Abstract

Standard enthalpies of formation of sulfides and sulfosalts on the quasi-binary Ag 2 S-Bi 2 S 3 join have been determined experimentally using the technique of high-temperature, direct synthesis drop calorimetry. This represents the first time that enthalpies of formation of bismuthinite (Bi 2 S 3 ) and of the sulfosalts matildite (AgBiS 2 ) and pavonite (AgBi 3 S 5 ) have been determined calorimetrically.For Ag 2 S (acanthite), beta -AgBiS 2 , alpha -AgBiS 2 , AgBi 3 S 5 , and Bi 2 S 3 , the enthalpies of formation at 298K are: -7.46 + or - 0.17, -19.53 + or - 1.6, -17.02 + or - 0.51, -52.45 + or - 0.89, and -32.31 + or - 0.57 kcal mole (super -1) , respectively. A comparison of our value for the enthalpy of formation of Ag 2 S with published values based on earlier calorimetric and galvanic cell data shows excellent agreement. The new value for Bi 2 S 3 is consistent with a number of published values derived from equilibrium studies.Mixing of end-member sulfides (Ag 2 S and Bi 2 S 3 ) to produce sulfosalts of intermediate composition shows that the enthalpy of mixing to produce beta matildite and pavonite is near zero (within experimental error); alpha matildite (the high-temperature disordered form) has a significant positive enthalpy of mixing, delta H mix0 [asymp] 3 kcal mole (super -1) ), indicating that it is entropy stabilized.

First Page Preview

First page PDF preview
You do not currently have access to this article.