Complex chemical reactions * A review

Complex responses have been observed in a wide range of chemical and engineering systems: In well-stirred and unstirred solution-phase, in gels, in heterogeneous catalysis and dissolution reactions, in gasand solid-phase combustion, widely in biological systems, accompanying phase transitions, in atmospheric kinetics and even in interstellar dust clouds. Complex behaviour does not, however, imply a necessarily complex underlying chemical mechanism. The appropriate feedback mechanisms, built on chainbranching, autocatalysis or self-heating, arise quite commonly in Nature. In many instances, it is appropriate "rst to attempt to "nd reduced mechanisms giving a semi-quantitative "t to observed responses. An example of such an approach applied to a model for complex oscillations of species concentrations in the mesosphere is presented, and the reduced model used to investigate the e!ects 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 exempli"ed by a study of the development of three-dimensional scroll waves in the Belousov}Zhabotinsky reaction. Such detailed information for a speci"c reaction also informs the general understanding of the class of &excitable media'. ( 1999 Elsevier Science Ltd. All rights reserved.