Discovery of oscillatory chemical reactions. Vibrational reactions Studying fluctuations in chemical reactions at school

The essence of oscillatory reactions. Mechanism and kinetics of vibrational reactions.

Content

1. INTRODUCTION…………………………………………………………...……..…3
2. Basic concepts……………………………………………………………4
3. History…………………………..………………………………………………5
4. Significance and scope…………………….……….…………8
5. Mechanisms of reactions………………………………………………………………………………………………10
6. Kinetics of oscillatory reactions……………………………………….…14
7. The order of the experiment………………………..…………….15
8. Experimental data…………………………………….……….18
9. Conclusion………………………………………………………………..23
10. Bibliography…………..………………………………..…………24

INTRODUCTION
Vibrational reactions are one of the most interesting and attractive branches of inorganic chemistry. Attracting close attention not only to chemists, but also to physicists, mathematicians, biophysicists and many others, they are a topical 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, having set up an experiment, get acquainted with oscillatory reactions in practice.

Basic concepts of oscillatory reactions

• Vibrational reactions- a class of redox reactions characterized by periodic fluctuations of intermediate substances and, as a result, fluctuations in color, temperature, flow rate, etc.

1. catalytic
2. homogeneous
3. Reactions catalyzed by enzymes
4. Reactions catalyzed by metal ions
5. Heterogeneous (reactions on solid catalysts)
6. 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.
• Oscillation period - the smallest period of time for which one complete oscillation occurs (that is, the system returns to the same state in which it was at the initial moment, chosen arbitrarily)

Reaction mechanisms
To date, several dozens of 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 of the reaction rate and concentrations of intermediate substances:

1. Stable fluctuations occur in most cases in open systems in which it is possible to keep the concentrations of the participating reactants constant.
2. An oscillatory reaction should include autocatalytic and reversible stages, as well as stages that are inhibited by the reaction products.
3. 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 discovered and studied oscillatory reactions. In this connection, it can perhaps be called one of the most studied reactions of this group. At the moment, in one way or another, the presence of eighty intermediate stages (and side reactions) occurring in the system has been confirmed.
One of the earliest and simplest reaction schemes was a scheme that consists of two stages:

1. Oxidation of trivalent cerium with bromate

1. And reduction of tetravalent cerium with citric acid

1. BrO 3 - + Br - + 2H + ↔ HBrO 2 + HBrO
2. HBrO 2 + Br - + H + ↔ 2HBrO
3. HBrO + Br - + H + ↔ Br 2 + H 2 O
4. Br 2 + HOOC(OH)C(CH 2 COOH) 2 → Br - + H + + HOOC(OH)C(CHBrCOOH)CH 2 COOH
5. BrO 3 - + HBrO 2 + H + ↔ 2BrO 2. + H2O
6. BrO2. + Ce 3+ + H + → HBrO 2 + Ce 4+
7. 2HBrO 2 ↔ BrO 3 - + HBrO + H +
8. HBrO + HOOC(OH)C(CH 2 COOH) 2 → H 2 O + HOOC(OH)C(CHBrCOOH)CH 2 COOH
9. 18Ce 4+ + HOOC(OH)C(CH 2 COOH) 2 + 5H 2 O → 18Ce 3+ + 6CO 2 + 18H +

So, let us consider the Ce 3+ / Ce 4+ oscillations in this system. Suppose we have a small, gradually increasing amount of Ce 4+ in solution, which means that the concentration of Br - is also small and grows 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 according to reactions (1) - (3). In this case, after reducing the concentration of Br - below a certain value, it will practically stop the reactions (2) and (3), leading to the accumulation of HBrO 2 . From which follows an increase in the concentration of Ce 4+ and a repetition of the cycle we have passed.

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, perhaps, first of all, 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 causes of fluctuations, it is enough to consider a simplified reaction mechanism, consisting of the eleven stages below:

1. IO 3 - + H 2 O 2 + H + → HIO 2 + O 2 + H 2 O
2. IO 3 - + HIO 2 + H + ↔ 2IO 2 . + H2O
3. HIO 2 + H 2 O 2 → HIO + O 2 + H 2 O
4. IO2. + Mn 2+ + H 2 O ↔ HIO 2 + MnOH 2+
5. 2HIO + H 2 O 2 → 2I - + 4O 2 + 4H +
6. MnOH 2+ + I - + H + ↔ I. + Mn2+ + H2O
7. HIO + I - + H + ↔ I 2 + H2O
8. 2HIO 2 → IO 3 - + HIO + H +
9. RH↔enol
10. HIO + enol → RI + H2O
11. I 2 + enol → RI + I - + H +

Kinetics of vibrational reactions

The order of the experiment
Belousov-Zhabotinsky reaction.

Reagents: Citric acid, potassium bromate, cerium(III) sulfate, sulfuric acid.
Utensils: Measuring cylinder 50 ml, heat-resistant glasses 300 ml and 100 ml, glass rod, spatula.
Equipment: Analytical balances, tiles.
To carry out the Belousov-Zhabotinsky reaction, it is necessary to prepare the following solutions and samples:

1. Prepare a solution of citric acid and heat it to 50 o C.
2. Add weighed portions of potassium bromate and cerium (III) sulfate, stir with a glass rod.
3. Remove grout from tiles.
4. Add sulfuric acid.

Briggs-Rauscher reaction.
Necessary reagents, utensils and equipment:
Reagents: Potassium iodate, sulfuric acid, malonic acid, manganese (II) sulfate, starch, hydrogen peroxide.
Utensils: measuring cylinder 50 ml, 2 cups 500 ml, 3 cups 100 ml, glass rod, spatula.
Equipment: Analytical balance, magnetic stirrer, magnet.
To carry out the Briggs-Rauscher reaction, it is necessary to prepare the following solutions:
Solution #1:

Solution #2:

Solution #3

The order of the experiment:

1. Prepare all necessary solutions.
2. 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.
3. Measure separately 25 ml of solution No. 2 and 40 ml of solution No. 3 into two other glasses.
4. Add, simultaneously, solutions No. 2 and No. 3 to solution No. 1.
5. Record the induction period and periods of oscillation.

