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, ΔsHm, 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, ΔsHm0, was put forward on the bases of Pitzer solution theory of mixed electrolytes. The values of ΔsHm0 and the sum of Pitzer parameters:
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]−, ΔHdis≈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, ΔdHm, in dissolution process of the IL.