Heat capacities and thermodynamic properties of ammonium and potassium thiocyanates from 5 to 340 K

Abstract

The heat capacities of ammonium and potassium thiocyanates were determined by adiabatic calorimetry over the range 5 to 340 K, and associated thermodynamic functions were computed from the results. At 298.15 K, the heat capacities Cp/cal K-1 mol-1 and entropies So/cal K-1 mol-1 are 29.98 and 33.52 for NH4SCN and 21.16 and 29.70 for KSCN. In both salts, the heat capacity contribution from libration of the thiocyanate ions may be represented by two Einstein functions up to 250 K. The torsional motion of the ammonium ion increases rapidly above 100 K and contributes more than 6 cal K-1 to the heat capacity above 250 K. Adjuvant data on potassium thiocyanate solutions lead to So = (34.23+/-0.3) cal K-1 mol-1 for SCN-(aq) at 298.15 K.