Chain reaction

A chain reaction is a sequence of reactions in which a reactive product or by-product from one step triggers further reactions, creating a self-amplifying chain of events through positive feedback. This mechanism is a primary way for systems not in thermodynamic equilibrium to release energy or increase entropy, moving toward a higher entropy state. Often, a system is blocked from directly releasing stored energy; a small initial energy release can, however, open a path for more releases in an expanding chain, leading to an explosive collapse until most of the stored energy is spent. Macroscopic metaphors include the snowball effect, where a small snowball triggers a larger one until an avalanche occurs, and the domino effect, where toppling one domino causes all others to fall. Chemically, this mirrors a spark igniting a forest fire; in nuclear physics, a single stray neutron can cause a prompt critical event, potentially leading to a reactor meltdown or nuclear explosion. The concept was first proposed for chemical reactions in 1913 by Max Bodenstein, who noted that reactions can produce unstable molecules that react more readily than the original reactants. In 1918, Walther Nernst used chain reactions to explain the quantum yield in hydrogen-chlorine photochemistry, where one photon initiates a long chain forming many product molecules. Later, J. A. Christiansen and Hendrik Kramers showed that chain reactions could start from thermal collisions and that branching—where one step produces two or more unstable molecules—leads to exponential growth and chemical explosions. Nikolay Semyonov later developed a quantitative theory, sharing a Nobel Prize with Cyril Hinshelwood. Typical steps include initiation (forming active particles like free radicals), propagation (a cycle where an active particle regenerates itself, sometimes with branching or chain transfer), and termination (loss of activity, e.g., radical recombination). The chain length is the average number of propagation cycles per initiation. The hydrogen-bromine reaction exemplifies this: initiation produces bromine radicals; propagation cycles through reactions with hydrogen and bromine; retardation occurs via a reverse step; and termination recombines radicals. This reaction exhibits fractional-order and mixed-order kinetics.