Complex chemical reactions - A review

Complex responses have been observed in a wide range of chemical and engine ering systems: In well-stirred and unstirred solution-phase, in gels, in he terogeneous catalysis and dissolution reactions, in gas- and solid-phase co mbustion, widely in biological systems, accompanying phase transitions, in atmospheric kinetics and even in interstellar dust clouds. Complex behaviou r does not, however, imply a necessarily complex underlying chemical mechan ism. The appropriate feedback mechanisms, built on chain-branching, autocat alysis or self-heating, arise quite commonly in Nature. In many instances, it is appropriate first to attempt to find reduced mechanisms giving a semi -quantitative fit to observed responses. An example of such an approach app lied to a model for complex oscillations of species concentrations in the m esosphere is presented, and the reduced model used to investigate the effec ts of longitudinal mixing in this system. Once a general understanding of a particular reaction system has been obtained, more detailed questions can be addressed. This is exemplified by a study of the development of three-di mensional scroll waves in the Belousov-Zhabotinsky reaction. Such detailed information for a specific reaction also informs the general understanding of the class of 'excitable media'. (C) 1999 Elsevier Science Ltd. All right s reserved.