Electron attachment and compound formation in flames. Part 1.—Electron affinity of BO2 and heats of formation of alkali metal metaborates

Abstract

Addition of boron to atmospheric-pressure fuel-rich H₂+ N₂+ O₂ flames seeded with alkali metals results in reductions in concentrations of free alkali metal atoms and in formation of negative boron-containing ions. The following enthalpy changes and equilibrium constants are inferred: Li + HBO₂⇌ LiBO₂+ H ΔH°₀=–21 ± 20 kJ mole^–1, K= 15 exp (2300/T). Na + HBO₂⇌ NaBO₂+ H ΔH°₀=+ 21 ± 20kJ mole^–1, K= 12 exp (–3000/T). K + HBO₂⇌ KBO₂+ H ΔH°₀=+ 8 ± 20 kJ mole^–1, K= 37 exp(–2500/T). HBO₂+ e^–⇌ H + BO^–₂ΔH₀°=+ 94 ± 20kJ mole^–1, K= 1.5 × 10³ exp (– 10000/T). These enthalpy changes, when combined with JANAF data for HBO₂, BO₂, H, K, Na and Li, lead to standard heats of formation at 0 K of –636 ± 25 kJ mole^–1 for LiBO₂(g), –644 ± 25kJ mole^–1 for NaBO₂(g) and –674 ± 25kJ mole^–1for KBO₂(g), and provide a value for the electron affinity of BO₂of 393 ± 20 kJ mole^–1.