………………...……..…3
- Basic concepts….……………………………………………………4
- History…………………………..……………………………………………………5
- Significance and scope…………………….……….…………8
- Reaction mechanisms………………………………………………………10
- Kinetics of oscillatory reactions…………………………………….…14
- Procedure for conducting the experiment………………………..…………….15
- Experimental data…………………………………….……….18
- Conclusion……………………………………………………………..23
- Bibliography…………..………………………………..…………24
INTRODUCTION
Vibrational reactions are one of the most interesting and attractive sections of inorganic chemistry. Attracting the close attention of not only chemists, but also physicists, mathematicians, biophysicists and many others, they are a pressing issue of modern science. Therefore, in my work I want to get acquainted with the history of oscillatory reactions, their practical application and the two most famous homogeneous oscillatory reactions, as well as understand their mechanisms and, by conducting an experiment, get acquainted with oscillatory reactions in practice.
Basic concepts of oscillatory reactions
- Oscillatory reactions- a class of redox reactions characterized by periodic fluctuations of intermediate substances and, as a consequence, fluctuations in color, temperature, flow rate, etc.
- Catalytic
- Homogeneous
- Enzyme catalyzed reactions
- Reactions catalyzed by metal ions
- Heterogeneous (reactions on solid catalysts)
- Non-catalytic, although it is more correct to call them autocatalytic (oxidation of aromatic compounds with bromate)
- The induction period is the time of primary formation and accumulation of the reaction catalyst.
- The period of oscillation is the shortest period of time during which one complete oscillation occurs (that is, the system returns to the same state in which it was at the initial moment, chosen arbitrarily)
The history of oscillatory reactions often begins with the German chemist and partly natural philosopher Friedlieb Ferdinand Runge. In 1850 and 1855, he published two books in succession, which described the colorful periodic structures that appear on filter paper when solutions of various substances are poured onto it one after another. Actually, one of them, “Matter in the Striving for Formation,” was “an album with pasted sheets of filter paper on which the corresponding reactions were carried out. For example, filter paper was soaked in a solution of copper sulfate, dried and again soaked in a solution of aluminum phosphate, drops of ferrous potassium sulfate were applied to the middle, after which the formation of periodic layers was observed.” After Runge, Raphael Liesegang enters the history of oscillatory reactions. In 1896, he published his experiments with rhythmic structures (Liesegang rings) resulting from the deposition of silver dichromate sediment in gelatin. Liesegang poured a heated gelatin solution containing potassium dichromate onto a glass plate. When the solution hardened, he applied a drop of silver nitrate solution to the center of the plate. The silver dichromate precipitate did not fall out in a continuous spot, but in concentric circles. Liesegang, familiar with Runge's books, was initially inclined towards a natural-philosophical and organismic explanation of the periodic process he obtained. At the same time, he reacted positively to the physical explanation of his “rings” given in 1898 by Wilhelm Ostwald, which was based on the concept of a metastable state. This explanation went down in history as the supersaturation theory.
Until now, we have not been talking about oscillatory chemical reactions themselves, but rather about periodic physicochemical processes, where a chemical transformation is accompanied by a phase transition. David Albertovich Frank-Kamenetsky came closer to chemical vibrations themselves, who began publishing his experiments on chemical vibrations in 1939. He described periodic phenomena during the oxidation of hydrocarbons: if, for example, mixtures of higher hydrocarbons are passed through a turbulent reactor, then periodic flashes (pulsations) are observed ) cold flame.
In 1949, a large article by I.E. was published in the Journal of Physical Chemistry. Salnikova, summing up his work, begun by joint research with D.A. Frank-Kamenetsky. In this article, the concept of thermokinetic oscillations was formed. During these oscillations, the temperature changes, and their necessary condition is a balance between the release of heat and its dissipation into the environment. And yet, the most powerful argument in favor of chemical vibrations was the article of Boris Pavlovich Belousov, which he tried unsuccessfully to publish twice - in 1951 and 1955. Although thermokinetic vibrations occur in homogeneous systems (unlike, say, Liesegang or oscillating chromium systems), they are provided by the physical (or physicochemical) process of thermocatalysis. Discovery of B.P. Belousova has practically completed an almost 150-year search for oscillatory modes in chemical processes. It was already a purely chemical vibrational reaction. In the 1950s, however, other events related to the Belousov reaction also occurred. After all, although the article by B.P. Belousov was rejected, information about his reaction was spread at the level of scientific folklore.
One of the recipients of this information was Simon Elyevich Shnol, who was already working on periodic processes in biochemistry. He was interested in the nature of chemical periodicity. Having received the manuscript of his article from Belousov in 1958, Shnol began experimenting with his reaction. And in 1961, he instructed his graduate student Anatoly Markovich Zhabotinsky to continue the work of B.P. Belousov, and he, conducting research first under the leadership of Shnol, and then independently of him, made a decisive contribution to elucidating the kinetics of the Belousov reaction and to its mathematical modeling. As a result, this reaction became known as the Belousov-Zhabotinsky reaction.
Reaction mechanisms
To date, several dozen homogeneous and heterogeneous chemical reactions have been studied. The study of kinetic models of such complex reactions made it possible to formulate a number of general conditions necessary for the occurrence of stable oscillations in reaction rates and concentrations of intermediate substances:
- Stable oscillations occur in most cases in open systems in which it is possible to maintain constant concentrations of the participating reagents.
- An oscillatory reaction must include autocatalytic and reversible steps, as well as steps that are inhibited by the reaction products.
- The reaction mechanism must include steps with an order higher than the first.
3HOOC(OH)C(CH 2 COOH) 2 + BrO 3 - Ce(3+/4+), H+→ Br - + 3CO 2 + 3H 2 O
The Belousov-Zhabotinsky reaction is the first of the open and studied oscillatory reactions. In this connection, it can perhaps be called one of the most studied reactions of this group. At the moment, the presence of eighty intermediate stages (and side reactions) occurring in the system has been confirmed in one way or another.
One of the very first and simplest reaction schemes was a scheme that consists of two stages:
- Oxidation of trivalent cerium with bromate
- And reduction of tetravalent cerium with citric acid
It, however, does not provide an understanding of how and as a result of which oscillations arise in the system, which leads us to consider the reaction mechanism proposed, in 1972, by Noyes and others:
- BrO 3 - + Br - + 2H + ↔ HBrO 2 + HBrO
- HBrO 2 + Br - + H + ↔ 2HBrO
- HBrO + Br - + H + ↔ Br 2 + H 2 O
- Br 2 + HOOC(OH)C(CH 2 COOH) 2 → Br - + H + + HOOC(OH)C(CHBrCOOH)CH 2 COOH
- BrO 3 - + HBrO 2 + H + ↔ 2BrO 2. +H2O
- BrO2. + Ce 3+ + H + → HBrO 2 + Ce 4+
- 2HBrO 2 ↔ BrO 3 - + HBrO + H +
- HBrO + HOOC(OH)C(CH 2 COOH) 2 → H 2 O + HOOC(OH)C(CHBrCOOH)CH 2 COOH
- 18Ce 4+ + HOOC(OH)C(CH 2 COOH) 2 + 5H 2 O → 18Ce 3+ + 6CO 2 + 18H +
So, let's consider the vibrations of Ce 3+ / Ce 4+ in this system. Let's say we have a small, gradually increasing amount of Ce 4+ in solution, which means that the concentration of Br is also small and increases due to reaction (10). Therefore, as soon as a certain critical concentration of Ce 4+ is reached, the concentration of Br - will increase sharply, which will lead to the binding of HBrO 2 stage (2), necessary for the catalytic oxidation of Ce 3+, stage (5), (6). It follows from this that the accumulation of Ce 4+ in the solution will stop and its concentration will decrease according to reactions (9), (10). A high concentration of Br will cause an increase in the rate of their consumption through reactions (1)-(3). In this case, after reducing the concentration of Br - below a certain value, it will practically stop reactions (2) and (3), leading to the accumulation of HBrO 2. This implies an increase in the concentration of Ce 4+ and a repetition of the cycle we have gone through.
Briggs-Rauscher reaction:
IO 3 - + 2H 2 O 2 + H + + RH Mn(2+/3+)→ RI + 2O 2 + 3H 2 O
Where RH is malonic acid, and RI is the iodine derivative of malonic acid.
This reaction was discovered in 1973. The essence of the reaction is the oxidation of malonic acid with iodate ions in the presence of hydrogen peroxide and a catalyst (Mn 2+/3+ ions). When starch is added as an indicator, fluctuations in the color of the solution are observed from colorless to yellow, and then to blue, caused by fluctuations in iodine concentrations. A complete study of the mechanism of the Briggs-Rauscher reaction is a complex and still unsolved problem, perhaps, first of all, a kinetic problem. According to modern concepts, the mechanism of this reaction includes up to thirty stages. At the same time, in order to understand the reasons for the oscillations, it is enough to consider a simplified reaction mechanism consisting of the eleven stages below:
- IO 3 - + H 2 O 2 + H + → HIO 2 + O 2 + H 2 O
- IO 3 - + HIO 2 + H + ↔ 2IO 2 . +H2O
- HIO 2 + H 2 O 2 → HIO + O 2 + H 2 O
- IO2. + Mn 2+ + H 2 O ↔ HIO 2 + MnOH 2+
- 2HIO + H 2 O 2 → 2I - + 4O 2 + 4H +
- MnOH 2+ + I - + H + ↔ I. + Mn 2+ + H 2 O
- HIO+ I - + H + ↔ I 2 + H2O
- 2HIO 2 → IO 3 - + HIO + H +
- RH↔ enol
- HIO + enol → RI + H2O
- I 2 + enol → RI + I - + H +
So, if the concentration of I is small (or these ions are absent in the solution, which corresponds to the initial moment of time), then in accordance with stage (5), and with further fluctuations and stage (11), as well as the reverse reaction of stage (7), they begin to accumulate in solution, which leads to a decrease (subject to availability) of the concentration of I 2. A decrease in the concentration of I 2 results in a decrease in the rate of accumulation of I - . At the same time, a high concentration of I - ions causes a greater rate of its consumption in the direct reaction of stage (7) and the increased concentration of I - decreases again, leading us to the beginning of this discussion and repetition of the described cycle.
