K.N. Roy

Determination of thermal properties of Cs2Cr2O7(s, 1) by high temperature Calvet calorimetry

Enthalpy increments have been determined for caesium dichromate in the temperature range 335 to 826 K using a high temperature Calvet micro calorimeter. A solid-solid transition has been observed at (620.5 ± 1.5 K) and the melting temperature was found to be (657.0 ±1.0) K. The corresponding enthalpy values are (15.6 ± 0.2) kJ/mol and (17.0 ± 0.22) kJ/mol. The results thus obtained are utilised for the evaluation of molar heat capacities, standard entropies and free energy functions for Cs2Cr2O7(s, l). The enthalpy increment values were fitted to a polynomial and can be represented by eqs. (1) to (3), respectively: (HTo − Ho298.15)(Cs2Cr2O7, s) (J/mol) = −6.410 × 104 + 1.939 × 102T(K) + 7.441 × 10−2T(K)2 (335 to 620.5 K), (1) (HoT − Ho298.15)(Cs2Cr2O7, s) (J/mol) = 1.313 × 106 − 4.137 × 103T(K) + 3.517T(K)2 (620.5 to 656 K), (2) (HoT − Ho298.15)(Cs2Cr2O7, l) (J/mol) = − 1.218 × 105 +3.890 × 102T(K) (657 to 826 K). (3)

Thermochemistry of caesium iodide and caesium chromate

Abstract Molar enthalpies of solution of caesium iodide and caesium chromate have been measured at 298.15 K using an isoperibol calorimeter. The molar enthalpies of solution at infinite dilution Δ sol H m ∞ (298.15 K) for CsI and Cs 2 CrO 4 are (33.33 ± 0.02) kJ · mol −1 and (31.37 ± 0.04) kJ · mol −1 , respectively. These values were used to calculate the standard enthalpy of formation at 298.15 K and the respective values for CsI and Cs 2 CrO 4 are −(348.27 ± 0.85) kJ · mol −1 and −(1429.98 ± 1.92) kJ · mol −1 .

Thermal properties of Na2MoO4(s) and Na2Mo2O7(s) by high-temperature calvet calorimetry in the temperature range 335 K to 760 K

Enthalpy increment Δ298.15 KTHmo measurements were made on Na2MoO4(s) and Na2Mo2O7(s) in the temperature range 335 K to 760 K by the drop method using a high-temperature Calvet calorimeter. The calorimeter was calibrated using an electrical method and synthetic sapphire SRM-720 (Al2O3). An on-line computer was used for acquiring and processing results from the calorimeter. The enthalpy increments for Na2MoO4(s) and Na2Mo2O7(s) were least-squares fitted to a polynomial with temperature and are given by: Na2MoO4(s): ΔT298.15 KH∘/(J·mol−1)±109=−38795+113.7(T/K)+0.0546(T/K)2, (335 to 520 K) ΔT298.15 KH∘/(J·mol−1)±111=−38598+142.8(T/K)+0.00458(T/K)2, (520 to 720 K) ΔT298.15 KH∘/(J·mol−1)±538=−105740+270.7(T/K) (720 to 760K) Na2Mo2O7(S): ΔT298.15 KH∘/(J·mol−1)±318=−62413105740+190.6(T/K)+0.0589(T/K)2, (335 to 760 K) The thermal properties of Na2MoO4 and Na2Mo2O7 were obtained using the above experimental values. These are the first experimental results on the thermal properties of these compounds.

Standard molar Gibbs free energy of formation of Na2ZrO3(s)

Abstract The standard molar Gibbs free energy of formation of Na2ZrO3(s) has been obtained by measuring equilibrium pressures of CO2 for the reaction: Na2CO3(s) + ZrO2(s) = Na2ZrO3(s) + CO2(g) by a static manometric method in the temperature range 878 to 1107 K. The CO2 pressures were least-squares analysed and can be represented by: lg ( p kPa ) = 7.470 − 7385.2( K T )±0.038, (878 to 1107 K ) . ΔfGmoNa2ZrO3(s) has been evaluated using ΔfGmos of CO2(g), ZrO2(s), and Na2CO3(s) and can be represented by Δ f G m o (Na 2 ZrO 3 , s , T) ( kJ · mol −1 ) = −1676.27+0.348( T K )±1.1 . Second- and third-law analyses yielded ΔfHmo(Na2ZrO3, s, 298.15 K) as −(1662.9±7.6) and −(1654.9±8.02) kJ · mol−1, respectively.

Thermodynamics of the vaporisation of thorium tetrachloride

Abstract Vapour pressures of solid and liquid ThCl4 have been measured in the temperature range 880 to 1037 K and 1045 to 1160 K both by transpiration and evaporation-temperature techniques. The vapour pressures obtained from the two methods agree well and so were combined to get the vapour-pressure equations for solid and liquid ThCl4. Melting temperature, evaporation temperature, and enthalpy of fusion at 1043 K for ThCl4 were evaluated to be 1043 K, 1205 K, and (14.84±0.89) kcalth mol−1 respectively. The values of the enthalpy of vaporization ΔHo(298.15 K) calculated from the vapour pressures for solid ThCl4 by second-law and third-law methods are (57.20±0.38) kcalth mol−1 and (52.97±0.34) kcalth mol−1.

