Belousov–Zhabotinsky reaction

The Belousov -Zhabotinsky reaction (BZR or BZ reaction ) is the classic example of a homogeneous chemical oscillator. It is often used to illustrate chaotic systems. It is a system comprising a plurality of chemical reactions, showing a temporal oscillation, which is very unusual for chemical reactions. Initially, the reaction is stopped for a measurement error or artifact, since the second law of thermodynamics seemed to prohibit such an operation. This set of physics states that (without supply of external energy) can not form a more ordered state from a disordered state by itself. This sentence is here but not applicable because it applies only to closed systems that are close to equilibrium. The Belousov -Zhabotinsky reaction is this very far, however, and may therefore exhibit this unusual behavior.

Classically, the Belousov -Zhabotinsky reaction is carried out in a Petri dish (pictured at right ), because it so nicely, eg by means of an overhead projector, model looks, which propagate as circular waves.

The principle of the chemical oscillator can be shown with other reaction systems, such as by means of the so-called Ioduhr ( Briggs -Rauscher reaction).

History

At the beginning of the 19th century oscillating chemical systems have been found and described. So reported Fechner 1828 via oscillating electrode processes. 1899 and 1900, then put Wilhelm Ostwald before a more detailed investigation of voltage and Korrosionsoszillationen of chromium in hydrochloric acid and iron in nitric acid. However, it was in all these oscillations are heterogeneous reactions. For example, based the reactions studied by Ostwald that at electrodes (solid / solid) form layers periodically out of solutions and dissolve again. This periodic fluctuations in the current flowing through the electrodes arise. 1920, Bray oscillation observed in the reaction of hydrogen with iodine and iodic acid as catalyst. It was suspected that this gas bubbles or dust grains formed the boundary surfaces, since one homogeneous oscillating systems considered excluded.

In 1950, Boris Pavlovich Belousov discovered ( Борис Павлович Белоусов ), the Belousov -Zhabotinsky reaction by chance. He was able to observe a change occurs periodically the color of the solution from yellow to colorless in the oxidation of citric acid with sulfuric acid bromate and cerium ions as a catalyst. Since this observation appeared for the same reason as in Bray as too improbable succeeded Belousov in 1959 to publish a short article about it. SE Schnoll recognized the importance of this reaction and instructed Anatoli Markowitsch Schabotinski ( Анатолий Маркович Жаботинский ) with the study of the phenomenon described.

Slow and non- Russian scientists showed interest in oscillating reactions, and began a comprehensive study of the phenomena associated with them. For example, spatial structures have been discovered ( circular patterns ) that can be formed in a thin layer of a solution of the Belousov -Zhabotinsky reaction.

1977 then received Ilya Prigogine received the Nobel Prize in Chemistry for his significant research in the field of thermodynamics. He looked far from equilibrium remote systems (so-called dissipative structures ), both in chemistry ( the Belousov -Zhabotinsky reaction belongs to this class of processes ) as well as in physics, biology (eg Lotka -Volterra model for predator-prey systems), and the sociology occur. After this Nobel Prize in 1980 Belousov ( posthumously ), Zhabotinsky and Zaikin, Krinsky and Ivanitzky together with the Lenin Prize, the highest scientific award of the Soviet Union, was honored with them.

Reactions

At the reaction solutions of four substances, potassium bromate, malonic acid, potassium bromide and concentrated sulfuric acid, and ferroin, or other redox indicator are involved. During the reaction the state of the indicator changes continuously between the reduced and oxidized form, which causes a typical color change. In ferroin as an indicator of the color between blue switches ( Ferriin, with Fe3 ) and red ( ferroin, with Fe2 ), with cerium between yellow ( Ce4 ) and colorless ( Ce3 ), with manganese between red ( Mn3 ) and colorless (Mn 2 ). The reaction does not proceed as long as desired, since both malonic acid and bromate are consumed.

During the reaction step, three different processes (A, B, and C), each having a plurality of reactions. Process A is a non- free radical, of the redox indicator is not involved. Essentially bromide is consumed and converted to Monobrommalonsäure. During this reaction Bromige acid, which is further reacted again formed.

Is much bromide consumed, this allows the reactions of the process B can run. This is radically and running with the redox indicator from. Bromige acid acts in a first reaction as an auto catalyst (see autocatalysis ), the concentrations being doubled to bromous acid per reaction.

For larger concentrations of these bromous acid to hypobromous acid reacts, so that an overall reaction for the process B of

Results.

This oscillation is possible, there must be another reaction in which the spent bromide is re-formed. This is the process C, react with the malonic acid, Monobrommalonsäure, hypobromite and the redox indicator with Bromidbildung.

Another bromide formed by the decomposition of the bromate hydroxymalonic acid to carbon dioxide and water.

Model for the course of the reaction

In the following a simple model for the BZR to be portrayed. The following picture is for illustrative purposes:

In the initial state A are adjacent to the starting materials ( bromate, malonic acid ), especially bromide and Ferroin present in the solution. Now I can run reaction that consumes the bromide and malonic acid brominated. The system is so in the state B in which virtually no longer present bromide. The bromide inhibits even the smallest concentrations of the drainage of the reaction II, so this in the beginning does not matter. Now they can, however, run and oxidizes the ferroin, which leads to a color change of the solution to blue. The system is now in state C, where the solution contains Ferriin and no bromide. The reaction III can now occur and reduces the Ferriin again Ferroin, whereby bromide is re-formed. Moreover, even formic acid as the product. This state corresponds to straight back to the initial state A.

This model is greatly simplified. There are works, in which up to 20 sub- equations are used to achieve a very accurate modeling of the system.

Mathematical models

There were developed several mathematical models to map the course of chemical oscillators. These include:

• Brusselator
• Oregonator

The Brusselator is very simple, but physically unrealistic. He delivers results that are very close to the BZR ( see figure). In addition, the system is relatively simple and can be mathematically well analyzed. Often you can simulate models for the BZR with a cellular automaton and thus obtains also models for the spatial patterns in the BZR.

Structural formulas involved substances

Ferroin