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.