Heats of formation of the fluoborates of lithium, sodium and potassium

Abstract

Heats of formation of LiBF₄(c) and NaBF₄(c) from solid alkali fluorides and boron trifluoride gas at 110°C(ΔH^°_{comb. 383}) have been measured directly. The heat of formation of KBF₄(c) from the constituent fluorides at 25°C has been determined from that of NaBF₄(c) by comparing the heats of solution in hydrofluoric acid of the pair, NaBF₄(c)+ KF(c), with that of the pair KBF₄(c)+ NaF(c). The following values have been obtained: ΔH^°_comb.298(LiBF₄)=–21.40–0.09 kcal/mole, ΔH^°_comb.298(NaBF₄)=–32.04±0.12 kcal/mole, ΔH^°_comb.298(KBF₄)=–43.13±0.22 kcal/mole. The errors are twice the standard deviation of the mean. With the known values of the heats of formation of BF₃(g), LiF(c), NaF(c) and KF(c), one arrives at the heats of formation of the fluoborates : ΔH^°_f298(LiBF₄)=–439.4±0.7 kcal/mole, ΔH^°_f298(NaBF₄)=–440.6±0.5 kcal/mole, ΔH^°_f298(KBF₄)=–450.2±0.4 kcal/mole.