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To investigate the principles of Non-Equilibrium Thermodynamics, an experimental and theoretical study of a chemical reacting system is made. Nitric oxide, iodine, chlorine, iodine monochloride, and nitrosyl chloride in the gas phase are the macroscopic chemical species which comprise this system.

The system displays a coupling behavior which is uncommon to chemical reacting systems. However, this behavior may be useful in explaining some biological phenomena. The observed coupling effect results in a phenomenological coefficient matrix which is not diagonal.

None of the macroscopic derivations of the Onsager reciprocity relations which are examined appear to be generally applicable. However, a limited derivation is proposed which not only identifies the phenomenological coefficients in the linear rate laws for chemical reacting systems but also indicates the validity of the Onsager relations. Linear transformations of the fluxes and forces which diagonalize or destroy the symmetry of the phenomenological coefficient matrix are not applicable to a chemical reacting system.

Photometric methods are used to experimentally determine the reaction velocities and chemical affinities of this system. The phenomenon of coupling is definitely exhibited by this system, as shown by the existence of the coupling coefficients, L[subscript 12] and L[subscript 21], and by the presence of a negative AV product. The applicability of linear phenomenological rate laws and also rate laws which include quadratic terms are investigated. The linear rate laws are better approximations as the system approaches equilibrium and may be useful at appreciable distances from equilibrium in certain cases. A conclusive test of the Onsager reciprocity relations could not be made.

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