Experiment
Belousov-Zhabotinsky reaction:
To carry out the reaction, a solution of citric acid (20 g per 80 ml of water) was prepared. For complete dissolution of citric acid, the solution had to be heated on an electric stove. Next, weighed portions of potassium bromate (8 g) and cerium sulfate III (1.5 g) were prepared and sequentially poured into a solution of citric acid. After stirring with a glass rod, sulfuric acid was added carefully, continuing stirring, after which fluctuations in the white-yellow color were recorded.

Briggs-Rauscher reaction:
Three solutions were prepared for the reaction: potassium iodate sulfate solution (c (KIO 3) \u003d 0.067 mol / l; c (H 2 SO 4) \u003d 0.053 mol / l) - 50 ml, starch solution of malonic acid with the addition of a catalytic amount of manganese sulfate two (c (MnSO 4) \u003d 0.0067 mol / l; c (CH 2 (COOH) 2) \u003d 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, in which the magnet was located, for 250 ml. The beaker was placed on a magnetic stirrer, which was subsequently turned on, and intensive stirring was turned on so that the color change occurred abruptly. Then, without stopping stirring, the contents of the beakers 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 stains - the oscillation period.

The induction period is 2 seconds.

Conclusion: As the reaction proceeds, a gradual increase in the period of oscillations is observed, which is especially clearly visible on the graph:

Conclusion
In this paper, oscillatory reactions and their properties were considered, in particular:

1. The field of application of oscillatory reactions in the modern world has been studied
2. The history of oscillatory reactions has been studied
3. The mechanisms of two oscillatory reactions are analyzed: Briggs-Rauscher

1. The Belousov-Zhabotinsky reaction mechanism was adapted for

1. A control synthesis was carried out for visual acquaintance with oscillatory reactions.

List of used literature

1. D. Garel, O. Garel "Vibrational chemical reactions" translated from English by L.P. Tikhonova. Publishing house "Mir" 1986. Page 13-25, 92-112.
2. A.M. Zhabotinsky "Concentration self-oscillations". Publishing house "Nauka" 1974. Page 87-89
3. OK. Pervukhin, Vibrational reactions. Toolkit". Publishing house of St. Petersburg State University 1999. Page 3-11.
4. S. P. MUShTAKOVA “Vibrational reactions in chemistry” Saratov State University named after V.I. N.G. Chernyshevsky
5. "Investigation of the conditions for the occurrence of an oscillatory regime in the process of oxidative carbonylation of phenylacetylene". Page 2-4.
6. I.D. Ikramov, S.A. Mustafin. "AN ALGORITHM FOR SEARCHING THE RATE CONSTANTS OF THE VIBRATIONAL RESPONSE BY THE EXAMPLE OF THE BELousOV-ZHABOTSKY REACTION". Bashkir chemical journal 2015
7. Pechenkin A.A. "The ideological significance of oscillatory chemical reactions"
8. Field R. J., Koros E., Noyes R. M., Oscillations in Chemical Systems II. Thorough Analisis of Temperal Oscillations in the Bromat-Cerium-Malonic Acid System., J. Amer. Chem. Soc., 94, 8649-8664 (1972).
9. Noyes R. M., Field R. J., Koros E., J. Amer. Chem. Soc., 94, 1394-1395 (1972).

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Studies of concentration fluctuations before the discovery of the reaction by Belousov

It turned out that one of the first publications on chemical vibrations dates back to 1828. In it, T. Fechner outlined the results of a study of the oscillations of an electrochemical reaction. In 1833, W. Herschel published a similar study of oscillations in a catalytic heterogeneous reaction. The most interesting is the work of M. Rosenskiöld, dating back to 1834. Its author quite by chance noticed that a small flask containing a little phosphorus emits quite intense light in the dark. There was nothing surprising in the very fact of the glow of phosphorus, but the fact that this glow was regularly repeated every seventh second was interesting. Rosenskiöld's publication gives a detailed study of bulb flickering. 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. Tsentnershwer also studied the effect of air pressure on periodic flashes of phosphorus. In his experiments, the flash period began from 20 sec. and decreased with decreasing pressure. At the same time, in England, chemists T. Thorp and A. Tatton observed periodic flashes of the oxidation reaction of phosphorus trioxide in a sealed glass vessel.

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 is dropped onto a glass plate coated with gelatin containing chromium peak, then the reaction product, precipitating, is located on the plate in concentric circles. Liesegang became fascinated with this phenomenon and studied it for almost half a century. It also found practical applications. In applied art, Liesegang's rings were used to decorate various products with imitation of jasper, malachite, agate, etc. Liesegang himself proposed the technology for making artificial pearls. Nevertheless, the discovery of Liesegang, which had a great resonance in scientific chemical circles, was not the first. And before him, chemical waves were studied, and in 1855 a book by F. Runge was published, in which numerous examples of such experiments were collected.

The list of such examples could be continued. Following these, oscillatory reactions were discovered at the interface between two phases. Of these, the most well-known are the reactions at the metal-solution interface, which received specific names - "iron nerve" and "mercury heart". The first of them - the reaction of dissolving iron (wire) in nitric acid - got its name because of the external resemblance to the dynamics of an excited nerve, noticed by V.F. Ostwald. The second, or rather one of its variants, is the decomposition reaction of H 2 O 2 on the surface of metallic mercury. In the reaction, periodic formation and dissolution of an oxide film on the mercury surface occurs. Fluctuations in the surface tension of mercury cause rhythmic pulsations of the drop, reminiscent of the beating of the heart. But all these reactions did not attract much attention of chemists, since the ideas about the course of a chemical reaction were still rather vague.

