Thermochemical studies and ion solvation enthalpies in formamide, N-methylformamide, and NN-dimethylformamide

Abstract

Heats of solution of alkali-metal and ammonium perchlorates, thiocyanates, and halides have been measured in the concentration range 0.003-0.25 mol l^-1 in formamide, (F), N-methylformamide, (NMF), and NN-dimethylformamide (DMF) at 26.9°. In some cases, the plots of heats of solution vs. the square root of concentration are linear and have been extrapolated to obtain standard heats of solution. In other cases, where the change in heat of solution with concentration is very small, standard heats of solution have been taken as the mean of four to six observations. Lattice energies of the salts which are not available have been calculated from Kapustinskii's equation. The combined ion-solvation enthalpies have been calculated by using the relation: ΔH°_solv=U+ΔH°_s. A method for obtaining ion-solvation enthalpies of the electrolytes has been suggested based on the modified Born model. Ion-solvation enthalpies of various ions have been evaluated in F, NMF, and DMF by this method. These values in F are in good agreement with absolute ion-solvation enthalpies obtained by an independent approach.