Enthalpy of Sublimation of Zinc and Cadmium; Correlation of ΔH° vs ΔS°; Comparison of Torsional and Knudsen Vapor Pressures

Abstract

The vapor pressures of solid zinc and cadmium have been determined directly by measurements of the torsional recoil of a suspended effusion cell (p) and indirectly by measurement of mass effusion (π). The results with R in calories/mole·degree and p and π in atmospheres are as follows: For zinc (610°–690°K) the torsional recoil yields lnp = − (30.370 ± 0.070) × 103 (RT)−1 + (27.215 ± 0.107)R−1, at 645°K logp = − 4.3423 ± 0.0011; the mass effusion yields lnπ = − (30.379 ± 0.126) × 103 (RT)−1 + (27.087 ± 0.193)R−1, at 645°K log π = − 4.3733 ± 0.0016; for cadmium (525°–590°K) the torsional recoil yields lnp = − (26.361 ± 0.123) × 103(RT)−1 + (27.135 ± 0.219)R−1, at 555°K logp = − 4.4501 ± 0.0017; the mass effusion yields lnπ = − (26.172 ± 0.139) × 103(RT)−1 + (26.672 ± 0.247)R−1, at 555°K logπ = − 4.4768 ± 0.0019. In these equations the cited errors are standard deviations generated in least-squares analyses. The measured pressures for zinc and cadmium are 1.075(± 0.01) and 1.063(± 0.01), respectively, times the equivalent mass effusion for molecular weight corresponding to monomer. This ratio appears to represent a demonstrable systematic difference between the two procedures. A brief survey of measurements for various material by others reveals that p is generally greater than π probably because the restituted momentum from the surroundings makes the former too high. A procedure based on ΔS°T(ΔH°0) is defined which recognizes systematic errors in lnp vs T−1 and which derives a value for ΔH°0 free of the inconsistencies frequently introduced in averaged values of RT lnp (the so-called third law procedure). For zero error in absolute entropies and for mass effusion, the results are for zinc ΔH°0 = 31.043 ± 0.054 kcal/mole and for cadmium ΔH°0 = 26.754 ± 0.062 kcal/mole.