On the formulation of microscopic rate equations

It is advocated that in the formulation of kinetic equations governing any complex process the terms representing opposite unit processes, whose rates must be equal at equilibrium in accordance with the principle of detailed balance should always be grouped in pairs. In approximate treatments such pairs must be struck out together. This makes for avoidance of thermodynamic error and the appreciation of terms which might otherwise be overlooked, and can give assistance in estimating the rates of particular unit processes. Incidentally it makes it easier to perceive the physical meaning of terms in the equations. The microscopic rate equations governing the populations of various sizes of adsorbed aggregates during heterogeneous nucleation and growth are considered as an example.