Kinetics of vibrational reactions
The problems of studying kinetics are, at the moment, the most complex and still unresolved issues of vibrational reactions. Due to the large number of interdependent and parallel processes occurring in this class of reactions, compiling systems of differential equations that give at least approximate values of the rate constants of intermediate stages becomes an extremely non-trivial task. And although there are now several simplified models that allow us to consider the main features of the complex behavior of oscillatory reactions, this topic seems to be quite poorly studied and therefore extremely interesting for subsequent generations of researchers. At the same time, despite this, in this work this section of the study of oscillatory reactions will not receive further development due to the lack of time and funds necessary for its study.Procedure for conducting the experiment
Belousov-Zhabotinsky reaction.
Reagents: Citric acid, potassium bromate, cerium(III) sulfate, sulfuric acid.
Utensils: 50 ml measuring cylinder, 300 ml and 100 ml heat-resistant glasses, glass rod, spatula.
Equipment: Analytical balances, tiles.
To carry out the Belousov-Zhabotinsky reaction, it is necessary to prepare the following solutions and samples:
- Prepare a solution of citric acid and heat it to 50 o C.
- Add a portion of potassium bromate and cerium (III) sulfate and stir with a glass rod.
- Remove the grout from the tiles.
- Add sulfuric acid.
Briggs-Rauscher reaction.
Necessary reagents, glassware and equipment:
Reagents: Potassium iodate, sulfuric acid, malonic acid, manganese (II) sulfate, starch, hydrogen peroxide.
Utensils: 50 ml measuring cylinder, 2 glasses per 500 ml, 3 glasses per 100 ml, glass rod, spatula.
Equipment: Analytical balance, magnetic stirrer, magnet.
To carry out the Briggs-Rauscher reaction, the following solutions must be prepared:
Solution No. 1:
Solution No. 2:
Solution No. 3
Experiment procedure:
- Prepare all necessary solutions.
- Pour 50 ml of solution No. 1 into a 500 ml beaker containing a magnet and place it on a magnetic stirrer. Turn it on.
- In two other glasses, separately measure 25 ml of solution No. 2 and 40 ml of solution No. 3.
- Add, simultaneously, solutions No. 2 and No. 3 to solution No. 1.
- Record the induction period and periods of oscillation.
Experiment
Belousov-Zhabotinsky reaction:
To carry out the reaction, a solution of citric acid was prepared (20 g per 80 ml of water). To completely dissolve the citric acid, the solution had to be heated on an electric stove. Next, weighed portions of potassium bromate (8g) and cerium III sulfate (1.5g) were prepared and sequentially poured into a solution of citric acid. After stirring with a glass rod, sulfuric acid was carefully added while continuing stirring, after which color fluctuations from white to yellow were recorded.
№ | Period, s | Color | № | Period, s | Color |
1 | 23 | white | 12 | 12 | yellow |
2 | 11 | yellow | 13 | 66 | white |
3 | 41 | white | 14 | 8 | yellow |
4 | 12 | yellow | 15 | 43 | white |
5 | 71 | white | 16 | 6 | yellow |
6 | 11 | yellow | 17 | 56 | white |
7 | 43 | white | 18 | 5 | yellow |
8 | 13 | yellow | 19 | 43 | white |
9 | 19 | white | 20 | 5 | yellow |
10 | 10 | yellow | 21 | 56 | white |
11 | 40 | white | 22 | 4 | yellow |
It is also worth noting the increase in the amount of gas released as the solution darkens.
Conclusion: Based on the recorded data, one can judge a stable decrease in the time spent in the solution of tetravalent cerium (which indirectly indicates a decrease in the pH environment, since the more acidic the environment, the more powerful the oxidizing agent cerium is and the less stable it is).
An amazing pattern was also discovered, since during the reaction not only the concentrations of intermediate substances fluctuate, but also the time of the periods of oscillation (damped harmonic oscillation):
Briggs-Rauscher reaction:
To carry out the reaction, three solutions were prepared: a sulfate solution of potassium iodate (c(KIO 3) = 0.067 mol/l; c(H 2 SO 4) = 0.053 mol/l) - 50 ml, a starch solution of malonic acid with the addition of a catalytic amount of manganese sulfate two (c(MnSO 4) = 0.0067 mol/l; c(CH 2 (COOH) 2) = 0.05 mol/l; starch 0.1%) - 25 ml and a seven-molar solution of hydrogen peroxide - 40 ml. Solution No. 1 was poured into a beaker containing a magnet, 250 ml. The glass was placed on a magnetic stirrer, which was subsequently turned on, and intensive stirring was turned on so that the color change occurred sharply. Then, without stopping stirring, the contents of the glasses with solutions No. 2 and No. 3 were added, simultaneously and quickly. The stopwatch measured the appearance of the first yellow color - the induction period and the beginning of the appearance of blue colors - the oscillation period.
The induction period is 2 seconds.
№ | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
Period, s | 13 | 12 | 14 | 12 | 13 | 14 | 13 | 14 | 14 | 15 | 15 | 16 |
№ | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 |
Period, s | 16 | 16 | 17 | 17 | 17 | 18 | 17 | 18 | 17 | 18 | 18 | 17 |
Conclusion: As the reaction progresses, a gradual increase in the oscillation period is observed, which is especially clearly visible in the graph:
Conclusion
In this work, oscillatory reactions and their properties were considered, in particular:
- The scope of application of oscillatory reactions in the modern world has been studied
- The history of oscillatory reactions has been studied
- The mechanisms of two oscillatory reactions are analyzed: Briggs-Rauscher
- The Belousov-Zhabotinsky reaction mechanism was adapted for
- A control synthesis was carried out to visualize the oscillatory reactions.
List of used literature
- D. Garel, O. Garel “Oscillatory chemical reactions” translation from English by L.P. Tikhonova. Publishing house "Mir" 1986. Page 13-25, 92-112.
- A.M. Zhabotinsky “Concentration self-oscillations”. Publishing house "Science" 1974. Page 87-89
- OK. Pervukhin “Oscillatory reactions. Toolkit". St. Petersburg State University Publishing House, 1999. Page 3-11.
- S. P. MUSHTAKOVA “Oscillatory reactions in chemistry” Saratov State University. N.G. Chernyshevsky
- "Study of the conditions for the occurrence of an oscillatory mode in the process of oxidative carbonylation of phenylacetylene." Page 2-4.
- I.D. Ikramov, S.A. Mustafina. “ALGORITHM FOR SEARCHING RATE CONSTANTS OF VIBRATIONAL REACTION BY THE EXAMPLE OF THE BELOUSSOV-ZHABOTINSKY REACTION.” Bashkir chemical journal 2015
- Pechenkin A.A. “The worldview significance of oscillatory chemical reactions”
- Field R. J., Koros E., Noyes R. M., Oscillations in Chemical Systems II. Thorough Analysis of Temperal Oscillations in the Bromat-Cerium-Malonic Acid Sistem., J. Amer. Chem. Soc., 94, 8649-8664 (1972).
- Noyes R. M., Field R. J., Koros E., J. Amer. Chem. Soc., 94, 1394-1395 (1972).
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Ministry of Education, Science, Youth and Sports
Theoretical Lyceum Petru Movila
Department
"Ability, work, talent"
Coursework in chemistry on the topic:
"Oscillatory chemical reactions"
Completed by: student of class 12A
Bolubash Irina
Teacher: Snidchenko M.A.
* Chisinau2007 *
1. Introduction:
a) Redox reactions
b) Oscillatory chemical reactions
2. History of the discovery of oscillatory reactions:
a) Studies of concentration fluctuations before discovery
reactions of B. P. Belousov
3. Theoretical part:
a) Mathematical model by A. Lotkoy
b) Study of the mechanism of oscillatory reactions
4. Experimental part
5. Conclusion
6. Application:
a) Recipes for some oscillatory reactions
b) Illustrations for the experiments performed
7. Literature
Introduction.
Chemistry is an experimental science. And therefore experiment as a method of scientific research has long firmly occupied a leading place among the methods of natural sciences. Experiment is the most important way to connect theory with practice when teaching chemistry and transform knowledge into beliefs. Therefore, revealing the cognitive significance of each experience is the main requirement for a chemical experiment.
Under the experiment (lat. "experiment" - "trial") understand the observation of the phenomenon being studied under certain conditions that make it possible to monitor the progress of this phenomenon and repeat it if these conditions are met. Chemical experiment occupies an important place in teaching chemistry, since through observations and experiments the diversity of the nature of substances is learned, facts are accumulated for comparisons, generalizations, and conclusions.
By conducting experiments and observing chemical transformations under various conditions, we are convinced that complex chemical processes can be controlled, that there is nothing mysterious in the phenomena, they obey natural laws, the knowledge of which makes it possible to widely use chemical transformations in practical human activity.
