THERMOCHEMISTRY OF THE AZIDE ANION - ASSIGNMENT OF DELTA(F)H(0)(N3-,G) USING VISCOSITY B-COEFFICIENT DATA

Uncertainty concerning the magnitudes of key thermodynamic values at the heart of azide chemistry has prevailed for a number of years. The ''direct' experimental determination of quantities like DELTA(f)H0(N3-,g), differ quite markedly (some 50 kJ mol-1) from the values indicated from indirect thermochemical cycles via lattice energies. The viscosity B-coefficients of alkali metal azides in aqueous solution are utilised, in conjunction with a well-established correlation, to estimate these important quantities from a new standpoint. The following values constitute the best estimates via this route: DELTA(f)H0(N3-,g) = 189 +/- 5.2 and DELTA(f)H0(N3.,g) = 450.5 +/- 11.8 kJ mol-1 (based on the averaged electron affinity of the azide radical, A(e)(N3.,g) = 261 +/- 5.6 kJ mol-1 from recent studies). Estimates of the lattice energies of the azide salts are presented. A value for the bond dissociation energy D0(H-N3) is assigned to be 388 +/- 13 kJ mol-1.