Chain Reaction (other)
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A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction,
positive feedback Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop where the outcome of a process reinforces the inciting process to build momentum. As such, these forces can exacerbate the effects ...
leads to a self-amplifying
chain of events A chain of events is a number of actions and their effects that are contiguous and linked together that results in a particular outcome. In the physical sciences, chain reactions are a primary example. Determinism ''Determinism'' is the philos ...
. Chain reactions are one way that systems which are not in
thermodynamic equilibrium Thermodynamic equilibrium is a notion of thermodynamics with axiomatic status referring to an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable ...
can release energy or increase entropy in order to reach a state of higher entropy. For example, a system may not be able to reach a lower energy state by releasing energy into the environment, because it is hindered or prevented in some way from taking the path that will result in the energy release. If a reaction results in a small energy release making way for more energy releases in an expanding chain, then the system will typically collapse explosively until much or all of the stored energy has been released. A macroscopic metaphor for chain reactions is thus a snowball causing a larger snowball until finally an avalanche results ("
snowball effect A snowball effect is a process that starts from an initial state of small significance and builds upon itself (an exacerbating feedback), becoming larger (graver, more serious), and also perhaps potentially more dangerous or disastrous (a vicio ...
"). This is a result of stored
gravitational potential energy Gravitational energy or gravitational potential energy is the potential energy an object with mass has due to the gravitational potential of its position in a gravitational field. Mathematically, it is the minimum Work (physics), mechanical work t ...
seeking a path of release over friction. Chemically, the equivalent to a snow avalanche is a spark causing a forest fire. In nuclear physics, a single stray neutron can result in a
prompt critical In nuclear engineering, prompt criticality is the criticality (the state in which a nuclear chain reaction is self-sustaining) that is achieved with prompt neutrons alone (without the efforts of delayed neutrons). As a result, prompt supercritic ...
event, which may finally be energetic enough for a nuclear reactor meltdown or (in a bomb) a nuclear explosion. Another metaphor for a chain reaction is the
domino effect A domino effect is the cumulative effect produced when one event sets off a series of similar or related events, a form of chain reaction. The term is an analogy to a falling row of dominoes. It typically refers to a linked sequence of events ...
, named after the act of
domino toppling Domino toppling refers to the process of arranging dominoes in a sequential layout, commonly known as a domino run, and subsequently knocking over the first domino in the sequence. This action triggers a chain reaction, where each domino in turn ...
, where the simple action of toppling one domino leads to all dominoes eventually toppling, even if they are significantly larger. Numerous chain reactions can be represented by a
mathematical model A mathematical model is an abstract and concrete, abstract description of a concrete system using mathematics, mathematical concepts and language of mathematics, language. The process of developing a mathematical model is termed ''mathematical m ...
based on
Markov chain In probability theory and statistics, a Markov chain or Markov process is a stochastic process describing a sequence of possible events in which the probability of each event depends only on the state attained in the previous event. Informally ...
s.


