CYANIDE THERMODYNAMICS .3. ENTHALPIES AND ENTROPIES OF IONIZATION OF WATER AND HYDROGEN-CYANIDE

The heats (enthalpy changes) associated with the ionization of water a nd of hydrogen cyanide have been determined by titration calorimetry a t 25 degrees C as a function of ionic strength up to 5 M in both NaCl and NaClO4 media. The enthalpy changes for both reactions exhibit a 'm edium effect' with Delta H being more positive in NaCl than in NaClO4 and with the difference becoming more pronounced with increasing ionic strength. This is attributed to the greater solvation of Cl- cf. CN- in aqueous solution. The present Delta H values are similar to previou s published results at high ionic strengths, and are in excellent agre ement with the well established literature values at infinite dilution . The present Delta H values were combined with literature stability c onstant data to calculate the corresponding entropies for the ionizati on of H2O and HCN as a function of ionic strength.