On the Use of Ultrasonic Absorption for the Determination of Very Rapid Reaction Rates at Equilibrium: Application to the Liquid Phase Association of Carboxylic Acids

Abstract

An equation is derived for the excess acoustic absorption in a liquid system due to the occurrence of a chemical reaction; the method used follows in part that used by Damköhler in the case of gases. It is shown that the gross reaction rate at equilibrium can in general be found once the frequency at which the maximum excess absorption per wavelength occurs has been found, provided thermodynamic data for the reaction are known. The application of this method to the study of the kinetics of very rapid reactions is discussed and the effects of other causes of absorption are considered. As another application of the results obtained the acoustic absorption of acetic and propionic acids is considered. A consistent interpretation of data in the literature is given on the assumption that the excess absorption is due to the perturbation of an equilibrium between monomeric and dimeric molecules. The mole fraction equilibrium constant K_N, and the heat of reaction for the dimerization are calculated. The values found for acetic acid are K_N(293°K)=61.97 and ΔH=−6.21 kcal/mole; for propionic acid, K_N(281.2°K)=3.040×10³ and ΔH=−9.33 kcal/mole. Rate constants and activation energies for the dissociation of the dimers are also calculated.