CYANIDE THERMODYNAMICS .4. ENTHALPIES AND ENTROPIES OF CYANIDE COMPLEXATION OF CU-I, AG-I, ZN-II AND CD-II

The enthalpy changes associated with the complexation of Cu-I, Ag-I, Z n-II and Cd-II by cyanide to form Cu(CN)(4)(3-), Ag(CN)(2)(-), Zn(CN)( 4)(2-) and Cd(CN)(4)(2-) have been determined by titration calorimetry in (wherever possible) both aqueous NaCl and NaClO4 media at an ionic strength of 1 mol 1-1 and 25 degrees C. The corresponding entropy cha nges were calculated by combining these data with stability constants from the literature. The Delta H values show little dependence on the medium but the Delta S values vary considerably. This is attributed to competitive complexation of some of the metal ions by Cl-. The great stability of the cyanide complexes in aqueous solution is shown to be due entirely to the favourable Delta H term as Delta S is negative. Po ssible reasons for these effects and their variation between metal ion s are discussed in terms of the solvation characteristics of the speci es involved.