Evaluation of the dissociation energy of Na₂ and K₂ molecules from the saturated vapor pressure of sodium and potassium

Conclusions

The values of D ⁰₀ obtained for sodium and potassium by the two methods are in good agreement. The value of D ⁰₀ for Na₂ molecules agrees within tolerances with other measurement results, with the exception of the value obtained by the luminescent irradiation method, which is known to be rough. The value of D ⁰₀ obtained by us for a K₂ molecule is in good agreement with the dissociation energy value obtained by the molecular beam method [12]. Spectroscopic measurements provide a much smaller value for D ⁰₀ , but according to Gaydon [13] they are not reliable, since they use too large an extrapolation of Berge-Sponer.

A slightly better agreement between the pressures calculated from (5) and the experimental values of sodium and potassium pressures are obtained in the values of D ⁰₀ and ΔH ⁰₁₀ calculated by the second method are used. Therefore, we can recommend the values of D ⁰₀ =17,920 ±400 kj/kmole (for an Na₂ molecule), of D ⁰₀ =53,750±400 kj/kmole (for a K₂ molecule), and corresponding heat of sublimation values of ΔH ⁰₁₀ =107,708 ±7 kj/kmole for sodium and of ΔH ⁰₁₀ =90,094 ±4 kj/kmole for potassium.

The tolerance of ±400 kj/kmole has been adopted by us so as to make the recommended value of D ⁰₀ agree with the results obtained by the molecular beam method for Na₂ and K₂, and by the spectrometric method for Na₂.