ScienceWeek

CHEMICAL DYNAMICS AND CHAIN REACTIONS

In this context, the term "chain reaction" refers to a series of reactions in which the product of each step is a reagent for the next step, the overall process thus continuing with explosive rapidity. Improved understanding of chain reactions led to better methods of plastics formation, and the analogous nuclear chain reaction led to the nuclear bomb and controlled nuclear energy. In general, A chemical chain reaction proceeds by a sequence subdivided into three stages: (a) Initiation, in which a reactive intermediate, which may be an atom, an ion, or a neutral molecular fragment, is formed, usually through the action of an agent such as light, heat, or a catalyst. (b) Propagation, whereby the intermediate reacts with the original reactants, producing stable products and another intermediate, whether of the same or different kind; the new intermediate reacts as before, so a repetitive cycle begins. (c) Termination, which may be natural, as when all the reactants have been consumed or the containing vessel causes the chain carriers to recombine as fast as they are formed, but more often is induced intentionally by introduction of substances called inhibitors or antioxidants.

Nikolay Semenov (1896-1986), discussed below, developed the theory of "branched chain reactions". Branched chain reactions are a form of chain reaction in which the number of chain carriers increases in each propagation. As a result the reaction accelerates extremely rapidly, sometimes being completed in less than 1 millisecond.

The following points are made by J. van Houten (J. Chem Educ. 2002 79:414):

1) The post-World War II era saw a remarkable increase in interest in chemical dynamics and the mechanisms of fast chemical reactions. In part, that interest can be attributed to research on fuels and munitions in the years leading up to and during the war. There also were many peaceful applications derived from better understanding of combustion processes in internal combustion engines, jet engines, and high explosives. Furthermore, technological advances in the first half of the 20th century had made it possible to study ever-faster reactions in ever-more detail.

2) In 1901 Jacobus van't Hoff (1852-1911) received the first Nobel Prize for "the discovery of the laws of chemical dynamics", and van't Hoff and the 1903 Nobel Laureate Svante Arrhenius (1859-1927) were responsible for the concept that molecules must collide with sufficient energy if they are to react. However, the dynamic behavior of many reactions could not be explained by the laws that van't Hoff had proposed. For example, phosphorus was known to glow in air (hence the term phosphorescence) but not in pure oxygen or in atmospheres with reduced oxygen partial pressure.

3) Photo-initiated reactions presented another paradox early in the 20th century -- the concept that a single photon could initiate a reaction in a single molecule was accepted in accordance with Planck's theory, however the ability of a single photon to initiate the reaction of literally millions of molecules did not seem to make sense. In 1913 Max Bodenstein (1871-1942) had first advanced the concept of reactive intermediates as part of a chain reaction mechanism. Bodenstein managed to obtain an empirical fit to the kinetics of the reaction H(sub2) + Br(sub2) --> 2HBr, and he found that iodine hindered the reaction. However, he could not find a correlation between the free energy and the kinetics of the reaction. Ten years later Christiansen and Kramers postulated that chain reactions might be initiated thermally as well as by light and, furthermore, if two or more reactive intermediates were produced then the reaction could branch, resulting in an explosion.

4) Christiansen and Kramers did not pursue their research on chain reactions, and investigations of the reaction of phosphorus vapor with oxygen were taken up in 1926 by two scientists, Chariton and Valta, working at the Leningrad Physico-Technical Institute headed by Nikolay Semenov (1896-1986). They showed that the reaction of phosphorus vapor with oxygen did, indeed, depend on the pressure of the gases, with an explosion occurring only at intermediate pressures, and with no reaction at very high or very low pressures. Bodenstein, who was considered to be the world's authority on chemical dynamics at the time, stated that the results were incomprehensible and must be wrong. However, the simple fact that the experimental results were incomprehensible using the available models of the time did not mean that the results were wrong. Semenov explained the results by showing that chain reactions lead to explosions when the chain branches, thereby increasing the concentration of the reactive intermediates and, consequently, the rate of the reaction. Furthermore, Semenov developed the mathematical relations that showed why the rate of branching, and hence the rate of the reaction, was facilitated at intermediate pressures but not at higher or lower pressures. Semenov received the Nobel Prize in Chemistry in 1956.