, Volume 8, Issue 5, pp 329-345

Ionization of aqueous benzoic acid: Conductance and thermodynamics

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Abstract

Molar conductances of dilute aqueous benzoic acid solutions are presented for temperatures from 5 to 80°C. The data have been analyzed to give acid dissociation constants as well as ΔH o, ΔS o, and ΔC p o for the ionization process and the limiting conductance of the benzoate ion. The conductance-viscosity product changes less than 4% over the temperature range, indicating that the interaction of the benzoate ion with the solvent changes little if at all with increasing temperature. The pK a(m) vs.T data show that ΔH o decreases quadratically while ΔC p o increases linearly withT although, over the 75°C range, ΔC p o increases only about 6 cal-mole−1 deg−1 around an average of −37 cal-mole−1deg−1. The acid dissociation constants as derived from the conductance-molal concentration analysis show an average uncertainty of about 0.1% and are fitted to within about 0.01% by the equation $$p{\text{K}}_{\text{a}} (m) = - 75.5422 + 3136.34/T + 28.7965 log T - 6.8139 {\text{x}} 10^{ - 3{\text{T}}} $$ whereT is the absolute temperature.