Processes forward-backward-translation

A local thermodynamic state is determined as elementary volumes at individual points for a nonequilibrium system. These volumes are small such that the substance in them can be treated as homogeneous and contain a sufficient number of molecules for the phenomenological laws to be applicable. This local state shows microscopic reversibility that is the symmetry of all mechanical equations of motion of individual particles with respect to time. In the case of microscopic reversibility for a chemical system, when there are two alternative paths for a simple reversible reaction, and one of these paths is preferred for the backward reaction, the same path must also be preferred for the forward reaction. Onsager s derivation of the reciprocal rules is based on the assnmption of microscopic reversibUity. The reversibility of molecular behavior gives rise to a kind of symmetry in which the transport processes are coupled to each other. Although a thermodynamic system as a whole may not be in equUibrium, the local states may be in local thermodynamic equilibrium, all intensive thermodynamic variables become functions of position and time. The local equilibrium temperature is defined in terms of the average molecular translational kinetic energy within the small local volume. 

Eqs. (5.75) - (5.81) relate properties of forward (A -> A) and backward (A A ) processes occurring at the same total energy E, and care must be taken not to apply them, by mistake, to such collisions occurring at equal translational energies. 

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