Calorimetrically measured enthalpies for the reaction of Hf with H(D)2 (g)

Abstract

The enthalpy for the direct reaction of H{sub 2} (g) with Hf has been measured by calorimetry for the first time at both moderate, 334 K, and elevated, 919 K, temperatures. The enthalpy for the reaction: 1/2 H{sub 2} (g) + 1(b{minus}a)HfH{sub a}({alpha}) {r_arrow} 1/(b{minus}a)HfH{sub b}({delta}) is {minus}70 {+-} 2.0 kJ/mol H at 334 K over a range of H contents from (H/Hf) = 0.5 to 1.5 with similar values found for D. The quantities {alpha} and {delta} are the coexisting phases and a and b are the corresponding (H/Hf) ratios, respectively. The magnitude of the enthalpy decreases from (H/Hf) = 0 to 0.5 and is then stable from 0.5 to 1.7. The value of {Delta}H{degree}{sub f} (HfH{sub 1.5}) = {minus}107.5 kJ/mol and {Delta}H{degree}{sub f} (HfH{sub 2.0}) = {minus}142.0 kJ/mol. In the elevated temperature range, calorimetric and equilibrium hydrogen pressure were determined over the range of H contents from 0 to 1.6. The enthalpy for the plateau reaction is {minus}74.5 kJ/mol H and after the two-phase region, {vert_bar}{Delta}H{sub H}{vert_bar} increases with the increase of (H/Hf) passing through a maximum at about (H/Hf) = 1.3.