Only in the second half of the XIX century. thermodynamics and chemical kinetics arose, which laid the foundation for a specific interest in oscillatory reactions and methods for their analysis. And at the same time, it was the development of equilibrium thermodynamics that at first served as a brake on the study of such processes. The point, apparently, was in the "inertia of previous knowledge." According to Professor Shnol, “an educated person could not imagine macroscopic order in the random thermal motion of a huge number of molecules: all molecules are in one state, then in another! As if to recognize the existence of a perpetual motion machine. This cannot be. This cannot be near the state of equilibrium, but only it was considered by the thermodynamics of those years.However, there are no restrictions on complex, including oscillatory, modes for nonequilibrium chemical systems, when the reactions have not yet been completed, and the concentrations of the reagents have not reached the equilibrium level But this circumstance escaped the attention of chemists ... It took an extraordinary intellectual effort to break out of the "iron fetters of complete knowledge" and investigate the behavior of systems far from equilibrium.

Nevertheless, already in 1910, the Italian A. Lotka, on the basis of an analysis of a system of differential equations, predicted the possibility of oscillations in chemical systems. However, the first mathematical models corresponded only to damped oscillations. Only 10 years later, Lotka proposed a system with two subsequent autocatalytic reactions, and in this model the oscillations could already be undamped.

However, the positions of physicists and chemists diverged here. One of the most striking achievements of physics and mathematics of the XX century. - Creation of the theory of vibrations. Great, universally recognized merits here belong to Soviet physicists. In 1928, graduate student A.A. Andronov, a future academician, made a presentation at the congress of physicists "Poincaré limit cycles and the theory of self-oscillations".

In the early 1930s At the Institute of Chemical Physics of the USSR Academy of Sciences, fluctuations in luminescence in "cold flames" similar to the vibrational luminescence of phosphorus vapor were discovered, which interested the famous physicist D.A. Frank-Kamenetsky, who explained these oscillations on the basis of Lotka's kinetic model. And in 1947, at the same institute, a dissertation on the topic "On the theory of periodic occurrence of homogeneous chemical reactions" written by I.E. Salnikov under the scientific guidance of Frank-Kamenetsky. This dissertation contained extensive information about more than a century of history of the study of chemical vibrations and the first results of their theoretical study using the methods of the theory of nonlinear vibrations developed by the school of Academician Andronov. But then her defense did not take place. According to Voltaire, "the works of Frank-Kamenetsky and Salnikov on chemical self-oscillations, presented in a dissertation, in a book and in a number of articles, were certainly innovative for the then chemical science. But few people understood this innovation. "Vibrational ideology" (Andronov's term) was alien to the non-oscillatory routine of chemical science and practice, and this can explain the fact that the works of Frank-Kamenetsky and Salnikov in the 1940s were received with hostility, and when the second discovery of chemical vibrations took place, no one remembered them. It remains a mystery whether Belousov had any idea about these works. In any case, his two papers do not cite the work of his predecessors.

Materials used:
him.1september.ru, Wikipedia, Nature magazine, scholarpedia.org, hopf.chem.brandeis.edu, online.redwoods.cc.ca.us, vivovoco.rsl.ru.

Dedenev Yuri

The reason for starting this work was the article “Vibrational reactions in chemistry” by S.P. Mushtakov from Saratov State University named after Chernyshevsky, published in the Soros Educational Journal (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 maximize the attention of students to the subject of chemistry, that is, not just the search for nuggets of chemistry enthusiasts, but also an attempt 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 say with confidence 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 CONFERENCE OF STUDENTS

"YOUNT, SCIENCE, CULTURE"

CHEMISTRY section

STUDY OF VIBRATIONAL CHEMICAL REACTIONS

Dedenev Yuri

Secondary school №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. The 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 page 5

1.3. Potential Process Applications 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) page 10

4+ p.10

3.3. Preparing and conducting an oscillatory reaction page 11

Chapter 4. Conclusion p.14

Literature p.18

Appendix p.19

Figure 1 page 19

Figure 2 page 20

Conventions

1. BZ - Belousov - Zhabotinsky

2. LK - Citric acid

3. MK - Malonic acid

4. BMA - Bromatmalonic acid

5. see - see

6. fig. - drawing

7.max - maximum

8. min - minimum

Introduction

The reason for starting this work was the article “Vibrational reactions in chemistry” by S.P. Mushtakov from Saratov State University named after Chernyshevsky, published in the Soros Educational Journal (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 maximize the attention of students to the subject of chemistry, that is, not just the search for nuggets of chemistry enthusiasts, but also an attempt 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 say with confidence 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 have set ourselves the following tasks:

1. Give a brief background to the discovery of oscillation reactions

2. Give a theoretical justification for the mechanisms of the oscillation reactions.

3. Carry out syntheses to obtain the necessary components from available chemical reagents

4. Carry out the oscillation 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, which manifests itself 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 repeated outbreaks were then repeatedly described by many researchers. In the 19th century, other oscillatory reactions were also 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 laid the foundation for a specific interest in oscillatory reactions and methods for their analysis. Predictions of the possibility of oscillations in chemical systems have been made since 1910 on the basis of A. Lotka's mathematical models. In 1921, W. Bray published an article in which the first oscillatory liquid-phase reaction is described in sufficient detail. Bray realized the connection between his discovery and Lotka's prediction. However, his work did not arouse interest for about 40 years. One of the reasons for such 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 forbids such fluctuations even far from equilibrium. In fact, most chemists believed that fluctuations in concentration in closed homogeneous systems are impossible, in other words, there are no purely chemical fluctuations.

1.2 Modern history of research on oscillatory processes

Studies of oscillatory chemical reactions in the liquid phase began in 1951, when B.P. Belousov discovered fluctuations in the concentrations of the oxidized and reducing forms of cerium in the reaction of the interaction of citric acid with bromate. The solution changed regularly from colorless to yellow (due to CeIV), then back to colorless (CeIII), and so on. Belousov conducted a fairly detailed study of this reaction and, in particular, showed that the period of oscillations greatly decreases with an increase in the 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. Belousov was continued by A.M. Zhabotinsky, who received fluctuations when using not only citric, but also malonic, as well as malic acids as a reducing agent in the Belousov reaction. A.M. Zhabotinsky carried out detailed studies of oscillations 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 about this amazing reaction went around the whole world, and in several laboratories (in the USSR, the USA and Western Europe) they began to intensively study the BZ reaction. Vibrational reactions have finally entered the chemistry lab.

