Why Are Addition Reactions to N2 Thermodynamically Unfavorable?

Thermochemical data are used to show that, of the 89.9 kcal/mol difference between the endothermicity of H-2 addition to N-2 (Delta H = 47.9 kcal/mol) and the exothermicity of H-2 addition to acetylene (Delta H = -42.0 kcal/mol), less than half is due to a stronger pi bond in N-2 than in acetylene. The other major contributor to the difference of 89.9 kcal/mol between the enthalpies of hydrogenation of N-2 and acetylene is that the pair of N-H bonds that are created in the addition of H-2 to N-2 are significantly weaker than the pair of C-H bonds that are created in the addition of H-2 to acetylene. The reasons for this large difference between the strengths of the N-H bonds in E-HN=NH and the C-H bonds in H2C=CH, are analyzed and discussed.