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

Publisher Website
Google Scholar
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 .

This publication has 20 references indexed in Scilit: