Thermochemistry and reactivity of the azides - II. Lattice energies of ionic azides, electron affinity and heat of formation of the azide radical and related properties

Abstract

The thermochemical data of part I, the heats of formation and solution of the alkali-metal (group 1 a ) azides, are used in conjunction with other data to derive values for the lattice energies of alkali-metal azides, the heat of formation of the azide radical, for the electron affinity and hydration heat of the azide ion. Calculations by previous workers of these magnitudes, which are not of course susceptible to direct measurement, have generally been erroneous. The lattice energies of the alkali azides (kcal mole -1 ) are: LiN 3 , 194; NaN 3 , 175; KN 3 , 157; RbN 3 , 152; CsN 3 , 146. For potassium, rubidium and caesium azides a term-by-term theoretical calculation of the lattice energy which allows for the non-spherical character of the azide ion supports these figures, which are based on experimental data of part I. The standard enthalpy of formation of the azide radical ∆ H 0 f (N 3G ) is estimated to be 116 kcal mole -1 . The electron affinity of the azide radical E (N 3G ) is 81 kcal mole -1 . These figures permit the evaluation of other lattice energies and the following values (kcal mole -1 ) have been obtained: NH 4 N 3 , 175; CuN 3 , 227; AgN 3 , 205; TlN 3 , 163·5; CaN 6 , 517; SrN 6 , 494; BaN 6 , 469 and PbN 6 , 516. From the enthalpy of formation of the azide radical the bond dissociation energies D ( X — N 3 ) in some covalent azides may be derived. D (H — N 3 ) is 96 kcal mole -1 and D (C— N 3 ) in aliphatic azides is about 83 kcal mole -1 .