Chain reaction

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 leads to a self-amplifying chain of events.

Contents 1 Chemical chain reactions 2 Nuclear chain reactions 3 Chain Reactions in Economics 4 Further examples 5 References 6 See also

[edit] Chemical chain reactions

In 1913 the German chemist Max Bodenstein first put forth the idea of chemical chain reactions. If two molecules react, not only molecules of the final reaction products are formed, but also some unstable molecules, having the property of being able to 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 1923, Danish and Dutch scientists Christian Christiansen and Hendrik Anthony Kramers, in an analysis of 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 in the traditional way classically 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 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 by Soviet physicist Nikolay Semyonov in 1934.^[1]. Semyanov shared the Nobel Prize in 1956 with Sir Cyril Norman Hinshelwood, who independently developed many of the same quantitative concepts.^[2]

The main steps of chain reaction are the following.

• Initiation (at this step an active particle, often a free radical, is produced);
• propagation (may comprise several elementary steps, as, for instance, reaction elementary acts, where the active particle through reaction forms another active particle which continues the reaction chain by entering the next elementary step); particular cases are:

* chain branching (the case of propagation step when more new active particles form in the step than enter it);

* chain transfer (the case in which one active particle enters an elementary reaction with the inactive particle which as a result becomes another active particle along with forming of another inactive particle from the initial active one);

• termination (elementary step in which active particle loses its activity without transferring the chain; e. g. recombination of the free radicals).

[edit] Nuclear chain reactions

A nuclear chain reaction was proposed by Leo Szilard in 1933, shortly after the neutron was discovered, but some years before nuclear fission was discovered. Szilard proposed to use neutrons from certain nuclear reactions in lighter isotopes, to induce further reactions in light isotopes that produced more neutrons. He did not envision nuclear fission as one of these reactions, since this reaction was not known at the time. Experiments he proposed using beryllium and indium failed.

Later, after nuclear fission was discovered in 1938, Szilard immediately realized the possibility of using neutron-induced fission as the particular nuclear reaction needed for a chain-reaction, so long as fission also produced neutrons. In 1939, with Enrico Fermi, Szilard proved this neutron-multiplying rection in uranium. In this reaction, a neutron plus a fissionable atom 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 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 reactors and atomic bombs.

[The above description is somewhat simplified. The crucial issue is whether enough of those secondary neutrons themselves produce a further fission. The nuclear chain reaction is described in significantly more detail in the article on Stan Ulam, who "discovered" or realized the details of the amplification concept, when fission by prompt fission neutrons were the mechanism].

Demonstration of a self-sustaining nuclear chain reaction was accomplished by Enrico Fermi and others, in the successful operation of Chicago Pile-1, the first artificial nuclear reactor, in late 1942.

[edit] Chain Reactions in Economics

In 1963 Friedman and Schwartz ^[3] proposed a positive feedback loop as a mechanism for catastrophic failures in economics: “It happens that a liquidity crisis in a unit fractional reserve banking system is precisely the kind of event that trigger- and often has triggered- a chain reaction. And economic collapse often has the character of a cumulative process. Let it go beyond a certain point, and it will tend for a time to gain strength from its own development as its effects spread and return to intensify the process of collapse”.

[edit] Further examples

• For example in a chemical reaction every step of H₂ + Cl₂ chain reaction consumes one molecule of H₂ or Cl₂, one free radical H· or Cl· producing one HCl molecule and another free radical.
• Electron avalanche process: Collisions of free electrons in a strong electric field releasing "new" electrons to undergo the same process in successive cycles.
• A cascading failure, a failure in a system of interconnected parts, for example a power transmission grid, where the service provided depends on the operation of a preceding part, and the failure of a preceding part can trigger the failure of successive parts.
• Polymerase chain reaction, a technique used in molecular biology to amplify (make many copies of) a piece of DNA by in vitro enzymatic replication using a DNA polymerase.

[edit] References

[edit] See also

IUPAC Gold Book - Chain reaction