Solid-state e.m.f. and calorimetric measurements on Cs2Cr2O7(I)

ΔfGmo(Cs2O7 l, T) has been determined in the temperature range 797 to 874 K using a solid-oxide-electrolyte galvanic cell of the type: Pt|Cs2Cr2O7(l)+Cs2CrO4(s)+Cr2O3(s)|0.85ZrO2+0.15CaO|air|Pt where p(O2) in air is taken to be 21.21 kPa. The measured e.m.f.s could be represented by: (E/mV) ± 0.5 = 160.0 − 0.08104(TK). Using the above equation and the reported values of ΔfGmo(Cs2CrO4, s) and ΔfGmo(Cs2Cr2O3, s), ΔfGmo(Cs2Cr2O7, l) calculated in the temperature range 797 to 874 K, is given by ΔfGmo(Cs2Cr2O7, l, T)/kJ·mol−1) ± 10 = −2023 + 0.5262 (TK). The melting and transition temperatures, molar enthalpies of fusion and of transition, and (Δ298.15KTHmo(Cs2Cr2O7, l) in the temperature range 662 to 826 K were determined using a high-temperature Calvet microcalorimeter. These results were used to evaluate ΔfGmo(Cs2Cr2O7, 298.15 K) and the value of −(2100 ± 10) kJ · mol−1 compares well with the values reported in the literature.

Thermochemistry of lithium chromate Li2CrO4(cr) and lithium molybdate Li2MoO4(cr)

The standard molar enthalpies of formation ΔfHom at the temperature T = 298.15 K of Li2CrO4(cr) and Li2MoO4(cr) have been determined using an isoperibol solution calorimeter. The value of ΔsolH∞m for Li2CrO4(cr) in water at T = 298.15 K was found to be −(45.77 ± 0.29) kJ·mol−1 and was used to obtain ΔfHom(298.15 K) as −(1393.7 ± 0.3) kJ·mol−1. The ΔsolHom of Li2MoO4(cr) and of {Li2O(cr) + MoO3(cr) in LiOH(aq, 0.1 mol·dm−3) at T = 298.15 K were used to obtain a value of −(1519.2 ± 2.2) kJ·mol−1 for ΔfHom for Li2MoO4(cr).

Standard molar enthalpies of formation at the temperature 298.15 K of iron telluride (FeTe0.9) and of nickel telluride (Ni0.595Te0.405)

Abstract The standard molar enthalpies of formation ΔfHmo of FeTe0.9 and Ni0.595Te0.405 have been obtained by measuring the enthalpy of reaction of each of the respective alloys and of their synthetic mixtures in the same solvent using an isoperibol solution calorimeter. Experiments with FeTe0.9 were carried out in (3.0 mol·dm−3 H2SO4 + 0.10 mol·dm−3 K2Cr2O7 + 0.01 mol·dm−3 MnSO4(aq) and with Ni0.595Te0.405 in (7 mol·dm−3 HNO3 + 5 mass per cent of H2SO4)(aq). The standard molar enthalpies of formation at the temperature 298.15 K of FeTe0.9 and Ni0.595Te0.405 were found to be −(26.08±0.36) kJ·mol−1 and −(23.46±0.53) kJ·mol−1, respectively.

Studies on (2UF4+H2 = 2UF3+2HF) and vapour pressure of UF3

Equilibrium constants for 2UF4(s)+H2(g) = 2UF3(s)+2HF(g) have been measured in the temperature range 967 to 1120 K. The results can be expressed in the form: log10Kθ = (6.35±0.17)−(12270±78)(KT). The results have been treated by second- and third-law methods to obtain ΔHo(298.15 K) and the values are (253.5±0.8) and (256.0±1.3) kJ·mol−1 respectively. The value of ΔSo(298.15 K) has been calculated by the second-law method to be 146.4 J·K−1·mol−1. The vapour pressure of UF3(s), measured by the transpiration technique in the range 1229 to 1367 K, can be expressed in the form: log10(pkPa) = (10.26±0.23)−(15666±302)(KT). The standard enthalpy of vaporization ΔHvo(298.15 K) and the standard entropy of vaporization ΔSvo(298.15 K) have been calculated to be (328.7±1.3) kJ·mol−1 and (193.9±0.9) J·K−1·mol−1 respectively. The vaporization results have also been used for the calculation of ΔHfo(UF3, g, 298.15 K) and ΔSfo(UF3, g, 298.15 K): −1165.4 kJ·mol−1 and 311.0 J·K−1·mol−1 respectively.

Preparation of uranium trifluoride and studies on its disproportionation

Abstract The reduction of UF 4 to UF 3 by molecular hydrogen has been studied. At hydrogen flow rates which yield a constant H 2 /HF ratio in the outlet gas stream, the optimum conversion temperature range is 1070–1125 K. Disproportionation studies on UF 3 have also been carried out in the temperature range 1169–1360 K. The variation of specific disproportionation rate ( k ) with temperature can be represented by