Determination of thermodynamic stability of lanthanum chloride hydrates (LaCl3⋠xH2O) by dynamic transpiration method

The decomposition pattern of LaCl3⋅7H2O(s) was studied by thermogravimetric technique. Thermal decomposition of LaCl3⋅7H2O(s) proceeded through the formation of LaCl3⋅3H2O(s), LaCl3⋅H2O(s) and LaCl3(s) in the temperature range 313 to 413 K. Thermodynamic stability of the LaCl3⋅7H2O(s) and its intermediate products were determined by measuring the vapor pressure of water over the compounds employing dynamic transpiration technique. Two independent sets of experiments were carried out to measure the vapor pressure of each step. The average vapor pressure of H2O(g) over LaCl3⋅7H2O(s), LaCl3⋅3H2O(s) and LaCl3⋅H2O(s) could be expressed by the relations: lnpH2O/atm (±0.02)=−7422 (±211)/T + 17.5 (±0.6) (327 ⩽ T/K ⩽ 334), lnpH2O/atm (±0.02) = −8287(±566)/T + 17.5 (±1.5) (365 ⩽ T/K ⩽ 369) and lnpH2O/atm (±0.01) = −8566(±346)/T + 16.4(±1) (391 < T/K < 396). The standard molar Gibbs energy of formation of LaCl3⋅7H2O(s), LaCl3⋅3H2O(s) and LaCl3⋅H2O(s) derived from the vapor pressure data are found to be: ΔfG°(LaCl3⋅7H2O,s) = −3226 (±13) + 1.6(±0.1)T (327 ⩽ T/K < 334), ΔfG°(LaCl3⋅3H2O,s) = −2006(±11) + 0.8(±0.1)T (365 ⩽ T/K ⩽ 369) and ΔfG°(LaCl3⋅H2O,s) = −1383(±10) + 0.4(±0.01)⋅T (391 < T/K < 396), kJ mol−1, respectively.