Heats of formation of some hydrates

Abstract

Heats of formation of hydrates may be measured by dissolving the hydrate and the corresponding anhydrate in water to attain the same final concentration and measure the difference in heats of solution.Measurements were made at 0 °C in a modified Bunsen ice calorimeter, well insulated, and surrounded with an ice–water mixture.The data obtained are a measure of the enthalpy involved in binding a mole of water into a crystal structure, to shed some light on the nature and mechanism of this binding. Several discrepancies are noted, in particular one involving [Formula: see text]. A method for adjusting calculations from 0 to 25 °C in the absence of heat capacity data is proposed.The following hydrates have been studied: LiCl•H2O, LiBr•2H2O, CaCl2•2 and •6H2O, MnCl2•1 and •2H2O, [Formula: see text], MnSO4•1 and •4H2O, MgSO4•1, •4(?), and •7H2O, and [Formula: see text].Additionally, the heat of hydration of hydrazine to N2H4•H2O has been checked.The formation of a coordinated hydrate, from liquid water, involves a heat effect which in the limit approaches 3 kcal mol−1. The first mole of water added may be most strongly bonded (6–10 kcal), possibly since alternate sites in the coordination sphere may be available to it.Values which differ markedly from these levels deserve a critical examination. Higher values may suggest the presence of OH−, or other structural variations, errors in the published data or mistaken identification of the hydrate.