STRUCTURE, EQUILIBRIUM, KINETICS

Every species of scientist seems to have particular preoccupations displayed by all members of the species and quite unknown to anybody else. For physical chemists, one of these is: ‘Are we talking about kinetics or equilibrium?’ Part of the difficulty here is that the word ‘equilibrium’ is used in a number of different senses by different kinds of scientist. Physical chemists use it in a rather strict sense, to mean ‘thermodynamic equilibrium’. This is a state of a system in which no macroscopic observables are changing with time, and there are no flows of energy or material through the system. In terms of biological systems, this means definitely dead; and such terms as ‘equilibrium evolution’ therefore tend to make physical chemists shudder.

Chemical reactions in a closed system, i.e. one to which no changes are being made by adding or removing material across its boundaries, proceed until equilibrium is reached. At that point nothing further happens. If, however, reactants are continuously being added and products removed, it is possible for the contents of the reaction vessel to reach time-independent concentrations and spatial distributions, the constancies of which are entirely dependent on continuous supplies and removals. These are known as ‘out-of-equilibrium steady-states’, and any pattern that one believes to be in the dynamically generated and dynamically maintained category is thereby counted among them. When we talk about it, we are discussing kinetics, not equilibrium.