However, the results of some chemical experiments are unexpected and do not fit into traditional ideas about the properties of substances or the patterns of chemical reactions. Such chemical transformations were called a problem experiment.
Even in ancient times, philosophers believed that any knowledge begins with surprise. The surprise caused by the new leads to the development of curiosity (sensitivity to problems in the world around us) with the subsequent formation of a sustainable interest in something. Surprise and, following it, a thirst for knowledge - this is fertile ground for studying a problem experiment, the formation of dialectical and systemic thinking, and the disclosure of creative potential.
The same state can be caused by a bright, impressive chemical experiment (problem experiment). In chemistry, the causes of problematic experiments are most often redox reactions.
Redox reactions
There are numerous criteria for classifying chemical reactions. One of the most important is a sign of changes in the oxidation states of elements. Depending on whether the oxidation states of elements change or remain the same, chemical reactions can be divided into redox reactions and those occurring without changing oxidation states.
Reactions that occur with changes in the oxidation states of elements (redox) are widely known. They play an important role in technology and nature, they underlie metabolism in living organisms, and the processes of oxidation, decay, fermentation, and photosynthesis are associated with them. Oxidation (and reduction) processes occur during fuel combustion, corrosion of metals, and electrolysis; with their help, metals, ammonia, alkalis and many other valuable products are obtained. Therefore, the study of redox reactions is included in school courses in inorganic and organic chemistry.
Let us recall the basic provisions associated with the concept of redox reactions.
Oxidation state corresponds to the charge that would appear on an atom of a given element in a chemical compound, if we assume that all electron pairs through which this atom is connected with others are completely shifted towards atoms of elements with higher electronegativity.
Oxidizer– a substance containing atoms or ions that accept electrons: Xm (oxidizing agent) + ne- = X(m-n), where m is the oxidation state of the element in the source substance, n is the number of electrons.
Reducing agent– a substance containing atoms or ions that donate electrons: Ym (reducing agent) - ne-= Y(m+n) .
Oxidation- the process of giving up electrons by an atom, molecule or ion, and the oxidation state of the element increases.
Recovery- the process of receiving electrons by an atom, molecule or ion, while the oxidation state of the element decreases.
Oxidation and reduction are coupled processes; the number of electrons donated by the reducing agent during its oxidation process is always equal to the number of electrons accepted by the oxidizing agent during its reduction.
Oscillatory chemical reactions
In this course work I will consider a special case of a problematic experiment, vibrational chemical reactions. Oscillatory reactions are a whole class of oxidation reactions of organic substances with the participation of a catalyst with redox properties. This process occurs cyclically, that is, it consists of multiple repetitions.
Vibrational chemical reactions were discovered and scientifically substantiated in 1951 by the Soviet scientist Boris Petrovich Belousov. B.P. Belousov studied the oxidation of citric acid during its reaction with sodium bromate in a solution of sulfuric acid. To enhance the reaction, he added cerium salts to the solution. Cerium is a metal with variable valence (3+ or 4+), so it can be a catalyst for redox transformations. The reaction is accompanied by the release of CO2 bubbles, and therefore it seems that the entire reaction mixture is “boiling.” And against the background of this boiling, B.P. Belousov noticed an amazing thing: the color of the solution periodically changed - it became yellow, then colorless. Belousov added a complex of phenanthroline with ferrous iron (ferroin) to the solution, and the color of the solution began to periodically change from purple-red to blue and back.
This is how the reaction that became famous was discovered. Now it is known all over the world, it is called the “Belousov-Zhabotinsky reaction”. A. M. Zhabotinsky did a lot to understand this amazing phenomenon. Since then, a large number of similar reactions have been discovered.
History of the discovery of oscillatory reactions.
IP Belousov made the discovery of the oscillatory chemical reaction while trying to create a simple chemical model of some stages of the system of key biochemical transformations of carboxylic acids in the cell. However, the first message about its discovery was not published. A reviewer for a chemical journal doubted the fundamental possibility of the reaction described in the article. Most chemists in those years believed that purely chemical vibrations did not exist, although the existence of oscillatory reactions was predicted in 1910 by A. Lotkoy on the basis of the mathematical theory of periodic processes.
The second attempt to publish the research results was made by the scientist in 1957, and again he was refused, despite the works of the Belgian physicist and physical chemist I.R. Prigogine that had appeared at that time. These works demonstrated the possibility and probability of oscillatory chemical reactions.
Only in 1959 was a short abstract about the discovery of a periodically acting oscillatory chemical reaction by B.P. Belousov published in the little-known publication “Collected Abstracts on Radiation Medicine.”
But the whole point is that when B.P. Belousov made his discovery, periodic changes in the concentration of reagents seemed to be a violation of the laws of thermodynamics. In fact, how can a reaction proceed either in the direct or in the opposite direction? It is impossible to imagine that the entire huge number of molecules in the vessel would be in one state or another (sometimes all “blue”, sometimes all “red”...).
The direction of the reaction is determined by the chemical (thermodynamic) potential - reactions are carried out in the direction of more probable states, in the direction of decreasing the free energy of the system. When a reaction in a given direction is completed, this means that its potential has been exhausted, thermodynamic equilibrium is achieved, and without energy expenditure, the process cannot spontaneously go in the opposite direction. And here... the reaction goes first in one direction, then in the other.
However, there was no violation of the law in this reaction. There were fluctuations—periodic changes—in the concentrations of intermediate products, rather than the starting reactants or final products. CO2 does not turn into citric acid in this reaction; this is indeed impossible. The reviewers did not take into account that while a system is far from equilibrium, many wonderful things may well happen in it. The detailed trajectories of the system from the initial state to the final state can be very complex. Only in recent decades have these problems been addressed by the thermodynamics of systems far from equilibrium. This new science became the basis of a new science - synergetics (the theory of self-organization).
Belousov's reaction, as noted above, was studied in detail by A. M. Zhabotinsky and his colleagues. They replaced citric acid with malonic acid. The oxidation of malonic acid is not accompanied by the formation of CO2 bubbles, so the change in the color of the solution can be recorded without interference by photoelectric devices. It later turned out that ferroin serves as a catalyst for this reaction even without cerium. B.P. Belousov, already in his first experiments, noticed another remarkable property of his reaction: when stirring stops, the color change in the solution spreads in waves. This propagation of chemical vibrations in space became especially clear when in 1970 A. M. Zhabotinsky and A. N. Zaikin poured the reaction mixture in a thin layer into a Petri dish. Bizarre figures are formed in the cup - concentric circles, spirals, “vortices”, spreading at a speed of about 1 mm/min. Chemical waves have a number of unusual properties. So, when they collide, they are extinguished and cannot pass through each other.
Concentration studies
vibrations before the discovery of the reaction by B. P. Belousov
But as history goes, B.P. Belousov’s discovery was by no means the first in world science. It turned out that one of the first publications on chemical vibrations dates back to 1828. In it, T. Fechner presented the results of a study of vibrations of an electrochemical reaction. The most interesting is the work of M. Rosenskiöld, dating back to 1834. Its author quite accidentally noticed that a small flask containing a little phosphorus, in the dark it emits a fairly intense light. There was nothing surprising in the fact that phosphorus glowed, but the fact that this glow was regularly repeated every seventh second was interesting. Forty years later, these experiments with the “flickering flask” were continued by the Frenchman M. Joubert (1874). He managed to observe the periodic formation of “luminous clouds” in a test tube. Twenty years later, the German scientist A. Zentnerschwer also studied the effect of air pressure on periodic phosphorus flashes. In his experiments, the period of flares began at 20 s and decreased with decreasing pressure.
A particularly bright page in the history of chemical vibrations is associated with the so-called Liesegang rings. In 1896, the German chemist R. Liesegang, experimenting with photochemicals, discovered that if lapis was dropped onto a glass plate coated with gelatin containing chromium, the reaction product, precipitating, was located on the plate in concentric circles. Liesegang became fascinated by this phenomenon and spent almost half a century researching it. Its practical application has also been found. In applied art, Liesegang rings were used to decorate various products with imitation jasper, malachite, agate, etc. Liesegang himself proposed the technology for making artificial pearls.
The list of such examples can be continued. Following these, vibrational reactions were discovered at the interface between two phases. Of these, the best known are reactions at the metal-solution interface, which have received specific names - “iron nerve” and “mercury heart”. The first of them - the reaction of dissolving iron (wire) in nitric acid - received its name due to its external similarity with the dynamics of an excited nerve, noticed by V.F. Ostwald. The second, or rather one of its variants, is the decomposition reaction of H2O2 on the surface of metallic mercury. In the reaction, periodic formation and dissolution of an oxide film on the surface of mercury occurs. Fluctuations in the surface tension of mercury cause rhythmic pulsations of the drop, reminiscent of a heartbeat. But all these reactions did not attract much attention from chemists, since ideas about the course of a chemical reaction were still quite vague.
Only in the second half of the 19th century. Thermodynamics and chemical kinetics arose, which laid the foundation for a specific interest in vibrational reactions and methods for their analysis.
