Thermodynamic properties of silicides: IV. Determination of the standard molar enthalpies of formation at the temperature 298.15 K of pentatungsten trisilicides of composition W5Si3and W5Si3.115by fluorine-combustion calorimetry

Abstract Two fully characterized specimens of pentatungsten trisilicide, nominally W 5 Si 3 , one believed to have been prepared by direct reaction of the elements in a sealed tube, and the other by are melting, have been studied by fluorine-combustion calorimetry. Chemical analyses of the samples played a pivotal role in this work, and their effects on the results are discussed in detail. The massic energies of combustion of the samples according to the reaction: W 5 Si 3 + x (cr) + (21 + 2 x )F 2 (g) = 5WF 6 (g) + (3 + x )SiF 4 (g), were measured calorimetrically. Results derived for the standard molar enthalpies of formation Δ f H m °(W 5 Si 3 + x ) at temperature T = 298.15 K and standard pressure p ° = 101.325 kPa are: −(126 ± 18) kJ·mol −1 for x = 0, and −(97 ± 20) kJ·mol −1 for x = 0.115. (Here, and throughout this paper, uncertainties are uncertainty intervals equal to twice the standard deviation of the mean.) The value for Δ f H m °(W 5 Si 3 , cr) agrees with estimates from Knudsen-effusion vaporization studies at elevated temperatures. There have been no previous reports of thermodynamic investigations of nonstoichiometric pentatungsten trisilicides.