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 by Poincare at the end of the 19th century, is their high sensitivity to the slightest external disturbances. Conducting research in this area opens up great prospects for the creation of fundamentally new methods for the analysis of trace amounts 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 the measurement of the oscillation frequency is one of the simplest and most accurately performed operations, self-oscillatory 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 occur in the heart muscle. At present, the kinetics of oscillatory reactions is a rapidly developing branch of knowledge that has arisen at the intersection of chemistry, biology, medicine, physics, and mathematics. At present, it has 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 initial and intermediate chemicals can be very suitable functional models for the study of chaotic processes in other fields of knowledge (science of the earth and other planets, solid state physics, nuclear physics and elementary particle physics). , engineering mechanics, etc.).

Chapter 2 Theoretical Prediction of Oscillation Reactions

2.1. Properties of the main components of the reaction.

The reducing agent must be readily oxidized by the oxidized form of the catalyst and must not react directly with the bromate. In addition, it is necessary that the reducing agent be easily brominated and the bromo derivatives decompose rather easily, releasing Br. These requirements are met by substances with an active methylene group. Reactions involving MK, BMK, LA are qualitatively similar.

Substances (primarily ions of variable valence) that are close to cerium ions both in terms of the redox potential and in the kinetics of oxidation and reduction reactions can be used as a catalyst.

The oxidation reactions of 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. Therefore, 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 decay of Y. This model has been 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 . Members k 0 A and k 3 y can describe both chemical reactions and linear transport processes in an open system.

The next model, studied by Lotka (1920) and later independently by Voltaire (Volterra, 1931), contains two successive autocatalytic reactions (this model is widely known in ecology as the “prey-prey” model. For example: A is the specific amount of grass, the supply of which considered inexhaustible; X is the population density of herbivores; Y is the population density of carnivores).

A X Y . 1.2

Assuming the same for scheme (1.2) as for scheme (1.1), Lotka and Voltaire obtained the following system of equations:

x \u003d k 1 Ax - k 2 xy, y \u003d k 3 xy - k 4 y.

Note that the mathematical description of these processes turned out to be rather complicated. 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 1930s to describe ecological processes.

As an example, we will use two interacting systems, one of which draws the energy, substance or other components it needs for development from the other (the 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 feed on hares, which in turn are food for wolves. As you know, if you have any set of living creatures, then under favorable conditions, their population will increase indefinitely. In fact, external factors, such as a lack of energy or food, limit this growth process. How does this happen in the example of wolves and hares?

Imagine that up to a certain point the interaction of two subsystems, that is, populations of wolves and hares, was balanced: hares (taking into account their natural replenishment) were just enough 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 the wolves and, therefore, their number. There was a fluctuation in the number of wolves. Moreover, the number of wolves and hares will change over time periodically around a certain average (equilibrium) value. Well-fed wolves will begin to multiply intensively, giving new offspring, which, on abundant food, quickly matures and gives new offspring. There is a situation when the hare is no longer able to feed all the wolves - the number of hares begins to fall, and the wolves (for the time being) continue to grow. Finally, the ecosystem is overpopulated with wolves, and the place for hares is 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 fallen to a minimum level at which they are almost elusive for wolves. The livestock of the latter, having passed through a maximum, begins to decline, and this reduction continues until a level is reached that is able to feed the hares with their minimum number. Now that the number of wolves has reached a minimum, there is no one to hunt for hares. Hares begin to breed, and the meager livestock of wolves can no longer keep track of them. The number of hares in a short time will reach a level that the grass is able to feed. Again there is an abundance of hares, and everything repeats 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 (under traction) very slowly from a dropping funnel with a tube reaching the bottom, with constant stirring. The resulting solution is saturated (under draft) with chlorine. The end of saturation is determined as follows. A sample of the solution (10 ml) is diluted with 10 ml of water, boiled until the complete removal of Br 2 and Cl 2 (starch iodine paper should not turn blue in liquid vapor) and add a drop of phenolphthalein solution. When fully saturated with chlorine, the sample solution should not turn red.

The reaction solution is cooled to 15 O C, the precipitated mixture of KS1O crystals is separated 3 and KC1 (300 - 350 g) and stir them with 150 ml of water for several hours. The remaining KBrO crystals 3 sucked off on a Buchner funnel, washed with 100 ml of water and separated. Get 200 - 240 g of crude potassium bromate.

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 obtaining the catalyst Ce 4+

In school chemistry labs, you can find cerium dioxide, which used to be part of the school's chemical reagent kit. The main task is to obtain any soluble cerium salt, in this case the easiest way is to obtain cerium sulfate (VI), for this it is necessary to subject the available cerium dioxide to the action of concentrated sulfuric acid during boiling. SEO 2 we will not dissolve in water, therefore it is necessary to act with sulfuric acid directly on the powder of cerium dioxide.

The reaction equation can be expressed as follows:

SeO 2 + 2H 2 SO 4 \u003d Ce (SO 4) 2 + 2H 2 O

A bright yellow solution of cerium (VI) sulfate is formed, then it can be evaporated on an evaporating dish until yellow crystals appear. If there is still no cerium dioxide, then you can get 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 when heated. The composition of silicon, from lighters, includes compounds of cerium (III) and (VI). However, it should 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 conduct of an oscillatory reaction.

Two solutions are prepared for the experiment. In the first case, a solution of cerium sulfate or nitrate (IV), 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 there. 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 stand out, and the intervals of alternating the color of the solution gradually increase in a strictly defined sequence: each subsequent interval is 10–15 seconds longer than the previous one, and the temperature of the solution also increases.

