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

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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, Kal , 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) [ΔH� = -10( �3) kJ mol-1 and ΔS�=-15( �9) J K-1 mol-1]; KKal = 1 .83(�0.04) mol dm-3 [ΔS� = 30(�1) kJ mol-1 and ΔS�= 104(�4) J K-1 mol-1]; Kal = 0.18( �0.2) mol dm-3 [ΔS�= 43(�4) kJ mol-l and ΔS�= 128(�13) J K-1 mol-1]. The high Kalis attributed to the participation of monodeprotonated SBD in hydrogen bonding. As the temperature increases, the overall equilibrium (given by KKal ) is driven towards DBD by a dramatic increase in the acidity ( Kal ) of SBD, an increase which more than offsets a concurrent decrease in K.

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