Chemical relaxation methods step perturbation

Figure 2.1 Shape of the perturbation (in full line) used in chemical relaxation methods and response of the system (dashed line) (A) step perturbation as in T-jump (B) rectangular perturbation as in shock-tube (0 = duration of tiie perturbation) (C) harmonic perturbation as in ultrasonic absorption relaxation.

Photolysis and pulse radiolysis are powerful methods for producing sizeable amounts of reactive transients whose physical and chemical properties may be examined. Structural information on the transient and the characterization of early (rapid) steps in an overall reaction can be very helpful for understanding the overall mechanism in a complex reaction. Chemical equilibria may be disturbed by photolysis or radiolysis since one of the components may be most affected by the beam and its concentration thereby changed. The original equilibrium will be reestablished on removing the disturbance and the associated change can be examined just as in the relaxation methods. The approach has been more effectively used in laser photolysis and since very short perturbations are possible the rates associated with very labile equilibria may be measured. 

The amplitudes of chemical relaxation processes are determined by the equilibrium concentrations (and strictly speaking, associated activity coefficients) and by thermodynamic variables appropriate for the particular perturbation method used. Thus, for example, an analysis of the amplitudes of relaxation processes associated with temperature jump measurements can lead to determination of the equilibrium constants and enthalpies associated with the mechanism under study. As might be anticipated from our previous discussion, the relaxation amplitudes are determined by normal mode thermodynamic variables which are linear combinations of the thermodynamic variables associated with the individual steps in the mechanism. The formal analysis of relaxation amplitudes has been developed in considerable detail [2, 5,7],... 

The experimental approaches we have described so far for the construction of mechanisms of oscillating reactions closely parallel the methods used by kineti-cists to study simpler systems—that is, measuring the rate laws and rate constants of the postulated elementary steps and comparing measurements of concentration vs. time data for the full system with the predictions of the mechanism. This is as it should be, since chemical oscillators obey the same laws as all other chemical systems. Another set of methods that has proved useful in kinetics studies involves perturbation techniques. One takes a system, subjects it to a sudden change in composition, temperature, or some other state variable, and then analyzes the subsequent evolution of the system. In particular, relaxation methods, in which a system at equilibrium is perturbed by a small amount and... 

These methods are applicable to chemical reactions at equilibrium. If the equilibrium is suddenly perturbed by a change in temperature, pressure, or some other parameter, then the system will relax toward the new equilibrium position. The rate constants for the forward and reverse steps can be determined from the rate of relaxation [19, 20]. 

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