Enthalpy of dissociation and hydration number of carbon dioxide hydrate from the Clapeyron equation

The enthalpies of the reactions in which carbon dioxide hydrate is dissociated to carbon dioxide vapor and either water or ice are determined by an analysis with the Clapeyron equation. The most important feature of the new analysis is the direct use of the Clapeyron equation rather than the Clausius-Clapeyron equation. The analysis takes into account the finite volumes of the condensed phases, the nonideality of the vapor phase, and the solubility of carbon dioxide in water. New data for the solubility in the vicinity of the (water + hydrate + vapor) coexistence curve are employed. The enthalpy change of the reaction CO2 . nH(2)O(s) = CO2(g)+nH(2)O(1) is found to vary from (63.6 +/- 1.8) kJ . mol(-1) to (57.7 +/- 1.8) kJ . mol(-1) between quadruple points Q1 and Q2, and the hydration number varies from (6.6 +/- 0.3) to (5.6 +/- 0.3) over the same range. The results are compared to values reported in the literature that were determined by various techniques. (C) 2003 Elsevier Science Ltd. All rights reserved.