During the demonstration or after the demonstration of the experiment for students, the mechanism of a chemical reaction can be explained in a simplified version, that is, as a redox process in which bromic acid (BA) acts as an oxidizing agent, and citric acid acts as a reducing agent:

KBrO 3 + H 2 SO 4 \u003d KHSO 4 + HBrO 3

9HBrO 3 + 2C 6 H 8 O 7 \u003d 9HBrO + 8H 2 O + 12CO 2

9HBrO + C 6 H 8 O 7 \u003d 9HBr + 4H 2 O + 6CO 2

The change in the color of the solution occurs under the action of catalysts - cerium compounds, which in turn also change the degree of oxidation, but up to a certain concentration of the ion, after which the reverse process occurs.

Chapter 4. Conclusion

For convenience of presentation, we first consider a simplified scheme of the self-oscillatory reaction. In the course of this reaction, fluctuations in the color of the solution are observed, caused by fluctuations in the concentration of cerium (VI). Fluctuations in the concentration of cerium (VI) are shown in Fig.2. These are relaxation oscillations, the period of which (T) is clearly divided into two parts: T1 - the phase of the decrease in the concentration of cerium (VI) and T2 - the phase of the increase in 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) brominate the LA. The resulting bromo 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 the autocatalytic reaction is that it proceeds with a variable concentration of the catalyst that increases during the course of the reaction. Therefore, the rate of the autocatalytic reaction in the initial period increases, and only at deeper stages of the transformation, as a result of a decrease in the concentration of the initial substances, the increase in the rate is replaced by a decrease.

The speed of autocatalytic processes, as the reagents are consumed, does not decrease, but increases without any contradiction with the law of mass action. The reaction mechanism is such that their intermediate or final products have an accelerating effect on the process. Therefore, their rate at the beginning is vanishingly small, but then grows along with an increase in the concentration of reaction products. According to modern terminology, such processes are referred to as processes with positive feedback. So, for example, if an intermediate or final product of a multistage process turns out to be its inhibitor, self-inhibition of the reaction will be observed - its rate will decrease faster. What decreases the concentration of the initial reagents.

In the reaction during the interaction of Ce4+ ions 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 flow 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 Ce 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 fast transition of Ce ions occurs 3+ in Ce 4+ and the solution returns to 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 is formed - ions that slow 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 upon 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 the bromide ion begins to decrease sharply. Then reaction (2) noticeably accelerates and the Ce(VI) concentration grows 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: interactions of citric acid with cerium (VI) ions and sulfuric acid (due to acidification of the solution and dissociation of cerium (VI) sulfate), the reaction mechanisms are not described because of their complexity, the products of these reactions are carbon monoxide (IV ), carbon monoxide (II), water and partially dimethyl ketone.

Now we can summarize 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 the occurrence of concentration fluctuations is the presence of feedbacks 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

1. A.M. Zhabotinsky Concentration fluctuations. M.: - Science. 1974.
2. Yu.V. Karyakin, I.I. Angels Pure chemicals. M.: - Chemistry. 1974.
3. B.N. Stepanenko Course of organic chemistry. M .: - Higher school. 1972.
4. ON THE. Ostapkevich Workshop on inorganic chemistry. M .: - Higher school 1987
5. V.N. Aleksinsky Entertaining experiments in chemistry. Moscow: Education, 1980.
6. Soros educational journal. No. 7.1997.

Application

[ce4+]

M - - - - - - -

N - - - - - - - - - - - - - - - - - - - - - - - - | - - - - - - - - - - - - - - - - - - - - - - - - - - -

| | |

| | |

| T 1 | T 2 |

| | |

| | t

| T |

| |

| |

Fig.1. Self-oscillations of the concentration of cerium (VI)

[ce4+]

Max - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Min - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

[Br]

Fig.2. Dependence of the change in the concentration of cerium (VI) on the concentration of bromide - ions.

VIBRATIONAL REACTIONS- a class of redox periodic reactions. The reaction mechanism is similar to that of a retaining latch device. For the first time such reactions were discovered in 1951 by the Moscow chemist B.P. Belousov.

Oscillatory reactions proceed with the participation of a catalyst (for the first time this was discovered in the course of a reaction in the presence of cerium ions) and usually consist of two stages.

Necessary conditions for the occurrence of such reactions:

a) the speed of the first stage should significantly exceed the speed of the second stage;

b) at the second stage, a compound should appear that inhibits the course of the first stage (it is called an inhibitor).

A similar reaction can be observed when mixing aqueous solutions of a cerium (III) salt (for example, cerium sulfate), potassium bromate KBrO 3 and bromomalonic acid HO (O) C - CH (Br) - C (O) OH. The reaction mass is acidified with sulfuric acid.

At the first stage, the trivalent cerium ion (which arose during the dissociation of the cerium salt) is reduced by the bromate anion (it is supplied by potassium bromate). In this case, the Ce(III) ion is oxidized to Ce(IV), which is externally noticeable by the change in the color of the reaction solution - Ce(III) ions in an aqueous solution are colorless, and Ce(IV) are yellow.

10Ce 3+ + 2BrO 3 – + 12H + = 10Ce 4+ + Br 2 + 6H 2 O (I)

At the next stage, the resulting Ce (IV) ion reacts with bromomalonic acid, oxidizing it:

4Ce 4+ + HO(O)C – CH(Br) – C(O)OH + 2H 2 O =

4Ce 3+ + HC(O)OH + 2CO 2 + 5H + + Br - (II)

In this case, cerium again becomes the Ce (III) ion and can again participate in reaction I. In this case, it plays the role of a typical catalyst, participates in the reaction, but is not consumed, nevertheless, the reaction will not proceed without it. Potassium bromate and bromomalonic acid are consumed during the reaction, cerium only transfers electrons from one reagent to another (the starting reagents are marked in black, and the reaction products are in red):

The peculiarity of this reaction is that at stage II, the bromine anion Br – . It inhibits, that is, inhibits stage I, but does not affect stage II. As a result, the products of stage II, primarily Ce 3+ ions, accumulate in the reaction system. At a certain moment, when these ions are accumulated in large quantities, bromine ions can no longer inhibit stage I, and it proceeds at a high rate. Ce(IV) ions reappear in the system and then participate in the slow stage II. Thus, bromine ions play the role of a trigger that prevents the first stage from starting up to a certain point. Outwardly, it looks like this (Ce(III) ions in an aqueous solution are colorless, and Ce(IV) are yellow): the reaction mass instantly turns yellow, and then slowly becomes colorless (Fig. 4, beaker No. 1). The color changes approximately every one and a half minutes, the time interval remains unchanged for several hours. If you gradually add consumable reagents, then such a “chemical clock” will work for a very long time. As the temperature rises, the time cycle of the oscillatory response shortens.

