Thermodynamics of proton dissociation from aqueous 1-propanoic and 1-butanoic acids at temperatures 278.15⩽( T/K)⩽393.15 and pressure p=0.35 MPa: apparent molar volumes and apparent molar heat capacities of aqueous solutions of the acids and their sodium salts

Abstract

We have determined apparent molar volumes V φ and apparent molar heat capacities C p, φ for aqueous solutions of 1-propanoic acid and 1-butanoic acid and of their sodium salts. Molalities m ranged from 1.0 to 0.01 mol· kg −1 and temperatures T ranged from 278.15 K to 393.15 K at a constant pressure of p=0.35 MPa. Density measurements from a vibrating-tube densimeter (DMA 512P, Anton Paar, Austria) were used to obtain V φ values. A twin fixed-cell, power-compensation, differential-output, temperature-scanning calorimeter (NanoDSC 6100, Calorimetry Sciences Corporation, American Fork, UT, USA) was used to obtain C p, φ values. We used regressions to empirical equations to describe ( V φ , m, T) and ( C p, φ , m, T) for each solute. Extrapolation of the fitted surfaces to the m=0 mol· kg −1 plane gave standard partial molar volumes V 2 0 and heat capacities C 0 p,2 . We have used literature values of V φ and C p, φ for aqueous NaCl, HCl, and NaOH solutions to calculate estimated Δ r V m and Δ r C p, m values for the volume and heat capacity changes for proton dissociation from each carboxylic acid as functions of m and T. We then determined the ionization enthalpies Δ r H m and equilibrium molality quotients Q a for these dissociations as functions of m and T by integrating (Δ r C p, m , m, T).