Thermochemistry of molybdates IV. Standard enthalpy of formation of lithium molybdate, thermodynamic properties of the aqueous molybdate ion, and thermodynamic stabilities of the alkali-metal molybdates

Abstract

The enthalpy changes of the processes: Moo 3+2 LiOH(aq, 0.2 mol dm −3= Li 2 MoO 4( AQ)+ H 2 O(L); δH=−(18.54±0.04) kcal th mol −1 and Li 2 MoO 4( c)= Li 2 MoO 4( aq); δH=−(7.61±0.01) kcal th mol −1 , have been determined in a solution calorimeter. These results, when combined with auxiliary thermochemical values, yielded the standard enthalpy of formation, ΔH f o(Li 2MoO 4, c, 298.15 K) = −(363.26 ± 0.12) kcal th mol −1. Based on thermodynamic and solubility data from the literature for sparingly soluble metal molybdates, ΔG f o(MoO 4 2−, aq, 298.15 K) was deduced to be −(200.0 ± 0.3) kcal th mol −1. This paper also includes estimates of ΔH f o for solid Rb 2WO 4 and Cs 2WO 4, and of the enthalpy of sublimation for Cs 2WO 4. It is also pointed out that ΔH f o(PbMoO 4) may be in error, and a value of −245 kcal th mol −1 is suggested.