There are other examples of oscillatory reactions. In the system described above, cerium ions can be replaced by iron ions. For this, a complex of Fe (II) sulfate with three molecules of phenanthroline is used, which is colored red in an aqueous solution (this complex is widely used for the quantitative determination of iron):

A similar Fe(III) complex, which appears as a result of oxidation, is colored blue; in the course of the reaction, the blue color instantly becomes red, which gradually turns into blue again (Fig. 4, beaker No. 2).

If we replace bromomalonic acid with citric [HOC(O)CH 2 ] 2 C(OH)C(O)OH, then in the presence of catalytic amounts of manganese salts, a system appears in which the color pulsates every two minutes (Fig. 4, glass No. 3) . Oxal-acetic acid HOC(O)CH 2 C(O)C(O)OH with cerium salts counts six-second intervals (glass No. 4). The time intervals in the animated figure are shown conditionally, the longest color change interval is in glass No. 3, the smallest is in glass No. 4

Soon after the discovery of such reactions, it was found that such processes are quite common. As a result, a general theory of oscillatory processes was developed, which include some gas-phase reactions (for example, the oxidation of hydrocarbons), heterophase oxidation of carbon monoxide, hydrogen, ammonia, ethylene on metal catalysts, and a number of polymerization processes. Oscillatory reactions determine the course of some of the most important biological processes: the generation of nerve impulses and the mechanism of muscle contraction.

Mikhail Levitsky

Ministry of Education, Science, Youth and Sports

Theoretical Lyceum Petru Movilă

department

"Ability, work, talent"

Coursework in chemistry on the topic:

"Vibrational Chemical Reactions"

Completed by: student of grade 12A

Bolyubash Irina

Lecturer: Snidchenko M.A.

* Chisinau, 2007 *

1. Introduction:

a) Redox reactions

b) Vibrational chemical reactions

2. The 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. Lotka

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 done

7. Literature

Introduction.

Chemistry is an experimental science. And therefore, the experiment as a method of scientific research has long firmly occupied a leading place among the methods of the natural sciences. The experiment is the most important way to connect theory with practice in teaching chemistry, turning knowledge into convictions. Therefore, the disclosure of the cognitive significance of each experience is the main requirement for a chemical experiment.

Under experiment (otlat. "experiment" - "trial") understand the observation of the phenomenon under study under certain conditions, allowing you to follow the course of this phenomenon and repeat it under these conditions. The chemical experiment occupies an important place in the teaching of chemistry, since through observations and experiments the diversity of the nature of substances is known, facts are accumulated for comparisons, generalizations, and conclusions.

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 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 have been called problem experiments.

Even at the time of antiquity, 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 surrounding world) with the subsequent formation of a stable interest in something. Surprise and, following it, the craving for knowledge - this is fertile ground for studying the 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, most often, are redox reactions.

Redox reactions

There are numerous criteria for the classification of chemical reactions. One of the most important is a sign of a change in the oxidation states of elements. Depending on whether the oxidation states of the elements change or remain, chemical reactions can be divided into redox reactions and those occurring without changing the oxidation states.

Reactions that occur with a change in the oxidation states of elements (redox) are widely known. They play an important role in technology and nature, they underlie the metabolism in living organisms, they are associated with the processes of oxidation, putrefaction, fermentation, and photosynthesis. The processes of oxidation (and reduction) occur during the combustion of fuel, corrosion of metals, electrolysis, with their help, metals, ammonia, alkalis and many other valuable products are obtained. Therefore, the study of redox reactions is provided for by school courses in inorganic and organic chemistry.

Recall the main 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, assuming that all electron pairs through which this atom is connected to others are completely displaced to atoms of elements with greater 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 initial 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 off electrons by an atom, molecule or ion, while 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 conjugate processes, the number of electrons given by the reducing agent in the process of its oxidation is always equal to the number of electrons accepted by the oxidizing agent in the process of its reduction.

Vibrational chemical reactions

In this term paper, I will consider a special case of a problematic experiment, oscillatory chemical reactions. Oscillatory reactions are a whole class of reactions of oxidation of organic substances with the participation of a catalyst with redox properties. This process proceeds 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 salt to the solution. Cerium is a metal with a 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 "boils". And against the background of this boiling, B.P. Belousov noticed an amazing thing: the color of the solution periodically changed - it became either yellow or colorless. Belousov added complexphenanthroline with ferrous iron (ferroin) to the solution, and the color of the solution began to change periodically from purple-red to blue and back.

Thus was discovered the reaction that became famous. 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.

The history of the discovery of oscillatory reactions.

I. P. Belousov made the discovery of an oscillatory chemical reaction in an attempt to create a simple chemical model of some stages of the system of key biochemical transformations of carboxylic acids in a cell. However, the first report of its discovery was not published. The reviewer of a chemical journal doubted the fundamental possibility of the reaction described in the article. Most chemists in those years believed that there were no purely chemical oscillations, 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 results of the study was made by the scientist in 1957, and again he was refused, despite the work of the Belgian physicist and physicochemist I. R. Prigozhin that appeared at that time. These works showed the possibility and probability of oscillatory chemical reactions.

It was not until 1959 that a short abstract was published on the discovery by B. P. Belousov of a periodically acting oscillatory chemical reaction in a little-known edition of the Collection of Abstracts on Radiation Medicine.

And the thing 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 forward or in the opposite direction? It is impossible to imagine that the entire huge number of molecules in the vessel were in one state, then in another (either all “blue”, then all “red” ...).

