Nernst suggested that the photon dissociates a Cl 2 molecule into two Cl atoms which each initiate a long chain of reaction steps forming HCl. This means that one photon of light is responsible for the formation of as many as 10 6 molecules of the product HCl. In 1918, Walther Nernst proposed that the photochemical reaction between hydrogen and chlorine is a chain reaction in order to explain what's known as the quantum yield phenomena. (In the new reaction, further unstable molecules are formed besides the stable products, and so on.)

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 1913, the German chemist Max Bodenstein first put forth the idea of chemical chain reactions. 4 Avalanche breakdown in semiconductorsД¬hemical chain reactions History.1.5 Acetaldehyde pyrolysis and rate equation.1.3 Detailed example: the hydrogen-bromine reaction.

Numerous chain reactions can be represented by a mathematical model based on Markov chains. In nuclear physics, a single stray neutron can result in a prompt critical event, which may finally be energetic enough for a nuclear reactor meltdown or (in a bomb) a nuclear explosion. Chemically, the equivalent to a snow avalanche is a spark causing a forest fire. This is a result of stored gravitational potential energy seeking a path of release over friction. 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.ДЄ macroscopic metaphor for chain reactions is thus a snowball causing a larger snowball until finally an avalanche results (' snowball effect'). 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. In a chain reaction, positive feedback leads to a self-amplifying chain of events.Д¬hain reactions are one way that systems which are not in thermodynamic equilibrium can release energy or increase entropy in order to reach a state of higher entropy. For other uses, see Chain reaction (disambiguation).ДЄ chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place.

This article is about chain reactions in chemistry and physics.

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