Belousov–Zhabotinsky reaction
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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 reaction is commonly used in scientific demonstrations, due to its ability to generate visually interesting patterns.
Chemical mechanism
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.[1][2]
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 color;
- 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 as the acid and potassium bromate as the source of bromine. The overall equation is:
- 3 CH2(CO2H)2 + 4 BrO−
3 → 4 Br− + 9 CO2 + 6 H2O
Many variants of the reaction exist. The only key chemical is the bromate oxidizer. The catalyst ion is most often cerium, but it can be also manganese, or complexes of iron, ruthenium, cobalt, copper, chromium, silver, nickel and osmium. Many different reductants can be used.
Procedure
NileRed made a good video on the reaction.
The main reagents needed for this reaction are: conc. sulfuric acid, sodium or potassium bromate, malonic acid, sodium bromide and ferroin. Three solutions are made from these reagents for the reaction:
- Solution 1: 67 ml H2O + 5 g NaBrO3 (or 5.55 g KBrO3) + 2 ml H2SO4
- Solution 2: 10 ml H2O + 1g malonic acid
- Solution 3: 10 ml H2O + 1 g NaBr
In a Petri dish, all the three solutions are added in the numerical order, in volumes of 6 ml, 1 ml and 0.5 ml respectively. When solution 3 is added, bromine forms, giving the solution a orange-ish color. The Petri dish can be mixed to homogenize the mixture, but this isn't required as the solution will turn colorless on its own eventually. 1 ml of ferroin is added in the dish and the solution is stirred manually to homogenize it. This causes it to change its color, from reddish-orange to blue, then purple-black, then brown, then finally brown-reddish.
After letting it sit for some time, colored spots will begin to appear. These spots grow into a series of expanding concentric rings or perhaps expanding spirals similar to the patterns generated by a cyclic cellular automaton. The colors disappear if the dishes are shaken, and then reappear. The waves continue until the reagents are consumed. An important note is that the solution film needs to be thin, if it's too thick, the reaction may not take place as it should. If the Petri dish used is too small, you can remove some of the liquid using a pipette or just use a very wide flat bottom dish instead of a Petri dish. The reaction can also be performed in a beaker using a magnetic stirrer.
One problem during the reaction is the formation of air bubbles at the surface, which "ruin" the visual of the reaction. Stirring the solution multiple times after reaction completion will remove the bubbles, but this causes the deplete the main reagents, causing fewer "ripples" to form.
