Entropies of tetrahedral M—phenyl species

Abstract

Tetraphenylarsonium tetraphenylborate, widely used as a reference electrolyte, is often related to neutral compounds isostructural with its constituent ions. Thermodynamic information concerning them is lacking. Standard molar entropies of the gaseous ions Ph₄B^–, Ph₄P⁺, and Ph₄As⁺, the species Ph₄C, Ph₄Si, Ph₄Ge, Ph₄Sn and Ph₄Pb, and the related compounds Ph₃CH and Ph₂CH₂ have been determined from calorimetric data where available and statistically from published spectroscopic and structural data. The latter calculation, deemed more reliable by us, gave the values (in J K^–1mol^–1): 654, 649 and 648 for the ions, 639, 661, 664, 680 and 694 for the neutral Ph₄M compounds, and 541 and 436 for the tri-and di-phenylmethanes. The standard partial molar entropy of the Ph₄B^– anion is 348 J K^–1mol^–1, hence its entropy of hydration is —306 J K^–1mol^–1. This value and the presumed similar one for Ph₄As⁺ indicate that these ions enhance the structure of water. The standard partial molar heat capacities of aqueous Ph₄P⁺, Ph₄As⁺ and Ph₄B^– are 1231, 1258 and 1051 J K^–1mol^–1, and those of the gaseous ions are 370, 372 and 367 J K^–1mol^–1, respectively. The heat-capacity changes on hydration also indicate these ions to be water-structure-makers.