Chemical chain reactions


History

In 1913, the German chemist
Max Bodenstein Max Ernst August Bodenstein (July 15, 1871 – September 3, 1942) was a German physical chemist known for his work in chemical kinetics. He was first to postulate a chain reaction mechanism and that explosions are branched chain reactions, later ...
first put forth the idea of
chemical A chemical substance is a unique form of matter with constant chemical composition and characteristic properties. Chemical substances may take the form of a single element or chemical compounds. If two or more chemical substances can be combin ...
chain reactions. If two molecules react, not only molecules of the final reaction products are formed, but also some unstable molecules which can further react with the parent molecules with a far larger probability than the initial reactants. (In the new reaction, further unstable molecules are formed besides the stable products, and so on.) In 1918,
Walther Nernst Walther Hermann Nernst (; 25 June 1864 – 18 November 1941) was a German physical chemist known for his work in thermodynamics, physical chemistry, electrochemistry, and solid-state physics. His formulation of the Nernst heat theorem helped ...
proposed that the
photochemical Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400  nm), visible (400–750&nb ...
reaction between
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
and
chlorine Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...
is a chain reaction in order to explain what is known as the ''
quantum yield In particle physics, the quantum yield (denoted ) of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system. \Phi(\lambda)=\frac Applications Fluorescence spectroscopy The fluorescence ...
'' phenomena. This means that one
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...
of light is responsible for the formation of as many as 106 molecules of the product
HCl HCL may refer to: Science and medicine * Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia * Harvard Cyclotron Laboratory, from 1961 to 2002, a proton accelerator used for research and development * Hollow-cathode lamp, a s ...
. Nernst suggested that the photon dissociates a Cl2 molecule into two Cl atoms which each initiate a long chain of reaction steps forming HCl.Laidler K.J., ''Chemical Kinetics'' (3rd ed., Harper & Row 1987) p.288-290 In 1923, Danish and Dutch scientists J. A. Christiansen and
Hendrik Anthony Kramers Hendrik Anthony "Hans" Kramers (17 December 1894 – 24 April 1952) was a Dutch physicist who worked with Niels Bohr to understand how electromagnetic waves interact with matter and made important contributions to quantum mechanics and statistica ...
, in an analysis of the formation of polymers, pointed out that such a chain reaction need not start with a molecule excited by light, but could also start with two molecules colliding violently due to thermal energy as previously proposed for initiation of chemical reactions by van' t Hoff. Christiansen and Kramers also noted that if, in one link of the reaction chain, two or more unstable
molecules A molecule is a group of two or more atoms that are held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemistry ...
are produced, the reaction chain would branch and grow. The result is in fact an exponential growth, thus giving rise to explosive increases in reaction rates, and indeed to chemical explosions themselves. This was the first proposal for the mechanism of chemical explosions. A quantitative chain chemical reaction theory was created later on by Soviet physicist
Nikolay Semyonov Nikolay Nikolayevich Semyonov , sometimes Semenov, Semionov or Semenoff (; – 25 September 1986) was a Soviet physicist and chemist. Semyonov was awarded the 1956 Nobel Prize in Chemistry for his work on the mechanism of chemical transformat ...
in 1934. Semyonov shared the Nobel Prize in 1956 with Sir
Cyril Norman Hinshelwood Sir Cyril Norman Hinshelwood (19 June 1897 – 9 October 1967) was a British physical chemist and expert in chemical kinetics. His work in reaction mechanisms earned the 1956 Nobel Prize in chemistry. Education Born in London, his parents we ...
, who independently developed many of the same quantitative concepts.http://nobelprize.org/nobel_prizes/chemistry/laureates/1956/press.html History of the chemical chain reaction from 1913 to the Nobel work recognized in 1956


Typical steps

The main types of steps in chain reaction are of the following types. *
Initiation Initiation is a rite of passage marking entrance or acceptance into a group or society. It could also be a formal admission to adulthood in a community or one of its formal components. In an extended sense, it can also signify a transformatio ...
(formation of active particles or chain carriers, often
free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabolic disorders Metabolism ...
s, in either a thermal or a photochemical step) *
Propagation Propagation can refer to: *Chain propagation in a chemical reaction mechanism *Crack propagation, the growth of a crack during the fracture of materials *Propaganda, non-objective information used to further an agenda *Reproduction, and other forms ...
(may comprise several elementary steps in a cycle, where the active particle through reaction forms another active particle which continues the reaction chain by entering the next elementary step). In effect the active particle serves as a catalyst for the overall reaction of the propagation cycle. Particular cases are: ** chain branching (a propagation step where one active particle enters the step and two or more are formed); **
chain transfer In polymer chemistry, chain transfer is a polymerization reaction by which the activity of a growing polymer chain is transferred to another molecule: \ce^\bullet + \ce^\bullet where • is the active center, P is the initial polymer chain, X i ...
(a propagation step in which the active particle is a growing
polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
chain which reacts to form an inactive polymer whose growth is terminated and an active small particle (such as a radical), which may then react to form a new polymer chain). * Termination (elementary step in which the active particle loses its activity; e. g. by recombination of two free radicals). The ''chain length'' is defined as the average number of times the propagation cycle is repeated, and equals the overall reaction rate divided by the initiation rate. Some chain reactions have complex
rate equation In chemistry, the rate equation (also known as the rate law or empirical differential rate equation) is an Empirical relationship, empirical Differential equation, differential Expression (mathematics), mathematical expression for the reaction rat ...
s with fractional order or
mixed order The Mixed Order () was a tactical formation originally used by ''demi-brigades'' of the French Revolutionary Army and then later by Napoleon's Grande Armée to great effect. First proposed by the French theorist Comte de Guibert, the Mixed O ...
kinetics.


