Significant discrepancies between van't Hoff and calorimetric enthalpies .3.

The study of a wide variety of reversible reactions in solution indicates that the enthalpy, Delta H-vH, which controls the temperature variation of the equilibrium constant for a reaction, can seldom, if ever, be taken to be independent of the temperature. It is also found in most cases that the values for Delta H-vH, properly evaluated as varying with the temperature, differ significantly from the values for the enthalpy, Delta H-cal determined by direct calorimetry under the same experimental conditions. In a continuing search for reactions which show agreement between Delta H-vH and Delta H-cal, we have studied by isothermal titration calorimetry the reactions of heptylamine with heptanoic acid in dodecane solution and of alpha-cyclodextrin with sodium heptanoate in aqueous solution.