Thermodynamic Properties and the Cohesive Energy of Calcium Ammoniate

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Abstract
The equilibrium pressure of ammonia over calcium ammoniate, Ca(NH3)6, has been measured at over 40 temperatures extending from − 10 to + 13°C. The standard enthalpy, free energy, and entropy of dissociation, Ca(NH3)6(s)⇌Ca(s)+6NH3(g), per mole of ammonia at 0°C have been determined to be 9.95 ± 0.10 kcal, 1.890 ± 0.010 kcal, and 29.5 ± 0.4 eu, respectively. Combining the enthalpy of dissociation with other thermodynamic data, the energy of the reaction Ca(NH3)6(s)→Ca(NH3)62+(g)+2e−(g) is found to be 234 kcal/mole of calcium ammoniate. Comparison of this value with the predictions of the modified Hartree model indicates that the valence electrons of calcium are more highly delocalized in calcium ammoniate than they are in calcium itself; calcium ammoniate is more nearly an ideal metal than calcium—at least in so far as its cohesive energy is concerned.

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