Thermodynamic Data Obtained from the Change of ΔH of a Normal Mode of Chemical Relaxation with Concentration

Abstract

When a system of coupled elementary reactions is perturbed from its equilibrium position by a change of some external parameter, the concentrations of the participants "relax" to their new equilibrium values with time-constants which are characteristic not of the individual elementary reactions, but rather of the "normal modes of chemical relaxation". Each normal mode can be considered as a linear combination of two or more elementary reactions, the coefficients depending both on rate constants and on concentrations. It can be "excited", for instance, by a sudden change of temperature,~T. Its degree of excitation of "amplitude" will be designated by y; it is the difference between the actual and equilibrium concentrations of that substance -or substances -whose stoichiometric coefficient in the normal mode is unity. Simple thermodynamics show that y at a time immediately after the jump, before relaxation has set in, is given by y =r~H~T/RT2 where r is an "amplitude factor" which depends on the stoichiometric coefficients and concentrations of the participants. Thermodynamically, a normal mode can be treated like an ordinary chemical reaction. Therefore, its~H simply follows from Hess' Law and is a suitable linear combination of the~H-values of the reactions which constitute the normal mode. If the coefficients with which the various reactions participate in the normal mode change with concentration, then~H of the normal mode will also change with concentration. Although all this is well known [1][2][3][4] and although it has often been found possible to improve the amplitude of a given reaction which has a low~H-value by coupling it with another system whosẽ H is high, the change of~H with a change of the coefficients does not seem to

Dynamic Analysis of Chemical Relaxation Effects: A Study of Salicylate-Indicator Systems by a T-Jump Apparatus with Laser Monitoring

“…The amplitudes of the relaxation effects should be associated, rather than to the individual steps, to special linear combinations of them denoted as normal modes of reaction or more simply normal reactions . The amplitude of the k th normal reaction, expressed as a change of absorbance, δ A k , following a jump of temperature δ T , is related to the relevant thermodynamic parameters by the relationship − where Γ k = (Σ N ik 2 / c i ) -1 is the amplitude factor whose magnitude depends on the stoichiometric coefficients N ik and on the equilibrium concentrations, c i , of the species taking part in the k th normal reaction …”