J . Phys. Chem. 1993, 97, 7876-7879
7876
Thermochemistry of the Azide Anion. Assignment of A@(NJ-,g) Using Viscosity &Coefficient Data Harry Donald Brooke Jenkins Department of Chemistry, University of Warwick, Coventry CV4 7AL, West Midlands, England, U.K. Received: March 5, 1993
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 AfHO(N3-,g), differ quite markedly (some 50 kJ mol-') 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: A@(Ns-,g) = 189 f 5.2 and AfHO(N3',g) = 450.5 f 11.8 kJ mol-' (based on the averaged electron affinity of the azide radical, Ae(N3',g) = 261 f 5.6 kJ mol-' from recent studies). Estimates of the lattice energies of the azide salts are presented. A value for the bond dissociation energy DO(H-N3) is assigned to be 388 f 13 kJ mol-'.
Introduction The key thermodynamic quantities in azide thermochemistry can be embodied into the two cycles shown in Figure 1 (centred on hydrogen azide, HN3) and in Figure 2 (concerned with monovalent azides, MN3). There are four consistency relationships that are relevant in selectingthe best thermochemical values for the free radical, N3' and anion, N3-, and for the bond energies in the azides: Ap(N,-,g) + Ae(N3',g) - D0(H-N3) = IP(W (1) whereAp(Ns-,g), Ae(N3',g), DO(H-N3) and IP(H) are the proton affinity, electronaffinity, H-N bond dissociationenergy (in HN3) and ionization potential of hydrogen as defined in Figure 1. The second relationship is a definition: Ae(N3',g) - A f P ( N 3 ' A + Af@(N
