# Chemical oscillator

In chemistry, a chemical oscillator is a complex mixture of reacting chemical compounds in which the concentration of one or more components exhibits periodic changes. They are a class of reactions that serve as an example of non-equilibrium thermodynamics with far-from-equilibrium behavior. The reactions are theoretically important in that they show that chemical reactions do not have to be dominated by equilibrium thermodynamic behavior.
In cases where one of the reagents has a visible color, periodic color changes can be observed. Examples of oscillating reactions are the Belousov–Zhabotinsky reaction (BZ reaction), the Briggs–Rauscher reaction, and the Bray–Liebhafsky reaction.

## History

The earliest scientific evidence that such reactions can oscillate was met with extreme scepticism. In 1828, G.T. Fechner published a report of oscillations in a chemical system. He described an electrochemical cell that produced an oscillating current. In 1899, W. Ostwald observed that the rate of chromium dissolution in acid periodically increased and decreased. Both of these systems were heterogeneous and it was believed then, and through much of the last century, that homogeneous oscillating systems were nonexistent. While theoretical discussions date back to around 1910, the systematic study of oscillating chemical reactions and of the broader field of non-linear chemical dynamics did not become well established until the mid-1970s.

## Theory

Thermodynamics :
The classical Carnot heat engine :
Branches
Classical
Statistical
Chemical
Quantum thermodynamics
Equilibrium / Non-equilibrium :
Laws
Zeroth
First
Second
Third :
Systems
Closed system
Open system
Isolated system
State
Equation of state
Ideal gas
Real gas
State of matter
Phase (matter)
Equilibrium
Control volume
Instruments
Processes
Isobaric
Isochoric
Isothermal
Adiabatic
Isentropic
Isenthalpic
Quasistatic
Polytropic
Free expansion
Reversibility
Irreversibility
Endoreversibility
Cycles
Heat engines
Heat pumps
Thermal efficiency :
System propertiesNote: Conjugate variables in italics
Property diagrams
Intensive and extensive properties
Process functions
Work
Heat
Functions of state
Temperature / Entropy (introduction)
Pressure / Volume
Chemical potential / Particle number
Vapor quality
Reduced properties :
Material properties
Property databases
Specific heat capacity 
$c=$
$T$$\partial S$$N$$\partial T$
Compressibility 
$\beta =-$
$1$$\partial V$$V$$\partial p$
Thermal expansion 
$\alpha =$
$1$$\partial V$$V$$\partial T$ :
Equations
Carnot's theorem
Clausius theorem
Fundamental relation
Ideal gas law
Maxwell relations
Onsager reciprocal relations
Bridgman's equations
Table of thermodynamic equations :
Potentials
Free energy
Free entropy
Internal energy $U(S,V)$Enthalpy $H(S,p)=U+pV$Helmholtz free energy $A(T,V)=U-TS$Gibbs free energy $G(T,p)=H-TS$ :
HistoryCulture
History
General
Entropy
Gas laws
"Perpetual motion" machines
Philosophy
Entropy and time
Entropy and life
Brownian ratchet
Maxwell's demon
Heat death paradox
Loschmidt's paradox
Synergetics
Theories
Caloric theory
Vis viva ("living force")
Mechanical equivalent of heat
Motive power
Key publications
An Inquiry Concerning the Source ... Friction
On the Equilibrium of Heterogeneous Substances
Reflections on the Motive Power of Fire
Timelines
Thermodynamics
Heat engines
ArtEducation
Maxwell's thermodynamic surface
Entropy as energy dispersal :
Scientists
Bernoulli
Boltzmann
Bridgman
Carathéodory
Carnot
Clapeyron
Clausius
de Donder
Duhem
Gibbs
von Helmholtz
Joule
Kelvin
Lewis
Massieu
Maxwell
von Mayer
Nernst
Onsager
Planck
Rankine
Smeaton
Stahl
Tait
Thompson
van der Waals
Waterston :
Other
Nucleation
Self-assembly
Self-organization
Order and disorder :
Category :
vte :
Chemical systems cannot oscillate about a position of final equilibrium because such an oscillation would violate the second law of thermodynamics. For a thermodynamic system which is not at equilibrium, this law requires that the system approach equilibrium and not recede from it. For a closed system at constant temperature and pressure, the thermodynamic requirement is that the Gibbs free energy must decrease continuously and not oscillate. However it is possible that the concentrations of some reaction intermediates oscillate, and also that the rate of formation of products oscillates.
Theoretical models of oscillating reactions have been studied by chemists, physicists, and mathematicians. In an oscillating system the energy-releasing reaction can follow at least two different pathways, and the reaction periodically switches from one pathway to another. One of these pathways produces a specific intermediate, while another pathway consumes it. The concentration of this intermediate triggers the switching of pathways. When the concentration of the intermediate is low, the reaction follows the producing pathway, leading then to a relatively high concentration of intermediate. When the concentration of the intermediate is high, the reaction switches to the consuming pathway.
Different theoretical models for this type of reaction have been created, including the Lotka-Volterra model, the Brusselator and the Oregonator. The latter was designed to simulate the Belousov-Zhabotinsky reaction.

## Types
### Belousov–Zhabotinsky (BZ) reaction

A Belousov–Zhabotinsky reaction is one of several oscillating chemical systems, whose common element is the inclusion of bromine and an acid. 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 the Briggs–Rauscher reactions and the BZ using the chemical ruthenium bipyridyl as catalyst can be excited into self-organising activity through the influence of light.
Boris Belousov first noted, sometime in the 1950s, that in a mix of potassium bromate, cerium(IV) sulfate, propanedioic acid (another name for 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 propanedioic acid to cerium(III) ions, which are then oxidized back to cerium(IV) ions by bromate(V) ions.

### Briggs–Rauscher reaction

The Briggs–Rauscher oscillating reaction is one of a small number of known oscillating chemical reactions. It is especially well suited for demonstration purposes because of its visually striking color changes: the freshly prepared colorless solution slowly turns an amber color, suddenly changing to a very dark blue. This slowly fades to colorless and the process repeats, about ten times in the most popular formulation.

### Bray–Liebhafsky reaction

The Bray–Liebhafsky reaction is a chemical clock first described by W. C. Bray in 1921 with the oxidation of iodine to iodate:
    5 H$_2$O$_2$ + I$_2$ → 2 IO$^−$ $_3$ + 2 H$^+$ + 4 H$_2$O
and the reduction of iodate back to iodine:
    5 H$_2$O$_2$ + 2 IO$^−$ $_3$ + 2 H$^+$ → I$_2$ + 5 O$_2$ + 6 H$_2$O