The anharmonicity correction to the zero-point energy change in the triatomic dihydride isotopic self-exchange equilibria, H2X+D2X=2HDX, is discussed. It is shown that the inclusion of the usually ignored term G0 in the expression for the anharmonic correction to the zero-point energy (G0 + 14Σxij) causes the anharmonicity correction to the zero-point energy change to be quite small for the cases considered (X is O, S, Se), but when this term is ignored the correction is found to be nonnegligible. Equilibrium constants are calculated, and it is found that the agreement between theory and experiment is considerably improved by including the G0 factor.
We measured the equilibrium constants KHDO = P2HDO/PH2O⋅PD2O, KHTO = P2HTO/PH2O⋅PT2O, and KDTO = P2DTO/PD2O⋅PT2O at 295.6°K (22.4°C), assuming the mass spectrometric sensitivities of the three species cancel in each expression of the equilibrium constant. The results obtained were KHDO = 3.82±0.05, KHTO = 3.66±0.06, and KDTO = 3.99±0.07. These values agreed with the current theoretical values of KHDO = 3.84, KHTO = 3.69, and KDTO = 3.97 at the same temperature. We discuss the discrepancy of the present value of KHDO as compared with previous experimental determinations, and we conclude that a direct measurement of the mass spectrometric sensitivity ratio for KHDO is still necessary to resolve the disagreement. We prepared the water mixtures that contained tritium by reacting the appropriate hydrogenic gases with oxygen over a platinum sponge catalyst.
Isotopic exchange equilibrium in the disproportionation reaction H2O+D2O=2 HDO was studied at 0° and 25° and over a range of deuterium compositions using a pulsed-molecular-beam mass spectrometer. The equilibrium constant K was found to be 3.75±0.07 at both temperatures. The pulsed-beam mass spectrometer was shown to be effective in minimizing the ``memory effect'' in the mass spectrometer. The experimental K is shown to be consistent with equilibrium constants for similar isotopic disproportionation reactions and with current theory.
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