Nitrogen bond enthalpy terms

A The calculation of the bond enthalpy term for nitrogen-fluorine bonds in NF3 is ... 

The P2 molecule, with a P=P triple bond, is unstable except at high temperatures, and P4 and other forms of phosphorus with P—P single bonds are more stable at ordinary temperatures. This behaviour is in contrast to that of nitrogen, which is stable only as N2 molecules. The difference between the two elements is a consequence of 2p-2p n bonding being more effective than 3p-3p n bonding, and of the N—N o bond being weaker than the P—P a bond. The bond enthalpy terms are listed in Table 11.3. 

Table 11.3 Molar bond enthalpy terms for some nitrogen and phosphorus bonds...

The bond dissociation enthalpies for the nitrogen-nitrogen bond in N2 and [N2] are 945 and 765kJmol respectively. Acconnt for this difference in terms of MO theory, and... 

There is also a relation between the amount of ionic character of a single bond and the enthalpy of formation of the bond. The amount of ionic character in percentage is roughly equal numerically to the heat of formation in kcaJ/mole. In applying this rule one must, of course, correct the heat of formation for the special stability of oxygen and nitrogen in their standard states, as expressed in Equation 3-13. This relation may be derived by expanding the exponential-function in Equation 3-15. The first term in the expansion, i(xA — zb)2, may be compared with the term 23 (xa — xb)2 of Equation 3-13. 

The class of compounds where the carbonyl is bonded to a nitrogen other than the anilino nitrogen is thermochemically poorly represented both in terms of the number of species that have been studied as well as in the qualitative and quantitative understanding of the results derived therefrom. The first species is A-(2-benzamidophenyl)aniline114, the enthalpy of formation for which as solid is —10 84 kJmol-1. The large error bar obscures any thermochemical conclusions we could draw. As documented in our archive, the enthalpy of formation of benzidine and its A-mono and A,A -diacetyl derivatives have all been reported in the same original paper115. From that data we deduce that the solid state disproportionation reaction 20 is exothermic by ca —81 kJmol-1. This is implausible. In a previous section we asserted that the enthalpy of formation of benzidine was substantially accurate. Does the error lie with either or both of the acetyl derivatives ... 

E3.24 The most important terms will involve the lattice enthalpies for the di- and the trivalent ions. Also, the bond dissociation energy and third electron gain enthalpy for nitrogen (Nj) will be large. See Section 3.11 for more details. 

The substance from which all nitrogen compounds are ultimately derived, NiCg), is unusually stable. One explanation of the limited reactivity of the N2 molecule is based on its electronic structure. As discussed in Chapter 10, the bond between the two N atoms in N2 is a triple covalent bond, which is unusually strong and difficult to break. In thermochemical terms, the enthalpy change associated with breaking the bonds in one mole of N2 molecules is very high—the dissociation reaction is highly endothermic. 

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