Ionic dissociation of aqueous hydrobromic acid. Part 1.—Estimate from vapour pressure and activity coefficient data

Abstract

The dissociation constant on the molal scale of a strong acid HX is usually evaluated from log K′_a(m)= 2 log m_s+ 2 log γ_s+ log f^°_HX– log p_HX– log 55.51, where m_s and γ_s are the stoichiometric molality and mean molal activity coefficient, f^°_HX the fugacity of the pure liquid acid and p_HX the partial vapour pressure of the aqueous acid, all at a given temperature. This quantity is only an apparent dissociation constant, however, since it contains as a factor the transfer activity coefficient of HX from the pure liquid HX under pressure p^°_HX to the infinitely dilute aqueous HX standard states. The data available for HX = HBr are examined, and lead to log (K′_a(m)/mol kg^–1)= 8.60 ± 0.06 at 25°C, compared with 8.56 ± 0.07 for HI, 6.07 ± 0.03 for HCl and 3.65 ± 0.05 for HNO₃. The apparent standard enthalpy of dissociation is ΔH′_a/kJ mol^–1=– 61 ± 3 for HBr compared with – 46 ± 4 for HCl. The degree of dissociation α of hydrobromic acid is estimated from a comparison of γ_s with the estimated activity coefficient of the ionized part, γ±. It falls below unity at m_s > 7.5 mol kg^–1, reaches ≈ 0.7 in the azeotropic acid (≈ 11 mol kg^–1), and as low as ≈ 0.3 in nearly saturated acid, 20 mol kg^–1. These estimates will be subject to revision when spectroscopically based values become available.