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 H₂ addition to N₂ (ΔH = 47.9 kcal/mol) and the exothermicity of H₂ addition to acetylene (ΔH = -42.0 kcal/mol), less than half is due to a stronger π bond in N₂ than in acetylene. The other major contributor to the difference of 89.9 kcal/mol between the enthalpies of hydrogenation of N₂ and acetylene is that the pair of N-H bonds that are created in the addition of H₂ to N₂ are significantly weaker than the pair of C-H bonds that are created in the addition of H₂ 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═CH₂ are analyzed and discussed.