Chain Reaction - Chemical Chain Reactions

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. Semyonov shared the Nobel Prize in 1956 with Sir Cyril Norman Hinshelwood, who independently developed many of the same quantitative concepts.

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).