Low temperature heat capacity data on K2ReCla are presented and the thermodynamic functions are tabulated. Of the four cooperative transitions exhibited by this compound with heat capacity maxima a t 11.9, 76.05, 103.4 and 110.9'1L, only the one a t the lowest temperature is the result of a magnetic transition. It is estimated that an entropy of approximately R In 2 is associated with each of the three remaining transitions. The solubility of KzReC16 in 0.01 M HC1 is 0.1689 m a t 25' and 0.0841 m a t 0'. The thermodynamic functions a t 298.15% (cal. deg.-l mole-l) are: C, O = 51.31, S O = 88.84 f 0.18, (HO -Hoa)/T = 37.970 and -(Fa -HoO))/T = 50.873. The entropy and entropy of formation a t 298.15"K. of the hexachlorohenate ion are: SO(ReCla-(aq.)) = 59.8 cal. deg.-l mole-' and ASP(ReCls-(aq.)) = -140.2 cal. deg.-l mole-I.The standard heat of solution is 10,397 cal. mole-' a t 25".
The standard enthalpy of hydrolysis of rhenium trichloride in alkaline solution to form hydrated rhenium sesquioxide has been observed to be -54.29 ± 0.14 kcal (gfw of ReCls)-1 at 25°. This result combined with other available thermal data gives for the enthalpy of formation AH°(Re203) = -119.2 kcal mol-1. The entropy of Re203 is estimated to be 32 cal deg-1 mol-1 which leads to AFf°(Re203) = -101.6 ± 0.6 kcal mol-1 for the free energy of formation at 25°. The electrode potentials of the half-reactions 2Re + 60H-= Re203 + 3H20 + 6e~and Re203 + 20H-+ 3H20 = 2Re02*2H20 + 2eare computed to be 0.333 ± 0.004 and 1.25 ± 0.01 V, respectively.
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