An Ultrasonic Investigation of a Chemical Kinetics Problem: The Dissociation of Magnesium Sulfate

New equations for the excess sound absorption per wavelength and the relaxation frequency have been derived. These equations are derived on the assumption that a dissociation process of the type (AB)⇌(A)+(B) is irreversibly perturbed by the sound field due to assumed pressure dependence of the equilibrium. These equations are applied to the problem of excess sound absorption in dilute magnesium sulfate solutions. By using mixtures of dioxane and water to vary the dielectric constant of the solvent the effect of varying the ion Coulombic interaction forces of the ions in solution has been studied experimentally. This work and the work of other investigators on the excess sound absorption in dilute magnesium sulfate solutions is explained quantitatively. The specific rates of dissociation and ion association for magnesium sulfate in solutions of water and water-dioxane have been determined. Good agreement is found between values of the mass action equilibrium constant determined by acoustic means and values determined by the usual conductivity method. The experimental activation energy of molecular dissociation is determined to be 6.5 kcal/mole while the activation energy of ion association is estimated to be 8.5 kcal/mole.