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A Belousov–Zhabotinsky reaction, or BZ reaction, is one of a class of reactions that serve as a classical example of non-equilibrium thermodynamics, resulting in the establishment of a nonlinear chemical oscillator.

The only common element in these oscillators is the inclusion of bromine and an acid. The reactions are important to theoretical chemistry in that they show that chemical reactions do not have to be dominated by equilibrium thermodynamic behavior. These reactions are far from equilibrium and remain so for a significant length of time and evolve chaotically. In this sense, they provide an interesting chemical model of nonequilibrium biological phenomena; as such, mathematical models and simulations of the BZ reactions themselves are of theoretical interest, showing phenomenon as noise-induced order. An essential aspect of the BZ reaction is its so called "excitability"; under the influence of stimuli, patterns develop in what would otherwise be a perfectly quiescent medium. Some clock reactions such as Briggs–Rauscher and BZ using tris(bipyridine)ruthenium(II) chloride as a catalyst can be excited into self-organising activity through the influence of light.

The discovery of the phenomenon is credited to Boris Belousov. In 1951, while trying to find the non-organic analog to the Krebs cycle, he noted that in a mix of potassium bromate, cerium(IV) sulfate, malonic acid, and citric acid in dilute sulfuric acid, the ratio of concentration of the cerium(IV) and cerium(III) ions oscillated, causing the colour of the solution to oscillate between a yellow solution and a colorless solution. This is due to the cerium(IV) ions being reduced by malonic acid to cerium(III) ions, which are then oxidized back to cerium(IV) ions by bromate(V) ions. Belousov made two attempts to publish his finding, but was rejected on the grounds that he could not explain his results to the satisfaction of the editors of the journals to which he submitted his results. Soviet biochemist Simon El'evich Shnoll encouraged Belousov to continue his efforts to publish his results. After Belousov's publication, Shnoll gave the project in 1961 to a graduate student, Anatol Zhabotinsky, who investigated the reaction sequence in detail; however, the results of these men's work were still not widely disseminated, and were not known in the West until a conference in Prague in 1968. The reaction can also be performed in a beaker using a magnetic stirrer. Andrew Adamatzky, a computer scientist in the University of the West of England, reported on liquid logic gates using the BZ reaction. The BZ reaction has also been used by Juan Pérez-Mercader and his group at Harvard University to create an entirely chemical Turing machine, capable of recognizing a Chomsky type-1 language. Strikingly similar oscillatory spiral patterns appear elsewhere in nature, at very different spatial and temporal scales, for example the growth pattern of Dictyostelium discoideum, a soil-dwelling amoeba colony. In the BZ reaction, the size of the interacting elements is molecular and the time scale of the reaction is minutes.

The mechanism for this reaction is very complex and is thought to involve around 18 different steps which have been the subject of a number of research papers. In a way similar to the Briggs–Rauscher reaction, two key processes (both of which are auto-catalytic) occur; process A generates molecular bromine, giving the red colour, and process B consumes the bromine to give bromide ions. Theoretically, the reaction resembles the ideal Turing pattern, a system that emerges qualitatively from solving the reaction diffusion equations for a reaction that generates both a reaction inhibitor and a reaction promoter, of which the two diffuse across the medium at different rates. One of the most common variations on this reaction uses malonic acid (CH2(CO2H)2) as the acid and potassium bromate (KBrO3) as the source of bromine.

Many different patterns can be observed when the reaction is run in a microemulsion.

Quick Facts

• In the BZ reaction, the size of the interacting elements is molecular and the time scale of the reaction is minutes.
• One of the most common variations on this reaction uses malonic acid (CH2(CO2H)2) as the acid and potassium bromate (KBrO3) as the source of bromine.
• The reactions are important to theoretical chemistry in that they show that chemical reactions do not have to be dominated by equilibrium thermodynamic behavior.
• This is due to the cerium(IV) ions being reduced by malonic acid to cerium(III) ions, which are then oxidized back to cerium(IV) ions by bromate(V) ions.
• The BZ reaction has also been used by Juan Pérez-Mercader and his group at Harvard University to create an entirely chemical Turing machine, capable of recognizing a Chomsky type-1 language.

Source material: Wikipedia - "Belousov–Zhabotinsky reaction". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.