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. Chain reactions are one way in which systems which are in thermodynamic non-equilibrium 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"). This is a result of stored gravitational potential energy 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 event, which may finally be energetic enough for a nuclear reactor meltdown or (in a bomb) a nuclear explosion.
Chemical chain reactions
HistoryIn 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 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 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. This means that one photon of light is responsible for the formation of as many as 106 molecules of the product HCl. 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 Christian Christiansen and Hendrik Anthony Kramers, 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 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 in 1934. Semyonov shared the Nobel Prize in 1956 with Sir Cyril Norman Hinshelwood, 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 stepsThe main types of steps in chain reaction are of the following types.
- Initiation (formation of active particles or chain carriers, often free radicals, in either a thermal or a photochemical step)
- Propagation (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:
- Termination (elementary step in which the active particle loses its activity; e. g. by recombination of two free radicals).
Detailed example: the hydrogen-bromine reactionThe 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.831
- Propagation (here a cycle of two steps)
- Retardation (inhibition)
- Termination 2 Br• → Br2
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.Laidler K.J., Chemical Kinetics (3rd ed., Harper & Row 1987) p.323-8
- *H + O2 → OH + O
- *O + H2 → OH + H
- In chain-growth polymerization, the propagation step corresponds to the elongation of the growing polymer 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 defined above may differ from the degree of polymerization of the product macromolecule.
- 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.