The direction of the reaction is determined by the chemical (thermodynamic) potential - the reactions are carried out in the direction of more probable states, in the direction of reducing the free energy of the system. When the reaction in this direction is completed, it means that its potential has been exhausted, thermodynamic equilibrium is reached, and without the expenditure of energy, spontaneously, the process cannot go in the opposite direction. And then ... the reaction goes in one direction or the other.

However, there was no violation of the laws in this reaction. There were fluctuations - periodic changes - in the concentrations of intermediate products, and not the initial reagents or final products. CO2 does not turn into citric acid in this reaction, this is actually 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 the system from the initial state to the final state can be very complex. Only in recent decades has the thermodynamics of systems far from equilibrium begun to deal with these problems. 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; therefore, the change in the color of the solution can be recorded without interference by photoelectric devices. Later it turned out that ferroin, even without cerium, serves as a catalyst for this reaction. B. P. Belousov already in the first experiments noticed another remarkable property of his reaction: when stirring is stopped, the change in color in the solution propagates in waves. This distribution of chemical vibrations in space became especially evident 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" that propagate 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
oscillations before the discovery of the reaction by B. P. Belousov

But as the story goes, the discovery of B.P. Belousov 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 outlined the results of a study of the oscillations of an electrochemical reaction. in the dark it emits quite intense light. There was nothing surprising in the very fact of the glow of phosphorus, 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. Tsentnershwer also studied the effect of air pressure on periodic flashes of phosphorus. In his experiments, the flash period 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 is dropped onto a glass plate coated with gelatin containing a chromium peak, the reaction product, precipitating, is located on the plate in concentric circles. Liesegang became fascinated with this phenomenon and studied it for almost half a century. It also found practical applications. In applied art, Liesegang's rings were used to decorate various products with imitation of jasper, malachite, agate, etc. Liesegang himself proposed a technology for making artificial pearls.

The list of such examples can be continued. Following the above, oscillatory reactions were discovered at the interface between two phases. Of these, the most well-known are the reactions at the metal–solution interface, which received specific names - “iron nerve” and “mercury heart”. The first of them - the reaction of dissolving iron (wire) in nitric acid - got its name because of the external resemblance to 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 mercury surface occurs. Fluctuations in the surface tension of mercury cause rhythmic pulsations of the drop, resembling a heartbeat. But all these reactions did not attract much attention of chemists, since ideas about the course of a chemical reaction were still rather vague.

Only in the second half of the XIX 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 "prey", for example, herbivores, and "predators" eating them (X and Y). Predators eat prey and reproduce - the concentration of Y increases, but to a certain limit, when the number of prey decreases sharply and predators die of hunger, the concentration of Y decreases. Then the surviving victims begin to multiply - the concentration of X grows. The surviving predators after this also multiply, the concentration of Y grows again, and so on many times. Periodic fluctuations in the concentration of reagents are observed. It is clear that the condition for such undamped (for a long time) fluctuations is the abundance of grass - the food of the victims. Lotka's equations were improved by V. Volterra. And the modern theory of oscillations was developed by Russian physicists L. I. Mandelstamm, A. A. Andronov, A. A. Witt, S. E. Khaikin, D. A. Frank-Kamenetsky. So for physicists and mathematicians, Belousovane's discovery was so amazing.

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 sufficient to imagine how bromomalonic acid is first formed from ismalonic acid, and in the further reaction, KBrO3 is converted into KBr. The anion Br-- inhibits the further oxidation of bromomalonic acid, and the oxidized form of the catalyst (quadrivalent cerium or ferric iron in combination with phenanthroline) accumulates. As a result, the accumulation of Br-- stops, and the oxidation of bromomalonic acid resumes ... Now it is clear that such a mechanism is far from complete. The number of intermediate products has reached forty, and the study continues.

In 1972, R. Noyes and colleagues 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), the bromate ion interacts with the bromide ion in the presence of H + to form bromous and hypobromic 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 the bromide ion and the synthesis of bromomalonic acid.

Group B reactions are possible only in the absence (low concentration) of the bromide ion. When the bromate ion reacts with bromous acid, the BrO2 radical is formed.

BrO-3+ HBrO2 + H+ > 2BrO2 + H2O (B1)

BrO2 reacts with cerium (III), oxidizing it to cerium (IV), and is itself reduced to bromic acid:

BrO2+ Ce3+ + H+ > HВrO2 + Ce4+ (B2)

Bromic acid decomposes into bromate ion and hypobromic acid:

2HBrO2> BrO-3 +HOBr + H+ (B3)

Hypobromic acid brominates malonic acid:

HOBr + CH2(COOH)2 > BrCH(COOH)2 + H2O (B4)

As a result of group B reactions, bromomalonic acid and tetravalent cerium are formed.

Fluctuations in the concentrations of the main components of the reaction: bromide and ferrin - in the phase space are represented as a closed line (limit cycle).

BrO-3+ 4Ce3+ + CH2(COOH)2 + 5H+ > BrCH(COOH)2 + 4Ce4+ + 3H2O (B)

The cerium (IV) formed in these reactions (reactions of group B):

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 a bromide ion, which is the more intense, the higher the concentration of bromomalonic acid. An increase in the concentration of the bromide ion leads to a cessation (dramatic slowdown) of the oxidation of cerium (III) to cerium (IV). In recent studies, cerium is usually replaced by ferroin.

This (incomplete) sequence of steps in the Belousov-Zhabotinsky reaction shows how complex this system is. So, it is enough to take into account the change in the concentration of all three-basic intermediate components of the reaction HВrO2 (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 bromous acid stops, and it quickly decomposes.

Fourth step– the process of ferriin decay, started at the 2nd step, 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, the fluctuations in the concentration of its reagents, and the patterns of propagation of concentration waves.

Experimental part:

The reaction of interaction of citric acid with potassium bromate:

Reagents:

1. KMnO4(potassium permanganate).

2. KBrO3(potassium bromide or potassium bromate).

3. H2SO4(concentrated).

4. Citric acid.

5. Distilled water.

Progress: A weighed portion of citric acid (2 g) was dissolved in 6 ml of H2O. A weighed portion of potassium bromide - 0.2 g was added to the resulting solution 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 complete dissolution of the reagents.

