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 t

摘要

We have determiend apparent molar volumes V_(phi) and apparent molar heat capacities C_(p,phi) 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 393.15K at a constant pressure of p = 0.35 MPa. Density measurements from a vibrating-tube densimeter (DMA512P, Anton Paar, Austria) were used to obtain V_(phi) values. A twin fixed-cell, power-compensation, differential-output, temeprature-scanning calorimeter (NanoDSC 6100, Calorimetry Sciences Corporation, American Fork, UT, USA) was used to obtain C_(p,phi) values. We used regressions to empirical equations to describe (V_(phi), m,T) and (C_(p,phi), 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_(p,2)~0. We have used literature values of V_(phi) and C_(p,phi) for aqueous NaCl, HCl, and NaOH solutions to calcualte estimated DELTA_rV_m and DELTA_rC_(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 DELTA_rH_m and equilibrium molality quotients Q_a for these dissociations as functions of m and T by integrating (DELTA_rC_(p,m),m,T).