Mathematical model by A. Lotkoy
The mathematical theory of oscillations in systems similar to chemical reactions was published back in 1910 by A. Lotka - he wrote a system of differential equations, from which the possibility of periodic regimes followed. Lotka considered the interaction of “preys”, such as herbivores, and the “predators” (X and Y) that eat them. Predators eat victims and reproduce - the concentration of Y increases, but up to a certain limit, when the number of victims sharply decreases and predators die of hunger - the concentration of Y decreases. Then the surviving victims begin to multiply - the concentration of X increases. The surviving predators then also reproduce, the concentration of Y increases again, and so on many times over. Periodic fluctuations in the concentration of reagents are observed. It is clear that the condition for such undamped (long-term) oscillations is the abundance of grass - the food of the victims. The tray equations were improved by V. Volterra. And the modern theory of oscillations was developed by Russian physicists L. I. Mandelstam, A. A. Andronov, A. A. Vitt, S. E. Khaikin, D. A. Frank-Kamenetsky. So for physicists and mathematicians, Belousovane’s discovery was so surprising.
Study of the mechanism of oscillatory reactions.
The detailed mechanism of the Belousov reaction is still not fully known. In the first works it seemed that the number of intermediate products was small. To explain the nature of the vibrations, it was enough to imagine how bromomalonic acid is first formed from ismalonic acid, and with a further reaction, KBrO3 is converted into KBr. The Br- anion inhibits further oxidation of bromomalonic acid, and the oxidized form of the catalyst (cerium tetravalent or ferric iron in combination with phenanthroline) accumulates. As a result, the accumulation of Br-- stops, and the oxidation of bromomalonic acid resumes... It is now clear that such a mechanism is far from complete. The number of intermediate products has reached fourty, and the study continues.
In 1972, R. Noyes and co-workers showed that the Belousov-Zhabotinsky reaction is the result of at least ten reactions that can be combined into three groups - A, B and C.
First (reaction group A), bromate ion reacts with bromide ion in the presence of H+ to form bromide and hypobromate acids:
BrO-3+ Br-- + 2H+ = HBrO2 + HOBr (A1)
hypobromic acid:
HBrO2+ Br-- + H+ = 2HOBr (A2)
Hypobromic acid, in turn, reacts with bromide ion, forming free bromine:
HOBr + Br--+ H+ = Br2 + H2O (A3)
Malonic acid is brominated with free bromine:
Br2+ CH2(COOH)2 = BrCH(COOH)2 + Br--+ H+ (A4)
As a result of all these reactions, malonic acid is brominated with free bromine:
BrO-3+ 2Br-- + 3CH2(COOH)2 + 3H+ =3BrCH(COOH)2 + 3H2O (A)
The chemical meaning of this group of reactions is twofold: the destruction of bromide ion and the synthesis of bromomalonic acid.
Group B reactions are possible only in the absence (low concentration) of bromide ion. When bromate ion reacts with bromous acid, the radical BrO2 is formed.
BrO-3+ HBrO2 + H+ > 2BrO2 + H2O (B1)
BrO2 reacts with cerium(III), oxidizing it to cerium(IV), and is itself reduced to bromide:
BrO2+ Ce3+ + H+ > HBrO2 + Ce4+ (B2)
Bromic acid breaks down into bromate ion and hypobromic acid:
2HBrO2> BrO-3 +HOBr + H+ (B3)
Hypobromic acid bromates malonic acid:
HOBr + CH2(COOH)2> BrCH(COOH)2 + H2O (B4)
As a result of the reactions of the B group, bromomalonic acid and tetravalent cerium are formed.
Fluctuations in the concentrations of the main components of the reaction: bromic acid and ferrin - in phase space are represented as a closed line (limit cycle).
BrO-3+ 4Ce3+ + CH2(COOH)2 + 5H+ > BrCH(COOH)2 + 4Ce4+ + 3H2O (B)
Cerium(IV) formed in these reactions (group B reactions):
6Ce4++ CH2(COOH)2 + 2H2O > 6Ce3+ +HCOOH + 2CO2 +6H+ (IN1)
4Ce4++ BrCH(COOH)2 + 2H2O > Br-- + 4Ce3++ HCOOH + 2CO2 + 5H+ (IN2)
The chemical meaning of this group of reactions is the formation of bromide ion, which is more intense the higher the concentration of bromomalonic acid. An increase in the concentration of bromide ion leads to a cessation (sharp slowdown) of the oxidation of cerium (III) to cerium (IV). In recent studies, cerium is usually replaced by ferroin.
From this (incomplete) sequence of stages of the Belousov-Zhabotinsky reaction, it is clear how complex this system is. Thus, it is enough to take into account the change in the concentration of all the tribasic intermediate components of the reaction HBrO2 (bromic acid), Br-- and ferroin (or cerium).
First step in the reaction - as a result of an autocatalytic reaction, bromous acid is formed (a fast, explosion-like process), ferroin is transformed into ferriin (the oxidized form of ferroin).
Second step– as a result of interaction with the organic component, ferrin begins to slowly transform back into ferroin, and at the same time bromide ion begins to form.
Third step– bromide ion is an effective inhibitor of the autocatalytic reaction (1st step). As a result, the formation of bromic acid stops and it quickly decomposes.
Fourth step– the process of ferriin decomposition, begun in step 2, is completed; bromide ion is removed from the system. As a result, the system returns to the state it was in before the 1st step, and the process is repeated periodically. There are several mathematical models (systems of differential equations) that describe this reaction, fluctuations in the concentration of its reagents and the patterns of propagation of concentration waves.
Experimental part:
Reaction of citric acid with potassium bromate:
Reagents:
1. KMnO4(potassium permanganate).
2. KBrO3(potassium bromate or potassium bromate).
3. H2SO4(concentrated).
4. Citric acid.
5. Distilled water.
Progress: A weighed portion of citric acid – 2 was dissolved in 6 ml of H2O. A weighed portion of potassium bromate was added to the resulting solution - 0.2 g and 0.7 ml of concentrated sulfuric acid was added. Then 0.04 g of potassium permanganate was added and the volume of the resulting solution was brought to 10 ml with distilled water. Mix thoroughly until the reagents are completely dissolved.
Observations: Immediately after adding KMnO4, the solution acquired a purple color and began to “boil.” After 25 s, with vigorous boiling, the color of the solution began to change to brown. As the reaction progresses, the solution gradually brightens - up to a light yellow color. After 3 minutes 45 seconds, a sharp darkening of the solution begins (similar to the diffusion of a high-density liquid), and after 40 seconds the solution becomes completely brown again. Then everything is repeated with a period of 4.5 minutes - 5 minutes. After a fairly long period of time, the reaction begins to slow down, and then stops altogether (yellow solution).
/>Oscillatory redox reactions:
Reagents:
1. FeSO4. 7H2O crystalline iron(II) sulfate heptahydrate or
Fe(NH4)2(SO4)2.6H2O(Mohr's salt) diammonium sulfate hexahydrate-
iron(II)
2. Ce(NO3)3.6H2O cerium(III) nitrate hexahydrate
3. KBr aqueous solution of potassium bromide (2 mol/l, or 12 g per 50 ml of water)
4. KBrO3 saturated solution of potassium bromate (about 10 g per 100 ml of water)
5. H2SO4 concentrated sulfuric acid
6. CH2(COOH)2 aqueous solution of malonic acid (5 mol/l, or 52 g in
100 ml water)
7. C12H8N2(phen)o-phenanthroline
8. distilled water
Crockery and cutlery: Polylux with screen, glass plate measuring 25 x 25 cm, Petri dish, 100 ml volumetric flask, 250 ml Erlenmeyer flask with ground stopper, six pipettes, burette, glass rod, wash, filter paper.
Description of the experience: To demonstrate the experiment, solutions A and B are pre-prepared.
Solution A – solution of ferroin – iron(II) complex with o-phenanthroline (phen). Add 0.70 g of iron(II) sulfate heptahydrate (or 0.99 g of Mohr's salt) and 1.49 g of o-phenanthroline into a 100 ml volumetric flask, adjust the volume of the solution to the mark with water and mix. The solution acquires a red color due to the formation of the phenanthroline iron(II) complex:
Fe2++ 3 phen = 2+
Solution B – solution of bromomalonic acid (prepared immediately before the demonstration). 3.3 ml of potassium bromide solution, 5 ml of malonic acid solution and 5 ml of concentrated sulfuric acid are introduced into a conical flask with a ground stopper. The resulting solution is titrated from a burette with a saturated solution of potassium bromate with stirring after adding each portion of the titrant, ensuring that the brown color disappears due to the release of bromine in a parallel commutation reaction:
BrO3–+ 5Br– + 6H+ = 3Br2 + 3H2O
3Br2+ 2CH2(COOH)2 + 2H2O = BrCH(COOH)2+ HCOOH + CO2 + 5HBr
The total volume of potassium bromate solution used for titration should be about 7.5 ml. The resulting bromomalonic acid is unstable, but it can be stored for some time at a temperature of 5100C.
To directly demonstrate the experiment, a Petri dish is placed on a glass plate covering the light window of the polylux, into which 10 ml of a saturated solution of potassium bromate, 4 ml of a solution of bromomalonic acid and 1.5 ml of a ferroin solution are successively added using pipettes. Within a few minutes, blue spots appear on the red background due to the formation of the phenanthroline iron(III) complex. 3+ as a result of oxidation of the corresponding iron(II) complex:
62++ 6H3O+ + BrO3– = 63++ 9H2O + Br–
This process is self-accelerating. The resulting complex 3+ oxidizes bromomalonic acid to form bromide ions:
43++ BrCH(COOH)2 + 7H2O =
= 2CO2+ 5H3O+ + Br– + HCOOH + 42+
The released bromide ions are inhibitors of the oxidation reaction of iron(II) complexes with bromate ions. Only when the concentration of complex ions 2+ becomes sufficiently high, the inhibitory activity of bromide ions is overcome, and the solution turns blue due to the formation of iron(III) complex. The process is repeated again and again, so the color of the solution periodically changes from blue to pink, or vice versa. The color change begins with the appearance of blue spots on a pink background, from which concentric waves of color diverge in all directions. Over time, the rate of color change decreases and, eventually, the process fades. In this case, you can observe the appearance of “black dots” on the screen - projections of bubbles of released carbon dioxide.