Observations: Immediately after the addition of KMnO4, the solution turned purple and began to "boil". After 25 s, with rapid boiling, the color of the solution began to change to brown. With the course of the reaction, the solution gradually brightens - up to a light yellow color. After 3 min 45 s, a sharp darkening of the solution begins (similar to the diffusion of a high density liquid), and after 40 s 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, then stops altogether (yellow solution).

/>Vibrational 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 an aqueous solution of potassium bromide (2 mol/l, or 12 g per 50 ml of water)

4. KBrO3 saturated potassium bromate solution (about 10 g per 100 ml of water)

5. H2SO4 concentrated sulfuric acid

6. CH2(COOH)2 an 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, 25x25 cm glass plate, Petri dish, 100 ml volumetric flask, 250 ml Erlenmeyer flask with ground stopper, six pipettes, burette, glass rod, washer, filter paper.

Experience description: To demonstrate the experiment, solutions A and B are preliminarily prepared.

Solution A – a solution of ferroin, a complex of iron(II) with o-phenanthroline (phen). 0.70 g of iron(II) sulfate heptahydrate (or 0.99 g of Mohr's salt) and 1.49 g of o-phenanthroline are added to a 100 ml volumetric flask, the volume of the solution is adjusted to the mark with water and mixed. The solution acquires a red color due to the formation of the iron(II) phenanthroline complex:

Fe2++ 3 phen = 2+

Solution B - a 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, achieving the disappearance of the brown color due to the release of bromine in a parallel switching reaction:

BrO3–+ 5Br– + 6H+ = 3Br2 + 3H2O

3Br2+ 2CH2(COOH)2 + 2H2O = BrCH(COOH)2+ HCOOH + CO2 + 5HBr

The total volume of the potassium bromate solution used for titration should be about 7.5 ml. The resulting bromomalonic acid is unstable, but it can be stored at a temperature of 5100C for some time.

To directly demonstrate the experiment, a Petri dish is placed on a glass plate covering the polylux light window, into which 10 ml of a saturated potassium bromate solution, 4 ml of a bromomalonic acid solution and 1.5 ml of a ferroin solution are successively added using pipettes. Within a few minutes, blue spots appear on a red background due to the formation of a phenanthroline complex of iron (III) 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 with the formation of 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 becomes blue due to the formation of an 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 change in color 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, the appearance of “black dots” - projections of bubbles of the released carbon dioxide can be observed on the screen.

The range of colors can be extended by adding a few crystals of cerium(III) nitrate hexahydrate to the Petri dish. Ce(NO3)3. 6H2O. Then, in addition to the blue and pink color, one 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 reaction rate increases, and the color change accelerates.

Note. Phenanthroline is a heterocyclic compound with two nitrogen atoms that have lone pairs of electrons and are capable of coordination. In complex compounds with iron O-phenanthroline plays the role of a bidentate ligand and forms strong chelate-type complexes.

Conclusion.

To date, the Belousov–Zhabotinsky reaction has taken its rightful place in world science. Every year, several international conferences on the dynamics of linear chemical systems are held in the world, and the words "BZ-reaction" (abbreviation: Belousov-Zhabotinsky reactions) are heard at dozens of other conferences devoted to problems of physics, chemistry, and biology.

The study of the Belousov-Zhabotinsky reaction, as I was 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 the study of a formidable violation of the heart - arrhythmias and fibrillations. And recently, experiments have been started with a light-sensitive modification of this reaction, when the dynamics in this system depends on the light intensity. It turned out that such a reaction can be used as a computer for storing and processing images. The light-sensitive modification of the Belousov-Zhabotinsky reaction can serve as a prototype of a computing complex, which may replace the computer.

On the other hand, oscillatory chemical reactions are a vivid example of self-organization in non-living nature, and in this sense there is not only a natural scientific, but also a philosophical significance. The fundamental changes in natural science that 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. , ranging 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 have a very different nature: chemical, mechanical, optical, electrical, etc.

Thus, applied research is becoming increasingly important, for example, in the field of modeling alternative means of information processing (in particular, the analysis of complex mosaics with gradation of object brightness). Another new direction of applied research is the study of the features 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 the heart tissue and brain tissues, processes occurring at the level of non-ecological systems), in its new field - synergetics (theory and self-organization), as well as experimental work initiated the development of the modern theory of dynamical systems. Although at present much of such reactions is already understood, however, the causes that cause oscillatory chemical processes remain unclear to the end.

At present, the kinetics of oscillatory reactions is a rapidly developing branch of knowledge that has arisen 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 even more I was struck that such an incredible significance, an impressive discovery for many years was not perceived by others, and simply was not understood by the great minds of that time. This discovery went through its thorny path, and, in the end, took its rightful place in world science. Asama, the possibility of such a reaction once again proves that in our world there is still a lot of unknown and unexplored.

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 much weaker. About 5 ml of a solution of all components must 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), oscillations 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: Required solutions:

- citric acid (40 g in 160 ml H2O)

- H2SO4(1:3).

And also hangings:

- KBrO3(16 g)

- Ce2(SO4)3(3-3.5 g)

Heat the citric acid solution to 40°-50°C, then pour out a sample of KBrO3. Put the beaker on a sheet of white paper and add a sample of Ce2(SO4)3 and a few ml of H2SO4. Alternation of colors immediately begins to occur: yellow > colorless > yellow, with a period of 1-2 minutes.

Recipe 4: Required solutions:

- 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 samples:

- MnSO4(1g) and some starch.

Pour everything into one glass (200-250 ml), add weighed portions, stir with a glass rod. There is an alternation of color: colorless > yellow > blue.

Bibliography.

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2. Shnol S.E. Knowledge is Power. 1994. No. 3. pp. 62-71.

3. Zhabotinsky A. M. Concentration self-oscillations. Moscow: Nauka, 1974.

4. Garel D., Garel O. Vibrational chemical reactions / Per. from English. M.:

5. Dubnishcheva 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.: UNITI-DANA, 1999, S. 78 - 87.

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