Chemical clock
Encyclopedia
A chemical clock 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
, resulting in the establishment of a nonlinear oscillator. 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, crossing a concentration threshold can lead to an abrupt color change in a reproducible time lapse. Examples of clock reactions are the Belousov-Zhabotinsky reaction
, the Briggs-Rauscher reaction
, the Bray-Liebhafsky reaction
and the iodine clock reaction
. The concentration of products and reactants of oscillatory chemical systems can be approximated in terms of damped
oscillation
s.
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 1970’s.
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-Zabotinskii reaction.
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 Briggs–Rauscher and 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 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.
to iodate
:
and the reduction
of iodate back to iodine:
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
chemical compounds in which the concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
of one or more components exhibits periodic changes.
They are a class of reactions that serve as an example of non-equilibrium thermodynamics
Non-equilibrium thermodynamics
Non-equilibrium thermodynamics is a branch of thermodynamics that deals with systems that are not in thermodynamic equilibrium. Most systems found in nature are not in thermodynamic equilibrium; for they are changing or can be triggered to change over time, and are continuously and discontinuously...
, resulting in the establishment of a nonlinear oscillator. The reactions are theoretically important in that they show that chemical reactions do not have to be dominated by equilibrium thermodynamic
Equilibrium thermodynamics
Equilibrium Thermodynamics is the systematic study of transformations of matter and energy in systems as they approach equilibrium. The word equilibrium implies a state of balance. Equilibrium thermodynamics, in origins, derives from analysis of the Carnot cycle. Here, typically a system, as...
behavior.
In cases where one of the reagents has a visible color, crossing a concentration threshold can lead to an abrupt color change in a reproducible time lapse. Examples of clock reactions are the Belousov-Zhabotinsky reaction
Belousov-Zhabotinsky reaction
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 oscillating systems is the inclusion of bromine...
, the Briggs-Rauscher reaction
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 colour changes: the freshly prepared colourless solution slowly turns an amber colour, suddenly...
, the Bray-Liebhafsky reaction
Bray-Liebhafsky reaction
The Bray–Liebhafsky reaction is a chemical clock first described by William C. Bray in 1921 and the first oscillating reaction in a stirred homogeneous solution. He investigated the role of the iodate , the anion of iodic acid in the catalytic conversion of hydrogen peroxide to oxygen and water by...
and the iodine clock reaction
Iodine clock reaction
The iodine clock reaction is a classical chemical clock demonstration experiment to display chemical kinetics in action; it was discovered by Hans Heinrich Landolt in 1886. Two colorless solutions are mixed and at first there is no visible reaction...
. The concentration of products and reactants of oscillatory chemical systems can be approximated in terms of damped
Damping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
oscillation
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
s.
History
The earliest scientific evidence that such reactions can oscillate was met with extreme scepticism. In 1828, G.T. FechnerGustav Fechner
Gustav Theodor Fechner , was a German experimental psychologist. An early pioneer in experimental psychology and founder of psychophysics, he inspired many 20th century scientists and philosophers...
published a report of oscillations in a chemical system. He described an electrochemical cell that produced an oscillating current. In 1899, W. Ostwald
Wilhelm Ostwald
Friedrich Wilhelm Ostwald was a Baltic German chemist. He received the Nobel Prize in Chemistry in 1909 for his work on catalysis, chemical equilibria and reaction velocities...
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 1970’s.
Theory
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
Brusselator
[Image:080205 Brusselator picture.jpg|thumb|right|350px|The Brusselator in the unstable regime. A=1. B=2.5. X=1. Y=0. The system approaches a limit cycle For B1+A^2 \,...
and the Oregonator
Oregonator
The Oregonator is a theoretical model for a type of autocatalytic reaction.The Oregonator is the simplest realistic model of the chemical dynamics of the oscillatory Belousov-Zhabotinsky reaction....
. The latter was designed to simulate the Belousov-Zabotinskii reaction.
BZ reaction
In a Belousov–Zhabotinsky reaction, the only common element in these oscillating systems is the inclusion of bromineBromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
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 Briggs–Rauscher and 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
Potassium bromate
Potassium bromate , is a bromate of potassium and takes the form of white crystals or powder.-Uses in baking:Potassium bromate is typically used as a flour improver , strengthening the dough and allowing higher rising. It is an oxidizing agent, and under the right conditions, will be completely...
, cerium(IV) sulfate
Cerium(IV) sulfate
Cerium sulfate, also called ceric sulfate, is a yellow to yellow/orange chemical compound. It exists as the anhydrous salt Ce2; a few hydrated forms are also known: Ce2 ⋅ xH2O, with x equal to 4, 8, or 12. Ceric sulfate is available commercially.-Properties:Ceric sulfate is moderately soluble in...
, propanedioic acid and citric acid
Citric acid
Citric acid is a weak organic acid. It is a natural preservative/conservative and is also used to add an acidic, or sour, taste to foods and soft drinks...
in dilute sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
, 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 iodineIodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
to iodate
Iodate
An iodate is a conjugate base of iodic acid. In the iodate anion, iodine is bonded to three oxygen atoms and the molecular formula is IO3−. The molecular geometry of iodate is trigonal pyramidal....
:
- 5 H2O2 + I2 → 2 IO3- + 2 H+ + 4 H2O
and the reduction
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
of iodate back to iodine:
- 5 H2O2 + 2 IO3- + 2 H+ → I2 + 5 O2 + 6 H2O