Detailed example: the hydrogen-bromine reaction

The reaction H2 + Br2 → 2 HBr proceeds by the following mechanism: Laidler K.J., ''Chemical Kinetics'' (3rd ed., Harper & Row 1987) p.291-4 P. Atkins and J. de Paula ''Physical Chemistry'' (8th ed., W.H. Freeman 2006), p.830-1 * Initiation : Br2 → 2 Br• (thermal) or Br2 + hν → 2 Br• (photochemical) : each Br atom is a free radical, indicated by the symbol "•" representing an unpaired electron. * Propagation (here a cycle of two steps) : Br• + H2 → HBr + H• : H• + Br2 → HBr + Br• : the sum of these two steps corresponds to the overall reaction H2 + Br2 → 2 HBr, with
catalysis Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
by Br• which participates in the first step and is regenerated in the second step. * Retardation (inhibition) : H• + HBr → H2 + Br• : this step is specific to this example, and corresponds to the first propagation step in reverse. * Termination 2 Br• → Br2 : recombination of two radicals, corresponding in this example to initiation in reverse. As can be explained using the
steady-state approximation In chemistry, a steady state is a situation in which all state variables are constant in spite of ongoing processes that strive to change them. For an entire system to be at steady state, i.e. for all state variables of a system to be constant, ...
, the thermal reaction has an initial rate of fractional order (3/2), and a complete rate equation with a two-term denominator ( mixed-order kinetics).


Further chemical examples

* The reaction 2 H2 + O2 → 2 H2O provides an example of chain branching. The propagation is a sequence of two steps whose net effect is to replace an H atom by another H atom plus two OH radicals. This leads to an explosion under certain conditions of temperature and pressure. ** H• + O2 → •OH + •O• ** •O• + H2 → •OH + H• * In
chain-growth polymerization Chain-growth polymerization (American English, AE) or chain-growth polymerisation (British English, BE) is a polymerization technique where monomer molecules add onto the active site on a growing polymer chain one at a time. There are a limited n ...
, the propagation step corresponds to the elongation of the growing
polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
chain. Chain transfer corresponds to transfer of the activity from this growing chain, whose growth is terminated, to another molecule which may be a second growing polymer chain. For polymerization, the
kinetic chain length In polymer chemistry, the kinetic chain length () of a polymer is the average number of units called monomers added to a growing chain during chain-growth polymerization. During this process, a polymer chain is formed when monomers are bonded toge ...
defined above may differ from the
degree of polymerization The degree of polymerization, or DP, is the number of structural unit, monomeric units in a macromolecule or polymer or oligomer molecule. For a homopolymer, there is only one type of monomeric unit and the ''number-average'' degree of polymeriza ...
of the product macromolecule. *
Polymerase chain reaction The polymerase chain reaction (PCR) is a method widely used to make millions to billions of copies of a specific DNA sample rapidly, allowing scientists to amplify a very small sample of DNA (or a part of it) sufficiently to enable detailed st ...
, a technique used in
molecular biology Molecular biology is a branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, biomolecular synthesis, modification, mechanisms, and interactio ...
to amplify (make many copies of) a piece of
DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
by ''
in vitro ''In vitro'' (meaning ''in glass'', or ''in the glass'') Research, studies are performed with Cell (biology), cells or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in ...
''
enzymatic An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as produc ...
replication using a
DNA polymerase A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to create t ...
.