The range of colors can be expanded by adding several crystals of cerium(III) hexahydrate nitrate to a Petri dish. Ce(NO3)3. 6H2O. Then, in addition to blue and pink colors, you can observe yellow (due to the formation of cerium(IV) compounds) or green color (due to the overlap of yellow and blue):
6Ce3++ BrO3– + 15H2O = 62++ Br– + 6H3O+
42++ BrCH(COOH)2 + 3H3O+ =
= 2CO2+ Br– + HCOOH + 4Ce3++ 9H2O
When heated, the rate of reactions increases and the color change accelerates.
Note. Phenanthroline is a heterocyclic compound with two nitrogen atoms possessing lone pairs of electrons and capable of coordination. In complex compounds with iron O-phenanthroline plays the role of a bidentate ligand and forms strong chelate-type complexes.
Conclusion.
By now, the Belousov–Zhabotinsky reaction has taken its rightful place in world science. Every year, several international conferences on the dynamics of nonlinear chemical systems are held around the world, and the words “BZ-reaction” (abbreviation: Belousov-Zhabotinsky reactions) are heard at dozens of other conferences devoted to problems in physics, chemistry, and biology.
The study of the Belousov-Zhabotinsky reaction, as I am convinced, is of great importance, because it has found application in various fields of science and technology. This reaction is used as a model for studying a serious disorder of the heart - arrhythmia and fibrillation. And recently, experiments have been started with a photosensitive modification of this reaction, when the dynamics in this system depend on the light intensity. It turned out that such a reaction can be used as a computing machine for storing and processing images. A light-sensitive modification of the Belousov-Zhabotinsky reaction can serve as a prototype of a computing complex that may replace computers.
On the other hand, oscillatory chemical reactions are a striking example of self-organization in extra-living nature, and in this sense there is not only natural scientific, but also philosophical significance. Fundamental changes in natural science, which gave rise to the so-called theory of self-organization, are largely due to the initial impetus given to it by Russian scientists at the turn of the 1950s–1960s, when Belousov discovered the redox chemical reaction. At the same time, striking analogies were discovered; it turned out that many natural phenomena , starting from the formation of galaxies to tornadoes, cyclones and the play of light on reflective surfaces, in fact, are processes of self-organization. They can be of a very different nature: chemical, mechanical, optical, electrical, etc.
Thus, applied research is gaining more and more importance, for example, in the field of modeling alternative means of information processing (in particular, the analysis of complex mosaics with gradation of brightness of objects). Another new direction of applied research is the study of the characteristics of polymerization in the BZh system or similar to it.
The complex spatio-temporal organization exhibited by the BZ system in the absence of mixing, over time, analogies were found in nature, in biological systems (for example: periodic processes of cellular metabolism, waves of activity in cardiac tissue and in brain tissue, processes occurring at the level of non-ecological systems), in its new field - synergetics (theory of self-organization), as well as experimental work initiated the development of the modern theory of dynamic systems. Although at present much of such reactions is already understood, the reasons causing oscillatory chemical processes remain unclear.
Currently, the kinetics of oscillatory reactions is a rapidly developing branch of knowledge that arose at the intersection of chemistry, biology, medicine, physics, and mathematics. It was very interesting for me to get acquainted with such unusual and, at first glance, impossible properties of living matter. But what struck me even more was that such an incredibly significant, impressive discovery was not perceived by others for many years, and was simply not understood by the great minds of that time. This discovery followed a peculiar path, and, in the end, took its rightful place in world science. The possibility of such a reaction once again proves that in our world there is still a lot of unknown and unstudied.
Application.
Recipes for some oscillatory reactions
Recipe 1: It is necessary to prepare solutions of the following substances based on their final concentrations: malonic acid 0.2 M; sodium bromate 0.3 M; sulfuric acid 0.3 M; ferroin 0.005 M. Ferroin can be replaced with divalent manganese or trivalent cerium sulfate, but the color intensity will be significantly weaker. About 5 ml of a solution of all components should be poured into a Petri dish so that the thickness of the liquid layer is 0.5-1 mm. After 3-8 minutes (transition period), vibrations and chemical waves can be observed.
Recipe 2: Pour the following solutions into a flat transparent cuvette in layers (1 ml):
- KBrO3(0.2 mol/l)
- malonic acid (0.3 mol/l)
- ferroin (0.003 mol/l)
- H2SO4(0.3 mol/l)
Place the cuvette on a sheet of white paper. The rate of reaction can be changed by adding alkali or acid.
Recipe 3: Solutions required:
- citric acid (40 g in 160 ml H2O)
- H2SO4(1:3).
And also attachments:
- KBrO3(16 g)
- Ce2(SO4)3(3-3.5 g)
Heat the citric acid solution to 40°-50° C, then pour in a sample of KBrO3. Place the glass on a piece of white paper and add a sample of Ce2(SO4)3 and a few ml of H2SO4. The alternation of colors immediately begins to occur: yellow > colorless > yellow, with a period of 1-2 minutes.
Recipe 4: Solutions required:
- H2O2(50 ml 30%)
- KIO3(7.17 g in 50 ml H2O)
- HClO4(30 ml diluted solution)
- malonic acid (3 g in 50 ml H2O). And weighed:
- MnSO4(1g) and a little starch.
Pour everything into one glass (200-250 ml), add a portion, stir with a glass rod. There is an alternation of color: colorless > yellow > blue.
Bibliography.
1. Aliev R., Shnol S. E. “Oscillatory chemical reactions.” Kinetics and catalysis. 1998. No. 3. P.130-133.
2. Shnol S.E. Knowledge is Power. 1994. No. 3. pp. 62-71.
3. Zhabotinsky A. M. Concentration self-oscillations. M.: Nauka, 1974.
4. Garel D., Garel O. Oscillatory chemical reactions / Transl. from English M.:
5. Dubnischeva T. Ya. Concepts of modern natural science. Novosi-
Birsk: YuKEA, 1997, pp. 683 – 697.
6. Concepts of modern natural science. Ed. V. N. Lavrinenko,
V. P. Ratnikova, M.: UNITY-DANA, 1999, pp. 78 - 87.
7. VavilinB.V."Self-oscillations in liquid-phase chemical systems."
Priroda, 2000, No. 5, pp. 19–25.
VIBRATIONAL, districts, during which there will be intervals. connections and the speed of the river fluctuate. Fluctuations m.b. periodic, in this case the values of c(t) oscillating (t - time) can be represented by a Fourier series:where a n, b n are coefficients of expansion of the function c(t) in rad (amplitudes of individual harmonic components), A n are complex amplitudes, w - oscillation frequency (i - imaginary unit). In general, the amplitudes and frequencies of oscillations can change over time (damped, increasing, modulated oscillations). Fluctuations will occur. conn. may be non-periodic or have a continuous spectrum.
