M.S. Samant

Thermodynamic investigation of the vaporization of molybdenum trioxide

Abstract The vapour pressure of MoO3(s) was measured using a novel thermogravimetric system. The total pressures obtained in this investigation agree well with the best assessed values from data in the literature. The enthalpy of sublimation at 1000 K obtained from this work is 365.2 ± 5.0 kJ mol−1 in close agreement with the assessed value of 369.1 ± 0.8 kJ mol−1. The vapour pressure data for pure MoO3(s) was obtained in the context of our investigations on the vaporization of transition metal molybdates.

A microthermogravimetric system for the measurement of vapour pressure by a transpiration method

Abstract A novel automatic recording microthermogravimetric system for the measurement of vapour pressures at high temperatures by transpiration method was designed and fabricated. The performance of the system was checked by measuring the vapour pressure of anhydrous cadmium chloride in flowing dry argon in the temperature range 713–833 K. The vapour pressure of cadmium chloride in this range could be expressed as The enthalpy of sublimation of CdCl 2 derived from this equation at the mean temperature of investigation was found to be 167.2 ± 2.2 kJ mol −1 and compared very well with the value of 166.5 ± 4.7 kJ mol −1 reported by Skudlarski et al. (J. Chem. Thermodyn. 19 (1987) 857) from their recent Knudsen effusion mass spectrometric measurements. The present system can be used for measurement of vapour pressures up to 1425 K and is the first of its kind in which the mass loss due to vaporization is monitored continuously during the conventional transpiration experiment. The system described in this paper effects considerable saving of time in the experiment and minimizes the errors associated with mass measurements in the conventional transpiration method currently in use.

The standard free energy of formation of ZrMo2O8 by the transpiration method

The standard free energy of formation for zirconium molybdate (ZrMo2O8) was derived from its vapour pressure measured in the temperature range 1029 to 1142 K employing the transpiration technique. ZrMo2O8 vaporizes incongruently according to the reaction n〈ZrMo2O8〉 → n〈ZrO2〉 + 2(MoO3)n, n = 3,4,5. The standard free energy of formation of ZrMo2O8 calculated from the partial pressure of (MoO3)3 in the vapour above ZrMo2O8 + ZrO2 mixture could be expressed by the relation ΔfG°〈ZrMo2O8〉 =(−2525.5±4.9) +(0.6115±0.0045)Tkj/mol (1029) < T/K < 1142). The values of ΔfG°〈ZrMo2O8〉 obtained in this work agree very well with those reported recently from solid electrolyte galvanic cell measurements.

Vaporization behaviour and thermodynamic stability of HfMo2O8

Abstract The standard Gibbs energy of formation for hafnium molybdate (HfMo2O8) was derived from its vapour pressure measured in the temperature range 1023 to 1185 K, employing the transpiration technique. HfMo2O8 vaporizes incongruently according to the reaction n〈HfMo2O8〉 → n〈HfO2〉 + 2(MoO3)n (n = 3, 4, 5). The standard Gibbs energy of formation of HfMo2O8 calculated from the partial pressure of (MoO3)3 in the vapour phase above the HfMo2O8 + HfO2 mixture could be expressed by the relation ΔfG°〈HfMo2O8〉 (kJ/mol) = −(2498.78 ± 4.06) + (0.6039 ± 0.0037)T (1023

Vaporization behaviour and thermodynamic stability of zirconium tellurate ZrTe3O8

Abstract The standard Gibbs energy of formation for zirconium tellurate ZrTe3O8 was derived from its vapour pressure measured in the temperature range 1008 to 1146 K, employing the transpiration technique. ZrTe3O8 vaporizes incongruently, according to the reaction 〈ZrTe3O8〉 ai 〈ZrO2〉 + 3(TeO2). The standard Gibbs energy of formation (ΔfG°) of 〈ZrTe3O8〉 calculated from the partial pressure of TeO2(g) in the vapour phase above the 〈ZrO2〉 + 〈ZrTe3O8〉 mixture can be represented by the relation ΔfG°〈ZrTe3O8〉(± 14.3 kJ mol−1) = −2168.3 + 0.801T(K) (1008 ≤ T/K ≤ 1146). The standard enthalpy of formation ΔfH°(298.15K) for 〈ZrTe3O8〉 derived from these data employing the estimated heat capacity for the compound was found to be −(2153.0 ± 18.3) kJmol−1, in good agreement with the value of −(2131.2 ± 9.64) kJmol−1 determined by isoperibol calorimetry.

A thermoanalytical study of solid state reactions between tellurium oxide and the oxides of zirconium and hafnium

Abstract The solid state reactions of tellurium oxide with zirconium and hafnium oxides were investigated employing thermogravimetry (TG), differential thermal analysis (DTA) and X-ray diffraction (XRD) techniques. The only compounds reported in these two pseudo-binary systems are ZrTe3O8 and HfTe3O8, respectively. From the results obtained in these studies it is concluded that these compounds can be synthesized in the solid state in the pure form from their binary component oxides by heating the stoichiometric mixtures to temperatures as low as 900 to 950K where the vaporization of the more volatile component TeO2, is insignificant.

The standard molar enthalpy of formation of ZrTe3O8

Abstract The molar enthalpies of solution of ZrTe 3 O 8 , TeO 2 , and ZrF 4 , in HF(aq) of concentration 10 mol.dm −3 have been measured using an isoperibol calorimeter. From the results and other auxiliary quantities, the standard molar enthalpy of formation of ZrTe 3 O 8 (s) has been determined to be Δ f H m °(298.15 K) = −(2119.2 ± 9.6)kJ·mol −1 .

The standard molar enthalpy of formation of HfTe3O8

Abstract The molar enthalpies of solution of HfTe 3 O 8 , TeO 2 and HfF 4 in 10 mol dm −3 HF(aq) have been measured using an isoperibol-type calorimeter. From these results and other auxiliary data, the standard molar enthalpy of formation of HfTe 3 O 8 (s) has been calculated to be Δ f H m 0 (298.15 K) = −2129.1 ± 10.1 kJ mol −1 . This value of enthalpy of formation of HfTe 3 O 8 is consistent with the free energy of formation of HfTe 3 O 8 determined in this laboratory by the transpiration technique.

The standard molar enthalpy of formation of ZrMo2O8

Abstract The molar enthalpies of a solution of ZrMo 2 O 8 , Mo, and ZrF 4 in 10 mol dm −3 HF(aq)+4.41 mol dm −3 H 2 O 2 (aq) have been measured using an isoperibol-type calorimeter. From these results and other auxiliary data, the standard molar enthalpy of formation of ZrMo 2 O 8 has been calculated to be Δ f H 0 m (298.15 K) = −2588.6±4.5 kJ mol −1 . This value of enthalpy of formation of ZrMo 2 O 8 is consistent with the second law of enthalpy of formation of ZrMo 2 O 8 determined in this laboratory by the transpiration technique.

The standard molar enthalpy of formation of HfMo2O8

The molar enthalpies of solution of HfMo2O8, Mo and HfF4 in (10 mol HF(aq) + 4.41 mol H2O2(aq)) dm−3 have been measured using an isoperibol type calorimeter. From these results and other auxiliary data, the standard molar enthalpy of formation of HfMo2O8(S) has been calculated to be ΔfHm(298.15K) = −2605.8 ± 5.9kJ mol−1. This value of enthalphy of formation of HfMo2O8 is consistent with the second law enthalpy of formation of HfMo2O8 determined in this laboratory by transpiration technique.