ENTHALPY OF SUBLIMATION OF ZINC AND CADMIUM - CORRELATION DELTAH DEGREES O VS DELTAS DEGREES - COMPARISON OF TORSIONAL AND KNUDSEN VAPOR PRESSURES

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 log p= -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+(27.13S±0.219)R-1, at 555°K log p=-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 log π=-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°r(Aff°0) is denned which recognizes systematic errors in Inp 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.