V.S. Iyer

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)

Standard molar Gibbs free energy of formation of NaCrO2 by e.m.f. measurements

Abstract The standard molar Gibbs free energy of formation of sodium chromite ΔfGmo(NaCrO2, s, T) has been determined in the temperature range 820 to 1006 K using a solid-electrolyte galvanic cell of the type: Pt|NaCrO 2 +Na 2 CrO 4 +Cr 2 O 3 |(1−x) ZrO 2 +x CaO|air|Pt where x = 0.15 and p(O2) in air is taken to be 21.21 kPa. The measured e.m.f.s. could be represented by (E/ mV )±2.8=605.8−0.4696(T/ K ) Using this equation and the reported standard molar Gibbs free energies of vormation of Na2CrO4 and Cr2O3, the standard molar Gibbs free energy of formation of NaCrO2 was calculated and is given by Δ f G° m ( NaCrO 2 , s,T ) ( kJ · mol −1 )±2.5=−856.1+0.1693(T/K)

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.

Calorimetric studies on uranium molybdate

Abstract Enthalpy increment measurements on UMoO6(s) have been carried out using a high-temperature Calvet micro-calorimeter in the temperature range 299 to 1000 K. The enthalpy increments were least squares analyzed using Shomate’s method. The complete thermodynamic information for UMoO6(s) has been computed. The enthalpy increment expression for UMoO6(s) as a function of temperature is given by H o (T)−H o (298.15 K )( J mol −1 )=−53928.8+158.65T( K )+21.443×10 −3 T 2 ( K )+14.077×10 5 /T( K ).

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.

Thermodynamic properties of Cr1 − xTe for x = 0.546 and 0.526

Abstract The vapour pressures of tellurium over the alloy Cr 0.474 Te 0.526 have been measured in the temperature range 1130 to 1217 K, and for chromium-rich alloys (Cr + Cr 1 − x Te, x = 0.546 and 0.526) in the temperature ranges 1055 to 1113 K and 1100 to 1260 K, respectively, using a Knudsen-effusion mass-loss technique. Using vapour pressures of tellurium over liquid tellurium, the change in chemical potential of Te in the alloy with respect to its standard state (Te, l) was calculated to be Δμ( Te in Cr 0.474 Te 0.526 , s , T)/( J · mol −1 ) ± 0.217 = −62412.5 + 7.52( T K ) . The molar Gibbs free energies of formation of Cr 0.454 Te 0.546 (low-temperature phase: Cr 3 Te 4 ) and Cr 0.474 Te 0.526 (high-temperature phase: Cr 1 − x Te) were determined from the vapour pressure of tellurium over the chromium-rich alloys and can be represented by Δ f G m o (Cr 0.454 Te 0.546 , s , T)/( J · mol −1 ) ± 338 = −74978 + 41.04( T K ) and Δ f G m o (Cr 0.474 Te 0.526 , s , T)/( J · mol −1 ) ± 211 = −39755.4 + 6.79( T K ) , respectively. The change in chemical potential of Cr in the alloy (Cr 0.474 Te 0.526 ) with respect to pure Cr(s) was calculated from the molar Gibbs free energy of formation of the alloy and the change in chemical potential of Te in the alloy with respect to pure Te(l).

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.

Thermodynamic studies on A2U4O12 and A2U4O13 (A = CsRb) system by emf and calorimetric measurements at high temperatures

Enthalpy increment measurements on A2U4O12 and A2U4O13 (A = CsRb) were carried out in the temperature range 347 to 804 K using a high temperature Calvet calorimeter. The enthalpy increments (H°T − H°298) in J/mol for Cs2U4O12, Cs2U4O13, Rb2U4O12 and Rb2U4O13 can be respectively represented by eqs. (1) to (4). (H°T − H°298)(Cs2U4O12) ± 1091 = − 9.210 × 104 + 325.3T + 0.1003T2 − 4.124 × 106 / T, (1) (H°T − H°298)(Cs2U4O13) ± 579 = − 1.309 × 105 + 555.1T − 0.0902T2 − 7.910 × 106 / T, (2) (H°T − H°298)(Rb2U4O12) ± 556 = 8.772 × 102 + 192.7T+0.0763T2 − 1.933 × 107 / T, (3) (H°T − H°298)(Rb2U4O13) ± 182 = − 1.257 × 105 + 422.5T + 1.114 × 10 − 3 / T2. (4) The molar Gibbs free energies of formation for Cs2U4O13 and Rb2U4O12 were obtained by measuring emfs in the temperature range 1019 to 1283 K and can be given by ΔfG°m(Cs2U4O13, s, T)(kJ/mol) ± 0.5= − 5902 + 1.390T(K), (5) ΔfG°m(Rb2U4O12, s, T)(kJ/mol) ± 0.3= − 5735 + 1.284T(K). (6) Thermal properties of the compounds were derived from the experimental data.

Enthalpy increment measurements of NaCrO2 using a high temperature Calvet calorimeter

Abstract Enthalpy increment measurements on NaCrO 2 (s) were carried out in the temperature range 323 to 839 K using a high temperature Calvet micro calorimeter. The enthalpy increment values were least-squares fitted with temperature with the constraint that (H T 0 − H 298 0 ) at 298.15 K equals zero and can be given by: ( H T 0 − H 298 0 )( J / mol ) ± 336 = −23515 + 75.364 T ( K ) + 0.01256 T 2 ( K ) (323 to 839 K). The first differential of the above equation with temperature gives the constant pressure molar beat capacity of NaCrO 2 (s), which is given by: C p 0 ( NaCrO 2 , s , T )( J / K mol ) = 75.364 + 0.02512 T ( K ). The thermal properties of NaCrO 2 (s) were calculated using the molar heat capacities from the present study and S 0 (298 K) from the literature.

Thermodynamic study of Ni3Te2 by e.m.f. measurements

Abstract The change in the chemical potential of mixing of Ni in the Ni0.595Te0.405 alloy has been determined in the temperature range 963 to 1178 K by a solid-oxide-electrolyte galvanic cell of the type: Pt | Au | Ni 0.595 Te 0.405 (s)+ NiO |0.85 ZrO 2 +0.15 CaO | air | Au | Pt . where p(O2) in air was taken to be 21.21 kPa. The e.m.f.s could be represented by: ( E mV ) ± 3.9 = 1256−0.5641 ( T K ) . From e.m.f.s, the relative and excess partial molar thermodynamic quantities of Ni have been evaluated.