Acetaldehyde pyrolysis and rate equation

The
pyrolysis Pyrolysis is a process involving the Bond cleavage, separation of covalent bonds in organic matter by thermal decomposition within an Chemically inert, inert environment without oxygen. Etymology The word ''pyrolysis'' is coined from the Gree ...
(thermal decomposition) of
acetaldehyde Acetaldehyde (IUPAC systematic name ethanal) is an organic compound, organic chemical compound with the chemical formula, formula , sometimes abbreviated as . It is a colorless liquid or gas, boiling near room temperature. It is one of the most ...
, CH3CHO (g) → CH4 (g) + CO (g), proceeds via the Rice-Herzfeld mechanism: *Initiation (formation of
free radicals In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired electron, unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemical reaction, chemi ...
): : CH3CHO (g) → •CH3 (g) + •CHO (g) k1 The methyl and CHO groups are
free radicals In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired electron, unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemical reaction, chemi ...
. *Propagation (two steps): : •CH3 (g) + CH3CHO (g) → CH4 (g) + •CH3CO (g) k2 This reaction step provides
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
, which is one of the two main products. : •CH3CO (g) → CO (g) + •CH3 (g) k3 The product •CH3CO (g) of the previous step gives rise to
carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
(CO), which is the second main product. The sum of the two propagation steps corresponds to the overall reaction CH3CHO (g) → CH4 (g) + CO (g),
catalyzed Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
by a methyl radical •CH3. *Termination: : •CH3 (g) + •CH3 (g) → C2H6 (g) k4 This reaction is the only source of
ethane Ethane ( , ) is a naturally occurring Organic compound, organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is List of purification methods ...
(minor product) and it is concluded to be the main chain ending step. Although this mechanism explains the principal products, there are others that are formed in a minor degree, such as
acetone Acetone (2-propanone or dimethyl ketone) is an organic compound with the chemical formula, formula . It is the simplest and smallest ketone (). It is a colorless, highly Volatile organic compound, volatile, and flammable liquid with a charact ...
(CH3COCH3) and
propanal Propionaldehyde or propanal is the organic compound with the formula CH3CH2CHO. It is the 3-carbon aldehyde. It is a colourless, flammable liquid with a pungent and fruity odour. It is produced on a large scale industrially. Production Propional ...
(CH3CH2CHO). Applying the Steady State Approximation for the intermediate species CH3(g) and CH3CO(g), the rate law for the formation of methane and the order of reaction are found: The rate of formation of the product methane is (1)... \frac = k_2\ce \ce For the intermediates (2)... \frac = k_1 \ce - k_2 \ce \ce + k_3 \ce - 2 k_4 \ce^2 = 0 and (3)... \frac = k_2 \ce \ce - k_3 \ce = 0 Adding (2) and (3), we obtain k_1 \ce - 2 k_4 \ce^2 = 0 so that (4)...\ce = \frac\ce^ Using (4) in (1) gives the rate law (5) \frac = \frac k_2 \ce^, which is order 3/2 in the reactant CH3CHO.


