Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

3 CHEMICAL RELAXATION METHODS 139Because of the very small effects expected and, consequently, the immense experimentaldifficulties involved in measuring them, techniques are not yet well developedwhich make use of these properties. The bulk of the work has made useof the disturbing influence of high-frequency sound-waves on reacting solutions,and the remainder of this section will discuss briefly the application of acousticalmethods to the study of chemical relaxation.3.3.1 Ultrasonic absorptionWhen sound-waves are propagated adiabatically through a liquid, the alternatingregions of high and low pressure are associated with slight increases and decreasesof temperature. If a chemical system whose position of equilibrium is pressure-or temperature-sensitive is present in the liquid, the disturbance caused bythe sound-waves may be used as a forcing function in the sense discussed above.It is of no consequence whether, in a given case, the system is reacting to the changein temperature or the change in pressure, since these two are completely coupled.Actually, for electrolytic reactions in water, e.g.,NH; +OH-NH, . H20the important parameter is AV while for reactions of nonele-trolytes, especially innon-aqueous solution (e.g., the dimerization of benzoic acid in toluene), AH ismore important.One of the parameters which can be measured directly is a, the attenuation constant+.a may be divided into several parts, the actual number depending on thenumber (n) of discrete chemical relaxation phenomena in the system under consideration.Thus, ,where ct,hem,n is that part of the attenuation constant attributable to the nth chemicalrelaxation process, while a,,, is the residual attenuation associated with dissipationproduced by the solvent or inherent in the design of the apparatus. The plot of loga/o’ us. log o has the typical “step” form (cf. Fig. 10(a)) and is shown in Fig. 11,for n = 2. The residual attenuation is sorgetimes determined in a separate experi-ment on the solvent alone. Strictly,for the solution, for example aned not be the same for the pure solvent asmay alter a,,, by tending tc break upt The decrease in energy for a plane sound-wave propagated in the direction of the x-axis isgiven by a factor e-zax, i.e., the energy absorption coefficient is characterized by 2%.References pp. 176-1 79