Studies on thermochemical properties of ionic liquids based on transition metal

A brown and transparent ionic liquid (IL), C₄mim]FeCl₄], was prepared by mixing anhydrous FeCl₃ with 1-butyl-3-methylimidazolium chloride (C₄mim]Cl]), with molar ratio 1/1 under stirring in a glove box filled with dry argon. The molar enthalpies of solution, Δ_s H _m, of C₄mim]FeCl₄], in water with various molalities were determined by a solution-reaction isoperibol calorimeter at 298.15 K. Considering the hydrolyzation of anion FeCl₄]⁻ in dissolution process of the IL, a new method of determining the standard molar enthalpy of solution, Δ_s H _m⁰, was put forward on the bases of Pitzer solution theory of mixed electrolytes. The values of Δ_s H _m⁰ and the sum of Pitzer parameters: and were obtained, respectively. In terms of thermodynamic cycle and the lattice energy of IL calculated by Glasser’s lattice energy theory of ILs, the dissociation enthalpy of anion FeCl₄]⁻, ΔH _dis≈5650 kJ mol⁻¹, for the reaction: FeCl₄]⁻(g)→Fe³⁺(g)+4Cl⁻(g), was estimated. It is shown that large hydration enthalpies of ions have been compensated by large the dissociation enthalpy of FeCl₄]⁻ anion, Δ_d H _m, in dissolution process of the IL.