Nuclear chain reactions

A ''nuclear'' chain reaction was proposed by
Leo Szilard Leo Szilard (; ; born Leó Spitz; February 11, 1898 – May 30, 1964) was a Hungarian-born physicist, biologist and inventor who made numerous important discoveries in nuclear physics and the biological sciences. He conceived the nuclear ...
in 1933, shortly after the neutron was discovered, yet more than five years before
nuclear fission Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactiv ...
was first discovered. Szilárd knew of ''chemical'' chain reactions, and he had been reading about an energy-producing
nuclear reaction In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two atomic nucleus, nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a t ...
involving high-energy protons bombarding lithium, demonstrated by
John Cockcroft Sir John Douglas Cockcroft (27 May 1897 – 18 September 1967) was an English nuclear physicist who shared the 1951 Nobel Prize in Physics with Ernest Walton for their splitting of the atomic nucleus, which was instrumental in the developmen ...
and
Ernest Walton Ernest Thomas Sinton Walton (6 October 1903 – 25 June 1995) was an Irish nuclear physicist who shared the 1951 Nobel Prize in Physics with John Cockcroft "for their pioneer work on the transmutation of atomic nuclei by artificially accelerate ...
, in 1932. Now, Szilárd proposed to use neutrons theoretically produced from certain nuclear reactions in lighter isotopes, to induce further reactions in light isotopes that produced more neutrons. This would in theory produce a chain reaction at the level of the nucleus. He did not envision fission as one of these neutron-producing reactions, since this reaction was not known at the time. Experiments he proposed using
beryllium Beryllium is a chemical element; it has Symbol (chemistry), symbol Be and atomic number 4. It is a steel-gray, hard, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with ...
and
indium Indium is a chemical element; it has Symbol (chemistry), symbol In and atomic number 49. It is a silvery-white post-transition metal and one of the softest elements. Chemically, indium is similar to gallium and thallium, and its properties are la ...
failed. Later, after fission was discovered in 1938, Szilárd immediately realized the possibility of using neutron-induced fission as the particular nuclear reaction necessary to create a chain-reaction, so long as fission also produced neutrons. In 1939, with Enrico Fermi, Szilárd proved this neutron-multiplying reaction in uranium. In this reaction, a
neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
plus a fissionable
atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
causes a fission resulting in a larger number of neutrons than the single one that was consumed in the initial reaction. Thus was born the practical
nuclear chain reaction In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series or "positive feedback loop" of thes ...
by the mechanism of neutron-induced nuclear fission. Specifically, if one or more of the produced neutrons themselves interact with other fissionable nuclei, and these also undergo fission, then there is a possibility that the macroscopic overall fission reaction will not stop, but continue throughout the reaction material. This is then a self-propagating and thus self-sustaining chain reaction. This is the principle for
nuclear reactor A nuclear reactor is a device used to initiate and control a Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...
s and
atomic bomb A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission or atomic bomb) or a combination of fission and fusion reactions (thermonuclear weapon), producing a nuclear expl ...
s. Demonstration of a self-sustaining nuclear chain reaction was accomplished by
Enrico Fermi Enrico Fermi (; 29 September 1901 – 28 November 1954) was an Italian and naturalized American physicist, renowned for being the creator of the world's first artificial nuclear reactor, the Chicago Pile-1, and a member of the Manhattan Project ...
and others, in the successful operation of
Chicago Pile-1 Chicago Pile-1 (CP-1) was the first artificial nuclear reactor. On 2 December 1942, the first human-made self-sustaining nuclear chain reaction was initiated in CP-1 during an experiment led by Enrico Fermi. The secret development of the react ...
, the first artificial nuclear reactor, in late 1942.


Electron avalanche in gases

An
electron avalanche An electron avalanche is a process in which a number of free electrons in a transmission medium are subjected to strong acceleration by an electric field and subsequently collide with other atoms of the medium, thereby ionizing them ( impact ioniz ...
happens between two unconnected electrodes in a gas when an electric field exceeds a certain threshold. Random thermal collisions of gas atoms may result in a few free electrons and positively charged gas ions, in a process called
impact ionization Impact ionization is the process in a material by which one energetic charge carrier can lose energy by the creation of other charge carriers. For example, in semiconductors, an electron (or hole) with enough kinetic energy can knock a bound elec ...
. Acceleration of these free electrons in a strong
electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...
causes them to gain energy, and when they impact other atoms, the energy causes release of new free electrons and ions (ionization), which fuels the same process. If this process happens faster than it is naturally quenched by ions recombining, the new ions multiply in successive cycles until the gas breaks down into a plasma and current flows freely in a discharge. Electron avalanches are essential to the
dielectric breakdown In electronics, electrical breakdown or dielectric breakdown is a process that occurs when an electrically insulating material (a dielectric), subjected to a high enough voltage, suddenly becomes a conductor and current flows through it. All ...
process within gases. The process can culminate in
corona discharge A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor (material), conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone ...
s, streamers,
leader Leadership, is defined as the ability of an individual, group, or organization to "", influence, or guide other individuals, teams, or organizations. "Leadership" is a contested term. Specialist literature debates various viewpoints on the co ...
s, or in a
spark Spark commonly refers to: * Spark (fire), a small glowing particle or ember * Electric spark, a form of electrical discharge Spark may also refer to: People * Spark (surname) * Jessica Morgan (born 1992; formerly known as Spark), female singe ...
or continuous
electric arc An electric arc (or arc discharge) is an electrical breakdown of a gas that produces a prolonged electrical discharge. The electric current, current through a normally Electrical conductance, nonconductive medium such as air produces a plasma ( ...
that completely bridges the gap. The process may extend huge sparks — streamers in
lightning Lightning is a natural phenomenon consisting of electrostatic discharges occurring through the atmosphere between two electrically charged regions. One or both regions are within the atmosphere, with the second region sometimes occurring on ...
discharges propagate by formation of electron avalanches created in the high potential gradient ahead of the streamers' advancing tips. Once begun, avalanches are often intensified by the creation of
photoelectron The photoelectric effect is the emission of electrons from a material caused by electromagnetic radiation such as ultraviolet light. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physi ...
s as a result of ultraviolet radiation emitted by the excited medium's atoms in the aft-tip region. The extremely high temperature of the resulting plasma cracks the surrounding gas molecules and the free ions recombine to create new chemical compounds. The process can also be used to detect radiation that initiates the process, as the passage of a single particles can be amplified to large discharges. This is the mechanism of a
Geiger counter A Geiger counter (, ; also known as a Geiger–Müller counter or G-M counter) is an electronic instrument for detecting and measuring ionizing radiation with the use of a Geiger–Müller tube. It is widely used in applications such as radiat ...
and also the visualization possible with a
spark chamber A spark chamber is a particle detector: a device used in particle physics for detecting electrically charged Subatomic particle, particles. They were most widely used as research tools from the 1930s to the 1960s and have since been superseded by ...
and other
wire chamber A wire chamber or multi-wire proportional chamber is a type of proportional counter that detects charged particles and photons and can give positional information on their trajectory, by tracking the trails of gaseous ionization. was located via ...
s.


