R. Pankajavalli

Standard gibbs energy of formation of ZrMo2O8

Abstract The oxide ZrMo2O8 is the only ternary compound existing in the Zr-Mo-O system as reported in the literature. To determine the standard Gibbs energy of formation of ZrMo2O8 (hexagonal) from the constituent binary oxides ZrO2(s) and MoO3(s), the emf of the galvanic cell Pt, ZrO2, MoO2, ZrMo2O8¦15 wt% Y2O3 stabilized ZrO2¦air (PO2 = 0.21 atm), Pt has been studied over the range 986 to 1206 K and emf has been determined to be (EI ± 1.56) (mV) = 772.42–0.29777T(K). Using the literature data, ΔG°f of MoO2(s) and MoO3(s), the ΔG°f,ox(ZrMo2O8) has been found to be ΔG°f,oX ± 13.2 (kJ/mol) = 25.73–0.06157T(K).

Thermodynamic stability of HfMo2O8 by the emf method

Abstract The emf of the galvanic cell, Pt, HfO2, MoO2, HfMo2O8 15wt% Y2O3 air (Po2 = 0.21 atm), Pt (I) stabilized ZrO2 has been measured over the range 982 to 1217 K to yield the least squares expression (E1 ± 2.05) (mV) = 873.29−0.29325T(K). From this expression, the standard Gibbs energy of formation ΔGf,oxo of HfMo2O8 from the constituent binary oxides has been calculated to be ΔGf,oxo (HfMo2O8) ± 13.4 (kJ/mol) = − 13.20−0.06332T(K). At a mean temperature of 1100 K, the ΔGf,oxo of HfMo2O8 is found to be 40 kJ mol−1 more negative than its zirconium analogue.

High-temperature stabilities of Rb3CrO4, Rb4CrO4 and Rb2Cr2O7 by solid electrolyte EMF

Abstract The oxygen potentials in the co-existing mixtures Rb 3 CrO 4 /Rb 2 CrO 4 /Cr 2 O 3 (I) and Rb 2 Cr 2 O 7 (1)/Rb 2 CrO 4 / Cr 2 O 3 (II) were measured by using solid electrolyte galvanic cells with a 15 mol% calcia-stabilized zirconia tube as the electrolyte and air ( P O 2 = 0.21 atm )/Pt as the reference electrode. The EMF values of these electrodes yielded the least-squares expressions ( E I ± 2.57) (mV) = 1216.81– 0.50062 T (K) (798–967 K), ( E I ± 3.45) (mV) = 1167.13 –0.44982 T (K) (994–1189 K) and ( E II ± 1.7) (mV) = 335.66–0.25697 T (K) (973–1153 K), respectively. From the expressions for E I the transition temperature T trans and the standard enthalpy of transformation, ΔG ° T , trans for Rb 2 CrO 4 were determined to be 978 K and 8.0 ± 1.0 kJ / mol , respectively. By making use of the standard Gibbs energy data for α-Rb 2 CrO 4 and Cr 2 O 3 from the literature, the standard Gibbs energies of formation of Rb 2 CrO 4 and Rb 2 Cr 2 O 7 (1) were determined to be ( ΔG ° f , T ( Rb 3 CrO 4 , s ) ± 2.22) ( kJ / mol ) = −1537.62 + 0.38342 T ( K ) and ( ΔG ° f , T ( Rb 2 Cr 2 O 7 , 1) ± 2.89) ( kJ / mol ) = −2049.89 + 0.55197 T ( K ). By combining with Gibbs energy and phase equilibrium data on Rb 4 CrO 4 assessed from the literature, the phase diagram for the Rb-Cr-O system at 773 K has also been proposed.

Thermodynamic stabilities of Cs2CrO4(β), Cs3CrO4 and M2Cr2O7(l) (where M = Na, K or Cs) by solid electrolyte emf method

Abstract The emf of the galvanic cells with Pt, Cs 2 CrO 4 (α or β)|Cr 2 O 3 |Cs 3 CrO 4 (cell I) and Pt, s 2 CrO 4 (β)|Cr 2 O 3 |Cs 2 Cr 2 O 7 (l) (cell II) against air|Pt reference electrode using 15 mol% calcia stabilized zirconia as the electrolyte were measured to be E I(a) (mV) = (1345.88 ± 3.68) − (0.53174 ± 0.03376) T (K); (860–967 K) E I(b) (mV) = (1266.47 ± 1.50) − (0.45345 ± 0.04351) T (K); (1052–1097 K) E II (mV) = (344.42 ± 1.84) − (0.25715 ± 0.0079) T (K); (1025–1189 K). Using the literature data for the standard Gibbs energies of formation of α-Cs 2 CrO 4 and Cr 2 O 3 together with the above numerical expressions for the emf values of cell I, the transition temperature and the standard enthalpy of α-to-β phase transformation in Cs 2 CrO 4 were found to be 1014 K and 12.8 kJ mol −1 . In addition, the Δ G f 0 (Cs 3 CrO 4 ) was derived to be as follows: (Δ G f 0 (Cs 3 CrO 4 ) ± 2.0) (kJ mol −1 = −1531.72 + 0.38044 T (K). The emf results from cell II led to the expression (Δ G f 0 (Cs 2 Cr 2 O 7 ,1 ± 3.35) (kJ mol −1 ) = − 2066.47 + 0.55062 T (K). By making emf measurements on galvanic cells with Pt, M 2 CrO 4 (β)|Cr 2 O 3 |M 2 Cr 2 O 7 (l) (where M = Na or K) against air|Pt as the reference electro configuration identical with cell II, the following expressions for Δ G f 0 of M 2 Cr 2 O 7 (l) were derived. (Δ G f 0 (Na 2 Cr 2 O 7 ,1 ± 2.0) (kJ mol −1 ) = −1942.68 + 0.51289 T (K), (Δ G f 0 (K 2 Cr 2 O 7 ,1 ± 2.5) (kJ mol −1 ) = −2037.35 + 0.5406 T (K). The third law treatment of the emf data on cells I and II yielded the values of −1549.8 ± 1.4 kJ mol −1 and −2108 ± 4 kJ mol −1 r for the standard enthalpies of formation, Δ H f298 0 of Cs 3 CrO 4 (s) and Cs 2 Cr 2 O 7 (s).