Fluctuations will occur. conn. - a relatively rare phenomenon observed in the course of certain complex operations. Elementary chem. districts are relaxation. processes that ensure a monotonous approach of the reacting system to the thermodynamic state. . For the occurrence of oscillations during homog. isothermal r-tion requires the presence of intervals. conn. and the interaction between them. In there are stationary states, in which the c(i) i-th intervals. conn. does not depend on time (with i =c 0 i). For small deviations of the system from the stationary state, the change with i is described by the sum of exponentials with complex indicators:
Quantities l i = g i +i w i, called characteristic numbers. In non-oscillation. sustainable systems l i are negative and real ( g i<0,
w
i =0). In these cases, usually instead l i use tenses t i =1/ l i. If the stationary state is close enough to the thermodynamic state. (Onsager's reciprocity relations are satisfied, see), then everything l i are real and negative (). In this case, the system approaches a stationary state without oscillations. In highly nonequilibrium systems l i can become complex numbers, which corresponds to the appearance of oscillations around a stationary state. At certain values of the parameters of a strongly nonequilibrium system (initial, t-ry, etc.), the stationary state may lose stability. Loss of stability of a stationary state is a special case of bifurcation, i.e. changes at a certain (bifurcation) value of the k.-l. number or type parameter dec. kinetic system modes. There are two simplest cases of bifurcation of a stable stationary state. In the first case one l i becomes positive. Moreover, at the bifurcation point ( l i =0) the initially stable state becomes unstable or merges with the unstable stationary state and disappears, and the system passes into a new stable state. In the space of parameters in the vicinity of this bifurcation there is a region where the system has at least three stationary states, of which two are stable and one is unstable. In the second case it works. part of one complex characteristic. numbers becomes positive. In this case, stable oscillations arise in the vicinity of the stationary state that has lost stability. After passing the bifurcation point, with further changes in the quantity parameter, the characteristics of oscillations (frequency, amplitude, etc.) can change greatly, but the quality. the type of system behavior is preserved. In chem. systems, instability can arise as a result of the acceleration of the river by its products or other types, substrate or cross inhibition (see), competition of the original substances for the intermediates. conn. and so on. In non-isothermal systems, the cause of instability can be self-acceleration of exothermic. stages of r-tion, and in electrochemical. r-tions exponential dependence of the speed of the r-tion on. The appearance of the simplest instabilities and corresponding kinetics. states of the system can be conveniently explained using the example of an enzymatic reaction with two
S 1 and S 2, one of which, for example. S 1, inhibits E:
S 01 D S 1 S 02 D S 2 S 1 +E 1 D S 1 E S 1 E+S 2 D S 1 E :
P S 1 E+S 1 D S 1 S 1 E
S 1 and S 2 can enter the system from the outside (for example, due to the influx in a flow reactor or through) or be formed as a result of slow homogeneities. r-tions S 0i D S i (i=1,2); product P is also removed, which does not affect the course of the process. S 1 E, S 1 S 2 E and S 1 S 1 E - enzyme-substrate complexes; occurs due to the formation of an inactive complex S 1 S 1 E. In this system there are 6 dynamic. variables: and , [E] and decomp. forms of enzyme-substrate complexes, and [E] + ++ = e - complete. Usually e<< и e<<, поэтому можно применить и представить фермент-субстратных комплексов как алгебраич. ф-ции . В результате поведение системы можно описать двумя дифференц. ур-ниями относительно и . Удобно использовать безразмерные переменные
s 1 =/K 1 and s 2 =/K 2 (K 1 and K 2 - Michaelis), parameters a 1 and a 2 - arrival rates, as well as dimensionless combinations of elementary stages e, b, g, d, ( and dimensionless time t . Then the differential The equations take the form:
Let us consider the case when this system has two stable stationary states - a bistable system, or trigger. If a 2 >> a 1 / e , i.e. r-tion speed S 02 D S 2 is very high compared to the speed of the S 01 D S 1 and the speed of the enzymatic reaction, then it is constant and equal to . In this case, the behavior of the system is described by only one equation (3.1). Dependencies d s l /d t from s 1 at different values a 1 are shown in Fig. 1, a. Dashed curves correspond to bifurcats. parameter values a-a " 1 and a : 1, and the curves between them intersect the abscissa three times. Intersection points correspond to stationary states s 1 01 , s 1 02 and s 1 03 , the average of which s 1 02 is unstable and separates the regions of attraction of stable states s 1 01
Rice. 1. Enzymatic system with three stationary states (biochemical trigger): a speed dependence d s 1 /d t changes in dimensionless S 1, from its value ( s 1 ) with decomposition speeds ( a 1 ) receipts ; The dotted line indicates the curves corresponding to the bifurcations. values a " 1 and a " " 1 ; 6 - dependence of stationary values s 0 1 from a 1 ; s 1 01 and s 1 0 3 stable, s 1 0 2 - unstable stationary states.
and s 1 0 3. On the stationary dependence curve s 1 0 from a 1 (Fig. 1, b) the region with three stationary states lies in the interval ( a " 1 , a "" 1). For forward and reverse slow parameter changes a 1 the system moves along different trajectories, i.e. hysteresis. It should be noted that the described bistability can be obtained in a system with a single-substrate solution, which behaves similarly to a two-substrate solution with a fixed one of .
In order for a system with one variable and bistability to become oscillatory, it is necessary to turn the parameter into a slow variable. In an enzymatic system with two such parameters, naturally, the second s 2. In this case, both equations (3) must be used to describe the system. Relative changes in S2 ( D /) will be slow compared to the relative changes in S l if >>. When passing to dimensionless parameters, this condition takes on the following form: a 1 ~ a 2 ~1, e <<1. На фазовой плоскости с координатами
s 1, s 2, the behavior of the system is qualitatively determined by the relative position of the null-isocline curves, on which the derivatives d s 1 / d t and d s 2 / d t are equal to 0 (Fig. 2, a). The intersection points of zero isoclines correspond to stationary states of the system. The dotted line shows the position of the null isocline d s 1 /d t =0 during bifurcation, accompanied by the appearance of stable oscillations (self-oscillations) of small amplitude. These oscillations correspond to a closed trajectory of the system - the so-called. limit cycle. Solid lines show null isoclines in a situation far from bifurcation, when the only stationary state of the system (point O in Fig. 2, a) is highly unstable and surrounded by a limit cycle ABCD. The movement of the system along this limit cycle corresponds to self-oscillations s 1 and s 2 with a large amplitude (see Fig. 2, b).
Rice. 2. Self-oscillations (stable oscillations) in a model enzymatic system: a-phase plane in coordinates s 1 - s 2 with null isoclines d s 1 /d t =0, d s 2 /d t =0; The dotted line shows the position of the null isocline d s 1 /d t =0, corresponding to oscillations. bifurcations, and a small limit cycle surrounding the unstable stationary state O, ABCD large limit cycle; b - self-oscillations s 1 and s 2 corresponding to the large limit cycle ABCD.
During the oscillatory periodic periods were observed. vibrations divers. shapes: sinusoidal, sawtooth, rectangular, etc.; modulated, quasiperiodic and stochastic. The periods of most oscillatory waves range from fractions of a second to tens of minutes. Liquid-phase vibrational ones include, for example, H 2 O 2 and S 2 O 4 2-, decomp. in-in halogen-oxygen compounds, and. Belousov-Zhabotinsky has been well studied, going in an aqueous solution, where HBrO 3 at variable oxidizes decomposition. org. conn., in particular malonic acid.
Gas-phase vibrational vibrations were discovered and studied in the presence of CO, CO and other compounds. In all cases, both the volumetric stages of the reaction and the breakage and nucleation of chains on the walls of the reactor, as well as the acceleration of the reaction due to heating of the system as a result of
exothermic stages (thermal). Purely thermokinetic are possible. self-oscillations, when thermal is the only cause of instability. The simplest thermokinetic model. oscillations in a flow reactor have the form: V 0:
IN :
P+Q. Here substance B enters an ideal flow reactor, where monomolecular exothermic reaction occurs. disintegration solution; the generated heat is removed through the reactor wall. The kinetics of this reaction is described by two differentials. equations relative to B and temperature T inside the reactor:
where [B 0 ] is given at the inlet to the reactor, T 0 is the temperature of the reactor wall, k is the coefficient. reaction update rate mixture in the reactor, h - coefficient. speed, Q - thermal effect of the r-tion, C r - at constant, r - density, E and A -
Dedenev Yuri
The reason for starting this work was the article “Oscillatory reactions in chemistry” by S.P. Mushtakov from Saratov State University named after Chernyshevsky, published in the Soros Educational Jornal No. 7 for 1997. In the school chemistry course there is not even a mention of the existence of this type of reaction; they are also called Belousov-Zhabotinsky reactions. The purpose of this work is to attract the maximum attention of students to the subject of chemistry, that is, not just to search for nuggets who are passionate about chemistry, but also to try to awaken hidden abilities in students that have not been openly manifested until now. To interest them, to instill a love for chemistry as one of the most interesting and beautiful sciences of our time, which hides the enormous potential of unexplored material, the ability to create new yet unknown substances. We can confidently say that the Kazan school of chemists is one of the strongest in Russia and therefore we would like it to be replenished with young, energetic and enthusiastic people who could instill a love for chemistry in others.
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RUSSIAN OPEN STUDENT CONFERENCE
"YOUTH, SCIENCE, CULTURE"
CHEMISTRY section
STUDY OF VIBRATIONAL CHEMICAL REACTIONS
Dedenev Yuri
Secondary school No. 105, 11th grade, Kazan
Scientific adviser:
Minnullin R.R., teacher of the II qualification category
Obninsk 2005
Symbols and abbreviations page 3
Introduction page 4
Chapter 1. History of the emergence and prospects of processes page 5
1.1. History of detection of oscillatory processes page 5
1.2. Modern history of process research p. 5
1.3. Possible prospects for the application of processes page 6
Chapter 2. Theoretical prediction of the possibility of a reaction page 7
2.1. Properties of the main components of the reaction page 7
2.2. The first mathematical models. Tray Systems page 7
Chapter 3. Experimental part page 9
3.1. Synthesis of potassium bromate (kaliumbromat) p.10
4+ page 10
3.3. Preparation and implementation of the oscillatory reaction page 11
Chapter 4. Conclusion p.14
Literature p.18
Appendix p.19
Figure 1 page 19
Figure 2 p.20
Legend
1. BZ - Belousov – Zhabotinsky
2. LA - Citric acid
3. MK - Malonic acid
4. BMK – Bromatemalonic acid
5. see – look
6. fig. - drawing
7. max - maximum
8. min – minimum
Introduction
The reason for starting this work was the article “Oscillatory reactions in chemistry” by S.P. Mushtakov from Saratov State University named after Chernyshevsky, published in the Soros Educational Jornal No. 7 for 1997. In the school chemistry course there is not even a mention of the existence of this type of reaction; they are also called Belousov-Zhabotinsky reactions.