Avalanche breakdown in semiconductors

An
avalanche breakdown Avalanche breakdown (or the avalanche effect) is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwis ...
process can happen in semiconductors, which in some ways conduct electricity analogously to a mildly ionized gas. Semiconductors rely on free electrons knocked out of the crystal by thermal vibration for conduction. Thus, unlike metals, semiconductors become better conductors the higher the temperature. This sets up conditions for the same type of positive feedback—heat from current flow causes temperature to rise, which increases charge carriers, lowering resistance, and causing more current to flow. This can continue to the point of complete breakdown of normal resistance at a semiconductor junction, and failure of the device (this may be temporary or permanent depending on whether there is physical damage to the crystal). Certain devices, such as
avalanche diode In electronics, an avalanche diode is a diode (made from silicon or other semiconductor) that is designed to experience avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current co ...
s, deliberately make use of the effect.


Living organisms

Examples of chain reactions in living organisms include excitation of neurons in epilepsy and
lipid peroxidation Lipid peroxidation, or lipid oxidation, is a complex chemical process that leads to oxidative degradation of lipids, resulting in the formation of peroxide and hydroperoxide derivatives.{{Cite journal , last1=Ayala , first1=Antonio , last2=Muñoz ...
. In peroxidation, a lipid radical reacts with oxygen to form a peroxyl radical (L• + O2 → LOO•). The peroxyl radical then oxidises another lipid, thus forming another lipid radical (LOO• + L–H → LOOH + L•). A chain reaction in glutamatergic synapses is the cause of synchronous discharge in some epileptic seizures.


See also

*
Cascading failure A cascading failure is a failure in a system of interconnection, interconnected parts in which the failure of one or few parts leads to the failure of other parts, growing progressively as a result of positive feedback. This can occur when a singl ...
*
Multiple-vehicle collision A multiple-vehicle collision (colloquially known as a pileup or multi-car collision), is a road traffic collision involving many vehicles. Generally occurring on high-capacity and high-speed routes such as freeways, they are one of the deadliest ...
*
Rube Goldberg machine A Rube Goldberg machine, named after American cartoonist Rube Goldberg, is a chain reaction–type machine or contraption intentionally designed to perform a simple task in a comically overcomplicated way. Usually, these machines consist of a s ...


References


External links


IUPAC Gold Book - Chain reaction
{{DEFAULTSORT:Chain Reaction Chemical kinetics Metaphors referring to objects Causality