Reactions for chemical systems far from equilibrium

Abstract

Reactions formulated for systems operating far from equilibrium may generate misleading conclusions if they contain an implied assumption of steady state. This assumption is introduced most commonly by eliminating an intermediate product from a pair of elementary reactions, condensing them into a single summary reaction. The consequences of using such inadequate reactions are illustrated through two well known model systems, the Lotka oscillator and the Brusselator. The Brusselator includes a termolecular reaction which could be generated by summarizing any of several sets of elementary reactions. When an elementary set is used in place of the termolecular reaction, the revised Brusselator fails to show its characteristic limit cycle. Both the Brusselator and the Lotka scheme have reactions in which the same substance appears both as reactant and as product, formed by eliminating some intermediate in a catalytic cycle. Including even short-lived intermediates alters the stability shown by the Lotka scheme, so that behaviour in the phase plane changes from a conservative cycle into an expanding spiral.