A mass spectrometric investigation of the vaporization thermodynamics and vapor composition of cadmium oxide

Abstract

Mass spectrometric Knudsen effusion vaporization experiments on CdO(s) were performed over the temperature range 886–1090 K. The results confirm directly that CdO(s) vaporizes by decomposition according to the reaction CdO(s) = Cd(g) + 1 2 O 2(g) The equilibrium constant for this reaction was determined to be log 10K = − (1.980 ± 0.016) × 10 4 T + (11.706 ±0.164) over the experimental temperature range. The second-law enthalpy change ΔH°(298.15 K) for the decomposition of CdO(s) derived from the present results was 92.82 ± 0.74 kcal mol −1 and the corresponding third-law value was 87.10 ± 2.03 kcal mol −1. Averaging the present third-law value for ΔH°(298.15 K) with third-law values derived from literature vapor pressures gave a recommended value for ΔHdg(298.15 K) of 87.89 ± 0.15 kcal mol −1. This result is in excellent agreement with ΔH°(298.15 K) = 87.92 ± 0.3 kcal mol −1 derived independently from the enthalpies of formation for CdO(s) and Cd(g). CdO(g) was not observed in the equilibrium vapor of CdO(s) for temperatures up to 1150 K. This result gave a revised lower limit for the enthalpy of formation for CdO(g), i.e. ΔH° f (298.15 K) ⩾ 27.2 kcal mol −1.