Oscillatory chemical reactions in closed vessels

Abstract

Abstract A skeleton kinetic scheme representing the simplest model for oscillatory chemical reactions in a closed vessel can be built around an autocatalytic feedback step precursor decay P → A k0p, uncatalysed step A → B k3a, autocatalysis A + 2B → 3B k1ab2, catalyst decay B → C k2b. The first intermediate A is formed via the slow decay of a reactant or precursor species P, initially in large excess. A is converted to B via two routes: a slow pseudo-first order process and a step in which B acts as its own catalyst. The autocatalyst B is then capable of a simple first order decay to a stable product C. The concentrations of the various species at first change steadily, with that of P decreasing while A, B and C increase. This period is followed by the onset and growth of oscillations in the concentrations of the intermediates A and B. The behaviour at long times, depends upon the uncatalysed conversion of A to B. Provided k3 is not taken as zero, the oscillations finally diminish in amplitude and die out leaving a steady decay of P, A and B until everything has been converted to C. The simplicity of the model allows the first self-consistent test of the ‘pool chemical approximation’, an approach commonly used in the analysis of mechanisms in closed systems in which the precursor concen­tration is assumed to be constant and set equal to its initial value. The results presented here reveal the range of applicability of the approxi­mation and show clearly how and why it can break down to give un­physical predictions.