Early Stages of the Hydrolysis of Chromium(III) in Aqueous Solution. V. Measurement of the Equilibrium Between Singly and Doubly Bridged Dimer

Abstract

Ion-exchange chromatography has been used to measure the equilibrium between singly bridged (SBD) and doubly bridged (DBD) forms of the hydrolytic dimer of chromium(III), at variable [H+] (0.008-0.97 M) and T (288.2-318.2 K) and constant I (1.0 M). From these measurements, the acid dissociation constant, K_al , for SBD, and the equilibrium constant, K, for interconversion between monodeprotonated SBD and DBD have been determined together with their corresponding thermodynamic constants. The results at 298.2K were as follows: K = 10.1( ±0.6) [ΔHº = -10( ±3) kJ mol^-1 and ΔSº=-15( ±9) J K^-1 mol^-1]; KK_al = 1 .83(±0.04) mol dm^-3 [ΔSº = 30(±1) kJ mol^-1 and ΔSº= 104(±4) J K^-1 mol^-1]; K_al = 0.18( ±0.2) mol dm^-3 [ΔSº= 43(±4) kJ mol^-l and ΔSº= 128(±13) J K^-1 mol^-1]. The high K_alis attributed to the participation of monodeprotonated SBD in hydrogen bonding. As the temperature increases, the overall equilibrium (given by KK_al ) is driven towards DBD by a dramatic increase in the acidity ( K_al ) of SBD, an increase which more than offsets a concurrent decrease in K.