Dissociation pressures and enthalpies of reaction in MgCl2 · nH2O and CaCl2 · nNH3☆

Abstract Chemical heat pumps based on reactions involving salt hydrates, ammoniates, or methanolates are currently being developed. The efficient operation of a chemical heat pump depends strongly upon such quantities as reaction energies and pressure-temperature relations. Therefore, dissociation pressures as a function of temperature and derived reaction energies are presented for MgCl 2 · n H 2 O (where n = 6.1, 6, or 4) and CaCl 2 · n NH 3 (where n = 8, 4, 2, or 1). Pressure measurements were made using a capacitance manometer over a temperature range of about room temperature to 470 K (the upper temperature varied depending on the composition being studied). By generating Φ o functions from available heat capacities complete second-law and third-law analyses have been done for MgCl 2 · 6H 2 O and MgCl 2 · 4H 2 O. Results for 1 6 MgCl 2 · 6H 2 O yielded enthalpies of dissociation at 298.15 K of (58.3 ± 0.2) and (58.6 ± 0.1) kJ mol −1 for second-law and third-law treatments, respectively. For MgCl 2 · 4H 2 O the values determined were (66.3 ± 1.2) and (67.0 ± 0.4) kJ mol −1 . Values for the enthalpies of dissociation of ( 1 n ) CaCl 2 · n NH 3 were also determined; they are (44.4 ± 0.3), (42.0 ± 0.3), (67.8 ± 0.8), and (78.2 ± 0.8) kJ mol −1 for CaCl 2 · 8NH 3 , CaCl 2 · 4NH 3 , CaCl 2 · 2NH 3 , and CaCl 2 · NH 3 respectively.