Heat and Entropy of Sublimation of Nickel Dichloride, Dibromide, and Di-iodide; Dissociation Energies of Gaseous NiCl2 and NiBr2

Abstract

The vapor pressures of solid anhydrous nickel dichloride, dibromide, and di-iodide have been measured directly by measurement of the torsional recoil of a suspended effusion cell and indirectly by measurement of mass effusion. The results with R in calories/mole·degree and p and π in atmospheres are as follows: NiCl2(550°–608°C): RT lnp = − (53.59 ± 0.41) × 103 + (43.70 ± 0.48)T, at 855°K logp = − 4.1472 ± 0.0020; RT lnπ = − (52.76 ± 0.40) × 103 + (42.63 ± 0.46)T, at 855°K logπ = − 4.1670 ± 0.0019. NiBr2(519°–584°C): RT lnp = − (53.01 ± 0.76) × 103 + (45.11 ± 0.95)T, at 820.16°K logp = − 4.2595 ± 0.0036; RT lnπ = − (51.97 ± 0.27) × 103 + (43.73 ± 0.33)T, at 820.16°K logπ = − 4.2843 ± 0.0014. Nil2(462°–505°C): RT lnp = − (35.58 ± 0.36) × 103 + (32.60 ± 0.52)T; RT lnπ = − (37.39 ± 0.24) × 103 + (34.93 ± 0.34)T. For the chloride and the bromide, the measured pressures are 1.05(± 0.02) times the equivalent mass effusion (π) for a molecular weight corresponding to the monomeric dihalide. This deviation from unity cannot be attributable to lack of information about the gaseous species but probably represents a demonstrable systematic difference between the two procedures. The free energies of bonding derived from the measurements are ΔF°b(NiCl2, g) = − 180 000 + 54.78T and ΔF°b(NiBr2, g) = − 131 000 + 38.22T both in calories/mole. The derived dissociation energies to the gaseous atoms are 7.8 eV for NiCl2(g) at 850°K and 5.7 eV for NiBr2(g) at 820°K. The absolute entropy of gaseous NiCl2 at 850°K is 85.4 eu, a value which yields an estimated 13.3 eu for the vibrational and electronic degrees of freedom. An analysis of the results for nickel di-iodide cannot be accomplished because significant but unknown amounts of gaseous NiI and I2 are present causing transport back and forth between the cell and its surroundings.