Thermodynamics of aqueous gallium chloride. Heats of solution and dilution at 25°C

Abstract

The heat of solution of GaCl₃ and heats of dilution of single GaCl₃ solutions in water and of mixed GaCl₃−HCl solutions in HCl solutions (with a fixed HCl concentration of 0.1337 mol-kg⁻¹ HCl) up to 4 mol-kg⁻¹ GaCl₃ were measured at 25°C. While in the acid solutions hydrolysis is suppressed to below 0.5% of total gallium concentration, the measurements in water allow evaluation of the effect of hydrolysis on the relative enthalpy. The Pitzer interaction model for excess properties of aqueous electrolytes was used to interpret the change in relative enthalpy with concentration. Pitzer parameters were derived by statistical inference using ridge regression. Their physical significance is supported by the heat of solution data. The measurements yield the following results for standard heats of formation and Pitzer parameters for the relative molar enthalpy at 25°C: $\begin{gathered} H_{GaCl_3 }^ \circ = - 722 kJ - mol^{ - 1} ; \beta _{L,GaCl_3 }^0 = - 1.52 \times 10^{ - 3} kg - mol^{ - 1} - K^{ - 1} ; \hfill \\ \beta _{L,GaCl_3 }^1 = 1.21 \times 10^{ - 2} kg - mol^{ - 1} - K^{ - 1} ; C_{L,GaCl_3 } = - 1.42 \times 10^{ - 4} kg^2 - mol^{ - 2} - K^{ - 1} ; \hfill \\ H_{GaOHCl_2 }^ \circ = - 492 kJ - mol^{ - 1} ; H_{Ga(OH)_2 Cl}^0 = - 706 kJ - mol^{ - 1} ; \hfill \\ \end{gathered}$

With these parameters the overall variance in the partial molar heat of solution at infinite dilution, extrapolated from the present experiments, is minimized to 0.35 kJ²-mol⁻², while the experimental apparent molar heats of dilution are reproduced on average within 2.7 kJ-mol⁻¹.