The purpose of this work is to attract the maximum attention of students to the subject of chemistry, that is, not just to search for nuggets who are passionate about chemistry, but also to try to awaken hidden abilities in students that have not been openly manifested until now. To interest them, to instill a love for chemistry as one of the most interesting and beautiful sciences of our time, which hides the enormous potential of unexplored material, the ability to create new yet unknown substances. We can confidently say that the Kazan school of chemists is one of the strongest in Russia and therefore we would like it to be replenished with young, energetic and enthusiastic people who could instill a love for chemistry in others. Therefore, we set ourselves the following tasks:
1. Give a brief background to the discovery of vibration reactions
2. Give a theoretical basis for the mechanisms of vibration reactions
3. Conduct syntheses to obtain the necessary components from available chemical reagents
4. Carry out the vibration reaction
Chapter 1. History of appearance and possible prospects
1.1.History of detection of oscillatory processes
For the first time, an oscillatory chemical reaction, manifested in the form of periodic flashes during the oxidation of phosphorus vapor, was observed by Robert Boyle at the end of the 17th century. These recurring outbreaks were then repeatedly described by many researchers. In the 19th century, other oscillatory reactions were discovered. However, they did not attract much attention, since chemical kinetics as a science did not yet exist. Only the emergence of thermodynamics and chemical kinetics gave rise to a specific interest in vibrational reactions and methods of their analysis. Predictions about the possibility of vibrations in chemical systems have been made since 1910 on the basis of mathematical models by A. Lotka. In 1921, W. Bray published an article in which the first oscillatory liquid-phase reaction was described in sufficient detail. Bray realized the connection between his discovery and Lotka's prediction. However, his work did not attract interest for about 40 years. One of the reasons for this indifference is the rather low level of development of methods for studying the mechanisms of complex chemical reactions. Another reason was the widespread belief that the second law of thermodynamics prohibits such fluctuations even far from equilibrium. In fact, most chemists believed that concentration fluctuations in closed homogeneous systems were impossible, in other words, there were no purely chemical fluctuations.
1.2.Modern history of studies of oscillatory processes
Research on oscillatory chemical reactions in the liquid phase began in 1951, when B.P. Belousov discovered fluctuations in the concentrations of oxidized and reduced forms of cerium in the reaction of citric acid with bromate. The solution regularly changed from colorless to yellow (due to CeIV), then again to colorless (CeIII), etc. Belousov conducted a fairly detailed study of this reaction and, in particular, showed that the period of oscillations greatly decreases with increasing acidity of the medium and temperature. The reaction was convenient for laboratory studies. The oscillations could be easily observed visually, and their period was in the range of 10–100 s, coinciding with the natural time scale of a human observer.
At the end of 1961, the work of B.P. Belousova was continued by A.M. Zhabotinsky, who obtained fluctuations when using not only citric, but also malonic and malic acids as a reducing agent in the Belousov reaction. A.M. Zhabotinsky conducted detailed studies of vibrations in a system with malonic acid, which turned out to be a more convenient reducing agent, since the reaction was not complicated by gas evolution. The news of this amazing reaction spread all over the world, and several laboratories (in the USSR, USA and Western Europe) began to intensively study the BZ reaction. Oscillatory reactions have finally entered chemistry laboratories.
1.3 Possible prospects for the use of oscillatory processes
Let us consider the prospects for the possible application of oscillatory chemical processes. A distinctive feature of such regimes, noted at the end of the 19th century by Poincaré, is their high sensitivity to the slightest external disturbances. Conducting research in this area opens up enormous prospects for creating fundamentally new methods for analyzing microquantities of substances.
The quantitative basis for the analytical determination of various microimpurities (and weak external influences) can be the dependence of the frequency (period) of oscillations on the concentration of reagents or catalyst. Since measuring the vibration frequency is one of the simplest and most accurately performed operations, self-oscillating chemical reactions can be used for analytical purposes.
A detailed study of the interaction of oscillations propagating from two spatially distant centers helped to understand the various types of arrhythmias that arise in the heart muscle. Currently, the kinetics of vibrational reactions is a rapidly developing branch of knowledge that arose at the intersection of chemistry, biology, medicine, physics, and mathematics. It has now been shown that chaotic regimes are observed in many areas of biology (in biochemistry, biophysics, the study of biorhythms, in the study of population dynamics, migration of organisms, etc.), ecology and, in the broadest sense of this concept, some social processes (change population, economic development). In many cases, relatively simple dynamic chemical systems with strictly controlled concentration changes of parent and intermediate chemicals can be very suitable functional models for studying chaotic processes in other fields of knowledge (earth and other planetary sciences, solid state physics, nuclear and particle physics , engineering mechanics, etc.).
Chapter 2 Theoretical prediction of vibration reactions
2.1. Properties of the main components of the reaction.
The reducing agent should be easily oxidized by the oxidized form of the catalyst and should not react directly with the bromate. In addition, it is necessary that the reducing agent be easily brominated and the bromine derivatives decompose quite easily, releasing Br. These requirements are met by substances with an active methylene group. Reactions involving MK, BMK, and LC are qualitatively close.
Substances (primarily ions of variable valence) close to cerium ions both in the value of the redox potential and in the kinetics of oxidation and reduction reactions can be used as a catalyst.
Oxidation reactions with halogen - oxygen compounds have similar kinetics. Therefore, it is natural to assume that chlorate and iodate can replace bromate. However, chlorate and iodate cannot replace bromate as an oxidizing agent. The redox potentials in the reactions of these compounds with various reducing agents (for example, halides) are close. However, the rates of oxidation reactions of the above catalysts with iodate and chlorate are much lower than the rates of oxidation with bromate. Consequently, bromate remains the only oxidizing agent in this class of reactions.
2.2. The first mathematical models of oscillatory chemical reactions
Tray Systems
Mathematical modeling of concentration oscillatory systems began with the work of Lotka (1910), which considered the system:
A X Y . 1.1
There is a reservoir A, a linear conversion of A to X, an autocatalytic conversion of X to Y, and a linear decline of Y. This model was applied by Lotka to describe both chemical and ecological systems. Lotka considered an open system, i.e. With from the very beginning he neglected the consumption of A and did not take into account the final products of the transformation Y. In addition, he described autocatalysis as an elementary reaction. These assumptions lead to the following system of equations:
x = k 0 A – k 1 xy, y = k 2 xy – k 3 y.
In the simplest case k 2 = k 1 . Terms k 0 A and k 3 y can describe both chemical reactions and linear transport processes in an open system.
The following model, studied by Lotka (1920) and later independently by Volterra (1931), contains two sequential autocatalytic reactions (this model is widely known in ecology as “prey-predator”. For example: A is the specific amount of grass, the supply of which considered inexhaustible; X – population density of herbivores; Y – population density of predators).
A X Y . 1.2
Assuming the same for scheme (1.2) as for scheme (1.1), Lotka and Woltera obtained the following system of equations:
x = k 1 Ax – k 2 xy, y = k 3 xy – k 4 y.
Note that the mathematical description of these processes turned out to be quite complex. It is no coincidence that theoretical works on oscillatory reactions continue to be published to this day, although the corresponding mathematical apparatus was developed at the end of the nineteenth century. Mathematical modeling led to unexpected results. It turned out that one of the simplest chemical schemes describing oscillations in a system of two successive autocatalytic reactions is mathematically identical to the equations that Voltaire used in the early 30s to describe environmental processes.
As an example, we will use two interacting systems, one of which draws the energy, matter or other components it needs for development from the other (a chemical analogue is an oscillatory reaction). This problem is called the predator-prey problem. For clarity, let’s imagine that wolves and hares live in some limited environment. In this ecological system, grass grows, which feeds hares, which in turn are food for wolves. As is known, if you have any set of living beings, then under favorable conditions their population will increase indefinitely. In fact, external factors, such as lack of energy or food, limit this growth process. How does this happen in the example of wolves and hares?
Let’s imagine that up to a certain point, the interaction of two subsystems, that is, the populations of wolves and hares, was balanced: there were just enough hares (taking into account their natural replenishment) to feed a certain number of wolves. Then, at the moment taken as zero of the time count, due to some fluctuation, the number of hares increased. This increased the amount of food for wolves and, therefore, their number. There was a fluctuation in the number of wolves. Moreover, the number of wolves and hares will change periodically over time around some average (equilibrium) value. Well-fed wolves will begin to multiply intensively, giving birth to new offspring, which quickly mature on abundant food and produce new offspring. A situation arises when the hare breeder is no longer able to feed all the wolves - the number of hares begins to fall, and the number of wolves (for the time being) continues to grow. Finally, the ecosystem is overpopulated with wolves, and hares have a place almost in the red book. Let's not rush to conclusions. Having become an ecological rarity, hares become difficult prey for wolves. The ecosystem is entering the next phase: the number of hares has already dropped to a minimum level at which they are almost elusive to wolves. The number of the latter, having passed through the maximum, begins to decline, and this reduction continues until a level is reached that the hares are able to feed with their minimum number. Now that the number of wolves has reached a minimum, there is no one to hunt the hares. The hares begin to breed, and the meager wolf population can no longer keep up with them. The number of hares will soon reach a level that the grass can feed. An abundance of hares appears again, and everything repeats itself again.
Chapter 3 Main experimental part
3.1. Synthesis of potassium bromate (kaliumbromat)
1050 ml of a filtered 30% solution of KOH (technical) is poured into a large porcelain glass and 110 g of bromine is poured very slowly (under draft) from a dropping funnel with a tube reaching to the bottom, with constant stirring. The resulting solution is saturated (under draft) with chlorine. The end of saturation is determined as follows. A solution sample (10 ml) is diluted with 10 ml of water, boiled until Br is completely removed 2 and Cl2 (iodine starch paper should not turn blue in the vapor of the liquid) and add a drop of phenolphthalein solution. When completely saturated with chlorine, the solution sample should not turn red.
The reaction solution is cooled to 15 O C, separate the precipitated mixture of KC1O crystals 3 and KS1 (300 - 350 g) and stir them with 150 ml of water for several hours. Remaining KBrO crystals 3 suction using a Buchner funnel, wash with 100 ml of water and separate. 200–240 g of crude potassium bromate are obtained.
