Knudsen effusion mass spectrometric determination of the dissociation energy of diniobium, Nb2(g), and the heat of sublimation of solid niobium

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Abstract
The dimer of niobium, Nb2(g), has been observed under thermal equilibrium conditions by the high temperature mass spectrometric technique, employing an unconventional design for the Knudsen cell. From the partial pressures of Nb and Nb2 measured in the 2545–2677°K temperature range, the dissociation energy of Nb2(g) has been determined to be D0°=503±10 kJ mole−1 or 120.2±2.4 kcal mole−1 and D°298=511±10 kJ mole−1 or 122.0±2.4 kcal mole−1. By combining D (Nb2) with the heat of vaporization of niobium, the heat of formation of Nb2(g) is derived as ΔH°f,298=916±12 kJ mole−1 or 218.9±2.9 kcal mole−1. Also, the heat of vaporization of Nb has been redetermined as ΔH°v,298=713±7 kJ mole−1 or 170.4±1.6 kcal mole−1 and ΔH°v,0=710±7 kJ mole−1 or 169.7±1.6 kcal mole−1. The dissociation energy of Nb2(g) is in good agreement with the value predicted previously and is consistent with the trends indicated for the dissociation energy of the diatomic molecules of the first and second transition series metals.