Enthalpy increments for α- and β-Na 2UO 4 and Cs 2UO 4 by drop calorimetry The enthalpy of the α to β transition in Na 2UO 4

Abstract

A high-precision drop-calorimetric system has been used to measure the enthalpies (relative to 298.15 K) of α- and β-Na 2UO 4 to 1300 K, and of Cs 2UO 4 to 1100 K. In the range 298 to 1193 K the enthalpy increments for α-Na 2UO 4 can be represented by the equation: H°(T)−H°(298.15 K={38.8476(T/ K)+3.09342×10 −3(T/ K) 2+5.01109×10 5(T/ K) −1−13538.12} cal thmol −1 . In the range 1193 to 1300 K the enthalpy increments of β-Na 2UO 4 can be represented by: H°(T)−H°(298.15 K={53.69844(T/ K)−23824.88} cal thmol −1 . Enthalpy increments for Cs 2UO 4 are represented over the temperature range 298 to 1100 K by: H°(T)−H°(298.15 K)={39.4076(T/ K+2.03432×10 −3(T/ K) 2+3.65323×10 5(T/ K) −1−13155.51} cal th mole −1 . Values of C p o( T), { H o( T) − H o(298.15 K), S o( T), and {G o (T) − H o(298.15 K) } T are tabulated. For the α to β transition in Na 2UO 4, a value of ΔH tr = 5.9 kcal th mol −1 at 1193 K has been deduced.