The synthesis can be expressed by the following chemical reaction equations:
Br 2 + 2KOH = KBrO + KBr + H 2 O
KBrO + Cl 2 + 4KOH = KBrO 3 + 4KCl + 2H 2 O
KBr + Cl 2 + 6KOH = KBrO 3 + 6KCl + 3H 2 O
3.2. Possible methods for preparing the Ce catalyst 4+
In school chemistry laboratories you can find cerium dioxide, which used to be part of the school chemistry set. The main task is to obtain any soluble cerium salt; in this case, the easiest way is to obtain cerium (VI) sulfate; for this it is necessary to expose the existing cerium dioxide to concentrated sulfuric acid by boiling. Seo 2 insoluble in water, so it is necessary to act with sulfuric acid directly on the cerium dioxide powder.
The reaction equation can be expressed as follows:
CeO 2 + 2H 2 SO 4 = Ce(SO 4 ) 2 + 2H 2 O
A bright yellow solution of cerium (VI) sulfate is formed, which can then be evaporated on an evaporation dish until yellow crystals appear. If there is still no cerium dioxide, then you can obtain a soluble cerium ion in the following way: you can use silicon from lighters, you need to take several of them and dissolve them in concentrated sulfuric acid while heating. Silicon from lighters contains cerium (III) and (VI) compounds. But it must be taken into account that the purity of the experiment may change due to the presence of impurities in the original component.
3.3. Preparation and implementation of an oscillatory reaction.
To conduct the experiment, two solutions are prepared. In the first case, a solution of cerium (IV) sulfate or nitrate; in this experiment, 1.0 g of freshly prepared cerium sulfate, dissolved in 15 ml of water and acidified with sulfuric acid, was used. In the second, citric acid is dissolved in 10 ml of hot water and potassium bromate is poured into it. To completely dissolve the substances, the mixture is slightly heated. The prepared solutions are quickly poured together and mixed with a glass rod. A light yellow color appears, which after 20 seconds. changes to dark brown, but after 20 seconds. turns yellow again. At a temperature of 45 O Such a change can be observed within 2 minutes. Then the solution becomes cloudy, bubbles of carbon monoxide (IV) begin to appear, and the intervals of alternating color of the solution gradually increase in a strictly defined sequence: each next interval is 10 - 15 seconds longer than the previous one, and the temperature of the solution also increases.
During the demonstration or after demonstrating the experiment to students, the mechanism of a chemical reaction can be explained in a simplified version, that is, as an oxidation-reduction process in which bromic acid (BA) plays the role of an oxidizing agent, and citric acid plays the role of a reducing agent:
KBrO 3 + H 2 SO 4 = KHSO 4 + HBrO 3
9HBrO 3 + 2C 6 H 8 O 7 = 9HBrO + 8H 2 O + 12CO 2
9HBrO + C 6 H 8 O 7 = 9HBr + 4H 2 O + 6CO 2
The color of the solution changes under the influence of catalysts - cerium compounds, which in turn also change the oxidation state, but up to a certain ion concentration, after which the reverse process occurs.
Chapter 4. Conclusion
For convenience of presentation, we will first consider a simplified diagram of a self-oscillatory reaction. During this reaction, fluctuations in the color of the solution are observed, caused by fluctuations in the concentration of cerium (VI). Fluctuations in cerium(VI) concentration are shown in Fig. 2. These are relaxation oscillations, the period of which (T) is clearly divided into two parts: T1 is the phase of decreasing concentration of cerium (VI) and T2 is the phase of increasing concentration. Accordingly, according to a simplified scheme, the reaction consists of two stages: in the first stage, tetravalent cerium is reduced with citric acid, Fig. 1.
OK
Ce4+ Ce3+ , (1)
in the second, trivalent cerium is oxidized by bromate
BrO3
Ce3+ Ce4+ (2)
The bromate reduction products formed in step (2) bromide LA. The resulting bromine derivatives of LA are destroyed with the release of bromine ions. Bromide is a strong inhibitor of reaction (2).
Any of its products can have a catalytic effect on the reaction.
This phenomenon is called autocatalysis. A characteristic feature of an autocatalytic reaction is that it is variable and the concentration of the catalyst increases during the reaction. Therefore, the rate of the autocatalytic reaction increases in the initial period and only at deeper stages of transformation, as a result of a decrease in the concentration of the starting substances, does the increase in rate give way to a decrease.
The speed of autocatalytic processes does not decrease as the reagents are consumed, but increases without any contradiction with the law of mass action. The mechanism of reactions is such that their intermediate or final products have an accelerating effect on the process. Therefore, their speed is initially vanishingly small, but then increases with increasing concentration of reaction products. According to modern terminology, such processes refer to processes with positive feedback. So, for example, if the intermediate or final product of a multistage reaction turns out to be its inhibitor, self-inhibition of the reaction will be observed - its speed will decrease faster. How the concentration of the initial reagents decreases.
In the reaction when Ce4+ ions interact with citric acid, they are reduced:
Ce 4+ + C 6 H 8 O 7 Ce 3+ + product (1)
The Ce3+ formed during the reaction must then react with the bromate ion:
Ce 3+ + BrO 3 Ce 4+ (2)
leading to a stationary distribution of cerium between oxidation states. However, reaction (2) is autocatalytic, and in it the self-accelerating course is preceded by an induction period, that is, the reaction does not start immediately. Therefore, during the induction period, almost all Ce ions 4+ go to Se 3+ . In this case, the color of the solution due to the absorption of light in the visible region of the spectrum by the Ce complex 4+ with citric acid, disappears. At the end of the induction period, a self-accelerating rapid transition of Ce ions occurs 3+ in Se 4+ and the solution regains its original color.
The periodic nature of the process can be explained as follows. As a result of reaction (1):
Ce(VI) + citric acid Ce(III) + product
bromide ions are formed, slowing down the reaction (2):
Ce(III) + HBrO 3 Ce(VI) + products.
However, the concentration of bromide in the system depends on the rate of the reaction, in which bromide is consumed due to interaction with bromate
(BrO 3 + Br Br 2 ). If the bromide concentration is high enough, then reaction (2) stops, since Ce(VI) is not regenerated during the oxidation of Ce(III) with bromate, and as a result, the catalytic cycle is interrupted. When the concentration of Ce(VI), which decreases as a result of reaction (1), reaches the minimum possible value, the concentration of bromide ion begins to decrease sharply. Then reaction (2) noticeably accelerates and the concentration of Ce(VI) increases to a certain value at which the bromide concentration begins to increase rapidly, thereby slowing down reaction (2). Then the whole cycle is repeated, Fig. 2.
In general, the reaction mechanism can be described by the following set of equations:
Process A
BrO 3 + 2Br + 3(CH 2 ) 2 C(OH)(COOH) 3 + 3H +
3BrCH(CH 2 )C(OH)(COOH) 3 + 3H 2 O
BrO 3 + Br + 2H + HBrO 2 + HOBr
HBrO 2 + Br + H + 2HOBr
HOBr + Br + H + Br 2 + H 2 O
Br 2 + (CH 2 ) 2 C(OH)(COOH) 3 BrCH(CH 2 )C(OH)(COOH) 3 + Br + H +
Process B
BrO 3 + 4Ce 3+ + (CH 2 ) 2 C(OH)(COOH)3 + 5H +
BrCH(CH 2 )C(OH)(COOH) 3 + 4Ce 4+ + 3H 2 O
BrO 3 + HBrO 2 + H + 2BrO 2 + H 2 O
BrO 2 + Ce 3+ + H + HBrO 2 + Ce 4+
2HBrO 2 BrO 3 + HOBr + H +
HOBr + (CH 2 ) 2 C(OH)(COOH) 3 BrCH(CH 2 )C(OH)(COOH) 3 + H 2 O
In addition to the above, there are also reactions: the interaction of citric acid with cerium (VI) ions and sulfuric acid (due to acidification of the solution and dissociation of cerium (VI) sulfate), we do not describe the reaction mechanisms due to their complexity, the products of these reactions are carbon monoxide (IV ), carbon monoxide (II), water and partially dimethyl ketone.
Now we can generalize everything that has been said and give a definition of oscillatory reactions: oscillatory reactions are periodic processes characterized by fluctuations in concentrations and, accordingly, conversion rates. The reason for concentration fluctuations is the presence of feedback between the individual stages of a complex reaction.
We sincerely hope that our work will attract the attention of many, and that it will be further developed and continued.
References
- A.M. Zhabotinsky Concentration fluctuations. M.: – Science. 1974.
- Yu.V. Karyakin, I.I. Angelov Pure chemicals. M.: – Chemistry. 1974.
- B.N. Stepanenko Course of organic chemistry. M.: – Higher school. 1972.
- ON THE. Ostapkevich Workshop on inorganic chemistry. M.: – Higher school 1987
- V.N. Aleksinsky Entertaining experiments in chemistry. M.: Education, 1980.
- Soros educational magazine. No. 7.1997.
Application
[Ce4+]
M - - - - - - -
N - - - - - - - - - - - - - - - - - - - - - - - - |- - - - - - - - - - - - - - - - - - - - - - - - - - -
| | |
| | |
| T 1 | T 2 |
| | |
| | t
| T |
| |
| |
Fig.1. Self-oscillations of cerium (VI) concentration
[Ce 4+]
Max - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Min - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[Br]
Fig.2. Dependence of changes in the concentration of cerium (VI) on the concentration of bromide ions.