Oscillating Reactions

In an oscillating chemical reaction, the concentrations of the reactants and products change with time in a periodic or quasi-periodic manner (i.e., they do not move directly or evenly toward their final concentrations). Chemical oscillators exhibit chaotic behavior, in which concentrations of products and the course of a reaction depend on the initial conditions of the reaction. Oscillating reactions are thought to play key roles in such diverse processes as biological morphogenesis and geologic stratigraphy.

Scientists have a long-standing fascination with the complexities of oscillating systems. In the seventeenth century, the English-Irish chemist Robert Boyle, reported the periodic "flaring up" of phosphorus in contact with the air. The classic modern example of an oscillating reaction is the Belousov-Zhabotinsky oscillating reaction that yields a red solution that turns blue at varying intervals of time. In a stirred vessel, the Belousov-Zhabotinsky reaction mixture will change color from red to blue dozens or hundreds of times before equilibrium is established. If the mixture is poured into a shallow vessel, the oscillation will be triggered at randomly spaced points and give rise to outgoing waves of alternating red and blue color.

Another example of an oscillating reaction is provided by the Bray reaction, the first identified homogeneous isothermal chemical oscillator, which is a complex reaction of iodate, iodine, and hydrogen peroxide. As hydrogen peroxide decomposes to oxygen and water, the resulting rate of the evolution of oxygen and I₂ vary periodically.

A distinguishing feature of oscillating reactions is the phenomena of autocatalysis. In autocatalytic reactions the increasing rate of reaction increases with the concentration of the reactants. Autocatalytic reactions eventually achieve a steady state (where the net production of products is zero) that can be determined by setting all the time derivatives equal to zero and solving the resulting algebraic equations for the concentrations of reactants and products. In oscillating reactions, small changes may result in a dramatic departure from the steady state.

Many skeptics of oscillating reactions dismisses these classic examples as aberrations due to contamination. Concerns that oscillating reactions could not exist because of apparent violations of thermodynamic laws have recently been refuted by careful studies that establish that oscillating reactions are in accord with thermodynamic laws. Oscillating chemical reactions are unlike the oscillations of a pendulum. Oscillating chemical reactions do not have to pass through an equilibrium point during each oscillating cycle. Although this seems counter-intuitive (outside of experience with the natural world) it is in perfect accord with quantum theory.

Because a closed system must eventually reach equilibrium, closed systems can sustain oscillating chemical reactions for only a limited time. Sustained oscillating reactions require an open system with a constant influx of reactants, energy and removal of products.

Oscillating reactions, a common feature of biological systems, are best understood within the context of nonlinear chemical dynamics and chaos theory based models that are used to predict the overall behavior of complex systems. A chaotic system is unpredictable, but not random. A key feature is that such systems are so sensitive to their initial conditions that future behavior is inherently unpredictable beyond some relatively short period of time. Accordingly, one of the goals of scientists studying oscillating reactions is to determine mathematical patterns or repeatable features that establish relationships to observable phenomena related to the oscillating reaction.

Oscillating systems can interact with interesting results. Much like waves can cancel one another out, two oscillating systems can interact to produce a steady state of oscillation. On the other hand, the joining of two systems already at steady state can cause rhythmogenesis in which the two systems will depart from the steady state.

The exact mechanisms (a series intermediate reactions or steps) of oscillating reactions are elusive and difficult to obtain for all but the simplest reactions. The chemical equations and mechanisms commonplace to stoichiometric chemistry describe only the overall reactions, they do not specify the molecular transformations that place between colliding molecules.