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

✍ Scribed by Iwona Tomaszkiewicz; G.A. Hope; Charles M. Beck; II; P.A.G. O’Hare

• Publisher: Elsevier Science
• Year: 1995
• Tongue: English
• Weight: 429 KB
• Volume: 27
• Category: Article
• ISSN: 0021-9614
• DOI: 10.1006/jcht.1995.0060

✦ Synopsis

Two fully characterized specimens of pentatungsten trisilicide, nominally W5Si3, one believed to have been prepared by direct reaction of the elements in a sealed tube, and the other by arc 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: W5Si3+x (cr) + (21 + 2x)F2(g) = 5WF6(g) + (3 + x)SiF4(g), were measured calorimetrically. Results derived for the standard molar enthalpies of formation DfH°m(W5Si3+x ) at temperature T = 298.15 K and standard pressure p°= 101.325 kPa are: -(126 2 18) kJ•mol -1 for x = 0, and -(97 2 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 DfH°m(W5Si3,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.

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