Portal de Periódicos da CAPES

Vaporization of Sodium Iodide and Thermochemistry of (NaI) 2 (g) and (NaI) 3 (g): An Experimental and Theoretical Study

1984; Volume: 88; Issue: 2 Linguagem: Inglês

10.1002/bbpc.19840880211

0005-9021

K. Hilpert,

Thermal and Kinetic Analysis

Abstract The evaporation of NaI(s) has been investigated by high temperature mass spectrometry and mass‐loss Knudsen effusion experiments with a micro recording vacuum balance. The gaseous phase in equilibrium with NaI(s) consists of NaI(g), (NaI) 2 (g), and (NaI) 3 (g). Partial pressures given by the equations log( p Pa) = –(9.479±0.13) 10 3 / T + 11.49±0.17 for (NaI)(g), log( p Pa) = –(10.953±0.06) 10 3 / T + 12.72±0.06 for (NaI) 2 (g), and log( p Pa) = –(13.543±0.11) 10 3 / T + 13.58±0.15 for (NaI) 3 (g) were determined in the temperature range of the measurements 674 to 843 K. Sublimation enthalpies were obtained in kJ mol −1 for the reactions i NaI(s) ⇄ (NaI) i (g) ( i = 1,2,3) (1) as Δ H 0 0 = 196.3±4.4 ( i = 1), Δ H 0 0 = 226.8±1.3 ( i = 2), and Δ H 0 0 = 284.8±6.6 ( i = 3). The enthalpy changes of the dimerization and trimerization reactions i NaI(g) ⇄ (NaI) i (g) ( i = 2,3) (2) are Δ H 0 0 = –165.8±8.9 ( i = 2) and Δ H 0 0 = –304.1±14.8 kJ mol −1 ( i = 3). The given enthalpy values are averages of the values determined with the second and third law method for each of the reactions Eqs. (1) and (2). Molecular parameters were estimated for (NaI) 3 (g) and enthalpy increments as well as Gibbs energy functions computed with these data are given. The enthalpy change of the trimerization reaction, Eq. (2) ( i = 3), was predicted with Pauling's model for alkali halides as Δ H 0 0 = –293.4 kJ mol −1 which agrees well with our experimental value. Entropies of sublimation to monomer, dimer, and trimer as well as dimerization and trimerization entropies were obtained at 750 K as 130.1, 148.0, 173.9, –112.2, and –216.4 J mol −1 K −1 , respectively. Appearance potentials of 8.7±0.1 (Na + ), 14.4±0.2 (I + ), 8.2±0.1 (NaI + ), 8.6±0.1 (Na 2 I + ), 7.5±0.1 (Na 2 I 2 + ), 8.3±0.1 (Na 3 I 2 + ), and 8.4±0.1 (Na 3 I 3 + ) were determined in V for the ions given in parentheses. The data obtained are discussed in the context of literature data.