The Thermodynamics of metal-cyanide interaction :|bII. The thermodynamics of Prussian blue and Turnbull's blue formation : III. A study of the chemistry of cobalt (II) cyanide ion reaction


ΔH° values are reported for the formation of Fe(CN)_6^4- (-85.77 ± 0.09 kcal/mole), Fe(CN)_6^3- (-70.14 ± 0.10 kcal/mole), Mn(CN)_6^4- (-36.76 ± 0.07 kcal/mole), Co(CN)_5^3- (-61.96 ± 0.15 kcal/mole), Ni(CN)_4^2- (-43.26 ± 0.08 kcal/mole), Zn(CN)_4^2- (-28.53 ± 0.10 kcal/mole), Cu(CN)_n^1-n (n = 2,3,4 - 29.1 ± 0.10, - 40.3 ± 0.10, and - 50.5 ± 0.01 kcal/mole respectively), PdBr_4^2- (-13.11 ± 0.06 kcal/mole), PdCl_4^2- (-5.50 ± 0.05 kcal/mole), Pd(CN)_4^2- (-92.14 ± 0.10 kcal/mole), Cd(CN)_4^2- (-28.50 ± 0.10 kcal/mole), Ag(CN)_2^- (-32.88 + 0.08 kcal/mole), and Ag(CN)_3^2- (-33.45 ± 0.10 kcal/mole) from their respective constituent ions in aqueous solution. A method is discussed for estimating the ligand field stabilization energy for low spin octahedral complexes from the measured ΔH° value. A comparison of the ΔH values for the formation PdX_4^2- (X = CN, Cl, Br) from their respective aqueous ions show that the ΔH value for Pd(CN)_4^2- is 75 kcal/mole more exothermic than the ΔH values for the formation of PdX_4^2- (X = Cl,Br). This large difference in ΔH values is discussed in terms of different bonding properties of the CN^-, Cl^- and Br^- ions. Evidence is presented which suggests that the nonstepwise formation of some cyano complexes results from ligand field effects. The ΔH° values for the formation of soluble Turnbull's and Prussian blue were measured. The resulting values are -15.08 ± 0.05 and +0.65 ± 0.01 kcal/mole, respectively. Soluble Prussian and Turnbull's blue were found to be thermochemically equivalent and to be noncolloidal in aqueous solution. The Co^2+ -CN^- system was investigated in dilute aqueous solution. The heat of interaction of Co^2+ and CN^- was studied as a function of the CN^-/Co^2+ ratio and the H^+ concentration. An enthalpy change of -30 kcal/mole results when the Co(CN)_5^3- ion reacts with one equivalent of H^+. A kinetic study was made of the decay of Co(CN)_5^3- under controlled pH conditions and as a function of Co(CN)_5^3- concentration. The following reactions were postulated as steps in the decay of Co(CN)_5^3- to form Co(CN)_5 H_2O^2- and H_2. Co(CN)_5^3- + H^+ = HCo(CN)_5^2- (1) 2HCo(CN)_5^2- = [ HCo(CN)_5 ]_2^4- (2) [ HCo(CN)_5 ]_2^4- = 2Co(CN)_5^2- + H_2 (3)



College and Department

Chemistry and Biochemistry



Date Submitted


Document Type





Calorimetry, Cyanides



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